A Path to Qualification of PET/MRI Scanners for Multicenter Brain Imaging Studies: Evaluation of MRI-Based Attenuation Correction Methods Using a Patient Phantom

Abstract

Positron emission tomography and magnetic resonance imaging (PET/MRI) scanners cannot be qualified in the manner adopted for hybrid PET and computed tomography (CT) devices. The main hurdle with qualification in PET/MRI is that attenuation correction (AC) cannot be adequately measured in conventional PET phantoms due to the difficulty in converting the MRI images of the physical structures (e.g., plastic) into electron density maps. Over the last decade, a plethora of novel MR-based algorithms have been developed to more accurately derive the attenuation properties of the human head, including the skull. Although very promising, none of these techniques has yet emerged as an optimal and universally adopted strategy for AC in PET/MRI.In this work, we propose a path for PET/MRI qualification for multicenter brain imaging studies. Specifically, our solution is to separate the head attenuation correction from the other factors that affect PET data quantification and use a patient as a phantom to assess the former. The emission data collected on the integrated PET/MRI scanner to be qualified should be reconstructed using both MR-and CT-based AC methods and whole-brain qualitative and quantitative (both voxelwise and regional) analyses should be performed. The MR-based approach will be considered satisfactory if the PET quantification bias is within the acceptance criteria specified herein. We have implemented this approach successfully across two PET/MRI scanner manufacturers at two sites.

Synthetic PET via Domain Translation of 3-D MRI

“…As the error in PET/MRI reconstruction is composed of errors involving prediction of the attenuation map and errors involving the reconstruction (e.g. choice of objective function), a standard approach is to measure the compound effect caused by the AC map by directly comparing PET volumes reconstructed with MRAC and CTAC voxel-wise and regionally [16], [35].…”

“…To benchmark systematic error arising from the reconstruction and re-projection in the experimental procedure, we also compare to the quantification error arising from using measured sinogram data y real corresponding to the true patient distribution x real (i.e. following the standard approach in [16]).…”

“…Consequently, the standard approach to evaluating PET/MRI performance is to utilize human subject datasets acquired on PET, CT, and MR. [11], [16]. This allows for a relative performance measure, by comparing the standardized uptake value (SUV) of MRAC-based PET reconstructions relative to PET reconstructions utilizing CT-based attenuation correction (CTAC) [16].…”

“…Consequently, the standard approach to evaluating PET/MRI performance is to utilize human subject datasets acquired on PET, CT, and MR. [11], [16]. This allows for a relative performance measure, by comparing the standardized uptake value (SUV) of MRAC-based PET reconstructions relative to PET reconstructions utilizing CT-based attenuation correction (CTAC) [16]. However, for sites to conduct such evaluations, numerous PET/CT/MR scans are required to characterize scanner and algorithm performance at operating points exhibiting natural imperfections that impact the physics of PET collection, such as those arising from detector characteristics, scattering, or unexpected attenuation [17].…”

A review of harmonization strategies for quantitative PET

“…Preliminary evaluations of MR-AC accuracy may be possible using these phantoms although they still cannot provide PET signals. On the other hand, Canata et al [ 103 ] proposed to use a patient as a phantom to assess the MR-AC accuracy using CT as the reference. PET/MR harmonization will be facilitated if a standard method that can verify the MR-AC accuracy is established.…”