A PET scan uses radiation to produce 3-D, color images of the inside of the human body. Rather than simply showing what an organ looks like, it can show how well it is working.
PET stands for positron emission tomography. The machine detects radiation that is emitted by a radiotracer which is injected into the body.
The images are reconstructed by computer analysis. Modern machines often use a CT (computer tomography) X-ray scan which is performed on a patient at the same time in the same machine.
PET scans can be used to diagnose a health condition, as well as for finding out how an existing condition is developing. PET scans are often used to see how effective an ongoing treatment is.
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Fast facts on PET scans
Here are some key points about PET scans. More detail and supporting information is in the main article.
- PET scans are often used to diagnose a condition or view how a condition is developing
- They are often used alongside X-rays or MRI scans
- PET scans are often used to investigate the following conditions: epilepsy, Alzheimer's disease, cancer, and heart disease
- A CT or MRI scan examines the size and shape of body organs and tissue; a PET scan looks at the function of those organs
- Patients are told to not consume any food for at least 4-6 hours before a scan, but to drink plenty of water
- PET scans are not painful
How a PET scan works
PET scans can be used to diagnose a health condition, as well as for finding out how an existing condition is developing.
Before carrying out a PET scan, a radioactive medicine is produced; this is then tagged to a natural chemical.
This natural chemical could be glucose, water, or ammonia. The tagged natural chemical is known as a radiotracer. The radiotracer is then injected into the human body.
When it is inside the body, the radiotracer will go to areas that use the natural chemical. For example, FDG (fluorodeoxyglucose - a radioactive drug) is tagged to glucose to make a radiotracer.
The glucose travels to those parts of the body that use glucose for energy. Cancers, for example, use glucose differently from normal tissue - so, FDG can show up cancers.
A PET scan detects the energy emitted by positively charged particles (positrons). As the radiotracer is broken down inside the patient's body, positrons are created. This energy appears as a 3-D image on a computer monitor.
The image reveals how parts of the patient's body function by the way they break down the radiotracer. A PET image will display different levels of positrons according to brightness and color.
When the image is complete, it will be examined by a radiologist who reports the findings to the doctor. A radiologist specializes in interpreting these types of images, as well as MRI scan, CT scan, ultrasound, and X-ray images.
Uses for PET scans
PET scans are used alongside X-rays or MRI scans. They are used to make a diagnosis or to get more data about a health condition. As mentioned above, they are also useful for finding out how effective current treatment is.
The biggest advantage of a PET scan, compared with an MRI scan or X-ray, is that it can reveal how a part of the patient's body is functioning, rather than just how it looks. Medical researchers find this aspect of PET scans particularly useful.
PET scans are commonly used to investigate the following conditions:
It can reveal which part of the patient's brain is being affected by epilepsy. This helps doctors decide on the most suitable treatments.
PET scans are very useful in helping doctors diagnose Alzheimer's disease. A PET scan that measures uptake of sugar in the brain can significantly improve the accuracy.
PET scans can show up a cancer, reveal the stage of the cancer, show whether the cancer has spread, help doctors decide on the most appropriate cancer treatment, and give doctors an indication of the effectiveness of ongoing chemotherapy.
A PET scan helps detect which specific parts of the heart have been damaged or scarred. Any faults in the working of the heart are more likely to be revealed with the help of a PET scan.
Researchers, especially those involved in how the brain functions, get a great deal of vital data from PET scans.
Differences between PET, CT, and MRI scans
A CT or MRI scan can assess the size and shape of body organs and tissue. However, they cannot assess function. A PET scan looks at function. In other words, MRI or CT scans tell you what an organ looks like, while a PET scan can tell you how it is working.
How to prepare for a PET scan
In most cases, the patient does not have to spend the night in the hospital when having a PET scan. The majority of patients will be told not to consume any food for at least 4-6 hours before the scan, but to drink plenty of water.
Some will be asked to refrain from consuming caffeine for at least 24 hours before the PET scan.
What happens during a PET scan?
MRI or CT scans tell you what an organ looks like, while a PET scan can tell you how it is working.
A small quantity of radiotracer will either be injected into the patient's arm or breathed in as a gas. The radiotracer may take anything from 30-90 minutes to reach the targeted part of the body.
While waiting for the radiotracer to reach its destination, the patient will normally be asked to stay still and not talk. Some patients may be given some medication, such as diazepam, to relax.
When a patient is ready, they will be taken to the room where the PET scan is and will lie down on a cushioned examination table.
The machine has a large hole that the table slides into. Then images of the body are taken. In many centers, the patient will be able to listen to music during the scan.
While the scan is in process, it is vital that the patient keeps as still as possible. Depending on which part of the body is being scanned, the whole process takes from 30-60 minutes.
If the patient feels unwell, a buzzer can be pressed to alert the staff. During the whole process, the patient is being watched by staff.
The process is not painful.
Most patients can go home as soon as the scan has been done. Doctors advise people to consume lots of liquids to flush the radioactive drugs out of their system more quickly. Experts say the radiotracers should have left the body completely within 3-4 hours.
Who should not have a PET scan?
Pregnant women and women who are breastfeeding should not have a PET scan as there is a risk for the baby. Any woman who is pregnant should tell her doctor straight away (before the scan).
Anybody who has just had a PET scan should stay away from pregnant women, babies, and young children for a few hours afterward.