Hybrid PET/MRI of Intracranial Masses: Initial Experiences and Comparison to PET/CT

Abstract

Simultaneous PET and MRI using new hybrid PET/MRI systems promises optimal spatial and temporal coregistration of structural, functional, and molecular image data. In a pilot study of 10 patients with intracranial masses, the feasibility of tumor assessment using a PET/MRI system comprising lutetium oxyorthosilicate scintillators coupled to avalanche photodiodes was evaluated, and quantification accuracy was compared with conventional PET/CT datasets. Methods: All measurements were performed with a hybrid PET/MRI scanner consisting of a conventional 3-T MRI scanner in combination with an inserted MRI-compatible PET system. Attenuation correction of PET/MR images was computed from MRI datasets. Diagnoses at the time of referral were low-grade astrocytoma (n 5 2), suspicion of low-grade astrocytoma (n 5 1), anaplastic astrocytoma (World Health Organization grade III; n 5 1), glioblastoma (n 5 2), atypical neurocytoma (n 5 1), and meningioma (n 5 3). In the glial tumors, 11 C-methionine was used for PET; in the meningiomas, 68 Ga-DOTATOC was administered. Tumor-togray matter and tumor-to-white matter ratios were calculated for gliomas, and tracer uptake of meningiomas was referenced to nasal mucosa. PET/MRI was performed directly after clinically indicated PET/CT examination. Results: In all patients, the PET datasets showed similar diagnostic image quality on the hybrid PET/MRI and the PET/CT studies; however, slight streak artifacts were visible in coronal and sagittal sections when using the higher intrinsic resolution of the PET/MRI insert. Prefiltering of images with a 4-mm gaussian filter at a resolution comparable to that of the PET/CT system virtually eliminated these artifacts. Although acquisition of the PET/MR images started at 30-60 min after PET/CT (20.4-min half-life of 11 C) acquisition, the signal-to-noise ratio was good enough, thus underlining the high sensitivity of the PET insert, compared with whole-body PET systems. The computed tumor-to-reference tissue ratios exhibited an excellent accordance between the PET/MRI and PET/CT systems, with a Pearson correlation coefficient of 0.98. Mean paired relative error was 7.9% 6 12.2%. No significant artifacts or distortions were detected in the simultaneously acquired MR images using the PET/MRI scanner. Conclusion: Structural, functional, and molecular imaging in patients with brain tumors is feasible with diagnostic imaging quality using simultaneous hybrid PET/MR image acquisition.

PET/MRI: Reliability/Reproducibility of SUV Measurements

“…The first study comparing PET quantification between PET/CT and PET/MR was performed for 11 C-methionine PET for gliomas and 68 Ga-DOTATOC in meningiomas. The authors concluded that the computed TBR exhibited an excellent accordance between PET/MR and PET/CT systems, with a correlation coefficient of 0.98 and a mean relative error of 7.9% [45].…”

Molecular neuroimaging with PET/MRI

“…The added value of hybrid PET/ MRI is the distinction between recurrence and scar in a, and the observation of significant PET uptake in both lesions in b, despite different Gd uptake patterns simultaneous PET/MRI, as compared with PET/CT, performed using the tracer 11 C-methionine [29,30]. The two studies, by the same group, investigated subjects with brain tumor, mainly astrocytomas, by PET/CT and subsequently simultaneous PET/MRI (dedicated brain system); no significant difference was found in the visual classification of positive/negative studies and quantitative ratios to reference tissues were comparable [29,30]. In addition, overall the quality of MRI was good: only a small percentage (about 10 %) of subjects showed major artifacts on some MR sequences and there was only one non-diagnostic echo planar sequence [30].…”

Whole-body PET/MRI in oncology: current status and clinical applications

“…Non-FDG tracers already used for clinical applications are 11 C-and 18 F-choline, 11 C-methionine and 18 F-FET, 18 F-DOPA, 68 Ga-DOTA-somatostatine analogs, 11 C-acetate, 18 F-FLT, and 124 I (Table I). Imaging of intracranial masses was performed on a hybrid PET/MRI system capable of simultaneous PET/MRI acquisition administering 11 C-methionine in patients with gliomas and 68 Ga-DOTA-somatostatine analogs in those with meningiomas [83]. Similar diagnostic image quality on the hybrid PET/MRI and the PET/CT studies was found, with excellent agreement between semiquantitative data (i.e., the tumor-to-reference tissue ratios) of the PET/MRI and PET/CT systems (r = 0.98) [82].…”