01: introduction to ultrasound george david, m.s. associate professor of radiology
TRANSCRIPT
01:01:
Introduction Introduction to to UltrasoundUltrasound
George David, M.S.Associate Professor of Radiology
Speakertransmits sound pulses
Microphonereceives echoes
Acts as both speaker & microphone› Emits very short sound pulse› Listens a very long time for returning echoes
Can only do one at a time
Voltage generated when certain materials are deformed by pressure
Reverse also true!› Some materials change dimensions
when voltage applied dimensional change causes pressure
change
› when voltage polarity reversed, so is dimensional change
V
What does your scanner know about the sound echoes it hears?
AcmeUltra-Sound
Co.
I’m a scanner, Jim,
not a magician.
Sound travels at 1540 m/s everywhere in body› average speed of sound in soft tissue
Sound travels in straight lines in direction transmitted
Sound attenuated equally by everything in body› (0.5 dB/cm/MHz, soft tissue average)
Sound travels at 1540 m/s everywhere in body› average speed of sound in soft tissue
Sound travels in straight lines in direction transmitted
Sound attenuated equally by everything in body› (0.5 dB/cm/MHz, soft tissue average)
Dot position ideally indicates source of echo
scanner has no way of knowing exact location› Infers location from
echo
?
Scanner aims sound when transmitting
echo assumed to originate from direction of scanner’s sound transmission
ain’t necessarily so
?
scanner assumes speed of sound is that of soft tissue› 1.54 mm/sec› 1540 m/sec› 13 usec required for echo object 1 cm from
transducer (2 cm round trip)
distance = time delay X speed of sound
1 cm13 sec
Handy rule
of thumb
Sometimes
?
soft tissue ==> 1.54 mm / sec
fat ==> 1.44 mm / sec
brain ==> 1.51 mm / sec
liver, kidney ==> 1.56 mm / sec
muscle ==> 1.57 mm / sec
•Luckily, the speed of sound is almost the same for most body parts
Correction needed to compensate for sound attenuation with distance
Otherwise dots close to transducer would be brighter
scanner assumes entire body has attenuation of soft tissue› actual attenuation
varies widely in body
• Fat 0.6
• Brain 0.6
• Liver 0.5
• Kidney 0.9
• Muscle 1.0
• Heart 1.1
Tissue Attenuation Coefficient (dB / cm / MHz)
Assumptions made by scanner cause artifacts when assumed conditions not true
All sound in body travels at same speed Sound travels only in straight lines Sound attenuated equally by everything
in body
All sound in body travels at same speed
Distance = Speed X Time Delay / 2
1380 m/s X 58usec / 2 = 4 cm
Actual Distance to interface
1540 m/s X 58usec / 2 = 4.47 cm
Calculated Distance to interface
AssumedAccurately Measured
Assumed speed
Actual speed
Actual object position
X Imaged object position
X
Incorrect dot placement can result in incorrect› Object placement› Object size› Object shape
Actual object position
X Imaged object position
X
Actual Object Position
X Position of Object on Image X
Refraction
Change in speed of sound causes beam to change direction
reflection from reflector “2” splits at “I”
some intensity re-reflected toward “2”
Result› later false echoes heard› scanner places dots
behind reflector “2”
1
2
Echo #1Echo #2Echo #3
Comet tail› dozens of multiple
reflections between transducer & reflector 2 reflectors
Mirror Image› common around strong
reflectors Diaphragm Pleura
Scanner emits 2nd pulse before all reflections received from 1st pulse
scanner assumes echo from 2nd pulse
places echo too close & in wrong direction
X
Actual Object Position
X Position of Object on Image
Sound attenuated equally by everything in body
Scanner assumes soft tissue attenuation
0.5 dB/cm per MHz
Attenuates more than .5 dB/cm/MHz
ShadowedReflector
http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag2-f2.htm
Attenuates less .5
dB/cm/MHz
Enhanced reflector
http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag6-f1.htm
difference between received & transmitted frequency
caused by relative motion between sound source & receiver
Frequency shift indicative of reflector speed
IN
OUT
change in pitch of as object approaches & leaves observer› train› Ambulance siren
moving blood cells› motion can be presented as sound or as an image
Doppler spectrum speckle Cause
› same as acoustic speckle› random constructive &
destructive interference from sound scattered in blood
duplicate vessel image visible on opposite side of strong reflector
Analogous to mirror image artifact Doppler data also duplicated
Femoral vein duplication in region of adductor canal.
Duplication of left vertebral artery in Doppler ultrasonography: arrows duplicated left vertebral arteries, LC left common carotid artery
Results in detection of improper flow direction
occurs because sampling rate too slow
Similar to wagon wheels rotating backwards in movies
All else staying equal, larger reflector speed must produce larger Doppler shift
77 X fD (kHz)v (cm/s) = -------------------------- fo (MHz) X cos
Lower operating frequency results in lower Doppler shift
77 X fD (kHz)v (cm/s) = -------------------------- fo (MHz) X cos
Constant
larger Doppler angle results in› Lower cos()› Lower Doppler shift
77 X fD (kHz)v (cm/s) = -------------------------- fo (MHz) X cos(
Constant
decrease imaging depth increase pulse repetition
frequency› Increases sampling rate› Lessens aliasing
BUT
› increases likelihood of range ambiguity for pulsed instruments
RangeAmbiguityTrade-offTriangle
Depth
Lines / Frame Frames / sec(dynamics)
operator instructs scanner to assume aliasing occurring› scanner does calculations based on operator’s assumption
scanner cannot independently verify