How Does PET CT Work?

PET CT is a combination of a PET scan and a CT scan obtained at one time, by a single piece of equipment. Positron emission tomography, or PET, is a painless diagnostic test that allows radiologists a unique view of the body's biological functions. The PET scan differs from an x-ray or CT image in that it looks at the body's metabolic activity and provides important information about its internal physiology. 

Almost all diseases alter the body's biological processes. PET is able to discover these changes in their earliest stages, often before any symptoms appear. Effective treatment plans can be initiated sooner with information on early developing cancers. PET can sometimes eliminate the need for other invasive procedures and, by correctly staging cancers, may prevent unnecessary surgical procedures. Cancer cells have higher metabolic rates than normal cells, and show up as denser areas on a PET scan. 

During a PET scan, a radioactive tracer is injected into the body and is tracked as it moves through organs. The tracer is a radiolabeled form of glucose known as fluorodeoxyglucose. The radiation exposure associated with PET is safe and lower than that associated with conventional CT scanning. The PET scanner detects and records the signals the tracers emit. These signals are then reassembled into actual images through a computer.

Unlike the PET scan, the CT scan very accurately evaluates anatomy. By combining these studies, physicians can take advantage of the strengths of both modalities. Before a scan begins, a radiopharmaceutical (tracer), which is comprised of a radiolabeled form of glucose, is injected into the patient. The patient then waits about 75 minutes for the tracer to distribute within the body.

Next the patient is placed on a table that moves into the scanner and undergoes a very fast CT scan. This takes less than a minute in most circumstances. The table then moves the patient into the PET scanning portion of the camera. The PET scanner consists of hundreds of radiation detectors that surround the patient. Using the emissions given off by the injected radionuclide, the PET scanner measures the amount of metabolic activity at a site in the body and a computer reassembles the signals into three-dimensional images of tissue function. The PET scan portion of the exam takes a little more than 20 minutes. The entire appointment lasts about 3 hours for most patients.