Purpose

To investigate tricomponent analysis of human cortical bone using a multipeak fat signal model with 3D ultrashort TE Cones sequences on a clinical 3T scanner.

Methods

Tricomponent fitting of bound water, pore water, and fat content using a multipeak fat spectra model was proposed for 3D ultrashort TE imaging of cortical bone. Three-dimensional ultrashort TE Cones acquisitions combined with tricomponent analysis were used to investigate bound and pore water T2∗ and fractions, as well as fat T2∗ and fraction in cortical bone. Feasibility studies were performed on 9 human cortical bone specimens with regions of interest selected from the endosteum to the periosteum in 4 circumferential regions. Microcomputed tomography studies were performed to measure bone porosity and bone mineral density for comparison and validation of the bound and pore water analyses.

Results

The oscillation of the signal decay was well-fitted with the proposed tricomponent model. The sum of the pore water and fat fractions from tricomponent analysis showed a high correlation with microcomputed tomography porosity (R = 0.74, P < 0.01). Estimated bound-water fraction also demonstrated a high correlation with bone mineral density (R = 0.70, P < 0.01).

Conclusion

Tricomponent analysis significantly improves the estimation of bound-water and pore-water fractions in human cortical bone.

PURPOSE:Quantitative susceptibility mapping (QSM) MRI is a tool that can characterize changes in susceptibility, an intrinsic property which is associated with compositional changes in the tissue. Current QSM estimation of cortical bone is challenging because conventional clinical MRI cannot acquire signal in cortical bone. This study aimed to implement Cones 3D ultrashort echo time MRI (UTE-MRI) for ex vivo QSM measurements in human tibial cortical bone, investigating the correlations of QSM with volumetric intracortical bone mineral density (BMD). MATERIALS AND METHODS:Nine tibial midshaft cortical bone specimens (25 mm long specimens cut at the mid-point of tibial shaft, 67 ± 20 years old, 5 women and 4 men) were scanned on a clinical 3 T MRI scanner for QSM measurement. The specimens were also scanned on a high-resolution micro-computed tomography (μCT) scanner for volumetric BMD estimation. QSM and μCT results were compared at approximately nine regions of interest (ROIs) per specimen. RESULTS:Average 3D UTE-MRI QSM showed significantly strong correlation with volumetric BMD (R = -0.82, P < 0.01) and bone porosity (R = 0.72, P < 0.01). Combining all data points together (77 ROIs), QSM showed significant moderate to strong correlation with volumetric BMD after correction for interdependencies in specimens (R = -0.70, P < 0.01). The corrections were required because the data points were not independent in each specimen. Similarly, the correlation between QSM and porosity was significant (R = 0.68, P < 0.01). CONCLUSIONS:These results suggest that the Cones 3D UTE-MRI QSM technique can potentially serve as a novel and accurate tool to assess intracortical bone mineral density whilst avoiding ionizing radiation.

PURPOSE:Quantitative imaging methods could improve diagnosis of rotator cuff degeneration, but the capability of quantitative MR and US imaging parameters to detect alterations in collagen is unknown. The goal of this study was to assess quantitative MR and US imaging measures for detecting abnormalities in collagen using an in vitro model of tendinosis with biochemical and histological correlation. METHOD:36 pieces of supraspinatus tendons from 6 cadaveric donors were equally distributed into 3 groups (2 subjected to different concentrations of collagenase and a control group). Ultrashort echo time MR and US imaging measures were performed to assess changes at baseline and after 24 h of enzymatic digestion. Biochemical and histological measures, including brightfield, fluorescence, and polarized microscopy, were used to verify the validity of the model and were compared with quantitative imaging parameters. Correlations between the imaging parameters and biochemically measured digestion were analyzed. RESULTS:Among the imaging parameters, macromolecular fraction (MMF), adiabatic T1ρ, T2*, and backscatter coefficient (BSC) were useful in differentiating between the extent of degeneration among the 3 groups. MMF strongly correlated with collagen loss (r=-0.81; 95% confidence interval [CI]: -0.90,-0.66), while the adiabatic T1ρ (r = 0.66; CI: 0.42,0.81), T2* (r = 0.58; CI: 0.31,0.76), and BSC (r = 0.51; CI: 0.22,0.72) moderately correlated with collagen loss. CONCLUSIONS:MMF, adiabatic T1ρ, and T2* measured and US BSC can detect alterations in collagen. Of the quantitative MR and US imaging measures evaluated, MMF showed the highest correlation with collagen loss and can be used to assess rotator cuff degeneration.

Cortical bone shows as a signal void when using conventional clinical magnetic resonance imaging (MRI). Ultrashort echo time MRI (UTE-MRI) can acquire high signal from cortical bone, thus enabling quantitative assessments. Magnetization transfer (MT) imaging combined with UTE-MRI can indirectly assess protons in the organic matrix of bone. This study aimed to examine UTE-MT MRI techniques to estimate the mechanical properties of cortical bone. A total of 156 rectangular human cortical bone strips were harvested from the tibial and femoral midshafts of 43 donors (62 ± 22 years old, 62 specimens from females, 94 specimens from males). Bone specimens were scanned using UTE-MT sequences on a clinical 3 T MRI scanner and on a micro-computed tomography (μCT) scanner. A series of MT pulse saturation powers (400°, 600°, 800°) and frequency offsets (2, 5, 10, 20, 50 kHz) was used to measure the macromolecular fraction (MMF) utilizing a two-pool MT model. Failure mechanical properties of the bone specimens were measured using 4-point bending tests. MMF from MRI results showed significant strong correlations with cortical bone porosity (R = -0.72, P < 0.01) and bone mineral density (BMD) (R = +0.71, P < 0.01). MMF demonstrated significant moderate correlations with Young modulus, yield stress, and ultimate stress (R = 0.60-0.61, P < 0.01). These results suggest that the two-pool UTE-MT model focusing on the organic matrix of bone can potentially serve as a novel tool to detect the variations of bone mechanical properties and intracortical porosity.

Tendons and bones comprise a special interacting unit where mechanical, biochemical, and metabolic interplays are continuously in effect. Bone loss in osteoporosis (OPo) and its earlier stage disease, osteopenia (OPe), may be coupled with a reduction in tendon quality. Noninvasive means for quantitatively evaluating tendon quality during disease progression may be critically important for the improvement of characterization and treatment optimization in patients with bone mineral density disorders. Though clinical magnetic resonance imaging (MRI) sequences are not typically capable of directly visualizing tendons, ultrashort echo time MRI (UTE-MRI) is able to acquire a high signal from tendons. Magnetization transfer (MT) modeling combined with UTE-MRI (i.e., UTE-MT-modeling) can indirectly assess macromolecular proton content in tendons. This study aimed to determine whether UTE-MT-modeling could detect differences in tendon quality across a spectrum of bone health. The lower legs of 14 OPe (72 ± 6 years) and 31 OPo (73 ± 6 years) female patients, as well as 30 female participants with normal bone (Normal-Bone, 36 ± 19 years), are imaged using UTE sequences on a 3T MRI scanner. Institutional review board approval is obtained for the study, and all recruited subjects provided written informed consent. A T1 measurement and UTE-MT-modeling are performed on the anterior tibialis tendon (ATT), posterior tibialis tendon (PTT), and the proximal Achilles tendon (PAT) of all subjects. The macromolecular fraction (MMF) is estimated as the main measure from UTE-MT-modeling. The mean MMF in all the investigated tendons was significantly lower in OPo patients compared with the Normal-Bone cohort (mean difference of 24.2%, p < 0.01), with the largest Normal-Bone vs. OPo difference observed in the ATT (mean difference of 32.1%, p < 0.01). Average MMF values of all the studied tendons are significantly lower in the OPo cohort compared with the OPe cohort (mean difference 16.8%, p = 0.02). Only the PPT shows significantly higher T1 values in OPo patients compared with the Normal-Bone cohort (mean difference 17.6%, p < 0.01). Considering the differences between OPo and OPe groups with similar age ranges, tendon deterioration associated with declining bone health was found to be larger than a priori detected differences caused purely by aging, highlighting UTE-MT MRI techniques as useful methods in assessing tendon quality over the course of progressive bone weakening.