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