Ultrasound Medical Imaging Science Fundamentals Sound waves Sound

Ultrasound Medical 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: The higher the frequency, the shorter the period. Units: 1/s The higher the frequency, the shorter the wavelength. Units: m Note: Propagation speed depends on the density and stiffness of the medium

• Human hearing range: 20 Hz to 20 KHz. • 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.

pressure • Driving force may be sinusoidal particles oscillate back and forth 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 • 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

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 = 2 d/t => d = (t*c)/2

• As round-trip increases, reflector’s distance increases as well • For c = 1540 m/s = 1. 54 mm/ms: 130 ms 39 ms 13 ms 0 1 2 3 4 5 6 7 8 9 10 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