4. Ultrasound

Question 1

** Ultrasound

Answer 1

a device that uses high-frequency sound waves to create images of the inside of the body

• pros: non invasive, low cost, portable
• medical freq: 1-10 MHz

Question 2

Ultrasound applications

Answer 2

abdominal imaging - tumors
cardiology
fetal growth

Question 3

Physics of Ultrasound

Answer 3

• small particles of matter move back and forth to generate the compression / expansion
• ultra sound waves travel through comp / exp in same direction (longitudinal wave)

Question 4

Reflection, Scattering, Refraction, Diffraction

Answer 4

Refl: change in direction when bounce off a barrier
S: multiple reflections in directions, low amp, due to high density of second medium
Refr: change in direction when pass between mediums
D: slight bending of light as passes edge of object

Question 5

Ultrasound waveform in body

Answer 5

1. pulse travels into tissue
2. reflected from tissue boundaries
3. echos return to transducer, repeat (echos 40-100 dB below original)
   - pulses must be spaced w enough time between to allow sound to hit target and return before next pulse

Question 6

Ultrasound transducer

Answer 6

• generated when electric field is applied to piezoelectric crystals on transducer’s surface
• electrical simulation caused mech distortion of crystals -> vibration -> sound waves
• image generated when pulse generated, transmitted into body, reflected and returns

Question 7

** Piezoelectric effect

Answer 7

• conversion of electric to mechanical energy

- common in cyrstalline materials with dipoles on each molecule

Question 8

** Attenuation of Ultrasound waves

Answer 8

• waves attenuate / become weaker when reflected
• attenuation due to absorption (conversion to heat), reflection, scattering
• 80% is absorption in soft tissue
• extent of attenuation depends on acoustic impedance of 2 tissues at the interface
• compensate, amplify signal intensity of returning echo (time gain compensation)

Question 9

** A-mode display

Answer 9

Amplitude

• scans line through body, return echos as f(depth)
• peaks: where pulse passes through new medium
• gives info about the distance between tissue boundaries
• cons: don’t know what direction echo came from, don’t know for sure what echo bounced off, don’t know what object generating echo looks like - obsolete

Question 10

** B-mode display

Answer 10

Brightness (babies)

• can produce 2D image, move transducer while object isn’t moving
• move transducer along straight line + pivot on axis
• echos appear as intensity mod dots - result: outline

Question 11

** M-mode display

Answer 11

Motion

• measures motion of tissue
• take repeated 1D scans, shows intensity of reflected echos at time instances -> possible to calculate rate of motion of tissue

Question 12

** Doppler Ultrasound Imaging

Answer 12

change frequency due to relative motion of source and object - used to measure blood mvmt

Question 13

** Ultrasound block diagram

Answer 13

Transducer Transmit / Receive Switch (pulses) -> Time Gain Compensation (user control) -> Beamformer -> Compression & detection (ctr) -> Signal Processing (ctr) -> scan convert -> image processing (ctr) -> display