1. Ultrasound

2. Reflection
The reflection of sound waves applies to frequencies above human hearing.
20 kHz is human maximum
Bats can reach 100 kHz
3.5 to 7 MHz are ultrasonic frequencies
Ultrasonic waves are short and make clean reflections.
Very little diffraction

3. Smile Autofocus cameras use ultrasonic waves.
Suppose a camera measures 12 ms.
Assume speed v = 340 m/s
Distance = vt
d= s * 340 m/s = 4 m
Twice the distance
The distance to the subject is 2 m.
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4. Acoustic Impedance Not all materials pass sound equally well.
Different speed v
Different density r
The acoustic impedance Z accounts for both effects.
Body Tissue Z (kg/m2 s) Air 426 Water 1.50 x 106 Blood 1.66 x 106 Muscle 1.70 x 106 Bone (avg) 6.0 x 106

5. Boundaries
At the boundary between two materials some wave is reflected.
Remaining transmitted
The intensity reflected depends on the impedances.
Reflection coefficient a

6. Medical Ultrasound Ultrasound transmits well though fluids.
It is reflected from bone and firm tissues.
The transducer receives the echo and translates the time to depth.

7. Multiple Images Each measurement reads a single line.
Multiple signals get a complete picture.

8. Sonogram Each separate echo is a single dot.
The multiple signal traces provide additional dots.
Our brain (or a computer) connects the dots to get the image.

9. Doppler Ultrasound Ultrasound is subject to the Doppler effect.
Frequency changes with speed
Frequency shifts are shown with color.
Correspond to different speeds.
Blood flow in a femoral artery with a blood clot