Measuring exposure as dose

1. Dose is measured in sievert ($Sv$), and because a sievert is a very large dose, medical exposures are usually quoted in millisievert ($mSv$) or microsievert ($\mu Sv$).
2. Two things fix the dose: how much energy the tissue absorbs from the radiation, and how damaging that particular type of radiation is for the same absorbed energy.
3. Alpha radiation is counted as far more damaging than gamma radiation for the same absorbed energy, because it produces its ionisations packed closely together in a small volume of tissue.
4. The dose a person receives builds up with time, so $dose=dose\; rate\times time$.
5. Doses add up over a lifetime, which is why records of past scans are kept and why a worker's exposure is totalled over months and years rather than judged one shift at a time.
6. For scale, the average person in the United Kingdom receives roughly $2.7mSv$ each year from background radiation, a chest X-ray adds about $0.02mSv$, and a whole-body computed tomography scan can add several $mSv$.
7. Because the harm is a matter of chance, there is no dose small enough to carry no risk at all, so the working rule everywhere is to keep every dose as low as is reasonably possible while still doing the job properly.

Justifying the exposure

1. Before any patient is exposed, a doctor has to judge that the expected benefit of the scan or the treatment outweighs the risk the radiation carries.
2. A scan that finds a tumour early can save a life, so a small increase in the lifetime risk of cancer is worth accepting; a scan that would tell the doctor nothing new is not.
3. Methods that use no ionising radiation, such as ultrasound or magnetic resonance imaging, are used instead wherever they would answer the same question.
4. The patient's record of previous exposures is checked, so that repeated scans of the same person are not ordered without good reason.
5. Extra care is taken with children and with pregnant patients, because dividing cells are the most easily damaged and a child has more years ahead in which a cancer could develop.

Limiting the dose to patients

1. Use the lowest intensity or activity that still produces a usable image, because the dose rate rises with the intensity of the beam.
2. Use the shortest exposure time, since the dose is the dose rate multiplied by the time for which the beam is on.
3. Narrow the beam with a collimator so that it covers only the part of the body being examined, which keeps the rest of the body out of the beam entirely.
4. Cover the organs just outside the beam with a lead apron or thyroid collar, because lead absorbs much of the scattered radiation that would otherwise reach them.
5. Check the settings and the patient's position before switching the beam on, because a poor image has to be retaken and a repeated scan doubles the dose.
6. Use sensitive digital detectors rather than old photographic film, since a detector that responds to fewer arriving photons needs a smaller dose to form the same image.
7. Where a radioactive substance is given to the patient, choose one whose activity falls away soon after the procedure, so the patient is not still being exposed days later.
8. Encourage the patient to drink plenty of fluid afterwards, so any such substance is removed from the body more quickly.
9. In treatment rather than imaging, a large dose is deliberately given to the tumour, so the precautions there are about protecting the healthy tissue around it rather than about reducing the dose to the target.

Protecting medical personnel

1. A patient may be scanned once or twice in a lifetime, but a radiographer works beside the same machine every day, so it is the total dose over a career that has to be controlled.
2. The strongest protection is to be somewhere else, so staff leave the room and operate the machine remotely from a control area.
3. Where they must stay in the room, they stand behind a lead-lined screen or a lead-glass window, which absorbs most of the radiation heading towards them.
4. The walls of an X-ray room are lined with lead or made of thick concrete, so that people in the corridor and in neighbouring rooms are shielded as well.
5. Staff who have to be at the bedside wear lead aprons, thyroid collars and lead gloves, and stand as far from the beam as the procedure allows.
6. Radioactive sources for treatment are moved with long handling tools and kept in shielded containers, so that no one holds a source in their fingers.
7. Work is shared out between staff, so no single person is repeatedly the one standing closest to the machine.
8. Every worker wears a dosimeter badge, which contains a film or a crystal that changes measurably in proportion to the radiation that has reached it.
9. The badges are collected and read at regular intervals, and each worker's running total is compared with the legal dose limit for the year.
10. If someone's total is climbing towards that limit, they are moved to work that involves no exposure, and the reason for the high reading is investigated.
11. A badge gives no protection of its own; it monitors the dose so that action can be taken before the exposure becomes unacceptable.