S15-09 SESSION 15: PLANNING/IMAGING - PART I BONE-SELECTIVE MRI AS A NONRADIATIVE ALTERNATIVE TO CT FOR CRANIOFACIAL IMAGING

Abstract

Introduction: CT, in contrast to conventional MRI, allows evaluation of craniofacial skeletal pathologies with high-resolution 3D visualization of cortical bone structures. However, ionizing radiation is of concern, particularly for pediatric patients. This study evaluates the feasibility of producing 3D human skull renderings using a novel bone-selective MRI technique, with comparison to CT. Methods: Dual-radiofrequency (RF) pulse and ultrashort echo time (UTE) sequence: two RF pulses are alternately applied and two echoes are acquired for each RF. The first echo captures signals with very short lifetimes (bone), while the second starts after a longer delay. Four echoes are obtained in total: ECHO11 (RFshortTEshort), ECHO12 (RFshortTElong), ECHO21 (RFlongTEshort), and ECHO22 (RFlongTElong). Two images are constructed: one combining the bone-selective echoes (ECHO11, ECHO21), and one combining the longer echoes (ECHO12, ECHO22). Bone proton magnetization exhibits a substantial level of signal decay during RFlong, while soft tissue retains nearly the same level of signal intensities over all echoes. Thus, subtraction of the two images enhances bone contrast by suppressing soft tissue signals. A human cadaver skull and a healthy adult human subject were scanned at 3 T field strength (Siemens Prisma, Erlangen, Germany) within 6 minutes. Semi-automatic segmentation of bone voxels was performed using ITK-SNAP, leading to 3D renderings of the skulls. For comparison, CT scans were performed with 1 mm isotropic resolution. The biometric accuracy was assessed by measuring eight anatomic distances in CT- and MRI-based 3D renderings, using Mimics software (Materialise®, Ghent, Belgium). Lin’s Concordance Correlation test was applied to assess agreement between measurements from MR-based and CT based 3D skull renderings. Results: Compared to CT, the 3D rendered MR images depict most craniofacial features (e.g., zygomatic arch), although some voxels were erroneously included or excluded in the renderings. MR-based measurements differed by mean of -6mm from CT-based measurements. Lin’s Concordance Correlation Coefficients for MR vs CT were 0.996 (cadaver, 95% CI 0.991–1.001), and 0.991 (adult subject, 95% CI 0.980–1.001). Conclusion: The proposed dual-RF dual-echo 3D UTE imaging technique produces high-resolution bone-specific images within a clinically feasible imaging time, leading to clear visualization of craniofacial skeletal structures. Concordance coefficients of 0.991–0.996 suggest good reliability of the method compared to CT. The method is being optimized for more accurate 3D renderings and evaluated in pediatric patients.