Lecture 6: Radiopharmacy Flashcards
How does a cyclotron work? (6 marks)
Best to sketch:
- Two hollow dees with a alternating charge applied to them.
- B-field going vertically through the dees.
- Charged particle, such as a proton, injected into the centre of the dees.
- Combination of the forces from the alternating charge on the dees accelerating the charged particle from one dee to the other, and the force applied from the B field.
- Results in a spiral motion and acceleration of the proton outward towards a target.
- Creates radionuclides via charged particle bombardment.
How do the products produced in a cyclotron decay and what can be produced? (5 marks)
- Products are generally proton rich and therefore decay via beta + or electron capture.
- Mainly used to produce small Z radionuclides such as F-18, O-15 (using in blood), C-11 (prostate).
How does a nuclear reactor work and how are radionuclides produced? (6 marks)
Best to sketch…
- Uses a graphite moderator to slow the neutrons produced and increase the chance of an interaction.
- Control rods can be inserted to slow down rate of fission.
- Generally operate via nuclear fission, where a chain reaction occurs where a neutron collides with a large atom such as U236 and causes it to fragment, or neutron capture. The daughter components produced can then be used as radionuclides.
- Fission RN = Mo99
- Neutron capture = Co60 (gamma knife)
How does U236 decay? (3 marks)
236U -> 90Sr + 144Xe + 2n
Why do we need RN generators? (1 mark)
To produce short-lived radionuclides on a longer time-scale.
Sketch what is meant by transient equilbrium? (4 marks)
- Plot showing the variation of activityof Tc-99m and Mo-99 in a Moly generator.
- Each time the generator is elutated the concentration of Tc-99m decreases, but then gradually increases again due to Mo decay.
- Equilibrium is reached for a brief period before the cow is milked again, hence transient.
- Mo99 has a HL of 66 hours.
How is a Tc-99m generator eluated? (6 marks)
Best to draw this one…
- Tube running in and out of shielded column.
- Saline solution is run through the tube.
- There is a Al oxide column which the MoO4(2-) is bound to.
- When the Mo decays to Tc-99m, the ion is more weakly bound (TcO4(-1)).
- This allows chloride ions to replace the ion and it is pulled through the generator
- There is a filter to catch any Al ions
How does a RN calibrator work? (2 marks)
It is essentially an ionisation chamber:
- gas contained in a volume, typically argon
- gas gets ionised by radiation which is then collected by polarised plates (there is a potential difference applied across them, the E-field causes the ions to separate)
For an ionisation chamber, what regions are there on a plot of voltage against charge collected? (5 marks)
Sketch it out:
- At low voltages we have a linear recombination region where the PD is too low and the charges produced recombine before they are detected.
- Then there is the flat ionisation chamber region where the response (amount of charge collected) does not vary with energy.
- Above this we have the proportional region where there is some avalanching in the detector but only when the ionisation event is close to the anode. Not useful as the charge produced varies with the voltage.
- Above this we enter the GM region where the avalanching is large enough to reach the eletrodes from any point.
- Above this point again we reach the continous discharge zone where the PD is so high that it causes spontaneous discharge even when there is no radiation.
What QC is carried out on radionuclide calibrators? (6 marks)
- Constancy check on a long lived HL source such as Cs-137, to ensure that response has not changed.
- Linearity test to ensure that readings are consistent at different dose rates: use the shielded method or a quickly decaying source such as Tc-99m.
- Accuracy by sending off to NPL for calibration.
- For sources, such as I-123, which emit low energies (~28 keV) and higher energies (159 keV), to use a calibrator we can use some copper shielding to block the lower energy x-rays. These vary largely on volume hence not wanted.
- In addition for sources which are beta emitters, we can measure the bremstrahlung produced to determine the absorbed dose, but issues with large variations in geomtry of source.
What radiopharmacy QA is carried out? (8 marks)
- Radionuclide purity: Mo breakthough test, as Mo emits much higher gammas, use shielding to detect this.
- Chemical purity: test for presence of Al ions, which should be collected by the filter. Test using indicator paper.
- Test for free pertechnatate, which has not bound to the pharmacutical: use chromotography. Free pertecnatate is carried up the paper and detected.
- Other tests: use settle plates to test for microbes, test pressure system is working by ensuring that there is a reasonable positive pressure being produced etc.
What three sources of RN impurities are there? (3 marks)
Decay products from parent, daughter and the manufacturing process.
