Understanding Combination PET/CT Technology

PET Technology Defined
Unlike conventional imaging procedures that measure the structure of an abnormality, PET measures the metabolic changes that occur in cells when disease is present. The atoms, molecules and cells of our body have particular behaviors and chemical processes. At the onset of disease, these behaviors change, often before a detectable physical difference like a tumor occurs.

This is because cancer cells behave differently than normal cells. PET technology allows us to detect these subtle changes and identify cancer at its most basic cellular level. PET is limited in that it doesn’t tell us the size or shape of the abnormality. Patients still need to undergo other procedures like CT scans for those answers.

CT Technology Defined
CT or Computed Axial Tomography, formerly known as a CAT scan, combines x-ray technology with advanced computer acquisition to uncover the precise form and location of an abnormality. CT is one of the primary tools of measurement in oncology evaluation. It is extremely fast and yields minutely thin, cross-sectional views (slices) of the body.

The Fusion of PET/CT Technology
PET, as we have learned, shows the miniscule changes in the body’s metabolism caused by the growth of abnormal cells. CT images allow physicians to pinpoint the exact location, size, and shape of the diseased tissue or tumor.

In the past, PET and CT images were performed independently, laid side by side and compared. However, difficulties arose when trying to match up the results of a CT scan done at a different time and location with the PET scan. Minor physiologic changes such as gas content in the stomach which occur from day to day can cause organs to shift slightly. Often times, the tables of the machines are different resulting in different positioning of the patient’s body. A slightly different angle of the head can be enough to pose problems in comparison.

The new PET/CT scanner acquires both pictures during the same exam all but eliminating these concerns. Sophisticated software then fuses the images. The result is a full body view showing the presence or absence of disease, how active it is, whether or not it has spread and precisely where and how large an abnormality is.

PET/CT at EPIC Imaging
In 1999, EPIC Imaging became the first facility in Oregon to acquire a dedicated PET scanner. In 2003, EPIC again became the first facility in the region to upgrade to the new combination PET/CT scanner. Later EPIC upgraded again to the first high definition PET/CT in the region. High definition PET/CT represents a dramatic advance in the technology, providing a clearer picture than was previously possible. The result is the earliest possible measure of the success of treatment and improved resolution to see lesions as small as 2mm. Scan times are 10 minutes or less.

How PET/CT Works
The CT portion of the exam is performed first. X-ray beams pass through the body from many different angles and sophisticated sensors measure the amount of radiation absorbed by the different tissues of the body. State-of-the-art software then uses the difference in the x-ray absorption of the tissues to form cross-sectional images or slices called tomograms. The sophisticated computer allows us to conduct accurate measurements and magnify and enhance images.

PET then measures the chemical changes that occur in the body when disease is present. The images are created with the help of three important tools: the injection of a radiopharmaceutical, the scanner and a computer. A safe, short-lived radiopharmaceutical, known as FDG, is injected into the body. FDG is tagged to a glucose molecule because cancer cells are highly metabolic and use more glucose (sugar) than normal cells. It is this increased glucose activity we are attempting to measure.

The scanner records the FDG as it accumulates in different cells in the body and sends that information to a computer. The computer reconstructs the signal into three-dimensional images of the body clearly demonstrating any increases in the glucose activity of the cells throughout the entire body and all of the body’s internal structures.

Once the images are acquired, the data is processed on a special workstation which performs three-dimensional fusion of the PET data with the CT data. The fused image represents the most comprehensive view of both the form and function of disease available today.

How We Use It
Most of the patients coming to EPIC are here to evaluate the progression of a cancer, check for a disease recurrence, to monitor the outcome of therapy or to identify whether tissue is cancerous or not. PET/CT can save patients unnecessary surgical procedures, recovery time and expense.

The future of PET/CT holds many more exciting applications. The technology continues to be recognized in other areas of medicine like neurology and cardiology as an important diagnostic tool. Current clinical applications include:

• Diagnosing and staging cancer
• Determining whether a tumor is benign or malignant
• Differentiating a recurrent brain tumor from radiation necrosis
• Diagnosing early dementias
• Localizing sources for intractable seizures