Faculty Bibliography

More than a decade after the introduction of compressed sensing (CS) in MRI, researchers are still working on ways to translate it into different research and clinical applications. The greatest advantage of CS in MRI is the reduced amount of k-space data needed to reconstruct images, which can be exploited to reduce scan time or to improve spatial resolution and volumetric coverage. Efficient data acquisition using CS is extremely important for compositional mapping of the musculoskeletal system in general and knee cartilage mapping techniques in particular. High-resolution quantitative information about tissue biochemical composition could be obtained in just a few minutes using CS MRI. However, in order to make this goal a reality, some issues still need to be addressed. In this article we review the current state of the art of CS methods for rapid compositional mapping of knee cartilage. Specifically, data acquisition strategies, image reconstruction algorithms, and data fitting models are discussed. Different CS studies for T₂ and T_1ρ mapping of knee cartilage are reviewed, with illustrative results. Future directions, opportunities, and challenges of rapid compositional mapping techniques are also discussed.

MR imaging is an indispensable instrument for the diagnosis of musculoskeletal diseases. In vivo MR imaging at 7T offers many advantages, including increased signal-to-noise ratio, higher spatial resolution, improved spectral resolution for spectroscopy, improved sensitivity for X-nucleus imaging, and decreased image acquisition times. There are also however technical challenges of imaging at a higher field strength compared with 1.5 and 3T MR imaging systems. We discuss the many potential opportunities as well as the challenges presented by 7T MR imaging systems and highlight recent developments in in vivo research imaging of musculoskeletal applications in general and cartilage, skeletal muscle, and bone in particular.

Purpose of Review: To describe the emergent role of ultra-high field (UHF) MR with respect to musculoskeletal MRI applications. Recent Findings: With the recent US Federal Drug Administration (FDA) and European Union (EU) approval of ultra-high field (UHF) MRI below 8T for clinical use, and the availability of clinical 7T MRI systems, there is a rising interest in the potential clinical and research applications in musculoskeletal MRI. Summary: With increases in field strength and SNR gains resulting in sharper and higher spatial resolution MRI images, there is increasing interest in UHF MRI. Although there are challenges and limitations in UHF, there are many new and unique musculoskeletal MR applications that UHF excels at such as morphological imaging, bone micro-architecture evaluation, biochemical imaging techniques such gagCEST, UTE/ZTE, T2 mapping, T2* mapping, T1rho mapping, and multi-nuclear imaging and spectroscopy/imaging techniques with ²³Na and ³¹P. The goal of this review is to highlight some of these recent findings in musculoskeletal MRI applications at UHF.

Background Bone remodels in response to mechanical loads and osteoporosis results from impaired ability of bone to remodel. Bone microarchitecture analysis provides information on bone quality beyond bone mineral density (BMD). Purpose To compare subchondral bone microarchitecture parameters in the medial and lateral tibia plateau in individuals with and without fragility fractures. Material and Methods Twelve female patients (mean age = 58 ± 15 years; six with and six without previous fragility fractures) were examined with dual-energy X-ray absorptiometry (DXA) and 7-T magnetic resonance imaging (MRI) of the proximal tibia. A transverse high-resolution three-dimensional fast low-angle shot sequence was acquired (0.234 × 0.234 × 1 mm). Digital topological analysis (DTA) was applied to the medial and lateral subchondral bone of the proximal tibia. The following DTA-based bone microarchitecture parameters were assessed: apparent bone volume; trabecular thickness; profile-edge-density (trabecular bone erosion parameter); profile-interior-density (intact trabecular rods parameter); plate-to-rod ratio; and erosion index. We compared femoral neck T-scores and bone microarchitecture parameters between patients with and without fragility fracture. Results There was no statistical significant difference in femoral neck T-scores between individuals with and without fracture (-2.4 ± 0.9 vs. -1.8 ± 0.7, P = 0.282). Apparent bone volume in the medial compartment was lower in patients with previous fragility fracture (0.295 ± 0.022 vs. 0.317 ± 0.009; P = 0.016). Profile-edge-density, a trabecular bone erosion parameter, was higher in patients with previous fragility fracture in the medial (0.008 ± 0.003 vs. 0.005 ± 0.001) and lateral compartment (0.008 ± 0.002 vs. 0.005 ± 0.001); both P = 0.025. Other DTA parameters did not differ between groups. Conclusion 7-T MRI and DTA permit detection of subtle changes in subchondral bone quality when differences in BMD are not evident.

Purpose To determine if 3-T magnetic resonance (MR) imaging of proximal femur microarchitecture can allow discrimination of subjects with and without fragility fracture who do not have osteoporotic proximal femur bone mineral density (BMD). Materials and Methods Sixty postmenopausal women (30 with and 30 without fragility fracture) who had BMD T scores of greater than -2.5 in the hip were recruited. All subjects underwent dual-energy x-ray absorptiometry to assess BMD and 3-T MR imaging of the same hip to assess bone microarchitecture. World Health Organization Fracture Risk Assessment Tool (FRAX) scores were also computed. We used the Mann-Whitney test, receiver operating characteristics analyses, and Spearman correlation estimates to assess differences between groups, discriminatory ability with parameters, and correlations among BMD, microarchitecture, and FRAX scores. Results Patients with versus without fracture showed a lower trabecular plate-to-rod ratio (median, 2.41 vs 4.53, respectively), lower trabecular plate width (0.556 mm vs 0.630 mm, respectively), and lower trabecular thickness (0.114 mm vs 0.126 mm) within the femoral neck, and higher trabecular rod disruption (43.5 vs 19.0, respectively), higher trabecular separation (0.378 mm vs 0.323 mm, respectively), and lower trabecular number (0.158 vs 0.192, respectively), lower trabecular connectivity (0.015 vs 0.027, respectively) and lower trabecular plate-to-rod ratio (6.38 vs 8.09, respectively) in the greater trochanter (P < .05 for all). Trabecular plate-to-rod ratio, plate width, and thickness within the femoral neck (areas under the curve [AUCs], 0.654-0.683) and trabecular rod disruption, number, connectivity, plate-to-rod ratio, and separation within the greater trochanter (AUCs, 0.662-0.694) allowed discrimination of patients with fracture from control subjects. Femoral neck, total hip, and spine BMD did not differ between and did not allow discrimination between groups. FRAX scores including and not including BMD allowed discrimination between groups (AUCs, 0.681-0.773). Two-factor models (one MR imaging microarchitectural parameter plus a FRAX score without BMD) allowed discrimination between groups (AUCs, 0.702-0.806). There were no linear correlations between BMD and microarchitectural parameters (Spearman ρ, -0.198 to 0.196). Conclusion 3-T MR imaging of proximal femur microarchitecture allows discrimination between subjects with and without fragility fracture who have BMD T scores of greater than -2.5 and may provide different information about bone quality than that provided by dual-energy x-ray absorptiometry.^©RSNA, 2018.

PURPOSE: To evaluate biexponential T2 relaxation mapping of human knee cartilage in vivo in clinically feasible scan times. MATERIALS AND METHODS: T2 -weighted magnetic resonance (MR) images were acquired from eight healthy volunteers using a standard 3T clinical scanner. A 3D Turbo-Flash sequence was modified to enable T2 -weighted imaging with different echo times. Series of T2 -weighted images were fitted using mono- and biexponential models with two- and four-parametric nonlinear approaches, respectively. RESULTS: Biexponential relaxation of T2 was detected in the knee cartilage in five regions of interest in all eight healthy volunteers. Short/long relaxation components of T2 were estimated to be 8.27 +/- 0.68 / 45.35 +/- 3.79 msec with corresponding fractions of 41.3 +/- 1.1% / 58.6 +/- 4.6%, respectively. The monoexponential relaxation of T2 was measured to be 26.9 +/- 2.27 msec. The experiments showed good repeatability with coefficient of variation root mean square (CVrms ) < 18% in all regions. The only difference in gender was observed in medial tibial cartilage, where the biexponential T2 in female volunteers was significantly higher compared to male volunteers (P = 0.014). Significant differences were observed in T2 relaxation between different regions on interest. CONCLUSION: Biexponential relaxation of T2 was observed in the human knee cartilage in vivo. The short and long components are thought to be related to the tightly bound and loosely bound macromolecular water compartments. These preliminary results of biexponential T2 analysis could potentially be used to increase the specificity for detection of early osteoarthritis by measuring different water compartments and their fractions. LEVEL OF EVIDENCE: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017.

PURPOSE/OBJECTIVE:To compare patellar instability with magnetic resonance imaging analysis using continuous real-time radial gradient-echo (GRE) imaging in the assessment of symptomatic patients and asymptomatic subjects. METHODS:Symptomatic patients with suspected patellofemoral maltracking and asymptomatic volunteers were scanned in real time by a radial 2-dimensional GRE sequence at 3 T in axial orientation at the patella level through a range of flexion-extension. The degree of lateral maltracking, as well as the associated tibial tubercle-trochlear groove distance and trochlea depth, was measured. Patellar lateralization was categorized as normal (≤2 mm), mild (>2 to ≤5 mm), moderate (>5 to ≤10 mm), or severe (>10 mm). The patellofemoral cartilage was also assessed according to the modified Outerbridge grading system. RESULTS:The study included 20 symptomatic patients (13 women and 7 men; mean age, 36 ± 12.8 years) and 10 asymptomatic subjects (3 women and 7 men; mean age, 33.1 years). The mean time to perform the dynamic component ranged from 3 to 7 minutes. Lateralization in the symptomatic group was normal in 10 patients, mild in 1, moderate in 8, and severe in 1. There was no lateral tracking greater than 3 mm in the volunteer group. Lateral maltracking was significantly higher in symptomatic patients than in asymptomatic subjects (4.4 ± 3.7 mm vs 1.5 ± 0.71 mm, P = .007). Lateral tracking significantly correlated with tibial tubercle-trochlear groove distance (r = 0.48, P = .006). There was excellent agreement on lateral tracking between the 2 reviewers (intraclass correlation coefficient, 0.979; 95% confidence interval, 0.956-0.990). CONCLUSIONS:The inclusion of a dynamic radial 2-dimensional GRE sequence is a rapid and easily performed addition to the standard magnetic resonance imaging protocol and allows dynamic quantitative assessment of patellar instability and lateral maltracking in symptomatic patients. With a paucity of reported data using this technique confirming that these results reach clinical significance, future work is required to determine how much lateral tracking is clinically significant. LEVEL OF EVIDENCE/METHODS:Level III, case control.

Purpose: To evaluate biexponential T2 relaxation mapping of human knee menisci in vivo in clinically feasible scan times Materials and Methods: The study was approved by the Institutional Review Board and informed consent was obtained from eight 8 healthy volunteers (4 male&4 female, mean age 30+/-4 years, mean weight 63+/-15 kg). T2-weighted MRI of the knee was acquired using a Siemens Prisma 3TMRI scanner. A 3D Turbo-Flash sequence was modified to enable T2- weighted imaging at 10 different echo times. Ten meniscal regions of interest were assessed: the red and white zones of the anterior and posterior horns, and the body of both the medial and lateral menisci. Series of T2-weighted images were fitted using mono- and biexponential models with two- and four- parametric nonlinear approaches, respectively. Results: Biexponential relaxation of T2 was detected in the knee menisci in 10 regions of interest in all eight healthy volunteers. Global short/long relaxation components of T2 were estimated to be 6.1 / 57.2 msec with corresponding fractions of 65.16% / 34.84%, respectively. The global monoexponential relaxation of T2 was measured to be 19.8 msec. Significant differences were observed in T2 relaxation between different regions of interest. Conclusion: Biexponential relaxation of T2 was observed in the human menisci in vivo. The short and long components are thought to be related to the tightly bound and loosely bound macromolecular water compartments respectively. These preliminary results of biexponential T2 analysis could potentially be used to increase the specificity for detection of early meniscal degeneration by measuring different water compartments and their fractions

Purpose: To determine the relationships between textural 3T MRI derived measures of proximal femur bone quality and age, body mass index (BMI), and fracture risk. Materials and Methods: The study had institutional review board approval. The proximal femurs of 23 female subjects were imaged on a 3T MRI scanner. Scanning parameters were TR/TE=37ms/4.92ms, flip angle= 250, bandwidth=130Hz/pixel, field of view=120 mm, in-plane voxel dimension=0.775mmx0.775mm, parallel imaging (GRAPPA) factor=2, slice thickness=2 mm. 40 coronal slices were segmented using an inhouse software program (Fire Voxel). We computed mean bone volume, mean signal intensity (SI), and mean signal intensity inhomogeneity (standard deviation/mean) within volumes of interest for total, cortical, and trabecular compartments of the proximal femur. We used standard statistical methods to correlate these parameters with age, BMI, and FRAX hip fracture score. Results: Subjects' mean age was 60.9 years (SD +/- 9.9) and mean BMI was 22.9 kg/m² (SD +/-4.3). The mean total hip T-score was -1.9 (SD +/- 1.2). Higher BMI correlated with lower cortical SI inhomogeneity (r=0.66, p=0.0005). Higher age correlated with lower SI inhomogeneity of total, trabecular, and cortical bone compartments (r=0.51, p=0.01; r=0.43, p= 0.03; and r=0.45, 0.03 respectively). Higher FRAX score also correlated with lower cortical and total bone SI inhomogeneity. (r=0.50, p=0.03; r=0.53, p=0.02) A higher total hip T-score correlated with lower total bone SI inhomogeneity (r=0.46, p=0.02). There was no significant correlation with cortical or trabecular bone SI inhomogeneity (p=0.4, p=0.097 respectively). Conclusion: Lower MRI-computed proximal femur SI inhomogeneity correlates with higher age, BMI, FRAX score and total hip BMDT-score. This may reflect the ability of textural metrics computed from 3TMRI to detect changes in bone quality with changes in age, BMI, and fracture risk. Larger cohort studies are needed to confirm these results