Whereas the scans acquired through Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) provide clear images of the structure and form within the body's interior, PET scans unveil functional details of its dynamic working. "This is the difference between an anatomical and a functional examination" explained Dr. Laurens Carp from the Nuclear Medicine Department at UZA. "Radiologists generate structural images by sending different external rays to the patient's body, their type depending on the kind of exam. In turn, nuclear medicine focuses on the visualisation of functional processes. PET equipment registers rays captured via the body after the patient is injected with a very small dose of radioactive tracer liquid. Both approaches are complementary. PET scans are only produced after less expensive diagnostic exams have been performed first, with the sole objective to acquire more relevant medical information", stated Dr. Carp.
Cancer cells multiply faster than normal cells, forcing them to spend more energy and glucose. In order to detect cancer cells and possible metastasis, physicians have to accurately map the glucose consumption in the body via PET. The tracer liquid consists of a "cold" and "hot" component. The body-specific cold component is selected according to the physical process or the body part which has to be examined. Glucose, for instance, is utilised to check the viability of the heart or to study the body's glucose metabolism. The warm component is a radioactive isotope, allowing the PET device to trace and visualise the distribution of the cold component throughout the patient's body. The UZA physicians exclusively use fluor-18 as tracer. The radiation level is very low and temporary as not to endanger the patient.
Once the body has sufficiently taken up the tracer, the patient is moved into the PET device where the body is scanned in phases via the detector crystals in the PET ring. UZA's Siemens Ecat Art equipment is able to scan the body part by part while the tracer is emitting positrons that immediately fuse with an electron. The total mass of both particles is transformed into the energy of two powerful centrifugal gamma rays. Each separate pair of gamma rays is registered by the detector crystals and the data is processed by the advanced PET software, generating an image of all the scanned body parts in which the tracer has penetrated. Cancer cells are more sensitive to the tracer because of their high level of glucose. In the PET image, the areas affected by cancer therefore show a darker colour than the healthy body parts.
PET constitutes a patient-friendly imaging modality without any unpleasant side effects. Applications range from tumour checking to view whether it is benignant or malignant, to the tracking down of metastasis, or defining the growth in the neighbourhood of a removed tumour to check out whether it is merely the effect of scarred tissue or the cancer that has returned. With PET scans, it is also possible to evaluate the success of a cancer therapy. The usefulness of PET imaging for early detection of the disease depends on the type of cancer. The traceability of lung and intestinal cancer is excellent but for organs like kidney or bladder, PET techniques are limited to visualising metastasis only. Since part of the tracer is removed in a natural way through these organs, its concentration is always very high in these areas and this automatically generates a darker colour in the image.
For opposite reasons, PET is barely helpful to visualise prostate tumours. In older men, the disease only slowly develops, causing cancer cells to have a glucose consumption too modest to be clearly visible on a PET scan. Body parts for which PET imaging shows satisfying results are the glands, skin, head, neck, brain, pancreas, esophagus, and breast. In contrast with CT or MR images, PET scans reveal little anatomical details. Sometimes, those details are crucial to figure out whether a tumour is located inside or just beside the organ. The combination of PET scans with CT or MR however remains difficult. Even the tiniest deviations in a patient's position during the various exams might lead to important differences in the position of the organs. Two images acquired with different modalities therefore cannot be superimposed just like that.
Efforts to develop software that allows a reliable fusion between images from different modalities have not been successful so far. An alternative is to build devices which are suitable to simultaneously generate PET and CT scans. In this way, it is possible to anatomically locate functional deviations, detected by PET, in a very precise manner. Professor Pierre Blockx, head of the UZA Department of Nuclear Medicine, however is not very inclined to buy such a device. "The PET-CT combination is just as expensive as a PET device plus a CT device, but with two separate scanners, our department is able to examine more patients. The PET-CT combination would only serve a small group of patients. Another factor to take into account is that the technical evolution of PET and CT is not running parallel. This would force us to buy a whole new combination each time a breakthrough in PET or in CT would be realised."
From a medical and technical point of view, the PET-CT combination looks very attractive but financially, it is not evident. "Given the fact that UZA already has 2 CT scanners, the PET-CT device would only be used for PET scans in daily practice", explained Dr. Blockx. "One can think of other expensive combinations, such as MR-CT which would perhaps be more useful to patients, or even PET-MR. This sounds all very promising but we should remain realistic. In the present situation, I would prefer that IT companies invest further in the development of reliable software for image fusion." Dr. Blockx also added that each technology has its limitations. The resolution of a PET scan amounts to some 7 millimetre. Small tumours or affections cannot be detected unless they take up the radioactive tracer in a highly elevated measure. "Of course, we would like to detect even one single cancer cell, but at the current stage, this is wishful thinking."