ULTRASOUND Flashcards
US matrix size
512x512, 1 byte/pixel, 0.25 MB
hypoechoic and hyperechoic image
mirror artifact image
causing the appearance of “liver” or “spleen” inside the lung
shadowing artifact image
-hard things like bone, stones
enhanceent artifact image
-liquid filled things like bladder, cyst
ring down artifact image
when fluid is trapped in a tetrahedron of air bubbles, the ultrasound waves reflect infinitely and result in an infinitely long vertical echogenic line.
comet tail artifact
Comet tail artifact is a form of reverberation artifact. In comet tail artifact the two reflective surfaces are closely spaced together (such as the bevel of a metallic needle). The reflective surfaces are so close that it is difficult to distinguish between each reflected echo.
Comet tail artifact is different from ring down artifact (described next) because the comet tail artifact dissipates with depth
lateral resolution compared to axial
4X worse than axial and becomes worse as distance from transudcer increases
what is lateral resolution determined by
US beam width
increasing lines per frame also increases resilution
assumptions in US
-echo depth ~ echo time
-sound travels in straight lines
-attenuation is uniform
Doppler effect- what objects have higher f?
-objects going toward detector
doppler shift formula
delta f = 2 v cos(theta) f/c
PFR for Doppler
must be 2X max expected doppler shift
PFR ~ 8 kHz
length of near field
D^2/4 lambda
D is transducer diameter
formula for PFR
frame rate * lines per image
each pulse is a line of sight
intensities for B mode vs M mode vs Doppler
B mode 10 mW/cm2
M mode 4X higher
Doppler 50x higher
duration of pulse
1 us
cavitation
creation and collapse of microscopic bubbles
thermal index of 1
increase in temperature of 1 degree celcius
what uses A ode imaging
ophthalmology
-depth on horizontal axis, echo intensity on vert
how many lines of sight in US image
100
PRF typical rate
4000 pulses/s, listening interval of 250 us
what corrects for attenuation at depth
depth gain compensation
T-M imaging
time on horizontal axis and depth on vertical axis
how many frames per second is typical
30
what does temporal resolution depend on?
PRF
line density
FOV
independent of pulse length
mechanical index
estimates change of inducing cavitation
what is line density
of lines per image/FOV
how to increase line density
reduce frame rate
but this reduces temporal resolution
it improves lateral resolution
harmonic imaging receives signals at what frequency
2X transmit frequency
elevational resolution
refers to planes perpendicular to the image plane
aka slice thickness
depends on height of transducer elements
can improve using 1.5D arrays
what can make something appear as reverse flow?
aliasing
reflected US intensity
((Z2-Z1)/(Z2+Z1))^2
Z is impedance
Impedance of air, tissue, bone, PZT
0.01 1, 5, 20
what organs scatter vs don’t scatter
-get scatter when object is smaller than wavelength
-kdiney, pancrease, spleen, and liver scatter
-bladder and cysts show black
does axial resolution vary with depth?
no, but lateral does
US spectral shift
plot Doppler frequency shift as a function of time
-gives info on blood flow
sound velocity in bone
4100 m/s
pulse duration and frequency relationship
delta f * delta t = 1/(2pi)
attenuation through tissue vs through bone
f0.5dB/cm/MHz for tissue, musle
f^20.5dB/cm/MHz
equation for lateral resolution
lambda * F/2a
F is focal length
2a is aperture of transducer
impedance of matching layer
Zmatching layer = root(Ztransducer*Zskin)
frequency where mechanical energy is best converted into electrical enerrgy
Q= (sound velocity in crystal)/(2*crystal thickness)
time it takes crystal to decay from resonant to unexcited
Q/(2pifres)
divergence of far zone
phi= arcsin(1.2lambda/D)
D=aperture diameter
types of averaged US intensity
temporal
pulse averaged
focused transducer
use curved crystal- get constructive interference at a point
phased array
-crystals can be excited independently
-by exciting crystals with certain time delays, can do depth focusing and steering
size of object versus lambda- effect on US image
d»> lambda- strong echoes
d«_space;lambda- scattering (speckle)
d~ lambda- graininess (structure)
time gain amplification is log or linear?
usually log
can sound waves travel in vaccuum?
no
how to get final attenuation in US
add up DBs
divergence of the far field
sin(theta)=1.22lambda/D
harmful US power
0.1W/cm2
narrow bandwidth
light damping, high Q
