Ultrasound physics Flashcards
which transducer to use
highest frequency
temporal resolution
frame rate
improved axial resolution
higher frequencies
along the axis of US beam
90 degrees to skin
Y-axis
depth resolution
improved lateral resolution
adjust focal zone
x axis - right left aspect of image
better with higher frequencies
improve temporal resolution
minimize depth
adjust frame rate
use narrow sector
benefits of compound imaging
reduces speckle
increases frame rate due to longer scan time
effect of harmonic imaging
decreases frame rate
acoustic power units
watts
watts =
joules/sec
intensity (thermal index/mechanical index)
power/beam area MW/cm2
mechanical index
likelihood of a non thermal bioeffect such as cavitation (compression and decompression of tissue)
17 mW/cm2 ophthalmic related to intensity of pulse max
value of peak negative pressure/sq root of acoustic center frequency
thermal index
ratio of the emitted acoustic power to the power to raise the temperature by 1 degree
acoustic power at depth of interest/ estimated power to elevate temp 1 degree
larger TI means larger heating potential
application specific exposure limits in mW/cm2 peripheral vascular cardiac fetal ophthalmic
720 mW/cm2 peripheral vascular
430 mW/cm2 cardiac
94 mW/cm2 fetal
17 mW/cm2 ophthalmic
according to J of US
0.23 W/cm2 (versus 1.9 for other organs)
ISPPA = spatial-peak pulse-average intensity
mechanical index
specular reflection
at smooth surfaces such as bone, diaphragm, tendons calcifications, valves, needles
angle dependent
mirror image artifact
90 degrees duplication at site of a strong reflector
far image is false image
1 reflector send it back to the other delay in sending back leads to duplicate
back scattering
associated with most tissues
less angle dependent
rough surface
Rayleigh scatter
blood - RBC’s smaller than wavelength
shadowing
decreased echoes behind strong reflector
refraction
reflector placed improperly
displaced laterally
greater change in impedance, greater bending
range ambiguity
echo received at different interval
reverberation
bouncing between 2 strong reflectors looks like multiple equally spaced lines
ex: comet tail artifact, ring down artifact
pencil probe Doppler
CW - no angle correction
duplex
B mode and Doppler
Doppler equation: change in frequency
(2 x transmitted frequency x velocity x cosine theta)/ speed of sound = 1540 m/s
Doppler equation: velocity
change in frequency x 1540 m/sec/ 2 x
frequency x cosine theta
cosine theta is smaller greater than 60 degrees making the velocity greater when dividing otherwise 1 at angle 0
cosine 0
1
cosine 30
.86
cosine 60
0.50
cosine 90
0
benefit pulsed wave
measure from depth
disadvantage pulsed wave
aliasing
can’t record super high velocities
number of crystals with Doppler
pulsed - 1
CW - 2 (can detect higher velocities)
advantage CW
picks up everything
better signal to noise
not range gated
higher velocities (2 crystals)
PRF (pulse repetition frequency)
SCALE
PRF to elicit aliasing
tips to decrease aliasing
1/2 PRF = Nyquist frequency
to avoid aliasing, PRF must be 2X frequency of signal detected
*****************************
adjust baseline and scale
try lower frequency
type of Doppler for color Doppler
pulsed wave
decreased angle
decreased velocity
to ascertain angle correctly
look at which vessel wall
back wall
p 21 of physics
drop image of MI and TI
tips to decrease aliasing
adjust scale and baseline
try lower frequency
contributors for aliasing
deeper vessels
higher velocities
volume flow rate
pressure difference x radius 4/
8 (PI) viscosity x length
volume flow rate
P1-P2/R
= pi (pressure difference) x radius 4/
8 x (viscosity) x (length)
Pouseille’s law
volume flow =
pressure/resistance
as radius decreases,
resistance increases
(P1-P2) π r4/
8 N L
N- viscosity
L - length
