Competitive advantage of PET/MRI

Abstract

Multimodality imaging has made great strides in the imaging evaluation of patients with a variety of diseases. Positron emission tomography/computed tomography (PET/CT) is now established as the imaging modality of choice in many clinical conditions, particularly in oncology. While the initial development of combined PET/magnetic resonance imaging (PET/MRI) was in the preclinical arena, hybrid PET/MR scanners are now available for clinical use. PET/MRI combines the unique features of MRI including excellent soft tissue contrast, diffusion-weighted imaging, dynamic contrast-enhanced imaging, fMRI and other specialized sequences as well as MR spectroscopy with the quantitative physiologic information that is provided by PET. Most evidence for the potential clinical utility of PET/MRI is based on studies performed with side-by-side comparison or software-fused MRI and PET images. Data on distinctive utility of hybrid PET/MRI are rapidly emerging. There are potential competitive advantages of PET/MRI over PET/CT. In general, PET/MRI may be preferred over PET/CT where the unique features of MRI provide more robust imaging evaluation in certain clinical settings. The exact role and potential utility of simultaneous data acquisition in specific research and clinical settings will need to be defined. It may be that simultaneous PET/MRI will be best suited for clinical situations that are disease-specific, organ-specific, related to diseases of the children or in those patients undergoing repeated imaging for whom cumulative radiation dose must be kept as low as reasonably achievable. PET/MRI also offers interesting opportunities for use of dual modality probes. Upon clear definition of clinical utility, other important and practical issues related to business operational model, clinical workflow and reimbursement will also be resolved.

Keywords: Hybrid; MRI; PET.

Figure 1

Schematic cross-sectional views of potential designs for combined PET/MR imaging systems: (a) tandem design with MR and PET mounted back-to back (similar to that in PET/CT instrumentation) to allow sequential rather than simultaneous acquisition, (b) insert design with PET imager (P) inserted between radiofrequency coil (R) and gradient set (G) of MR imager, and (c) fully integrated design with two imagers in same gantry. Radiofrequency coil, gradient set, PET imager and patient bed (B) are shown for all configurations. Reproduced with permission from Ref. 15.

Figure 2

MR and fused PET/MR high-spatial resolution images of three 23-year-old volunteers, two men and one woman, acquired with the same PET imager (Siemens Biograph mMR; Siemens Medical Solutions, Erlangen, Germany). (a) In first male volunteer, coronal 7-T T2*-weighted gradient-echo MR image (750/21; flip angle, 30°) and (b) fused PET/MR image of hippocampal region show subhippocampal resolution separating metabolic function of region cornu ammonis 4 (CA4) from surrounding structures including parahippocampal gyrus (PHG) and fusiform face area (FFA). (c) In second male volunteer, coronal 7-T T1-weighted three-dimensional magnetization-prepared rapid-acquisition gradient-echo MR image (4000/5.3/1000; flip angle 10°) and (d) fused PET/MR image of thalamic region show subthalamic resolution, allowing for structural and anatomic quantification of individual nuclei. Thalamic nuclei (yellow): centromedian thalamic nucleus (*), parafascicular thalamic nucleus (**), magnocellular part of medial dorsal thalamic nucleus (***), medial dorsal thalamic nucleus (MD), dorsal superficial nucleus (DSF), pulvinar (PUL), ventral lateral thalamic nucleus (VL), ventral posterior lateral thalamic nucleus (VPL). Fiber tracts (white): superior cerebellar peduncle (scp), capsule of red nucleus (cr), fasciculus retroflexus (fr), body of fornix (bfx), cerebellorubrothalamic fibers (1). Other structures (red): lateral geniculate nucleus (LG), zona incerta (ZI), substantia nigra (SN), ventral tegmental area (VTA), red nucleus (R). (e) In female volunteer, midline sagittal 7-T T2*-weighted gradient-echo MR image (750/16.8; flip angle, 30°) and (f) fused PET/ MR image through brainstem show detailed anatomy and metabolic function of raphe nuclei. Raphe nuclei: dorsal (d), superior central (sc), pontine (p), medullary (including magnus, obscurus, and pallidus) (m). mamillary body (MB), thalamus (T), red nucleus (R), inferior colliculus (IC). Reproduced with permission from Ref. 15.

Figure 3

A 51-year-old man with suspected intrathoracic sarcoidosis had an arachnoid cyst incidentally identified by FDG PET/CT. A, Axial fusion images of integrated FDG PET/MRI show normal glucose metabolism adjacent to the cortex of a voluminous arachnoid cyst. B, Functional MRI obtained after right (green) versus left (red) hand finger tapping demonstrates large areas of activation between inferior frontal gyrus and postcentral gyrus. There was no neurological deficit due to this functional reorganization. Functional MRI superimposed on morphological T1-weighted 3-dimensional MRI demonstrate apparent extracortical overflow of the activation area in the right hemisphere presumed secondary to increased blood oxygen levels in large superficial cortical veins draining the very thin motor cortex. Pink areas represent artifacts associated with inhomogeneities in the magnetic correction field map. Reproduced with permission from Lippincott Williams & Wilkins; Hubele F, Imperiale A, Kremer S, Namer IJ. Clin Nucl Med 2012; 37:982–983.

Figure 4

89-year-old woman with probable Lewy body dementia. MR images show diffuse atrophy, without regional specificity, whereas FDG PET shows significant posterior cortical hypometabolism, with preservation of posterior cingulate and precuneus metabolism, typically associated with Lewy body dementia. DaTSCAN results (not shown) were supportive for Lewy body disease. Reproduced with permission from Ref. 49.

Figure 5

8 year old girl with chronic seizures. Co-registered independently acquired ¹⁸F-FDG PET and 3D SPGR MR axial, coronal, and sagittal images demonstrate left open schizencephaly.

Figure 6

Integration of delayed enhancement MRI, PET perfusion and αvβ3 integrin expression images in a patient with re-perfused myocardial infarction 2 weeks previously. A, D Four chamber view (A) and two chamber view (B) show delayed enhancement (arrows) extending from the anterior wall to the apical region. B, E Fusion of ¹³N-ammonia PET/MR images show severely reduced myocardial blood flow in the region of delayed enhancement (arrows). C, F Focal ¹⁸F-RGD activity colocalized to the infarcted area as seen by delayed enhancement MRI. ¹⁸F-RGD activity corresponds to the regions of severely reduced ¹³N-ammonia activity, reflecting the extent the of αvβ3 expression within the infarcted area noted on delayed contrast enhanced MR. Reproduced with permission from Springer: Nekolla SG, Martinez-Moeller A, Saraste A. PET and MRI in cardiac imaging: from validation studies to integrated applications. Eur J Nucl Med Mol Imaging 2009; 36 (Suppl 1):S121–S130.

Figure 7

49 year old man with left leg weakness. Axial, coronal, and sagittal ¹⁸F-FDG PET (top row), contrast enhanced 3D SPGR (middle row) and co-registered PET/ MR images demonstrate an asymmetrical enhancing medial precentral gyrus lesion with a cystic component and prominent surrounding white matter edema. Hybrid images document significantly increased glucose metabolism within the most enhancing region, a Grade 4 glioma.

Figure 8

69-year-old man with right thalamic glioblastoma. Axial, coronal, and sagittal contrast enhanced T1 weighted MR (left column), inline hybrid ¹⁸F-fluoroethyltyrosine (FET) PET/MR (center column), and FET PET (right column) demonstrate significant ring contrast enhancement and intense FET uptake (estimated SUVmax is 3.4). Reproduced with permission from Ref. 49.

Figure 9

A 70-year-old man with an elevated level of prostate-specific antigen (17.7 ng/mL) and prior negative transrectal biopsy. Top row, left to right: ¹¹C-choline PET at 5 minutes, contrast enhanced CT, and hybrid PET/CT demonstrates slight enhancement of the right prostate apex with increased choline uptake. Middle row, left to right: ¹¹C-choline PET at 30 minutes, T2 weighted MR, and fully integrated hybrid PET/MR (Siemens Biograph mMR; Siemens Medical Solutions, Erlangen, Germany) shows vague T2 increase and continued prominent choline uptake. Lower row, left to right: Apparent diffusion coefficient MR, early perfusion MR, and area under curve within 60 seconds documents restricted diffusion, increased perfusion with early wash-in of contrast agent. Re-biopsy of the ventral periphery of the right apex demonstrated high-grade prostatic intraepithelial neoplasia. Reproduced with permission from Lippincott Williams & Wilkins; Takei T, Souvatzoglou M, Beer AJ, Ambros J, et al. A Case of Multimodality Multiparametric 11C-Choline PET/MR for Biopsy Targeting in Prior Biopsy-Negative Primary Prostate Cancer. Clin Nucl Med 2012; 37:918–919.