JNM: Hybrid PET/MRI of intracranial tumors comparable to PET/CT
Structural, functional and molecular imaging in patients with brain tumors is feasible with diagnostic imaging quality using simultaneous hybrid PET/MRI acquisition, according to a study published in this month’s Journal of Nuclear Medicine.

Andreas Boss, MD, and colleagues from the department of diagnostic and interventional radiology at Eberhard Karls University of Tübingen in Tübingen, Germany, compared the feasibility of tumor assessment using a PET/MRI system comprising lutetium oxyorthosilicate scintillators coupled to avalanche photodiodes in 10 patients with intracranial masses, and quantified the accuracy with conventional PET/CT datasets.

All measurements were performed with a hybrid PET/MRI scanner consisting of a conventional 3T MRI scanner in combination with an inserted MRI-compatible PET system.

Boss and colleagues used two different approaches for attenuation correction when comparing the data from the two systems (PET/MRI versus PET/CT). The CT scan was used as the basis for the calculation of the attenuation map in the PET/CT component, whereas in the PET/ MRI system the attenuation map was calculated from MRI data using an algorithm combining pattern recognition and atlas registration.

At the time of referral, two patients were diagnosed with low-grade astrocytoma, three had meningioma, one had suspicion of low-grade astrocytoma, two had glioblastoma, one had anaplastic astrocytoma (World Health Organization grade III) and one had atypical neurocytoma.

The researchers used 11C-methionine for PET in the glial tumors and 68Ga-DOTATOC in the meningiomas. Tumor–to–gray 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.

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. The mean paired relative error was 7.9 percent and no significant artifacts or distortions were detected in the simultaneously acquired MR images using the PET/MRI scanner, according to Boss and colleagues.

The authors concluded that hybrid PET/MRI of intracranial tumors using 11C-methionine or 68Ga-DOTATOC can be reliably performed and the image quality and quantitative data achieved using PET/MRI is similar to that using PET/CT.

In addition, “the combination of PET with MRI instead of CT offers many advantages such as higher soft-tissue contrast; reduced radiation exposure; and advanced MRI techniques such as perfusion imaging, diffusion imaging, and MR spectroscopy,” noted the authors.

Boss and colleagues predicted that in future, reliable and fast partial volume and motion correction methods may be implemented to significantly improve PET quantitation accuracy using the optimally coregistered MRI data.