Basics

Question 1

What is the preferred emission for radionuclide imaging and why?

Answer 1

Gamma rays with energy range 80-500keV, or annihilation photons at 511keV. Gamma rays of these energies can penetrate through soft tissues in the body so can be detected from deep-lying organs. They can also be stopped efficiently by dense scintillators and can be adequately shielded with reasonable thicknesses of lead.
Alpha particles and electrons (beta particles, Auger electrons, conversion electrons) cannot penetrate more than a few millimeters of tissue so cannot escape from the body to reach the external radiation detectors.

Question 2

Give examples of radionuclides used in SPECT

Answer 2

• 99m-Tc ~ bone imaging ~ T1/2 = 6 hours
• 123-I ~ thyroid imaging ~ T1/2 = 13.22 hours
• 201-Tl ~ cardiac imaging ~ T1/2 = 3.04 days

Question 3

Give examples of radionuclides used in PET

Answer 3

• 11-C ~ 20.33 mins ~ prostate cancer
• 13-N ~ 9.97 mins ~ myocardial blood flow
• 15-O ~ 2.03 mins ~ cerebral blood flow
• 18-F ~ 109.73 mins ~ Alzheimer’s disease

Question 4

Give an example of a radionuclide used in therapy

Answer 4

I-131 ~ 8.02 days ~ hyperthyroidism

Question 5

For NM energies, what is the typical range for beta particles?

Answer 5

<= 1mm

• b+ lose energy (slow down) just as b- do but then annihilate in collision with an atomic electron (2x511keV photons)

much less for alpha particles!

Question 6

What are the interaction mechanisms for photons?

(y-annihilation photons and x-rays)

Answer 6

• Photoelectric absorption (interaction with atomic e-)
• Compton scatter (interaction with ‘free’ e-)
• Rayleigh/coherent scatter (interaction with entire atom)
• Pair production (produces e+/e- pair)

Question 7

What is the photoelectric effect mechanism?

Answer 7

An atomic absorption process in which an atom absorbs all the energy of an incident photon. A photoelectron is ejected

Question 8

How does the probability of the photoelectric effect scale with E and Z?

Answer 8

PE = (Z3)/(E3.p)

Z=atomic no. of material; E=incident photon energy; p=material density

Question 9

What is compton scatter?

Answer 9

A collision between a photon and a loosely bound outer shell orbital electron. The interaction looks like a collision between the photon and a ‘free’ electron. A compton recoil electron is ejected and a scattered photon of lower energy is emitted at some scattering angle theta

Question 10

When does pair production occur?

Answer 10

When a photon interacts with the electric field of a charged particle. Usually the interaction is with an atomic nucleus but occasionally it is with an electron.
Photon energy is converted into an electron-positron pair + KE. Positron will eventually interact with a free electron and produce a pair of 511keV annihilation photons.

Question 11

What energy does the initial photon need to have in pair production?

Answer 11

> = 1.022MeV

Question 12

What is coherent/Rayleigh scatter?

Answer 12

Scattering interactions that occur between a photon and an atom as a whole. Because of the great mass of an atom, very little recoil energy is absorbed by the atom and the photon is deflected with essentially no loss of energy

Question 13

At what energies is Compton scattering important?

Answer 13

< 50keV