Accuracy of MR-based attenuation: first experience with a whole-body PET/MR system

Combined PET/MR systems for whole body investigations have been developed recently and are now starting to become available. One of the first of these systems (Gemini TF PET/MR, Philips) has been installed at our site. PET/MR is expected to provide new possibilities, notably in the area of quantitative bimodal functional imaging.
Quantitative PET imaging requires attenuation correction (AC), which is straightforward for PET/CT but not for PET/MR. In order to ensure quantitative accuracy of the PET data in the absence of a measured AC, one needs to use MR-based AC (MRAC), which relies on a combination of templates and accurate segmentation and tissue type identification of a suitable MR scan. Here, we report on a first evaluation of MRAC in phantoms as well as in patient investigations with the new system.

Methodik/Methods: A NEMA IEC Body Phantom was used. Data were evaluated regarding quantitative accuracy of background and sphere activity as well as homogeneity of the background. Reconstructed resolution was assessed as well. Evaluation of MRAC in patients is performed by direct comparison of MRAC with a transmission based AC (TMAC) acquired with an ECAT Exact HR+ (Siemens).

Ergebnisse/Results: Background inhomogeneity was found to be < 4% across the whole phantom. The reconstructed activity in the sphere inserts deviates from the known true values by < 5%. No artifacts related to erroneous AC could be observed in these measurements. The reconstructed resolution (FWHM) in these measurements was determined to be (5.01 - 5.48) mm. Patient studies will be performed in the coming months and evaluated as described above. The corresponding results will be reported.

Schlussfolgerungen/Conclusions: The vendor provided MRAC algorithms yielded very good results in our first phantom measurements both with respect to quantitative accuracy as well as regarding general image quality. The upcoming patient investigations will show whether these findings can be confirmed in whole body applications as well.