Radionuclide imaging and therapy (3.10.6)
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Medical tracers are radioactive substances used to assess the function of various tissues and organs. Unlike X-ray imaging, which primarily shows structure, medical tracers can also reveal the structure and function of different organs.
Medical tracers (radiopharmaceuticals) consist of:
- a radioactive isotope (radionuclide) that emits detectable radiation (gamma rays or positrons),
- A carrier molecule that is used by the body, such as glucose or water, and
- A linker protein that binds the radioactive isotope to the carrier molecule.
The tracer is administered to the body either by injection or oral ingestion. The tracer circulates and accumulates in target tissues based on the carrier’s biological behaviour.
The diagram below shows the structure of a radiopharmaceutical.
The radiation emitted by the radionuclide is detected by special sensors, and an image of the organ under investigation is constructed using complex computer algorithms.
There are three main medical tracers used in medical imaging:
- Technetium-99m: Widely used radioisotope due to its half-life, which is long enough for imaging after uptake, and short enough to limit patient radiation dose.
- Iodine-131: An isotope suitable for assessing and treating thyroid conditions, as the thyroid naturally concentrates iodine.
- Indium-111: An isotope used to label white blood cells and certain antibodies to detect infections, inflammations, or locate specific targets.
Properties of these three medical tracers are shown in the table below:
Medical tracers have an effective biological half-life, which reflects both physical decay and biological clearance.
The body metabolises and eliminates the carrier substances bound to the radionuclide, thereby decreasing their concentration over time.
- The rate at which the body metabolises substances is known as the rate of excretion. This biological clearance is described by the biological half-life
- The radionuclide also undergoes physical radioactive decay, described by the physical half-life
- These two processes combine to give the effective half-life which determines how long detectable radiation persists in the body.
The effective half-life of a medical tracer is calculated by:
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Radionuclide imaging and therapy
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The physical half-life of technetium-99m is too short (approximately 6 hours) for it to be transported; it will simply decay before it can be used as a medical tracer. To ensure on-demand availability, hospitals receive technetium generators that contain its parent radionuclide, molybdenum-99.
Molybdenum-99 has a longer half-life than technetium-99m (66 hours) and so is more suitable for transport. The process of producing technetium-99m using a generator is:
- Molybdenum-99 is adsorbed onto an aluminium oxide column.
- The molybdenum decays, producing technetium-99m, which does not bind strongly to the aluminium oxide column.
- A saline solution placed into the generator flushes out any technetium-99m. This solution, which contains technetium-99m, is then injected into the patient or combined with another substance to create a different tracer.
A gamma camera detects gamma radiation emitted by medical tracers injected into a patient.
- Lead collimator: A block of lead with thousands of vertical holes, filters any off-axis gamma rays, so only those parallel to the holes travel through and are detected.
- Scintillator crystal: The crystal, often sodium iodide, emits flashes of visible light photons when a gamma ray strikes it.
- Photomultiplier tubes (PMTs): Photons from the scintillator crystal strike the photocathode in the PMT and these emit electrons via the photoelectric effect. The PMT then converts these into electrical signals by accelerating electrons through a series of dynodes, multiplying at each stage and producing a large cascade of electrons.
- Electrical circuits: The PMT electrical signals are combined and transmitted to a computer, which then constructs an image.
- Lead shield: A block of lead surrounds the detector so radiation from other sources cannot enter the camera and interfere with the image.
The lead collimator in a gamma camera helps produce a sharp, high-resolution image by controlling the direction of incoming gamma rays. Without a collimator, gamma rays would enter from all directions, leading to a blurred, low-resolution image. The diagram below shows the basic structure of a collimator:
The collimator contains an array of small holes, allowing only gamma rays travelling along specific paths to reach the detector. It works by absorbing off-axis gamma rays, preventing them from contributing to the image:
- The smaller the holes in the collimator, the higher the resolution, but the lower the sensitivity.
- The larger the holes, the lower the resolution, but the greater the sensitivity.
The photons produced by the scintillator crystal need to be amplified and converted into an electrical signal. The diagram below shows the internal structure of a single photomultiplier tube from a gamma camera:
- A single visible light photon from the scintillator hits the photocathode to produce a single photoelectron.
- The photoelectron is then accelerated towards the first dynode, an electrode with a potential difference of .
- When the photoelectron collides with the dynode, it produces additional electrons known as secondary electrons.
- These are then accelerated to the next dynode, which is held at a higher potential difference. Each electron produces, on average, four secondary electrons at each dynode – creating an exponential growth of electrons.
- The electrons are all collected at an anode at the end, producing a current that passes through a resistor and generates a small voltage pulse.
Positron emission tomography (PET) scans utilise radiopharmaceuticals that emit positrons.
- The patient is injected with a biologically active molecule, such as fluorodeoxyglucose labelled with a positron-emitting radionuclide with a short half-life. Examples of these radionuclides include nitrogen-13, oxygen-15, and fluorine-18. The medical tracer is allowed time to distribute throughout the body to the target organs.
- Each positron emitted by the radionuclide annihilates with a nearby electron in the organs, emitting two high-energy gamma rays that travel in opposite directions.
- A ring of gamma detectors around the patient detects the photon pairs, and a computer constructs a map of the body’s radioactivity.
A computer synthesises the data from all detectors to create a 3-D image of the body. The image uses varying colours and brightness to highlight the concentration of the medical tracer absorbed by different organs; higher concentrations are indicated by brighter colours.
The annihilation of an electron-positron pair during a PET scan produces two gamma rays. The energy of these two gamma rays can be calculated using Einstein’s mass–energy equation:
The electron and positron have the same mass The resulting photons have zero mass. Therefore:
Thus, the total energy released during the annihilation reaction is:
Substituting the numerical values, we get
Convert joules to MeV. and so
Therefore, the energy of each photon will be half of this energy:
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Radionuclide imaging and therapy
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Question walkthrough
Radionuclide imaging and therapy
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Ionising radiation damages cells and can lead to cancer. However, there are times when this property can be useful in treating cancer.
The table below compares external radiotherapy using high energy X-rays to internal radiotherapy using beta emitters:
Contrast media are easily seen on X-ray images; these materials have a high proton number , and hence a high attenuation coefficient. Common contrast materials:
- barium
- iodine (
Soft tissue has a composition of different atoms, which has an average proton number, , and has a low attenuation coefficient.
The correlation between the attenuation coefficient of a material and proton number is due to the photoelectric effect: this is the dominant absorption mechanism at X-ray frequencies of In general there is a proportionality of:
Contrast media are used to distinguish between different tissues in the body that look very similar in normal X-ray imaging.
In medical X-ray imaging, barium and iodine are commonly used as contrast media.
Iodine is used as a contrast medium in liquids, for example, to observe blood flow. An organic iodine compound is injected into the bloodstream. In the X-ray image, low attenuated intensities indicate where the blood is flowing well, and areas with high attenuation (where the iodine is collecting) indicate poor blood flow.
Barium sulphate is used as a contrast medium in the digestive system. A liquid ‘barium meal’ is swallowed by the patient. Similarly, areas where the barium builds up will be highly attenuated and can locate blockages.
A computed axial tomography scan or CAT scan is a medical imaging technique that uses X-rays to analyse a patient’s body structure.
It comprises the following key elements:
- Rotating X-ray source: an X-ray tube rotates around the patient. It produces a thin, fan-shaped X-ray beam that passes through the body, targeting specific slices or sections.
- Ring of detectors: a ring of detectors encircling the patient capture the X-rays after they pass through the body. As the X-ray beam rotates, the detectors measure the amount of X-rays absorbed by different tissues, which varies based on the attenuation ability of different materials.
To generate images on a computer, software and displays are used.
- Computer software: data from the detectors is sent to a computer. Using specialised software, the computer processes this data, performing complex mathematical calculations to generate detailed images of the scanned body part.
- Display: processed data is converted into visual images that can be displayed on a monitor. These images show detailed cross-sections (slices) of the body, allowing healthcare professionals to make accurate diagnosis.
There are advantages of using CAT (computed axial tomography) scans over traditional X-ray medical imaging:
- Cross-sectional imaging: CAT scans provide detailed cross-sectional slices of the body, offering a better view of internal structures. X-ray images are 2D, which can make it difficult to distinguish overlapping tissues.
- Better soft tissue contrast: CAT scans are superior at visualising soft tissues, such as the brain and blood vessels, which are hard to differentiate on X-ray images.
- Diagnostic accuracy: CAT scans can provide more extensive information for healthcare professionals to use for diagnosis, making their conclusions more accurate.
PET scans are useful for imaging the body as well as its functions. However, the use of ionising radiation does present some disadvantages.
The table gives some advantages and disadvantages of PET scans:
There are multiple imaging techniques used in medical imaging. The table below compares the various imaging methods used.
