Measurement of mechanical properties of bone material in vitro by ultrasound reflection: methodology and comparison with ultrasound transmission

Characterization of Structural Bone Properties through Portable Single-Sided NMR Devices: State of the Art and Future Perspectives

Nuclear Magnetic Resonance (NMR) is a well-suited methodology to study bone composition and structural properties. This is because the NMR parameters, such as the T2 relaxation time, are sensitive to the chemical and physical environment of the 1H nuclei. Although magnetic resonance imaging (MRI) allows bone structure assessment in vivo, its cost limits the suitability of conventional MRI for routine bone screening. With difficulty accessing clinically suitable exams, the diagnosis of bone diseases, such as osteoporosis, and the associated fracture risk estimation is based on the assessment of bone mineral density (BMD), obtained by the dual-energy X-ray absorptiometry (DXA). However, integrating the information about the structure of the bone with the bone mineral density has been shown to improve fracture risk estimation related to osteoporosis. Portable NMR, based on low-field single-sided NMR devices, is a promising and appealing approach to assess NMR properties of biological tissues with the aim of medical applications. Since these scanners detect the signal from a sensitive volume external to the magnet, they can be used to perform NMR measurement without the need to fit a sample inside a bore of a magnet, allowing, in principle, in vivo application. Techniques based on NMR single-sided devices have the potential to provide a high impact on the clinical routine because of low purchasing and running costs and low maintenance of such scanners. In this review, the development of new methodologies to investigate structural properties of trabecular bone exploiting single-sided NMR devices is reviewed, and current limitations and future perspectives are discussed.

Influence of cancellous bone microstructure on ultrasonic attenuation: a theoretical prediction

Background

Quantitative ultrasound has been used for the assessment of cancellous bone status. The attenuation mechanisms of cancellous bone, however, have not been well understood, because the microstructure of cancellous bone is significantly inhomogeneous and the interaction between ultrasound and the microstructure of cancellous bone is complex. In this study, a theoretical approach was applied to investigate the influence of the microstructure of cancellous bone on ultrasonic attenuation.

Results

The scattering from a trabecular cylinder was significantly angle dependent. The dependencies of the ultrasonic attenuation on frequency, scatterer size, and porosity were explored from the theoretical calculation. Prediction results showed that the ultrasonic attenuation increased with the increase of frequency and decreased linearly with the increase in porosity, and the broadband ultrasound attenuation decreased with the increase in porosity. All these predicted trends were consistent with published experimental data. In addition, our model successfully explained the principle of broadband ultrasound attenuation measurement (i.e., the attenuation over the frequency range 0.3–0.65 MHz was approximately linearly proportional to frequency) by considering the contributions of scattering and absorption to attenuation.

Conclusion

The proposed theoretical model may be a potentially valuable tool for understanding the interaction of ultrasound with cancellous bone.

Magnetic Resonance-Guided Focused Ultrasound in Neurosurgery: Taking Lessons from the Past to Inform the Future

Magnetic resonance-guided focused ultrasound (MRgFUS) is a new emerging neurosurgical procedure applied in a wide range of clinical fields. It can generate high-intensity energy at the focal zone in deep body areas without requiring incision of soft tissues. Although the effectiveness of the focused ultrasound technique had not been recognized because of the skull being a main barrier in the transmission of acoustic energy, the development of hemispheric distribution of ultrasound transducer phased arrays has solved this issue and enabled the performance of true transcranial procedures. Advanced imaging technologies such as magnetic resonance thermometry could enhance the safety of MRgFUS. The current clinical applications of MRgFUS in neurosurgery involve stereotactic ablative treatments for patients with essential tremor, Parkinson's disease, obsessive-compulsive disorder, major depressive disorder, or neuropathic pain. Other potential treatment candidates being examined in ongoing clinical trials include brain tumors, Alzheimer's disease, and epilepsy, based on MRgFUS abilities of thermal ablation and opening the blood-brain barrier. With the development of ultrasound technology to overcome the limitations, MRgFUS is gradually expanding the therapeutic field for intractable neurological disorders and serving as a trail for a promising future in noninvasive and safe neurosurgical care.

Assessment and classification of mechanical strength components of human femur trabecular bone using texture analysis and neural network

In this work the mechanical strength components of human femur trabecular bone are analyzed and classified using planar radiographic images and neural network. The mechanical strength regions such as Primary Compressive, Primary Tensile, Secondary Tensile and Ward Triangle in femur trabecular bone images (N = 100) are delineated by semi-automatic image processing procedure. First and higher order texture parameters and parameters such as apparent mineralization and total area associated with the strength regions are derived for normal and abnormal images. The statistically derived significant parameters corresponding to the primary strength regions are fed to the neural network for training and validation. The classifications are carried out using feed forward network that is trained with standard back propagation algorithm. Results demonstrate that the apparent mineralization of normal samples is always high as (71%) compared to abnormal samples (64%). Entropy shows a high value (7.3) for normal samples and variation between the mean intensity and apparent mineralization for the primary strength zone is statistically significant (p < 0.0005). The classified outputs are validated by sensitivity and specificity measurements and are found to be 66.66% and 80% respectively. Further it appears that it is possible to differentiate normal and abnormal samples from the conventional radiographic images. As trabecular architecture in the human femur is an important factor contributing to bone strength, the procedure adopted here could be a useful supplement to the clinical observations for bone loss and fracture risk.

High frequency ultrasound prediction of mechanical properties of cortical bone with varying amount of mineral content

In this study, we evaluate if high frequency ultrasound impedance measurements can predict the mechanical properties of bones where the amount of bone mineral is varied. The motivation stems from the potential utility of ultrasound as a noninvasive technique to evaluate and monitor the mechanical properties of bone during treatment of diseased states where the ratio of mineral content to organic matrix content could change (e.g., metabolic bone diseases, osteoarthritis, osteogenesis imperfecta, fracture healing). Eleven cortical bovine femur samples, which were taken along the long axis of femur, were used in each group. Bone samples with reduced mineral content (estimated to be 21% and 35% less than the control) were obtained by immersing samples into fluoride ion solution for 3 and 12 d. Control and fluoride treated samples were first tested mechanically in tension. Acoustic impedances of the mechanically tested samples were obtained by using scanning acoustic microscopy (SAM). Results from mechanical tests indicate that the tensile elastic modulus of the samples was highly correlated to the yield strength (r(2) = 0.94, p < 0.01) and to the ultimate strength (r(2) = 0.75, p < 0.01). SAM results indicate that the acoustic impedances were significantly correlated to the elastic modulus (r(2) = 0.85, p < 0.01), yield strength (r(2) = 0.86, p < 0.01) and ultimate strength (r(2) = 0.70, p < 0.01). These results show that ultrasonic techniques could potentially be used to predict the in vivo ultimate strength of bone tissue caused by changes in mineral content.

Instrumentation for in vivo ultrasonic characterization of bone strength

Although it has been more than 20 years since the first recorded use of a quantitative ultrasound (QUS) technology to predict bone fragility, the field has not yet reached its maturity. QUS has the potential to predict fracture risk in several clinical circumstances and has the advantages of being nonionizing, inexpensive, portable, highly acceptable to patients, and repeatable. However, the wide dissemination of QUS in clinical practice is still limited and suffering from the absence of clinical consensus on how to integrate QUS technologies in bone densitometry armamentarium. Several critical issues need to be addressed to develop the role of QUS within rheumatology. These include issues of technologies adapted to measure the central skeleton, data acquisition, and signal processing procedures to reveal bone properties beyond bone mineral quantity and elucidation of the complex interaction between ultrasound and bone structure. This article reviews the state-of-the art in technological developments applied to assess bone strength in vivo. We describe generic measurement and signal processing methods implemented in clinical ultrasound devices, the devices and their practical use, and performance measures. The article also points out the present limitations, especially those related to the absence of standardization, and the lack of comprehensive theoretical models. We conclude with suggestions of future lines and trends in technology challenges and research areas such as new acquisition modes, advanced signal processing techniques, and modelization.

Predictions of the modified Biot-Attenborough model for the dependence of phase velocity on porosity in cancellous bone

The modified Biot-Attenborough (MBA) model for acoustic wave propagation in porous media has been found useful to predict wave properties in cancellous bone. The present study is aimed at applying the MBA model to predict the dependence of phase velocity on porosity in cancellous bone. The MBA model predicts a phase velocity that decreases nonlinearly with porosity. The optimum values for input parameters of the MBA model, such as compressional speed c(m) of solid bone and phase velocity parameter s(2), were determined by comparing the predictions with previously published measurements in human calcaneus and bovine cancellous bone. The value of the phase velocity parameter s(2)=1.23 was obtained by curve fitting to the experimental data for 53 human calcaneus samples only, assuming a compressional speed c(m)=2500 m/s of solid bone. The root-mean-square error (RMSE) of the curve fit was 15.3m/s. The optimized value of s(2) for all 75 cancellous bone samples including 22 bovine samples was 1.42 with a value of 55 m/s for the RMSE of the curve fit. The latter fit was obtained by using of a value of c(m)=3200 m/s. Although the MBA model relies on the empirical parameters determined from experimental data, it is expected that the model can be usefully employed as a practical tool in the field of clinical ultrasonic bone assessment.

Is the Kramers-Kronig relationship between ultrasonic attenuation and dispersion maintained in the presence of apparent losses due to phase cancellation?

Phase cancellation effects can compromise the integrity of ultrasonic measurements performed with phase sensitive receiving apertures. A lack of spatial coherence of the ultrasonic field incident on a phase sensitive receiving array can produce inaccuracies of the measured attenuation coefficient and phase velocity. The causal (Kramers-Kronig) link between these two quantities in the presence of phase distortion is investigated using two plastic polymer materials, Plexiglas and Lexan, that exhibit attenuation coefficients that increase linearly with frequency, in a fashion analogous to that of soft tissue. Flat and parallel plates were machined to have a step of a thickness corresponding to an integer number of half wavelengths within the bandwidth investigated, 3 to 7 MHz. Insonification of the stepped portion of each plate produces phase cancellation artifacts at the receiving aperture and, therefore, in the measured frequency dependent attenuation coefficient. Dispersion predictions using two different forms of the Kramers-Kronig relations were performed for the flat and the stepped regions of each plastic plate. Despite significant phase distortion and a detection system sensitive to these aberrations, the Kramers-Kronig link between the apparent attenuation coefficient and apparent phase velocity dispersion remains intact.

Callus mineralization following distraction osteogenesis of the mandible monitored by scanning acoustic microscopy (SAM)

BACKGROUND

Scanning acoustic microscopy uses ultrasound to analyse histomorphology of tissues with microscopic resolution and delivers data about physical properties of the specimen.

MATERIAL AND METHODS

Bony consolidation was monitored by scanning acoustic microscopy in 12 embedded specimens of dog mandibles after distraction osteogenesis. Increasing mineralization was detected by measurements of acoustic impedance (Z).

RESULTS

There was a strong correlation between acoustic impedance and time of consolidation. Measurements of the speed of sound (v) provided specific information about non-mineralized zones of the distracted area. Distribution of density in the distracted area could be reconstructed by using the measurements of acoustic impedance and speed of sound.

CONCLUSION

The method seems suitable for studying bone remodelling qualitatively and quantitatively.

Reduction in normalized bone elasticity following long-term bisphosphonate treatment as measured by ultrasound critical angle reflectometry

Using an improved version of ultrasound critical angle reflectometry, the bone quality of cortical and trabecular bone was assessed in vivo by measuring elastic moduli (normalized for bone density) at both principal axes, referred to as the minimum and maximum normalized elasticities. The measurements were made in 30 normal premenopausal women, 30 normal postmenopausal women, 22 untreated postmenopausal women with osteoporosis, 74 postmenopausal women with osteoporosis or osteopenia on bisphosphonate treatment, and 32 patients with renal transplantation (16 women and 16 men) taking steroids. Cortical elasticity was higher than trabecular elasticity; both declined slightly and non-significantly with age in normal women. Among untreated postmenopausal women with osteoporosis, cortical maximum normalized elasticity (E(cmax)) remained within 95% prediction intervals of normal women. Among patients on bisphosphonate, E(cmax) was low in the majority of patients. E(cmax) was significantly more depressed among those taking the drug > or =3 years than <3 years (22.1% below normal premenopausal women versus 17.2%, P =0.001), and among those with incident non-spinal fractures than without (75.9 vs. 81.5%, P =0.008). E(cmax) was independent of bone mineral density at the calcaneus. Most patients with renal transplantation had low E(cmax), with a mean 20.8% below the normal premenopausal mean. Qualitatively similar findings were found with cortical minimum elasticity and with trabecular minimum and maximum elasticities. Thus, the material bone quality of cortical and trabecular bone may be impaired following bisphosphonate treatment, as in renal transplantation on steroids.

Optimal prediction of bone mineral density with ultrasonic measurements in excised human femur

Bone mineral density (BMD) measured with dual energy X-ray absorptiometry (DXA) techniques is the current gold standard for osteoporotic fracture risk prediction. Quantitative ultrasound (QUS) techniques in transmission measurements are, however, increasingly recognized as an alternative approach. It is feasible to select different QUS methods, one type being optimized to assess microarchitectural properties of bone structure and another to assess BMD. Broadband ultrasonic attenuation (BUA) and ultrasonic velocity (UV) measured on the proximal human femur have been shown to be both significantly correlated with BMD. However, a great diversity of algorithms has been reported to measure the time-of-flight used to derive UV values. The purpose of this study was to determine which procedure results in the optimal BMD prediction at the proximal femur from ultrasound measurements. Thirty-eight excised human femurs were measured in transmission with a pair of focused 0.5-MHz central frequency transducers. Two-dimensional scans were performed and radiofrequency (RF) signals were recorded digitally at each scan position. BUA was estimated and eight different signal processing techniques were performed to estimate UV. For each signal-processing technique UV was compared to BMD. We show that the best prediction of BMD was obtained with signal-processing techniques taking into account only the first part of the transmitted signal (r2BMD-SOS = 0.86). Moreover, we show that a linear multiple regression using both BUA and speed of sound (SOS) and applied to site-matched regions of interest improved the accuracy of BMD predictions (r2BMD-SOS/BUA = 0.95). Our results demonstrate that selecting specific signal-processing methods for QUS variables allows optimal assessment of BMD. Correlation is sufficiently high that this specific QUS method can be considered as a good surrogate of BMD.

Emerging concepts in osteoporosis and bone strength

OBJECTIVE

Osteoporosis is a systemic skeletal disorder characterized by compromised bone strength and increased fracture risk. The factors that contribute to bone strength include bone mineral density (BMD) and bone quality, which encompasses factors such as bone turnover, microarchitecture, mineralization, and geometry. The objective of this paper was to review the factors that contribute to bone strength and osteoporosis.

RESEARCH DESIGN

A MEDLINE search of English language journals between 1 January 1995 and 1 March 2005 was conducted using the term 'osteoporosis' combined with 'bone strength' or 'bone quality'. Reference lists of pivotal studies and reviews were also examined. Studies were otherwise not excluded on the basis of quality or size, the aim being to present an overview of research conducted to date on osteoporosis and bone strength.

RESULTS

While there is a relationship between BMD and fracture risk, evidence suggests that BMD measurements reflect only 1 component of bone strength. For example, small changes in BMD produced by osteoporosis treatments do not fully explain the reductions in fracture risk observed after initiation of therapy, and substantial fracture risk reduction is observed before peak increases in BMD are achieved. In addition to their effects on BMD, anti-resorptive therapies for osteoporosis (i.e., bisphosphonates, selective estrogen receptor modulators, calcitonin, and estrogen) produce positive effects on bone turnover, microarchitecture, and/or mineralization, all of which can contribute to the reductions in fracture risk observed with these agents. Anabolic agents such as teriparatide also appear to have beneficial effects on bone strength independent of bone mass. New, non-invasive, high-resolution imaging methods, such as magnetic resonance imaging and computed tomography, may offer a comprehensive assessment of bone quality in the future.

CONCLUSIONS

The development of clinical tools that assess bone quality independent of BMD will be essential to advance our assessment of fracture risk and response to osteoporosis treatment.

In vitro speed of sound measurement at intact human femur specimens

Quantitative ultrasound has been recognized as a useful tool for fracture risk prediction. Current measurement techniques are limited to peripheral skeletal sites. Our objective was to demonstrate the in vitro feasibility of ultrasonic velocity measurements on human proximal femur and to investigate the relationship between velocity and bone mineral density (BMD). Sound velocity images were computed from 2-D scans performed on 38 excised human femurs in transmission at 0.5 MHz. Different regions-of-interest were investigated. Dual x-ray absorptiometry scans have been achieved for BMD measurements in site-matched regions. Our study demonstrates the feasibility of ultrasonic velocity measurements at the hip with reasonable precision (coefficient of variation of 0.3%). The best prediction of BMD was reached in the intertrochanter region (r(2) = 0.91, p < 10(-4)), with a residual error of 0.06 g/cm(2) (10%). Because BMD measured at the femur is the best predictor of hip fracture risk, the highly significant correlation and small residual error found in this study suggest that speed of sound measurement at the femur might be a good candidate for hip fracture risk prediction.

Relationship among MRTA, DXA, and QUS revisited

Inexpensive, commercially produced devices that directly measure bone strength in vivo are not currently available. Mechanical response tissue analysis (MRTA), a unique prototype device, is an in vivo vibrational test that measures transverse bending stiffness (a measure of whole bone strength expressed as the product of estimated Young's modulus of elasticity and cross-sectional moment of inertia, EI, Nm2) at ulna midshaft. We compared speed of sound (SOS; [m/s]) in ulna cortical bone using a commercially available axial transmission quantitative ultrasound (QUS) device with EI using MRTA. Dual-energy X-ray absorptiometry (DXA) was used to provide an estimate of ulna size (cm2), bone mineral content (BMC; [g/cm]) and areal bone mineral density (BMD; [g/cm2]). The objective of the study was to determine if ulna SOS--alone or in combination with BMD from DXA--was correlated with ulna EI, thus becoming a surrogate measure of transverse bending stiffness, and thus whole bone strength. Data were collected from 138 female volunteers (18-86 yr). EI and SOS were significantly correlated, r = +0.218, p = 0.01, but r2 was very low, 4.8%. SOS and total ulna BMD were combined to estimate elastic modulus, which correlated with EI, r = +0.377, p < 0.0001; however, the correlation was not significantly better than with SOS alone. We conclude that axial transmission QUS is not a strong surrogate in vivo technique for estimating transverse bending stiffness.

Is ultrasound of bone relevant for corticosteroid-treated patients? A comparative study with bone densitometry measured by DEXA

Corticosteroid treatment diminishes bone mass and alters bone quality. The objective was to evaluate bone in corticosteroid-treated patients and controls and in fractured and non-fractured patients treated with corticosteroids using both X-ray densitometry (DEXA) and ultrasound. We evaluated 34 women aged 58 +/- 14 years (X +/- SD), who had been on long-term low dose prednisone therapy for at least 6 months, and who had never received specific treatment for osteoporosis. Bone mineral density of total skeleton (TS), lumbar spine (LS), femoral neck (FN), and vertebral morphometry (MXA) were measured by DEXA. Speed of sound (SOS), broadband ultrasound attenuation (BUA) and stiffness were measured using an Achilles Plus system. Forty-two healthy women served as controls. Both densitometric and ultrasound parameters in the patients were significantly diminished compared with controls: TS: P < 0.002, LS: P < 0.025, FS: P < 0.005, Stiffness: P < 0.001, BUA: P < 0.002 and SOS: P < 0.002. The percentage of patients with a Z score below -2 was higher in Stiffness and BUA: 38% and 47%, respectively, compared with a range of 16-24% in the other parameters (P < 0.05 BUA vs. DEXA measurements). Eleven patients with previous bone fracture had values lower than the non-fractured patients, both according to DEXA and ultrasound measurements, but the difference was only significant for BUA (P < 0.02). BUA of the calcaneus was more effective in detecting the specific skeletal alterations and fracture risk of the group of patients receiving chronic corticosteroid treatment.

Accuracy of elastic property measurement in mandibular cortical bone is improved by using cylindrical specimens

Ultrasonic determination of elastic properties in human craniofacial cortical bone is problematic because of a lack of information about the principal material axes, and because the cortex is often thinner than in long bones. This study investigated solutions that permit reasonable determination of elastic properties in the human mandible. We tested whether ultrasonic velocities could be reliably measured in cylindrical samples of aluminum and mandibular bone, and the effects of reduced specimen thickness. Results indicted that (1) varying shape had minimal effects on ultrasonic velocities or derived elastic properties, and (2) ultrasonic velocities have relatively increased measurement error as propagation distances decreased. The increased error in velocity measurements of mandibular cortical specimens of less than 1.2 mm in thickness should be considered when assessing the reliability of single measurements.

Guided ultrasonic waves in long bones: modelling, experiment and in vivo application

Existing ultrasound devices for assessing the human tibia are based on detecting the first arriving signal, corresponding to a wave propagating at, or close to, the bulk longitudinal velocity in bone. However, human long bones are effectively irregular hollow tubes and should theoretically support the propagation of more complex guided modes similar to Lamb waves in plates. Guided waves are attractive because they propagate throughout the bone thickness and can potentially yield more information on bone material properties and architecture. In this study, Lamb wave theory and numerical simulations of wave propagation were used to gain insights into the expected behaviour of guided waves in bone. Experimental measurements in acrylic plates, using a prototype low-frequency axial pulse transmission device, confirmed the presence of two distinct propagating waves: the first arriving wave propagating at, or close to, the longitudinal velocity, and a slower second wave whose behaviour was consistent with the lowest order Lamb antisymmetrical (A0) mode. In a pilot study of healthy and osteoporotic subjects, the velocity of the second wave differed significantly between the two groups, whereas the first arriving wave velocity did not, suggesting the former to be a more sensitive indicator of osteoporosis. We conclude that guided wave measurements may offer an enhanced approach to the ultrasonic characterization of long bones.

Contrast and velocity ultrasonic tomography of long bones

Our objective is to derive quantitative sound speed images of cortical bone using ultrasonic transmission tomography. Cortical bone is a highly refracting medium, i.e., the sound velocity changes abruptly across the interface between soft tissue and bone. It results in a loss of data compared to classical tomography in soft tissues. In order to correct for degradation by refraction effects, the classical acquisition procedure of projection data is modified: the transducers are oriented according to Snell's law of refraction with the aim of optimizing the sound propagation as parallel longitudinal rays inside the bone. This strategy allows the subsequent application of straight-ray reconstruction by the backprojection technique, which is a classical procedure in x-ray tomography. The method is validated with Plexiglas solid cylinders and tubes immersed in water. Improved sound velocity images are then derived using conventional Radon transform of the experimental time-of-flight data. The method is then extended to in vitro human femur immersed in water. The geometry of the bone cross-section is reconstructed from measurements using ultrasonic reflection tomography. The result is then introduced in the calculation of the position and orientation of the transducers, which are associated with the parallel acoustical paths in bone in the transmission measurements. The procedure leads to significant restoration enhancement over the non corrected image. The mean value of the velocity of 3,200 ms(-1) in the cortical shell is consistent with the values known from literature. These preliminary quantitative images using combined reflected and transmission ultrasound show promise for bone imaging.

North American male reference population for speed of sound in bone at multiple skeletal sites

Alternatives to dual-energy X-ray absorptiometry (DXA) have been sought to increase access to low-cost osteoporosis risk assessment. Early quantitative ultrasound (QUS) systems measured speed of sound (SOS) and broadband ultrasound attenuation (BUA) at the calcaneus, and these were demonstrated to be good predictors of hip fracture risk. Recent studies have demonstrated the usefulness of other peripheral sites to assess bone status. The Sunlight Omnisense (Sunlight Medical, Rehovot, Israel) is a portable, inexpensive QUS device capable of multiple-site SOS measurement. To provide a robust male reference database, 588 healthy Caucasian males aged 20-90 yr were recruited from 6 centers across North America. SOS measurements were taken at the distal 1/3 radius, proximal third phalanx, midshaft tibia, and fifth metatarsal. A female reference database has previously been collected at North American sites. The results indicate that SOS in males exhibits an age-related decline beginning in the fifth decade at the radius, phalanx, and metatarsal, whereas the tibial SOS remains nearly constant until the ninth decade. Although females reach a higher-peak SOS than males at most sites, SOS is higher in males at all sites after the sixth decade, as a result of a more gradual decline in SOS. Longitudinal monitoring of healthy men should be performed to confirm these cross-sectional results.

Bone material ultrasound velocity is predictive of whole bone strength

In humans, bone strength is assessed indirectly by the noninvasive measurement of structure or mass. Recent clinical application of an ultrasonic critical-angle reflectometry technique (UCR) has demonstrated the measurement of the regional and directional distribution of mechanical stiffness. This study investigates the specific question: are these measurements of a local material level property predictive of the strength of whole bone? Maximum values of pressure wave velocity and breaking strength were recorded at two locations (midshaft and base of neck) on rat femurs from growing rats. The results demonstrate a strong empirical relationship between material-level ultrasound (US) velocity and whole bone mechanical strength. However, the US velocity at a specific bone site can be used to assess bone strength at that site only, explaining discrepancies in other published studies that negate a relationship between strength and US velocity. The results indicate an important role for US velocity measurement in clinical evaluation of bone health.

Estimation of wrist fracture load using phalangeal speed of sound: an in vitro study

This study aimed to evaluate the ability of speed of sound (SOS) measured at the phalanges to estimate simulated wrist fracture load and stress. SOS was measured along the proximal phalanges of the second, third and fourth fingers using an ultrasound (US) system operating in axial transmission mode. The bone mineral density (BMD) of the radius and the phalanges was also measured with quantitative computed tomography (QCT) and dual x-ray absorptiometry (DXA), and the combined cortical thickness (CCT) of the phalanges was measured from hand radiographs. After the measurements were completed, the radius was excised from the cadaver, embedded in polymethylmethacrylate and tested to failure on a servohydraulic testing machine. The configuration of the radius was chosen to simulate a fall onto the hand. Linear regression analysis showed a highly significant correlation between SOS (r = 0.76-0.94, p < 0.001), CCT (r = 0.86-0.90, p < 0.001) and BMD (r = 0.92-0.96, p < 0.0001) in the three proximal phalanges measured. SOS, BMD and CCT were significant predictors of fracture load (r = 0.60-0.69, p < 0.03) and stress (r = 0.65-0.77, p < 0.02). Cortical area and bone mineral content (BMC) of the radius were consistently higher predictors of fracture load (r = 0.76-0.82, p < 0.01 for area and r = 0.78-0.88, p < 0.01 for BMC) than BMD. The correlation of BMC and area was poorer with fracture stress. In a step-wise regression analysis using both phalangeal BMD and SOS, only SOS remained a significant predictor of fracture stress. In forward stepwise regression analysis, both cortical area and SOS were entered into the regression model to estimate fracture load. Only SOS remained significant in the model for estimating fracture stress. Phalangeal BMD was only entered in the combined model with the cortical area at the 4% site (r = 0.84, p = 0.002). Phalangeal SOS is a useful parameter in the assessment of bone status of the radius.

Bone material elasticity in a murine model of osteogenesis imperfecta

To investigate the source of bone brittleness in the disease osteogenesis imperfecta (OI), biomechanical properties have been measured in the femurs from a homozygous (oim/oim) mutant mouse model of OI, its heterozygous littermates, and wild-type animals. The novel technique of ultrasound critical-angle reflectometry (UCR) was used to determine bone material elasticity matrix from measurements of the pressure and shear wave velocity at different orientations about selected points of the bone specimens. This nondestructive method is the only available means for obtaining measurements of this nature from a single surface. The ultrasound pressure wave velocity showed an increased isotropy in the homozygous compared to the wild-type specimens. This was reflected in a significant decrease in the principal elastic modulus measured along the length of the oim/oim bones (E33) while the modulus along the width (E11) did not change significantly, compared to wild-type specimens. The Poisson's ratio, v12, also had a significantly increased value in oim/oim bones. Measurements of these parameters in heterozygous animals generally fell between those from homozygous and control mice. The differences in the elasticity components in oim/oim bones indicate an altered stress distribution and a modified elastic response to loads, compared to normal bone.

Three-dimensional spatial compounding of ultrasound scans with weighting by incidence angle

A three-dimensional (3D) ultrasound imaging system has been used to study spatial compounding of images acquired with different scanhead positions and orientations. A compounding algorithm has been developed that assigns regional weights depending on the local incidence angle of the ultrasound beam. Compound scans were performed of bones in vitro and the shoulder rotator cuff in volunteer subjects. Border measurements (peak value and width) were compiled as a function of ultrasound beam incidence angle and compared for single views and for maximum, mean and weighted mean compounding techniques. The weighted mean produces less variability than that of the maximum and mean for both intensity and border width. The weighted method also demonstrates less blurring of borders than the maximum and mean methods. Surfaces derived from the weighted reconstructions exhibited fewer gaps and fewer spurious connections between surfaces, which could be of particular importance for automated image analysis.

Computerized radiographic texture measures for characterizing bone strength: a simulated clinical setup using femoral neck specimens

We are investigating computerized methods to ultimately characterize bone trabecular pattern from clinical skeletal radiographs. In this paper, we present a "phantom" for potential use in the development and evaluation of computerized methods for characterizing radiographic trabecular patterns and ultimately bone strength. Femoral neck specimens were excised during total hip arthroplasties from subjects exhibiting a range of diseases. To mimic the femoral neck in vivo, a "simulated clinical" setup was implemented in which specimens were exposed under conditions that yielded radiographs similar in appearance to standard pelvis radiographs. Fourier-based and fractal-based texture measures were used in the computer analysis; including RMS variation, first moment of the power spectrum, angular-dependent forms of these measures, and fractal dimension. The texture measures obtained from the "simulated clinical" specimen films correlated modestly with those from direct exposure "verification" films of the specimens (r= 0.59-0.69; p<0.0001). From our study, we conclude that the femoral neck specimen "phantoms" may be useful in the development and evaluation of computerized methods for analyzing bone trabecular patterns from skeletal radiographs. The use of a phantom that simulates the clinical radiographic examination allows for repeat exposures without the concern of excessive radiation exposure to a patient.

An in vitro investigation of the dependence on sample thickness of the speed of sound along the specimen

To measure the speed of sound (SOS), most quantitative ultrasound (QUS) devices use the transmission mode, whereby two transducers are placed on opposite sides of the sample. This mode is limited to a few specific skeletal sites because of the varying configuration of bone geometry and varying amounts of overlying soft tissue at most other sites. The aim of this study was to address the dependence of SOS measured along the sample on the thickness and composition of the bone sample. Bovine samples from mid-femur and trochanter, and perspex phantoms were used. We prepared the perspex samples in the shapes of blocks and cylinders to investigate the effect of wall thickness on SOS. The thickness of the blocks was decreased in decrements of 1 mm; a 22 mm diameter hole was drilled through the cylindrical samples and the hole size was gradually increased. The second configuration was also used with the bovine samples. For each experimental set-up five SOS measurements were acquired, with the probe aligned along the sample and a mean value computed. All measurements were taken with castor oil as the coupling agent, and in the cylindrical cases, the oil was used to fill the tube. The measurement precision determined as the root mean square coefficient of variation (RMSCV) was determined to be 0.14% and 0.65% for perspex and bovine samples respectively. The measured SOS on the perspex phantom (2760+/-4 m/s) was within the published values for bulk velocity. It was observed that for both perspex and bovine samples the SOS was independent of sample wall thickness greater than the wavelength (2.2 mm, 2.7 mm and 3.5 mm for perspex, trochanter and mid-femur respectively). The SOS decreased with sample wall thickness smaller than the wavelength in concordance with theoretical predictions. The SOS values obtained for bovine samples reflected either totally cortical (mid-femur) or a composite of cortical and cancellous bone (trochanter).

Bone elasticity and ultrasound velocity are affected by subtle changes in the organic matrix

The mechanical competence of bone can be studied through the measurement of the components of its material elasticity, a property which can vary both in magnitude and in dependence upon orientation (anisotropy). While it is known that the elasticity is largely determined by the mineral constituents of the bone matrix, it is nonetheless clear that it must be also dependent upon the remaining constituents of bone material. In this work, the influence of organic components on the elasticity is explored by altering specific constituents of the bone matrix to varying degrees. This study addresses two questions: first, are the resulting changes in elasticity strongly or weakly dependent upon direction, and second, are they substantially dependent upon the nature and magnitude of the induced matrix alteration? To answer these questions, we performed different chemical manipulations of the bone matrix and measured the changes in elasticity and velocity using the technique of ultrasound critical angle reflectometry. Altering the properties of the organic matrix resulted in substantial and complex changes in the elasticity of bone. The observed changes were strongly dependent upon direction, could not be explained by changes in density alone, and varied strongly with the specific chemical treatment of the matrix. Immersion in urea selectively affected protein components of the organic matrix and resulted in reversible changes in velocity and elasticity, while removal of collagen caused anisotropic decreases and removal of all organic matter caused a collapse of all components of the elasticity. In conclusion, this study confirms that the organic matrix exerts a profound influence on the elasticity and indicates that the measurement of elastic properties at multiple directions is necessary in the assessment of bone mechanical competence.

Reflection ultrasound velocities and histomorphometric and connectivity analyses: correlations and effect of slow-release sodium fluoride

To better understand how structural and functional bone properties contribute to the changes in bone biomechanical properties revealed by ultrasound critical angle reflectometry (UCR) analysis, we measured both UCR velocities and histomorphometric properties in bone biopsy specimens from 33 osteoporotic patients before and following intermittent slow-release sodium fluoride (SRNaF) and continuous calcium citrate administration. Mean skeletal fluoride exposure was 17 months, and mean skeletal fluoride content was 0.203 +/- 0.088 SD% bone ash. Intermittent SRNaF and continuous calcium citrate promoted significant increases in trabecular thickness (122 +/- 18 SD microm to 131 +/- 20, p = 0.020), mineral apposition rate (0.79 +/- 0.26 to 1.05 +/- 0.40 microm/day, p = 0.014), and a significant decline in eroded surface (3.9 +/- 1.6 to 2.8 +/- 1.4%, p = 0.002). There were also significant increases in node number (0.193 +/- 0.100 to 0.368 +/- 0.245, p < 0.01) and node-to-node strut length (0.076 +/- 0.087 to 0.191 +/- 0.173, p < 0.01) relative to total cancellous area. Cortical UCR velocity did not change but cancellous velocity significantly increased by 97 m/s following therapy (p = 0.0005). When compared against the significant changes in bone histomorphometry and connectivity, the sum of both cancellous and cortical ultrasound velocities was significantly correlated with node number/area (R2 = 0.305, p < 0.0001) and node-to-node strut length/area (R2 = 0.372, p < 0.0001) and to a lesser extent with mineral apposition rate (R2 = 0.106, p = 0.032). Multiple regression analysis demonstrated that 40% of the variance in the sum of the UCR velocities can be accounted for by the variability in these histomorphometric and connectivity parameters. There were no significant correlations between the sum of cortical and cancellous ultrasound velocities and cancellous bone volume (R2 = 0.014, p = 0533), trabecular thickness (R2 = 0.012, p = 0.47), or bone mineral density (R2 = 0.003, p = 0.80). These observations indicate that velocity measurements with the UCR methodology show an improvement in bone elasticity associated, in part, with an improvement in the rate of bone mineralization and an improvement in bone quality at the structural level as shown by microarchitecture.

Utility of ultrasound to assess risk of fracture

Traditional assessments of bone properties have utilized densitometry techniques such as Dual Energy X-ray Absorptiometry (DXA). Recently, quantitative ultrasound (QUS) has been introduced as an alternative method of assessing bone properties. Advantages of QUS over X-ray techniques include low costs, portability, and nonionizing radiation. Proponents of QUS have claimed that this technology can provide information not only about the density but also about the structure and mechanical properties of bone. There are two major questions that need to be answered for those who seek to diagnose bone disorders with ultrasound: (1) what does quantitative ultrasound actually measure, and, even more importantly, (2) what is its clinical utility? In this review we will briefly examine the first question and will focus on the utility of ultrasound in clinical trials to discriminate between fractures and non-fractures and to predict the risk of fractures.

Ultrasound critical-angle reflectometry (UCR): a new modality for functional elastometric imaging

This paper discusses the measurement of velocity in a solid based on the analysis of the amplitude and phase of ultrasound waves reflected by a solid, a technique called ultrasound critical-angle reflectometry (UCR). To this end, the complete formulation of ultrasound wave reflection and refraction from a liquid-solid interface is described. Differences between this formulation and previously published ones are briefly discussed. Based on this analysis it is in particular possible to measure by this technique not only pressure but also, for the first time in such studies, shear wave velocities, an experimentally confirmed result. The measurement of the complete stiffness matrix of a transversely isotropic solid, specifically cortical bone, by applying UCR elastometry to any point on the solid's surface is demonstrated. Finally this method is extended to functional elastometric imaging. The techniques presented in this paper offer new opportunities for applications of UCR imaging to the assessment of bone metabolism, formation and disease and also the analysis of composite materials in general.

Quantitative ultrasound techniques for the assessment of osteoporosis: expert agreement on current status. The International Quantitative Ultrasound Consensus Group

Quantitative ultrasound (QUS) methods have been introduced in recent years for the assessment of skeletal status in osteoporosis. The performance of QUS techniques has been evaluated in a large number of studies. Reviewing existing knowledge, an international expert panel formulated the following consensus regarding the current status of this technology. To date, evidence supports the use of QUS techniques for the assessment of fracture risk in elderly women. This has been best established for water-based calcaneal QUS systems. Future studies should include the predictive validity of other QUS systems. Additional clinical applications of QUS, specifically the assessment of rates of change for monitoring disease progression or response to treatment, require further investigation. Its low cost and portability make QUS an attractive technology for assessing risk of fractures in larger populations than may be suitable or feasible for bone densitometry. Additional investigations that assess innovative QUS techniques in well defined research settings are important to determine and utilize the full potential of this technology for the benefit of early detection and monitoring of osteoporosis.

Peripheral measurement techniques for the assessment of osteoporosis

Peripheral measurement techniques have been the first to be developed for the assessment of osteoporosis, and they remain useful. Besides traditional approaches such as radiographic absorptiometry (RA), radiogrammetry, and single-photon absorptiometry (SPA), new peripheral approaches have been developed that offer powerful ways to assess skeletal status in osteoporosis. These include single x-ray absorptiometry (SXA), peripheral dual x-ray absorptiometry (pDXA), peripheral quantitative computed tomography (pQCT), quantitative ultrasound (QUS) techniques, and magnetic resonance imaging (MRI) approaches. This review describes the current role of peripheral imaging techniques vis-à-vis their central imaging counterparts. Peripheral measurement techniques are attractive because equipment cost is substantially lower, radiation exposure is small, and the devices require less space and sometimes are even portable. Additionally, QUS and MRI offer the potential to measure aspects of bone status beyond the limits of bone densitometry. Peripheral techniques represent important diagnostic methods for the assessment of osteoporosis.

Measurement of shear-wave velocity by ultrasound critical-angle reflectometry (UCR)

There exists a growing body of research that relates the measurement of pressure-wave velocity in bone to different physiological conditions and treatment modalities. The shear-wave velocity has been less studied, although it is necessary for a more complete understanding of the mechanical properties of bone. Ultrasound critical-angle reflectometry (UCR) is a noninvasive and nondestructive technique previously used to measure pressure-wave velocities both in vitro and in vivo. This note describes its application to the measurement of shear-wave velocity in bone, whether directly accessible or covered by soft tissue.

Effect of a growth hormone treatment on bone orthotropic elasticity in dwarf rats

A refinement of the current ultrasonic elasticity technique was used to measure the orthotropic elastic properties of rat cortical bone as well as to quantify changes in elastic properties, density, and porosity of the dwarf rat cortex after a treatment with recombinant human growth hormone (rhGH). The ultrasonic elasticity technique was refined via optimized signal management of high-frequency wave propagation through cubic cortical specimens. Twenty dwarf rats (37 days old) were randomly assigned to two groups (10 rats each). The dwarf rat model (5-10% of normal GH) was given subcutaneous injections of either rhGH or saline over a 14-day treatment period. Density was measured using Archimedes technique. Porosity and other microstructural characteristics were also explored via scanning electron microscopy and image analysis. Statistical tests verified significant decreases in cortical orthotropic Young's (-26.7%) and shear (-16.7%) moduli and density (-2.42%) concomitant with an increase in porosity (+125%) after rhGH treatments to the dwarf model (p < 0.05). A change in material symmetry from orthotropy toward planar isotropy within the radial-circumferential plane after GH treatments was also noted. These results demonstrate some alteration in bone properties at this time interval. Structural implications of these changes throughout physiological loading regimens should be explored.

Reductions in bone strength after fluoride treatment are not reflected in tissue-level acoustic measurements

Acoustic velocity measurements are used to estimate tissue-level bone strength after fluoride therapy for osteoporosis. However, acoustic measurements provide information about elasticity, not strength, and bone elasticity does not necessarily correlate with bone strength at a tissue level. The current study was undertaken to evaluate the effects of fluoride treatment on tissue-level acoustic velocities, and to determine the relationship between acoustic velocity and bone strength measured in the femur, femoral neck, and spine. Young adult rabbits were treated with either 0 or 100 parts per million of fluoride in their drinking water for six months. After treatment, the bones were harvested for measurement of tissue fluoride, bone strength, and acoustic properties. Acoustic velocities were measured in the femoral midshaft using an acoustic microscope with a 50 MHz transducer. Both longitudinal and transverse velocities were measured. After the initial acoustic measurements the bone specimens were treated to remove either the organic matrix or mineral, and the acoustic measurements were repeated. Fluoride treatment increased bone fluoride levels 7-8 fold and reduced all biomechanical parameters. Most notably the fracture force of the femoral neck was reduced by 25% (p < 0.005), and the fracture stress of the L-5 vertebra was reduced by 19% (p < 0.05). Fluoride treatment had no significant effect on any of the measured acoustic velocities. The elastic anisotropy of the bone was decreased by demineralization (p < 0.0001) and increased by removal of the organic matrix (p < 0.0001), but unaffected by fluoride treatment. Acoustic measurements were not correlated with bone strength in the femoral neck or femoral midshaft. There was a positive correlation between the longitudinal velocity measured in the femur and the vertebral fracture stress, but this was the only positive association between acoustic velocities and strength measurements. These data cast doubt on the utility of high frequency (>2 MHz) acoustic measurements for evaluating the efficacy of fluoride therapy, especially in the hip.

Advances in the noninvasive assessment of bone density, quality, and structure

Recent advances in the development of methods to assess the skeleton noninvasively have contributed to screening for risk of osteoporosis, early detection of the disease, and effective monitoring of its progression and response to therapy. The capability now exists to evaluate the peripheral, central, or entire skeleton as well as the trabecular bone or cortical bone envelopes accurately and precisely, with the capacity to determine bone strength and predict fracture risk. In this article we examine the current and future capabilities of quantitative computed tomography (QCT), quantitative ultrasound (QUS), and magnetic resonance microscopy (muMR) to assess architectural and densitometric properties of the skeleton to enhance the prediction of fracture risk.

Effect of slow-release sodium fluoride on cancellous bone histology and connectivity in osteoporosis

We have previously demonstrated that a treatment regimen of slow-release sodium fluoride (SRNaF) and continuous calcium citrate increases lumbar bone mass, improves cancellous bone material quality, and significantly reduces vertebral fracture rate in osteoporotic patients. In order to assess whether such treatment also improves trabecular structure, we quantitated cancellous bone connectivity before and following 2 years of therapy with SRNaF in 23 patients with osteoporosis and vertebral fractures. In addition, we performed bone histomorphometry on the same sections used for connectivity measurements. There was a significant increase in L2-L4 bone mineral density during therapy (0.827 +/- 0.176 g/cm2 SD to 0.872 +/- 0.166, p = 0.0004). Significant histomorphometric changes were represented by increases in mineral apposition rate (0.6 +/- 0.4 microns/d to 1.1 +/- 0.7, p = 0.0078) and adjusted apposition rate (0.4 +/- 0.3 microns/d to 0.6 +/- 0.4, p = 0.016). On the other hand, trabecular spacing significantly declined (from 1375 +/- 878 microns to 1052 +/- 541, p = 0.05). Two-dimensional quantitation of trabecular struts on iliac crest histological sections disclosed significant increases in mean node number per mm2 of cancellous tissue area (0.22 +/- 0.12 vs. 0.39 +/- 0.27, p = 0.0077), the mean node to free-end ratio (0.23 +/- 0.21 vs. 0.41 +/- 0.46, p < 0.05), and in the mean node to node strut length per mm2 of cancellous area (0.098 +/- 0.101 vs. 0.212 +/- 0.183, p < 0.01). There were no significant changes in any of the measurements associated with free-end number or free-end to free-end strut length.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of reflection and transmission ultrasonic techniques for measurement of cancellous bone elasticity

A reflection ultrasonic technique, which offers several advantages over transmission ultrasonic techniques, has been described for elastic property measurement of bone. Ultrasonic velocities from specimens of cancellous bone were compared using a reflection ultrasound technique and the more traditional transmission ultrasonic technique. The two techniques were found to yield velocities which were reasonably well correlated (r2 = 0.74). However, a statistical difference was found between the line of identity and the regression between transmission and reflection velocities (due to an offset in the intercept). In spite of differences in intercept between velocities measured by the two techniques, significant correlation was found between the two methods, suggesting that the reflection technique can measure wave velocities meaningful to bone elasticity.

Three quantitative ultrasound parameters reflect bone structure

We investigated whether quantitative ultrasound (QUS) parameters are associated with bone structure. In an in vitro study on 20 cubes of trabecular bone, we measured broadband ultrasound attenuation (BUA) and two newly defined parameters--ultrasound velocity through bone (UVB) and ultrasound attenuation in bone (UAB). Bone mineral density (BMD) was measured by dual X-ray absorptiometry (DXA) and bone structure was assessed by microcomputed tomography (microCT) with approximately 80 microns spatial resolution. We found all three QUS parameters to be significantly associated with bone structure independently of BMD. UVB was largely influenced by trabecular separation, UAB by connectivity, and BUA by a combination of both. For a one standard deviation (SD) increase in UVB, a decrease in trabecular separation of 1.2 SD was required compared with a 1.4 SD increase in BMD for the same effect. A 1.0 SD increase in UAB required a reduction in connectivity of 1.4 SD. Multivariate models of QUS versus BMD combined with bone structure parameters showed squared correlation coefficients of r2 = 0.70-0.85 for UVB, r2 = 0.27-0.56 for UAB, and r2 = 0.30-0.68 for BUA compared with r2 = 0.18-0.58 for UVB, r2 < 0.26 for UAB and r2 < 0.13 for BUA for models including BMD alone. QUS thus reflects bone structure, and a combined analysis of QUS and BMD will allow for a more comprehensive assessment of skeletal status than either method alone.

Quantitative ultrasound of the heel: correlation with densitometric measurements at different skeletal sites

To assess the utility of quantitative ultrasound (QUS) of the heel for osteoporosis screening, we studied a group of 170 early postmenopausal women using both QUS of the heel and dual-energy X-ray absorptiometry (DXA) at the spine, hip, forearm, and whole body. On the basis of the linear regression results between QUS and DXA, a 95% bone mineral density (BMD) estimate confidence range was defined. Correlation coefficients between the QUS measurements and DXA ranged from 0.26 to 0.63. The confidence ranges for the estimated BMD based on a QUS measurement of the heel were large, such that an estimation of skeletal BMD at any of the DXA sites measured was not possible. For example, an estimate of the normative anteroposterior spine BMD (i.e. the T-score or the Z-score) based on a calcaneal ultrasound reading would have an error of +/- 1.9 standard deviations. Results for predicting the normative BMD of the other DXA regions were similar, with expected errors ranging from +/- 1.4 to +/- 2.0 standard deviations. We therefore conclude that QUS is not suited for the screening of early postmenopausal women for low axial or peripheral BMD. However, QUS may have a role as an independent predictor of fracture by measuring skeletal properties in addition to bone density.