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:
f T1
= The higher the frequency, the shorter the period. Units: 1/s
The higher the frequency, the shorter the wavelength. Units: m
f
= c λ
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
pressure time
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 frequencies 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
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:
• As round-trip increases, reflector’s distance increases as well
• For c = 1540 m/s = 1.54 mm/µs:
0 1 2 3 4 5 6 7 8 9 10 13 µs
39 µs 130 µs
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
