Echo Physics 101 Flashcards
Fundamental Imaging
Based on the reflection of transmitted frequency. US waves pass through tissue twice
Harmonic Imaging
harmonic frequency is generated as the US signal propagates through the tissue. It is single-pass imaging and therefore reduces artifacts. Useful for imaging deeper structures
Reverberation - define
how to fix
more distant to true object
comet/ring down
straight line through probe center
fix: decrease gain
alternate imaging plane
Acoustic shadowing
how to fix
alternative imaging
increase gain or adjust TGC
mirror artifact
more distant than true object
decrease gain
refraction artifact
at same distance from probe
decrease gain
use alternative imaging planes/avoid refracting structure
side lobe artifact
linear, symmetric at both sides of object
same direction from probe (arc-like in radial direction)
apply color doppler
decrease gain
beam width artifact
at same distance from probe
true object/doppler signal outside imaging plane
adjust focal zone
alternative imaging plane
near field clutter
apply color doppler, reducing scale
alternative imaging plane
doppler shift equation
[2xreflector speed x incident freq x cos(theta)]/propagation speed
doppler shift is directly related to _
blood cell speed
freq of transducer
cos of angle bet. flow and sound beam
doppler shift is inversely related to _
speed of sound in medium
what does the 2 in doppler equation represent
double doppler shift : 1st is when sound strikes cell, 2nd is from moving cell reflecting wave back to transducer
in order to accurately determine velocity…
the angle between direction of flow and sound beam must be known
velocity (measured) is as related to Doppler shift…
= true velocity x cos (theta)
Doppler shift definition
a change or variation in the frequency of sound as a result of motion bet. sound source and the receiver
Doppler frequency
difference between received and transmitted frequencies
Positive doppler shift
when source and receiver are approaching each other
reflected freq > transmitted freq
Negative doppler shift
when source and receiver are moving apart
reflected freq < transmitted freq
typical values for audible sound
20 Hz to 20 kHz
doppler US uses these transducers
2 to 10 MHz
Demodulation
extracts Doppler freq from transducer freq and is performed by a demodulator
bi-directional doppler is analyzed with
phase quadrature processing
doppler shift alternate equation
received - transmitted freq
relation bet. velocity and doppler shift
direct
duplex US
simultaneous imaging and Doppler
disadvantage of CW Doppler
range ambiguity - echoes arise from entire length of overlap between transmit and receive beams
range = depth
Nyquist limit
prf/2 (in kHz)
aliasing appears when
doppler shift exceeds the Nyquist limit
5 ways to Eliminate Aliasing
use CW
use lower freq transducer
(reduces doppler shift and shrinks spectrum)
select new view with shallower sample vol (increases PRF and Nyquist limit)
increase scale
baseline shift
PW Type of Transducer
minimum 1 crystal
range resolution
limit of max velocity
uses damped, low Q, wide bandwidth transducer
CW Type of Transducer
minimum of 2 crystals
range ambiguity
unlimited max velocity
uses UNdampled, high Q, low bandwidth transducer (allows for higher sensitivity to small Doppler shifts)
Color Doppler is based on PULSED US and is subject to…
range resolution/specificity
aliasing
Color doppler provides
info on direction of flow and is semi-quantitative
knowledge of angle not really important
what kind of velocities does Color Doppler report?
average velocities ie. mean velocities
relationship bet. color Flow and doppler shift
doppler shifts are coded into colors and superimposed on a 2D image
Packet
multiple pulses
Multiple ultrasound pulses are needed to accurately
determine red blood cell velocities by hDoppler
small packet
less accurate Doppler
less sensitive to low velocity flow
higher frame rate and improve temporal resolution
LARGE packet
more accurate doppler
more sensitive to low velocity flow
lower frame rate, reduced temporal resolution
packet size must balance between
accurate velocity measurements and temporal resolution
Spectral Analysis
is performed to extract or identify the
individual frequencies making up the complex signal. It is
used to interpret individual velocities in the signal.
methods of spectral analysis for cw pw vs. color Doppler
FFT for PW and CW
Autocorrelation for color (less accurate but faster than FFT)
Lateral Resolution
determined by…
best with…
LATA
determined by Beam Width
best with narrowest beam
Lateral Resolution changes with
depth
best at focus
Lateral resolution
In Near Field, best with
smallest diameter crystal
Lateral resolution
In Far Field, best with
largest diameter crystal and highest frequency (largest divergence)
Axial Resolution
determined by…
best with…
determined by pulse length
best with shortest pulse
highest freq and fewest cycles
Axial Resolution
changes with
same at all depths
does not change
Axial Resolution
In Near Field, best with
shortest pulse
Axial Resolution
In Far Field, best with
shortest pulse
Duty Factor definition
percentage of time that an echo machine is actually transmitting a pulse into the body
approximate time transmitting vs. listening in average US machine
0.2% of the time transmitting and 99.8% of the time “listening” for returning signals
Duty Factor formula
[Pulse Duration/PRP] x100
TDI vs. Blood flow Doppler
TDI signals - high amplitude (power output and gain are low), low velocities
Blood flow Doppler - high velocity and high frequency, low amplitude
in TDI, low amp, high freq signals filtered out
frequency determined by
sound source
wavelength determined by
sound source and medium
relationship of stiffness and density to speed
stiffness directly related to speed
density indirectly related to speed
pulse duration formula
cycles in pulse x period
pulse duration definition
time from start of pulse to the end of that pulse
actual time the pulse is “ON”
time usec
relationship between pulse duration and sonographer
cannot be altered by sonographer
does not change with depth
determined by transducer
SPL
length or distance the entire pulse occupies in space
distance from start to end of one pulse
SPL determined by…
source and medium
SPL relationship to sonographer
cannot be changed by sonographer
SPL formula
cycles x wavelength
mm
PRP definition
from start of one pulse to the start of the next one
one pulse duration + one listening time
PRP determined by
imaging depth
directly related
PRP relationship to sonographer
changed by sonographer
adjusting depth of view changes listening time
the deeper, the longer the PRP
PRF definition
number of pulses created by the system in one second
PRF determined by
imaging depth
inverse relationship
PRF relationship to sonographer
can be changed by sonographer
by adjusting the PRP, PRF is changed
Duty Factor relationship to sonographer
changed by sonographer when imaging depth is changed
shallow vs. deep image and Duty Factor
shallow - high DF
deep - low DF
very low MI imaging…
minimizes microbubble destruction in the near field
(permits detection of apical abnormalities)
relationship spatial resolution and frequency
higher frequency means better spatial resolution and shorter wavelength
Short pulses are seen with…
Shorter pulses result in better…
seen with higher frequency or with transducers that dampen the pulse signal so that there is less ringing
result in better axial resolution
