For several decades PET has demonstrated its value in providing non-invasive information of tissue at the molecular level. Although the value of PET lies in its high sensitivity tracking of biomarkers in vivo, it lacks precise anatomical information. This problem was addressed with the introduction of the PET/CT scanners in 2001. The PET/CT scanner has since gained widespread use and has had an enormous impact on the staging and treatment of especially patients with cancer. However, introducing PET/MR, can, perhaps, approach some of the limitations applying to PET/CT as well as providing a new tool for molecular imaging.
1) During diagnostic work up and follow up schemes in patients with good prognosis, especially pediatric patients, the amount of ionizing radiation from repeated PET/CT scans is a matter of concern [4
]. Due to the absence of ionizing radiation in MRI, PET/MR will reduce the dose associated with the examination substantially by eliminating the radiation dose from the CT, which generally accounts for approx. half of the dose associated with a PET/CT scan. This will be of especially importance in the handling of pediatric patients, but also adult cancer patients with a good prognosis, where the dose from repeated PET/CT scans can sum up to substantially amounts of ionizing radiation. Thus, in future scenarios the availability of PET/MR and an increased number of salvage therapies might render the clinicians more amenable to more frequent follow up with imaging.
2) CT provides excellent structural information, being a prerequisite for the widespread use of the PET technology. However, it has some limitations with regard to soft tissue and in areas with complex anatomy, i.e. the head and neck area and in the pelvis. MRI is widely used in the radiological imaging, as it provides excellent soft tissue differentiation, and in this aspect is considered superior to CT, allowing more precise radiographic measurements of tumor location, size and invasion. In addition to routine anatomical MR imaging a variety of MR acquisition sequences are available which can yield images of biophysical, pathophysiological or functional properties of tissues.
3) In PET/CT the duration of the CT scan is typically less than 1-2 minutes, whereas the PET scan lasts approx. 15-20 min from skull base to thighs. Thus, most of the time the CT scanner is not in use and PET acquisition governs the total scan time. With PET/MR both modalities acquire data simultaneously and total scanning time is normally governed by MR imaging, so that a longer PET acquisition time could be without time loss [6
]. This could make it possible to reduce the dose from the PET tracer without hampering sensitivity.
4) In the present PET/CT scanners CT and PET are acquired sequentially and not simultaneously. Sequential data acquisition and subsequent image fusion can hamper the possibility for accurate quantification and image interpretation due to misalignment. This is especially relevant in abdominal or thoracic studies, due to respiratory movements and bowel motion [7
]. Furthermore simultaneous imaging will also make it possible to examine the patient in exactly identical metabolic state and condition (). The sequential image acquisition also eliminates any temporal correlation between the two modalities, such as CT perfusion measurements and PET tracer kinetics.
Figure 2 Sagittal PET/CT and PET/MR images of a patient with cervical cancer (yellow arrow) and one pathological pelvic lymph node (green arrow): A: CT-scan, B: FDG-PET scan performed on the PET/CT scanner, C: Fused PET/CT image –note the mismatch between (more ...)
5) Acquiring PET and MRI data simultaneously allows essentially perfect temporal correlation of acquired data sets from both modalities. Basically, MRI already provides a large variety of protocols which selectively enhance contrast and thus visual discrimination among different tissue in vivo, and which can be used for dynamic contrast enhanced imaging, diffusion weighted imaging, functional MRI (fMRI) etc. Therefore, the combination of PET with MRI provides many advantages, which go far beyond simply combining functional PET information with structural MRI information. Some of the challenges will be to adapt the MR protocols to the PET acquisition time for each bed position and choose the right combination of PET tracer and MR imaging protocols.
Currently, no clinical indication for PET/MR has been established. In this paper we will focus on potential applications for, primarily simultaneous, PET/MR in the field of research and in the treatment of patients with extra cerebral solid tumors. Methods and tracers for use with the PET technology will be familiar to most readers of this journal; thus we will in the following give a short introduction to a number of different, more functional MRI techniques, which have shown promising results in the treatment of patients with solid tumors and could be applied together with PET increasing the amount of information about the tissues of interest.