Physics, Radiation and Cameras

Radiation - introduction & terminology

Radioactivity is a natural process in which ionising radiation is emitted from unstable atoms known as radioisotopes. Nuclear medicine makes use of this ionising radiation to produce an image showing the distribution of a radioisotope within a patient.

There are three main types of radiation that can be emitted in radioactive decay:

alpha particles

beta particles

gamma-rays

Gamma-ray emitting radioisotopes are suitable for use in imaging.

Alpha emitting radioisotopes are not used in nuclear medicine. Beta particles can only penetrate through a few millimetres of tissue, so their radiation will all be absorbed in the local organ. This makes beta emitters useful for therapy, but beta particles cannot be used for imaging because they can’t escape from the patient. Gamma-rays are much more penetrating and most will escape from inside a patient as long as their energy is above about 100 keV (100 thousand electron volts). This makes gamma-ray emitting radioisotopes suitable for use in imaging.

The strength of a radioactive source is measured by its activity in megabequerels (MBq). The activity will gradually decay away and the time taken for the activity to halve is known as the half-life. For most applications a half-life of a few hours is convenient for imaging without leaving activity in the patient for an unnecessary length of time. In nuclear medicine, the most commonly used radioisotope is 99mTc, which emits gamma-rays with an energy of 140 keV and has a half-life of 6 hours.

Risks of radiation

People are exposed to natural background ionising radiation every day and the average dose across the UK from this is approximately 2 mSv per year.

Ionising radiation is so called because it has sufficient energy to remove electrons from atoms in tissue. This can lead to damaged cells in tissues and organs with a consequent risk that this can eventually lead to cancer developing. The risk increases with the dose that is absorbed. The measure of dose is usually quoted in millisievert (mSv), with typical doses from nuclear medicine imaging procedures in the range of 1 to 15 mSv. Studies on radiation risk estimate that the probability of a fatal cancer developing from an exposure to 1 mSv of ionising radiation is approximately 50 in a million. People are exposed to natural background ionising radiation every day and the average dose across the UK from this is approximately 2 mSv per year.

The Gamma Camera

The light is detected by an array of photomultiplier tubes and the electronics work out whereabouts in the crystal the gamma ray was absorbed. This enables a computer to build up an image showing the distribution of radiopharmaceutical in the patient.

The gamma camera is a specialised device that is used to detect and produce images from the gamma-rays emitted from a patient. Gamma-rays pass through holes in a collimator and are absorbed by a scintillation crystal where their energy is converted to a small flash of light. The light is detected by an array of photomultiplier tubes and the electronics works out whereabouts in the crystal the gamma ray was absorbed. This enables a computer to build up an image showing the distribution of radiopharmaceutical in the patient. Gamma cameras only acquire one, or sometimes two, planar images at one time, but by rotating the camera around the patient and acquiring a series of images from many directions, it is possible to generate a 3-dimensional tomographic image. This technique, called Single Photon Emission Tomography (SPECT), is commonly used in nuclear cardiology to produce cross sectional images of perfusion to the myocardium.

Planar and SPECT imaging

Planar imaging

Planar images may be static (showing a fixed distribution of radiopharmaceutical), dynamic (showing how the distribution changes with time) or ECG gated (showing how the distribution changes during each heart beat).

Gamma cameras have one, or sometimes two, detectors allowing them to produce images of radiopharmaceutical distribution viewed from just one, or two, particular directions. These are called planar images. Planar images may be static (showing a fixed distribution of radiopharmaceutical), dynamic (showing how the distribution changes with time) or ECG gated (showing how the distribution changes during each heart beat). In nuclear cardiology rapid dynamic images can be used to observe the passage of a bolus of radiopharmaceutical through the heart in a first pass study. Gated images of a pharmaceutical that remains in the blood can be used to measure cardiac ejection fraction.

SPECT imaging

By rotating the camera around the patient and acquiring a series of planar images from many directions, a computer can be used to reconstruct a 3-dimensional tomographic image of the myocardium.

The gamma camera can be used to assess myocardial perfusion by imaging the distribution of a radiopharmaceutical such as tetrofosmin or sestamibi that is fixed in the heart muscle in proportion to local blood flow. However, planar images are difficult to interpret because of overlap between anterior and inferior walls of the heart. By rotating the camera around the patient and acquiring a series of planar images from many directions, a computer can be used to reconstruct a 3-dimensional tomographic image of the myocardium. This technique is called Single Photon Emission Tomography (SPECT) and it makes it much easier to detect areas of reduced perfusion without interference from the other wall. If ECG gating is also used, then gated SPECT images can show how the heart wall moves during the cardiac cycle and also be used to calculate left ventricular ejection fraction.

Radiopharmaceuticals

A radiopharmaceutical is a radioisotope chemically bound to a pharmaceutical. For a nuclear medicine procedure the patient is injected with a small amount of an appropriate radiopharmaceutical. The pharmaceutical is designed to concentrate in a particular organ or biological process, and the radioisotope which is attached to it allows the distribution of the pharmaceutical to be detected. This can therefore produce a map of where the pharmaceutical has concentrated. For example, if the pharmaceutical concentrates in the myocardium, its distribution will reflect myocardial blood flow. In nuclear cardiology the most commonly used radiopharmaceuticals that are labelled with 99mTc are tetrofosmin (marketed as Myoview) and sestamibi (Cardiolite).