PHYSICS - US Flashcards
PACS (acronym)
Picture Archiving and Communication System
DICOM (acronym)
Digital Imaging and COmmunications in Medicine
Speed of sound in soft tissue
1540 m/s
Terms for areas of high and low pressure created by sound waves
compression and rarefaction, respectively
Bone - higher or lower speed of sound
higher speed of sound (dense, less compressible)
Air - higher or lower speed of sound
lower speed of sound (less dense, more compressible)
Change in decibels equivalent to a 50% loss in sound intensity
-3 dB
Half value thickness definition
thickness of tissue that attenuates sound intensity by 3 dB (50%)
Strength of returning echoes is influenced by…
magnitude of impedence difference and angle of incidence
Distance traveled in US
twice the depth of the reflector (lesion)
No refraction occurs if…
incident waves are perpendicular to tissue boundary or no impedence difference between tissues
Snell’s Law
angle of refraction increases with increasing speed difference between tissues and increasing angle of incidence
“Edge shadowing”
refraction artifact (distal to curvilinear surface); computer assumes linear progression of sound waves
Specular scatter
a.k.a. smooth scatter (not really scatter, just reflection); occurs when reflector dimensions are larger than wavelength
Non-specular scatter
a.k.a. diffuse scatter; occurs when reflector dimensions are smaller than wavelength
Non-specular scatter increases with…
decreasing wavelength (smaller waves “see” more small irregular surfaces which cause scatter)
Relationship between scatter and frequency
directly proportional; increased TF => decreased wavelength => increased scatter
Attenuation increases with…
TF and tissue depth
Attenuation coefficient for soft tissue
0.5 (dB/cm)/MHz
Effect of increasing frequency on HVT
decreased HVT (less tissue required to attenuate a higher frequency beam by 50%)
Components of transducer
piezoelectric crystals, dampening block, matching layer
Frequency at which maximum intensity waves are produced
center (resonance) frequency
Transducer crystal thickness is equal to…
half of the wavelength (or wavelength is equal to 2x crystal thickness)
Effect of thinner crystal on frequency
thinner crystal => smaller wavelength => higher transducer frequency
Effect of thicker crystal on frequency
thicker crystal => longer wavelength => lower transducer frequency
Thin dampening block used for…
Doppler imaging (longer SPL, narrow bandwidth)
Thick dampening block used for…
B-mode (shorter SPL, broader bandwidth)
Low Q
thick dampening block; good for B-mode, broad bandwidth
High Q
thin dampening block; good for Doppler, narrow bandwidth
Purpose of matching layer
to minimize impedence differences between transducer and patient (gel also helps with this)
Optimal matching layer thickness
1/4 of wavelength (or 1/2 of crystal thickness)
Activation type where crystal groups are pulsed sequentially
linear array activation (linear or curvilinear probes)
Sector transducers
a.k.a. phased array transducers
Activation type where crystal groups are pulsed simultaneously
phased array activation (firing times can be adjusted to create constructive and deconstructive effects)
Fresnel zone
a.k.a. near field
Fraunhofer zone
a.k.a. far field
Length of near field influenced by…
transducer frequency and crystal diameter
Divergence in far field influenced by…
transducer frequency and crystal diameter
Effect of higher TF on near field and far field
longer near field, less divergence in far field (better lateral resolution)
Effect of increased crystal diameter on near field and far field
longer near field, less divergence in far field (better lateral resolution)
Best lateral resolution at the…
focal zone
Spatial pulse length (SPL) definition
of waves per pulse; generally 2 waves (so 2 * wavelength)
Formula for axial resolution
SPL / 2 - note that smaller axial resolution is better
Pulse repetition period (PRP) definition
time between the beginning of subsequent pulses
Relationship between PRP and depth of FOV
directly related; greater PRP => increased depth of FOV
Relationship between PRF and depth of FOV
inversely related; greater PRF => decreased depth of FOV
How to: correct aliasing in spectral Doppler
increase PRF, increase Doppler angle, decrease TF, increase the scale
Relationship between PRF and frame rate
increased PRF => increased frame rate
Disadvantage of broadband transducers
spectral broadening; this is why thin dampening blocks are used for Doppler imaging
Advantages of broadband transducers
produce smaller SPLs, can use multiple frequencies, can perform harmonic imaging
Bandwidth definition
range of frequences produced by a transducer
Minimum required separation to differentiate two adjacents objects
1/2 of the SPL; a.k.a. the AXIAL RESOLUTION
How to: get a smaller SPL (better axial resolution)
increase frequency, thicker dampening block, broad bandwidth
T/F - axial resolution is depth dependent
false - axial resolution is NOT depth dependent
Factors affecting lateral resolution
beam width, transducer frequency, scan line density
Relationship between beam width and lateral resolution
thinner beam => better lateral resolution
T/F - lateral resolution is depth dependent
true - lateral resolution is depth dependent
Elevational resolution is dependent on…
transducer element thickness
Maximum frame rate equation
PRF / # of scan lines per frame
Effect of increasing number of scan lines
improved lateral resolution, slower frame rate
Effect of decreasing PRF
increased depth of FOV, slower frame rate
Effect of using multiple focal zones
better lateral resolution, slower frame rate
Pixel depth for B-mode, M-mode, and color Doppler
8 bits for B-mode and M-mode, 24 bits for color Doppler
Setting where echoes that are multiples of the center frequency are collected
harmonics
Benefits of harmonics
improved axial and lateral resolution, increased SNR, decreased reverberation and side lobe artifact
Drawback of harmonics
echoes are attenuated more rapidly (decreased visualization of deep tissues), more shadowing
Benefits of compound imaging
edge sharpening, less shadowing
Cyst appears more like a hypoechoic mass with…
compound imaging
Hypoechoic mass appears more cystic with…
harmonics
Positive Doppler shift indicates flow in which direction?
towards transducer; positive Doppler shift = increase in frequency
Negative Doppler shift indicates flow in which direction?
away from transducer; negative Doppler shift = decrease in frequency
Ideal Doppler angle
30-60 degrees (relative to long axis of vessel)
High frequency transducers are more or less sensitive to blood flow?
more sensitive
Magnitude of Doppler shift is proportional to…
cos(theta), velocity of flowing blood, and TF
How to: increase sensitivity for slow flowing blood
decrease PRF, increase TF, switch to power Doppler, smaller Doppler angle
Wall filter
only displays Doppler shifts above a set threshold; removes artifacts, but may also remove signal from slow flowing blood
Doppler technique that uses a single gate to yield a spectrum of Doppler shifts
spectral Doppler
Doppler technique that displays an average of Doppler shifts
color Doppler
Doppler technique that displays the total number of Doppler shfits
power Doppler
Doppler technique(s) that demonstrate direction of flow
spectral and color Doppler
Doppler technique(s) that are susceptible to aliasing
spectral and color Doppler
Effect of increased power
increased SNR, brighter image, greater depth of FOV; may result in artifacts, risk for potential bioeffect
ALARA prefers increasing power or gain?
gain (no extra energy imparted to patient)
Time gain compensation
progressive amplification of returning echoes from increasing depths
Persistence definition
frame averaging to decrease noise; drawback is decreased temporal resolution
Thermal index (TI)
maximal increase in temperature secondary to energy deposition
TI for OB imaging
<0.7
TI where US should not exceed 30 min
1.0-1.5
TI where US should not exceed 1 min
2.5-3.0
TI where US should not be used
> 3.0
Mechanical index (MI)
likelihood of cavitation
Relationship between MI and frequency
inversely related; high frequency => lower MI
FDA limits for MI
1.9 for an adult, 1.0 for OB
Cavitation type resulting in tissue damage - stable or transient
transient cavitation
Cavitation is most likely to occur with ____ frequency and ____ pressure
low frequency and high pressure
All US equipment required to display TI and MI by who?
FDA
Tissue damage is proportional to…
TI, MI, and exposure time
1st trimester US recommendations
no Doppler, keep TI <1.0; scanning uterine arteries is ok
Equipment testing QC interval
semi-annual (per ACR)
Artifact: echoes outside main beam are erroneously placed in main beam
side lobe artifact; due to radial expansion of piezoelectric crystals; occurs more with linear array transducers
Artifact: duplicated SMA
refraction artifact; computer assumes linear progression of sound waves
Nyquist limit
1/2 of the PRF; Doppler shifts above Nyquist limit result in aliasing
Burst of color (Doppler) filling the screen
flash artifact; due to transducer or patient motion
Effect of increasing power (or transmit gain) on resolution
increased power => beam widening => worse lateral resolution
Relationship between Doppler imaging and axial resolution
longer SPLs are required to determine Doppler shifts => worse axial resolution
Risk-benefit discussion required at what TI and MI?
TI >1.0 and MI >0.5
Difference between reverberation and comet tail artifact
distance between reflective surfaces in comet tail artifact is <1/2 SPL
Artifact: multiple evenly spaced lines in the axial direction
reverberation artifact
Typical depth of penetration for 3 MHz
20 cm
Typical depth of penetration for 10 MHz
6 cm
Effect of increasing TF on Doppler shift, sensitivity to slow flow, and aliasing
increased TF => increased magnitude of detected Doppler shift, increased sensitivity to slow flow, more prone to aliasing
Determinants of impedance
density and speed of sound in a given tissue
Sound intensity reduction with -10, -20, and -30 dB
reduction to 10%, 1%, and 0.1 %, respectively
TF affects axial resolution, lateral resolution, or both?
both
Effect of frequency on tissue heating and cavitation
increasing frequency => increases heating, decreases cavitation
Cause of posterior acoustic enhancement and shadowing
material attenuates sounds less than or greater than that of surrounding tissue
Cause of tissue vibration artifact
turbulent flow
Speed displacement artifact
echoes traveling through an area of decreased speed (e.g. fat) are erroneously placed at increased depth (due to longer round trip time)
