@@Ultrasound

Doppler effect

A change in frequency observed when the sound source and observer are in relative motion away from or toward each other

Doppler shift

The magnitude of the frequency change in hertz when sound and observer are in relative motion away from or toward each other.
* Quantified

 

 

Physical principles

• Sound and ultrasound are created by pressure on a medium, causing it to compress and expand
  * Travels longitudinally
  * Cannot travel through vacuums
  * Speed depends on the nature of medium, not on frequency or amplitude
• Intermittent pulses of sound waves
  * Generated by electrically induced deformation in piezoelectric quartz crystals

Frequency

• Frequency used: 2.5-15MHz
  * Increase frequency --> Increased resolution (to 1mm) but decreased penetration (10-25cm)

Reflection

• Sound waves passing through tissues of differing densities causes reflection of part of the sound energy
• Loudness of reflection is interpreted as intensity
• Delay of reflection is interpreted as distance from the probe
  * Sound assumed to travel at 1540m/s in tissue at 37C

NB:

• Human ear can hear from 20Hz to 20kHz
• Ultrasound >20kHz
• Infrasound < 20Hz

 

Different modes

A-mode ("Amplitude" scan)

• Simplest
• Echo signals separated by time intervals
• Peaks in amplitude (??of reflection) correspond to reflective interfaces
• Time taken before reflection is sensed corresponds to distance from the probe
• Used to measure eye axial length

B-mode ("Brightness" modulation)

• Same as A-mode, but using brightness to display intensity of signal of reflection, rather than using amplitude of waves.
  * i.e. one dimensional

M-mode (time-positioning)

• Repeated B-mode scan, drawn against time
  * i.e. one-dimensional image of tissue against time
• Vertical axis is the depth of reflecting surfaces
• Movement would be shown
• Good for valve motions

2-D

• Multiple crystals or moving crystal
• Sequential B-mode pulses sweeping up to 90degrees across a plane
• Displayed as a single 2D image
• Up to 30 images per second
• Moving in real time

Pulse wave Doppler

?? Double check. I think wavelength changes as well as frequency

Principle:
* Alteration in the frequency of reflected sound due to the relative velocity of the source of reflection

Velocity of sound = frequency x wavelength

--> c = f0 x lambda

After doppler shift

--> velocity of sound + 2xspeed of movement = frequency of reflection x wavelength

--> c+2v = fr x lambda

Derived from these two equation

--> V = (fr-f0)c/2cos(angle)x f0

Thus,

Motion velocity of the object equals to
Increase in frequency (i.e. doppler shift), times sound velocity,
Divided by 2 x cos (angle) x original frequency

Duplex

Combination of Doppler detection with real-time pulse-echo imaging.