Using 31P-MRI of hydroxyapatite for bone attenuation correction in PET-MRI: proof of concept in the rodent brain

BACKGROUND The correction of γ-photon attenuation in PET-MRI remains a critical issue, especially for bone attenuation. This problem is of great importance for brain studies due to the density of the skull. Current techniques for skull attenuation correction (AC) provide indirect estimates of cortical bone density, leading to inaccurate estimates of brain activity. The purpose of this study was to develop an alternate method for bone attenuation correction based on NMR. The proposed approach relies on the detection of hydroxyapatite crystals by zero echo time (ZTE) MRI of 31P, providing individual and quantitative assessment of bone density. This work presents a proof of concept of this approach. The first step of the method is a calibration experiment to determine the conversion relationship between the 31P signal and the linear attenuation coefficient μ. Then 31P-ZTE was performed in vivo in rodent to estimate the μ-map of the skull. 18F-FDG PET data were acquired in the same animal and reconstructed with three different AC methods: 31P-based AC, AC neglecting the bone and the gold standard, CT-based AC, used to comparison for the other two methods. RESULTS The calibration experiment provided a conversion factor of 31P signal into μ. In vivo 31P-ZTE made it possible to acquire 3D images of the rat skull. Brain PET images showed underestimation of 18F activity in peripheral regions close to the skull when AC neglected the bone (as compared with CT-based AC). The use of 31P-derived μ-map for AC leads to increased peripheral activity, and therefore a global overestimation of brain 18F activity. CONCLUSIONS In vivo 31P-ZTE MRI of hydroxyapatite provides μ-map of the skull, which can be used for attenuation correction of 18F-FDG PET images. This study is limited by several intrinsic biases associated with the size of the rat brain, which are unlikely to affect human data on a clinical PET-MRI system.