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utrasound Flashcards

1
Q

Ultrasound machine and speed of sound?

A

assumes constant in every medium 1540 m/s

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2
Q

Change in wavelength and frequency between media?

A

Frequency doesn’t change

Wavelength changes in media

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3
Q

Interacting waves

A

‘constructive’ or ‘destructive’ effects can be used to shape and steer beam

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4
Q

Relative intensity dB

A

Reducing to 10% = -10dB

to 1% = -20dB

to 0.1% = -30dB

-3dB = 50% loss of signal intensity

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5
Q

US half-value thickness

A

Tissue thickness that reduces US intensity by 3dB

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6
Q

Impedance

A

Z = density x speed of sound

reflection based in differences in impedance

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7
Q

Refraction

depends on ?

clinical example

A

Angle of incidence

change in speed

along edges of tissue/fluid interfaces like the GB you see ‘bending’ of the beam

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8
Q

Specular reflection and frequency

what happens to scatter with frequency?

A

Shorter wavelengths see surfaces as more ‘rough’, non-specular. So higher frequency = more scatter

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9
Q

Absorption =

A

Sound energy turned to heat, increases with frequency

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10
Q

‘attenuation’

A

loss of intensity from both scatter and absorption

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11
Q

rough attenuation calc for ‘soft tissue’

dB and cm

A
  1. 5 dB per cm per MHz
  2. 5 (dB/cm)/MHz
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12
Q

Attenuation and frequency

A

Proportional

a 2 MHz beam will have twice the attenuation of a 1MHz beam

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13
Q

frequency and HVT

A

(3dB reduction)

HVT decreases with increasing frequency

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14
Q

What determines strength of echoes?

A

angle and impedance

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15
Q

Unit for impedance?

A

Rayl

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16
Q

Piezoelectric material

A

Can be quartz, usually PZT (lead-zinc-titanate)

molecular arrangement of electrical dipoles can be compressed, disturbed and measured

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17
Q

Operating frequency of a transducer dependent on?

A

speed of sound in, and thickness of the piezoelectric material

ONLY WAY TO CHANGE FREQUENCY IS TO CHANGE PROBE

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18
Q

Crystal/transducer thickness and frequency

A

thickness of transducer = 1/2 wavelength

lower frequency = thicker

higher frequency = thinner

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19
Q

Dampening block

where?

what?

A

Sits behind the crystal and absorbs backward directed US energy.

Also dampens transducer vibration, shortens spatial pulse length, preserving axial resolution

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20
Q

Thickness of dampening block

Dampening introduces a broadband frequency spectrum

A

Thin (ding)

called high Q, long pulse length

NARROW bandwidth

Thick (thud)

Low Q, short pulse length

BROAD bandwidth

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21
Q

When is good for low or heavy damping?

A

Low damping (thin) = narrow bandwidth, used for DOPPLER, to preserve velocity information

High damping (thick) = High spatial (axial) resolution (fewer interference effects and more uniformity)

Review, thick = LOW Q

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22
Q

Matching layer

what is

made of

optimal thickness??

A

Between crystal and patient to minimize differences in acoustic impedance

made of stuff intermediate in impedance between soft tissue and transducer material

optimal thickness = 1/4 the wavelength

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23
Q

Linear (sequenced) transducers

Width of transducer =

A

individual elements firing and receiving on their own

(no steering or interference)

Width of transducer = width of the sum of individual elements

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24
Q

Linear arrays are good for?

A

Peds, superficial things (carotids, leg veins, testicles, thyroids)

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25
curve-linear
still 'linear', each element operates on its own scan lines diverge deeper into image abdominal imaging
26
Phased array good for?
hive mind groups of elements firing in multiples with interference patterns used to steer the beam CAN BE MADE SMALLER THAN LINEAR Good for limited windows (between ribs)
27
near field, far field, focal zone
near and far fall on either side of focal zone
28
Near field (fresnel zone) Near field length depends on?
Near field converges **Higher frequency** = LONGER NEAR FIELD **Larger diameter element** = LONGER NEAR FIELD Longer near field = less divergence of far field
29
"stand off pad"
For very superficial things low impedance barrier to scan superficial things (moves them from near zone to focal zone, BEST lateral resolution)
30
Three US spatial dimensions
Axial, lateral, elevation (slice thickness)
31
Axial resolution returning echoes
Returning echoes need to be SEPARATE and not overlap minimim required sparation is **1/2 the spatial pulse length**
32
Spatial pulse length =
cycles per pulse multiplied by wavelength length x number Two objects closer than **1/2 SPL** will NOT be resolved as separate objects (axial)
33
Lateral resolution
Dependent on skinny beam. Skinniest at focal zone. Shitty lateral resolution both near and far
34
Lateral and axial res vs depth
Axial depends on SPL, NOT DEPTH Lateral worse at increasing depth past focal zone
35
Gain and lateral res
**Gain widens the beam** **WORSE lateral res**
36
Elevation res (slice thickness)
Similar to lateral res TYPICALLY WEAKEST Depends on **HEIGHT OF TRANSDUCER ELEMENTS** Source of volume averaging before and after the focal zone
37
IMPROVING AXIAL RES
Shorter pulses (smaller SPL) MORE DAMPING (low Q) Higher frequency (shorter wavelength)
38
Better lateral res
Put in focal zone narrow beam minimal gain (GAIN WIDENS BEAM) Phased array (multiple focal zones) INCREASE line density (lines per cm)
39
Improving elevation res
fixed focal length across surface of array minimize slice thickness (phase excitation of outer to inner arrays)
40
US Artifacts Side lobe artifact example/usual? Worst with which 'ducer?
When a **strong reflector falls out in the side of the beam** Transducer assumes that echoes originate from main beam, **places strong reflector out on the side into the central field**, moreso when central field is something ANECHOIC **pseudo sludge in GB** **WORSE with LINEAR ARRAY TRANSDUCERS**
41
US Artifacts Beam width artifact usually seen where? fix?
similar to side lobe, strong reflector in far field picked up by highly diverged beam will be placed in main beam Usually **seen as bladder with peripheral echoes** FIX = **adjusting focal zone to ROI**, placing transducer at center of image
42
US Artifacts Reverb
2 parallel highly reflective surfaces
43
US Artifacts comet tail
**kind of reverb** parallel reflectors are closer than 1/2 SPL, therefore **not resolved as separate (triangle fading down)**
44
US Artifacts Ring down
fluid trapped between air bubbles creates a nearly continuous sound wave back to probe **LINE or Series of parallel bands posterior to a collection of gas**
45
US Artifacts Mirror image
Kinda like reverb with bouncing but results in duplication deep to strong reflector CLASSIC (almost always) = **liver parenchyma where lung should be at liver/lung interface** "trapped behind a strong reflector"
46
US Artifacts Velocity related - machine assumes 1540 m/sec Speed displacement
speed of sound slows down in fat relative to liver Liver edge imaged behind some fat, beam takes longer to get back, assumed to be further, **results in discontinuous, posteriorly displaced liver border**
47
US Artifacts Refraction artifact
Returning echo gets **refracted right before detection** **object can then be displayed as** **1 wider than it should be** **2 misplaced to the side** **3 duplicated** **CLASSIC = DUPLICATED SMA deep to rectus muscles and fat** (move transducer and it will be single)
48
MODES A mode
Amplitude historical used by optho now processed info from the reciever vs time gives amplitude as a function of time
49
MODES B
Brightness conversion of a line of info to brightness-modulated dots on a display. proportional relationship of brightness to echo signal amplitude
50
MODES M
Motion B mode info is used to display the echoes from a moving organ from a fixed transducer and beam position **M mode is 4x greater than B mode** **Pulsed doppler is 20x greater than B mode**
51
Doppler angle ideal why
30-60 something cosign zero would be ideal, but less than 20 there's too much refraction 90 gives no flow and possibly a mirror image
52
pulsed wave (spectral) doppler transducer
utilizes a single transducer for both reception and transmission flow velocity varies giving a spectrum of doppler shifts instead of a single frequency
53
Color doppler how? angle?
obtains samples of each pixel multiple times then displays the average shift doppler angle not as important since info is semi-quantitative
54
Power doppler aliasing? angle?
VERY sensitive for flow without info on direction still get color but each pixel registers total number of frequency shifts NO ALIASING NO DEPENDENCE ON ANGLE, CAN BE MEASURED PERPENDICULAR
55
Doppler artifacts Aliasing doppler shift is greater than ?
Greater than Nyquist frequency **1/2 pulse repetition frequency** **1/2 PRF \> shift to avoid aliasing** **Ex. SHIFT = 3.5 kHz, need PRF of 7kHz to avoid aliasing** **PRF needs to be double the shift**
56
Doppler artifacts How to limit aliasing
Decrease doppler shift **lower frequency transducer doppler angle closer to 90** INCREASE PRF INCREASE THE SCALE
57
Flash color artifact
transducer or patient motion FETAL KICK
58
Color bleed
color extending beyond vessel wall FIX = decrease color gain
59
Image optimization Output power (transmit gain)
Increases brightness by adjusting strength of sound pulse LOSE LATERAL RES - WIDENING BEAM
60
Image Optimization Receiver gain
Increases brightness AFTER IT RETURNS transmit vs receiver gain ? ALARA says Receiver Gain first
61
Image Optimization Time Gain Compensation
makes top and bottom uniform now automated
62
Harmonics
Receiving at second harmonic frequency harmonics not produced in near field IMPROVES lateral resolution REDUCED reverb LOSE depth
63
Compound imaging
Imaging an object in multiple different directions electronic steering of the beam **sharpens edges** and **gets rid of posterior shadowing** (can make a cyst look solid)
64
Harmonics vs compound imaging example with a breast 'nodule' Normal Harmonics Compound
**Normally** hypoechoic with blurry margins, reverb and some shadowing **Harmonics**- MORE ANECHOIC, WORSE SHADOWING, NO REVERB **Compound**- hypoechoic, **SHADOWING GONE, Sharp margins** **Compound makes cystic look solid** **Harmonics makes solid look cystic**
65
US Safety Thermal index
max temperature rise based on a homogenous tissue model with given instrument parameters
66
US Safety Mechanical index
How likely it is that cavitation will occur considering peak rarefaction pressure and frequency. ## Footnote **Most relevant with contrast enhanced US**
67
Cavitation
Sonically generated activity in compressible bodies composed of gas or vapor Stable - bubbles already present, they expand and contract Transient cavitation - bubbles collapse, shock waves ripple causing tissue damage
68
Thermal induced damage
worse at higher frequency rise in temp slows 2/2 conduction and perfusion Damage has a THRESHOLD, none til a certain temp
69
Cavitation most likely at?
Lower frequency and higher pressure
70
Wat deposits most heat?
Spectral doppler
71
Bad numbers for MI and TI?
risk benefit decision when **TI \> 1.0** **MI \> 0.5** **Til MIS**
72
Fetus and US Where is thermal damage most likely? rules
soft tissue - bone interface (brain and spinal cord) USE different index (BONE INDEX) after 10 weeks 1st trimester no pulsed doppler (color, spectral, power) M mode used for HR instead TI under 1.0
73
General TI guidelines \<0.7 1. 0-1.5 2. 5-3.0 \>3.0
\< 0.7 = general rec for OB 1. 0-1.5 = DONT exceed 30 mins 2. 5 - 3.0 = NOT exceed 1 minute Greater than 3.0, NONE ATALL
74
Mirror image artifact what is wrong assumption
all echoes return after a single reflection

physics factoids (8 decks)

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