Medical diagnostic ultrasound - physical principles and imaging
Ultrasound Medical Imaging
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Transcript of Ultrasound Medical Imaging
Ultrasound Medical Imaging
Imaging Science Fundamentals
Sound waves
• Sound waves are mechanical waves. Not in electromagnetic spectrum!
• Just like light waves, sound waves transmit energy, and can be described in terms of wavelength, period, speed, and amplitude
• Also can be described in terms of pressure, density, particle displacement
Sound waves (2)
• Frequency, period, and amplitude are determined by the source
• Wave speed and changes in density, pressure, and particle displacement are determined by the medium
• Wavelength depends on both source and medium
Some relationships:
Tf
1 The higher the frequency, the shorter the
period. Units: 1/s
The higher the frequency, the shorter the wavelength. Units: mf
c
Note: Propagation speed depends on the density and stiffness of the medium
• Human hearing range: 20 Hz to 20KHz.
• Sound requires a medium in which to travel. In the following diagram assume particles are separated by springs.
When a particle is pushed, the disturbance is transmitted to the others by springs.
• Driving force may be sinusoidal particles oscillate back and forth
pres
sure
time
Period
Sound and Human Hearing
Fast-rate changes (oscillations) in air pressure arriving at ear are detected by eardrum and transmitted to the brain, where they are interpreted as sounds.
• Ultrasound is any sound emitted at a frequency > 20 KHz.
–Medical ultrasound uses freqencies in the MHz range
• Sound speed in tissue: c 1540 m/s –similar to speed of sound in water–need to know c, because it is used to generate image
• In lungs, because of gas, propagation speed is lower. In bone, because it’s a solid, propagation speed is higher.
–Ultrasound doesn’t penetrate lungs & bone very well
Ultrasound
What kinds of waves are used in imaging?
• Pulsed ultrasound. A few cycles of ultrasound
• Produced by applying electric pulses to transducer
Pulse duration Pulse repetition period
Imaging with ultrasound:
• Sound wave is altered by tissue through which it passes.•At boundaries between structures (e.g., different tissue types) it is partially reflected•To determine distance, we need to know propagation speed; pulse round-trip time (from signal emission to “echo” detection) then determines distance: c = 2d/t => d = (t*c)/2
• As round-trip increases, reflector’s distance increases as well
• For c = 1540 m/s = 1.54 mm/s:
0 1 2 3 4 5 6 7 8 9 10
13 s
39 s
130 s
cm
Imaging with ultrasound:
• Ultrasound pulse simultaneously encounters several scatterers several echoes are generated simultaneously
+
Interference
Imaging with ultrasound:• Interference produces a “dotted” pattern (“speckles”)
–does not directly represent echoes, but rather how they recombine at the detector