PET Scan: What it is and What to Expect

Your doctor has referred you for a PET scan. What exactly is a PET scan and what should you expect?

A PET scan, known more formally as Positron Emission Tomography, is a diagnostic nuclear medicine procedure which produces functional images of various body processes. Though often combined with a Cat Scan or MRI, the information provided by PET differs significantly. While CT (Cat Scan) and MRI primarily provide superior images of the anatomy, PET will measure body functions such as oxygen use, blood flow, and metabolism. This information can basically allow the physician to distinguish between a malignant or non malignant mass. Put in a simpler terms, a Cat Scan may show the presence of a mass in the body. A PET scan can determine if the mass is cancerous. Many PET scanners today utilize a combination of PET and CT. When performed together as one procedure they can more accurately localize an area of abnormal metabolic activity than when done separately.

To produce functional images, the PET scan uses a small amount of radioactive material called a radiopharmaceutical or radiotracer. While the amount of radiation in the tracer is too low to have any adverse effects to the body, women who are pregnant or breastfeeding may not be candidates for PET scanning. Dependent upon the body system being imaged, the radiopharmaceutical is either injected, swallowed, or inhaled by the patient. The radioactive material will move through the body and collect in the organs or tissues that have been targeted for the study. The scanner or gamma camera will detect the radioactivity emitted by the target area of the patient and convert it to an electrical signal which is then processed by the computer. The final result is a digitally produced three dimensional image of the scanned area of the patient’s body. An area seen of increased radiopharmaceutical activity or “hot spot” on the image can often indicate a cancerous area.

For the patient, other than a simple IV injection if called for, the PET scan itself is painless. In general, instructions will be given when the exam is scheduled and the patient will be asked to sign a consent form. On the day of the procedure the patient will be instructed to not eat or drink anything for several hours prior to the exam and to wear loose comfortable clothing. To begin the exam, the patient will first be given the radioactive tracer which can take up to an hour to collect in the desired area of the body. When ready the patient will be taken into the scanning room and instructed to lie on a table and remain as still as possible. The table will slowly slide several times through a large donut shaped hole in the machine called a gantry. Unlike MRI, the machine does not make any unusually loud noises and the patient will not remain in the gantry for long periods of time. The examination typically takes from thirty minutes to an hour, after which patients are advised to resume normal activities and to drink several glasses of water to flush the radiotracer from their system.

PET scans are most frequently indicated in people with cancer, heart disease, and central nervous system disorders and can often provide an earlier diagnosis than other imaging modalities. They are of particular use in cancer patients as a method of diagnosing cancer, determining whether a cancer has spread, evaluating cancer’s response to treatment, or confirming a recurrence. In heart disease the PET scan can be useful in assessing the damage to the heart after a heart attack, determining blood flow to the heart, and preplanning for angioplasty or coronary bypass procedures. PET scans can show which areas of the brain show the most activity during specific tasks and are helpful in evaluating seizures, tumors, and other central nervous system abnormalities.

It is important for the patient to understand that, as extremely useful as PET scan images may be, they are just one of many tools your physician will use to determine your diagnosis and treatment. Not all cancers will show up on a PET scan, nor will all PET scans provide and immediate an accurate diagnosis.

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