Transcript Medical Ultrasonography

Medical
Ultrasonography

Objectives
1. Be able to discuss the physics of ultrasound
2. Be able to describe the operation of ultrasonic transducers
3. Be able to list and describe the operation of ultrasonic
instruments
4. Be able to inspect and evaluate the performance of ultrasonic
instrument
17-3 What is ultrasound ?
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Medical ultrasound systems operate at frequencies of up to
10MHz or more
An ultrasonic wave is acoustical:is a mechanical wave in a
gaseous,liquid,or solid medium
The mechanical wave consists of alternating areas of higher and
lower pressure
Distinguish with the radio signals
and the ultrasound signals
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Radio signals: are electromagnetic waves
i.e:
If an alternating current(ac) oscillation of,say,2500kHz,were
connect to an appropriate antenna,then radio would be
launched.
Ultrasound signals: are acoustical
i.e:
If that same 2500kHz ac signal were applied to an ultrasound
transducer,then an acoustical signal would be launched.
17-4 Physics of sound and ultrasound
waves
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In radio signals:the velocity is about 300,000,000m/s
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In ultrasound signals:the velocity =
F 
17-4-2 Reflection,refraction,diffraction,and
scattering phenomena
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Reflection and refraction of waves
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Diffraction:is a bending of the direction of propagation that
occurs when a wave impinges on an object of different
density embedded within,and surrounded by,the incident
media.
diffraction: can distort the direction information
of the signal.
17-4-3 Specular reflection,diffuse
reflection,and scattering
The first –pulse arrival circuit
17-4-4 Acoustical impedance
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Acoustical impedance:is a measurement of its opposition to the
propagation of sound waves
17-5 Ultrasound transducer
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In ultrasound,two transducer functions are recognized:
(1) conversion of ac oscillations into acoustical vibratioon
(2) conversion of acoustical oscillations back into electrical
oscillations
These two functions are the transmit and receive transducers
The piezoelectric element
Inverse square law
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Inverse square law:the power density drops according to the
square of the distance
The resolution of the ultrasonic signal
1.
2.
The pulse width: sets the maximum longitudinal resolution
also called range resolution
The beamwidth: sets sets the azimuthal or angular resolution
17-6 Absorption and attenuation of
ultrasound energy
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Look the eq(17-9) in the textbook
The value of attenuation by absorption depends on both the
acoustical impedance and the frequency used.
Figure 17-12 shows how soft-tissue absorption losses rise with
frequency
17-6-1 Time-gain compensation
17-7 Scan modes and scanning systems
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A-scan mode: uses a stationary transducer to fire a pulse into
tissue
B-scan
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B-scan mode: use the same time base as the A-scan but but
plots the strength of the returning signal as changes in
brightness
i.e: a strong reflection is brighter than the weaker reflection
Back-and-forth scan method
2-D B-scan mode
Time-motion(T-M) mode
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M-mode: is essentially an A-scan but with successive looks at
the target created by scanning the time base vertically.
look at the figure17-19
17-7-1 Electronically scanned phased
array transducers
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The ESPA angle = eq 17-12
The ESPA combined output voltage = eq 17-13
Alter the direction of the combine
wavefront
ESPA’s grating lobes problem
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The angle at which grating lobes appear = eq 17-14
The grating lobes will cause spurious returns to to be displayed,
some suppression will reduce this situation(especially signal
strength display)
17-9 Doppler effect
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Doppler effect: is a change of frequency that occurs when the
receiver and transmitter move relative to each other
The frequency shift = eq 17-16
17-10 Transcutaneous Doppler flow
detectors
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If the blood were motionless,the frequency shift would be
zero ,so the return wave frequency will be identical to the
incident wave frequency.
But moving blood produces a shift that is proportional to the
blood velocity.
A frequency shift of shift of approximately 200Hz at 10MHz
corresponds to a blood velocity of approximately 6 cm/s.
17-11 Flowmeters
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The average flow velocity = eq 17-17
Doppler flowmeter
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This transducer uses the frequency change of a wave scattered
from particulate matter flow in the vessel
Look at the eq 17-19
17-12 Ultrasonic blood pressure
measurement
Pneumatic system
17-13 Echoencephalography
R-L and L-R patterns
Solid and fluid-filled masses
Summary
1 Medical ultrasound tends to use frequency in the 2-10MHz
2 Ultrasound waves obey the reflection ,refraction,diffraction,and
scatter rules of ordinary wave behavior.
3 Several factors affect biological interaction with
ultrasound :frequency,irradiation time,beam intensity,and duty
cycle.
4 Most ultrasound transducer are piezoelectric crystals.