Ultrasound Physics

Question 1

Velocity of sound in average soft tissue

Answer 1

1540 m/sec

Question 2

Wavelength equation

Answer 2

Velocity = frequency x wavelength
m/sec = 1/sec x m

Question 3

Wavelength of 3MHz sound wave in soft tissue?

Answer 3

0.5mm

Question 4

Mechanical index =

Answer 4

= peak negative pressure / (square root of frequency)

Question 5

Intensity =

Answer 5

power / cross-sectional area

units: watts/cm2

Question 6

% reflected =

Answer 6

= (difference in impedence)2/(total impedence)2 x 100

Question 7

Effect of frequency on scatter

Answer 7

decrease frequency -> decrease scatter (eg decrease echo smoke)
increase frequency -> increase scatter (eg see thrombus better)

Question 8

Refraction

Answer 8

bending of wavefront as sound passes between media with difference velocities. Ratio of sin of angles = ratio of velocities

Question 9

Diffraction

Answer 9

spreading or divergence of a sound beam: greater with a smaller source

Question 10

Attenuation

Answer 10

loss of intensity (due to reflection, scatter, absorption) as sound wave passes through medium

Question 11

Most commonly used piezoelectric material in diagnostic ultrasound transducers

Answer 11

PZT = lead zirconate titanate

Question 12

Types of ultrasound transducers:

Answer 12

1) Narrow bandwidth = high Q-factor. Ring freely, more efficient.
2) Wide bandwidth = low Q-factor. Short ultrasound pulses, wide range of frequencies sent/received.

Question 13

Pulse repetition period

Answer 13

Time between pulses

Directly related to imaging depth

Question 14

Pulse repetition frequency =

Answer 14

= 1/PRP

Inversely related to imaging depth

Question 15

Duty factor

Answer 15

Fraction of PRP that transducer is emitting sound

= pulse duration / pulse repetition period

Question 16

Speed of echo beam

Answer 16

13 microsec/cm, roundtrip

Question 17

Axial resolution =

Answer 17

= (cycles x wavelength) / 2

Question 18

Simplified doppler

Answer 18

velocity of object proportional to doppler shift, inversely related to frequency of source

Question 19

Nyquist limit =

Answer 19

= PRF/2

Higher velocities can be measured at shallower depth and lower frequency

Question 20

Wavelength of ultrasound in soft tissue =

Answer 20

= 1.54 / f (MHz)
1 MHz = 1.54mm
2 MHz = 0.77mm
3 MHz = 0.51mm
5 MHz = 0.31

Question 21

3 dB attenuation = ? reduction in intensity

Answer 21

to 1/2

Question 22

10 dB attenuation = ? reduction of intensity

Answer 22

to 1/10

Question 23

20 dB attenuation = ? reduction of intensity

Answer 23

to 1/100

Question 24

Frequency range for audible sound

Answer 24

20Hz - 20kHz

Question 25

Frequency range for ultrasound

Answer 25

> 20,000 Hz

Question 26

Ways to improve temporal resolution:

Answer 26

1. Decrease image depth
2. Decrease image display width
3. Decrease # focal points
4. Decreasing line intensity

Question 27

Determinants of lateral resolution

Answer 27

• width of beam (focal zone)

- space between objects

Question 28

Determinants of axial resolution

Answer 28

• pulse length. Better at higher frequency

Question 29

PRF =

Answer 29

= 77,000 / depth

Question 30

how to resolve aliasing:

Answer 30

1) use a continuous-wave transducer, 2) increase the PRF (scale) to the maximum setting for the depth you are imaging at, 3) switch to a high pulse repetition frequency, 4) utilize a lower frequency transducer and 5) adjust the baseline to allow for imaging the maximum velocity

Question 31

ultrasound period

Answer 31

= time to complete one cycle