Physics/knobs/doppler Flashcards
Class 1 indications for TEE from 1996
1 Rescue tool
2 surgical repair of valves, HCM, dissection
3 eval complex valve replacements
4 congenital lesions requiring cpb
5 surgical intervention for endocarditis
6 placement of intracardiac devices and monitoring position
7 evaluation of pericardial window procedures
2010 update on indications
1 Cardiac and Thoracic surgery
2 Noncardiac rescue and monitoring
3 Critical care
absolute contraindications to TEE
esophageal stricture
transesophageal fistula
esophageal trauma
esophagectomy/esophagogastrectomy
relative contraindications to TEE
barretts
hiatal hernia
large escending aortic aneurysm
unilateral vocal cord paralysis
Precautions for TEE in high risk patients
consider other imaging
obtain GI consult
use smaller probe
limit exam and unnecessary probe manipulation
Piezoelctric and reverse piezoelectric effect
piezoelectric- sound waves strike crystal which is converted into electricity
reverse effect- voltage applied to crystal which is converted to sound waves
Imaging modes
A mode= amplitude mode (strength = amplitude)
b mode= brightness mode
m mode = motion mode
2d = multiple m mode lines
3d = pyramid of m mode lines
M mode frame rate and brightness
frame rate = 1000
brighness = strength of signal
HOCM M mode
premature systolic closure of aortic valve, and fluttering
sound waves
mechanical longitudinal waves but often talked about as transverse wave
wave properties
period
frequency
pulse duration
pulse repetition period
pulse repetition frequency= 2x nyquist limit and determines temporal resolution
wavelength
spatial pulse length=
amplitude= max acoustic variable- avg acoustic variable. Higher amp is stronger pulse
power = amount of work/time
intensity = power/area and determines bioeffects
spatial resolution
Axial>lateral>elevational
axial- longitudinal, range, depth, determined by 1/2 spatial pulse length
lateral- determined by beam witdth, known as transverse, angular, azimuthal
elevational- determined by beam heighth
audible sound frequency and ultrasound frequency
20-20KHz audible
above 20KHz ultrasound
Determinants of temporal resolution
how much something moves
frame rate ( # pulses, line density, image depth, sector width)
Pulse repetition frequency
proportional to frame rate and temporal resolution
2x Nyquist limit
Optimize image tips
decrease depth
narrow sector width
place focal point at ROI
Gain
no bioeffects, no power change
amplifies returning signals
time gain compensation
compensates for attenuation with depth
lateral gain compensation
compensates for attentuation in lateral position, corrects enhancement artifact
compression
reduces dynamic range of ultrasound signals-> leads to brighter brights, darker darks, less shades of gray, highly contrasted image
dynamic range
inverse of compression, and more shades of gray with increase
Doppler shift
simply a change in frequency. Frequency received - frequency transmitted
blood flow in parallel with ultrasound beam will cause change in frequency
VCos(theta)2Ft/C is dopper shift equation
significant error when theta greater than 20-30
continuous wave doppler
one crystal always sending
one crystal always listening
PRF = infinity and therefore high nyquist limit and no aliasing
measures high velocities but range ambiguity , duty factor 100%
Pulsed wave doppler
emits pulse , waits, listens for echo from sample gate/sample volume
time = distance
used to calculate SV, AVA, dimensionless index (independent of patient size), diastolic function
advantages = range resolution
limitations=aliasing/limited max velocity
nyquist limit
same as max doppler shift
= 1/2 PRF
reduce aliasing
lower transmitted frequency
decrease depth of gates (increase PRF)
shift baseline
use continuous wave
Increase PRF (NL=1/2PRF)
color flow doppler
form of pulsed wave doppler
Blue away , red towards
Variance map looks at laminar vs turbulent (left laminar , right turbulent)
decreasing box size will increase frame rate by making machine do less work
rapid precise visualization and assessment of flow and regurgitation
limitations= aliasing, decreased temporal resolution and velocity measurements are estimates
cos of 30 , 45, 60, 90
0 =1
30=sqrt(3)/2
45=sqrt(2)/2
60=sqrt(1)/2
90 = 0
high pass wall filter
filters out low velocities
used for blood flow velocities
200-800 Hz
can affect mean and peak velocities and prevent detection of onset and determination of blood flow
low pass wall filter
filters out high velocities/frequencies
used for Tissue doppler
allows low velocity , high amplitude signals
Reject filter
used in 2d imaging, filters low amplitude signals indicative of ‘noise’
5 functions of receiver
amplification
compensation
compression
demodulation
rejection (AKA suppression, threshold)
Power Doppler
Energy mode or color angio
shows flow but no direction or velocity
low frame rate and susceptible to flash artifact
unaffected by angle unless 90, aliasing, and its sensitive to low flow
parameters determined by ultrasound source and medium
anything with length like wavelength
parameters determined by sound source only
Anything with time units (seconds) and strength
Only parameter determined by medium
velocity
Parameters determining velocity
Increased stiffness and decreased density
PWD changes to peak E and decel time from atria into ventricle
Peak E increases as it goes through valve into ventricle
E wave decel time decreases as it goes into ventricle through valve
wave between E and A wave on MV inflow
L wave - indicates impaired relaxation and elevated LAP
simplified bernoulli for pressure gradient
= 4v^2
what percentage of peak velocity is the velocity at which pressure half time occurs
71%
Pressure half time definition
time it takes to go from max pressure gradient to half max pressure gradient
Lesions for PHT, utility and limitations
Aortic regurgitation
Mitral stenosis
used to determine size of hole
Formula is 220/PHT for MS
limitations: debate it shouldn’t be used when not rheumatic valves
AI effect on mitral PHT
decreases pressure half time causing underestimation of MS
lv compliance effect on PHT
stiffer ventricle shortens PHT and underestimates MS
Impaired relaxation on PHT
increases PHT, overestimates MS
AI PHT cutoff
> 500 ms mild
200-500 ms moderate (slope>2m/s?
<200 ms severe (slope >3 m/s)
Uses for tissue doppler
diastolic function- use lateral e’ -> E/e’
systolic function- s’ should be greater than 8 cm/s, <5 cm/s is bad
ischemia, constrictive pericarditis
RV function - measure TA velocity
what is post systolic shortening
seen in ischemia on TDI
velocities occurring during isovolumic relaxation time
What is annular reversus
seen in constrictive pericarditis when lateral e’ < septal e’
Limitations of tissue doppler
angle dependent (should be <20)
average over 3 cycles to reduce error
MAC / MV tethering
myocardial performance index
= IVCT + IVRT / ET
usually less than 0.39
measure of both systolic and diastolic function
DIlated CM usually >0.59
a’ and e’/a’ values
a’ <10 good
e’/a’ <1 bad
isovolumic acceleration (IVA)
max isovolumic velocity/ IVCT and is used for systolic function
Usually 1-2 m/s2 normal
Te’
time for onset of e’ wave. prolonged in diastolic dysfunction
TE
Time from R on QRS to E inflow
Te’-TE is prolonged with diastolic dysfunction
TDI for ischemia
S’ decreases, e’/a’ <1
PSS, e’ decrease, prolong Q to peak s’
TDI in CP and tamponade vs RICM
Normal in both, annular reversus in CP
decreased in RICM
Best view for RV TDI
transgastric RV I/O or RV inflow
RVs’ less than 10 is abnormal in young healthy person
tricuspid closure opening time (TCO)
Same as IVCT + ET + IVRT and used to get RV MPI
MPI above 0.5 is predictave of instability and mortality
Tissue doppler for strain
Strain and strain rate can be calculated
(V2-V1)/distance between the two = strain rate
Determinants of frame rate
line density
# pulses per line
image depth
sector width
amplitude
Difference between avg value and max value of an acoustic variable.
POwer and intensity are proportional to amplitude squared
power
rate at which work is performed or energy is transferred
measured in watts or j/sec
intensity
power/area
determines bioeffects
mechanisms of bioeffects
thermal- limit to max of 1C rise in local tissue temp
cavitation
vibration
max SPTA for focused and unfocused beam
unfocused <1W/cm2
focused<100 mW/cm2
What causes most heating of all US modes
PWD
mechanical index
measure of US to produce cavitation
harmonic frequencies
caused by shrinking and expanding of bubbles
allows for better imaging of areas that werent imaged well before
arise from non linear behavior
Mechanical index depends on frequency and pressure
Increases with lower frequency and stronger pressure
Mechanical index
strongest harmonics and resonance leading to cavitation seen when MI > 1
harmonics and resonance with MI 0.1-1
no harmonics with MI <0.1 - only backscatter and linear behavior
Harmonics
tissue harmonics- US travels through tissue and frequency changes into harmonic frequency. strength grows as it goes deeper
contrast harmonics- echo contrast with microbubbles. Harmonic imaging causes bubbles to shrink and then expand leading to cavitation
Resonance
uneven shrinking and expanding of microbubbles
Time- wave properties
period
frequency
pulse duration
PRP
PRF- higher frame rate and better temporal resolution
Determined by the sound source
wave properties determined by medium
velocity. Increased with increased stiffness and decreased density
sound travels faster in dense materials because of its stiffness not density
Impedance
attenuation
increases with increasing depth and increasing frequency
over 80% of attenuation is from absorption in tissues
Impedance
resistance to sound traveling through medium
Z = density (p) x velocity (v)
pzt > matching layer > gel > mucosa
incident intensity
= reflected + transmitted intensity
Curie temp
temp that can change crystal to no longer produce ultrasound waves
damping material
also called backing material
decreases ringing
decreases SPL and improves axial resolution
decreases sensitivity to reflected echoes
decreases pulse duration
increases bandwidth
decreases Q factor ( RF/ bandwidth)
Frequency determination for PWD
V/2T
frequency determination for CWD
electrical frequency of voltage applied to crystal
q factor
ability of trasnducer to emit a clean pulse with narrow bandwidth
damping decreases q factor
matching layer
layer between crystal and skin/tissue
anatomy of a sound beam
focus is at the minimum diameter of the beam and is where best lateral resolution is
near field ( fresnel) = r (crystal)^2/ wavelength
far field ( fraunhofer)
focal zone is near the focus
focusing ultrasound
lens
curved crystal
focusing mirror
electronic (used by TEE)
range resolution
describes axial resolution
but also describes object at specific depth as with PWD
ultrasound system
master synch
pulser
transducer
receiver and processor
storage
functions of receiver
amplification- enlargement of returning signal (gain)
compensation- makes all echoes from similar objects appear with similar brightness (tgc, lgc)
compression - reduces range of signals from smallest to largest
demodulation- rectification and smoothing
rejection - filters low amplitude signals
artifacts
due to US assumptions (sound travels in straight line, reflections are along main axis, intensity of reflection corresponds to reflectors scattering strenth)
reverberation
rungs on ladder or blurred comet tail / ringdown
refraction artifact
assumes that us travels in straight line. Places object that is off to side in straight line and deeper
side and grating lobes
assumes us only travels in main axis.
Strong reflectors in side lobe path will place it in main axis
acoustic shadowing/echo dropout
strong reflectors that do not allow penetration of US beam
mirror artifact
similar to reflection artifact
pericardium can act as strong reflector
raleigh scattering
occurs when the reflectors dimensions are much smaller than the wavelength of the US
sound wave equally redirected in all directions
backscatter
AKA diffuse reflection in which the object has irregular surface and allows for imaging at suboptimal angle, but its weak.
snells law
determines refraction
sin transmission / sin incidence = v2/v1
if velocity transmitted is less then angle is smaller than incident angle
refraction
requires angle that is not 90
requires v2 does not equal v1
oblique reflection
reflection angle the same as incident angle when it does occur
shallow focal length
high density, low impedance, thickest crystal, lowest diameter
temporal resolution depends on 2 things
how much the object moves
the frame rate which depends on several things
frame rate depends on several factors
line density = # of scan lines/ image ( as line density increases , frame rate goes down)
# foci / line = pulses per scan line (increase pulse per scan will decrease frame rate)
imaging depth (less depth = increase frame rate)
1540 m/s in mm/microsec
1.54 mm/microsec
backing material
decreases transducer sensitivity to reflected echoes
improves axial res by decreasing SPL
Decreases Q factor = RF/BW (increases BW)
Pulse effect on bandwidth
shorter pulses = larger bandwidth
velocity of wave formula
v=frequency x wavelength
primary form of attention in tissue
absorption
intensity reflection coefficient
determined by acoustic impedance
=reflected intensity/ incident intesity
affected by stiffness, density, velocity of two media
largest to smallest impedance
pzt = matching layer > matching layer > gel > skin
q factor for imaging transducer
lower number better quality. Imaging transducers have short pulses that contain a broad range of requencies and lead to a low q factor
propagation speed artifact
assumes us travels exactly at 1540. If it travels too fast or too slow then the structure will be placed at improper depth
most transducers have what frequency
2-15 MHz
range ambiguity
results when emission of pulse happens before all previous pulses have been received back.
lowest to highest velocity in media
air<lung<fat<soft tissue< bone
Formulas for pressure , intensity, amplitude, power
pressure dB=20 Log p2/p1
Intensity dB= 10 Log I2/I1
Amplitude dB = 20 Log A2/A1
Power dB= 10 Log P2/P1
Continuous mode unfocused transducer focus size
Focus = transducer diameter / 2
Spectral doppler change from LA to mitral leaflet tips
peak E increases and decel time shortens (steeper decel)
Curie temp changes to crystal
causes it to become depolarized and lose piezoelectric properties
