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: 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 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 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:  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