ABSTRACT
Mechanical strength and fracture resistance of trabecular bone (TB) are largely determined by the structural arrangement of individual trabeculae. Fast 3D spin-echo approaches are preferable to gradient echoes in that they are less sensitive to local induced gradients at the bone/marrow interface caused by magnetic susceptibility difference between the two tissues. FLASE is a 3D pulse sequence that serves this purpose. Here, we present a new pulse sequence dubbed FLADE (fast low-angle dual spin-echo) that overcomes some of the limitations inherent to FLASE, such as sensitivity to artifactual stimulated echoes. The double-echo sequence features a flip angle <90 degrees allowing for TR << T(1). The second phase-reversal pulse has the dual function of creating a second echo and restoring inverted longitudinal magnetization. The prolonged TR, made possible by sampling only half of k(z)-space, is used to collect navigator echoes in adjacent slabs for sensing subpixel translational displacements. FLADE is shown to provide SNR comparable to FLASE while having narrower point-spread function and being more robust to imperfections in the nonselective 180 degree pulses. Structural parameters derived from the in vivo images with the two pulse sequences are highly correlated, therefore suggesting that clinical data obtained with either pulse sequence can be merged.
Subject(s)
Bone and Bones/anatomy & histology , Magnetic Resonance Imaging/methods , Humans , Imaging, Three-DimensionalABSTRACT
The mechanical competence of trabecular bone is significantly determined, next to material density, by its three-dimensional (3D) structure. Recent advances in micromagnetic resonance imaging (micro-MRI) acquisition and processing techniques allow the 3D trabecular structure to be analyzed in vivo at peripheral sites such as the distal radius and tibia. The practicality of micro-MRI-based noninvasive virtual bone biopsy (VBB) for longitudinal studies of patients hinges on the reproducibility of the derived structural parameters, which largely determine the size of the effect that can be detected at a given power and significance level. In this paper, the reproducibility of micro-MRI-derived trabecular bone structure measures was examined by performing repeat studies in six healthy subjects in whom the distal aspects of the radius and tibia were scanned with a 3D spin-echo sequence at 137 x 137 x 410 microm3 voxel size. Bone volume fraction (BV/TV) and digital topological analysis (DTA) structural parameters including the topological bone surface-to-curve ratio (SCR) and topological erosion index (TEI) were evaluated after subjecting the raw images to a cascade of processing steps. The average coefficient of variation was 4-7% and was comparable for the two anatomic sites and for all parameters measured. The reliability expressed in terms of the intraclass correlation coefficient ranged from 0.95 to 0.97 in the radius and 0.68 to 0.92 in the tibia. Error analysis based on simulations suggests involuntary patient motion, primarily rotation, to be the chief source of imprecision, followed by failure to accurately match the analysis volumes in repeat studies.
