Diagnostic methods in medicine (6.5.2)
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A radioisotope (or radionuclide) is an unstable isotope of an element that undergoes radioactive decay, emitting alpha, beta, or gamma radiation to achieve a more stable state.
Radioisotopes used in medical imaging must have a short half-life to ensure high activity and be detectable within the body. This allows small amounts of the radioisotope to be used to create an image, ensuring patient exposure is kept to a minimum.
Gamma-emitting isotopes are ideal for medical imaging due to their low ionisation and ability to penetrate the patient. Alpha and beta sources are not used in medical imaging because they are highly ionising, they can damage the cells in the body and lead to cancer.
Two common examples of radioisotopes used in medicine are:
- Fluorine-18: Produced in hospitals using a particle accelerator and used in PET scans.
- Technetium-99m: Produced by natural decay of molybdenum-90. Used to monitor the function of major organs, e.g. heart, lungs, brain, liver and kidneys.
The image below shows the molybdenum-90 decay chain where technetium-99m is produced:
Molybdenum-90 beta-minus decays to form technetium-99m. The ‘m’ refers to metastable, meaning the nucleus remains in a high-energy state. Technetium-99m then beta-minus decays to technetium-99, which has a very long half-life of 210,000 years.
To ensure the radioisotopes reach the desired location, they are attached to a targeting molecule using a linker (a chemical group that joins the radioisotope to the targeting molecule), which binds to the target protein on a cell membrane’s surface.
The combination of a radioisotope, linker, and targeting molecule is known as a radiopharmaceutical or a medical tracer. The diagram below shows the basic structure of a medical tracer:
It is useful to note that medical tracers can “label” a target in different ways, depending on the target molecule’s location. Some tracers bind to cell-surface receptors, whilst others are absorbed into cells by crossing the cell surface membrane. In both cases, the tracer accumulates more in the target area, making it easier to detect.
Question walkthrough
Writing the Beta-Plus Decay Equation for Fluorine-18
Write the nuclear decay equation for the beta-plus decay of the medical tracer isotope fluorine-18.
A gamma camera detects gamma photons emitted by medical tracers (usually containing Technetium-99m) injected into a patient. The diagram below shows the main components of a gamma camera:
When a gamma photon passes through the collimator and strikes the scintillator crystal (often sodium iodide), it may produce a brief flash of visible light. These light photons are detected by photomultiplier tubes, which convert the faint light into an electrical signal and amplify it.
The signals are then processed by the electrical circuits and sent to a computer, which uses them to work out where in the scintillator each gamma photon was detected. By repeating this many times, the computer builds up an image showing the distribution (concentration) of the medical tracer in the body.
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.
A gamma camera is used to diagnose conditions by detecting gamma photons emitted from a patient’s body after they have been injected with a medical tracer. It monitors the function of various organs, such as the heart, lungs, liver, kidneys, and brain. It helps identify abnormalities such as reduced blood flow, blockages, or cancerous tumours by showing areas with abnormal tracer concentration.
For example, technetium-99m can be traced through the body using the gamma camera. Technetium-99m emits gamma photons, that can be detected as they exit the body, allowing the location of the radioisotope to be determined. Organs with lower concentrations of the medical tracer typically signify malfunction.
It is useful to note that technetium-99m can be combined with sodium and oxygen to create , a compound that targets brain cells and is used in brain imaging. Different medical tracers are designed to target different parts of the body.
Some of the advantages and disadvantages of using a gamma camera for medical imaging are listed below:
Advantages:
- Non-invasive imaging method.
- Provides functional information about the organs, whereas X-rays and CT scans mainly show structure.
- Medical tracers can be tailored to target specific tissues or organs, allowing for highly focused imaging of the affected areas.
Disadvantages:
- Image resolution may be affected by scattered photons from the body or low scintillator efficiency.
- The patient is exposed to a small dose of radiation.
- Gamma cameras are expensive to purchase and maintain, and the need for radiopharmaceuticals makes the procedure costly and less accessible.
Fluorine-18 is a radioactive isotope used as a medical tracer in Positron Emission Tomography (PET) scans.
Fluorine-18 beta-decays and emits a positron , as shown in the nuclear notation below:
Fluorine-18 decays with a half-life of approximately minutes. So, it must be made in the hospital where the patient is located or at a nearby specialised site with a particle accelerator.
One production method for fluorine-18 is to accelerate protons to high speeds and collide them with oxygen-18 nuclei. This produces fluorine-18 nuclei and neutrons. The process is shown in the nuclear equation below:
Oxygen-18 is non-radioactive and easy to obtain since approximately of natural oxygen consists of this isotope.
All particles have a corresponding antiparticle with the same mass but opposite charge and opposite values of key quantum numbers (for example, lepton number or baryon number), while other properties such as spin are the same.
When a particle and its corresponding antiparticle collide, they annihilate each other. The mass of both the particle and antiparticle is converted into energy, typically in the form of photons.
The annihilation of a positron and an electron creates two gamma photons. The gamma photons travel in opposite directions in order to conserve momentum, as shown below:
- If the particle and antiparticle are at rest in the centre-of-mass frame, the photons have equal energy and travel in opposite directions to conserve momentum.
- If the pair has net momentum in the centre-of-mass frame, the photons are still emitted so that total momentum is conserved, but not necessarily in the opposite direction.
The majority of PET scanners use the medical tracer fluorodeoxyglucose (FDG). This compound is similar to naturally occurring glucose but has been combined with radioactive fluorine-18. The body treats FDG the same as regular glucose; therefore, when it is injected into a patient, it accumulates most in tissues that have a high respiration rate.
Gamma photons are produced when the positrons emitted from the decay of fluorine-18 annihilate with nearby electrons within the patient’s body. The gamma detectors detect these gamma photons produced via annihilation.
The image below shows a basic schematic of a PET scanner:
- The patient lies on a table surrounded by a ring of many gamma detectors. Each gamma detector comprises a sodium iodide scintillator crystal and a photomultiplier tube.
- Every gamma photon emanating from the patient’s body that collides with the scintillator crystal produces a voltage pulse.
- The ring of gamma detectors can determine the location of the annihilation from the difference in time at which the two gamma rays (gamma photons) strike the diametrically opposite detectors.
- The computer receives all the voltage pulses produced at the detectors and analyses them to generate a 3-D image on a display.
- The different concentrations of the medical tracer are highlighted in different colours and brightness on the display, allowing the analyser to determine where uptake was greatest or lowest.
PET scanners may also use carbon monoxide as a medical tracer, with the carbon atom labelled with carbon-11 rather than stable carbon-12. Carbon-11 undergoes beta decay, emitting a positron, and has a half-life of approximately 20 minutes.
Carbon monoxide binds to haemoglobin molecules found in red blood cells. Thus, it can be transported around the body and its concentrations monitored using a PET scanner. This provides information about blood flow in the body, such as a bleed in the brain or obstruction in the heart.
Some of the advantages and disadvantages of using PET scans for medical imaging are listed below:
Advantages:
- PET scanners are noninvasive and can be used to diagnose different types of cancer, monitor brain function, and assist with complex surgeries.
- PET scans are able to monitor brain function, so they can help identify early onset of brain disorders such as Alzheimer’s disease.
- Can assess the effectiveness of new medicines and drugs on tissues and organs. They allow researchers and doctors to see how a drug is absorbed, distributed, and metabolised in real time, helping to determine its efficacy and potential side effects before widespread clinical use.
Disadvantages:
- PET scans are very expensive due to the facilities required to produce the medical tracers necessary for the scans.
- Typically found only at larger hospitals.
- Only those patients with complex medical problems are given PET scans.
When a hospital is being equipped with a PET scanner, it must carefully consider the following issues:
- High initial cost: PET scanners are very expensive to purchase and install. Hospitals must justify the cost based on the number of patients who are expected to benefit from the service.
- Operational costs: In addition to the initial purchase price, significant ongoing operating costs exist, including the production and purchase of radioactive tracers and the need for specialised staff to operate the scanner.
- Training and expertise: PET scanning requires skilled technicians and medical professionals to interpret the results. Hospitals must invest in training and potentially hire new staff, which increases operational costs.
- Maintenance: PET scanners require regular maintenance and servicing, which can be costly and time-consuming. If the machine malfunctions, it can severely affect the hospital’s diagnostic capabilities.
- Limited availability of tracers: Because radioactive tracers have short half-lives, hospitals must either produce them on-site or rely on external suppliers, which can lead to supply chain issues or testing delays.