A potential major application of simultaneous avalanche photodiode–based positron emission tomography (PET)/magnetic resonance imaging (MRI) systems are quantitative brain studies for cerebral blood flow measurements in combination with blood-oxygen-level–dependent or perfusion MRI, requiring a high performance for both modalities. Thus, we evaluated PET quantification accuracy and homogeneity for 2 different simultaneous PET/MRI systems (whole–body and brain scanner) compared with those of a state-of-the-art PET detector (PET/computed tomography) using phantom studies. In addition, we investigated the long-term stability of PET and quality of functional MRI measurements of a clinical whole-body PET/MRI scanner.
Materials and Methods
Phantom measurements were conducted using spheres filled with [18F]-fluoride distributed in a homogeneous cylinder phantom at different positions inside the PET field of view. Recovery values and standard deviations were extracted from resulting PET images. The influence of magnetic resonance–based attenuation correction and that of activity outside the PET field of view on the recovery values of these spheres was evaluated. Furthermore, long-term PET stability of the whole-body PET/MRI system was assessed by evaluating position profiles, energy spectra, count rates, and recovery values from [68Ge]-phantom scans. Functional MRI applicability was tested in accordance with the functional Biomedical Information Research Network procedure.
The BrainPET system showed high recovery values (up to 99%) but also increased variability (up to 7.4%). Significant underestimations in PET quantification near activity outside the PET field of view were found (up to 80%). Using magnetic resonance–based attenuation correction led to an underestimation in PET activity of approximately 7%. In distinction, the whole-body PET/MRI system revealed performance similar to the PET/computed tomographic scanner (recovery values up to approximately 60% with a variability of approximately 4%). Long-term stability and fMRI performance of the whole-body PET/MRI scanner showed no degradation compared with stand-alone systems.
Homogeneity and accuracy of avalanche photodiode-based PET detectors is comparable with those of the state-of-the-art detectors based on photomultiplier tubes. However, attenuation correction on PET/MRI systems has to be adapted carefully for quantitative PET measurements. The BrainPET system needs improved scatter correction to perform quantitative brain studies. The whole-body PET/MRI scanner, however, is applicable for quantitative brain studies.