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PET/MRI for Neurologic Applications

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PET/MRI for Neurologic Applications Journal of Nuclear Medicine, published on November 9, 2012 as doi:10.2967/jnumed.112.105346 CONTINUING EDUCATION PET/MRI for Neurologic Applications Ciprian Catana1, Alexander Drzezga2, Wolf-Dieter Heiss3, and Bruce R. Rosen1 1Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts; 2Nuklearmedizinische Klinik und Poliklinik der Technischen UniversitätMünchen, Klinikum rechts der Isar, Munich, Germany; and 3Max Planck Institute for Neurological Research, Cologne, Germany Learning Objectives: On successful completion of this activity, participants should be able to (1) understand the methodologic opportunities created by simultaneous PET/MRI, (2) explain how this novel imaging modality may benefit existing neurologic applications, and (3) design novel investigations for using simultaneous PET/MRI to address unmet scientific and medical needs. Financial Disclosure: Dr. Catana has received honoraria from Siemens Healthcare for lectures. The authors of this article have indicated no other relevant relationships that could be perceived as a real or apparent conflict of interest. CME Credit: SNMMI is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to sponsor continuing education for physicians. SNMMI designates each JNM continuing education article for a maximum of 2.0 AMA PRA Category 1 Credits. Physicians should claim only credit commensurate with the extent of their participation in the activity. For CE credit, participants can access this activity through the SNMMI Web site (http:// www.snmmi.org/ce_online) through December 2015. sumption for some studies, this may not be the case more PET and MRI provide complementary information in the study generally. For example, a subject’s mental state may change of the human brain. Simultaneous PET/MRI data acquisition on time frames from minutes to even seconds while physio- allows the spatial and temporal correlation of the measured logic and metabolic changes can occur on the order of min- signals, creating opportunities impossible to realize using utes in some disease conditions such as acute ischemic stand-alone instruments. This paper reviews the methodologic stroke or migraine. Likewise, rapid changes in baseline phys- improvements and potential neurologic and psychiatric appli- iology can occur with some therapeutic interventions, such as cations of this novel technology. We first present methods for improving the performance and information content of each neurooncologically directed treatments with antiangiogenic modality by using the information provided by the other agents. technique. On the PET side, we discuss methods that use the One means to address such potential pitfalls is through simultaneously acquired MRI data to improve the PET data the simultaneous collection of MRI and PET data. The quantification. On the MRI side, we present how improved PET feasibility of simultaneous PET and MRI data acquisition quantification can be used to validate several MRI techniques. for human studies was first demonstrated in 2007, and Finally, we describe promising research, translational, and ½ : clinical applications that can benefit from these advanced tools. proof-of-principle brain data (Fig. 1) were collected using Fig 1 Key Words: simultaneous PET/MRI; multimodal imaging; a prototype MRI-compatible PET insert—called BrainPET— neurology positioned inside a commercially available 3-T MRI Trio J Nucl Med 2012; 53:1–10 system (Siemens Healthcare Inc.) (1). In 2010, a fully inte- DOI: 10.2967/jnumed.112.105346 grated PET/MRI scanner also became available for human whole-body imaging (Biograph mMR; Siemens) (2). Simultaneous PET/MRI allows for both spatial and temporal correlation of the signals, creating opportunities impossible to realize using sequentially acquired data. The RI and PET provide complementary anatomic, phys- M features of this new technology may be particularly iologic, metabolic, and functional information about the appealing to applications in neuroscience and translational brain. Pooling information obtained with each modality neurologic and psychiatric research, considering that MRI has long been performed through a parallel analysis of the sequentially acquired data and, more commonly today, represents the first-line diagnostic imaging modality for by using software coregistration techniques. However, un- numerous indications and that a great number of specific derlying such studies is the assumption that no significant PET tracers are available today to assess functional and changes in physiologic or cognitive conditions have oc- molecular processes in the brain. In the study of physio- curred between the 2 examinations. Although a good as- logic brain function, simultaneous acquisition may allow improved in vivo assessment and cross-correlation of Received Aug. 15, 2012; revision accepted Oct. 19, 2012. several neuropsychologic events, such as changes in For correspondence or reprints contact: Ciprian Catana, Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, hemodynamics, including quantitative assessment of cere- Massachusetts General Hospital and Harvard Medical School, Building bral blood flow (CBF), volume (CBV), and oxygenation; 149, Room 2301, 13th St., Charlestown, MA 02129. E-mail: [email protected] neurovascular coupling; inflammation and microglial acti- Published online nnnn. vation; ischemia; necrosis; and apoptosis. The understand- COPYRIGHT ª 2012 by the Society of Nuclear Medicine and Molecular Imaging, Inc. ing of the pathophysiologic mechanisms underlying disease PET/MRI FOR NEUROLOGIC APPLICATIONS • Catana et al. 1 jnm105346-sn n 11/9/12 Copyright 2012 by Society of Nuclear Medicine. FIGURE 1. First simultaneous PET/MRI study in 66-y-old healthy volunteer. MRI RGB sequences included T2-weighted turbo spin echo, echo planar, time-of-flight MR angiog- raphy, and MR spectroscopy. PET image displayed was reconstructed from 20-min emission data recorded at steady state after injection of 370 MBq of 18F-FDG. Data were acquired on BrainPET prototype (Siemens). MPRAGE 5 magnetization-prepared rapid gradient echo; MRS 5 MR scintigraphy; TSE 5 turbo spin echo. (Reprinted with per- mission of (1).) states and the clinical evaluation of various disorders of the success) or for correcting for head movements (3–11). central nervous system over time might also be improved, However, most of these methods require a relatively un- such as quantitative measurements of pathologic processes obstructed view of the optical sensors from outside the in brain cancer before and after treatment; neurodegener- scanner—not typically the case on an integrated PET/MRI ative diseases; and acute and chronic stroke, including scanner because of the radiofrequency coils. neuroplasticity associated with stroke recovery. In addition, In an integrated scanner, the MRI data acquired simul- other disorders associated with changes in mental status, taneously with the PET data can be used to obtain high- such as depression, dementia, schizophrenia, and obsessive– temporal-resolution rigid-body motion estimates, most compulsive disorders, might be investigated in new ways, often in the background of the actual MRI data acquisition. combining anatomic, functional, and metabolic measure- One such method consists of repeatedly acquiring anatomic ments during identical examination conditions. data and coregistering the individual MRI volumes. When In this article, we discuss the methodologic opportunities fast imaging with EPI or spiral sequences is performed, created by simultaneous PET/MRI data acquisition and such methods provide temporal updating of spatial informa- describe how this novel imaging modality might benefit tion in times typically from 2 to 6 s. These methods are neurologic and psychiatric applications. particularly attractive because they allow the simultaneous acquisition of functional MRI (fMRI) and PET data, which is METHODOLOGIC AND SCIENTIFIC ADVANTAGES of interest for several research applications. For more Simultaneous acquisition immediately brings to mind the conventional acquisition methods (e.g., high-resolution ana- possibility of improving the performance and information tomic imaging), motion estimates with very high temporal content of one instrument using the information obtained resolution (e.g., every 20 ms) can be obtained using embedded from the other instrument. This street does not need to be navigator pulses (12). Their potential use to correct the PET 1-way: the perspective to combine the strengths of both data in very short frames could be particularly important for techniques for mutual improvement appears tempting. The performing motion correction in the early phases of a dynamic accuracy of the PET estimates might be improved by PET study, when frames as short as 1 s are often used to including the MRI information as the structural framework sample the radiotracer arterial input function (AIF). underlying the distribution of the PET signal. Reciprocally, Proof-of-principle MRI-assisted PET rigid-body motion the strength of PET to provide absolute quantitative correction studies have already been performed on healthy information might help validate several MRI techniques volunteers using the BrainPET scanner (Fig. 2) (13). MRI- ½Fig: 2 in vivo. Next, we give examples of methodologic improve- based motion correction has the potential to improve PET ments made possible via simultaneous
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