Acoustic and ultrasonic tissue characterization--assessment of osteoporosis

Radiofrequency Echographic Multispectrometry (REMS) can Overcome the Effects of Structural Internal Artifacts and Evaluate Bone Fragility Accurately

PURPOSE

This study measured bone mineral density (BMD) in a Japanese population using the novel non-ionizing system using radiofrequency echographic multispectrometry (REMS) and compared the results with those obtained using traditional dual-energy X-ray absorptiometry (DXA). We aimed to identify any discrepancies between measurements obtained using these instruments and identify the influencing factors.

METHODS

This cross-sectional study examined patients with osteoporosis treated at a single center from April to August 2023. We examined BMD assessment by DXA and REMS in lumbar spine and proximal femur. Patients were categorized into two groups: those with discrepancies between lumbar spine BMD measured by DXA and REMS, and those without. Semiquantitative evaluation of vertebral fractures and abdominal aortic calcification scoring were also performed and compared between the two groups, along with various patient characteristics.

RESULTS

A total of 70 patients (88.6% female; mean age 78.39 ± 9.50 years) undergoing osteoporosis treatment were included in the study. A significant difference was noted between DXA and REMS measurement of BMD and T-scores, with REMS recording consistently lower values. The discrepancy group exhibited a higher incidence of multiple vertebral fractures and increased vascular calcification than the non-discrepancy group. Multivariate analysis indicated that diabetes mellitus, severe vertebral fractures, and increased abdominal aortic calcification scores were significantly associated with discrepancies in lumbar spine T-scores.

CONCLUSION

This study suggests that REMS may offer a more accurate measurement of BMD, overcoming the overestimation of BMD by DXA owing to factors such as vertebral deformities, abdominal aortic calcification, and diabetes mellitus.

Bone quantitative susceptibility mapping using a chemical species-specific R2* signal model with ultrashort and conventional echo data

PURPOSE

To develop quantitative susceptibility mapping (QSM) of bone using an ultrashort echo time (UTE) gradient echo (GRE) sequence for signal acquisition and a bone-specific effective transverse relaxation rate ( R2*) to model water-fat MR signals for field mapping.

METHODS

Three-dimensional radial UTE data (echo times ≥ 40 μs) was acquired on a 3 Tesla scanner and fitted with a bone-specific signal model to map the chemical species and susceptibility field. Experiments were performed ex vivo on a porcine hoof and in vivo on healthy human subjects (n = 7). For water-fat separation, a bone-specific model assigning R2* decay mostly to water was compared with the standard models that assigned the same decay for both fat and water. In the ex vivo experiment, bone QSM was correlated with CT.

RESULTS

Compared with standard models, the bone-specific R2* method significantly reduced errors in the fat fraction within the cortical bone in all tested data sets, leading to reduced artifacts in QSM. Good correlation was found between bone CT and QSM values in the porcine hoof (R²  = 0.77). Bone QSM was successfully generated in all subjects.

CONCLUSIONS

The QSM of bone is feasible using UTE with a conventional echo time GRE acquisition and a bone-specific R2* signal model. Magn Reson Med 79:121-128, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Micro-scale finite element modeling of ultrasound propagation in aluminum trabecular bone-mimicking phantoms: A comparison between numerical simulation and experimental results

The present study investigated the accuracy of micro-scale finite element modeling for simulating broadband ultrasound propagation in water-saturated trabecular bone-mimicking phantoms. To this end, five commercially manufactured aluminum foam samples as trabecular bone-mimicking phantoms were utilized for ultrasonic immersion through-transmission experiments. Based on micro-computed tomography images of the same physical samples, three-dimensional high-resolution computational samples were generated to be implemented in the micro-scale finite element models. The finite element models employed the standard Galerkin finite element method (FEM) in time domain to simulate the ultrasonic experiments. The numerical simulations did not include energy dissipative mechanisms of ultrasonic attenuation; however, they expectedly simulated reflection, refraction, scattering, and wave mode conversion. The accuracy of the finite element simulations were evaluated by comparing the simulated ultrasonic attenuation and velocity with the experimental data. The maximum and the average relative errors between the experimental and simulated attenuation coefficients in the frequency range of 0.6-1.4 MHz were 17% and 6% respectively. Moreover, the simulations closely predicted the time-of-flight based velocities and the phase velocities of ultrasound with maximum relative errors of 20 m/s and 11 m/s respectively. The results of this study strongly suggest that micro-scale finite element modeling can effectively simulate broadband ultrasound propagation in water-saturated trabecular bone-mimicking structures.

The finite element method for micro-scale modeling of ultrasound propagation in cancellous bone

Quantitative ultrasound for bone assessment is based on the correlations between ultrasonic parameters and the properties (mechanical and physical) of cancellous bone. To elucidate the correlations, understanding the physics of ultrasound in cancellous bone is demanded. Micro-scale modeling of ultrasound propagation in cancellous bone using the finite-difference time-domain (FDTD) method has been so far utilized as one of the approaches in this regard. However, the FDTD method accompanies two disadvantages: staircase sampling of cancellous bone by finite difference grids leads to generation of wave artifacts at the solid-fluid interface inside the bone; additionally, this method cannot explicitly satisfy the needed perfect-slip conditions at the interface. To overcome these disadvantages, the finite element method (FEM) is proposed in this study. Three-dimensional finite element models of six water-saturated cancellous bone samples with different bone volume were created. The values of speed of sound (SOS) and broadband ultrasound attenuation (BUA) were calculated through the finite element simulations of ultrasound propagation in each sample. Comparing the results with other experimental and simulation studies demonstrated the capabilities of the FEM for micro-scale modeling of ultrasound in water-saturated cancellous bone.

Prediction of trabecular bone qualitative properties using scanning quantitative ultrasound

Microgravity induced bone loss represents a critical health problem in astronauts, particularly occurred in weight-supporting skeleton, which leads to osteopenia and increase of fracture risk. Lack of suitable evaluation modality makes it difficult for monitoring skeletal status in long term space mission and increases potential risk of complication. Such disuse osteopenia and osteoporosis compromise trabecular bone density, and architectural and mechanical properties. While X-ray based imaging would not be practical in space, quantitative ultrasound may provide advantages to characterize bone density and strength through wave propagation in complex trabecular structure. This study used a scanning confocal acoustic diagnostic and navigation system (SCAN) to evaluate trabecular bone quality in 60 cubic trabecular samples harvested from adult sheep. Ultrasound image based SCAN measurements in structural and strength properties were validated by μCT and compressive mechanical testing. This result indicated a moderately strong negative correlations observed between broadband ultrasonic attenuation (BUA) and μCT-determined bone volume fraction (BV/TV, R²=0.53). Strong correlations were observed between ultrasound velocity (UV) and bone's mechanical strength and structural parameters, i.e., bulk Young's modulus (R²=0.67) and BV/TV (R²=0.85). The predictions for bone density and mechanical strength were significantly improved by using a linear combination of both BUA and UV, yielding R²=0.92 for BV/TV and R²=0.71 for bulk Young's modulus. These results imply that quantitative ultrasound can characterize trabecular structural and mechanical properties through measurements of particular ultrasound parameters, and potentially provide an excellent estimation for bone's structural integrity.

Numerical simulation of effective phase velocity and attenuation of shear elastic wave propagation in unidirectional composite materials

In this paper, a simple simulation approach is presented for calculating the effective phase velocity and attenuation coefficient of elastic shear waves propagating in composite materials with randomly distributed unidirectional inclusions. As an application of the developed numerical approach, the phase velocities and attenuation coefficients of the coherent waves in four different types of composite material are simulated for various incident frequencies up to π. Numerical results are compared with theoretical predictions obtained from three representative theoretical models. While all theoretical results agree very well with numerical values at low incident frequencies, the discrepancies increase with the increased incident frequency and volume fraction of inclusions. It has been found that within the frequency and volume fraction ranges considered in this work, the generalized self-consistent model by Kanaun and Levin [18] seems to provide the most accurate estimations.

Quantitative ultrasound predicts bone mineral density and failure load in human lumbar vertebrae

BACKGROUND

Quantitative ultrasound is in widespread clinical use for assessment of bone quality at peripheral skeletal sites, but has not yet been applied to those sites in the axial skeleton, such as the spine and hip, where osteoporotic fractures are common.

METHODS

Ultrasound measurements were made in 11 cadaveric vertebrae and relationships with bone mineral density and failure load were investigated. An ultrasonic imaging system was used to measure speed of sound, broadband ultrasonic attenuation, and attenuation at a single frequency, through the vertebral body in the sagittal plane. Ultrasonic measurements were averaged over a region of interest centrally within the vertebral body, and were calculated with and without normalization for bone size. Vertebral bone mineral density was measured in antero-posterior and lateral projections using dual energy X-ray absorptiometry. Compressive mechanical testing was performed to determine vertebral failure load.

FINDINGS

Bone mineral density correlated with failure load (r=0.74-0.78, all P<0.01), and with quantitative ultrasound (r=0.63-0.82, P=0.038-0.004), in line with previous studies. Of the ultrasonic measurements, those parameters not normalized for bone size gave the highest correlations with failure load, ranging from r=0.71 (P=0.021) for speed of sound to r=0.93 (P<0.001) for attenuation. When ultrasonic measurements were normalized for bone size, the correlations with both failure load and bone mineral density were lower.

INTERPRETATION

These results confirm the feasibility of vertebral quantitative ultrasound in vitro, and indicate that ultrasound does provide information on both bone mineral density and failure load. The predictive performance of ultrasonic measurements for failure load was comparable to or greater than that of bone mineral density, suggesting that ultrasound has the potential to be at least as useful as mineral density in the assessment of vertebral bone. Normalizing ultrasonic measurements for bone size reduced the strength of correlations because both bone mineral density and bone strength reflect bone size to a certain extent.

Bone fractal analysis

Fractal analysis is a quantitative method used to evaluate complex anatomic findings in their elementary component. Its application to biologic images, particularly to cancellous bones, has been well practiced within the past few years. The aims of these applications are to assess changes in bone and the loss of spongious architecture, indicate bone fragility, and to show the increased risk for fracture in primary or secondary osteoporosis. The applications are very promising to help complete the studies that can define bone density (bone mineral density by dual energy x-ray absorptiometry or quantitative computed tomography), and also have the capacity to distinguish the patients with a high or low risk for fracture. Their extension to the clinical fields, to define a test for fracture risk, is still limited by difficult application to the medical quantitative imaging of bones, between correct application at superficial bones and unreliable application to deep bones. The future evolution and validity do not depend upon fractal methods but upon well-detailed imaging of the bones in clinical conditions.

Multiphase nature and structure of biomaterials studied by ultrasounds

The paper discusses the applicability of a two-phase model of saturated porous materials for a description of the results of broadband ultrasonic studies of wave parameters in bovine trabecular bone. The analysis is focused on the role of the internal structure of the materials in the propagation of dilatational waves within the frequency range with a significant attenuation of wave energy due to absorption and scattering. The applicability of ultrasonic studies for the determination of characteristic macro- and micro-structural parameters of biomaterials using a model-based approach is considered.