Nuclear medicine is a term used for medical procedures that involve the introduction of a radioactive substance into a body. There are three uses for this. Firstly, the bulk of the work for most nuclear medicine departments is imaging; secondly, for therapeutic reasons; and thirdly, for non-imaging tests. All three are covered in more detail below.
The process of imaging, generally, breaks down like this:
- A radioactive isotope, usually technetium 99m (99mTc)1, is attached to a pharmaceutical.
- The pharmaceutical used is dependant on the organ or system to be studied. This combination is called a radiopharmaceutical.
- The radiopharmaceutical is introduced into the patient, usually by IV2 injection.
- Often a delay is necessary for the radiopharmaceutical to reach its target.
In this entry 99mTc will be used as an example. That is not to say that is the only isotope used - nuclear medicine uses chromium-51, krypton-81m and many others. The process, however, is fairly similar.
One example of the imaging performed in nuclear medicine is a heart scan.
Images are taken using a gamma camera. This is a machine capable of detecting the gamma rays given off by 99mTc and converting them into an image.
The Gamma Camera
A gamma camera is, simply put, a machine used in nuclear medicine that can detect gamma rays and turn them into an image.
The workings of a gamma camera are best described by following the path of a gamma ray. Gamma rays (a form of electromagnetic radiation) are emitted by the radioactive isotope previously introduced to the patient (see above). The gamma ray exits the patient and soon reaches the camera.
The first part it encounters is the collimator. Its purpose is to ensure that the only gamma rays detected by the camera are those travelling perpendicular to the face of the camera. If it were not there then even a small point of radiation would produce an image that covered the whole screen and be of no use in imaging.
The collimator is a relatively simple device. It is a large lead 'mesh'; any ray travelling at an angle other than perpendicular is stopped by the walls. Any ray perpendicular to the face can pass straight through. The larger the hole the less exactly perpendicular the ray has to be. Smaller holes mean better resolution but sometimes resolution has to give way to time, which is decreased with greater hole size.
The gamma ray then strikes a crystal. This crystal converts the gamma ray to light photons that are useful in later stages. The crystal is sodium iodide (NaI) 'doped' with traces of thallium (Tl). The crystal gives off flashes of ultraviolet light.
The light from the crystal strikes a photo-multiplier tube (PMT). Firstly this converts the light photons to electrons. The electrons are accelerated down a series of increasingly high potential differences (voltages) creating more electrons as they go. The electrons are collected by an anode at the far end and generate a signal. This passes into the processing equipment - a topic in itself.
Nuclear Medicine therapy exploits the fact that radioactivity is harmful. Many readers will be familiar with the concept of radiotherapy; the use of beams of radiation to treat cancer by killing the cancerous cells. Nuclear Medicine therapies help in a similar way.
A pharmaceutical is selected that will target the harmful cells. A radioactive element is incorporated into it and the 'radiopharmaceutical' is given to the patient.
Probably the simplest example is the treatment of hyperthyroidism. Hyperthyroidism is a condition where the thyroid gland is overactive. The treatment for this is suprisingly simple, kill some of the thyroid cells. This is achieved by introducing a radioactive isotope to the thyroid gland. For this particular treatment this is relatively simple. As part of the physiological processes of the thyriod gland it takes up iodine, that we all ingest, from the blood. So what the technicians in nuclear medicine do is give the patient a tablet that is a radioactive isotope of iodine (131I)3. The 131I is chemically the same as the natural 127I and so follows the same physiological paths and is absorbed into the thyroid. The beta radiation emitted by131I is harmful to the tissue it is absorbed into so killing some of the thyroid cells. Beta radiation does not travel far in tissue so the surrounding tissue is relativly unharmed.
The non-imaging tests performed in Nuclear Medicine are all quite similar. They all involve the introduction of radiopharmaceuticals (see above) into the body and then measuring the levels of radioactivity present at a later time.
The radiopharmaceutical's introduction is performed in one of two ways. A blood sample may be taken, the radiopharmaceutical combined with the blood (or part of it) and then the blood re-injected. This method is most commonly used where the test is interested in one part of the blood. The other possible method is the injection of the radiopharmaceutical directly into a vein where it combines with the blood while inside the body. The later counting can sometimes be performed with external counting devices pointed at the patient or, more commonly, require blood (or other body fluid) samples to be taken.
A good example of this system is a test of Glomerular4 Filtration Rate, or GFR. This is a measure of how fast your kidneys filter things, like drugs, out of the blood. This is an important factor in preparing doses for treatment such as chemotherapy5. For this test a radiopharmaceutical is injected into a patient. Blood samples are later taken, for example 2,3 and 4 hours after injection. The plasma from these blood samples is separated because that is the part of the blood the radiopharmaceutical is present in. The plasma samples are then put in a device that 'counts' how much radiation is present. The rate at which the amount of radiation, and therefore the radiopharmaceutical, is dropping, is calculated. This rate is an indicator of the rate at which blood is being filtered by the kidneys. The final result gives a GFR in millilitres per minute (ml/min).
Technetium 99m is, in Nuclear Medicine, the most commonly used radioactive element. It is useful for several reasons:
- It can be easily combined with several pharmaceuticals.
- It gives off gamma rays at 140keV which is a good match to the sensitivity range of the Gamma Camera.
- Its half-life of six hours is long enough to allow practical imaging but not so long that the patient, public and environment are over-burdened with radiation.
- It is a pure gamma emitter.