Nuclear Medicine (Medical Physics series, part 2)

Nuclear medicine, in a nutshell, involves injecting - or otherwise dosing - people with radioactive substances. The rest of diagnostic radiology deals with images created using an external radiation source; nuclear medicine is very much internal. One exciting result of this is that while x-rays, MRI and the like show anatomy brilliantly, nuclear medicine is able to show the functional state of an organ or tissue.

For example, this image shows the pelvis and legs of a patient with arthritis. You can see the worst affected areas (the left knee and ankle); the increased cell activity engaged in repairing the damage caused by arthritis means that the cells take up more of the radioactive tracer, and show up more brightly on the scan. In addition, you can see the patient's bladder - the standard route of excretion for many substances, radioactive or otherwise!

As with all areas of medical physics, nuclear medicine involves a lot of quality assurance: physicists test equipment weekly, and monthly, and six monthly; they carry out detailed surveys of equipment prior to first clinical use, and after maintenance, and any time something seems to not quite be working correctly. Most equipment tests in nuclear medicine are associated with image quality - checking that a camera's spatial and contrast resolution aren't degrading to a point where clinical use is compromised, for example. Some non-imaging equipment is used as well, though, for example well calibrators (used to measure the activity of radiation in a patient injection) or gamma counters (used to quantify the radiation emitted by blood samples), and these need testing too.

On the clinical side of things, nuclear medicine physicists look at quantitative (or semi-quantitative) analysis of data. Sometimes that data involves images; assessment of the change in activity (recorded counts, pixel value) within an area over time, for example, or the total activity taken up in an area. I did a project during my placement in nuclear medicine looking at the (then current) protocol for calculating thyroid uptake index (a measure of thyroid function) vs. two different methods using commercial software, aiming to assess the differences and the potential impact of changing the protocol. Sometimes there are no images at all - several pages of my training portfolio concentrated solely on calculating glomerular filtration rate - a measure of kidney function - and I could bore you about it for days.

In addition to the above, the radiopharmacy - responsible for producing individual patient doses of the required pharmaceutical (different tracers for different purposes) - will have physicists involved somewhere. Depending on the department, it may be a physics-led radiopharmacy, or it may be led by pharmacists and supported by physicists; in either case, physicists will advise on radiation protection issues and ensure that the department complies with the relevant legislation.

The final area of nuclear medicine is radionuclide therapy (RNT): actually treating conditions using radiopharmaceuticals. One of the most common conditions treated in this way is an over-active thyroid. The thyroid takes up iodine, so by giving patients a radioactive form of iodine, it's possible to kill cells in the thyroid - reducing function to a normal level - without killing cells elsewhere in the body¹. Physicists are required by law to be involved in RNT, and may administer the radio-isotope or provide advice and information to patients about to undergo RNT.

RNT can also be used to treat disorders where cells are growing out of control, such as polycythemia vera, a disorder in which too many blood cells are produced, and even metastases from cancer². As well as benign thyroid conditions, radioiodine is also used to treat thyroid cancer, for which patients are treated with a high activity of radioiodine and so have to spend three or four days as an in-patient in a lead-shielded room. This minimises the exposure of the general public (including family and friends) to radiation, and physicists are instrumental in calculating just how long the patient should stay in hospital, and how long visitors may be present for on each day.

Nuclear medicine physicists also need to consider the scope and impact of a lot of legislation, perhaps more - at least in number of applicable statutory instruments! - than in any other area of medical physics. Legislation, however, is complex enough to require its own post.

Next time: diagnostic radiology, the purpose of which you might well be able to glean from the name...!

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1. Technically, this isn't quite true: of course some cells in the rest of the body will be affected. But it's a numbers game, and the huge affinity of the thyroid for iodine means that very little of it reaches the rest of the body.

2. For a particularly exciting use of RNT in metastatic cancer, look up Alpharadin: its use in prostate cancer patients was so effective that the trial was actually ended early.

Medical Physics: An Introduction (Medical Physics series, part 1)

In December 2009, a dear friend of mine asked me a very big question. 'Tell us all about medical physics,' she said. 'What's it about?'

At this point, I had only known of the existence of medical physics for about six months. Approximately 30 seconds after finding out about it, I wanted to do it, and by December I had arranged two work experience visits and a tour of a third department, but I still had no idea how to describe it.

I've now been in the job around two and a half years, and it's still hard to describe. It's so big! It really is. My standard answer has become 'anything involving radiation in hospitals', and that's a fair simplification, but doesn't really explain to anyone what I do all day. It also tends to lead to a stock series of responses, including 'oh, you're a radiographer?' (no) 'oh, you're a radiologist?' (no) and 'oh, you fix [insert medical machine here]??' (no). In order: radiographers push buttons which make images (or treat patients, in radiotherapy); radiologists are doctors specialising in radiology (medical imaging), and engineers fix machines. (We just tell the engineer it's broken...)

Wikipedia¹ helpfully describes medical physics as follows: Medical physics is generally speaking the application of physics concepts, theories and methods to medicine. This must be very helpful for people who don't understand the adjectival suffix, but not otherwise. What physics concepts?

Well, as I said above, mainly radiation; to be specific, ionising radiation. That is, x-rays, radioactive materials, etc. Since that's the bulk of it, and that's what I'm working in, I'll come to the details of that later.

But as well as ionising radiation, we also work with non-ionising radiation: lasers (used to break up kidney stones, remove tattoos, etc.), ultrasound (obviously used in imaging, but also in interventional medicine), UV dermatological treatments (for psoriasis and certain skin cancers), and magnetic resonance imaging (MRI).

The main roles in a lot of this are quality assurance, and safety. We test machines; we look for patterns in response, for abnormalities, and we try to explain them. (Then we call the engineers...) We advise doctors, and other medical staff, on the safety issues with various types of non-ionising radiation (don't shine lasers into your eyes; don't take metallic power tools into an MRI room²; in terms of ultrasound, well, there's very little safety to advise on, which is one of the reasons I personally find it dull).

We also try to optimise imaging: obtaining the best quality image possible (for a definition of quality which depends on the clinical purpose), and writing computer programs to help with image processing and analysis. This is also to do with safety, in terms of reducing the dose of radiation needed to achieve an appropriate image quality.

In ionising radiation, there are three main areas: nuclear medicine, radiotherapy, diagnostic radiology; and covering all of these, radiation protection. I'll explain these in more detail in following posts.

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1. en.wikipedia.org/wiki/Medical_physics [quotation retrieved 14/03/2013]. ↩

2. This really happened; it destroyed a very expensive machine which had only just been installed. ↩