What Is A PET Scan? How Does A PET Scan Work?

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Main Category: MRI / PET / Ultrasound
Also Included In: Radiology / Nuclear Medicine
Article Date: 22 Jun 2009 - 10:00 PDT

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A PET scan uses radiation, or nuclear medicine imaging, to produce 3-dimensional, color images of the functional processes within the human body. PET stands for positron emission tomography. The machine detects pairs of gamma rays which are emitted indirectly by a tracer (positron-emitting radionuclide) which is placed in the body on a biologically active molecule. The images are reconstructed by computer analysis. Modern machines often use a CT X-ray scan which is performed on the 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.

How does a PET scan work?

Radiotracer - Before carrying out a PET scan, a radioactive medicine is produced in a cyclotron (a type of machine). The radioactive medicine 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 inserted into the human body.

When it is inside the radiotracer will go to areas inside the body that use the natural chemical. For example, FDG (fluorodeoxyglucose - a radioactive drug) is tagged to glucose to make a radiotracer. The glucose goes into 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.

Detecting positrons - A PET scan detects the energy emitted by positively-charge particles (positrons). As the radiotracer is broken down inside the patient's body positrons are made. This energy appears as a 3-dimensional image on a computer monitor.

The image - The image reveals how parts of the patients 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 his/her findings to a doctor. A radiologist is a doctor who specializes in interpreting these types of images, as well as MRI scan, CT scan, Ultrasound and X-ray images.

Why are PET scans required?

PET scans are generally used alongside X-rays or MRI (magnetic resonance imaging) scans. Doctors use PET scans as a complementary test to these main ones. They are used to make a diagnosis or to get more data about a health condition. As mentioned above, they are also useful in finding out how effective current treatment is. The use of combined imaging technologies may hold the key to stopping - and even preventing - heart attacks, a study revealed.

The biggest advantage of a PET scan, compared to 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:

• Epilepsy - it can reveal which part of the patient's brain is being affected by epilepsy. This helps doctors decide on the most suitable treatments.MRI and/or CT scans are recommended for people after a first seizure, this study explains.


• Alzheimer's disease - it is very useful in helping the doctor diagnose Alzheimer's disease. A PET scan that measures uptake of sugar in the brain significantly improves the accuracy of diagnosing a type of dementia often mistaken for Alzheimer's disease, a study revealed.


Interesting related articles:

What is MRI? How does MRI work?

What is a CT scan? What is a CAT scan?
• Cancer - 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 on the effectiveness of ongoing chemotherapy. A PET scan several weeks after starting radiation treatment for lung cancer can indicate whether the tumor will respond to the treatment, a study showed. This article looks at whether PET scans are beneficial during cancer diagnosis, staging and monitoring.


• Heart disease - 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. A study revealed how comprehensive diagnosis of heart disease based on a single CT scan is possible.


• Medical research - researchers, especially those involved in how the brain functions get a great deal of vital data from PET scans.

What is difference between a PET scan and CT or 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 is looks like, while a PET scan can tell you how it is working.

What happens during a PET scan?

In most cases, the patient does not have to spend the night in hospital when he/she comes in for a PET scan.

The majority of patients will be told not to consume any food for at least four to six hours before the scan, but to drink plenty of water. Some will be asked to refrain from consuming caffeine for at least 24 hours prior to the PET scan.

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 minutes to 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 the patient is ready he/she 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 which 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 keep as still as possible. Depending on which part of the body is being scanned, the whole process takes from about 30 to 60 minutes.

If the patient feels unwell he/she can press a buzzer which alerts 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 three to four hours after entering the body.

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 after the scan.

Written by Christian Nordqvist
Copyright: Medical News Today
Not to be reproduced without permission of Medical News Today