The World of Nuclear Science

Medicine

Nuclear science is widely applied in medicine. The versatility of nuclear techniques allows for a range of applications in medical diagnosis and treatment of bodily conditions. In the United States alone, over 3500 hospitals perform 10 million nuclear medicine procedures each year.

In medicine, substances containing radioisotopes are given the specialised name radiopharmeceuticals. Many nuclear medicine procedures rely upon radiopharmeceuticals to work.

Radiopharmeceuticals can act as tracers. (See Science Applications for more about this.) A huge array of different compounds are found in the human body, and doctors can follow these through the body using radiopharmeceuticals. Radiopharmeceuticals are simply the ordinary compounds found within the body - except they are synthesised to contain an atom which is radioactive. Thus, a radiopharmeceutical is just a normal compound that has been radioactively 'tagged', and can thus be tracked with radiation measuring devices.

Consider a molecule of glucose (a type of sugar found in the body). When it is artificially synthesised as a tracer, scientists replace a stable carbon atom with a radioactive one, consequently tagging it. This resultant tagged molecule is a radiopharmeceutical. The trail of radiation that it emits while it passes through the body can be monitored using Geiger meters or scintillation counters. This trail can then be mapped into a computer to show doctors how the compound is processed by the body. The tracing technique is particularly useful for monitoring digestion of food. It has also been used in the study of how amino acids, critical substances in our bodies, are formed.

Radioisotopes can also be used to calculate the permeability of cell walls - that is, how easily cell walls can be penetrated. Radioactively tagged molecules are injected into the fluid surrounding the cell. The presence of radioactivity inside or outside of the cell is monitored over time, allowing the permeability to be determined. For example, nerve impulses in our bodies involve sodium and potassium ions passing through 'sodium-potassium pumps'. By placing radioactive sodium ions outside cell walls of nerve cells and monitoring the level of radioactivity, the activity of the pump can be determined.

Radiopharmeceuticals can also be used to determine activity of organs. Specific compounds concentrate in certain parts of the body. Therefore in the analysis of a certain part of the body, the patient is given the corresponding radiopharmeceutical that will accumulate most there.

Once in the body, the distribution of the radioactively labelled compounds is monitored by a network of "gamma cameras" around the patient, which work by detecting the radiation from the compounds. The data collected by the cameras is processed by a computer, allowing doctors to analyse the patient's body. High concentrations of the radioactively tagged compound in or around an organ can indicate overactivity; low concentrations can indicate underactivity.

One example of this is the common test for thyroid gland functionality. Patients are given a capsule of often iodine-123, and a scan takes place usually on the next day to assess the condition of the thyroid.

Magnetic resonance imaging is based on the vibrations that nuclei experience when bombarded by radio waves in a magnetic field. See Nuclear Magnetic Resonance in science.

The alternative name, magnetic resonance imaging (MRI) arises from a modification of nuclear magnetic resonance (NMR). The word "nuclear" is omitted so as to avoid the negative connotations. In fact, NMR has nothing to do with radiation or radioactivity and is relatively safe compared to other medical imaging techniques. "Nuclear" is relevant because of the process' relation with nuclei.

Magnetic resonance imaging is used as an analytical tool in scanning various parts of the body. It is especially used in brain scans and is one of the best techniques known for such procedures. The technique is also used in obtaining highly accurate images of body tissues - much more precise than computer aided tomography (CAT) scans or ultrasound. The resolution of magnetic resonance images typically is of the order of 0.5 to 1 mm, one of the most precise techniques available.

In a hospital, magnetic resonance imaging machines consist of a large hollow cylinder where the patient is placed. In this cylinder, there are several kilometres of wire wrapped around in a coil. When a current is passed through this wire, a very strong magnetic field is generated, and is especially concentrated in the centre of the cylinder. This provides the magnetic field for the resonance of nuclei.

The patient to be scanned is placed in the cylinder, and then the machine generates a magnetic resonance image, which is then used by doctors to determine the patient's condition.

sample magnetic resonance image of a brain scan

Radiation can cause cancer. Normally, cells that are exposed to radiation die. However, if the radiation does not kill the cell, they cell may be mutated by the radiation, and can start to reproduce. These multiplying mutant cells constitute cancer.

However, although radiation can cause cancer, it can also be used to treat it. Rapidly multiplying cancer cells are particularly prone to radiation. Doctors can administer radiation to kill these cancerous cells. However, some surrounding normal cells are unavoidably killed as well, due to the large doses of radiation required. Thus patients who undergo radiation therapy also experience side effects such as radiation sickness. To try to minimise these side-effects, narrow beams of radiation are used, and the beam continuously rotates around the body, minimising the radiation to normal areas but concentrating it around the cancerous region.

Cross-section view of how a radiation beam rotates around a point so that radiation is concentrated there, but minimised everywhere else.

Radiation is also used to sterilise surgical equipment and bandages. By bombarding the object to be sterilised with radiation, bacteria and viruses can be killed. This procedure is very common in hospitals for cleaning reusable surgical equipment between operations and sterilising blood for transfusions. It is also used in the manufacture of disposable medical products such as bandages and syringes where sterilisation is required before sale or use. Radiation-based sterilisation is especially useful in sterilising heat-sensitive or steam-sensitive materials.

The technique of single photon emission tomography, also known as SPET, involves administering the patient with a radiopharmeceutical, and then rotating a gamma camera or other radiation detector around the patient to detect the emitted radiation from many different angles. These different angles are put together by a computer to produce an image useful for the doctors.
The general concept of moving a detector around a patient to gain data from different angles is known as computer-aided tomography, or CAT. Thus SPET is a type of CAT.

Another technique called positron emission tomography, or PET, makes use of positron emitting atoms such as carbon-11, nitrogen-13 and oxygen-15. These atoms are again incorporated into radiopharmeceuticals that concentrate themselves in the part of the body to be studied once injected or consumed by the patient. When these atoms beta-decay, the emitted positron will move and eventually collide with an electron elsewhere in the body. Once this happens, the positron and electron are both destroyed, producing two gamma rays. These resultant gamma rays radiate out in exactly opposite directions.

A ring of detectors is placed around the patient, and when the gamma rays are detected, a computer can calculate the time difference and therefore work out where the annihation event occurred. However, with today's technology, these time differences cannot be measured accurately enough to generate useful data. (The best possible resolution is about 8cm). Nevertheless, if this could be refined, the biggest advantage of PET would be the lower scanning time and that a lower dose of radiopharmeceutical could be used. This is because the detection in PET is done capturing two gamma rays at once with one large circular detector, whereas with other CAT techniques one small detector that rotates around the patient is used.