Clinical Evaluation of Zero-Echo-Time MR Imaging 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 imaging is, however, quite challenging, because of the low proton density and fast decay time of bone tissue. The clinical evaluation of a novel, recently published method for zero-echo-time (ZTE)-based MR bone depiction and segmentation in the head is presented here. Methods: A new paradigm for MR imaging bone segmentation, based on proton density-weighted ZTE imaging, was disclosed earlier in 2014. In this study, we reviewed the bone maps obtained with this method on 15 clinical datasets acquired with a PET/CT/MR trimodality 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 and qualitative scoring of anatomic accuracy by an experienced radiologist and nuclear medicine physician were performed. Results: The average Jaccard 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 dualecho ultrashort echo time, for which the Jaccard 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 anatomic detail, such as the inner ear, remain a challenge. Conclusion: This is the first, to our knowledge, 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 anatomic accuracy for, for example, PET attenuation correction.

Comparison of atlas-based techniques for whole-body bone segmentation

“…In whole-body imaging, almost all proposed methods, except joint attenuation-activity reconstruction techniques, rely on prior knowledge present in atlas images to predict bone from MRI. Moreover, owing to long acquisition time, application of ultra-short echo time (UTE) ( Keereman et al, 2010 ) or zero time echo (ZTE) ( Delso et al, 2015 ) sequences are still limited to brain imaging (single bed position). Atlas-guided segmentation has been successfully applied in various image segmentation tasks using different imaging modalities, particularly for cases with very low contrast to the surrounding tissues ( Lorenzo-Valdés et al, 2004 ).…”

Clinical Evaluation of Zero-Echo-Time Attenuation Correction for Brain 18F-FDG PET/MRI: Comparison with Atlas Attenuation Correction

“…In addition, the mMR has a clinical dual ultrashort echo (UTE)-based segmentation method (8) and a prototype modelbased method (7). In turn, the SIGNA has a clinical atlas-based method and a prototype zero-echo-time (ZTE)-based segmentation method (5,16,17). The clinical atlas-AC on SIGNA is comparatively accurate in supratentorial regions, whereas not accurate enough in the infratentorial regions (5).…”

Quantitative analysis of MRI-guided attenuation correction techniques in time-of-flight brain PET/MRI

“…Initial UTE-based AC studies have reported quantification errors of less than 5% (Aasheim et al, 2015); however, the inhomogeneous and imprecise classification of bones, particularly in the presence of diamagnetic susceptibility effects at air/bone or air/soft-tissue interfaces (Delso et al, 2014) can give rise to errors in the range 4%-17% in different regions of the brain (Dickson et al, 2014). Recently, Delso et al (2015) demonstrated that ZTE-based bone segmentation outperforms its UTE-based counterpart with reduced segmentation errors. However, these two MRI sequences are time consuming (between 3 and 6 min for low and high resolution U/ZTE acquisitions (Delso et al, 2015)), which currently limits their adoption in clinical practice.…”

“…Recently, Delso et al (2015) demonstrated that ZTE-based bone segmentation outperforms its UTE-based counterpart with reduced segmentation errors. However, these two MRI sequences are time consuming (between 3 and 6 min for low and high resolution U/ZTE acquisitions (Delso et al, 2015)), which currently limits their adoption in clinical practice. The assignment of constant LACs to each tissue class is another source of error in segmentation-based AC techniques, since inter/intrapatient variability of attenuation coefficients is not accounted for.…”

“…However, neglecting bone in the resulting attenuation maps can give rise to substantial errors in quantification of brain PET images (5%-10% (Andersen et al, 2014), 10%-29% (Dickson et al, 2014)) and lesions seated close to bones (2%-31% (Samarin et al, 2012, Arabi et al, 2015). In brain PET/MRI imaging, ultrashort echo-time (UTE) (Keereman et al, 2010, Catana et al, 2010 and more recently zero echo-time (ZTE) , Delso et al, 2015 MR sequences have been developed to specifically delineate bones and include them in the attenuation maps. Initial UTE-based AC studies have reported quantification errors of less than 5% (Aasheim et al, 2015); however, the inhomogeneous and imprecise classification of bones, particularly in the presence of diamagnetic susceptibility effects at air/bone or air/soft-tissue interfaces (Delso et al, 2014) can give rise to errors in the range 4%-17% in different regions of the brain (Dickson et al, 2014).…”