Basic Physics of Medical Ultrasound     

Ultrasound is high-frequency vibration that travels through tissue at high speed approximately of 1540 m/s.

In medical ultrasound, vibrations in the range 20 000 to 50 000 000 cycles/s are used.

The term frequency is the rate of vibration and the unit hertz (Hz)

we use frequencies in the range 20 kilohertz (20 kHz) to 50 megahertz (50 MHz).

In medical ultrasound the source is a piezoelectric crystals in the transducer driven to vibrate by an applied fluctuating voltage and convert electrical to mechanical energy and vice versa

The great majority of medical ultrasound machines generate short bursts or pulses of vibration, e.g. each three or four cycles in duration by applying voltage for a short time produce pulsed wave.

For a continuous wave an alternating (oscillating) voltage is applied continuously.

The push–pull action of the transducer causes regions of compression and rarefaction to pass out from the transducer face into the tissue. These regions have increased or decreased tissue density.

A waveform can be drawn to represent these regions of increased and decreased pressure to generate an ultrasound wave

The distance between equivalent points on the waveform is called the wavelength and the maximum pressure fluctuation is the wave amplitude

If ultrasound is generated by a transducer with a flat face, regions of equal compression or rarefaction will lie in planes as the vibration passes through the medium generating Plane waves.

if the transducer face is convex or concave the wavefront will be convex or concave.

At a particular point, the tissue is oscillating rapidly back and forward about its rest position.

The number of oscillations per second is the frequency of the wave.

The speed with which the wave passes through the tissue is very high close to 1540 m/s for most soft tissue

Pulses can be transmitted and echoes collected very rapidly, enabling images to be built up in a fraction of a second.

Echoes produced by transmitted ultrasound pulses at tissue interfaces are the basic source of information in diagnostic ultrasound.

The speed of sound, c, is simply related to the frequency, f, and the wavelength, λ, by the formula:

C = f λ

Longitudinal waves or compressional wave

Acoustic waves oscillations of the particles of the medium are in the same direction as the wave travel. This type of wave is calle longitudinal wave and it gives rise to regions of increased and decreased pressure.

Transverse or shear waves

When oscillations are perpendicular to the direction of wave travel, they are called transverse or shear waves

The speed of ultrasound in soft tissue depends on its rigidity and density, the more rigid a material, the higher the speed.

All diagnostic instruments measure the time of echo return after the instant of pulsed ultrasound transmission and then use the speed in tissue to convert this time into the tissue depth of the reflecting structure.