Option - Chapter 6

Question 1

What is the biological, physical and effective half-life of a radioisotope?

Answer 1

Biological - of a substance in the body is the time taken for half of that substance to be removed from the body.
Physical - of a radioactive isotope is the time taken for the activity of the radioactive isotope to decrease to half its initial activity.
Effective - of a radioactive isotope in the body is the time taken for the activity of the radioactive isotope in the body to reduce to half its initial value.

Question 2

What does a radioactive isotope need to be and why? When used for diagnostic purposes

Answer 2

It needs to be:
1, Gamma radiation - Alpha would be too easily absorbed and difficult to detect outside the body and would cause unnecessary exposure to ionising radiation. Beta would also be too difficult to detect outside the body and cause unnecessary exposure.
2, Have an effective half-life, long enough to allow enough time to get measurements but short enough to ensure it doesn’t remain in the body
3, Be identical to one of the chemical elements in the solution and decay to a stable isotope.

Question 3

How are most radioisotopes produced?

Answer 3

By exposing a stable isotope to neutron radiation in a nuclear reactor or from a particle accelerator. They become unstable and emits gamma radiation.

Question 4

What is a metastable state?

Answer 4

An excited state of the nuclei of an isotope that lasts long enough after alpha and beta emission for the isotope to be separate from the parent isotope.

Question 5

Describe how a technetium generator works?

Answer 5

The sodium chloride solution goes into the generator, it is passed through the ion exchange column which contains ammonium molybdate in ammonia (which has been exposed to neutron radiation serval days earlier to make a significant number of molybdenum nuclei unstable). After serval days, sufficient Tc-99m nuclei that have formed exchange with chlorine ions when the solution is passed through the column. The molybdenum ions don’t exchange, so the solution that emerges for the column contains Tc-99m ions and no molybdenum ions.

Question 6

What is a gamma-ray camera designed to do?

Answer 6

It is designed to detect gamma radiation from sites in the body where a gamma-emitting isotope is located.

Question 7

What does a gamma-ray camera consist of?

Answer 7

It consists of a sodium iodide crystal behind a lead collimator with photomultiplier tubes behind the crystal.

Question 8

What happens when a gamma ray passes into the gamma-ray camera?

Answer 8

It is used to detect gamma radiation. The photocathode within the tube faces a scintillation crystal which causes light photons be to emitted when gamma photons are incident upon it, these are then emitted towards the photocathode inside the tube. These light photons reach the photocathode and causes electrons into the tube, these electrons are then emitted electrons are attracted to the dynodes causing secondary emission of electrons at each dynode. This results in the number of electrons reaching the anode is much greater than without the dynodes.

Question 9

Why is a photomultiplier better than a Geiger counter at detecting gamma radiation?

Answer 9

It is more effective and can detect gamma photons at a faster rate.

Question 10

On a photomultiplier, what is the outside pulse from the anode proportional to?

Answer 10

It is proportional to the energy of the incident gamma photons.

Question 11

What must happen before a PET scan takes place?

Answer 11

The patient must drink water containing a small amount of a positron-emitting isotope such as the fluorine isotope 18F9

Question 12

How does a PET scanner work?

Answer 12

The patient is placed in the middle of the scanner. Around them is a detector ring. Each position from the drink travels less than 1mm in the patient before meeting an electron and annihilation occurs. The detector ring detects the two photons and is able to pinpoint the location of the positron-emitting isotope in the body.

Question 13

Give three examples of a radioactive isotope?

Answer 13

Iodine, Indium, Fluorine, technetium, fluorine and sodium.

Question 14

What are the two safest type of scanner and why?

Answer 14

MRI scanner and an ultrasound because they both use non-ionising radiation and therefore do not harm body tissue. There also requires no special storage facilities.

Question 15

What two types of scanner produce the best images?

Answer 15

CT scanners and MRI scanners give better resolution than ultrasound, but PET scanners and gamma scans give the best resolution.

Question 16

Which type of scanner is more convenient and widely used?

Answer 16

An ultrasound scanner is the most widely used and is quicker than ionising radiation scans and MRI scans when detail isn’t needed.

Question 17

What is X-ray therapy?

Answer 17

Where X-rays are used to destroy cells such as tumours.

Question 18

How does X-ray therapy work?

Answer 18

A collimated beam of high-energy X-rays directed at the will body to destroy cells. Gamma radiation isotope such as cobalt-60 is used to treat deeper tumours.

Question 19

Why can’t normal X-ray tubes to used to treat deeper tumours?

Answer 19

Because they are unable to penetrate deep enough, they only have a single anode and are unable to operate above 300kV because their insulation breaks down at any higher PD.

Question 20

Describe how a medical linear accelerator works?

Answer 20

Electrons are emitted from a heated filament in a metal tube called a waveguide. They are then accelerated towards a positive anode that has a hole through which some of the electrons pass to enter the waveguide tube. The electrons that enter the waveguide are then accelerated along it, by pulses of microwaves. A bending magnet is used to deflect the electrons onto a metal target, which stops electrons, causing them to emit X-rays in the process. A lead block with a narrow hole drilled through it is placed in the path of the X-rays so that a narrow beam of X-rays emerge through the hole.

Question 21

What is the maximum kinetic energy which can be achieved by X-ray photons from an accelerator?

Answer 21

25MeV

Question 22

Name one advantage of an electron accelerator?

Answer 22

They can be switched on and off, enabling exposure to be controlled.

Question 23

What is a radioactive implant used for?

Answer 23

Destroying malignant cells in a tumour.

Question 24

What are the positives of a radioactive implant?

Answer 24

No need for surgery can be removed and inserted using a needle.