Echo Principles Flashcards
Ultrasound interaction with tissues (4 types)
Reflection: creates ultrasound images
Scattering: basis of Doppler ultrasound
Refraction: used to focus us waves
Attenuation: loss of signal strength in tissues
Ultrasound descriptors (4 characteristics)
Frequency : cycles per sec= Hz, 1000 cycles/sec = 1 MHz
Propagation velocity: 1540 m/sec in blood
Wavelength: propagation velocity/frequency
Amplitude: decibels or dB
Tissue penetration and frequency
Greatest with lower frequency transducer (2-3 MHz)
Frequency and resolution
Greatest (about 1 mm) with higher frequency transducer (5-7.5 MHz)
6 dB change
Amplitude logarithmic : 6 dB change doubling or halving of signal amplitude
Acoustic impedance factors (2)
Tissue density
Propagation velocity
Ultrasound reflection factors (3)
Greatest with smooth tissue boundaries with different impedences and perpendicular to tissue interface
Scattering best characteristics
Doppler occurring with small structures scatters generating Doppler signals- velocities best when parallel to flow
Frequency:
Definition
Example
Clinical implication
Number of cycles per second in wave
Eg: transducer frequencies MHz 1,000,000c/sec
Doppler KHz 1,000 cycle/sec
Clinical implication: different transducer freq for specific application- affects tissue penetration, image res and Doppler signal
Velocity of propagation:
Definition
Example
Clinical implication
Ultrasound speed thru tissue
Average velocity in soft tissue about 1540m/sec
Velocity similar in soft tissue myocardium,liver,fat, but lower in lung and much higher in bone
Wavelength:
Definition
Example
Clinical implication
Distance between ultrasound waves Wavelength =prop vel/ freq Ex: shorter with higher freq and longer with lower freq Resolution best with shorter wavelength Depth is greatest with lover wavelength
Amplitude:
Definition
Examples
Clinical implication
Height of ultrasound wave or loudness dB
Log scale: 80 dB 10,000 fold and 40dB 100 fold increase
Wide range of amplitude can be displayed using greyscale for imaging and spectral Doppler
Acoustic impedence:
Definition
Examples
Clinical implication
Tissue specific defined by density (p) and prop velocity (c) z = p x c
Eg: lung low density, slow prop velocity, bone high density fast prop velocity
US reflected by boundaries of acoustic impedence- blood vs myocardium
Reflection:
Definition
Examples
Clinical implication
Return of ultrasound signal to transducer from smooth tissue boundary
Reflection used for 2D images
Greatest when perpendicular to surface
Scattering:
Definition
Example
Clinical implication
Radiation of ultrasound in multiple directions from small structures such blood cells
Eg: change in freq of signals scattered from moving blood cells basis for Doppler ultrasound
Clinical implication : amplitude is 100-1000 less than reflected
Refraction:
Definition
Example
Clinical implication
Deflection of ultrasound waves from straight path due to different acoustic impedence
Eg: used in transducer design to focus us beam
Causes double image artifacts
Attenuation:
Definition
Examples
Clinical implication
Loss in signal strength due to absorption of ultrasound energy by tissues
Higher frequencies have more attenuation (less penetration)
Lower freq transducer needed for apical views in larger patients
Resolution:
Definition
Examples
Clinical implication
The smallest resolvable distance between two specular reflectors on ultrasound image
Resolution has 3 dimensions:
1. Along length of beam (axial) 2. Lateral across the image (azimuthal) 3. Elevation plane
Eg: axial most precise, therefore measurements best along length of beam
Bandwidth
Transducer design
Wider gives better axial resolution
Pulse length
Burst length
Higher freq signal can be transmitted in short pulse length
Short pulse length improves axial resolution
Pulse rep freq
Number of transmission-receive /sec
PRF decreases with depth due to time needed for sig to get to transducer
Affects resolution and frame rate
M mode sample time
1800/sec
Doppler modalities
Pulsed: sample velocities timed for depth- limited velocities
Color flow imaging:
Continuous wave: can measure high velocities- cannot localize depth
Doppler effect:
definition
example
clinical implication
Freq change of ultrasound scattered from moving target
V=c x delta F/ 2F (cos theta)
Shift from 1 - 20 kHz
Assumes theta cos = 1
PRF:
definition
example
clinical implication
Number of pulsed transmitted / sec
Limited by time needed to reach and return
Max velocity measure able with pulsed Doppler is1m/ sec@ 6cm depth
Nyquist Limit
Max freq shift measurable with pulsed Doppler
= PRF/2
The greater the depth, the Lower max velocity measurable
Velocity error
20 deg vs 60 deg
20 deg 6%
60 deg 50%
Ultrasound exposure
Thermal index: ratio of transmitted power to power needed to increase temp 1 deg C
Mechanical index:
Ratio of peak rarefaction pressure to square root of transducer frequency
Contrast bubbles
About size of rbc
2-8 micron rbc 6-8 micron
Bubble behavior
Mi low, mid, high
Low- linear
Mid nonlinear back scatter
Disruptive - transient harmonic
Contrast echo apical dropout
Correct by decreasing MI
Focal misplacent
Contrast swirling
MI too high
Low volume contrast
Artifact types:
Distant: parallel : reverberation
Opposite motion : mirror image
Same distance: beam width, side lobe, refraction
Dissection v artifact
Independent motion
Attached
Flow divider
4 chamber to 5 chamber
Transducer anterior
Ohm law
P=Q x R
Laplace wall p x r / m
RV measurements abl
Basal > 4.2 Mid > 3.5 Rv long > 8.6 Rv place prox > 3.3 Rvot psax > 2.
Qualitative RV size
No < 2/3 LV
Pulmonary embolism
Echo Sens
30-40% Rv size incr, Rv ef depresses New tr Rv thrombus McConnell sign
RA pressure est
3,8,15
0-5, 5-10,> 15
TAPSE
Volume vs pressure
Increase with vol overload
Decrease with pressure overload
IVC
0-5 if < 2.1 collapse > 50%
10-20 if > 2.1 with < 50 % collapse
Flying W
Severe pul htn
No a wave
RVOT accel time
Nl > 120 msec
Mean pap = 79-0.45 x accel time
< 90 msec then peak > 60 mmhg
LVEDD
Measure LV axis at tip MV leaflet
Relative wall thickness
2 x pwt/ lvedd
Concentric hypertrophy:
Eccentric
Concentric remodeling
Conc increased mass, increase RWT
Eccentric - increased mass, decreased RWT
Conc remodeling- nil mass, increased RWT
Echo lvsv error
Underestimates - don’t include trabeculations
EF: Nl Mild Mod Severe
> 55%
Mild:45-54%
Moderate: 31-44%
Severe: <30%
LA volume
4 chamber area x length
2 chamber area x length
0.85 x A1 x A2/ L (shorter of two lengths)
LA size
Mild 29-33
Mod 34-39
Severe> 40
TGC
Evens brightness
Suppresses strong near field, boots weak farfield
Decrease frequency
Increases depth
Decreases resolution
McConnell sign
Rv dilated
Apex hyperdynamic
Mid RV akinetic
Prosthetic MV mismatch
Eoa mild < 1.2cm/m2
Severe < 0.9 cm/ m2
Follow up echo prosthetic valves
2-4 yrs after replacement
Bio prosthetic 5 yr
Mech - no routine check
Pseudodyskinesis
Inferior
Diastolic flattening - associated ascites, diaphragm
HCM dimension
15 mm LV wall
13-14 mm borderline
Mid sys knotch
m mode with SAM vs early closure with membrane
HCM gradient
Peak instantaneous gradient
Sig if > 30 mmhg
Intervention if > 50
Pseudo SAM
SAM after posterior wall contraction
Cyclic pressure changes for sound
Rarefaction and compressions
In longitudinal wave particles move parellel to direction of wave
Perpendicular to direction is how articles move in transverse wave
Crest and trough describes transverse wave-sound waves are longitudinal
How to resolve aliasing in HCM with pulse wave in llvot?
- Continuous wave
- Increase prf scale
- Switch to high prf
- Use lower freq transducer
- Adjust baseline
Def of ultrasound period
Time to complete one cycle
0.06. - 0.05 micro seconds 10 to -6
Ultrasound beam that is narrowist in far field?
Crystal size and freq
Larger crystals diverge less in far fields
Higher frequency diverges less in far fields
PRF of 15 KHz - what is max shift?
Nyquist Is prf/2 so 7.5
Variance map vs velocity map
Variance - adds green to depict turbulence
Artifact in gray scale 2 d- step to disprove
Change depth setting -range artifact
Color flow nest step
Refraction artifact
Misplacent of object in image due to change in direction At non perpendicular boundaries with different impedance
Artifact : hyper intense signal behind a low attenuating structure as I fluid filled
Enhancement :
Artifact resulting in placement of echo genic lines equally space in fluid filled structure?
Reverberation
From multiple echos created by two structures close to each other- reflection of us between structures before next pulse generated
Ghost images off axis
Grating lobe artifact
Dues to division of small transducer face into large number of small elements- the small elements produce us energy at high singles compared to main beam
Best lateral resolution
lateral resolution best where beam narrowest
..?Wavelength of sound in soft tissue from 3 MHz transducer
Speed 1540 m/ sec
1.54/ 3. = 0.51 mm
Duty factor
% of time echo machine actually transmitting pulse
Most of time spent listening
0.2% time transmitting and 99.8% listening
If pulse duration is 1 microsec and prf 1 ms, what is duty factor ?
Pulse duration / prf x 100
0.0000001 / 0.001 (x100%)= 0.1%
What is maximum duty factor?
100%
Reverberation
Multiple echos created between 2 close structures - lines equally spaced from reflection of energy between before next pulse generated
Accuracy
True positive + true neg/
All tests
Harmonic imaging probs
Makes MV look thicker
ROA from PISA
Pisa = 2 x 3.14 x rsq
Pisa x vel alias = roa x vel MR
Rv = roa x VTI
SV = 3.14 x r sq x VTI
Vegetaions
Upstream side of valve
Abnl irregular mass
Chaotic motion
Near length zone
NLZ = diam sq x f /4 wavelength
Truncus arteriosis av types
Tri and quadra cusp
