Medical Imaging

Question 1

Define a contrast material and give an example

Answer 1

Used to improve the visibility of internal structures with low absorption coefficients
Iodine.

Question 2

What happens in Compton scattering

Answer 2

Photons with energy range (0.5-5.0Mev)
Interacts with an electron and the electron is ejected.
The photon is scattered with reduced energy.
Momentum is conserved.

Question 3

What happens in pair production

Answer 3

Energy greater than 1.02 Mev

The photon disappears and the EMR energy produces an electron and positron pair.

Question 4

Why is the photoelectric effect used in Hospitals?

Answer 4

Requires energy less than 100kev

Hospitals use machines with 30-100kv supplies.

Question 5

Explain how a gamma camera works

Answer 5

• Collimator- gamma photons in axis of lead tubes are absorbed.
• Scintillator- gamma photon produces photons of visible light
• Photomultiplier tubes- electrons produced by photons of light which send electrical impulses to a computer that creates an image.

Question 6

Explain how a PET scan works

Answer 6

• FDG injected into patient
• Annihilation of electron and positron
• 2 gamma photons are produced that travel in opposite direactions
• patient is surrounded by gamma detectors which use the time delay of the photons to pinpoint area of increased activity
• a 3D image is produced by a computer

Question 7

Explain how the Piezoelectric effect produces ultrasound

Answer 7

• crystals such as quartz are compressed/stretched produce an emf.
• or when an external pd is applied across the crystal, its stretches/compresses no more than 0.1%
• sound waves with frequency over 20kHz

Question 8

Define impedance matching

Answer 8

When two subtsances have similar values of acoustic impedance.
eg coupling gel and skin

Question 9

What happens in a simple scatter

Answer 9

Energy 1-20keV

-X-ray photon interacts with electron and bounces off electron without change of energy

Question 10

Describe how an A-scan works

Answer 10

A single transducer sends ultrasound impulses through bone/lens

• Each pulse of ultrasound is partially relfected between two tissues
• the voltage signals are attenuated
• the time interval between the transducer and tissues is used to calculate distance with speed of ultrasound known.

Question 11

Describe how a B-scan works

Answer 11

• The transducer is moved over patients skin
• each position of the transducer produces a row of dots
• the brightness of the dot is proportional to the intensity of reflected ultrasound
• collection of dots produced a 2d image from high speed computer