Medical (M6)

Question 1

Acoustic impendance

Answer 1

The product of the density of a substance and the speed of ultrasound in that substance.
Z = pc
SI unit: kgm-2s-1

Question 2

Impendence/Acoustic Matching

Answer 2

The use of two substances with similar acoustic impendences to minimise reflection of ultrasound at the boundary between them.

Question 3

Intensity reflection coefficient

Answer 3

The ratio of reflected intensity over incident intensity for ultrasound incident at a boundary.

Question 4

A-scan

Answer 4

The simplest type of ultrasound scan.
A single transducer records along a straight line to determine the thickness of bone or distance between lens and retina.
It measures the time between sending the signal and receiving the echo.
A = amplitude

Question 5

B-scan

Answer 5

Transducer is moved over the patient’s skin to create a 2D image. For each position of the transducer the computer produces a row of dots on the screen. The brightness of the dot is proportional to the intensity of the reflected ultrasound.
B = brightness

Question 6

Piezoelectric effect

Answer 6

The production of an emf by some crystals such as quartz and lead zirconate titanate (a ceramic) when they are compressed, stretched, twisted or distorted.

Question 7

Ultrasound frequency

Answer 7

Minimum 20kHz
Medical imaging uses 1-15MHz

Question 8

Ultrasound transducer

Answer 8

High frequency alternating p.d. applied across a crystal repeatedly compresses and expands the crystal. Frequency of p.d. is the natural frequency of crystal so it resonates and produces an intense ultrasound signal.
Emits pulses of ultrasound ~5000 per second

Incident ultrasound vibrates the crystal, creating an alternating emf across the ends of the crystal which is detected by a circuit.

Question 9

Coupling gel

Answer 9

Fills air gaps between the transducer and the skin to ensure that almost all the ultrasound enters the patient’s body.
Impendence matching - if acoustic impendences are too different most ultrasound will be reflected by the skin.

Question 10

Simple scatter

Answer 10

Photon just rebounds elastically off electron

Question 11

Photoelectric effect scatter

Answer 11

X-ray photon disappears and removes an electron from the atom

Question 12

Compton scattering

Answer 12

The X-ray photon is scattered by an electron, its energy is reduced and the electron is ejected from the atom

Question 13

Pair production

Answer 13

The X-ray photon disappears to produce an electron-positron pair