There are three types of PET available in the marketplace today: PET-only, PET plus CT fusion (PET+CT) and PET/CT. PET-only is the older technology where no anatomical imaging is done. With PET+CT, PET is done on a PET-only system followed by a CT scan in a standalone CT scanner, with the two image sets fused using specialized software. With PET/CT, PET and CT are acquired, nearly simultaneously, in the same gantry. Although the steps from PET-only, to PET+CT to PET/CT may seem small, the impact is profound and far reaching. PET/CT is considerably more than the sum of its parts.
One considerable advantage of PET/CT is that the time the patient must lie motionlessly in the scanner is reduced from about 40 minutes to 20 minutes. The reason for this is that the CT part of PET/CT also is used to correct for attenuation in the PET image. Like all nuclear medicine, with PET the x-rays originate from radioisotopes that are inside the body. Some of those x-rays are absorbed by the body (attenuated) which distorts the PET image. It is absolutely essential that this distort is corrected. Prior to PET/CT, PET scanners had the equivalent of a very crude CT scanner built-in that used the PET detectors as crude CT detectors. This CT-equivalent was not only very slow, but the “images” it produced were non-diagnostic and the patient was exposed to additional radiation. With PET/CT, these attenuation correction images can be acquired in a few seconds rather than a few minutes and they are fully diagnostic. In addition, this increase in speed dramatically increases patient tolerance and reduces motion artifacts.
PET/CT is also a more accurate test. With FDG PET, there are many sources of normal variants in the uptake of FDG (see Shreve 1999). The addition of fully diagnostic CT provides an anatomical context which greatly reduces the false positive rate. PET+CT fusion claims to do this, but since the patient must be moved between the PET and the CT scans, it is very difficult to get the patient in the same position in each scanner. This significantly reduces the accuracy of the co-registration of the two image sets.
Finally, PET/CT provides accurate, anatomical landmarks to guide biopsy, surgery and even radiation treatment planning.
In conclusion, PET/CT is faster and more accurate all at the same price. If there is PET/CT in your area, there is no reason to do PET.