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Ultrasound Transducer. Speakertransmits sound pulses. Microphonereceives echoes. Acts as both speaker
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1. Resident Physics Lectures 01:
Ultrasound Basics Principles
2. Ultrasound Transducer 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
3. Piezoelectric Principle 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
4. US Transducer Operation alternating voltage (AC) applied to piezoelectric element
Causes
alternating dimensional changes
alternating pressure changes
pressure propagates as sound wave
5. Ultrasound Basics What does your scanner know about the sound echoes it hears?
6. What does your scanner know about echoed sound? How loud is the echo?
7. What does your scanner know about echoed sound? What was the time delay between sound broadcast and the echo?
8. What else does your scanner know about sound echoes? Direction sound was emitted
9. What else does your scanner know about echoed sound? The sounds pitch or frequency
10. What Does Your Scanner Assume about Echoes(or how the scanner can lie to you) 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)
11. Luckily These Are Close Enough to Truth To Give Us Images 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)
12. Ultrasound Display B-scan (Brightness Mode) Image
series of gray shade dots
For each dot, scannermust calculate
position
Gray shade
13. Images from Echos
14. Dot Placement on Image Dot position ideally indicates source of echo
scanner has no way of knowing exact location
Infers location from echo
15. Dot Placement on Image Scanner aims sound when transmitting
echo assumed to originate from direction of scanners sound transmission
aint necessarily so
16. Positioning Dot Dot positioned along assumed line
Position on assumed line calculated based upon
speed of sound
time delay between sound transmission & echo
17. Distance of Echo from Transducer Time delay accurately measured by scanner
distance = time delay X speed of sound
18. What is the Speed of Sound? scanner assumes speed of sound is that of soft tissue
1.54 mm/msec
1540 m/sec
13 usec required for echo object 1 cm from transducer (2 cm round trip)
19. So the scanner assumes the wrong speed? Sometimes
20. Gray Shade of Echo Ultrasound is gray shade modality
Gray shade should indicate echogeneity of object
21. How does scanner know what gray shade to assign an echo? Based upon intensity (volume, loudness) of echo
22. Gray Shade Loud echo = bright dot
Soft echo = dim dot
23. Complication Deep echoes are softer (lower volume) than surface echoes.
24. Gray Shade of Echo Correction needed to compensate for sound attenuation with distance
Otherwise dots close to transducer would be brighter
25. Depth Correction
26. Echos Gray Shade Gray Shade determined by
Measured echo strength
accurate
Calculated attenuation
27. Attenuation Correction scanner assumes entire body has attenuation of soft tissue
actual attenuation varies widely in body
28. Ultrasound Display One sound pulse produces
one image scan line
one series of gray shade dots in a line
Multiple pulses
two dimensional image obtained by moving direction in which sound transmitted
29. Moving the Sound Beam electronically
phased or pulsed transducer arrays
30. Scan Patterns Linear
beam translated
moved sideways
produces rectangular image
sector
beam pivoted
produces pie-shaped image