Clinical evaluation of zero echo time MRI for the segmentation of the skull

Abstract

MR-based attenuation correction is instrumental for integrated PET/MR imaging. It is generally achieved by segmenting MR images into a set of tissue classes with known attenuation properties (e.g. air, lung, bone, fat, soft tissue). Bone identification with MR is, however, quite challenging, due to the low proton density and fast decay time of bone tissue. The clinical evaluation of a novel, recently-published method for zero TE (ZTE) based MR bone depiction and segmentation in the head is presented here. Methods: A new paradigm for MRI bone segmentation, based on proton density-weighted ZTE imaging, has been disclosed earlier this year. In this study we reviewed the bone maps obtained with this method on fifteen clinical datasets acquired with a PET/CT-MR tri-modality setup. The CT scans acquired for PET attenuation correction purposes were used as reference for the evaluation. Quantitative measurements based on the Jaccard distance between ZTE and CT bone masks were performed, as well as qualitative scoring of anatomical accuracy by an experienced radiologist and nuclear medicine physician. Results: The average overlap distance between ZTE and CT bone masks evaluated over the entire head was 52±6% [range 38-63%]. When only the cranium was considered, the distance was 39±4% [range 32-49%]. These results surpass previously reported attempts with dual-echo UTE, for which the overlap distance was in the 47-79% range (parietal and nasal regions, respectively). Anatomically, the calvaria is consistently well segmented, with frequent but isolated voxel misclassifications. Air cavity walls and bone/fluid interfaces with high anatomical detail, such as the inner ear, remain a challenge. Conclusion: This is the first clinical evaluation of skull bone identification based on a ZTE sequence. The results suggest that proton density-weighted ZTE imaging is an efficient means of obtaining high-resolution maps of bone tissue with sufficient anatomical accuracy for, e.g. PET attenuation correction.