• 10/15/2004
  • Landis K. Griffeth, MD, PhD
  • Cure Magazine, Fall 2004

PET/CT is the latest innovation in oncology imaging, but is, in reality, just the combination, in a single device, of the two most useful imaging instruments in cancer care. In order to understand the benefits of a PET/CT device, we first must consider the two pieces separately.

Computerized tomography (CT) scanning has been the primary tool of oncologic imaging for three decades. It is unrivaled in demonstrating the precise anatomy of the structures within the body, including tumors. CT works by passing X-rays through the body, while the X-ray source and the X-ray detector (on the opposite side of the body) swing in a circle around the patient’s body. The scanner measures the absorption of the X-rays along all these angles passing through the body and, using special computer programs, constructs images that are “slices” through the body, showing great anatomical detail.

A general term for imaging slices of the body is tomography. Virtually all tomographic types of imaging studies today are computerized, but CT was the first of this type. Advances in CT scanners in recent years have made it such that these scanners can now make very high-resolution images (very thin slices through the body) and can cover large parts of the body very quickly as the patient on the table travels through the “doughnut” of the CT scanner gantry.

The limitation of CT is that, as accurate as it is for showing the internal anatomy of the body, the anatomy is frequently not the whole story. For example, many times, especially after treatment, residual live tumor cannot be distinguished from dead tumor or scar tissue on CT. In other cases, CT often cannot distinguish tumor from normal tissues.

Positron emission tomography (PET) offers an alternative means to look at such processes. Though PET has only seen widespread use in the past six to eight years, it actually was invented in the 1970s. PET, like most other nuclear medicine studies, images the physiology and biochemistry of the body rather than the anatomy.

With PET, the body is encircled by multiple rings of PET detectors, which record the “emission” of a small amount of radiation from special radioactive atoms (“positron” emitters) that are chemically attached to biologically active molecules and administered to the patient. Thus, the PET machine makes “tomographic” images (slices) by detecting the radiation “emission” from “positron” emitters.

The vast majority of current oncologic PET studies utilize a compound called F18-fluorodeoxyglucose, or FDG, which allows us to trace the rate of glucose metabolism in the cells. Since most cancer cells use more glucose than most normal cells, tumors typically show up as hot spots on the PET scan.

PET has recently shown to be extremely useful in managing many types of cancer and will likely see increasing use in the future, especially for follow-up after therapy. Overall, for the tumors for which PET scanning is approved, PET generally shows a 20 to 30 percent improvement in accuracy of cancer imaging over CT alone.

The downside of PET is that its images do not have the high resolution of CT (they are fuzzier than CT images) and they don’t outline the anatomy of the body very well. Thus, PET might show that an abnormality suspicious for cancer is present, but might not show exactly where it is. CT, on the other hand, might not show it at all.

One way to overcome this is to try using special computer software to “register” or “fuse” the two different data sets, so that the hot spot on PET can be referenced to a location on the CT. This approach has some benefits, but this technique often has limited accuracy and does not do the job of localizing the abnormalities as well as an expert PET reader can do simply by visually comparing the PET with the CT. The best-of-both-worlds approach is to combine both scanners together in one gantry. Using precise calibration, this means the CT and PET images, obtained sequentially on the same scanner, are aligned almost perfectly, allowing for a small amount of inevitable patient motion during the scanning session.

This combined data set has proved very helpful in interpretation of the PET images. PET alone proves very accurate, but PET/CT is about 4 to 8 percent more accurate than PET in, for example, staging of cancer. However, the precise localization of tumor, which can be especially important if biopsy or radiotherapy is needed, has been shown to be up to 20 to 40 percent higher in some studies. Moreover, the confidence of the interpretation of the PET/CT (e.g., classifying a finding as “definitely abnormal” versus “probably abnormal”) is significantly higher with PET/CT.

As with all other advanced diagnostic and therapeutic techniques, the expert performing the procedure is still much more important than the technology itself. Even with PET/CT, there are many pitfalls to image interpretation that can only be recognized after considerable training and experience.

There are other issues related to PET/CT. By combining the two machines in one, the “doughnut” through which the patient must pass is much thicker than with either machine alone. While this still does not approach the “tunnel effect” of the typical MRI scanner, this can be a problem for claustrophobic patients. Latest-generation PET/CT scanners have partially addressed this problem by widening the hole of the doughnut, but claustrophobic patients generally would benefit from a mild relaxant.

A side benefit of current PET/CT scanners is that scan time is usually shorter than with a PET machine, with scans often completed in 30 minutes or so (depending on patient size and other factors), as opposed to the typical 50 to 60 minutes for a typical PET. Patients should remember, though, that the “uptake time” between the injection of FDG and the beginning of imaging would remain about 60 to 90 minutes. Only the scanning time is shorter.

As the equipment and, especially, the computer software of this new combined modality advance, it is likely even shorter and more convenient imaging protocols will be possible.

Landis K. Griffeth, MD, PhD, is the director of nuclear medicine at Baylor University Medical Center in Dallas and medical director of North Texas Clinical PET Institute.