Module 2 - Transducers, Ultrasound modes and Beam forming. Flashcards
1
Q
What happens when voltage is applied to a piezo electric crystal ? (PZT or LZT in the case of ultrasound transducers)
A
It produces strain (3 reversed) across opposite faces of the crystal.
2
Q
What is strain?
A
the change in shape due to applied stress (this can be voltage or pressure) It is proportional to applied voltage.
3
Q
What is the piezo electric effect?
A
is the production of a voltage on the crystal when it is deformed by returning ultrasound pulses or waves.
4
Q
What is the inverse piezo electric effect?
A
The production of a dimensional change in the crystal when a voltage is applied. This effect is used to produce ultrasound pulses or waves.
5
Q
What happens if the frequency of a voltage applied to a PZT crystal changes?
A
The CW ultrasound wave possesses this new frequency.
6
Q
What happens to an LZT crystal when a voltage spike is applied? (in terms of oscillation and frequency)
A
It will oscillate at it’s natural (resonance) frequency.
7
Q
What determines the natural (resonance) frequency of an LZT crystal?
A
The crystals physical dimensions. The thickness of the crystal is equal to precisely ½ the wavelength of the resonance oscillation produced.
8
Q
What are the equations for determining the natural resonance frequency of a crystal given it’s dimensions?
A
λ = 2L and if the speed of sound in the piezo-electric material is c = f λ then we can determine the frequency as f = c/ λ = c/2L Where L = LZT thickness f = frequency Hz
9
Q
What is the duty factor (DF) of an LZT crystal?
A
The duty factor (DF) or duty cycle is the time that the LZT crystal produces a sound wave pulse compared to the total time.
10
Q
What is the Pulse Repetition period (PRP)?
A
the time delay between each voltage pulse to the LZT crystal. Measured in seconds.
11
Q
What is the pulse repetition frequency (PRF)?
A
The pulse repetition frequency, PRF or PRR, is the reciprocal of the pulse repetition period. The PRF is measured in Hz, and may also be called the pulse repetition rate (PRR). It is the total number of pulses transmitted each second. The PRR is limited by the time it takes ultrasound to travel in tissues as the machine must not transmit again until all detectable echoes from the previous pulse have been received.
12
Q
What is the equation relating PRP and PRF?
A
PRF = 1/PRP
13
Q
What effect does depth of penetration have on the PRF?
A
If the depth of penetration is small a high PRF can be used so the frame rate will also be high. Conversely a large depth of penetration will cause the machine to use a low PRF.
14
Q
What is a transducer?
A
A device that transforms one kind of energy into another. IN an ultrasound transducer the piezo-electric crystal acts as both as an input and output transducer utilizing the inverse and normal piezo electric effect.
15
Q
List the components of an ultrasound transducer.
A
LZT crystal (transducer), Electrodes, Matching layer, damping block, housing, insulating material.
16
Q
What are the role of the electrodes in an ultrasound probe?
A
Electrodes are connected to the front and back of the crystal surface to provide the excitation pulse and to transmit the output voltage signal upon the crystal receiving the ultrasound echo.
17
Q
What is the role of the damping block in an ultrasound probe?
A
A damping block is attached to the rear surface of the transducer to reduce the time duration of the ultrasound wave following the applied voltage spike. It also;
- Absorbs ultrasound waves of the back of the transducer
- Has a similar acoustic impedance to the crystal to optimise transmission and increase energy removal
- has a sloped rear surface to increase the attenuation path length and prevent reflection
- constructed of highly absorbing material
18
Q
What is the role of the matching layer in an ultrasound probe?
A
The matching layer optimises transition of the sound from the transducer to the patient. The acoustic impedance of the LZT crystal differs significantly from that of skin/soft tissue. The layer of matching material is added to more nearly match the acoustic impedance of the human skin so that as much of the ultrasound energy as possible passes into the patient.
Note the thickness of the PZT is generally 1/2 the wavelength of the ultrasound produced and the thickness of the matching layer is generally 1/4 the wavelength of the ultrasound produced and is sometimes called the quarter wave matching layer.
The matching layer further dampens the transducer and so contributes to broadening the bandwidth and shortening the pulse duration.
19
Q
What is the role of the transducer housing in an ultrasound probe?
A
Maintains electrical safety and is made of plastic.
20
Q
What is the role of the insulating material in an ultrasound probe?
A
An acoustic and electrically insulating material is placed between the piezoelectric crystal and the transducer housing to reduce the risk of electric shock and isolate the crystal from any vibration due to movement of the transducer.
21
Q
What is transducer damping?
A
a material (damping element, damping material) bonded to the back of the piezoelectric element of a transducer to limit the duration of vibration.
22
Q
Why is a transducer damped?
A
To shorten the impulse response of the transducer to generate short signals.
23
Q
Why is it important for the transducer to generate short signals? (eg why is a transducer damped explained)
A
The backing material has the effect of decreasing ringing of the PZT crystal. (limiting duration of vibration/shortening impulse response) It therefore shortens the spatial pulse length and so improves axial resolution (this is why the backing material is used). The backing material also has the effect of widening the bandwidth and therefore decreasing the quality factor. (because q factor is the central frequency divided by the bandwidth) Thus, diagnostic transducers are wide bandwidth, low quality factor transducers.
24
Q
List why is it important for the transducer to generate short signals?
A
decreasing ringing of the PZT crystal limiting duration of vibration/shortening impulse response shortens the spatial pulse length improves axial resolution widening the bandwidth decreasing the quality factor
25
What is the bandwidth of an ultrasound transducer?
The range of frequencies produced by the transducer
26
How do you calculate the Q factor of an ultrasound transducer?
Central frequency (natural/resonance)divided by the bandwidth.
27
Define sensitivity of a transducer
The ability of the transducer to detect reflected ultrasound and generate an electrical signal.
28
What is the central frequency of a transducer?
The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency.
29
What happens when a transducer is highly damped?
Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range (bandwidth) provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration.
30
What is the equation for calculating bandwidth?
B = fU – fL (Hz)
If the resonance frequency is denoted by f0 and the lower and upper half-power (or half-energy) frequencies are written as fL and fU respectively, then the bandwidth, B, is defined as (above)
31
What is the equation for determining quality factor?
This is a measure of the sharpness of the resonance, and can be expressed relating bandwidth to the resonance frequency.
Q = f0/B = f0/(fU - fL)
The Q-factor is thus a dimensionless quantity that is large if the damping is small, and small if the damping is large.
32
What are the range of desired Q factors?
For CW situations you want an ultrasound transducer with Q around 700 to 800, while pulse ultrasound transducers have a Q in the range 2 to 3.
33
Explain what is meant by the spatial pulse length, SPL, and state approximately how many wavelengths this involves
The spatial pulse length, SPL, is the pulse length in space. SPL = 2-5 λ.
34
What is echo time?
the total time for a round trip of the ultrasound pulse.
35
What is the equation used to determine echo time and how can it be manipulated?
Δt = x/c = 2d/c
a total distance, x, with x = 2d, (out and back)
Where c = 1540 m/s
d = c Δt/2
36
Why is the echo time and thus distance travelled calculations important?
From this echo time measurement, the distance d is determined, and this is the basis of all pulse wave (PW) diagnostic ultrasound imaging.
37
Why is maximum depth important and what equation is used to calculate it?
The received echo can only be sensibly recognised and interpreted if it returns to the transducer during the next ‘off’ time of the duty cycle. The maximum depth of a reflection interface that can be unambiguously calculated correctly is given by
dMAX = (c PRP)/2 = c/(2 PRR)
38
Differentiate between the piezo-electric effect and the inverse piezo-electric effect
The inverse piezo-electric effect is the production of a dimensional change in the crystal when a voltage is applied. This effect is used to produce ultrasound pulses or waves.
The piezo-electric effect is the production of a voltage on the crystal when it is deformed by returning ultrasound pulses or waves.
39
What other way can you ensure a LZT crystal oscillates at a frequency, say, 10 MHz?
Apply a voltage across it pulsing at a frequency of 10 000 000Hz
40
Why is damping used in a diagnostic ultrasound transducer, and what forms of damping are incorporated?
Damping is used to produce a pulse of the correct duration, PD, and pulse length, SPL.
The damping used is a combination of mechanical and electronic damping.
41
What are typical pulse durations?
A typical transmit pulse lasts 3-5 wavelengths (Spatial pulse length) and so the pulse duration will be 3-5 times the period of the ultrasound wave. (The period is the duration of a single cycle) The overall time is referred to as the pulse duration.
A spatial pulse length is about 3-5 wavelengths.
And as such a pulse duration is about 3-5 periods.
42
What are typical Spatial Pulse lengths
A typical transmit pulse lasts 3-5 wavelengths (Spatial pulse length)
43
What are typical q-values?
Q-factor. This is a measure of the sharpness of the resonance, and can be expressed relating bandwidth to the resonance frequency.
Q = f0/B = f0/(fU - fL)
The Q-factor is thus a dimensionless quantity that is large if the damping is small, and small if the damping is large.
For CW situations you want an ultrasound transducer with Q around 700 to 800, while pulse ultrasound transducers have a Q in the range 2 to 3
44
why is it important to have a series of ultrasound pulses in diagnostic imaging and not a continuous sinusoidal wave exiting the transducer and passing into the patient.
The duration of the transmit pulse will be extended and so the image quality will be poor. The machine will not be able to resolve objects close together as the returning echoes will be received continuously.
45
What effect does transmit pulse length have on resolution?
The shorter the transmit pulse, the better the resolution
46
How can a short transmit pulse be achieved?
a) transmitting only a few cycles and b) using as high ultrasound frequency as possible. This is because the higher the frequency the shorter the period and so the total pulse duration (PD) will be shorter.
T = 1/f PD = T x (3-5)
Bandwidth B = 1/PD
This means as the pulse gets shorter the bandwidth gets larger and vice versa.
In other words, if you want a very short pulse a bigger range of frequencies must be combined
47
How is an ultrasound image made?
The ultrasound LZT (also referred to as PZT), piezo-electric, crystal produces a continuous wave (CW) if excited by a sinusoidal, continuous, voltage (the inverse piezo-electric effect). With a voltage pulse (spike) applied, the crystal rings at the natural frequency. Heavy damping (electronic and mechanical) results in an acoustic pulse with a SPL (spatial pulse length) of about 2-5 wavelengths. This results in a pulse duration, PD, of about 2-5 periods.
The natural frequency of the crystal is determined by its thickness, L, as λ = 2L. A CW response has a well-defined frequency, fresonance, while a damped response is represented by a broad bandwidth, B, centred about fresonance, and a low quality factor, Q.
The voltage pulses occur at a rate described by the pulse repetition rate (or frequency), PRR = 1/PRP, where PRP is the pulse repetition period (or time, the time between adjacent pulses).
Received ultrasound echoes are turned into voltage signals (the piezo-electric effect) where the voltage is the same shape as the received sound pressure time variation. The echo interface is calculated as the distance d = ½Δt c from the crystal transducer, where Δt is the echo time and c = 1540 m/s. This equation embodies a number of approximations that give rise to image artefacts.
48
What is axial direction?
Moving away from the transducer and into the patient
49
How does the intensity change when moving through the patient and in different parts of the beam?
Beam intensity changes as it moves away from the transducer and into the patient
Beam intensity changes across the beam width which is the lateral direction.
Beam intensity changes across the beam and perpendicular to the scan plane (slice thickness or elevation direction)
50
What is the effective beam profile?
the region where the beam intensity is within 20 dB (ie greater than 1%) of the maximum intensity
51
What is the theoretical beam profile?
Defines the total extent of the beam, including the very low level intensities at the edge of the beam.
52
What are the two regions of the beam profile?
the Fresnel Zone (near field) and the Fraunhofer (far field) zone
53
How is good lateral resolution achieved?
by a narrow ultrasound beam width
54
Explain ultrasound beam profiles for both a plane transducer and a focused transducer
Beam intensity changes as it moves away from the transducer and into the patient (axial). Beam intensity changes across the beam width which is the lateral direction. Beam intensity changes across the beam and perpendicular to the scan plane (slice thickness or elevation direction)
The effective beam profile defines the region where the beam intensity is within 20 dB (ie greater than 1%) of the maximum intensity
The theoretical beam profile defines the total extent of the beam, including the very low-level intensities at the edge of the beam
The beam profile results from diffraction and interference effects, and may be divided into two regions, the Fresnel Zone (near field) and the Fraunhofer (far field) zone.
55
Explain the appropriate intensity distribution in an ultrasound beam for both a plane transducer and a focused transducer
The intensity of the beam varies both along the axis of the beam and perpendicular to the axis of the beam.
The intensity fluctuations along and perpendicular to the axis of the beam within the Fresnel zone are a result of the interference and diffraction effects. These effects are no longer evident in the Fraunhofer zone, where the beam starts to diverge, producing a gradual decrease in intensity as the beam energy is spread over an increasing area.
56
What is the idea behind focusing?
is to narrow the ultrasound beam (in both lateral direction and elevation/slice thickness direction).
57
How can focusing be achieved?
By using an acoustic lens (external, mechanical focusing), or by curving the face of the individual crystal element (internal focusing).
58
How does focusing effect the beam profile?
This reduces the beam width in the focal region, but tends to increase it at other depths.
59
How does focusing effect beam intensity?
It concentrates the intensity of the beam in the focal region, with the maximum intensity at the focal depth, and reducing the intensity at other depths.
60
What are the classifications of beam focusing?
Weak, medium and strong.
61
What are focusing classifications based on?
The ratio of the focal depth (FD) to the Fresnel Zone length:-
The Depth of focus, FD, may be approximated by A, the radius of curvature for the lens or curved crystal.
62
Quantify the classifications of focusing
Weak focussing A / T > 1/2
Medium focussing 1/2 > FD / T > 1/6
Strong focussing 1/6 > FD / T
63
Define axial resolution
the ability of the transducer to resolve objects in the axial plane.
in ultrasound refers to the ability to discern two separate objects that are longitudinally adjacent to each other in the ultrasound image.
Axial resolution is defined by the equation: axial resolution = ½ ⨉ spatial pulse length
64
Define lateral resolution
The ability to discern to separate objects that are lateral to each other. At any particular depth into the patient it is just the beam width at that point.
65
Define slice Thickness
The width of the beam in the elevation direction (or perpendicular to the axial direction) It is determined by the element aperture.
66
Explain what is meant by the terms near field and far field in the beam radiating from a transducer.
The diffraction limit is the narrowest possible beamwidth that can be achieved at each depth.
The depth at which the transition from near zone to far zone occurs is called the near zone length and the transition point is the diffraction limit.
The near zone length is important because it is only possible to focus a transducer within the near zone. To focus a transducer at a given depth it is therefore important that its near zone length is greater than its depth.
Focusing the beam will be more successful if the frequency is high and or the aperture is large as per the Near zone length equation NZL = A^2/4 λ
Near field is also called the Fresnel zone and far field is also called the Fraunhofer zone. Near field refers to the area from the transducer up until which the beam begins to diverge and far field refers to the area after the beam has diverged.
67
What is the Focal Zone?
The focal zone is the area of the beam where the beam intensity is within 6dB of maximum intensity or ¼ of the maximum intensity at the focus.
68
Explain why it is important that the structures of interest lie within the focal zone.
The focal zone is the area around the focus where the beam intensity is within 6dB of the maximum intensity.
This is the area where the beam is narrowest providing optimal lateral resolution and of highest intensity providing higher intensity echoes.
69
A focussed beam may be classified as being weak, medium or strong focussing. Describe the general differences in beam profile between these three classifications.
The weakly focussed beam exhibits minimal narrowing in the focal zone, and extends for a greater range of depths. The strongly focussed beam, in contrast, is much narrower at the focus, but this narrowing does not extend far, and the beam is markedly divergent beyond the focal region, reducing the intensity and penetration of the beam
70
What effect does pulse duration have on bandwidth?
Decreasing the pulse duration increases bandwidth. Increasing the pulse duration decreases bandwidth.
71
What size should the matching layer be?
The matching layer must be a quarter of the wavelength.
72
What is a display frame?
A display frame is a single 2D display. It is a product of all the scan lines producing echoes, those echoes returning and the data processed to form a single image. In any one frame there could be more than 150 scan lines
73
What is frame time?
The time it takes to create a display frame.
If there are M scan lines, we already know that each scan line requires a certain time to acquire all the depth information (and the time per scan line depends on the depth reached and the pulse repetition period).
The total time, ΔT, to gather all this information for one frame is the frame time, FT
FT = ΔT = M × PRP
74
What is the equation to calculate frame rate?
FR = 1/FT = 1/(M × PRP) = PRF/M
75
What two parameters determine frame rate?
the pulse repetition period and the number of scan lines used
76
What will decreasing the PRP do?
(increasing the pulse repetition frequency) increases the frame rate, but limits the depth that can be scanned.
77
What will decreasing the number of scan lines do?
increases the frame rate, but degrades the quality of the image as less of the patient section is actually scanned, or the same section at poorer spatial resolution.
78
Explain how the frame rate depends on data and processing time
Frame rate is not only dependent on the PRF(PRR) and the amount of scan lines it is also dependent on the amount of time it takes for software to interpret the data received and display it as an image. (which we shall assume is included in the above expression for simplicity)
79
Define what is meant by real-time display in terms of frame rate
With an image that is refreshed 20 times per second or more,(or has a frame rate of 20 frames per second or greater) our eyes and brain produce an effect of a continuously changing image. A video rather than a flickering image.
If we can produce a 2-D diagnostic image at a frame rate of 20 Hz or higher, we say the image presentation is in real-time.
80
What frame rate constitutes a real time display?
20Hz or higher
81
What is the line of sight of an ultrasound pulse?
Line of site is the pulse direction of the one beam of ultrasound pulse emitted into a patient during A-mode imaging
82
What is echo return time?
Echo return time is the time it takes an emitted pulse to enter the patient, be reflected and return to the transducer.
83
What is echo amplitude?
Echo amplitude – is a representation of the intensity of the echo. The larger the amplitude the larger the intensity. For one echo on the ultrasound scan line the return pulse looks a little like a scaled down (in magnitude) version of the original pulse of about 2-5 wavelengths
84
What are the axes of a mode display?
A-mode display will show echo amplitude as a function of depth.
Echo amplitude being the y axis and depth being the x axis
85
Describe a B-mode image
B-mode stands for brightness mode. The amplitude of the return signal/echo is displayed as brightness, and instead of a single scan line there are many lines displayed at once in the 2-D display. Using a transducer array, the scan lines are generated across the lateral width of the array in rapid succession, based on the PRF(PRR)
86
What determines axial, lateral and spatial resolution?
Axial resolution is determined by the SPL, while lateral resolution is determined by the ultrasound beam width. The spatial resolution (slice thickness) in the elevation plane is primarily determined by the crystal dimension D.
87
Define the focus of the beam.
Focus - Beam focussing refers to creating a narrow point in the cross-section of the ultrasound beam called the focal point. It is at the focal point where the lateral resolution of the beam is the greatest. Before the focal point is the near field or Fresnel zone, where beams converge. Distal to this focal point is the far field or Fraunhofer zone where beams diverge
88
What does the brightness of a B-mode display indicate?
A representation of the amplitude of the returned signals/echos. The higher the signal the brighter it is represented.
89
What are the axes of B mode display?
The vertical scale (Y axis) is calibrated in axial depth (determined from the echo times, ▲t) while the horizontal scale (X axis) is the lateral distance across the patient for that section. So axial depth as a function of lateral distance.
The displayed brightness is the z-axis of the display and is determined by the amplitude of the returned signal.
90
What is M mode scanning?
M-mode means motion scanning. The M-mode 2-D display shows depth in the (Y axis) vertical direction against time in the ( x axis) horizontal direction. Structures that are stationary relative to the probe will be shown at a constant depth (horizontal line) while structures moving toward and away from the probe (eg heart walls) will move up and down the screen (as their depth is changing)
M mode thus displays information about the amount of movement of individual structures and how that changes with time.
91
Define temporal resolution
Temporal resolution - in ultrasound represents the extent to which an ultrasound system is able to distinguish changes between successive image frames over time (i.e. movement).
Temporal resolution is chiefly determined by the image frame rate of the system (measured in Hertz), which may vary depending on a number of factors. Overall, an increased frame rate equates to an increased likelihood of discerning rapid movements (e.g. valve leaflets in echocardiography), and thus improved temporal resolution
92
What are the axes of m mode display?
The M-mode 2-D display shows depth in the (Y axis) vertical direction against time in the ( x axis) horizontal direction
93
What is m mode use in cardiac imaging?
M-mode is utilised for its superior temporal and axial resolution; the temporal resolution is most useful in delineating the path of structures moving at a high velocity, as well as their timing (e.g. in relation to the cardiac cycle). The axial resolution is also vital in echocardiography, as it allows the resolution of delicate cardiac structures (e.g. valve leaflets) which are difficult to resolve, especially with transthoracic echocardiography. It can also show the maximum and minimum diameters of the heart chambers and the movement of valve leaflet as the valve opens and closes
94
Distinguish between frame, frame time and frame rate
A frame is the complete echo data (in image format) from one scan across the transducer elements. The frame time, FT, is the time it takes to acquire and process this echo data and have a frame completed, the frame rate, FR, is the number of frames produced per second; FR = 1/FT.
95
What is the essential material property that gives rise to a reflecting interface in diagnostic imaging?
Acoustic impedance. A boundary between two tissues with different acoustic impedance, Z. As we are talking about a reflection the boundary/interface should be of far greater extent than the wavelength.
96
Understand single transducer element technology?
A single transducer element has a width D. The scan line is the direction of the emitted beam, and any received echo in the transducer off time is assumed to have travelled along this scan line back to the transducer. The scan line is the ray that defines the centre of the beam, the central ray. The beam width tells us about the lateral extent, while the slice thickness is the elevation plane extent of the beam.
97
What does 'transducer array' mean?
Transducer array refers to the fact that a large number of cuts are made in the transducer. This transforms the transducer into an array of tiny identical transducer elements.
98
What does an array do that a single element can not?
The array is what makes it possible to focus and steer the ultrasound beam.
99
Describe a transducer array operation
The elements are narrow. Small gaps are between each that only project half way into the transducer and are filled with an insulating material. The front and rear surface of each element are coated with a thin metal layer and each has a separate wire connection and a single wire makes contact to the front face.
Even though the insulating material only projects half way into the transducer material each element operates as a separate transducer, transmitting and receiving independent of its neighbouring elements.
100
What is the difference between mechanically and electronically steered beams?
The Annular array (circular elements of different radii) uses mechanical means to steer the beam e.g. a motor and rotating wheel assembly can be used.
An electronically focussed beam in an annular phased array is achieved by applying delays between firing each element, firing the outer ring element first, and then the next element, firing them sequentially until the central disc element is fired.
Sp one uses physical means and one use electronic controlled phased pulses.
101
Describe a linear array
The Linear array uses overlapping groups of elements, stepping one element along the array to fire successive beams. The beams produced are all parallel, producing a rectangular field of view.
102
Describe a curved array
Curved arrays are similar in construction to the linear array, with many individual elements that are shaped into a convex form that for some purposes allows a better ‘fit’ to the external surfaces that have to be scanned, and also produce a field of view with diverging lines of sight, that provides a wider field of view at depth.
The operation of the curved array transducer is similar to the linear array, using overlapping groups of elements for each successive line of sight, and time delays in transmission and reception to provide electronic beam focussing within the scan plane. Elevation plane focussing is achieved by mechanical means.
103
Describe a phased array
The Phased array transducer uses time delays to steer the beam. In phased arrays, the probe is far more compact, using electronic beam steering to provide the full cover of the 2-D scan. The sector display of a phased array thus has a much smaller initial footprint than a conventional curved array as, in theory, all scan line central rays emanate from a single point; from the central element in the phased array
104
How is a beam electronically steered and focussed?
Ultrasound beams are electronically steered and focussed by having elements transmit along the array at different times and upon reception the data is processed to accurately display the image taking into account transmitting and receiving delays.
The user will set the depth of focus and the machine calculates the necessary delays to produce that depth of focus
The outermost elements will have the shortest delay as they transmit first and the central element will have the longest delay and therefore transmits last. The pulse will arrive simultaneously and in phase at a point at the focus depth.
105
Explain reception focusing and steering.
The principle of reception focussing and steering is very similar to that used for transmitting. An echo returning from a point in soft tissue will arrive at slightly different times across the array. The machine uses electronic delays to compensate for these differences so the echoes will be simultaneous and in phase when added together electronically for processing. So similar to transmitting the echo arrives at the centre element first and so requires the longest delay and the echoes arrive at the outer elements last and so require the shortest delay.
106
What is the important difference between transmit and receive focussing?
The transmitted beam has a single depth of focus set by the user.
Receive focus is done automatically by the machine and is dynamic. At each instant the machine knows what depth the echoes are coming from (using the constant of an echo travelling 1cm per 13us.
The machine adjusts the receive focussing delays so that at each instant the beam is correctly focussed at the depth from which the echoes are coming
Similarly, the number of elements used to receive echoes and thus the receive aperture is dynamically varied with depth and this is known as dynamic aperture
107
How do side lobes occur?
Additional beams that occur either side of the desired ultrasound beam. They are generally weaker than the main beams by a factor of 100 (20dB) they are still capable of contributing echoes and can therefore cause artifacts.
Since a machine cannot know that the echo from the object comes from a sidelobe it displays it across the midline of the beam.
As the beam is scanned the object will be displayed in its correct location by the main beam and also to the left and right of it’s true location by the sidelobes. This is referred to as side lobe artifact.
108
How do grating lobes occur?
Are another type of side lobe. They are important because if they occur, they are higher in amplitude than the previously described side lobes. Grating lobes can be eliminated whereas side lobes can only be reduced.
Grating lobes form due to constructive interference of beams from two adjacent transducer elements when they are exactly one cycle apart.
109
How can grating lobes be eliminated?
by ensuring the geometry of the transducer does not allow this. Grating lobes cannot occur so long as the distance from one transducer element to the next is less than half the wavelength.
110
Define apodisation
Used to reduce side lobe artifact. On transmission and reception of echoes the machine deemphasises the contribution of the outer transducer elements. This means the transmit pulse amplitude for these elements is less (compared with the amplitude of other elements) and the receive amplification applied to receive signals is less compared to others. This significantly reduces the amplitude of the side lobes and so makes side lobe artifacts less evident. This comes at the cost of a slight beamwidth increase of the main beam
111
Define dynamic focusing (reception focusing)
The transmitted beam has a single depth of focus set by the user. Receive focus is done automatically by the machine and is dynamic. At each instant the machine knows what depth the echoes are coming from (using the constant of an echo travelling 1cm per 13us).
The machine adjusts the receive focussing delays so that at each instant the beam is correctly focussed at the depth from which the echoes are coming
112
Define dynamic aperture
Similarly to dynamic focusing, the number of elements used to receive echoes and thus the receive aperture is dynamically varied with depth by he machine.
113
Explain how multiple focal zones are produced
As you can alter the focal length of the ultrasound beam, it is possible to arrange for the one beam line to have a number of focal lengths to improve the lateral resolution, firing multiple pulses along each line of sight, each focussed at a different depth. This comes at the expense of the frame rate which decreases significantly for each additional focal zone added
114
What are the various methods used to improve the beam profile?
multiple focal zones dynamic aperture, dynamic receive aperture, dynamic focussing and apodisation
115
Array transducers comprise a number of piezoelectric elements each electrically and acoustically isolated. Briefly discuss the role of this isolating material.
The isolating material ensures that each crystal element operates independently, allowing a small-time delay to be introduced between firing adjacent elements. This is essential for the application of electronic beam steering and focussing.
116
What methods are used to provide beam focussing in the elevation plane?
Internal focusing is normally done by curving the crystals themselves. External focusing (more common now) is by use of an acoustic lens on the face-late of the transducer housing. Neither of these methods allows one to vary the focal length – electronic focusing does
