Hybrid PET/MRI in Cerebral Glioma: Current Status and Perspectives

Abstract

Advanced MRI methods and PET using radiolabelled amino acids provide valuable information, in addition to conventional MR imaging, for brain tumour diagnostics. These methods are particularly helpful in challenging situations such as the differentiation of malignant processes from benign lesions, the identification of non-enhancing glioma subregions, the differentiation of tumour progression from treatment-related changes, and the early assessment of responses to anticancer therapy. The debate over which of the methods is preferable in which situation is ongoing, and has been addressed in numerous studies. Currently, most radiology and nuclear medicine departments perform these examinations independently of each other, leading to multiple examinations for the patient. The advent of hybrid PET/MRI allowed a convergence of the methods, but to date simultaneous imaging has reached little relevance in clinical neuro-oncology. This is partly due to the limited availability of hybrid PET/MRI scanners, but is also due to the fact that PET is a second-line examination in brain tumours. PET is only required in equivocal situations, and the spatial co-registration of PET examinations of the brain to previous MRI is possible without disadvantage. A key factor for the benefit of PET/MRI in neuro-oncology is a multimodal approach that provides decisive improvements in the diagnostics of brain tumours compared with a single modality. This review focuses on studies investigating the diagnostic value of combined amino acid PET and ‘advanced’ MRI in patients with cerebral gliomas. Available studies suggest that the combination of amino acid PET and advanced MRI improves grading and the histomolecular characterisation of newly diagnosed tumours. Few data are available concerning the delineation of tumour extent. A clear additive diagnostic value of amino acid PET and advanced MRI can be achieved regarding the differentiation of tumour recurrence from treatment-related changes. Here, the PET-guided evaluation of advanced MR methods seems to be helpful. In summary, there is growing evidence that a multimodal approach can achieve decisive improvements in the diagnostics of cerebral gliomas, for which hybrid PET/MRI offers optimal conditions.

Preoperative 11C-Methionine PET-MRI in Pediatric Infratentorial Tumors

“…The present study reported that, although high-grade lesion may show heterogeneous conventional MRI features, in WHO grade 4 tumors herein, the level of MET uptake was higher, and ADC values were lower than in WHO grade 1 tumors, which agrees with previous studies. [25][26][27][28][29] Moreover, a meta-analysis focusing on glioma grading using 11 C-MET PET in adults found a sensitivity of 94% and a specificity of 89% to differentiate low-from high-grade glioma. 30 This clinical indication is underlined in the EANM/EANO/RANO guidelines for adult patients, 5 as a negative scan excludes a grade 3/4 glioma, but it can also exclude a lymphoma or metastasis.…”

“…Combined 11 C-MET PET and MRI is used in brain tumors to perform targeted biopsies and to define the tumor limit 8,17 ; the complementary information provided by MRI and PET parameters in pediatric brain tumors was assessed by a limited number of studies, and none used MET. 7,18,31 Herein, the MET uptake was compared with the ADC min , which is used as a marker of dense tissue with high cellularity, small extracellular space, and high nuclear/cytoplasmic ratio.…”

“…CE reflects the blood-brain barrier disruption, but MET PET uptake reflects the L-type amino acid transporter 1 expression 34 ; hence, the MET uptake provides additional information to the CE MRI, with clinical implication. For example, in addition to conventional CE MRI that might be negative, baseline 11 C-MET PET is particularly useful to characterize low-grade glioma. 35 Moreover, in pediatric high-grade glioma, it delineates the non-contrast-enhancing tumor regions, which are at increased risk for recurrence.…”

“…21 All patients fasted for ≥4 hours and were scanned on a PET-MRI scanner (Biograph mMR, Siemens). The mean ± standard deviation injected dose of 11 C-MET was 186 ± 60 MBq (5 ± 1.6 mCi), and the PET evaluation was based on the summed PET data obtained between 20 and 40 minutes after injection. The PET images were reconstructed iteratively using 3D ordinary Poisson-ordered subsets expectation maximization algorithm, incorporating the system point spread function with 3 iterations of 21 subsets.…”

“…Hybrid imaging, using a PET-MRI scanner, appears particularly suitable for children [7][8][9] ; in brain tumors, it is useful to avoid 2 examinations, 10 and therefore a potential need for general anesthesia. 11 In addition, compared with PET/CT, it allows a lower radiation exposure, 12,13 which is particularly important in children.…”