In vivo bone marrow lipid characterization with line scan Carr-Purcell-Meiboom-Gill proton spectroscopic imaging

Estimating bone mineral density using MRI in medicine and dentistry: a literature review

OBJECTIVES

Studies performed in the medical area have shown that an indirect diagnosis of bone mineral density (BMD) is feasible by assessing the amount of bone marrow fat with non-ionizing magnetic resonance imaging (MRI). In dentistry, radiographic methods are still the most used for alveolar bone diagnosis. The present literature review aimed at addressing the role of MRI in assessing BMD in medicine and dentistry.

METHODS

MEDLINE and EMBASE databases were searched for articles published up to 2019.

RESULTS

A total of 428 potentially eligible papers were screened. Of these, 397 were excluded after title, abstract and keyword assessment, yielding 31 papers that potentially met the inclusion criteria. Eleven studies were then excluded because their full texts did not discuss the role of MRI in the indirect diagnosis of BMD. As a result, a total of 20 studies were finally identified as eligible for inclusion in this literature review. Most studies found satisfactory accuracy of MRI for indirectly assessing BMD by quantifying bone mineral fat (BMF). However, only one of these studies was on dentistry.

CONCLUSION

Within the limitations of this study, the present findings suggest that MRI is accurate to indirectly estimate bone density by assessing BMF, and could be clinically relevant during dental treatment planning.

Bone marrow fat contributes to insulin sensitivity and adiponectin secretion in premenopausal women

PURPOSE

Bone marrow fat is a functionally distinct adipose tissue that may contribute to systemic metabolism. This study aimed at evaluating a possible association between bone marrow fat and insulin sensitivity indices.

METHODS

Fifty obese (n = 23) and non-obese (n = 27) premenopausal women underwent proton magnetic resonance spectroscopy to measure vertebral bone marrow fat content and unsaturation index at L4 level. Abdominal visceral, subcutaneous fat, and epicardial fat were also measured using magnetic resonance imaging. Bone mineral density was measured by dual-energy X-ray absorptiometry. Body composition was assessed by bioelectrical impedance analysis. Fasting serum glucose, insulin, lipids, adiponectin were measured; the insulin resistance index HOMA (HOMA-IR) was calculated.

RESULTS

Bone marrow fat content and unsaturation index were similar in obese and non-obese women (38.5 ± 0.1 vs. 38.6 ± 0.1%, p = 0.994; 0.162 ± 0.065 vs. 0.175 ± 0.048, p = 0.473, respectively). Bone marrow fat content negatively correlated with insulin and HOMA-IR (r = -0.342, r = -0.352, respectively, p = 0.01) and positively with high density lipoprotein cholesterol (r = 0.270, p = 0.043). From a multivariate regression model including lnHOMA-IR as a dependent variable and visceral, subcutaneous, epicardial fat, and bone marrow fat as independent variables, lnHOMA-IR was significantly associated with bone marrow fat (β = -0.008 ± 0.004, p = 0.04) and subcutaneous fat (β = 0.003 ± 0.001, p = 0.04). Bone marrow fat, among the other adipose depots, was a significant predictor of circulating adiponectin (β = 0.147 ± 0.060, p = 0.021). Bone marrow fat unsaturation index negatively correlated with visceral fat (r = -0.316, p = 0.026).

CONCLUSIONS

There is a relationship between bone marrow fat content and insulin sensitivity in obese and non-obese premenopausal women, possibly mediated by adiponectin secretion. Visceral fat does not seem to regulate bone marrow fat content while it may affect bone marrow fat composition.

Bone marrow segmentation based on a combined consideration of transverse relaxation processes and Dixon oscillations

The aim of this study was to demonstrate that gradient-echo sampling of single spin echoes can be used to isolate the signal from trabecular bone marrow, with high-quality segmentation and surface reconstructions resulting from the application of simple post-processing strategies. Theoretical expressions of the time-domain single-spin-echo signal were used to simulate signals from bone marrow, non-bone fatty deposits and muscle. These simulations were compared with and used to interpret signals obtained by the application of the gradient-echo sampling of a spin-echo sequence to image the knee and surrounding tissues at 1.5 T. Trabecular bone marrow has a much higher reversible transverse relaxation rate than surrounding non-bone fatty deposits and other musculoskeletal tissues. This observation, combined with a choice of gradient-echo spacing that accentuates Dixon-type oscillations from chemical-shift interference effects, enabled the isolation of bone marrow signal from surrounding tissues through the use of simple image subtraction and thresholding. Three-dimensional renderings of the marrow surface were then readily generated with this approach - renderings that may prove useful for bone morphology assessment, e.g. for the measurement of femoral anteversion. In conclusion, understanding the behavior of signals from bone marrow and surrounding tissue as a function of time through a spin echo facilitates the segmentation and reconstruction of bone marrow surfaces using straightforward post-processing strategies that are typically available on modern radiology workstations.

Quantification of bone marrow water and lipid composition in anterior cruciate ligament-injured and osteoarthritic knees using three-dimensional magnetic resonance spectroscopic imaging

PURPOSE

To quantitatively evaluate longitudinal changes in water and lipid in knee bone marrow with and without bone marrow edema-like lesions (BMELs) in subjects with acutely ruptured anterior cruciate ligaments (ACLs) or osteoarthritis (OA) using three-dimensional magnetic resonance spectroscopic imaging (3D MRSI).

MATERIAL AND METHODS

Ten ACL and 10 OA subjects who presented with BMEL and seven BMEL-free controls were scanned at 3T. All ACL and OA subjects had one-year follow-up scans. 3D MRSI was acquired in BMEL and adjacent bone marrow, and water content (WC) and unsaturated lipid index (UI) were calculated in each region of interest.

RESULTS

At baseline, ACL BMEL WC was significantly higher than ACL non-BMEL, OA BMEL, and control WC; ACL non-BMEL WC, ACL BMEL UI, and OA BMEL WC were significantly higher than control. ACL BMEL WC decreased significantly one year post-reconstruction; UI decreased non-significantly (p=0.09). No significant changes in OA BMEL or ACL and OA non-BMEL WC and UI were observed.

CONCLUSION

3D MRSI is a powerful method of quantitatively assessing the biochemical composition of bone marrow in OA and ACL-injured knees, which may serve as imaging markers to improve comprehension of primary and secondary OA pathology.

Time course of osteonecrosis in rabbit articular intercalated bone: line scan spectroscopic imaging and correlation with histology

PURPOSE

Magnetic resonance (MR) imaging offers the highest sensitivity for detecting bone necrosis. We evaluated osteonecrosis in rabbit models by calculating the percentage of fat to (fat + water) [F/(F+W)] on MR spectroscopy (MRS) and compared MR spectroscopy and imaging findings with corresponding histological results.

METHODS

To model the natural course of articular osteonecrosis, we removed the fourth tarsal bone in 45 rabbits, froze it for 5 min in liquid nitrogen to produce complete cellular necrosis, and then replaced the bone into the knee joint. We performed Carr-Purcell-Meiboom-Gill proton spectroscopic imaging to assess necrotic bone at 3 days and one, 2, 3, 4, 8, 12, 16, and 20 weeks after osteonecrosis and calculated the percentage of F/(F+W) of each bone. We also performed conventional T1- and T2-weighted imaging and compared all data to histological findings to analyze the natural course of necrosis.

RESULTS

T1-weighted MR imaging demonstrated obvious low signal intensity at 2 to 8 weeks and recovery at 12 to 20 weeks, whereas T2-weighted imaging demonstrated inconsistent intensities throughout the period. The postoperative percentage of F/(F+W) measured using line scan MRS decreased to 8.88% at 3 weeks, 6.22% at 8 weeks, and 34.40% at 20 weeks results that were mostly consistent with MR imaging findings. Histological findings demonstrated complete absence of osteocyte nuclei and loss of osteoid-osteogenesis at 3 to 8 weeks. Recovery of bone marrow was identified as an increase in the area of fat after 12 weeks.

CONCLUSION

Osteonecrosis delineated by T1-weighted MR imaging demonstrated fat content in the bone marrow that correlated with histology. The present MRS modality can be used to calculate the percentage of F/(F+W) of osteonecrosis to enable objective assessment of recovery and quantification of osteonecrosis to provide a numerical value for osteonecrosis.

Fraction of unsaturated fatty acids in visceral adipose tissue (VAT) is lower in subjects with high total VAT volume - a combined 1 H MRS and volumetric MRI study in male subjects

Visceral adipose tissue (VAT) is thought to play an important role in the pathogenesis of obesity and insulin resistance. However, little is known about the composition of VAT with regard to the amount of mono- (MUFAs) and polyunsaturated fatty acids (PUFAs) in triglycerides. Volume-selective MRS was performed in addition to MRI for the quantification of VAT. Analysis comprised proton signals from the vinyl-H group (H-C=C-H), including protons from MUFA+PUFA, and diallylic-H, i.e. methylene-interrupted PUFAs. The methyl (-CH(3) ) resonance, which is the only peak with a defined number of protons/triglyceride (n=9), served as reference. Twenty male subjects participated in this prospective study and underwent MRS of VAT on a 3-T whole-body unit. Spectra were recorded by a single-voxel stimulated echo acquisition mode (STEAM) technique (TE/TM/TR=20/10/4000 ms; volume of interest between 20 × 25 × 20 and 30 × 30 × 20 mm(3); 48-80 acquisitions depending on the size of the volume of interest; bandwidth, 1200 Hz). Post-processing was performed by a Java-based magnetic resonance user interface (jMRUI; AMARES). The volume of VAT was quantified in a separate session on a 1.5-T imager a few days prior to the MRS session by T(1) -weighted imaging. The relative amount of VAT was calculated as a percentage of body weight (%VAT). Ratios of vinyl-H to -CH(3) and diallylic-H to -CH(3) were calculated. All spectra recorded from VAT were of high quality, enabling reliable quantification of the mentioned resonances. %VAT and vinyl-H/CH(3) varied over a broad range (2.8-8.3% and 0.45-0.64, respectively). A strong negative correlation between %VAT and vinyl-H/CH(3) was found (r= -0.92), whereas diallylic-H/CH(3) alone was clearly less well correlated with %VAT (r= -0.21). The composition of VAT shows strong interindividual variations. The greater the total amount of VAT, the less unsaturated the fatty acids. This is a preliminary result in mainly obese male subjects, and it remains to be determined whether this correlation holds for other cohorts of different age, gender and body mass index. Furthermore, changes in VAT composition during weight loss or different forms of diet have yet to be examined.

Is 3T-MR spectroscopy a predictable selection tool in prophylactic vertebroplasty?

This study was designed to confirm relationships between decrease of bone mineral density and increase of marrow fat and to delineate, through MR spectroscopy, vertebral body at high risk for compression fracture onset to justify prophylactic vertebroplasty. We enrolled 127 women: 48 osteoporotic, 36 osteopenic, and 43 normal subjects, who underwent DXA and MR examination of spine. Then, we selected 48 patients with at least two acute osteoporotic vertebral fractures with interposed normal "sandwich" vertebrae; all patients underwent MR examination of spine. Significant statistical differences were found among "Fat Fraction" (FF) values in normal, osteopenic, and osteoporotic subjects: 59.8 ± 5.1%; 64.8 ± 4.4%; and 67.1 ± 3.3%. A mild, significant, negative correlation was observed between T-score and vertebral fat content (r = - 0.585; P = 0.0000). In the second part of the study, 9 new vertebral fractures were observed in 48 patients (19%): 6 were "sandwich" vertebrae (12.5%), and 3 were located in distant vertebral body. The mean FF in sandwich fractured vertebrae was 72.75 ± 1.95 compared with the FF of the nonfractured sandwich, and distant control vertebrae were 61.83 ± 3.42 and 61.42 ± 3.64. We found a significant statistical difference between fractured and nonfractured vertebrae (P < 0.001). The results of this study suggest that MR spectroscopy could be a reliable index to predict the risk of new compression vertebral fracture and could be used for vertebroplasty planning contributing to clarify the possibility to add prophylactic PVP to standard treatment.

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

PURPOSE

To quantitatively assess bone marrow edema-like lesion (BMEL) and the overlying cartilage in osteoarthritis (OA) or anterior cruciate ligament (ACL)-injured knees using magnetic resonance imaging (MRI) and spectroscopic imaging (MRSI).

MATERIALS AND METHODS

Eight healthy controls and 30 patients with OA and other injuries who showed BMEL were scanned at 3.0T. A regression model was constructed to automatically calculate the volume of BMEL. Normalized T(1rho) z-scores were calculated within BMEL-overlying cartilage. Three-dimensional (3D) MRSI was acquired in the BMEL and surrounding bone marrow.

RESULTS

The mean T(1rho) z-score was significantly higher in BMEL-overlying cartilage than that in surrounding cartilage in the lateral tibia of patients with ACL tears (P < 0.001). Significantly elevated water and unsaturated lipids, and decreased saturated lipids were observed in BMEL. The volume of elevated water correlated with the volume of BMEL. Water content was significantly higher within BMEL than that outside BMEL. The unsaturation index outside BMEL was significantly higher in patients with ACL tears than that in OA.

CONCLUSION

3D MRSI and T(1rho) mapping provide tools to quantitatively evaluate BMEL in OA and knee injury. This may allow us to better understand pathophysiology and evolution of injuries and degenerative conditions of the knee.

Osteoporosis is associated with increased marrow fat content and decreased marrow fat unsaturation: a proton MR spectroscopy study

PURPOSE

To use proton magnetic resonance spectroscopy ((1)H-MRS) to evaluate vertebral marrow fat, and to determine whether bone density correlates with fat content and fat unsaturation levels in postmenopausal women.

MATERIALS AND METHODS

Fifty-three women (mean age = 70 years) underwent dual energy x-ray absorptiometry and (1)H-MRS, and 12 young female controls (mean age = 28 years) underwent (1)H-MRS of the lumber spine. Water and lipid peak amplitudes were measured to calculate fat content and fat unsaturation index. Spearman's correlation tests and a t-test comparison of means were applied.

RESULTS

(1)H-MRS was successful in 15 normal, 15 osteopenic, and 20 osteoporotic subjects, and in all controls. Marrow fat content was significantly elevated in osteoporotic (65.5% +/- 10%) and osteopenic (63.5% +/- 9.3%) subjects compared to normal subjects (56.3% +/- 11.2%) and young controls (29% +/- 9.6%). The fat unsaturation index was significantly decreased in osteoporotic (0.091 +/- 0.013) and osteopenic (0.097 +/- 0.014) subjects compared to normal subjects (0.114 +/- 0.016) and young controls (0.127 +/- 0.031). A good inverse correlation was observed between the fat content and the unsaturation index (r(s) = -0.53, P < 0.0001).

CONCLUSION

Osteoporosis is associated with increased marrow fat. As marrow fat increases, saturated lipids appear to increase preferentially to unsaturated lipids.

Failure of trabecular bone with simulated lytic defects can be predicted non-invasively by structural analysis

Pathologic fracture is a significant risk for patients afflicted with metastatic or benign skeletal tumors. The quandary for physicians who treat these patients is that after making the diagnosis they must try to predict the load bearing capacity of the involved bone and the fracture risk from images seen in radiological examinations. Since bone fails at a relatively constant strain independent of density we demonstrate that using a mechanics of materials approach that the cross-sectional structural properties of the bone most affected by the lytic defect governs the load bearing capacity of the entire bone. Homogeneous cylindrical cores of trabecular bone were harvested from the vertebral bodies of whale spines, and prepared with circular or slotted through-hole defects of varying sizes to simulate lytic skeletal tumors. Each specimen was imaged using quantitative computed tomography (CT), dual energy X-ray absorptiometry (DXA), and magnetic resonance imaging (MRI) to obtain data for calculating cross-sectional structural properties: axial, flexural, and torsional rigidity. The specimens were then divided into groups uniformly distributed with respect to defect sizes and shapes, and subjected to uniaxial tension, four-point bending or torsion until failure. A strong positive relationship was found between measured tensile yield loads, bending, and torsional yield moments vs. axial, flexural and torsional structural rigidities respectively, calculated from QCT, DXA, and MRI data [QCT: tension r2=0.951 , bending r2=0.909, torsion r2=0.914; DXA: tension r2=0.926, bending r2=0.841, torsion r2=0.916 (p<0.001); MRI: tension r2=0.916; bending r2=0.856, torsion r2=0.852]. For cylindrical cores of trabecular bone with simulated lytic defects, the load bearing capacity of the entire core was directly proportional to the axial, bending, or torsional rigidity at the weakest cross-section through the core containing the defect. Therefore structural rigidity analysis of cross-sectional geometric data measured non-invasively by QCT, DXA, and MRI of bones containing lytic defects may be used to predict the load bearing capacity of the involved bone and the relative fracture risk in vivo.

Spectroscopic imaging of bone marrow composition in vertebral bodies

The proton spectroscopic imaging technique that uses read gradient during acquisition was used for the measurement of the proton spectra in the lumbar and thoracic part of the spine of a patient with breast cancer without known skeletal metastases. The bone marrow fat/water ratios were evaluated in the same location before and after chemotherapy treatment. The results were corrected for relaxation effects. The fat/water ratios showed a significant increase as a consequence of the bone marrow degradation process due to chemotherapy. The proposed spectroscopic imaging technique offers rapid acquisition of proton spectra from large volumes of the vertebral bodies.

Temperature monitoring with line scan echo planar spectroscopic imaging

A new magnetic resonance imaging method, line scan echo planar spectroscopic imaging (LSEPSI), is shown capable of providing rapid, internally referenced temperature monitoring from water and fat chemical shifts.

METHODS

Orthogonal 90 degrees and 180 degrees slice selective RF pulses inclined by 45 degrees from the image plane solicit a spin echo from a tissue column. The echo is read by asymmetric sampling of 32 gradient echoes spaced 1.4-1.8 ms apart. Sixty-four adjacent columns are sequentially sampled in 4.2-6.4 s with 4,096 voxels sampled with voxel volumes of 0.08-0.13 cm3. Mixed mayonnaise/water phantoms were used to correlate LSEPSI-derived chemical shifts and thermocouple-based temperature measurements from 23 to 60 degrees C with a 1.5 T scanner. Measurement artifacts unrelated to temperature were investigated with the phantom, as was the feasibility of applying the sequence in human breast in vivo.

RESULTS

The correlation between LSEPSI and thermocouple-based temperature measurements in the phantom was excellent (r2>0.99). Field drifts affecting the temperature measurements using the water peak alone were corrected by using the water/lipid peak difference. The sequence had an average temperature resolution of 1.4 degrees C in the phantom. The frequency difference measurement reduced the sensitivity to artifacts related to temperature. Both water and lipid peaks were detectable throughout many locations in the breast, suggesting the applicability of LSEPSI in this organ.

DISCUSSION

T1-saturation losses occur in conventional and echo-planar based 2D CSI sequences using phase encoding methods with short TR periods. These losses are eliminated when individual columns are sampled in snapshot fashion with LSEPSI since the effective TR becomes the time between scans rather than excitations. T1 saturation can make small spectral peaks difficult to detect at high temperatures and generally lowers the signal-to-noise ratio of the spectra. The rapid acquisition and insensitivity to T1 saturation effects make LSEPSI an attractive technique for monitoring thermal therapies in breast using the internally referenced fat/water frequency separation.

Continuous distribution analysis of marrow 1 H magnetic resonance relaxation in bone

Longitudinal and transverse NMR relaxation of 1H nuclei were studied in vitro on fresh animal femur samples. A large number of data points were taken, starting at 100 micros for T(1) by inversion-recovery, at 200 micros for T(2) by single-echo sequences, and at 600 micros for T(2) by CPMG echo-trains. Quasi-continuous distributions of relaxation times were computed, giving wide distributions for all samples. Bulk marrow removed from the medullary cavity showed T(2) distributions from about 20 ms to 600 ms and T(1) distributions from about 40 ms to 2 s. The 1H nuclei in trabecular bone samples, where marrow is confined, may show long tails for T(2) at relaxation times down to 250 micros, the origin of which is still not known. These tails are absent in bulk marrow from the medullary cavity. The differences observed in T(1) distributions among trabecular bone samples are in accordance with the different marrow compositions. Discrete exponential fits were computed also, and in most cases four discrete exponential components were required to fit the experimental data adequately. However, the discrete components do not seem to correspond to any physically distinguishable separate compartments.

An automated iterative algorithm for water and fat decomposition in three-point Dixon magnetic resonance imaging

An iterative, outlier exclusion, second-order surface fitting algorithm has been developed to solve the well-known phase wraparound problem associated with in vivo applications of the three-point Dixon magnetic resonance imaging method. The technique was optimized for speed by reducing the problem to a pair of planar fits. The spatial misalignment between water and fat components due to the chemical shift was handled on a subpixel level by invoking the shift theorem of Fourier transformation. From the chemical shift corrected water and fat images, high quality recombined MR images were generated. The algorithm was validated in both phantom and patient studies. In vivo breast images and pelvic images are provided as a demonstration of the method.

Spin-Echo planar spectroscopic imaging for fast lipid characterization in bone marrow

Lipid characterization of bone marrow in vivo with proton magnetic resonance spectroscopy was performed using Spin-Echo Planar Spectroscopic Imaging sequences. The methods are shown capable of rapidly generating two-dimensional chemical shift imaging data sets suitable for measuring lipid indices that reflect unsaturation levels among triglycerides, as demonstrated in oil phantoms and bone marrow from a healthy volunteer. The volume coverage, spatial resolution, acquisition speed, and spectral characteristics of Spin-Echo Planar Spectroscopic Imaging should make it attractive for clinical studies of diseases affecting normal lipid chemical composition.

Optimization of chemical shift selective suppression of fat

Strategies to optimize flip angles for chemical shift selective fat suppression are discussed. Mathematical models for fat suppression in spoiled gradient recalled acquisition, spin echo, and RARE, which incorporate steady state conditions and multiple spectral components of fat, are developed. The optimal suppression flip angle is found to be larger than that determined with a single fat component model by more than 10 degrees due to contributions from unflipped components such as olefinic and glycerol protons that lie outside the suppression band.