6.5.2 Diagnostic methods in medicine

Radioactive isotopes have to be placed inside the patient and their radiation is detected from the outside. Gamma emitters are ideal sources as gamma photons are the least ionising and can also penetrate through the patient and be detected externally. Radioisotopes that are used for medical imaging must have a short half-life to ensure high enough activity from the source so only a small amount is needed for the image to form (this is also important as the patient is not subjected to a high dosage of radiation after the procedure). Radioisotopes such as fluorine-18 (used in PET scans) are produced artificially on-site (as they have a short half-life). Technetium-99m (produced by the natural radioactive decay of molybdenum-99) can be used to monitor the function of major organs such as the heart, liver, lungs, kidneys, and brain.

Radioisotopes are chemically combined with elements that will target the desired tissue to make a radiopharmaceutical (this is a medical tracer). E.g Tc-99m can be combined with sodium and oxygen to produce NaTcO4 (this will target the brain once injected). Its progress through the body can be traced using a gamma camera as the Tc-99m emits gamma photons. The concentration of Tc-99m can identify irregularities in the function of the body.


Fluorine-18 is a radiopharmaceutical used in PET scans (positron emission tomography). It has a half life of ~110 minutes and it will decay into a nucleus of oxygen-18, a positron, a neutrino, and a gamma photon. It has to be made in a laboratory near the hospital or with a particle accelerator (e.g high speed protons collide with oxygen-18 nuclei to produce fluorine-18 nuclei and neutrons.


The gamma camera
The gamma camera is a diagnostic tool that detects gamma photons emitted from radioactive nuclei injected into the patient, and an image is constructed which indicates the concentration of the tracer in the body. The gamma photons travel towards the collimator, any arriving at an angle are absorbed by the tubes so only those travelling along the axis of the tubes reach the scintillator. The scintillator is usually sodium iodide. A single photon striking the scintillator produces thousands of visible light photons (but not all the gamma photons produce these flashes as there is only a 1/10 chance a gamma photon will interact with the scintillation. The photons of visible light travel through the light guide into photomultiplier tubes. These are arranged in a hexagonal pattern and a single photon is converted into an electrical pulse. The outputs of each photomultiplier tube is connected to a computer and the electrical impulses are processed to locate the impacts of the photons on the scintillator. These impact positions construct a high quality image showing the concentrations of the tracer in the patient and the final image is displayed on a screen.

Gamma cameras produce an image that shows the function and processes of the body rather than the anatomy (like an x-ray).


Positron emission tomography
PET scans can be used to construct a detailed 3D image with gamma radiation (instead of X-rays). More often than not the radiopharmaceutical fluorodeoxyglucose (FDG) is used as it is similar to naturally occurring glucose but tagged with a radioactive fluorine-18 atom in place of an oxygen atom. Our bodies treat FDG as normal glucose and incorporate it into tissues with a high rate of respiration. Its activity can be monitored using gamma detectors. Carbon monoxide (with the carbon-11 isotope) can also be used as a radiopharmaceutical for PET scans. This emits a positron and has a half life of ~20 minutes. It attaches to haemoglobin in red blood cells (meaning it can be transported in the blood).

Pet scanners work as follows:

• the patient lies on a horizontal table surrounded by gamma detectors
• each detector consists of a photomultiplier tube and sodium iodide scintillator and produces a voltage pulse/signal for every gamma photon incident at its scintillator
• the detectors are connected to a computer
• the patient is injected with FDG
• the pet scanner detects the gamma photons emitted when positrons (from decaying Fl-18) annihilate with electrons inside the patient
• photons detected by the scanner come from the annihilation of these positrons, not from the gamma photons emitted by Fl-18 decaying. The annihilation of a positron and electron produces 2 gamma photons that are travelling in opposite directions (momentum is conserved)
• the computer determines the point of annihilation from the difference in arrival times of the photons at two diametrically opposite detectors and the speed of the photons (3x10^8)
• the voltage signals from the detectors are fed into the computer which analyses and manipulates the signals to form an image on a display screen
• different concentrations of the tracer show up as areas of different colours and brightnesses

Okay so we need to know the issues raised when equipping a hospital with an expensive scanner, such as a PET scanner. The advantages and disadvantages are as follows:

• Advantages
• non-invasive technique
• help diagnose different types of cancers/plan complex heart surgery/observe function of the brain
• help doctors to identify the onset of certain disorders of the brain (e.g. Alzheimer's)
• can be used to assess the effect of new medicines and drugs on organs
• Disadvantages
• very expensive due to the facilities required to produce the medical tracers
• only found at larger hospitals
• only patients with complex health problems are recommended for a PET scan.