Ultrasound Imaging

Ultrasound is sound above the range of human hearing, above about 20 kHz. Medical imaging typically uses 1 to 15 MHz.

Generation and detection — a piezoelectric crystal changes shape when a voltage is applied, so an alternating voltage makes it vibrate and emit ultrasound. The effect works in reverse: a returning pulse deforms the crystal and generates a voltage. The same transducer both transmits and receives.

How the image is formed — a short pulse is sent in, reflections return from the boundaries between tissues, and the time delay gives the depth from d = vt/2. The factor of two matters: the pulse travels there and back.

Acoustic impedance — the property determining how much is reflected at a boundary:

Z = ρ c

the density multiplied by the speed of sound in the medium.

The larger the difference in Z at a boundary, the greater the fraction reflected. A small difference, as between two soft tissues, reflects a little and transmits the rest — which is exactly what imaging needs, since the pulse must penetrate further to reach deeper structures.

Why coupling gel is essential — the impedance mismatch between air and skin is enormous, so almost 100% of the ultrasound would reflect at the surface and none would enter the body. The gel has an impedance close to that of skin, excluding the air and allowing the pulse through. Without it the scan shows nothing at all.

A-scan — a single transducer gives a one-dimensional plot of echo amplitude against depth. Used for simple distance measurements, such as eye scans.

B-scan — an array of transducers, or one swept across the body, builds a two-dimensional image, with echo strength shown as brightness. This is the familiar image of a fetus.

Resolution and penetration trade off — higher frequency means shorter wavelength and finer detail, but greater absorption and so less depth. A shallow structure is scanned at high frequency; a deep one at low frequency and lower resolution.

Doppler ultrasound — a moving reflector, such as blood, shifts the frequency of the returned pulse. The shift gives the speed of flow, so blood velocity can be measured without entering the body.

Example: ultrasound uses no ionising radiation at all, which is why it is the routine choice in pregnancy where an X-ray would be unacceptable. It is also real-time, showing a beating heart as it beats. Its weakness is that it will not pass through bone or gas, so the lungs and the adult brain are effectively invisible to it.