Beyer, Thomas

Ratib, Osman

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EDITORIALWhole-body hybrid PET/MRI: ready for clinical use?Osman Ratib & Thomas BeyerPublished online: 5 April 2011# Springer-Verlag 2011Since its introduction in 1998 [1], combined PET/CT hasgained clinical acceptance as an oncology imaging modal-ity providing superior accuracy in tumour staging andassessment of oncology therapy response compared to PETor CT alone [2, 3] and to both imaging modalities viewedside-by-side [4, 5]. The PET technology available incombined PET/CT has advanced significantly over theyears and now supports emission imaging of the torso fromhead to mid-thigh with isotropic sub-centimetre resolutionin 10 min, or less. The integration of high-end multidetectorCT in dual modality tomographs marks a trend towardsperforming contrast-enhanced diagnostic CT studies, whichhas progressively increased the clinical relevance ofintegrated PET/CT imaging protocols. Undisputedly, thesechanges in diagnostic pathways require the close collabo-ration and even cross-specialist training of nuclear medicinespecialists and radiologists.With the concurrent evolution of MR imaging technol-ogies and sequences, MR studies can provide a wealth ofdiagnostic information regarding soft-tissue analysis, tu-mour detection, tissue characterization and functionalimaging. It is not uncommon today for patients to undergoan MR study in addition to a PET/CT study as part of theirclinical workup for detection and staging of cancers if soft-tissue characterization and morphological information thatCT alone cannot provide are required. It is, therefore, quiterational to anticipate that combined PET/MR examinationswill potentially become more valuable than PET/CTimaging followed by a complementary MR examination[6] in PET indications where MR outperforms CT. It haseven been suggested that whole-body MR imaging(WBMRI) could replace PET/CT [7]. However, such thesetypes of enthusiastic reports on the excellent diagnosticperformance of stand-alone MR imaging are based mostlyon using diffusion weighted (DW) sequences that showparticularly high sensitivity at the cost of low specificity,and, therefore, may be of limited value in a cost-conscioushealth-care system.Technical innovation in combined PET/MRThe idea of combining PET and MR arose around thesame time that PET/CT was conceptualized. In the early1990s Simon Cherry and Paul Marsden saw a need forPET/MR in small-animal imaging studies to combinehigh soft-tissue contrast with molecular informationprovided by PET [8, 9]. PET/MR was destined to remainin the preclinical arena for another decade until, in 2006,the first simultaneous MR and PET images of the humanbrain were acquired [10]. A major challenge for bringingPET and MR technologies physically together is theinterferences between the high-field magnets and theelectronics of the PET scintillation detectors [11]. Onesolution for PET detectors to be compatible with MR, is toreplace photomultipliers with avalanche photodiodes(APD) that are less sensitive to magnetic fields, or to usealternative solid-state technologies, which have beenproposed in prototype animal PET systems [12]. It isO. Ratib (*)Department of Medical Imaging and Information Sciences,University Hospital of Geneva,24, rue Micheli-du-Crest,1205 Geneva, Switzerlande-mail: osman.ratib@hcuge.chT. Beyercmi-experts GmbH,Zurich, SwitzerlandEur J Nucl Med Mol Imaging (2011) 38:992–995DOI 10.1007/s00259-011-1790-4worth noting that, unlike combined PET/CT, dual modal-ity PET/MR started out in the preclinical arena beforebeing adopted clinically, much supported by industry.Today three concepts for human PET/MR systems arevividly and often controversially discussed: separate PET,or PET/CT and MR gantries operated in different rooms(A), PET and MR gantries arranged in the direction of themain scanner axis with a patient handling system mountedin between (B), and a fully integrated PET/MR systemwith simultaneous acquisition (C).The integrated design (C), first presented in 2006, andalso the most challenging, is based on a PET detector ringdesigned as an insert that can be placed inside a 3-T MRscanner (Siemens Healthcare). This prototype system(BrainPET) was anticipated for brain imaging only [10].The PET insert has an internal diameter of 35.5 cm andcomprises LSO (lutetium oxyorthosilicate) detector blocksmade of 2.5-mm crystals coupled to an array of APDs thathave been shown to be operational in magnetic fields of upto 7 T [8]. Four such units are now in research operation atthe University of Tübingen, Forschungszentrum Jülich,MGH Boston and Emory University Atlanta.The coplanar PET/MR concept (B), first presented in2010, is based on a tandem design of a whole-body time-of-flight (TOF) PET system and a 3-T MR system (PhilipsHealthcare) with a rotating table platform in between.Through minor modifications of the PET detector system(e.g. orientation of the photomultiplier tube, minor shield-ing) the PET gantry can be operated in close proximity tothe 3-T MR system. Two such whole-body units have beeninstalled and tested in clinical routine in Mount SinaiHospital in New York and at the University Hospital ofGeneva, and have been certified since January 2011 forroutine clinical use.Design (A) was proposed by GE Healthcare in late2010. It is available only as prototype technology andhas been installed and tested at the University Hospitalof Zurich. This design is based on a combination of adual modality PET/CT and a 3-T MR system that areoperated side-by-side in separate adjacent rooms; patientsare shuttled from one system to the other without gettingoff the bed.In an extension to the integrated design concept (C), asimilar system was proposed in late 2010 that merges awhole-body PET with a 3-T MR system to allowsimultaneous torso and whole-body imaging. Just like theBrainPET PET/MR prototype, this system is based onLSO-APD PET detector technology that is integrated intothe MR gradient coil system offering a 60-cm gantryopening (versus a 35-cm gantry opening for the brainprototype). Since December 2010 two prototypes have beeninstalled in clinical settings at the Technical University ofMunich and the University of Tübingen.PET/MR or PET/CT?Since the introduction of PET/CT into clinical practice in1998, clinical evidence for the diagnostic superiority ofPET/CT over CT and PET acquired separately has beengrowing. This pertains to a number of indications, primarilyin oncology [13–15]. It is conceivable that similar advan-tages over stand-alone imaging could be expected fromcombining PET with high-resolution MR images. However,the acquisition of whole-body MR data with adequateanatomical resolution and within a reasonable total exam-ination time, matching that of a whole-body PET/CT studyremains a challenge. As such, the existence of PET/CT maybe the biggest challenge to the concept of PET/MR, both interms of the diagnostic accuracy and logistical benefits.Nonetheless, in several instances the information pro-vided by MR cannot be obtained by CT [16]. For example,MR is advantageous in soft-tissue characterization, such asin parenchymal organs, the brain and bone lesions.Therefore, the concept of combining PET and MRcontinues to be of interest and clinical attention willincrease in particular as a combination of whole-body MRtechniques with specific tracer imaging in PET may provideincremental diagnostic accuracy. In addition, further clini-cally relevant information can be gathered by MR withinthe same examination procedure. For example, advancedMR imaging techniques can provide functional informa-tion, for example MR spectroscopy (MRS), diffusion-weighted and perfusion imaging, and functional MRimaging (fMRI). As an example, MR breast imaging hasa high sensitivity in the detection of breast lesions but arelatively low specificity [17]. In comparison, [18F]FDGPET/CT imaging has gained a significant role in thedetection and staging of breast cancer patients with ahigher specificity than MR mammography [18, 19]. Fusionof MR and PET data should, therefore, combine the highsensitivity of MR imaging for detection of breast lesionsand the high specificity of PET for differentiating a benignfrom a malignant lesion [6]. Other domains for combinedPET/MR are likely to be prostate cancer and head/neckcancer imaging. Finally, replacing the CT in PET/CT withMR could significantly reduce the effective patient expo-sure, although this may be less relevant in oncology and inelderly patients given their reduced life expectancy. It is,however, beneficial in younger patients with potentiallycurable disease and in patients with non-oncologicalindications.Despite the promising potential of dual modality PET/MR, a number of methodological and clinical concernsawait resolution before PET/MR can become more widelyadopted in clinical routine. First, this relates to thederivation of attenuation correction (AC) factors fromPET/MR data. Several techniques for calculation MR ACEur J Nucl Med Mol Imaging (2011) 38:992–995 993based on MR image segmentation of different tissuecharacteristics of the body have been implemented andtested [20], but numerous technical challenges for accurateAC remain, including compensation for MR image trunca-tion and correction for RF coils and accessories. Second,PET/MR imaging protocols have a direct impact on patientthroughput. Adding emission acquisition time and multi-sequence clinical MR imaging time could result in totalexamination times that are significantly longer than those ofa comparable PET/CT study. Therefore, the combination ofPET and MR in a single imaging modality requiresmodified imaging protocols that yield competitive exami-nation times. Otherwise, two already long examinationtimes will add up to an examination time not acceptable inclinical routine. While advanced MR imaging sequencescan provide relevant diagnostic information, they cannot beapplied to whole-body imaging. Specific imaging sequen-ces may have to be restricted to organs and body sectionsthat benefit most from such imaging protocols, such asbody regions with an abnormal metabolic pattern as seen onPET. Conversely, with higher efficiency PET systems usingTOF technology, imaging time can be reduced essentiallyby half while maintaining equivalent or better image qualitythan non-TOF PET systems. This should allow shorter PETimaging times that leave more flexibility for additionalcomplementary MR sequences to be performed. Recently ithas been suggested that TOF can also be helpful inreducing the effects of inconsistent data, such as erroneousnormalization, poor scatter estimates and mismatched orinaccurate attenuation maps [21].Most clinical protocols today require between two andfive additional MR series with dedicated protocols onspecific organs or body sections. These additional sequen-ces can also be completed in under 30 minutes, allowing adiagnostic PET/MR scan to be completed within an hour.While such a study is longer than a standard diagnosticPET/CT study, it is still shorter than the total time that aPET/CT and an MRI study would take today if performedon two separate systems at different times. This clearlybenefits patients who need both PET/CT and MR studies intheir clinical workup. We also believe that using the resultsof the PET study to identify areas where additional MRsequences must be applied could significantly reduce thetime for a diagnostic MR scan by limiting the number andextent of these additional MR sequences by applying themonly to limited areas where suspicious PET uptake isidentified. Other challenges for the routine use of PET/MRinclude methodological aspects, such as the fact that CT isthe modality of choice for the characterization of lunglesions. In some patients with metal implants and pace-makers, MR imaging may not be employed at all. Also, thenumber of patients unable to undergo the study due toclaustrophobia is higher with MR than with CT.Overall, we believe that in oncology applications MRimages can bring additional relevant diagnostic informationto PET images and the combination of the two modalitiescan offer more accurate and specific diagnostic perfor-mance than PET/CT alone [6]. Coregistered anatomy andmetabolic images should enable better staging and moreaccurate lesion identification in a variety of tumour entitiessuch as liver and bone metastases, brain tumours, andprostate, gynaecological, breast and head and neck cancers.In thoracic tumours, however, PET/CT will still bepreferred. However, the added diagnostic value providedby MR over CT and PET must be proven to be valuable andvaluable in the context of a presumably longer imagingtime if PET/MR is performed instead of PET/CT.In follow-up studies of patients undergoing specificoncological treatment, PET/MRI could become the modal-ity of choice offering a more accurate and quantitativeassessment of patient tumour response to treatment. Specialprotocols focused on specific body regions could use muchshorter MR sequences than those used for diagnosticpurposes. Repeated PET/MRI studies as part of patienttreatment monitoring would have the major advantage ofmuch lower radiation exposure than conventional PET/CTused in most cases today. With the emergence of newtracers, it is also expected that more specific and accuratePET protocols will emerge for diagnosis and follow-up ofcancer, and also for prediction and assessment of patientresponse to treatments [22]. The combination of PET andMR images may be ideal for anatomical and metabolicassessment of protocols and clinical pathways for oncology[8].While hybrid PET/MR may certainly not replace PET/CT in all cases, it has the potential to improve workflowand diagnostic accuracy in patients who already requireboth a PET/CT and an MR examination as part of theirclinical workup.References1. Townsend DW, Beyer T, Kinahan PE, Meltzer CC, Brun T, NuttR, et al. The SMART Scanner: A combined PET/CT tomographfor clinical oncology. Radiology. 1998;209P:169–70.2. Collins CD. 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Whole-body hybrid PET/MRI: ready for clinical use?

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Whole-body hybrid PET/MRI: ready for clinical use?

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