Positron Emission Tomography (PET)

PET scan is an imaging technique which produces images of the body by detecting radiation or gamma rays emitted from the body after the patient is injected with radioactive substance. When a positron emitted by a radioactive substance bombards an electron in the tissue, two gamma rays are emitted in the opposite directions as shown in the diagram below:

These rays are detected by a detector. When the gamma ray strikes the detector, it emits a pulse of light. This is amplified in photomultiplier tube and the resulting signal goes to the computer for processing.

The patient is injected with a radio pharmaceutical in the vein of the arm. In some instances, the patient may be asked to inhale the radio-pharmaceutical.

Radio pharmaceutical is a radionuclide combined with Sugar, Fluorodeoxyglucose (FDG). The radioactive atom is either carbon-11, Flourine-18, Oxygen-15 or Nitrogen-13. When the radioactive isotope decays or transforms to another stable isotope, it emits positrons or positively charged particles. The half-life for the different isotopes is given in the table below:

Sugar attached to the radionuclide is used more by tumour cells as compared to healthy cells because of higher cell-metabolism. Hence the output of the detector from this region is large. The patient is placed in a ring of detectors in the PET brain scan as shown in the figure below:

A pair of detectors is placed symmetrically at 180°. Since the scintillation of light is mapped, the process is called scintigraphy.

Scanning

PET scan combines nuclear scanning with CT scanning (Computed Tomography). When a PET scan of the brain is done, an image of a thin slice across the brain is obtained. The table on which the patient is placed is moved and the process is repeated to obtain images of a large number of thin slices.

Images of the brain are mapped into thin slices and they are then reconstructed by the computer to give the entire picture of the brain. The radiation pulses during a fixed interval of time are displayed on the computer screen as a dot. The intensity of the dot will be a measure of the intensity of the signal. The picture thus shows various shades of grey, with black of zero count and white for higher count of pulses. To obtain a good contrast, the same image is displayed in false colours of the rainbow e.g. Yellow, Green, Blue and Violet. Violet and Black represent lowest activity.

Patient Preparation for the PET test

A catheter is inserted in the arm of the patient and an injection with the radio tracer is administered. If O-15 is the tracer used, recording lasts for one minute and thirty seconds. Another injection can be administered after 8-10 minutes. If the experiment is not completed a maximum of four injections can be repeated.

The patient should not eat or drink for 8-12 hours before the test. He should not consume any beverages containing alcohol or caffeine or smoke or take medicines 24 hours before the test. Diabetic patients should take less insulin.

The entire procedure takes 1 – 3 hours depending upon the radioactive tracer and the organ being studied. After the test, the patient is asked to take a lot of fluids to eliminate the tracer from the body. It generally takes 6 – 24 hours for elimination.

PET instrumentatin system

The instrumentation system consists of:

• Cyclotron
• Bio-synthesizer
• Scanner detector
• Computer

Cyclotron

The cyclotron is a machine which is used to produce radioisotopes, which are then used in the bio-synthesizer to produce radio-pharmaceuticals.

The cyclotron consists of a huge chamber which produces radioisotopes of a substance. It first produces protons and gets them accelerated along a circular orbit by means of a powerful electromagnetic field. When the protons gain a large energy, they bombard a target at the speed of light. The target is transformed into a radioactive unstable isotope as a result of a nuclear reaction. The material chosen is a naturally occurring substance in the body like Carbon, Oxygen or Nitrogen.

Bio-synthesizer

It attracts Fluorodeoxyglucose (FDG) to the radionuclide to form a radiopharmaceutical.

For the study of the brain, O-15 water is used. C-11 or O-15 carbon monoxide is used for cerebral volume. C-11 or F-18, N–Methylspiperone is used for mapping dopamine and serotonin.

Scanner

It is large, ring-shaped with detectors. When the patient is placed inside a scanner, he may feel claustrophobic.

Computer

The computer terminal is located outside the room. It collects the data from the scanner and the software helps to reconstruct images with false colours to highlight areas of large consumption of glucose.

Advantages of PET scan

PET scans are performed on the brain, heart or any tissue for detecting cancer etc.

Cancer

For cancer of the breast, brain, lungs, colon and prostrate, PET is more sensitive than CT scan or MRI. It can detect alterations in the biochemical processes prior to the disease or before changes in anatomy are apparent on other imaging tests such as CT or MRI scans. It is particularly useful in evaluating cancer treatment.

Heart

It can detect decreased blood flow which can indicate scarred tissue or damage caused due to heart attack.

It is used in thallium stress test, where images of the heart are taken when the patient is exercising on a treadmill and then taken when at rest after exercise. Both images are compared to reveal changes in blood flow to the working heart.

Brain

Area of reduced oxygen can indicate stroke, while reduced glucose metabolism can indicate some forms of dementia. Abnormal forms of blood flow and oxygen can indicate a brain tumour.

You can also read: The Introduction to Medical Diagnostics Techniques

Risks

There is a slight risk of damage to cell or tissue. However, the risk is very low. Soreness or swelling can develop at the site of the injection.

Single Photo Emission Computed Tomography (SPECT)

The SPECT scan is similar to the PET scan except that the radio nuclide is different and has a large decay time (half-life period). In this scanning, the radioactive substances used are iodine-123, Xenon-133 and Technetium-99. When a positron strikes an electron, it emits a single gamma ray instead of two rays at 180°. Hence for each ray, there is only one detector. The image that is obtained with SPECT gives fewer details as compared to a PET image. However, this technique is less expensive as compared to PET.