Ultrasound Physics Flashcards
Velocity of sound in average soft tissue
1540 m/sec
Wavelength equation
Velocity = frequency x wavelength m/sec = 1/sec x m
Wavelength of 3MHz sound wave in soft tissue?
0.5mm
Mechanical index =
= peak negative pressure / (square root of frequency)
Intensity =
power / cross-sectional area
units: watts/cm2
% reflected =
= (difference in impedence)2/(total impedence)2 x 100
Effect of frequency on scatter
decrease frequency -> decrease scatter (eg decrease echo smoke)
increase frequency -> increase scatter (eg see thrombus better)
Refraction
bending of wavefront as sound passes between media with difference velocities. Ratio of sin of angles = ratio of velocities
Diffraction
spreading or divergence of a sound beam: greater with a smaller source
Attenuation
loss of intensity (due to reflection, scatter, absorption) as sound wave passes through medium
Most commonly used piezoelectric material in diagnostic ultrasound transducers
PZT = lead zirconate titanate
Types of ultrasound transducers:
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.
Pulse repetition period
Time between pulses
Directly related to imaging depth
Pulse repetition frequency =
= 1/PRP
Inversely related to imaging depth
Duty factor
Fraction of PRP that transducer is emitting sound
= pulse duration / pulse repetition period
Speed of echo beam
13 microsec/cm, roundtrip
Axial resolution =
= (cycles x wavelength) / 2
Simplified doppler
velocity of object proportional to doppler shift, inversely related to frequency of source
Nyquist limit =
= PRF/2
Higher velocities can be measured at shallower depth and lower frequency
Wavelength of ultrasound in soft tissue =
= 1.54 / f (MHz) 1 MHz = 1.54mm 2 MHz = 0.77mm 3 MHz = 0.51mm 5 MHz = 0.31
3 dB attenuation = ? reduction in intensity
to 1/2
10 dB attenuation = ? reduction of intensity
to 1/10
20 dB attenuation = ? reduction of intensity
to 1/100
Frequency range for audible sound
20Hz - 20kHz
Frequency range for ultrasound
> 20,000 Hz
Ways to improve temporal resolution:
- Decrease image depth
- Decrease image display width
- Decrease # focal points
- Decreasing line intensity
Determinants of lateral resolution
- width of beam (focal zone)
- space between objects
Determinants of axial resolution
- pulse length. Better at higher frequency
PRF =
= 77,000 / depth
how to resolve aliasing:
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
ultrasound period
= time to complete one cycle
