1. Resident Physics Lectures
01:
Ultrasound Basics Principles
George David, M.S.
Associate Professor of Radiology

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
Microphone
receives echoes
Speaker
transmits sound pulses

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 V

4. US Transducer Operation
alternating voltage (AC) applied to piezoelectric element
Causes
alternating dimensional changes
alternating pressure changes
pressure propagates as sound wave

5. I’m a scanner, Jim, not a magician.
Ultrasound Basics
What does your scanner know about the sound echoes it hears?
I’m a scanner, Jim, not a magician.
Sound
Acme
Ultra-
Sound
Co.
Echo

6. What does your scanner know about echoed sound?
How loud is the echo?
inferred from intensity of electrical pulse from transducer

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 sound’s 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, scanner must calculate
position
Gray shade

13. Images from Echos
Let's see how this works!

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 scanner’s sound transmission
ain’t 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
distance

18. What is the Speed of Sound?
distance = time delay X speed of sound
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)
13 msec
1 cm
Handy rule of thumb

19. So the scanner assumes the wrong speed?
Sometimes
Luckily, the speed of sound is almost the same for most body parts
soft tissue ==> 1.54 mm / msec
fat ==> 1.44 mm / msec
brain ==> 1.51 mm / msec
liver, kidney ==> 1.56 mm / msec
muscle ==> 1.57 mm / msec ?

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
Loud
Soft

23. Complication
Deep echoes are softer (lower volume) than surface echoes.
Loud
Soft ?

24. Gray Shade of Echo
Correction needed to compensate for sound attenuation with distance
Otherwise dots close to transducer would be brighter

25. I'd like to see an example!
Depth Correction
                        
I'd like to see an example!

26. Echo’s Gray Shade Gray Shade determined by Measured echo strength
accurate
Calculated attenuation
Who am I?
Charles Lane

27. Attenuation Correction
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)

28. Ultrasound Display One sound pulse produces Multiple pulses
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
Focus
Arrows indicate timing variations.
Activating top & bottom elements earlier than center ones focuses beam.

30. Scan Patterns Linear sector beam translated produces rectangular image
moved sideways
produces rectangular image
sector
beam pivoted
produces pie-shaped image

31. Th’ Th’ Th’ Th’ That’s All Folks