Showing papers by "Sharmila Majumdar published in 2008"

In vivo T(1rho) mapping in cartilage using 3D magnetization-prepared angle-modulated partitioned k-space spoiled gradient echo snapshots (3D MAPSS).

Abstract: For T(1rho) quantification, a three-dimensional (3D) acquisition is desired to obtain high-resolution images. Current 3D methods that use steady-state spoiled gradient-echo (SPGR) imaging suffer from high SAR, low signal-to-noise ratio (SNR), and the need for retrospective correction of contaminating T(1) effects. In this study, a novel 3D acquisition scheme-magnetization-prepared angle-modulated partitioned-k-space SPGR snapshots (3D MAPSS)-was developed and used to obtain in vivo T(1rho) maps. Transient signal evolving towards the steady-state were acquired in an interleaved segmented elliptical centric phase encoding order immediately after a T(1rho) magnetization preparation sequence. RF cycling was applied to eliminate the adverse impact of longitudinal relaxation on quantitative accuracy. A variable flip angle train was designed to provide a flat signal response to eliminate the filtering effect in k-space caused by transient signal evolution. Experiments in phantoms agreed well with results from simulation. The T(1rho) values were 42.4 +/- 5.2 ms in overall cartilage of healthy volunteers. The average coefficient-of-variation (CV) of mean T(1rho) values (N = 4) for overall cartilage was 1.6%, with regional CV ranging from 1.7% to 8.7%. The fitting errors using MAPSS were significantly lower (P < 0.05) than those using sequences without RF cycling and variable flip angles.

Meniscal Measurements of T1ρ and T2 at MR Imaging in Healthy Subjects and Patients with Osteoarthritis

Abstract: Purpose: To prospectively evaluate differences in T1ρ (T1 relaxation time in the rotating frame) and T2 values in the meniscus at magnetic resonance (MR) imaging in both patients with varying degrees of osteoarthritis (OA) and healthy control subjects. Materials and Methods: The study was institutional review board approved and HIPAA compliant. Written informed consent was obtained from all subjects. T1ρ and T2 measurements were performed at 3.0-T MR imaging in 60 subjects deemed to be healthy (n = 23; mean age, 34.1 years ± 10.0 [standard deviation]; age range, 23–59 years), having mild OA (n = 27; mean age, 52.5 years ± 10.9; age range, 32–69 years), or having severe OA (n = 10; mean age, 61.6 years ± 11.6; age range, 50–86 years). Semiautomatic segmentation was performed to generate T1ρ and T2 maps of the menisci. Clinical findings were assessed by using Western Ontario and McMaster Osteoarthritis (WOMAC) questionnaires. Differences in T1ρ and T2 values between the three subject groups were calculated ...

Assessment of bone tissue mineralization by conventional x-ray microcomputed tomography: comparison with synchrotron radiation microcomputed tomography and ash measurements.

Abstract: Assessment of bone tissue mineral density (TMD) may provide information critical to the understanding of mineralization processes and bone biomechanics. High-resolution three-dimensional assessment of TMD has recently been demonstrated using synchrotron radiation microcomputed tomography (SRmuCT); however, this imaging modality is relatively inaccessible due to the scarcity of SR facilities. Conventional desktop muCT systems are widely available and have been used extensively to assess bone microarchitecture. However, the polychromatic source and cone-shaped beam geometry complicate assessment of TMD by conventional muCT. The goal of this study was to evaluate muCT-based measurement of degree and distribution of tissue mineralization in a quantitative, spatially resolved manner. Specifically, muCT measures of bone mineral content (BMC) and TMD were compared to those obtained by SRmuCT and gravimetric methods. Cylinders of trabecular bone were machined from human femoral heads (n = 5), vertebrae (n = 5), and proximal tibiae (n = 4). Cylinders were imaged in saline on a polychromatic muCT system at an isotropic voxel size of 8 microm. Volumes were reconstructed using beam hardening correction algorithms based on hydroxyapatite (HA)-resin wedge phantoms of 200 and 1200 mg HA/cm3. SRmuCT imaging was performed at an isotropic voxel size of 7.50 microm at the National Synchrotron Light Source. Attenuation values were converted to HA concentration using a linear regression derived by imaging a calibration phantom. Architecture and mineralization parameters were calculated from the image data. Specimens were processed using gravimetric methods to determine ash mass and density, muCT-based BMC values were not affected by altering the beam hardening correction. Volume-averaged TMD values calculated by the two corrections were significantly different (p = 0.008) in high volume fraction specimens only, with the 1200 mg HA/cm3 correction resulting in a 4.7% higher TMD value. MuCT and SRmuCT provided significantly different measurements of both BMC and TMD (p < 0.05). In high volume fraction specimens, muCT with 1200 mg HA/cm3 correctionteg resulted in BMC and TMD values 16.7% and 15.0% lower, respectively, than SRmuCT values. In low volume fraction specimens, muCT with 1200 mg HA/cm3 correction resulted in BMC and TMD values 12.8% and 12.9% lower, respectively, than SRmuCT values. MuCT and SRmuCT values were well-correlated when volume fraction groups were considered individually (BMC R2 = 0.97-1.00; TMD R2 = 0.78-0.99). Ash mass and density were higher than the SRmuCT equivalents by 8.6% in high volume fraction specimens and 10.9% in low volume fraction specimens (p < 0.05). BMC values calculated by tomography were highly correlated with ash mass (ash versus muCT R2 = 0.96-1.00; ash versus SRmuCT R2 = 0.99-1.00). TMD values calculated by tomography were moderately correlated with ash density (ash versus muCT R2 = 0.64-0.72; ash versus SRmuCT R2 = 0.64). Spatially resolved comparisons highlighted substantial geometric nonuniformity in the muCT data, which were reduced (but not eliminated) using the 1200 mg HA/cm3 beam hardening correction, and did not exist in the SRmuCT data. This study represents the first quantitative comparison of muCT mineralization evaluation against SRnuCT and gravimetry. Our results indicate that muCT mineralization measures are underestimated but well-correlated with SRmuCT and gravimetric data, particularly when volume fraction groups are considered individually.

Quantitative Assessment of Bone Marrow Edema-Like Lesion and Overlying Cartilage in Knees With Osteoarthritis and Anterior Cruciate Ligament Tear Using MR Imaging and Spectroscopic Imaging at 3 Tesla

Abstract: BONE MARROW EDEMA PATTERN is defined as an area of high signal intensity in T2-weighted, fat-saturated magnetic resonance (MR) images or in short inversion time inversion-recovery (STIR) images. These lesions are present in knee osteoarthritis (OA) (1-3) and acute knee injuries, frequently associated with anterior cruciate ligament (ACL) tear (4-6). Although termed as “edema,” these lesions have shown surprisingly little edema based on histopathologic examination (7). Instead, this increase in signal has been attributed to a number of other factors, including abnormal trabeculae, bone marrow necrosis, swelling of fat cells, and marrow bleeding. Therefore, in this study, we term these lesions as bone marrow edema-like lesion (BMEL). While MR findings of BMEL are common, our knowledge concerning their natural history and significance is limited. In ACL tears, BMEL indicates a so-called bone bruise or impression fracture due to translational injury, where the anterolateral femur impacts the posterolateral tibia (kissing contusions) when the ACL is ruptured. A number of studies have proposed that the overlying cartilage has sustained irreversible injury during impact, and thus, cartilage degeneration can continue to occur despite the fact that functional stability of the knee is restored following ACL reconstruction (8,9). In OA, BMEL has been associated with the severity and progression of OA (2,10), and pain in OA (10,11). However, the causal relationship between these lesions and the disease progression is unclear. Better understanding of the interrelation of cartilage and the bone immediately under it is also necessary. Recent advances in MRI allow quantitative measures of volumetric and biochemical changes of cartilage prior to macroadaptive changes seen on traditional radiographs. In particular, T1ρ has been proposed as an attractive candidate to probe the early changes in cartilage extracellular matrix (ECM) during early stages of cartilage degeneration (12-15). T1ρ describes the spin-lattice relaxation in the rotating frame. It provides low-frequency information and probes changes in macromolecules. T1ρ relaxation rate (1/T1ρ) has been shown to decrease linearly with decreasing proteoglycan (PG) content in ex vivo bovine patellae (16). In vivo studies have also shown increased cartilage T1ρ values for patients with OA (17-19). One difficulty in studying BMEL lies in quantification of these lesions due to its ill-defined shape in MR images. Most previous imaging studies used qualitative (presence of high signal or not) (4,6,11), or semiquantitative evaluations (scoring the size of bone bruise volume) (1,10). Recent studies on measuring volume of BMEL manually or semiautomatically using MR images provide potential for quantitative evaluation of such lesions (5,20-22). Such quantification of BMEL may allow better classification of these lesions and potentially a better prediction to their prognosis. Furthermore, proton MR spectroscopy (MRS) provides a noninvasive method for quantifying biochemical or metabolic changes in tissues. 3D MR spectroscopic imaging (MRSI) will further allow examining the spatial distribution of the metabolic levels. In the case of bone marrow, the water and lipids contents are of interest (23-26). Different compartments of lipids can be also investigated using MRS. Of particular interest are indices that provide measures of the unsaturation levels among the triglycerides, which may have potential medical applications (27,28). MRS has been used widely in cerebral imaging and other tissues/organs such as prostate, breast, and muscle. However, few studies have investigated MRS in knee bone marrow (29). The objective of this study was to apply quantitative MRI and 3D MRSI methods to assess BMEL in patients with OA and ACL tears. Specifically, the goals were: 1) to establish an automatic method to calculate the volume of BMEL; 2) to quantify water and lipids changes in BMEL using 3D MRSI; and 3) to assess composition changes in cartilage overlying areas of BMEL using measurements of T1ρ relaxation time.

Lactic acid and proteoglycans as metabolic markers for discogenic back pain.

Abstract: Study Design.Disc tissue was removed at surgery from 9 patients with discogenic pain and 9 deformity patients with scoliosis undergoing anterior and posterior spinal fusion. These samples were then analyzed using ex vivo proton high resolution magic angle spinning (HR-MAS) NMR spectroscopy to produc

Prevalence of pathologic findings in asymptomatic knees of marathon runners before and after a competition in comparison with physically active subjects-a 3.0 T magnetic resonance imaging study.

Abstract: To determine the prevalence of pathologic findings in asymptomatic knees of marathon runners before and after a competition in comparison with physically active subjects. To compare the diagnostic performance of cartilage-dedicated magnetic resonance imaging (MRI) sequences at 3.0 T. Ten marathon runners underwent 3.0 T MRI 2–3 days before and after competition. Twelve physically active asymptomatic subjects not performing long-distance running were examined as controls. Pathologic condition was assessed with the whole-organ magnetic resonance imaging score (WORMS). Cartilage abnormalities and bone marrow edema pattern (BMEP) were quantified. Visualization of cartilage pathology was assessed with intermediate-weighted fast spin-echo (IM-w FSE), fast imaging employing steady-state acquisition (FIESTA) and T1-weighted three-dimensional (3D) high-spatial-resolution volumetric fat-suppressed spoiled gradient-echo (SPGR) MRI sequences. Eight of ten marathon runners and 7/12 controls showed knee abnormality. Slightly more and larger cartilage abnormalities, and BMEP, in marathon runners yielded higher but not significantly different WORMS (P > 0.05) than in controls. Running a single marathon did not alter MR findings substantially. Cartilage abnormalities were best visualized with IM-w FSE images (P < 0.05). A high prevalence of knee abnormalities was found in marathon runners and also in active subjects participating in other recreational sports. IM-w FSE sequences delineated more cartilage MR imaging abnormalities than did FIESTA and SPGR sequences.

Diagnostic performance of in vivo 3-T MRI for articular cartilage abnormalities in human osteoarthritic knees using histology as standard of reference

Abstract: The purpose of this study was (1) to evaluate the sensitivity, specificity and accuracy of sagittal in vivo 3-T intermediate-weighted fast spin-echo (iwFSE) sequences in the assessment of knee cartilage pathologies using histology as the reference standard in patients undergoing total knee replacement, and (2) to correlate MR imaging findings typically associated with osteoarthritis such as bone marrow edema pattern (BMEP) and cartilage swelling with histological findings. Tibial plateaus and femoral condyles of eight knees of seven patients were resected during surgery, and sagittal histological sections were prepared for histology. Preoperative MRI findings were compared to the corresponding region in histological sections for thickness, surface integrity and signal pattern of cartilage, and histological findings in areas of BMEP and swelling were documented. The overall sensitivity, specificity and accuracy were 72%, 69% and 70% for thickness, 69%, 74% and 73% for surface and 36%, 62% and 45% for intracartilaginous signal pattern. For all cases of BMEP on MRI subchondral ingrowth of fibrovascular tissue and increased bone remodeling were observed. MRI using fat-saturated iwFSE sequences showed good performance in assessing cartilage thickness and surface lesions, while signal changes of cartilage were not suited to characterize the severity of cartilage degeneration as validated by histology.

Assessment of trabecular bone structure comparing magnetic resonance imaging at 3 Tesla with high-resolution peripheral quantitative computed tomography ex vivo and in vivo

Abstract: Summary In vivo high-resolution peripheral quantitative micro-CT (HR-pQCT) is a new modality for imaging peripheral sites like the distal tibia and the distal radius, providing structural bone parameters. Comparing HR-pQCT with MRI, we found that both modalities are capable of offering meaningful information on trabecular structure.

In Vivo T1ρ Quantitative Assessment of Knee Cartilage After Anterior Cruciate Ligament Injury Using 3 Tesla Magnetic Resonance Imaging

Abstract: The anterior cruciate ligament (ACL) is the most commonly torn ligament in the knee, with more than 80,000 injuries occurring annually in the United States alone.1 The rupture of the ACL frequently occurs in young active individuals. The injury is usually a result of a valgus and internal rotational torque to the knee. During the injury, the knee can sublux after ACL rupture and the lateral femoral condyle (LFC) can impact the posterior aspect of the lateral tibia, resulting in the classic bone marrow edema-like lesions (BMELs) or osteochondral lesions within the lateral compartment, also known as the “kissing lesions.”2 The prevalence of BMEL in patients with ACL injuries has been reported to be up to 70% to 80%.3,4 The BMEL is usually located in the middle portion of the lateral femoral condyle and the posterior portion of the lateral tibial plateau. Despite the high prevalence of these BMEL with ACL ruptures, little is known about the clinical consequences of these findings or about their relationship with local and global cartilage degeneration. Magnetic resonance imaging (MRI) is a useful diagnostic tool for ACL tears and related injuries.5 BMEL detected with MRI has been described previously as focally signal hyperintensity in the bone marrow on fat-suppressed T2-weighted MR images or in short inversion time inversion-recovery images due to multiple factors such as abnormal trabeculae, bone marrow necrosis, marrow hemorrhage, and marrow edema.6 The formation of bone bruises has been shown to cause substantial damage to the articular cartilage leading to cell death of a significant number of chondrocytes.7 Despite ACL reconstruction to correct instability, a significant proportion of patients develop clinical symptoms of osteoarthritis (OA) 5 to 10 years postinjury.8,9 These findings suggest that initial injury to the articular cartilage may predispose the joint to the development of OA. Therefore, a detailed topographic analysis of the articular cartilage is important to identify local cartilage degeneration after ACL injury. Quantitative MRI is a noninvasive technique for assessing acute knee injuries and can play an important role in determining treatment options and evaluating surgical interventions. In cartilage, the proteoglycan (PG) is largely responsible for the high elasticity and resilience of tissue. The PG content of cartilage can be probed using spin lattice relaxation in the rotating frame: T1ρ-weighted imaging.10 This particular quantitative method based on spin-locking technique reflects the slow motion interactions between motion-restricted water molecules and their local macromolecular environment. T1ρ mapping has recently been proposed as a potential of MRI to reflect changes in the biochemical composition of cartilage with early OA, such as PG loss.11–13 Several recent ex vivo14 and in vivo15 studies investigated this relaxation technique in human OA, suggesting that T1ρ relaxation time increases with the degree of OA. Our previous case report on quantitative MRI has demonstrated that cartilage injury was present overlying BMEL after ACL injuries.16 In that report, elevated T1ρ relaxation time was found over the lateral tibia confirmed with arthroscopic findings of cartilage injury. This study prompted our current clinical evaluation on determining the severity of cartilage injury over BMEL. The goal of this study was to assess cartilage overlying areas of BMEL versus surrounding cartilage and to analyze defined cartilage compartments using T1ρ quantification in patients with acute ACL tears. The hypothesis of the study is that the cartilage degeneration can occur during acute ACL injuries. We also hypothesized that there can be a different T1ρ quantification at various regions of the knee. In this study, we compared T1ρ quantification in patients with acute ACL injuries versus healthy individuals.

In vivo ultra-high-field magnetic resonance imaging of trabecular bone microarchitecture at 7 T.

Abstract: Purpose To investigate the feasibility of 7T magnetic resonance imaging (MRI) to visualize and quantify trabecular bone structure in vivo by comparison with 3T MRI and in vivo three-dimensional (3D) high-resolution peripheral quantitative computed tomography (HR-pQCT). Materials and Methods The distal tibiae of 10 healthy volunteers were imaged. Therefore, fully balanced steady state free precession (bSSFP) and spin-echo (bSSSE) pulse sequences were implemented and optimized for 7T. Structural bone parameters, such as apparent bone-volume over total-volume fraction (app.BV/TV), apparent trabecular plate separation (app.TbSp), apparent trabecular plate thickness (app.TbTh), and apparent trabecular plate number (app.TbN), were derived. Results All structural trabecular bone parameters correlated well (r > 0.6) between 7T and 3T, and between 7T and HR-pQCT (r > 0.69), with the exception of app.TbTh, which correlated modestly (r = 0.41) between field strengths and very low with HR-pQCT (r < 0.16). Regarding absolute values, app.TbN varied only 4% between field strengths, and only 0.6% between 7T and HR-pQCT. App.TbSp correlated best between 7T and HR-pQCT (r = 0.89). Using bSSSE, significant smaller trabecular thickness and significant higher trabecular number were found compared to bSSFP. Conclusion We concluded that imaging and quantification of the trabecular bone architecture at 7T is feasible and preferably done using bSSSE. There exists great potential for ultra-high-field (UHF) MRI applied to trabecular bone measurements. J. Magn. Reson. Imaging 2008;27:854–859. © 2008 Wiley-Liss, Inc.

Feasibility and reproducibility of relaxometry, morphometric, and geometrical measurements of the hip joint with magnetic resonance imaging at 3T.

Abstract: Purpose To test the feasibility of in vivo magnetic resonance T1ρ relaxation time measurements of hip cartilage, and quantify the reproducibility of hip cartilage thickness, volume, T2, T1ρ, and size of femoral head measurements. Materials and Methods The hip joint of five human healthy volunteers, one subject with mild hip osteoarthritis (OA) and one subject with advanced hip OA, was imaged with magnetic resonance imaging (MRI) at 3T. Hip cartilage thickness, volume, T1ρ, and T2 were quantified, as well as the size of the femoral head. All imaging and analysis procedures were performed twice for the healthy volunteers to assess reproducibility. Results In vivo MR T1ρ measurements of hip cartilage at 3T were feasible as demonstrated by high quality images and relaxation time maps. High levels of reproducibility were obtained for measurements of hip cartilage thickness (CVSD = 2.19%), volume (CVSD = 3.5%), T2 (CVSD = 5.89%), T1ρ (CVSD = 2.03%), and size of femoral head (CVSD = 0.49%). Mean T2 and T1ρ relaxation time values for human healthy subjects were 28.38 (±2.66) msec and 38.72 (±3.84) msec, respectively. Mean T2 and T1ρ relaxation time values for subjects with OA were 34.78 (±8.36) msec and 44.07 (±0.99) msec, respectively. T2 and T1ρ values increased from the deep to the superficial layers. Conclusion Qualitative and quantitative results indicate that the MRI techniques presented in this study may be applied clinically to patients with OA of the hip to investigate these parameters at different stages of disease. J. Magn. Reson. Imaging 2008;28:227–235. © 2008 Wiley-Liss, Inc.

Proximal Femur Specimens: Automated 3D Trabecular Bone Mineral Density Analysis at Multidetector CT—Correlation with Biomechanical Strength Measurement

Abstract: Purpose: To prospectively evaluate an automated volume of interest (VOI)-fitting algorithm for quantitative computed tomography (CT) of proximal femur specimens, correlate bone mineral density (BMD) with biomechanically determined bone strength in vitro, and compare that correlation with those observed at dual-energy x-ray absorptiometry (DXA) measurement of BMD. Materials and Methods: The study was compliant with institutional and legislative requirements; donors had dedicated their body for education and research before death. Multidetector CT and DXA scans were acquired in 178 proximal femur specimens harvested from human cadavers (91 women, 87 men; mean age at death, 79 years ± 10.2; range, 52–100 years). An automated VOI-fitting algorithm was used to calculate BMD and bone mineral content (BMC) in the head, neck, and trochanter from CT findings and pixel distribution parameters. The femur failure load (FL) was determined by using a mechanical test. Quantitative CT BMD, quantitative CT pixel distribut...

Randomized Trial of Once-Weekly Parathyroid Hormone (1-84) on Bone Mineral Density and Remodeling

Abstract: Context: Daily PTH administration increases bone mineral density (BMD) and reduces fracture risk. However, cost and compliance significantly limit clinical use. Objective: Our objective was to determine whether less frequent PTH administration increases lumbar spine BMD. Participants, Design, and Setting: Fifty postmenopausal women ages 45–70 yr with femoral neck BMD T-score between −1.0 and −2.0 participated in a double-blind, randomized, placebo-controlled trial at St. Joseph Hospital, Bangor, ME. Intervention: Subjects received sc injections of daily PTH(1-84) (100 μg) or placebo for 1 month, followed by weekly injections (PTH or placebo) for 11 months. Outcomes: Change in lumbar spine dual-energy x-ray absorptiometry areal BMD (primary) was assessed. Secondary outcomes included volumetric BMD at spine and hip by quantitative computed tomography, trabecular bone microarchitecture by magnetic resonance imaging of distal radius, and biochemical bone turnover markers. Results: At 12 months, lumbar spine areal BMD increased 2.1% in PTH-treated women compared with placebo (P = 0.03). Vertebral trabecular volumetric BMD increased 3.8% in PTH-treated women compared with placebo group (P = 0.08). PTH-treated women also had higher distal radial trabecular bone volume, number, and thickness compared with placebo-treated women (P < 0.04). After 1 month of daily PTH, N-terminal propeptide of type I collagen (P1NP) was markedly increased compared with placebo (P < 0 .0001), and a difference persisted, although lessened, throughout the study. Bone resorption indices were unchanged in PTH-treated women and were reduced in the placebo group. Conclusion: Once-weekly PTH after 1 month of daily treatment increases spine BMD, radial trabecular bone, and bone formation markers in postmenopausal women. These results suggest that less frequent alternatives to daily PTH dosing for 2 yr could be effective. Additional studies are required to define the optimal frequency of PTH administration.

New techniques for cartilage magnetic resonance imaging relaxation time analysis: Texture analysis of flattened cartilage and localized intra- and inter-subject comparisons

Abstract: MR relaxation time measurements of knee cartilage have shown potential to characterize knee osteoarthritis (OA). In this work, techniques that allow localized intra- and inter-subject comparisons of cartilage relaxation times, as well as cartilage flattening for texture analysis parallel and perpendicular to the natural cartilage layers, are presented. The localized comparisons are based on the registration of bone structures and the assignment of relaxation time feature vectors to each point in the bone-cartilage interface. Cartilage flattening was accomplished with Bezier splines and warping, and texture analysis was performed with second-order texture measures using gray-level co-occurrence matrices (GLCM). In a cohort of five normal subjects the performance and reproducibility of the techniques were evaluated using T1rho maps of femoral knee cartilage. The feasibility of creating a mean cartilage relaxation time map is also presented. Successful localized intra- and inter-subject T1rho comparisons were obtained with reproducibility similar to that reported in the literature for regional T2. Improvement of the reproducibility of GLCM features was obtained by flattening the T1rho maps. The results indicate that the presented techniques have potential in longitudinal and population studies of knee OA at different stages of the disease.

Rapid in vivo musculoskeletal MR with parallel imaging at 7T.

Abstract: The purpose of this work was to implement autocalibrating GRAPPA-based parallel imaging (PI) for in vivo high-resolution (HR) MRI of cartilage and trabecular bone micro-architecture at 7T and to evaluate its performance based on comparison of MR-derived morphology metrics between accelerated and conventional images and comparison of geometry factor measures between 3T and 7T. Using an eight channel coil array for trabecular MRI at the ankle, a higher maximum feasible acceleration (R) = 6 and lower geometry factor values than that at 3T were observed. The advantages of two-dimensional acceleration were also demonstrated. In knee cartilage and bone acquisitions, feasibility of PI with a dual-channel quadrature coil was investigated. Robust quantification of bone and cartilage metrics could be derived from accelerated ankle and knee acquisitions. PI can enhance the clinical feasibility of in vivo bone and cartilage HR-MRI for osteoporosis and osteoarthritis at 7T. Magn Reson Med, 2008. © 2008 Wiley-Liss, Inc.

Magnetic resonance imaging of iron oxide labelled stem cells: applications to tissue engineering based regeneration of the intervertebral disc

Abstract: Minimally-invasive monitoring of regeneration in diseased tissue is an important aspect of stem cell therapy. Magnetic resonance imaging (MRI) based tracking of cells labelled with ferumoxides has the potential for non-invasive in vivo detection and longitudinal assessment of implanted cells. Cells labelled with ferumoxides appear as hypointense regions on MR images and thus can be distinguished from the surroundings. Application of this methodology to intervertebral disc degeneration (IVD), and detection of labelled cells implanted into the disc for tissue regeneration was examined. Mesenchymal stem cells labelled with a ferumoxide contrast agent were imaged in vitro to quantitatively characterize the signal intensity loss using MRI relaxation parameters (T1, T2, and T2*). To determine whether labelled cells could be detected within scaffolds suitable for implantation, labelled cells were seeded within both natural and synthetic polymers and imaged using MRI. Labelled cells were loaded within poly(ethylene glycol) hydrogels and imaged in vitro using both MRI and confocal microscopy. Labelled cells were also loaded into fibrin gels, and detected ex vivo within rat IVDs using MRI. Lastly, the effect of ferumoxide labelling on cell viability was investigated. Quantitatively, labelled cells demonstrate the greatest signal intensity loss and contrast on T2*-weighted images. Labelled cells can be detected in both synthetic and natural polymers, and can be distinguished from the native tissue environment of the rat IVD. Finally, labelling does not significantly impair cell viability. Consequently, this technique shows promise as a potential method for in vivo longitudinal tracking of stem cell based regeneration of the IVD.

Magnetic resonance imaging for osteoporosis

Abstract: Osteoporosis is characterized by a decrease in bone strength and an increased propensity to fracture. The routine assessment of skeletal integrity is carried out using dual X-ray absorptiometry (DXA), which is a measure of areal bone density, and does not capture all factors contributing to bone strength. In addition to bone mass, density, and content, the trabecular and cortical bone architecture, mineralization, micro-fracture, and damage repair also contribute to bone strength. Site-specific bone structure information would significantly contribute to further understanding the determinants of fracture risk, and influence optimization of therapy. Considerable effort is being expended in developing techniques to assess trabecular bone micro-architecture non-invasively. Three-dimensional techniques that reveal trabecular bone structure are emerging as important tools for defining some features of bone quality not assessed by DXA. Microcomputed tomography (μCT) has recently been developed to provide high resolution images of trabecular architecture. This method is routinely used in specimen evaluation and has recently been extended to in vivo animal [1] and human extremity imaging [2]. A more recent development in the assessment of trabecular bone structure is the use of magnetic resonance imaging (MRI) techniques that serve as a surrogate for bone biopsies at multiple anatomic sites. Trabecular bone consists of a network of rod-like elements interconnected by plate-like elements, immersed in bone marrow that is composed partly of water and partly of fat. The percentage of fat and water in bone marrow varies and depends on the skeletal site. Magnetic susceptibility of trabecular bone is substantially different from that of bone marrow. This gives rise to susceptibility gradients at every bone–bone marrow interface. Magnetic inhomogeneity arising from these susceptibility gradients depends on the static magnetic field strength, number of bone–bone marrow interfaces, and the size of individual trabeculae. Techniques to quantify these effects and relate the effect size to trabecular bone have been developed [3]. However, the real potential of obtaining in vivo images of trabecular bone, as shown for the tibia in Fig. 1, is perhaps the most exciting contribution that MRI can make in the area of osteoporosis. Improved coils and increased field strengths have made it possible to obtain highresolution MRI in plane resolutions starting at ~150 μm and slice thickness starting at 300 μm (see Fig. 1 in which trabecular bone is depicted as dark striations amidst bright bone marrow). Images such as these may be quantified to characterize the complex trabecular bone network. Several in vitro studies relating the MRI-derived measures of trabecular structure to bone strength have been conducted [4, 5]. In vivo imaging studies at the distal radius, calcaneus, tibia, and the proximal femur at field strengths ranging from 1.5–7 Tesla have also been conducted in humans. Images have been obtained using gradient echoand spin echo-based techniques, and the trade-off between coverage, signal-to-noise ratio, total imaging time, and field strength make this an active area of research and clinical applications. Following image acquisition several computerized image processing steps are undertaken. These may include region-of-interest extraction, registration with baseline images for longitudiSkeletal Radiol (2008) 37:95–97 DOI 10.1007/s00256-007-0412-5

Feasibility of Measuring Trabecular Bone Structure of the Proximal Femur Using 64-Slice Multidetector Computed Tomography in a Clinical Setting

Abstract: We studied the feasibility of cancellous bone structure assessment of the proximal femur using multidetector computed tomography (MDCT) in an simulated in vivo experimental model. The proximal femur of 15 intact human cadavers was examined using 64-row MDCT using a thin-section protocol with an in-plane spatial resolution of 273 μm. High-resolution peripheral quantitative computed tomography (HR-pQCT) of the isolated specimens with a voxel size of 82 μm served as a standard of reference. Trabecular bone structure and optimized textural parameters were calculated in MDCT images and compared to measures obtained by HR-pQCT. Significant correlations between MDCT- and HR-pQCT-derived values for bone fraction (r = 0.87), trabecular separation (r = 0.66), and number (r = 0.53) were found. Parameters derived from textural analysis performed better in predicting trabecular separation (up to r = 0.86) and number (up to r = 0.83). Trabecular thickness could not be quantified correctly using MDCT, most likely due to its limited resolution. Individual parameters for assessement of trabecular microarchitecture can be measured using MDCT-derived imaging studies and a simulated in vivo setup. Thus, in vivo assessment of bone architecture in addition to BMD may be feasible in clinical practice.

Three-dimensional image registration of MR proximal femur images for the analysis of trabecular bone parameters.

Abstract: This study investigated the feasibility of automatic image registration of MR high-spatial resolution proximal femur trabecular bone images as well as the effects of gray-level interpolation and volume of interest (VOI) misalignment on MR-derived trabecular bone structure parameters For six subjects in a short-term study, a baseline scan and a follow-up scan of the proximal femur were acquired on the same day For ten subjects in a long-term study, a follow-up scan of the proximal femur was acquired 1 year after the baseline An automatic image registration technique, based on mutual information, utilized a baseline and a follow-up scan to compute transform parameters that aligned the two images In the short-term study, these parameters were subsequently used to transform the follow-up image with three different gray-level interpolators Nearest-neighbor interpolation and B -spline approximation did not significantly alter bone parameters, while linear interpolation significantly modified bone parameters ( p 001 ) Improvement in image alignment due to the automatic registration for the long-term and short-term study was determined by inspecting difference images and 3D renderings This work demonstrates the first application of automatic registration, without prior segmentation, of high-spatial resolution trabecular bone MR images of the proximal femur Additionally, inherent heterogeneity in trabecular bone structure and imprecise positioning of the VOI along the slice (anterior–posterior) direction resulted in significant changes in bone parameters ( p 001 ) Results suggest that automatic mutual information registration using B -spline approximation or nearest neighbor gray-level interpolation to transform the final image ensures VOI alignment between baseline and follow-up images and does not compromise the integrity of MR-derived trabecular bone parameters used in this study

Analyser-based tomography images of cartilage.

Abstract: Analyser-based imaging expands the performance of X-ray imaging by utilizing not only the absorption properties of X-rays but also the refraction and scatter rejection (extinction) properties. In this study, analyser-based computed tomography has been implemented on imaging an articular cartilage sample, depicting substructural variations, without overlay, at a pixel resolution of 3.6 microm.

Three-dimensional image registration of MR proximal femur images for the analysis of trabecular bone parameters

Abstract: This study investigated the feasibility of automatic image registration of MR high-spatial resolution proximal femur trabecular bone images as well as the effects of gray-level interpolation and volume of interest (VOI) misalignment on MR-derived trabecular bone structure parameters. For six subjects, a baseline scan and a follow-up scan of the proximal femur were acquired on the same day. An automatic image registration technique, based on mutual information, utilized a baseline and a follow-up scan to compute transform parameters that aligned the two images. These parameters were subsequently used to transform the follow-up image with three different gray-level interpolators. Nearest neighbor interpolation and b-spline approximation did not significantly alter bone parameters, while linear interpolation significantly modified bone parameters (p<0.01). Improvement in image alignment due to the automatic registration was determined by visually inspecting difference images and 3D renderings. This work demonstrates the first application of automatic registration, without prior segmentation, of high-spatial resolution trabecular bone MR images of the proximal femur. Additionally, effects due to imprecise analysis volume alignment are investigated. Inherent heterogeneity in trabecular bone structure and imprecise positioning of the VOI along the slice (A/P) direction resulted in significant changes in bone parameters (p<0.01). Results suggest that automatic mutual information registration using nearest-neighbor gray-level interpolation to transform the final image ensures VOI alignment between baseline and follow-up images and does not compromise the integrity of MR-derived trabecular bone parameters.

Studying the effect of noise on the performance of 2D and 3D texture measures for quantifying the trabecular bone structure as obtained with high resolution MR imaging at 3 Tesla

Abstract: 3.0 Tesla MRI devices are becoming popular in clinical applications since they render images with a higher signal-tonoise ratio than the former 1.5 Tesla MRI devices. Here, we investigate if higher signal-to-noise ratio can be beneficial for a quantitative image analysis in the context of bone research. We performed a detailed analysis of the effect of noise on the performance of 2D morphometric linear measures and a 3D nonlinear measure with respect to their correlation with biomechanical properties of the bone expressed by the maximum compressive strength. The performance of both 2D and 3D texture measures was relatively insensitive to superimposed artificial noise. This finding suggests that MR sequences for visualizing bone structures at 3T should rather be optimized to spatial resolution (or scanning time) than to signal-to-noise ratio.