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Clinical evaluation of zero-echo-time MR imaging for the segmentation of the skull
Gaspar Delso1, Florian Wiesinger2, Laura I Sacolick2
1MR Applications and Workflow, GE Healthcare, Waukesha, Wisconsin gaspar.delso@usz.ch.
Unlabelled:
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 dual-echo 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.
Insights
This study evaluates a new zero-echo-time (ZTE) MRI method for bone segmentation in the head. The technique shows promise for accurate bone depiction, crucial for PET/MR attenuation correction.
Area of Science:
- Medical Imaging
- Radiology
- Biophysics
Background:
- Magnetic Resonance (MR) imaging is vital for PET/MR attenuation correction.
- Accurate segmentation of bone tissue in MR images is challenging due to low proton density and fast decay.
- Zero-echo-time (ZTE) imaging offers a potential solution for improved bone depiction.
Purpose of the Study:
- To clinically evaluate a novel proton density-weighted ZTE MR imaging method for bone segmentation in the head.
- To assess the accuracy of ZTE-based bone maps compared to CT scans for PET attenuation correction.
Main Methods:
- A proton density-weighted ZTE MR imaging sequence was used to acquire bone maps.
- Fifteen clinical datasets from a PET/CT/MR trimodality setup were analyzed.
- Quantitative (Jaccard distance) and qualitative assessments were performed comparing ZTE bone masks to CT references.
Main Results:
- The average Jaccard distance between ZTE and CT bone masks was 52% for the entire head and 39% for the cranium.
- These results are superior to previous ultrashort echo time methods.
- The calvaria was well segmented, though air cavities and bone/fluid interfaces presented challenges.
Conclusions:
- This is the first clinical evaluation of skull bone identification using ZTE sequences.
- Proton density-weighted ZTE imaging provides high-resolution bone maps with adequate accuracy for PET attenuation correction.
