Chapter 16 - Diagnostic Methods In Medicine

Question 1

Two reasons a radionuclide should have a short half life?

Answer 1

• it will give out its radiation quickly so that only a small amount is needed to form an image in the gamma camera
• any radionuclide that remains in the patient will soon decay away, ensuring they are not exposed to hazardous levels of radiation

Question 2

Write equations for technetium-99m?

Answer 2

Book

Question 3

The radionuclide must decay fast. How do hospitals stop it decaying before they use it?

Answer 3

Hospital buys a supply of Mo-99, which is produced in a nuclear reactor (HL 67hours)
The Mo-99 produces Tc-99m at a predictable rate, and this can be extracted for use with patients

Question 4

Define metastable?

Answer 4

Describes a nucleus that is in an unstable but relatively long-lived state.

Question 5

Define radiopharmaceutical?

Answer 5

A substance tagged with a radioisotope and which targets a specific organ or tissue in the body.

Question 6

Define tracers?

Answer 6

Radioactive substances used to investigate the function of organs of the body.

Question 7

Use of fluorine-18?

Answer 7

Bone imaging

Question 8

Use of technetium-99m? (3)

Answer 8

Bone growth
Blood circulation in lung, brain and liver
Function of heart and liver

Question 9

Use of iodine-123? (2)

Answer 9

Function of thyroid

Function of kidney

Question 10

Use of xenon 133?

Answer 10

Function of lung

Question 11

Explain how a gamma camera works? (4 steps)

Answer 11

1) gamma photons pass through collimator
2) gamma beam strikes scintillator crystal which makes a flash of light
3) light detected by photomultiplier tubes which send electrical pulse for each photon received
4) electrical signals from photomultipliers are processed by the computer and it then generates a high quality image

Question 12

2 uses of gamma camera?

Answer 12

• Bone scan (a static study in which a single image is produced a suitable time after injection)
• Kidney scan (a dynamic study in which a series of images will be made of the kidneys to see how well they are functioning)

Question 13

What radiation must be emitted from a radionuclide and why?

Answer 13

Only gamma, alpha and beta will be absorbed by the body and are extremel dangerous.

Question 14

Define positron emission tomography?

Answer 14

A technique where gamma ray photons from electron-positron annihilations are detected to produce a 3D image of the body.

Question 15

Explain briefly how positron emission tomography works?

Answer 15

Radiopharmaceuticals injected contain radioisotopes which emit positrons (and gamma). Each positron collides with an electron and the two are annihilated. Their mass is released as energy in the form of two gamma photons at 180degrees to each other, and 90degrees to the e+ and e-.

Question 16

Explain how the PET scanner works? (6)

Answer 16

The patient takes the radiopharmaceutical which emits positrons. It will accumulate in particular tissues. The patient lies in a ring of gamma detectors. These detect the gamma photons which are coming from inside the patient but going in OPPOSITE directions. The times at which they arrive are compared and from this the position they were emitted can be determined. Gradually a 3D image of the distribution of the tracer is built up.

Question 17

Uses of PET scanning? (2)

Answer 17

• Showing up cancerous tissue

- Showing up normal bodily functions like brain activity

Question 18

Define spin and what is it?

Answer 18

A quantum property of many atomic or sub-atomic particles.

It is a property which causes nuclei to behave like tiny magnets in a magnetic field.

Question 19

Explain spin for protons (assuming a Hydrogen nucleus because it is present in all tissues)?

Answer 19

Proton has positive charge, it’s spin means it behaves like a magnet with N and S poles.
When strong magnetic field is applied, most protons line up with field (low energy state), a few go the other way (higher unstable energy state).

Question 20

Define precession?

Answer 20

The movement of the axis of a spinning object (proton) around another axis.

Question 21

Define Larmor frequency?

Answer 21

The frequency of precession of nuclei in an external magnetic field.

Question 22

Larmor frequency equation?

Answer 22

ω0 = δΒ0

Question 23

Explain how a photomultiplier tube works?

Answer 23

Incident gamma photon strikes scintillator, photo of light released, this releases an electron from the photocathode by the photoelectric effect. The electron is then accelerated to +100V electrode and on impact releases 2/3 electrons. This continues until there is lots of electrons.

Question 24

What is ω0, B0 and δ?

Answer 24

W0 = Larmor frequency (angular frequency of precession) (rads^-1)

B0 = magnetic flux density (T)

δ = Gyromagnetic ratio

Question 25

How can you work out the normal precession frequency f0? Also other equation to join precession frequency and angular frequency of precession?

Answer 25

f0 = δB0/2π

Also

ω0 = 2πf0