utrasound Flashcards
Ultrasound machine and speed of sound?
assumes constant in every medium 1540 m/s
Change in wavelength and frequency between media?
Frequency doesn’t change
Wavelength changes in media
Interacting waves
‘constructive’ or ‘destructive’ effects can be used to shape and steer beam
Relative intensity dB
Reducing to 10% = -10dB
to 1% = -20dB
to 0.1% = -30dB
-3dB = 50% loss of signal intensity
US half-value thickness
Tissue thickness that reduces US intensity by 3dB
Impedance
Z = density x speed of sound
reflection based in differences in impedance
Refraction
depends on ?
clinical example
Angle of incidence
change in speed
along edges of tissue/fluid interfaces like the GB you see ‘bending’ of the beam
Specular reflection and frequency
what happens to scatter with frequency?
Shorter wavelengths see surfaces as more ‘rough’, non-specular. So higher frequency = more scatter
Absorption =
Sound energy turned to heat, increases with frequency
‘attenuation’
loss of intensity from both scatter and absorption
rough attenuation calc for ‘soft tissue’
dB and cm
- 5 dB per cm per MHz
- 5 (dB/cm)/MHz
Attenuation and frequency
Proportional
a 2 MHz beam will have twice the attenuation of a 1MHz beam
frequency and HVT
(3dB reduction)
HVT decreases with increasing frequency
What determines strength of echoes?
angle and impedance
Unit for impedance?
Rayl
Piezoelectric material
Can be quartz, usually PZT (lead-zinc-titanate)
molecular arrangement of electrical dipoles can be compressed, disturbed and measured
Operating frequency of a transducer dependent on?
speed of sound in, and thickness of the piezoelectric material
ONLY WAY TO CHANGE FREQUENCY IS TO CHANGE PROBE
Crystal/transducer thickness and frequency
thickness of transducer = 1/2 wavelength
lower frequency = thicker
higher frequency = thinner
Dampening block
where?
what?
Sits behind the crystal and absorbs backward directed US energy.
Also dampens transducer vibration, shortens spatial pulse length, preserving axial resolution
Thickness of dampening block
Dampening introduces a broadband frequency spectrum
Thin (ding)
called high Q, long pulse length
NARROW bandwidth
Thick (thud)
Low Q, short pulse length
BROAD bandwidth
When is good for low or heavy damping?
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
Matching layer
what is
made of
optimal thickness??
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
Linear (sequenced) transducers
Width of transducer =
individual elements firing and receiving on their own
(no steering or interference)
Width of transducer = width of the sum of individual elements
Linear arrays are good for?
Peds, superficial things (carotids, leg veins, testicles, thyroids)
curve-linear
still ‘linear’, each element operates on its own
scan lines diverge deeper into image
abdominal imaging
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)
near field, far field, focal zone
near and far fall on either side of focal zone
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
“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)
Three US spatial dimensions
Axial, lateral, elevation (slice thickness)
Axial resolution
returning echoes
Returning echoes need to be SEPARATE and not overlap
minimim required sparation is 1/2 the spatial pulse length
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)
Lateral resolution
Dependent on skinny beam. Skinniest at focal zone. Shitty lateral resolution both near and far
Lateral and axial res vs depth
Axial depends on SPL, NOT DEPTH
Lateral worse at increasing depth past focal zone
Gain and lateral res
Gain widens the beam
WORSE lateral res
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
IMPROVING AXIAL RES
Shorter pulses (smaller SPL)
MORE DAMPING (low Q)
Higher frequency (shorter wavelength)
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)
Improving elevation res
fixed focal length across surface of array
minimize slice thickness (phase excitation of outer to inner arrays)
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
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
US Artifacts
Reverb
2 parallel highly reflective surfaces
US Artifacts
comet tail
kind of reverb
parallel reflectors are closer than 1/2 SPL, therefore not resolved as separate (triangle fading down)
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
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”
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
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)
MODES
A mode
Amplitude
historical
used by optho now
processed info from the reciever vs time
gives amplitude as a function of time
MODES
B
Brightness
conversion of a line of info to brightness-modulated dots on a display.
proportional relationship of brightness to echo signal amplitude
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
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
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
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
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
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
Doppler artifacts
How to limit aliasing
Decrease doppler shift
lower frequency transducer doppler angle closer to 90
INCREASE PRF
INCREASE THE SCALE
Flash color artifact
transducer or patient motion
FETAL KICK
Color bleed
color extending beyond vessel wall
FIX = decrease color gain
Image optimization
Output power (transmit gain)
Increases brightness by adjusting strength of sound pulse
LOSE LATERAL RES - WIDENING BEAM
Image Optimization
Receiver gain
Increases brightness AFTER IT RETURNS
transmit vs receiver gain ?
ALARA says Receiver Gain first
Image Optimization
Time Gain Compensation
makes top and bottom uniform
now automated
Harmonics
Receiving at second harmonic frequency
harmonics not produced in near field
IMPROVES lateral resolution
REDUCED reverb
LOSE depth
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)
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
US Safety
Thermal index
max temperature rise
based on a homogenous tissue model with given instrument parameters
US Safety
Mechanical index
How likely it is that cavitation will occur considering peak rarefaction pressure and frequency.
Most relevant with contrast enhanced US
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
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
Cavitation most likely at?
Lower frequency and higher pressure
Wat deposits most heat?
Spectral doppler
Bad numbers for MI and TI?
risk benefit decision when
TI > 1.0
MI > 0.5
Til MIS
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
General TI guidelines
<0.7
- 0-1.5
- 5-3.0
>3.0
< 0.7 = general rec for OB
- 0-1.5 = DONT exceed 30 mins
- 5 - 3.0 = NOT exceed 1 minute
Greater than 3.0, NONE ATALL
Mirror image artifact
what is wrong assumption
all echoes return after a single reflection
