Topic 21: imaging with US Flashcards
(47 cards)
1
Q
Explain how image is produced
A
- US images = display difference in acoustic impedance of different tissues
- Variation in density/compressibility of different tissues
1) Ultrasonic pressure wave = transmitted from transducer along single line of sight into body
2) As pressure wave propagates = echoes reflected from objects along line of sight
3) Returning echoes received by transducer
2
Q
Describe how image is displayed
A
- Mapping echo magnitude as brightness in image
- Mapping arrival time as distance along line
- Repeated along many lines of sight = 2D image
3
Q
Describe go-return time
A
- AKA time of flight
- Travel time of journey = accurately position reflector
- Elapsed time from pulse creation → pulse reception = time of flight
- Used to determine reflector depth = measuring distance in diagnostic imaging
4
Q
What is the relationship between TOF + distance pulse travels?
A
- US system’s programmed with average speed of sound in soft tissue
- If average speed known = TOF + distance pulse travels = directly related
5
Q
How is depth of reflector calculated?
A
Depth = 1.54 x go return time / 2
6
Q
What is the 13μs rule?
A
- Applies when sound travels through soft tissue
- For every 13μs of go return time = object creating reflection = 1 cm deeper in body
7
Q
Give the 2 functions of transducer
A
1) Transmitter = produces ultrasound by converting electrical energy → acoustic
2) Receiver = detects US waves + converst acoustic energy →electric
8
Q
Define bandwidth
A
- Range of frequencies transducers able to produce
9
Q
Explain piezoelectric effect
A
- Certain crystals change dimensions when under electric field + change back when removed
- When compressed = generate electric potentials
10
Q
Explain piezoelectric effect in transducer
A
- Changes in polarity of voltage applied to transducer = change in thickness
- Polarity change = expanding/contracting
- Causes increase/decrease in pressure
- Produces US = transmitted into body
- Pressure change caused by returing US = converted to electric energy signals = transferred to computer = ultrasound
11
Q
What is the function of crystal size?
A
- Wavelength of emitted US
12
Q
What are transducers made of?
A
1) Lead-zirconate-titanate = PZT
2) PVDF
3) Composite
13
Q
Describe the polarization of PZT
A
- Exposed to strong electric field + heated substantially
- Curie temp = 360°C
- If PZT heated above = piezoelectric properties destroyed = depolarization
14
Q
Describe the relationship between PZT speed of sound + frequency
A
- PZT speed + frequency directy related
- Speed faster = frequency higher
15
Q
What is the speed of speed in piezoelectric material?
A
4-6 mm/μs
16
Q
Describe the relationship between PZT thickness + frequency
A
- Thinner active elements in transducer = higher frequency
- PZT thickness + frequency = inversely related
17
Q
Give the equation relating frequency + thickness
A
𝒇𝒓𝒆𝒒𝒖𝒆𝒏𝒄𝒚 = 𝒔𝒐𝒖𝒏𝒅 𝒔𝒑𝒆𝒆𝒅 𝒊𝒏 𝑷𝒁𝑻/ 𝟐 × 𝒕𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔
18
Q
Define receiver
A
- Detects + amplifies weak signals from different depths
- Can selectively amplify returning signals = match dynamic range of display
19
Q
Define TGC
A
- Difference in echo strength compensated by time gain compensation
20
Q
What is the relationship between attenuation + frequency?
A
- Attenuation of sound = proportional to frequency = constant for certain tissues
21
Q
What does TCG control allow operator to do?
A
- Selective amplification from deeper structures
- Suppression of signals from superficial structures
- Most important user control = big impact on quality image
22
Q
Give the factors effecting US beam
A
1) Curvature of transducer
2) Pressure waves
3) Acoustic lens
23
Q
Give the 5 terms of US beam
A
1) Focus
2) Near zone
3) Focal zone
4) Focal length/near zone length
24
Q
Describe ultrasound fields
A
1) Beam exists probe = near/Fresnel zone
2) Beam narrows = until narrrowed fully = focus = start of far/Fraunhofer zone
3) Distance from transducer-focus = focal length
4) Beam starts to widen
25
Describe the beam diameter in relation to location
- Transducer = equals transducer diameter
- Focus = half transducer diameter
- 2 near zone lengths = equal transducer diameter
- Deeper than 2 zone lengths = wider than transducer diameter
26
What focuses transducers?
- Curved piezo electric crystal
- Acoustic lens
27
Define phased array
- Transmitting + receiving signals = individually delayed in time
28
Describe beamforming
- Electronically steering each sequence of acoustic pulses through plane being imaged
- Produces 2D/3D map = scattered echoes on display
29
What is the relationship between frequency and quality of resulting image?
- Frequency of beam affects quality of resulting image to great degree
- Higher frequency = longer near field + less divergence in far field
- Better resolution in small structures
30
Give the factors affecting beamsteering
1) Transducer diameter
2) Transducer frequency
3) Mechanical focus = focused/unfocused
31
What is the relationship between transducer diameter and fields?
- Small diameter = near field shorter + diverges more
32
What is the relationship between transducer frequency + fields?
- Higher frequency = longer near fields
- Higher = deeper focus
- Overcome by small diameter + high frequency crystal
33
What is the calculation for focal depth?
𝒅𝒊𝒂𝒎𝒆𝒕𝒆𝒓 × 𝒇𝒓𝒆𝒒𝒖𝒆𝒏𝒄𝒚 / 6
34
How is focus achieved by phased array?
- Appropriate timing of individual elements = leading edges of the wavelets can
produce concentrically curved wave front
- resultant ultrasonic beam focuses on a given point from the transducer
35
Give 3 planes of spatial resolution
1) Depth/axial resolution
2) Lateral resolution
3) Elevation resolution
36
Give 2 factors in selecting frequency for US
1) Depth
2) Spatial resolution
37
Describe what different frequencies are used for
- Low = deep in abdomen
- High = finer details but close to surface
38
Describe axial resolution
- Determines ability to separate 2 objects along axis of beam
- Min distance between 2 structures = still produce 2 distanct echoes on an image
- Determined by = pulse length = mm
- Higher transducer frequency = better resolution
39
Define pulse length
- Product of wavelength + number of cycles in pulse
40
Give the 2 calculations for axial resolution
Pulse length / 2
Wavelength x cycles in pulse / 2
41
Describe lateral resolution
- Ability to distinctly identify 2 structures very close together when side by side/perpendicular to beam axis
- Min distance between 2 structures but can still tell apart
- Determined by width of US beam = mm
- Controlled by focusing beam
42
What is better resolution for clinical?
- Axial > lateral = US pulses shorter
43
Describe shadow artefacts
- US + high density structure
- Surface = hyperechoic
- Deep to bone/gallstones = anechoic
- Black area = shadow
- Used to identify gallstones
- Disadvantage = can obstruct view of deeper structures
44
Describe enhancement artefacts
- US + low density fluid filled structure
- More echogenic than surrounding tissue = enhanced
- Used to differenciate between fluid filled cyst + tumor
- Cyst = enhanced
45
Describe mirror image artefacts
- US + reflective curved surface e.g. diaphram/liver
- Part of US indirect trajectory when returning = wrongly interpreted as deeper structure
- Causes mirror image
- Will not form if pleural effusion present
- Disadvantage = mirror image of liver misinterpreted as consolidated lung
46
Describe reverberation artefact
- US + 2 closely apposed reflective surfaces e.g. parietal/visceral pleura
- Creates parallel linear artefacts at intervals
- A-lines = in lungs = used to assess dyspnoea
47
Describe refraction artefacts
- US + fluid-filled curved structure = deflected from path
- Anechoic artefact projects from edge of structure + extends to far-field
