Main abstract

Even though PET-MR has been around for slightly more than a decade, it is not highly available in all PET imaging centers worldwide due to the PET-MR’s high installation and maintenance costs and its acceptance in clinical purposes. Many PET-MR scanners are still being used for research purposes. Thus, seeking experience in setting up PET-MR protocols can be quite challenging and is limited to the research data accessibility and the type of research scans being done. This protocol-setting process can be quite daunting for first-timers and is also difficult for scans involving new tracers without a comparison study.
According to an article in the Journal of Nuclear Medicine’s (JNM) August 2016 issue, “since its first commercial installation in 2011, the number of facilities housing PET/MRI systems has increased to 70 worldwide.” In JNM’s August 2016 article, the authors found out from the different PET/MRI facilities, only a few centers use them for only either clinical (13 percent) or research purposes (19 percent) and the rest do a mixed approach of both clinical and research purposes. It is thus safe to conclude that currently, most PET-MR scanners are still being used for research purposes.
It is natural for the set up of the PET-MR scan protocol to be similar to the PET-CT protocol as the latter has been more established in the molecular imaging field and some PET tracers are used in both PET-MR and PET-CT imaging. Of course, for those PET tracers with shorter half-life, such as C11, it is preferably done on PET-CT scanners for faster scan time.
Creation of this checklist is based on working with the Siemens Biograph mMR 3-Tesla machine. This scanner can perform PET acquisition and MRI imaging simultaneously. This dual function serves to save time and allows the patient or subject to finish the scan in the shortest possible time. The downside is when the PET acquisition is being performed over a body region, e.g., the brain, the scanner cannot perform MR imaging of other body part and can only perform imaging of the brain until the PET acquisition of the brain is completed. This is due to ‘fixing’ the bed to a body part and preventing the scanner bed to move before the PET acquisition is completed.
From the beginning, when there is a new PET tracer study, one needs to decide if an internal dosimetry study needs to be incorporated before the actual study. It usually involves the acquisition of multiple beds covering from vertex of the head to the thighs. This will take at least 6 beds for an average adult height. It is recommended to do at least 3 timepoints of dosimetry on the SPECT scanner to show the bio-distribution uptake of the new tracers in the human being. However, as MR imaging is not as fast as SPECT acquisition, the PET timing per bed needs to take into account of MR shimming and the PET-MR requirement to do a MR Attenuation Correction (AC) sequence for the reconstruction of the PET AC images. One tip is, if the PET-MR scanner allows, for Dosimetry scans, or when dynamic PET acquisition is required, do not start PET acquisition with ‘Start MR’ at the same time. In the planning, choose PET acquisition to start when you have clicked on the save and go button. Do not trigger PET acquisition with ‘Start MR’ at the same time as MR sequences will take time to shim and this will delay the start of PET acquisition time.
It is recommended to do PET acquisition starting from the thigh to the vertex (caudal-to-cranial approach) as the patient’s bladder will continue to be filled with urine due to the patient’s physiologic urinary function The distended bladder might obscure lesions and show as a ‘hot spot’ as time passed. Hence, it is imperative to acquire the patient’s pelvic region as soon as possible rather than starting from cranial-caudad direction.
The main discussion will elaborate on more learning points with protocol checklist.
As there is no denying that PET-CT imaging has been more established than PET-MR imaging for clinical and research purposes, this checklist hopes to boost the users’ confidence in setting up the PET-MR protocols. As more PET-MR radiographers and researchers have established their PET-MR protocols and tested them well with the checklist, they would have saved time in streamlining their protocols and are strongly encouraged to share their protocols with others for data comparison and future improvement.