The physics of ultrasound Flashcards
(242 cards)
1
Q
What is B mode ultrasound?
A
Many 2D images can be generated per minute, and a moving image of the heart is made = real-time or B mode ultrasound.
Fig 1.1
2
Q
What is M-mode image=
A
By sending out only one sound beam instead of many, only the structures associated with that one beam are seen; producing an M-mode image. The structures associated with that one line through the heart keep scrolling on the screen as the heart continues to contract and relax. The M-mpde imager displays depth on the vertical axis and time along the horizontal axis.
Fig 1.2
3
Q
What is Doppler?
A
Doppler is used in diagnostic ultrasound to provide information on blood flow (spectral and color-flow Dopller) or myocardial motion (tissue Doppler imaging-TDI) of the heart and its vessels. Specific locations can be selected. As in M-mode, the horizontal axis represents time, while the vertical axis represents velocity.
Fig 1.3
4
Q
Sound waves travel in…… lines within the medium
A
longitudinal
5
Q
Sound waves: The molecules along that longitudinal course of movement in their pathway are alternately compressed (molecules move closer together) and rarefacted (molecules are ………)
A
spread apart
The time required for one complete compression and rarefaction to occur is one cycle.
(Figure 1.4).
6
Q
What is wavelength?
A
The distance in millimeters that the sound wave travels during 1 cycle is its wavelength
7
Q
What determines the length of a cycle?
A
The source of the sound. transducers generate the sound in diagnostic ultrasound. For any given transducers the wavelength is constant.
8
Q
What is the frequency?
A
The number of cycles per second is the frequency of the sound wave.
Fig 1.5
9
Q
Frequecency is measured in?
A
Hertz (Hz)
10
Q
1 Hz equals ……….per second.
A
once cycle
11
Q
Ultrasound has a frequency greater than …….cycles per second
A
20 000 cycles per second, which is beyond the range of human hearing
12
Q
Since frequency is the number of complete cycles per second; the higher the frequency of the sound wave the ……………..must be.
A
shorter the wavelength
13
Q
A 5.0 - megahertz (MHz) transducer transmits ………….. cycles per second at 0.31 millimeters (mm) per cycle, while a 2.0 - MHz transducer transmits only ……….. cycles per second at 0.77 mm per cycle.
A
A 5.0 - megahertz (MHz) transducer transmits 5 million cycles per second at 0.31 millimeters (mm) per cycle, while a 2.0 - MHz transducer transmits only 2 million cycles per second at 0.77 mm per cycle.
Table 1.1 lists wavelengths for sound generated at various frequencies.
14
Q
The speed of sound (V) depends upon the …………. and ……….. of the medium through which it is traveling.
A
The speed of sound (V) depends upon the density and stiffness of the medium through which it is traveling.
15
Q
Increased density allows sound to travel faster. Does the velocity of sound change within a homogeneous substance? Is the velocity dependent or independent of frequency?
A
Increased density allows sound to travel faster. The velocity of sound does not change within a homogeneous substance and is independent of frequency
(Figure 1.6 ). Table 1.2 lists the speed of sound in various tissues.
16
Q
The speed of sound through air is very …… because of its low density, while bone allows sound to travel at relatively …….. speeds.
A
The speed of sound through air is very slow because of its low density, while bone allows sound to travel at relatively high speeds.
17
Q
The average velocity of a sound wave in soft tissue is ……….. meters per second regardless of transducer frequency.
A
The average velocity of a sound wave in soft tissue is 1,540 meters per second regardless of transducer frequency (Figure 1.7 ).
18
Q
Velocity is calibrated into the ultrasound machine, which then calculates the distance (D) to cardiac structures based upon how long it takes to receive reflected echoes (T):
Hur är formeln?
A
D = V x T/2
19
Q
Does transducer frequency affect the speed of sound in tissues?
A
No
20
Q
The number of cycles per second is the frequency of the sound wave. Frequency is measured in Hertz (Hz). One Hz equals one cycle per second.
Hur många cyklar är 5 Hz?
A
5 cycles/sec
Fig 1.5
21
Q
Increased tissue density allows sound to travel faster. Why will sound generated by a 2.5 - MHz transducer and a 5.0 - MHz transducer have the same velocity within the same tissues?
A
Sound generated by a 2.5 - MHz transducer and a 5.0 - MHz transducer will have the same velocity within the same tissues since the speed of sound is not affected by frequency. Fig 1.6
Se tabel 1.1 and 1.2
22
Q
Sound travels through soft tissues at an average velocity of ………… m/sec regardless of transducer frequency.
A
Sound travels through soft tissues at an average velocity of 1,540 m/sec regardless of transducer frequency.
The time required to travel 1 cm at 1,540 m/sec is 6.5 microseconds ( μ sec) one way and 13 μ sec round trip.
Fig 1.7
23
Q
The time (T) required to travel 1 cm is ……….. microseconds or ……..microseconds round trip.
A
The time (T) required to travel 1 cm is 6.5 microseconds or 13 microseconds round trip.
Even though sound must travel through various tissues with slightly different velocities during an echocardiographic exam, the equipment is calibrated for the average speed of sound in soft tissues (1,540 meters per second). Structures are displayed on a monitor at the calculated depth, and an image of the heart is created. This always creates some degree of error in calculating true structure depth, but the error is generally negligibl
24
Q
What is Acoustic Impedance?
A
Acoustic impedance is the opposition or resistance to the flow of sound through a medium.
25
Impedance depends upon the density and stiffness of the medium. Is impedance dependent or independent of frequency?
Impedance is independent of frequency.
Very stiff or hard materials are hard to compress and rarefact. Therefore, although increased density increases the speed of sound, if the ability to compress and rarefact a sound wave is limited, the impedance or resistance to sound transmission is high.
26
Contradictory as it sounds, the higher the density and the greater the velocity of sound through a medium, the ........... the resistance is to sound transmission.
Contradictory as it sounds, the higher the density and the greater the velocity of sound through a medium, the greater the resistance is to sound transmission.
Table 1.3 lists the acoustical impedance of sound in various tissues. Because of its stiffness and inability to compress and rarefact molecules easily, bone has high impedance, while air, because its molecules are easily compressed and rarefacted, has low impedance.
27
High ................. is what produces a high degree of sound reflection at bony or air interfaces, creating a shadow on the ultrasound image beyond the bone or air due to lack of further sound transmission.
High acoustical impedance is what produces a high degree of sound reflection at bony or air interfaces, creating a shadow on the ultrasound image beyond the bone or air due to lack of further sound transmission.
28
Reflection of sounds depends upon (3)?
1: acoustical mismatch: The greater the difference in acoustical properties the greater the degree of reflection.
2. angle of incidence: Sound striking an organ perpendicularly will have a large amount of sound reflected straight back to the transducer.
3. Depends upon reflecting structure ’ s size: Must be at least 1/4 size of the wavelength. Higher frequency transducers can reflect sound from smaller structures.
29
What is reflection?
Sound that is turned back at a boundary within a medium. These reflected echoes are called specular echoes. When an interface between two tissues with different acoustical impedances is reached, a portion of the sound is reflected back to the transducer. The rest continues on through the tissues.
30
The greater the difference in acoustical impedance the .............. the degree of reflection.
The greater the difference in acoustical impedance the greater the degree of reflection.
For the same reason, if two boundaries have little or no acoustical mismatch they will not be identifi ed as two different tissues. Therefore interfaces between muscle and fluid, as in the heart, will reflect sound at different intensities while the cells within the homogeneous muscle itself will reflect sound with similar strengths.
All interfaces between muscle and blood - filled chambers in the heart have slightly brighter boundaries on the ultrasound image because of this increased reflection. The interface between tissue and air has an even greater difference in acoustical impedance, and therefore the pericardial sac around the heart is always one of the brightest structures on the ultrasound image.
31
Why is gel placed between the transducer and skin surface?
The gel placed between the transducer and skin surface is used to prevent the large degree of reflection ordinarily seen between a tissue and air interface.
32
The angle at which sound strikes the reflective surface (the angle of incidence) determines the angle of reflection. The angle of reflection is equal to the angle of ...............
The angle at which sound strikes the refl ective surface (the angle of incidence) determines the angle of refl ection. The angle of reflection is equal to the angle of incidence (Figure 1.8 ).
33
When sound is directed perpendicular to a structure the angle of incidence is zero and the sound is reflected straight back to the transducer. If the angle of incidence is 50 ° , then the angle of reflection will be also be 50 ° . When the angle of incidence is 90 ° or parallel to the interface: How will the reflection be?
When the angle of incidence is 90 ° or parallel to the interface, no sound will be refl ected back to the source.
This principle tells us that the best two - dimensional and M - mode cardiac images are obtained when sound is directed perpendicular to the tissues.
34
Not all sound is reflected however, and some continues on through the tissues. These sound waves are refracted if the two tissues are different. What is refraction?
(Figure 1.8 ): Refraction is the change in direction of sound as it travels from one medium to another.
This is similar to what happens when light waves in water create a distorted image.
35
The ............... the mismatch in acoustical impedance between the two tissues the greater the degree of refraction.
The greater the mismatch in acoustical impedance between the two tissues the greater the degree of refraction.
As the refracted sound beam travels in a new direction, the angle of reflection with respect to the original source is different, and posi- tional errors can result since the transducer thinks the received sound is coming from the same direction as the sound waves it generated earlier.
36
The errors produced by refraction during an examination create few problems unless the refracted beam has to travel a great distance. An angle of 1 or 2 degrees at the top of the refracting tissue can result in a several millimeter error in position by the time it reaches the far side of a deep structure. When the two mediums differ enough to create a refractive angle of greater than ...... ° (as with soft tissue and bone) then an image is not generated beyond the second structure.
When the two mediums differ enough to create a refractive angle of greater than 90 ° (as with soft tissue and bone) then an image is not generated beyond the second structure.
37
Figure 1.8 : The angle of reflection is equal to?
The angle of reflection is equal to the angle at which sound strikes the tissue. Sound that is directed perpendicular to the tissue is reflected straight back to the transducer producing the best images.
38
Sound is refracted when it crosses a boundary between two different tissues. The greater the ................... in acoustical properties between the two tissues, the greater the degree of refraction.
Sound is refracted when it crosses a boundary between two different tissues. The greater the difference in acoustical properties between the two tissues, the greater the degree of refraction.
39
Reflection of sound is not only dependent upon the acoustical mismatch of two tissues but also .............?
Reflection of sound is not only dependent upon the acoustical mismatch of two tissues but also upon the structure’ s size. The structure must be at least one quarter the size of the wavelength for reflection to occur..
40
The short 0.21 - mm wavelengths of a 7.5 - MHz transducer are reflected from structures that are as small as 0.05 mm in thickness, while structures must be at least 0.19 - mm thick for the 0.77 - mm wavelengths of a 2.0 - MHz transducer to be reflected. High frequency transducers then, provide higher resolution images: why?
Because smaller structures reflect their sound waves.
41
↑ Frequency = .... Wavelength = .. Resolution
↑ Frequency = ↓ Wavelength = ↑ Resolution
42
↓ Frequency = .. Wavelength = ... Resolution
↓ Frequency = ↑ Wavelength = ↓ Resolution
43
Structures that are small and irregular with respect to the sound wave do not reflect sound but rather .................
Structures that are small and irregular with respect to the sound wave do not refl ect sound but rather scatter it in all directions without regard for the angle of incidence (Figure 1.9 ).
Some of this scattered sound is directed back to the sound source and is what allows ultrasound to give us information about tissue character. Scattered sound is important for the generation of images from objects with large angles of incidence to the sound beam or small structure - like cells.
44
Scattered sound is important in ......
In tissue characterization
45
Sound traveling through a medium is weakened by reflection, refraction, scattering, and absorption of heat by the tissues. This loss of energy is called ..........
Attenuation
46
High frequency sound attenuates to a greater degree than lower frequency sound. Why?
High frequency sound attenuates to a greater degree than lower frequency sound because its wavelength allows it to interact with more structures.
This is the reason the deep bass sounds of an orchestra carry farther than the high - pitched sounds. The large degree of attenuation with high frequency sound leaves less energy available for continued transmission through the medium.
47
↑ Frequency = ..... Depth
↑ Frequency = ↓ Depth
48
↓ Frequency = ..... Depth
↓ Frequency = ↑ Depth
49
What is the half-power distance of a tissue?
The half-power distance of a tissue is the distance sound will travel through it before half of the available sound energy has been attenuated.
Table 1.4 lists the half - power distances of various tissues at two different frequencies. The data in this table clearly show that low frequency sound waves are able to penetrate tissues deeper than higher frequency sound waves.
50
Air attenuates half of the sound energy within 0.05 centimeter (cm) when a 2.0 - MHz transducer is used. What happens with sound energy after 0.05 cm?
Therefore, although the density of air creates less impedance for sound, little sound energy is left for image generation from soft tissues after 0.05 cm.
Gel is used to eliminate the air between the transducer and skin, which would otherwise attenuate sound dramatically.
51
What is tissue harmonic imaging?
When ultrasound is transmitted at one frequency and returned at twice or more the transmitted frequency, it is called tissue harmonic imaging.
52
Sound waves change from their sinusoidal shape as they travel through tissues to nonsinusoidal waves. What is this caused by?
This is caused by changes in pressure, with the higher pressure portion of the sound waves traveling faster than the slower portions.
These nonsinusoidal waves contain additional frequencies in multiples of the fundamental or originating frequency. These even and odd multiples of the fundamental frequency are called harmonic frequencies.
53
When using harmonic imaging the fundamental frequency is filtered out and second harmonic waves are used to generate the ultrasound image. This imaging mode is used to enhance what?
This imaging mode is used to enhance the definition of endocardial borders and reduces the generation of artifacts, especially in patients with poor acoustic windows.
54
Poor image quality is usually the result of factors like fat, muscle, and fibrosis that are present before the sound beams have even entered the tissue of interest.
These factors create variations in...?
These factors create variations in the speed of sound and create distortion of the sound beam and the resulting ultrasound image.
55
Where are the harmonic frequencies created?
The harmonic frequencies are created in the chest from the reflected sound and not at the chest wall where most artifacts originate; this results in the alleviation of imaging artifacts especially side lobe artifact. It also enhances contrast resolution of the ultrasound image.
The result is improved image quality with reduced artifact generation, enhanced endocardial details, improved contrast, and decreased noise.
(There are some patients in which harmonic imaging does not improve image quality because of frequency dependent attenuation of sound).
56
Transducers are the source of sound in diagnostic ultrasound. Transducers contain piezoelectric crystals that are deformed by electrical voltage and generate sound. These crystals, often called elements, are also able to receive sound and convert it back into electrical energy. The.............of the crystal dictates the basic operating frequency of the transducer.
The thickness.
57
Wavelength will be one-half of the element thickness so decreased crystal thickness produces ......... wavelengths and .......... frequencies.
Wavelength will be one-half of the element thickness so decreased crystal thickness produces shorter wavelengths and higher frequencies.
58
Transducers used in pulsed - echo applications do not transmit sound ................... They send sound waves out in ...............................This is called pulsed ultrasound.
Transducers used in pulsed - echo applications do not transmit sound continuously. They send sound waves out in short bursts and receive sound the remainder of the time. This is called pulsed ultrasound.
59
The number of pulses per second is referred to as the .................... which is measured in ........
The number of pulses per second is referred to as the pulse repetition frequency (PRF). PRF is measured in Hz.
The PRF for example would be 10 Hz if there are 10 pulses per second (Figure 1.10 ).
60
Each pulse may have any number of cycles, but in diagnostic ultrasound, there are generally ........... cycles per pulse.
Each pulse may have any number of cycles, but in diagnostic ultrasound, there are generally two or three cycles per pulse.
The number of cycles per pulse is controlled by damping materials within the transducer.
61
The duration of a pulse, measured in microseconds, and pulse length, measured in mm, decreases if the frequency of the sound wave .................. since the wavelengths are shorter (Figure 1.10 ). By the same token, ............. frequency sound waves have longer wavelengths so pulse duration and length are increased.
The duration of a pulse, measured in microseconds, and pulse length, measured in mm, decreases if the frequency of the sound wave increases since the wavelengths are shorter (Figure 1.10 ). By the same token, lower frequency sound waves have longer wavelengths so pulse duration and length are increased.
62
Accurate ultrasound images can only be generated if all reflected and scattered echoes are received at the transducer before the next pulse is generated. The transducer assumes that the echoes it receives are products of its last burst. If an echo has not been received before the next burst and it arrives at the transducer shortly after the second burst, then the instrument “ thinks ” very little time has elapsed since it was transmitted and received. Since time is used along with the speed of sound in tissues to determine structure depth, a structure that is actually deeper will be displayed closer to the body surface (Figure 1.11 ). Pulse repetition frequency must ................ as deeper structures are imaged for accurate depth assessment.
Pulse repetition frequency must decrease as deeper structures are imaged for accurate depth assessment.
63
A sound wave must be transmitted, reflected, and received by the transducer before the next pulse is generated. The number of pulses per second is the ...............
A sound wave must be transmitted, refl ected, and received by the transducer before the next pulse is generated. The number of pulses per second is the pulse repetition frequency. Pulse repetition frequency must decrease for accurate structure localization when interrogating deeper structures.
64
Sound beams generated by transducers are three dimensional. They not only have pulse length and duration but they also have beam ...........and..........
Sound beams generated by transducers are three dimensional. They not only have pulse length and duration but they also have beam widths and thicknesses.
65
Beam diameter determines the ........ within the scan plane and the........... perpendicular to the scan plane.
Beam diameter determines the width within the scan plane and the thickness perpendicular to the scan plane.
66
Sound beams do not remain the same width as they travel through a medium. In an unfocused transducer the sound beam starts out with a width equal to the transducer diameter and, as it travels through the tissues, it...................
In an unfocused transducer the sound beam starts out with a width equal to the transducer diameter and, as it travels through the tissues, it diverges (Figure 1.12 ).
67
The distance from the transducer element to where it diverges is the beam’ s ............. The area beyond the near field is the ..........
The distance from the transducer element to where it diverges is the beam’ s near field. The area beyond the near field is the far field.
68
Near field length is directly proportional to the beam diameter and inversely proportional to wavelength (Figure 1.12 ). For two transducers of the same frequency, the .................... will be longer for the transducer with the larger diameter.
Near field length is directly proportional to the beam diameter and inversely proportional to wavelength (Figure 1.12 ). For two transducers of the same frequency, the near field will be longer for the transducer with the larger diameter.
69
For two transducers with the same diameter, the near field will be longer for the ............... frequency transducer.
For two transducers with the same diameter, the near field will be longer for the higher frequency transducer.
70
Near field =
radius2 /wavelength
71
Larger beam width =
longer near field
72
shorter wavelength =
longer near field
73
Far field divergence is also dependent upon transducer size. ............ diameter transducers produce less divergence in the far field.
Larger diameter transducers produce less divergence in the far field.
74
.......... frequency transducers with ........... diameters therefore produce the longest near field and the narrowest far field (Figure 1.12 ).
High frequency transducers with large diameters therefore produce the longest near field and the narrowest far field (Figure 1.12 ).
75
When a curved element or lens is used, the beam can be focused and beam width will ............ throughout the entire near field and create a ............. zone, but beam width will diverge rapidly beyond this focal point (Figure 1.13 ).
When a curved element or lens is used, the beam can be focused and beam width will decrease throughout the entire near field and create a focal zone, but beam width will diverge rapidly beyond this focal point (Figure 1.13 ). Many transducers today have variable focal zones that the examiner can set.
76
If multiple pulses are generated and each pulse is set to a different focal zone; then an .............. focal zone can be created.
If multiple pulses are generated and each pulse is set to a different focal zone; then an elongated focal zone can be created. The transducers simply ignores echoes returning from depts other than the focal depth for any given pulse.
77
Up to this point only single sound beams have been considered. ...................... sound beam is used to generate an M-mode image of the heart.
Up to this point only single sound beams have been considered. A single sound beam is used to generate an M-mode image of the heart. This beam travels through the cardiac structures and a one-dimensional image is generated.
78
B-mode or 2-D imaging uses an array (group) of crystals that are electronically triggered to generate sound waves. It is important to recognize that each sound beam generated by a transducer is affected by...? (4)
pulse length,
beam width,
focal length,
PRF.
79
Linear array transducers have multiple elements arranged in .............
Linear array transducers have multiple elements arranged in a row. Sequences of elements are electronically stimulated at one time (i.e. elements one through four, then elements two through five, etc) with each group producing one scan line. This producers a high quality image with increased line density within the generated image.
80
Fig 1.12: The distance from a transducer element to where the beam diverges is referred to as the .......... The area beyond that is the ............
Fig 1.12: The distance from a transducer element to where the beam diverges is referred to as the near field. the area beyond that is the far field.
81
Fig 1.13. A sound beam can be focused by using a .............or................ This decreases beam width within the near field.
Fig 1.13. A sound beam can be focused by using a curved element or lens. This decreases beam width within the near field.
82
Linear array transducers can be modified into curvilinear formats.
Linear array transducers can be modified into curvilinear formats.
83
Phased array transducers stimulate each crystal with a small time interval (less than a ...............) between them and they are directed through the tissues at slightly different ........ (phased).
Phased array transducers stimulate each crystal with a small time interval (less than a microsecond) between them and they are directed through the tissues at slightly different angles (phased). (fig 1.14). This produces a sector image and these transducers are often called electronic sector transducers.
Rapidly stimulating these elements over and over again in sequence produces the moving cardiac images we call real-time ultrasound.
84
What is resolution?
Resolution is the ability to identify 2 objects as different.
85
Pulse length, beam width, beam diameter, focal length, and PRF are important physical aspects of transducers that affect the .......................(3) resolution of ultrasound images.
Pulse length, beam width, beam diameter, focal length, and PRF are important physical aspects of transducers that affect the axial, lateral, and temporal resolution of ultrasound images.
86
What is axial resolution?
Ability to differentiate between 2 structures along the length of the sound beam. Axial resolution is also called depth or longitudinal resolution. The smaller the axial resolution is, the better the detail of the image.
87
What is lateral resolution?
Ability to resolve 2 structures in the plane perpendicular to the sound beam
88
What is temporal resolution?
Ability to resolve structures with respect to time, keeping up with the actual events.
89
Transducers .......... plays an important role in axial resolution.
Transducers frequency plays an important role in axial resolution.
90
Axial resolution is equal to half the pulse length; that is?
Axial resolution is equal to half the pulse length; that is 2 structures cannot be closer than half the pulse length to each other in order to be distinguished as 2 separate things.
91
Remember that pulse length depends upon.......? (2)
The wavelength of the sound and upon the number of cycles per pulse.
When one or both of these is reduced axial resolution improves. (Fig 1.15)
92
Is the axial resolution better with 7.5 MHz frequency sound or with 3.5 MHZ frequency sound? Why is it so?
Wavelength decreases as frequency of sound increases, so axial resolution is better with 7.5 MHz.
93
Pulse length and duration are shortened by adding .........?
Pulse length and duration are shortened by adding damping materials within the transducer or electrical damping with the equipment.
94
A pulse may have any number of cycles (generally .....or.... in echo).
A pulse may have any number of cycles (generally 2 or 3 in echo).
95
Pulse length decreases with higher frequency sound because of?
shorter wavelengths
96
Pulse length increases with....?
with lower frequency sound
97
Better axial resolution=
Better image detail. Axial resolution improved with increased frequency and decreased pulse length. 2 things must be farther apart than one-half the pulse length to be identified as 2 different structures. Fig 1.15
98
Axial resolution is equal to?
Half the pulse length
99
Higher frequency transducers have better ........resolution
Better axial resolution
100
Higher frequency = ....resolution
Higher resolution
101
Lower frequency =
Lower resolution
102
Lateral resolution is the ability to ?
The ability to resolve 2 structures as distinct and different in a plane perpendicular to the sound wave and depends upon beam width. 2 structures that fall within the beam width will not be differentiated while 2 structured that are farther apart than the beam width will be identified as separate.
(the axial resolving powers of the system may differentiate 2 structures that fall within beam width when they are offset in depth. Fig 1.16.
103
Lateral resolution is equal to?
Equal to beam width and improves with smaller beam widths.
104
Beam width is affected by?
1) Focusing the sound waves generated by a transducer
2) Transducer diameter
3) Transducer frequency
105
The narrower the beam width the better the ability to differentiate between ....?
The narrower the beam width the better the ability to differentiate between 2 structures in a plane perpendicular to the sound beam. Fig 1.16
106
Beam width varies along the length of the sound wave but is at is narrowest at the focal zone in focused transducers. Lateral resolution is best (smallest) at.....
At the focal zone.
107
What happens with 2 structures that are side by side within the boundaries of the beam width?
2 structures that are side by side within the boundaries of the beam width will not be resolved as 2 different structures. Fig 1.16.
If they are offset a little in depth however, they may be resolved as 2 different structures based upon axial resolving powers of the transducer (fig 1.16).
108
Lateral resolution improves with ............. beam widths.
Lateral resolution improves with narrower beam widths
109
Lateral resolution is usually narrowest at ............ zones of .............. transducers.
Lateral resolution is usually narrowest at focal zones of focused transducers.
110
Lateral resolution is best within...?
Lateral resolution is best within near field where beam width is narrowest.
111
A high frequency transducer will have better lateral resolution than a lower frequency transducer of the same size. Why?
A high frequency transducer will have better lateral resolution than a lower frequency transducer of the same size because of its longer near field.
112
Long narrow near fields allow more ........................ of the heart to be images; creating less ambiguity about the source of the returning echoes (lateral position errors).
Long narrow near fields allow more specific areas of the heart to be images; creating less ambiguity about the source of the returning echoes (lateral position errors).
113
Longer near field length, focused transducer beams, and less far field divergence also improve image quality by increasing beam ...........
Longer near field length, focused transducer beams, and less far field divergence also improve image quality by increasing beam strength. Stronger beams increase the degree of reflection and can travel farther before all the sound is attenuated.
114
The number of real-time images produced per minute is referred to as the ...........
frame rate
115
The frame rate is dependent upon?
The PRF ( pulse repetion frequency)
116
The faster the frame rate, the faster the .......
The faster the frame rate, the faster the PRF
117
Faster frame rates produces better
Temporal resolution (resolution with respect to time)
118
Logically; rapidly moving structures require fast frame rates in order to prevent ......
slow motion or freeze frame images of cardiac motion.
119
Temporal resolution is dependent upon frame rate. reduce .........in order to improve the frame rate
sector width and reduce image depth
120
Sector transducers that emit multiple pulses with varying focal zones per scan line must wait until ........?
Sector transducers that emit multiple pulses with varying focal zones per scan line must wait until all sound has returned before generating the next set of pulses. Otherwise range or depth ambiguity results. In doing so the frame rate and temporal resolution of the generated 2 D image is reduced.
Interrogation of deep structures also requires a slower frame rate ans less temporal resolution is possible.
It is possible to increase PRF in both of these settings by reducing sector width and/or image depth since less time is required before the next frame can be produced.
121
Doppler allows detection and analysis of .....
moving blood cells or myocardium
122
Doppler tells us about the direction, velocity, character, and timing of .......or .......
of blood flow or muscle motion
123
The hemodynamic information provided by Doppler echo allows definitive diagnosis in most cardiac examinations by giving information about: (4)
Direction
Velocity
Character
Timing
124
4 types of Doppler used during av echo exam:
```
Pulsed wave (PW) Doppler
Continous wave (CW) Doppler
Color flow (CF) Doppler
Tissue Doppler imaging (TDI)
```
125
PW Doppler is ....specific
Site specific: It can be directed and set to sample flow at very specific places within the heart.
126
Limitation with PW Doppler?
Limited in its capacity to detect higher frequency (velocity) shifts
127
CW Doppler has the ability to detect higher frequency shifts: Any limits in flow velocity detection?.
CW Doppler has the ability to detect higher frequency shifts and therefore can record high-flow velocities with virtually no limits.
128
Limitation with CW Doppler
Not site specific: blood cells are examined all along the sound beam. Since sound is continuously transmitted and received in CW Doppler, it is not possible to select and interrogate at specific depths within the heart. However, although this may sound like a disadvantage, the information provided by CW Dopller is very valuable.
129
Color flow Doppler is a form of PW Doppler. Frequency shifts are encoded with varying hues and intensities of color. Flow information is very vivid, and detection of abnormal flow is easier with CW Doppler although ...................information is limited
Quantitative.
CF Doppler: color codes the various velocities and directions of flow.
130
TDI uses pulsed-wave Doppler to interrogate myocardial motion and velocities. It is used to assess both .................and..........myocardial function and synchronicity
Systolic and diastolic myocardial function and synchronicity
131
The Doppler effect: Christian Dopppler found that all types of waves (light, sound, etc) change in wavelength when there is a change in position between...?
......the source of the wave and the receiver of the wave.
132
Using sound; if you were moving toward a sound source; the pitch or frequency of that sound would increase, and if you were moving away from that sound source, the frequency would decrease.
What is the Doppler shift?
The change in wavelength (pitch and frequency) when there is a change in position between the sound source (sound that is transmitted) and the reflecting structure (blood cells in this case) (sound that is received).
133
What happens when the source and the reflecting surface are both stationary?
When the source and the reflecting surface are both stationary, the transmitted (incident) and reflected wavelengths are equal (Fig 1.17).
134
What happens when the reflecting structure is moving toward the source?
Sound waves are encountered more often, resulting in an increased number of waves (= increased frequency) being reflected back toward the source.
135
What happens when the reflecting structure is moving away from the source?
they travel ahead of the transmitted wave front and sound waves are encountered less frequently resulting a decreased number of sound waves (lower frequency) reflected back to the source.
136
Positive frequency shift?
Cells moving toward the transducer reflect an increased number of sound waves, and so the received frequency is greater than the transmitted frequency.
137
Negative frequency shift?
Cells moving away from the transducer reflect fewer sound waves, and the received frequency is less than the transmitted frequency.
138
Everyday ex of Doppler shift include any loud sound moving toward or away from you such as sirens, trains, marching bands, etc.
The sound of a siren as it approaches you will increase in pith (frequency increases) and then as it passes you, the pith will decrease (frequency decreases).
Doppler radar uses this principle when policemen determine the speed of your car; Howecome?
Since the frequency shift is used to determine velocity.
Doppler radar is also used in forecasting weather
139
The Doppler shift as we utilize it in diagnostic ultrasound is?
The difference in frequency transmitted by the transducer and received frequency reflected from blood cell.
140
The Doppler-derived frequency shift (fd) is equal to reflected frequency minus transmitted frequency. Therefore; objects moving toward the source result in ............. frequency shifts while objects moving away from the source result in ............frequency shifts.
Therefore; objects moving toward the source result in positive frequency shifts while objects moving away from the source result in negative frequency shifts.
141
The site (gate) for Doppler flow interrogation is selected by the examiner and is represented on the Doppler display as a line (baseline). Positive frequency shifts (flow moving .............. the transducer) produce waveforms up from the baseline while negate frequency shifts (flow moving ...............from the transducer) produce downward deflection of the Doppler tracings.
Positive frequency shifts (flow moving toward the transducer) produce waveforms up from the baseline while negate frequency shifts (flow moving away from the transducer) produce downward deflection of the Doppler tracings.
Figure 1.18.
These images are called spectral tracings.
142
The velocity range is split between the positive and negative directions of flow. What happens when the baseline is located in the middle of the spectral display?
The total velocity range is displayed equally above and below the baseline (Figure 1.19).
When the baseline is moved all the way to the top of the image, the entire velocity range is allocated to downward flow. When it is moved to the bottom of the image, the entire velocity range is allocated to upward flow.
143
PW Doppler: pulsing the sound waves allows a transducer to act as receiver for the signal only during the time interval specified by a sample depth. With PW Doppler the transducer will record frequency shifts only during ...........................
With PW Doppler the transducer will record frequency shifts only during the time interval dictated by the depth of the sample site ignoring all other returning echoes.
Fig 1.20
New sound waves will not be transmitted until the transducer has received the echoes from the previous burst.
144
PW Doppler: The ability to measure velocity within a small cell at a specified depth along the ultrasound beam is referred to as ...................., and the site at which sampling is set to occur is referred to as the ..............
The ability to measure velocity within a small cell at a specified depth along the ultrasound beam is referred to as range resolution, and the site at which sampling is set to occur is referred to as the gate. The gate is manually set by the examiner while watching a 2D image.
145
CW Doppler: continuously sends out sound and continuously receives sound. Is it possible to range gate CW Doppler?
No, because the transducer has no way of detecting the depth of the reflected signal. CW Doppler detects frequency shifts all along the ultrasound beam with no range resolution.
146
CW Doppler is steered in one of two ways:
1) .............. CW systems use a cursor representing the Doppler sound beam. The cursor is placed over the 2D image and frequency shifts are calculated all along the beam.
2)..................CW systems use a dedicated CW probe without the luxury of a 2D image.
1) Imaging CW systems use a cursor representing the Doppler sound beam. The cursor is placed over the 2D image and frequency shifts are calculated all along the beam.
2) Non-imaging CW systems use a dedicated CW probe without the luxury of a 2D image.
These systems require recognition of characteristic flow profiles.
147
Velocities along the beam vary, and a full spectrum of frequency shifts is detected with CW Doppler. When CW Doppler is used properly, the highest ....................long the line of interrogation are recorded.
When CW Doppler is used properly, the highest velocities along the line of interrogation are recorded.
Figure 1.21.
The highest velocities are generally what is of interest and diagnostically important. Lower velocity flow found along the Doppler line of interrogation are hidden within the higher flow profiles.
Flow patterns for the various valves and vessels in the heart are very characteristic and usually are easily identified with both PW and CW Doppler.
148
Doppler ultrasound can determine blood cell velocity within the heart or in peripheral vessels based upon the Doppler shift. Blood cell velocity (V) is determined using the following formula:
se Equation 1.3 s 18
Shows that V is equal to the speed of sound in tissues (C) times the frequency shift (fd) in kHz, divided by the transmitting frequency of the transducer f0 (2.5, 3.5, 5.0 etc) times the cosine of 0 where 0 where 0 is equal to the intercept angle of the ultrasound beam with respect to the blood flow.
The speed of sound in tissues is a constant (1.540 m/sec), leaving the interrogation angle, 0, and transducer frequency as variables that can be controlled.
149
Velocity measurement: Accurate measurements are affected by? (2)
1. Transducer frequency
| 2. Intercept angle
150
Angle of Interrogation: The closer to ............... the transmitted wave is with the direction of blood flow being interrogated, the more accurate the velocity measurement.
The closer to parallel the transmitted wave is with the direction of blood flow being interrogated, the more accurate the velocity measurement.
(Fig 1.22)
151
When the Doppler equation is changed to calculate for the frequency shift, you can see that the cosine of the intercept angle directly affects the frequency shift (fd)
Equation 1.4
Since the speed of sound in tissues (C) and the transmitting frequency (f0) is known, the calculated frequency shift and therefore the calculated flow velocity is directly dependent upon the cosine of the intercept angle. The cosine of 0 grader (symbol) is one. The value increases, and by the time an angle of 90 grader is reached, the cosine is zero.
152
Table 1.5 lists the cosines for several angles. Larger intercept angles and cosines of less than one falsely decrease the recorded frequency shift of blood flow. Generally, interrogation angled greater than ....-...... grader are considered unacceptable.
Generally, interrogation angled greater than 15-20 grader are considered unacceptable.
The graph in Figure 1.23 shows the relationship between the cosine of the angle of incidence with respect to blood flow and the calculated velocity for a 5.0 MHz transducer.
153
Can velocity be overestimated?
Velocity cannot be overestimated, just underestimated, when interrogation angles with respect to flow become larger than zero.
154
PW Doppler measures the frequency shift at very specific locations within the heart. Just like 2D and M-mode imaging, the reflected signal must be received before the next pulse is transmitted or there will be ambiguity in the recorded signals. The time interval between pulses must be ..... times the sample depth and is also referred to as the ................
The time interval between pulses must be 2 times the sample depth and is also referred to as the pulse repetition frequency (PRF).
155
Decreased PRF decreases the ..................... shift that can be accurately measured.
Decreased PRF decreases the Doppler frequency shift that can be accurately measured.
Fig 1.24 shows how sampling frequency affects your perception of events. As the sampling frequency decreases, information is lost. Time on the clock in Figure 1.24 is perceived correctly until the sampling frequency decreases to two times per minute. At that rate it is not possible to determine whether the hand on the clock is moving clockwise or counterclockwise. This is similar to what happens in movies when wheels on vehicles appear to rotate backwards.
156
The sampling rate (PRF) must be at least 2 times the frequency shift for unambiguous flow information to be received by the transducer. Equation 1.5 states the maximum Doppler shift that can be recorded accurately is equal to one-half the PRF.
Doppler shift = 1/2 PRF
Equation 1.5
157
The best Doppler recordings at any given depth are obtained with .................. frequency transducers.
The best Doppler recordings at any given depth are obtained with lower frequency transducers.
158
................. of the PRF is referred to as the Nyquist limit. When the Nyquist limit is exceeded signal ambiguity results. This ambiguity is called ...................
One-half of the PRF is referred to as the Nyquist limit. When the Nyquist limit is exceeded signal ambiguity results. This ambiguity is called aliasing.
Figure 1.25 shows an aliased Doppler display.
159
When the Nyquist limit is exceeded ny larger degrees, the aliased signal no longer displays the characteristic flow profile and direction can no longer be determined (Figure 1.26). Switching to ............. allows flows velocities that exceed the Nyquist limit to be recorded accurately.
Switching to CW Doppler allows flows velocities that exceed the Nyquist limit to be recorded accurately.
Equation 1.6 is used to determine the maximum velocity a PW system can record accurately without aliasing of a given transducer frequency (f0) and sampling depth (D).
The equation 1.6 shows that the maximum velocity that can be recorded at any given depth with no ambiguity is inversely proportional to transducer frequency.
160
The best recordings of higher velocity jets at any given depth are obtained from a ................ frequency transducer. This is opposite of what produces the best M-mode and 2D exams where higher ..................... transducers produce the best images.
The best recordings of higher velocity jets at any given depth are obtained from a lower frequency transducer. This is opposite of what produces the best M-mode and 2D exams where higher frequency transducers produce the best images.
161
Table 1.6 lists the maximum velocities that can be accurately recorded at a variety of depths and transducer frequencies.
Table 1.6 lists the maximum velocities that can be accurately recorded at a variety of depths and transducer frequencies.
162
Effect of Sampling (Gate) Depth
Equation 1.6 also shows that the maximum velocity that can be recorded without aliasing is inversely proportional to depth for any given transducer frequency.
163
What to do about Aliasing?
1. Move the baseline up or down
2. Find an imaging plane where less depth is necessary
3. Use a lower transducer frequency
4. Switch to CW Doppler
164
When the Nyquist limit is exceeded, aliasing occurs. When the Nyquist limit is not exceeded by degree, we can still see the typical flow profile; however, it wraps around the image. This type of aliasing can be eliminated by moving the baseline up or down in order to see the entire flow profile. Fig 1.25
Fig 1.25
165
Normal blood flow is typically ........
laminar
166
Laminar blood flow: All blood cells within a vessel, outflow tract, or chamber are moving in the same direction with very similar flow velocities.
Vessel and chamber walls do create friction for the blood cells moving adjacent to their surface and velocities are generally somewhat slower along the ................. of the flow stream than in the center of the flow stream.
Vessel and chamber walls do create friction for the blood cells moving adjacent to their surface and velocities are generally somewhat slower along the periphery of the flow stream than in the center of the flow stream.
Nevertheless velocities are similar enough that a velocity profile is produced that has little variance.
167
PW Doppler always appears hollow with little spectral broadening when the following occurs:
1) Flow is laminar,
2) intercept angles are close to zero,
3) the Nyquist limit is not exceeded
The signals are hollow when flow is laminar because there is little variance in velocity.
(Figure 1.27)
168
Spectral ........... is the filling in of the typically hollow wave-form
Spectral broadening is the filling in of the typically hollow wave-form. (Figure 1.28)
CW Doppler displays always show spectral broadening because of the many velocities detected along the CW sound beam. PW Doppler may she spectral broadening when gain is too high, when intercept angles are large or when flow becomes turbulent
169
Spectral broadening in a pulsed-wave signal may be due to....(3)
1) improper gain settings,
2) a large intercept angle,
3) non-laminar (turbulent) flow.
170
PW: When velocities dramatically exceed the Nyquist limit, normal flow profiles are lost and it is impossible to determine flow direction or velocity (Fig 1.26)
PW: When velocities dramatically exceed the Nyquist limit, normal flow profiles are lost and it is impossible to determine flow direction or velocity. Switching to CW Doppler allows the high velocity flow of mitral regurgitation to be recorded accurately. (Fig 1.26)
171
When flow becomes abnormal it is generally turbulent. Turbulent flow has blood cells moving in many directions and at variable velocities. This kind of flow is seen with stenotic lesions, shunts, and valvular regurgitation. Doppler signals produced from turbulent flow have a lot of spectral broadening because of the many velocities and flow directions present in the jet. Why does CW Doppler always shows spectral broadening?
CW Doppler always shows spectral broadening even when flow is laminar because flow velocities detected all along the transmitted sound beam vary tremendously. (Fig 1.28)
172
Color flow Doppler is a form of ........ Doppler. RT images and CF mapping are done at the same time with alternating scan lines dedicated toward RT image generation and Doppler signals.
Color flow Doppler is a form of PW Doppler. RT images and CF mapping are done at the same time with alternating scan lines dedicated toward RT image generation and Doppler signals.
173
Remember that PW Doppler is range gated in that a specific sampling site i chosen and the ultrasound machine ignores signals that come back from any other point along the line of interrogation. This can be done by knowing the speed of sound i .............. and the depth of the ...........
Remember that PW Doppler is range gated in that a specific sampling site i chosen and the ultrasound machine ignores signals that come back from any other point along the line of interrogation. This can be done by knowing the speed of sound i tissues and the depth of the gate.
174
CF mapping involves the analysis of information all along hundreds of interrogate lines, each with hundreds of gates, until a wedge is filled with color. Each gate sends frequency shift information back to the ............ This frequency shift info is sent to a ................, which calculates the mean velocity, direction, and location of blood cells at each gate. Info from each gate is assigned a color and position on the image.
CF mapping involves the analysis of information all along hundreds of interrogate lines, each with hundreds go gates, until a wedge is filled with color. Each gate sends frequency shift information back to the transducer. This frequency shift info is sent to a processor, which calculates the mean velocity, direction, and location of blood cells at each gate. Info from each gate is assigned a color and position on the image.
Fig 1.29.
175
Blood flow is color mapping is perceived by the machine as either moving toward the transducer or away from it via a negative or positive frequency shift. By convention, flow moving toward the sound source is plotted in hues of ........., and flow moving away from the transducer is mapped in shades of ............. although this can be changed by the operator.
Blood flow is color mapping is perceived by the machine as either moving toward the transducer or away from it via a negative or positive frequency shift. By convention, flow moving toward the sound source is plotted in hues of red, and flow moving away from the transducer is mapped in shades of blue although this can be changed by the operator. Fig 1.30
176
No flow generates no ..............., and no color is assigned.
No flow generates no frequency shift, and no color is assigned.
177
Enhanced color maps, available in most equipment, display .............. information as well as .........
Enhanced color maps, available in most equipment, display flow velocity information as well as direction.
178
Color range from deep ......... for slow flow to bright ........... for rapid blood flow toward the transducer. Slow blood flow away from the transducer is mapped in deep .......... colors while more rapid flow away from the transducer is mapped in shades of light b........ and .........
Color range from deep red for slow flow to bright yellow for rapid blood flow toward the transducer. Slow blood flow away from the transducer is mapped in deep blue colors while more rapid flow away from the transducer is mapped in shades of light blue and white.
179
CF Doppler quality is dependent upon 2 important factors; ................ and ................... of the transducer.
As with spectral Doppler the ......... of the sound source dictates the maximal velocity, which can be accurately mapped at any given depth before aliasing occurs.
CF Doppler quality is dependent upon 2 important factors; pulse reception frequency and frequency of the transducer. As with spectral Doppler the frequency of the sound source dictates the maximal velocity, which can be accurately mapped at any given depth before aliasing occurs.
180
Aliasing in CF Doppler involves a reversal of color and the result is a mosaic or mixing of the blue and red hues.
Aliasing can occur while using ........ frequency transducers when in actuality there is normal flow and the aliasing is only a function of transducer frequency. The aliasing would be eliminated if a .......... frequency transducer were used.
Aliasing in CF Doppler involves a reversal of color and the result is a mosaic or mixing of the blue and red hues. (Fig 1.31: Aliasing secondary to high velocity or turbulent flow is displayed as a mosaic of color).
Aliasing can occur while using high frequency transducers when in actuality there is normal flow and the aliasing is only a function of transducer frequency. The aliasing would be eliminated if a lower frequency transducer were used.
181
CF Doppler: May aliasing be seen even when flow is normal?
Aliasing occurs at lower velocities due to sampling time requirements. Therefore; aliasing may be seen even when flow is normal.
182
........ maps are found in many ultrasound machines (Fig 1.32). These machines map turbulent flow in hues other than blue or red, typically green.
Variance maps are found in many ultrasound machines (Fig 1.32). These machines map turbulent flow in hues other than blue or red, typically green.
(All CF images in the textbook by Boon use enhanced and variance color maps.
183
Frame rate refers toP
Frame rate refers to the number of times a B-mode or color-flow image is generated per minute.
184
A frame rate of at least ....... times per minute is required for smooth transitions and the appearance of a continuously moving image.
A frame rate of at least 15 times per minute is required for smooth transitions and the appearance of a continuously moving image.
185
CF Doppler: Frame rate in a 2D image is equal to?
Frame rate in a 2D image is equal to PRF divided by scan lines per color sector (which is superimposed upon a 2D image).
186
The width of the color sector can be altered by the operator. Decreasing the color wedge ............. the amount of time necessary for sampling and ........... the fram rate.
The width of the color sector can be altered by the operator. Decreasing the color wedge decreases the amount of time necessary for sampling and increases the fram rate.
187
The operator can also eliminate the real time image; which extends beyond the width of the color sector. This also decreases the time necessary for image generation and enhances ....... mapping.
The operator can also eliminate the real time image; which extends beyond the width of the color sector. This also decreases the time necessary for image generation and enhances CF mapping.
188
Many machines allow the operator to decrease the depth of the color wedge. Does this has an effect on the fram rate?
This typically has no effect on fram rate on most ultrasound machines since total image depth is unchanged. It merely decreases the info the mind has to process by eliminating processed info from the display.
189
What is the packet size?
The numer of times each line of sound (within a color sector) is sampled is referred to as its packet size.
(Fig 1.34)
Increasing packet size improves image quality (because more samples can be taken) and fills in the color display, but this is at the expense of frame rate as more time is necessary.
190
Packet sizes can be selected by the operator on some equipment. Decreasing packet size will ................ your frame rate but decrease sampling time. Information may be lost with very short sampling times. This may be necessary however with rapid heart rates. Increasing packet size will increase the time required for sampling and ................ the fram rate, but it will be able to map velocities and color with greater color filling.
Packet sizes can be selected by the operator on some equipment.
Decreasing packet size will increase your frame rate but decrease sampling time (decrease the number of times each scan line is sampled): so color info is not as complete, but frame rate is higher.
Information may be lost with very short sampling times. This may be necessary however with rapid heart rates. Increasing packet size will increase the time required for sampling and decrease the fram rate, but it will be able to map velocities and color with greater color filling.
191
How can CF imaging be optimized?
1. Decrease transducer frequency
2. Decrease color sector width
3. Eliminate real-time image
4. Increase packet size: Decreases fram rate however.
5. Decrease packet size: Decreases sampling time and good for high heart rates but may lose information
192
CF Doppler: Variance maps display turbulent (aliased) flow by ....
By adding green to the mapped flow
193
CF Doppler: Enhanced maps mix and reverse color and all the hues when ......
when flow becomes turbulent.
194
CF Doppler: Small color sector: ..... time required.
| = ................ frame rate.
CF Doppler: Small color sector: Less time required.
= increased frame rate.
Fig 1.33. Better temporal resolution is then possible.
195
Large color sector = .........time required.
| = .............. frame rate.
Large color sector = more time required
| = decreased frame rate.
196
Eliminate image outside color sector = .......... time required.
= ......................... frame rate.
Eliminate image outside color sector = less time required.
= increased frame rate.
Improved color flow mapping.
197
Tissue Doppler Imaging -TDI (or tissue Doppler echo-TDE) involves .........?
Acquiring myocardial velocities.
198
While blood cells reflect ........ amplitude signals at high velocity, myocardial motion has .............-amplitude signals but low velocity.
While blood cells reflect low amplitude signals at high velocity, myocardial motion has high-amplitude signals but low velocity.
199
Standard Doppler interrogation of blood flow filters out ........ velocity signals. TDI however bypasses the ....... velocity filter.
Standard Doppler interrogation of blood flow filters out low velocity signals. TDI however bypasses the low velocity filter.
200
TDI can employ PW signals only or can be used in conjunction with ........ Doppler.
TDI can employ PW signals only or can be used in conjunction with CF Doppler.
201
Color TDI uses a ........ sector of color (to keep frame rates high) placed over a section of myocardium.
Color TDI uses a narrow sector of color (to keep frame rates high) placed over a section of myocardium.
202
A PW gate can be placed anywhere over the color sect after the fact from stored video loops.
A PW gate can be placed anywhere over the color sect after the fact from stored video loops (Fig 1.35)
203
When using PW Doppler TDI, the spectral .......... is placed over a color Doppler sector in the area of interest on the myocardium and a spectral trace of myocardial motion is displayed in real-time.
When using PW Doppler TDI, the spectral gate is placed over a color Doppler sector in the area of interest on the myocardium and a spectral trace of myocardial motion is displayed in real-time. (Fig 1.36)
204
The advantage of PW TDI to color TD?
The advantage to color TDI is that myocardium anywhere under the color sector can be interrogated, after the fact all at the same time, and compared.
205
Limitations of PW TDI ?
PW TDI is limited to recording myocardial velocities taken i RT under the gate.
PW TDI, however, provides the highest temporal and velocity range resolution.
206
TDI uses both apical, poarasternal, and transverse images of the heart.
A typical TDI display shows systolic myocardial motion directed ............ into the ventricular chamber and diastolic motion directed ........... from the center of the chamber on transverse or long-axis parasternal images.
TDI uses both apical, poarasternal, and transverse images of the heart.
A typical TDI display shows systolic myocardial motion directed centrally into the ventricular chamber and diastolic motion directed away from the center of the chamber on transverse or long-axis parasternal images.
207
On an apical 4-chamber view of the heart, systolic motion is directed ............ the transducer, while during diastole, myocardial motion is directed .................. the transducer.
On an apical 4-chamber view of the heart, systolic motion is directed upward to the transducer, while during diastole, myocardial motion is directed away from the transducer.
208
Color-tissue Doppler uses a narrow sect of color placed over myocardium. Off-line analysis allows gates to be placed over the myocardium anywhere within the color sector. Myocardial velocity corresponding to the selected gate or gates is displayed
Color-tissue Doppler uses a narrow sect of color placed over myocardium. Off-line analysis allows gates to be placed over the myocardium anywhere within the color sector. Myocardial velocity corresponding to the selected gate or gates is displayed.
Fig 1.35
209
PW tissue Doppler also uses a narrow sector of color placed over myocardium. A PW gate is placed over the myocardium anywhere within the color sector. Instantaneously myocardial velocity corresponding to the selected gate is displayed. Myocardial velocity from only one gate at a time can be displayed.
PW tissue Doppler also uses a narrow sector of color placed over myocardium. A PW gate is placed over the myocardium anywhere within the color sector. Instantaneously myocardial velocity corresponding to the selected gate is displayed. Myocardial velocity from only one gate at a time can be displayed. Fig 1.36
210
Give ex of some common echo artifacts
Inability to resolve 2 structures as separate entities both in the lateral and longitudinal planes.
Timing artifacts related to temporal resolution.
Aliasing
Other artifacts are created as a result of patient movement, and respiration, improper gain settings, peripheral sound beams, or strong reflectors.
211
What can be done to minimize echo artifacts?
Selecting the most appropriate transducer for the application will minimize these artifacts.
Using harmonic imaging during the echo examination helps eliminate many artifacts.
212
Respiratory motion moves the transducer farther and closer to the cardiac structures. The resulting M-mode image will have excessive cardiac motion simply due to this perceived cardiac motion by the equipment when in actuality it is only movement of the transducer with respect to the heart. Movement of the transducer closer to and farther away from the heart is interpreted by the equipment as cardiac motion.
Respiratory motion moves the transducer farther and closer to the cardiac structures. The resulting M-mode image will have excessive cardiac motion simply due to this perceived cardiac motion by the equipment when in actuality it is only movement of the transducer with respect to the heart. Movement of the transducer closer to and farther away from the heart is interpreted by the equipment as cardiac motion. Fig 1.37.
213
What is temporal artifacts/inaccuracy?
Temporal artifact is the inaccurate timing of CF Doppler information. Temporal inaccuracy is often created secondary to reparatory movement. Color is encoded for a chamber or vessel at a location on the screen that has moved before the info is processed. This usually appears as a sheet of color. Frame rates that are too slow for sampling in hearts with rapid heart rates will result in color being placed over the heart during the wrong phase of the cardiac cycle.
Fig 1.38.
214
What is a side lobe artifact?
All transducers produce a central beam. They also all produce peripheral beams. These peripheral beams are directed laterally with respect to the central beam. When they intercept structures and are reflected back to the transducer, the equipment does not have the ability to recognize that the info presented did not come from the central beam. Lateral structures are then superimposed upon centrally located structures (such as within an echo-free space like a chamber in a RT image) and are called side lobe artifacts. Fig 1.39.
215
Are side lobe artifacts always easily detected?
Side lobes are much weaker than central beams and thus returning echoes are also weaker. Often the potential artifact is not even seen since main beam structures have stronger reflections. When other structures are not there to overshadow them, however, they are easily visible. The most common place to see side lobe artifact is within a dilated left atrium or ventricle. This empty space allows eager side lobe echoes to be displayed. Fig 1.40.
When side lobe reflections are seen, they typically appear as a curved line extending from the side of the sector into a fluid-filled space or an irregular density within a chamber. Make sure you see a structure in several imaging planes to eliminate possible misinterpretation of these reflections.
216
What is a reverberation artifact?
Reverberation artifact occurs when strong reflectors are encountered within the thorax. These structures send such strong echoes back to the transducer that the sound is both received by the transducer and reflected from it. The same sound beams travel through the heart again, and when they are sent back to the transducer for a second time they are perceived as having taken twice as long. This produces a mirror image below the first one. Fig 1.41.
217
When does reverberation artifact/mirror image artifact commonly occur?
Reverberation artifact often occurs in cardiac imaging when strong reflectors like the pericardium and lung interface cause echoes to bounce back and forth.
Reverberation artifact can also be produced between two strong reflectors within the thorax or heart. The sound may bounce back and forth between these two highly dense structures one to several times before traveling all the way back to the transducer. Multiple images of the same strutters are created, each equally spaced and deeper in the image.
218
How can reverberation artifacts be minimized?
Reverberation artifact can be minimized by making sure depth settings are adequate for the heart size and not so deep was to see a double image.
219
What is a mirror image artifact?
When spectral Doppler flow shows up on both side of the baseline, it is refereed to as mirror image artifact. Fig 1.42.
Depending upon where blood flow is being sampled, it is still possible to decide which flow direction is the correct one. This artifact is created ny high gain settings creating a situation similar to reverberation artifact. It may also be produced as a result of large angles of incidence with respect ot blood flow.
220
Summary: Transducers sent out sound waves that travel in cycles. The number of cycles per second determines transducer frequency. High frequency transducers generate more cycles per second and so have ........ wavelengths.
Transducers sent out sound waves that travel in cycles. The number of cycles per second determines transducer frequency. High frequency transducers generate more cycles per second and so have shorter wavelengths.
221
The average speed of sound, sent out from transducers) in soft tissue is .......m/second ?
1,540 m/second
222
Is the average speed sent out from transducers dependent of transducer frequency?
No, the speed is independent of transducer frequency.
223
Acoustic impedance increases with increased tissue ......... and ............ This is also independent of transducer frequency.
Acoustic impedance increases with increased tissue density and stiffness. This is also independent of transducer frequency.
224
An acoustical difference between .................. causes sound to be reflected back to the transducer.
An acoustical difference between two tissues causes sound to be reflected back to the transducer.
225
Bone, being very stiff and dense, impedes the flow of sound tremendously and reflects almost ...... sound.
Bone, being very stiff and dense, impedes the flow of sound tremendously and reflects almost all sound.
226
Scattered sound generates the images from within ................. tissues where acoustical properties are similar and structures are small and irregular with respect to wavelength.
Scattered sound generates the images from within homogenous tissues where acoustical properties are similar and structures are small and irregular with respect to wavelength.
227
A structure must be at least ............ the size of the wavelength to be reflected.
A structure must be at least one-quarter the size of the wavelength to be reflected.
228
High frequency sound with shorter wavelengths can reflect sound from smaller structures and produce better images.
Drawback with high frequency transducers?
High frequency sound with shorter wavelengths can reflect sound from smaller structures and produce better images.
Sound attenuates rapidly with these short wavelengths, however, since they interact with more structures. High frequency transducers therefore create high-resolution images but lose strength rapidly and cannot penetrate as far into tissues as sound from low frequency transducers.
229
Transducers contain piezoelectric crystals, which function to generate sound and receive sound. the sound is sent out in pulses and the number of pulses per second is the ....... of the transducer?
Pulse repetition frequency
230
One of the factors affecting axial resolution is the .....?
The pulse length of a transducer.
231
Higher frequency transducers will have better axial resolution since their ...... wave and pulse lengths can differentiate smaller structures.
Higher frequency transducers will have better axial resolution since their short wave and pulse lengths can differentiate smaller structures.
232
.......... resolution is dependent upon several factors one of which is beam width. The wider the beam width the poorer the lateral resolving power since structures must be farther apart than the beam width to be differentiated.
Lateral resolution is dependent upon several factors one of which is beam width. The wider the beam width the poorer the lateral resolving power since structures must be farther apart than the beam width to be differentiated. Therefore, lateral resolution is best with smaller diameter transducers.
High frequency transducers have longer near fields before the beam diverges, and this enhances the lateral resolution of deeper structures.
233
Current technology allows multiple focal points along each sound beam in a phased array or annular array transducer. Correct timing of cardiac motion and flow is dependent upon .................. resolution.
Current technology allows multiple focal points along each sound beam in a phased array or annular array transducer. Correct timing of cardiac motion and flow is dependent upon temporal resolution.
As multiple focal zones are used or as deeper structures are imaged, the pulse repetition frequency is decreased.
This may not allow fast enough processing of information, and temporal resolution will suffer.
Decreasing the width of the RT sector will increase the PRF as will decreasing the depth of interrogation and using no more than one focal zone.
234
What is the Doppler shift?
The change in frequency between sound transmitted by the transducer and sound received by the transducer is the Doppler shift.
235
Blood cells moving toward the transducer will create a positive frequency shift and be displayed above the baseline, while blood moving away from the transducer produces a negative frequency shift and flow profiles below the baseline.
Blood cells moving toward the transducer will create a positive frequency shift and be displayed above the baseline, while blood moving away from the transducer produces a negative frequency shift and flow profiles below the baseline.
236
PW Doppler is range gated in that it samples blood at indicated sites within the heart. It is however limited in the highest velocity it is capable of measuring accurately.
PW Doppler is range gated in that it samples blood at indicated sites within the heart. It is however limited in the highest velocity it is capable of measuring accurately.
237
Continuous-Wave Doppler samples blood flow all along the sound beam and while not site specific, it has no .......... limit.
Continuous-Wave Doppler samples blood flow all along the sound beam and while not site specific, it has no Nyquist limit.
238
The Nyquist limit is dependent upon....?
The Nyquist limit is dependent upon PRF and thus transducer frequency. The Nyquist limit for any transducer frequency decreases as blood flow is sampled at increasing depths. Lower frequency transducers are capable of accurately recording higher flow velocities at any given depth before aliasing occurs.
239
Accurate velocity measurements are also dependent upon having an angle of interrogation that is ...............with flow. Deviations of the Doppler beam away from parallel result in underestimation of flow velocities.
Accurate velocity measurements are also dependent upon having an angle of interrogation that is parallel with flow. Deviations of the Doppler beam away from parallel result in underestimation of flow velocities.
240
............ flow creates a Doppler signal with little variance in velocity and little spectral broadening. Spectral broadening is seen when flow becomes turbulent or when CW Doppler is used since many frequency shifts are received by the transducer.
Laminar flow creates a Doppler signal with little variance in velocity and little spectral broadening. Spectral broadening is seen when flow becomes turbulent or when CW Doppler is used since many frequency shifts are received by the transducer.
241
CF Doppler is a form of PW Doppler. Frequency shift info is encoded with color. The most commonly used color map uses a blue away and red toward (BART) format.
Aliasing typically occurs at ............ velocities in CF Doppler because of the increased time necessary for flow analysis at multiple gates. Aliasing in CF Doppler results in a mosaic of color. As in spectral Doppler, .................frequency transcoders will increase the Nyquist limit.
CF Doppler is a form of PW Doppler. Frequency shift info is encoded with color. The most commonly used color map uses a blue away and red toward (BART) format.
Aliasing typically occurs at lower velocities in CF Doppler because of the increased time necessary for flow analysis at multiple gates. Aliasing in CF Doppler results in a mosaic of color. As in spectral Doppler, lower frequency transcoders will increase the Nyquist limit.
242
Artifacts are common in diagnostic ultrasound and may be created by...?
The physical properties of transducers
Patient-related factors
Tissue characteristics
Operator-related errors.
Selecting the most appropriate transducer for the exam, calming the patient, and realizing the limitations of diagnostic ultrasound will help eliminate many of these problems or allow intelligent decision making.
