Anatomic Evaluation of 3-Dimensional Ultrashort-Echo-Time Bone Maps for PET/MR Attenuation Correction

Abstract

Ultrashort-echo-time (UTE) sequences have been proposed in the past for MR-based attenuation correction of PET data, because of their ability to image cortical bone. In the present work we assessed the limitations of dual-echo UTE imaging for bone segmentation in head and neck imaging. Sequentially acquired MR and PET/CT clinical data were used for this purpose. Methods: Twenty patients referred for a clinical oncology examination were scanned using a trimodality setup. Among the MR sequences, a dual-echo UTE acquisition of the head was acquired and used to create tissue R2 maps. The different undesired structures present in these maps were identified by an experienced radiologist. Global and local measurements of the overlap between R2-based and CT-based bone masks were computed. Results: UTE R2 maps displayed a nonfunctional relation with CT data. The obtained bone masks showed acceptable overlap with the corresponding CT data, in the case of the skull itself (e.g., 47% mismatch for the parietal region), with decreased performance in the base of the skull and in the neck (e.g., 78% for the maxillary region). Unwanted structures were detected, both anatomic (e.g., sternocleidomastoid, temporal, and masseter muscles) and artifactual (e.g., dental implants and air-tissue interfaces). Conclusion: It is indeed possible to estimate the anatomic location of bone tissue using UTE sequences. However, using pure parametric maps for attenuation correction may lead to bias close to certain anatomic structures and areas of high magnetic field inhomogeneity. More sophisticated approaches are necessary to compensate for these effects. Twel ve years after its commercial introduction, integrated PET/CT has evolved into one of the major imaging procedures in oncology, infection imaging, and cardiology. However, PET/CT has several limitations, both technical and diagnostic, that have recently led to the emergence of hybrid PET/MR scanners (1-3). Among other advantages, PET/MR imaging offers superior soft-tissue contrast (e.g., higher sensitivity for small liver metastases, additional information for lesion characterization, and better depiction of pelvic structures) and lower radiation exposure to the patient. There are, however, several technical and clinical challenges that have to be solved before this modality can reach its full potential.One of the main topics of controversy is how the attenuation correction of the PET images should be addressed (4-6). PET/CT machines provide the possibility of generating the annihilation photon attenuation map of the patient from a fast low-dose CT scan. This has replaced the former slow measurement of this map by means of external 511-keV radiation sources, thus significantly reducing the total scan time.Several approaches are currently being investigated to generate valid annihilation photon attenuation maps from fast MR sequences (7-11). Comprehensive reviews of current MR-based attenuation correction methods, their advantages and limitations, are available in the literature (12,13).MR-...

Clinical Evaluation of Zero-Echo-Time MR Imaging for the Segmentation of the Skull

“…Contrary to previously reported skull segmentation studies based on UTE sequences (27), the presence of tendons on the bone masks obtained with ZTE was minor. Nasal cartilage was partially included in the bone masks.…”

“…The bone masks yielded by ZTE postprocessing were evaluated relative to CT, the current gold standard for bone imaging. Quantitative evaluation using the Jaccard distance shows better performance than previously reported for dual-echo UTE (27), which ranged between 47% and 79%, as opposed to the 38%-63% obtained in this study.…”

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

“…This might be some advantage when compared with the UTE-AC, which is one of the most popular segmentation-based methods in clinical head PET/MR study (8). The artifact sometimes causes the error in the calculation of T2 relaxation time on UTE-AC because of reduced signal intensities in the first UTE echo just after switching (9,29). There is no fair-comparison study between UTE-AC and ZTE-AC in the clinical setting because each method has been developed by a different vendor.…”

Hybrid PET/MR Imaging: An Algorithm to Reduce Metal Artifacts from Dental Implants in Dixon-Based Attenuation Map Generation Using a Multiacquisition Variable-Resonance Image Combination Sequence

“…Alternative MR imaging sequences such as a UTE data acquisition (23) or even the combination of UTE with MAVRIC (24) have been suggested for improved AC in PET/MR imaging. The main advantage of a UTE sequence would be the detection of signal from bone, thereby facilitating bone segmentation in MR imaging-based AC.…”