Interrelationships between 3-T-MRI-derived cortical and trabecular bone structure parameters and quantitative-computed-tomography-derivedbone mineral density

Can magnetic resonance imaging accurately and reliably measure humeral cortical thickness?

Background

Historically, imaging osseous detail in three dimensions required a computed tomography (CT) scan with ionizing radiation that poorly visualizes the soft tissues. The purpose of this study was to determine the accuracy and reliability of ultrashort echo time (UTE) magnetic resonance imaging (MRI) in measuring humeral cortical thickness and cancellous density as compared with CT.

Methods

This was a comparative radiographic study in nine cadavers, each of which underwent CT and UTE MRI. On images aligned to the center of the humeral shaft, anterior, posterior, medial, and lateral humeral cortical thickness was measured 5, 10, and 15 cm distal to the top of the head. Cancellous density was measured as signal within a 1-cm diameter region of interest in the center of the head, the subtuberosity head, the subarticular head, and the subarticular glenoid vault. Glenoid cortical thickness was measured at the center of the glenoid. Cortical measurements were compared using mean differences and 95% confidence intervals, paired Student’s t-tests, and intraclass correlation coefficients (ICCs). We compared cancellous measurements using Pearson’s correlation coefficients. For all measurements, we calculated interobserver and intraobserver reliability using ICCs with 0.75 as the lower limit for acceptability.

Results

With regard to accuracy, for humeral cortical thickness measurements, there were no significant differences between MRI and CT measures, and ICCs were >0.75. The glenoid cortical thickness ICC was <0.75. There was no significant correlation between the cancellous signal on MRI and on CT in any region. For both MRI and CT, interobserver reliability and intraobserver reliability were acceptable (ie, >0.75) for almost all humeral cortical thickness measures.

Conclusion

UTE MRI can reliably and accurately measure humeral cortical thickness, but cannot accurately measure cancellous density or accurately and reliably measure glenoid cortical thickness.

Update on bone density measurements and their interpretation in children and adolescents

Following the increased awareness about the central role of the pediatric age in building bone for life, clinicians face more than ever the necessity of assessing bone health in pediatric subjects at risk for early bone mass derangements or in healthy children, in order to optimize their bone mass accrual and prevent osteoporosis. Although the diagnosis of osteoporosis is not made solely upon bone mineral density measurements during growth, such determination can be very useful in the follow-up of pediatric patients with primary and secondary osteoporosis. The ideal instrument would give information on the mineral content and density of the bone, and on its architecture. It should be able to perform the measurements on the skeletal sites where fractures are more frequent, and it should be minimally invasive, accurate, precise and rapid. Unfortunately, none of the techniques currently utilized fulfills all requirements. In the present review, we focus on the pediatric use of dual-energy X-ray absorptiometry (DXA), quantitative computed tomography (QCT), peripheral QCT (pQCT), and magnetic resonance imaging (MRI), highlighting advantages and limits for their use and providing indications for bone densitometry interpretation and of vertebral fractures diagnosis in pediatric subjects.

Deficits in Trabecular Bone Microarchitecture in Young Women With Type 1 Diabetes Mellitus

The pathophysiological mechanism of increased fractures in young adults with type 1 diabetes mellitus (T1DM) is unclear. We conducted a case-control study of trabecular bone microarchitecture and vertebral marrow adiposity in young women with T1DM. Thirty women with T1DM with a median age (range) age of 22.0 years (16.9, 36.1) attending one outpatient clinic with a median age at diagnosis of 9.7 years (0.46, 14.8) were compared with 28 age-matched healthy women who acted as controls. Measurements included MRI-based assessment of proximal tibial bone volume/total volume (appBV/TV), trabecular separation (appTb.Sp), vertebral bone marrow adiposity (BMA), and abdominal adipose tissue and biochemical markers of GH/IGF-1 axis (IGF-1, IGFBP3, ALS) and bone turnover. Median appBV/TV in cases and controls was 0.3 (0.22, 0.37) and 0.33 (0.26, 0.4), respectively (p = 0.018) and median appTb.Sp in T1DM was 2.59 (2.24, 3.38) and 2.32 (2.03, 2.97), respectively (p = 0.012). The median appBV/TV was 0.28 (0.22, 0.33) in those cases with retinopathy (n = 15) compared with 0.33 (0.25, 0.37) in those without retinopathy (p = 0.02). Although median visceral adipose tissue in cases was higher than in controls at 5733 mm(3) (2030, 11,144) and 3460 mm(3) (1808, 6832), respectively (p = 0.012), there was no difference in median BMA, which was 31.1% (9.9, 59.9) and 26.3% (8.5, 49.8) in cases and controls, respectively (p = 0.2). Serum IGF-1 and ALS were also lower in cases, and the latter showed an inverse association to appTbSp (r = -0.30, p = 0.04). Detailed MRI studies in young women with childhood-onset T1DM have shown clear deficits in trabecular microarchitecture of the tibia. Underlying pathophysiological mechanisms may include a microvasculopathy.

Preclinical assessment of a new magnetic resonance-based technique for determining bone quality by characterization of trabecular microarchitecture

The utility of HR-CT to study longitudinal changes in bone microarchitecture is limited by subject radiation exposure. Although MR is not subject to this limitation, it is limited both by patient movement that occurs during prolonged scanning at distal sites, and by the signal-to-noise ratio that is achievable for high-resolution images in a reasonable scan time at proximal sites. Recently, a novel MR-based technique, fine structure analysis (FSA) (Chase et al. Localised one-dimensional magnetic resonance spatial frequency spectroscopy. PCT/US2012/068284 2012, James and Chase Magnetic field gradient structure characteristic assessment using one-dimensional (1D) spatial frequency distribution analysis. 7932720 B2, 2011) has been developed which provides both high-resolution and fast scan times, but which generates at a designated set of spatial positions (voxels) a one-dimensional signal of spatial frequencies. Appendix 1 provides a brief introduction to FSA. This article describes an initial exploration of FSA for the rapid, non-invasive characterization of trabecular microarchitecture in a preclinical setting. For L4 vertebrae of sham and ovariectomized (OVX) rats, we compared FSA-generated metrics with those from CT datasets and from CT-derived histomorphometry parameters, trabecular number (Tb.N), bone volume density (BV/TV), trabecular thickness (Tb.Th) and trabecular separation (Tb.Sp). OVX caused a reduction of the higher frequency structures that correspond to a denser trabecular lattice, while increasing the preponderance of lower frequency structures, which correspond to a more open lattice. As one example measure, the centroid of the FSA spectrum (which we refer to as fSAcB) showed strong correlation in the same region with CT-derived histomorphometry values: Tb.Sp: r -0.63, p < 0.001; Tb.N: r 0.71, p < 0.001; BV/TV: r 0.64, p < 0.001, Tb.Th: r 0.44, p < 0.05. Furthermore, we found a 17.5% reduction in fSAcB in OVX rats (p < 0.0001). In a longitudinal study, FSA showed that the age-related increase in higher frequency structures was abolished in OVX rats, being replaced with a 78-194% increase in lower frequency structures (2.4-2.8 objects/mm range), indicating a more sparse trabecular lattice (p < 0.05). The MR-based fine structure analysis enables high-resolution, radiation-free, rapid quantification of bone structures in one dimension (the specific point and direction being chosen by the clinician) of the spine.

Validation of quantitative magnetic resonance imaging-based apparent bone volume fraction in peri-articular tibial bone of cadaveric knees

BACKGROUND

In the knee, high-resolution magnetic resonance (MR) imaging has demonstrated that increased apparent bone volume fraction (trabecular bone volume per total volume; BV/TV) in the peri-articular proximal medial tibia is associated with joint space narrowing and the presence of bone marrow lesions. However, despite evidence of construct validity, MR-based apparent BV/TV has not yet been cross-validated in the proximal medial tibia by comparison with a gold standard (e.g., micro-computed tomography [microCT]). In this cadaveric validation study we explored the association between MR-based apparent BV/TV and microCT-based BV/TV in the proximal peri-articular medial tibia.

METHODS

Fresh cadaveric whole knee specimens were obtained from individuals 51 to 80 years of age with no knee pathology other than osteoarthritis. Ten knees were collected from five cadavers within 10 hours of death and underwent a 3-Tesla MR exam including a coronal-oblique 3-dimensional fast imaging with steady state precession (3D FISP) sequence within 36 hours of death. The specimens were placed in a 4% paraformaldehyde in phosphate buffer within 58 hours of death. After preservation, a subchondral region from the tibial plateau was collected and underwent microCT imaging with a voxel size of 9 μm x 9 μm x 9 μm. A single reader analyzed the microCT images in a similar volume of interest as selected in the MR measures. A different reader analyzed the MR-based trabecular morphometry using a custom analysis tool. To analyze the MR-based trabecular morphometry, a rectangular region of interest (ROI) was positioned on the 20 central images in the proximal medial tibial subchondral bone. The primary outcome measures were MR-based and microCT-based trabecular BV/TV in the proximal medial tibia.

RESULTS

The MR-based apparent BV/TV was strongly correlated with microCT-based BV/TV (r=0.83, confidence interval=0.42 to 0.96), despite the MR-based apparent BV/TV being systematically lower than measured using microCT.

CONCLUSIONS

MR-based apparent BV/TV in the proximal peri-articular medial tibia has good construct validity and may represent an alternative for CT-based BV/TV.

Cylinders or walls? A new computational model to estimate the MR transverse relaxation rate dependence on trabecular bone architecture

OBJECTIVE

Bone density is distributed in a complex network of interconnecting trabecular plates and rods that are interspersed with bone marrow. A computational model to assess the dependence of the relaxation rate on the geometry of bone can consider the distribution of bone material in the form of two components: cylinders and open walls (walls with gaps). We investigate whether the experimentally known dependence of the transverse relaxation rate on the trabecular bone structure can be usefully interpreted in terms of these two components.

MATERIALS AND METHODS

We established a computer model based on an elementary computational cell. The model includes a variable number of open walls and infinitely long cylinders as well as multiple geometric parameters. The transverse relaxation rate is computed as a function of these parameters. Within the model, increasing the trabecular spacing with a fixed trabecular radius is equivalent to thinning the trabeculae while maintaining constant spacing.

RESULTS

Increasing the number of cylinder and wall gap elements beyond their nearest neighbors does not change the transverse relaxation rate. Although the absolute contribution to the relaxation due to open walls is on average more important than that due to cylinders, the latter drops off rapidly. The change on transverse relaxation rate is larger for changing cylinder geometry than for changing wall geometry, as it can be seen from the effect on the relaxation rate when trabecular spacing is varied, compared to varying the size of wall gaps.

CONCLUSION

Our results provide strong evidence that trabecular thinning, which is associated with increasing age, decreases the relaxation rates. The effect of thinning plates and rods on the transverse relaxation can be understood in terms of simple cylinders and open walls. A reduction in the relaxation rate can be seen as an indication of thinning cylinders, corresponding to reduced bone stability and ultimately, osteoporosis.

Cortical and trabecular bone structure analysis at the distal radius-prediction of biomechanical strength by DXA and MRI

The purpose of this study was to investigate whether the combination of dual-energy X-ray absorptiometry (DXA)-based bone mass and magnetic resonance imaging (MRI)-based cortical and trabecular structural measures improves the prediction of radial bone strength. Thirty-eight left forearms were harvested from formalin-fixed human cadavers. Bone mineral content (BMC) and bone mineral density (BMD) of the distal radius were measured using DXA. Cortical and trabecular structural measures of the distal radius were computed in high-resolution 1.5T MR images. Cortical measures included average cortical thickness and cross-sectional area. Trabecular measures included morphometric and texture parameters. The forearms were biomechanically tested in a fall simulation to measure absolute radial bone strength (failure load). Relative radial bone strength was determined by dividing radial failure loads by age, body mass index, radius length, and average radius cross-sectional area, respectively. DXA derived BMC and BMD showed statistically significant (p < 0.05) correlations with absolute and relative radial bone strength (r ≤ 0.78). Correlation coefficients for cortical and trabecular structural measures with absolute and relative radial bone strength amounted up to r = 0.59 and r = 0.74, respectively, (p < 0.05). In combination with DXA-based bone mass, trabecular but not, cortical structural measures, added in multiple regression models significant (p < 0.05) information in predicting absolute and relative radial bone strength (up to R adj = 0.88). Thus, a combination of DXA-based bone mass and MRI-based trabecular structural measures most accurately predicted absolute and relative radial bone strength, whereas structural measures of the cortex did not provide significant additional information in combination with DXA.

Reproducibility of trabecular bone structure measurements of the distal radius at 1.5 and 3.0 T magnetic resonance imaging

The purpose of this study was to assess and compare the reproducibility of trabecular bone structure measurements of the distal radius at 1.5 and 3.0 T magnetic resonance imaging (MRI). Root mean square reproducibility errors ranged from 0.69% to 4.94% at 1.5 T MRI and from 0.38% to 5.80% at 3.0 T MRI. Thus, reproducibility errors of trabecular bone structure measurements are overall in an acceptable range and similar at 1.5 and 3.0 T MRI.

Determination of vertebral and femoral trabecular morphology and stiffness using a flat-panel C-arm-based CT approach

The importance of assessing trabecular architecture together with bone mineral density to determine bone stiffness and fracture risk in osteoporosis has been well established. However, no imaging modalities are available to assess trabecular architecture at clinically relevant sites in the axial skeleton. Recently developed flat-panel CT devices, however, offer resolutions that are potentially good enough to resolve bone architecture at these sites. The goal of the present study was to investigate how accurate trabecular architecture and stiffness can be determined based on images from such a device (XperCT, Philips Healthcare). Ten cadaver human C3 vertebrae, twelve T12 vertebrae and 12 proximal femora were scanned with XperCT while mimicking in-vivo scanning conditions and compared to scans of the same bones with microCT. Standard segmentation and morphology quantification algorithms were applied as well as finite element (FE) simulation based on segmented and gray value images. Results showed that mean trabecular separation (Tb.Sp) and number (Tb.N) can be accurately determined at all sites. The accuracy of other parameters, however, depended on the site. For T12 no other structural parameters could be accurately quantified and no FE-results could be obtained from segmented images. When using gray-level images, however, accurate determination of cancellous bone stiffness was possible. For the C3 vertebrae and proximal femora, mean bone volume fraction (BV/TV), Tb.Sp, Tb.N, and anisotropy (C3 only) could be determined accurately. For Tb.Th, structure model index (SMI, femur only), and anisotropy good correlations were obtained but the values were not determined accurately. FE simulations based on segmented images were accurate for the C3 vertebrae, but severely underestimated bone stiffness for the femur. Here also, this was improved by using the gray value models. In conclusion, XperCT does provide a resolution that is good enough to determine trabecular architecture, but the signal to noise ratio is key to the accuracy of the morphology measurement. When the trabeculae are thick e.g. in the femur or the noise is low, e.g. cervical spine, architecture and stiffness could be determined accurately, but when the trabeculae are thin and the noise is high, e.g. thoracic spine, architecture could not be determined accurately and the connectivity was lost and hence no mechanical properties could be calculated directly.