Influence of marrow on ultrasonic velocity and attenuation in bovine trabecular bone

Mechanisms of Interaction of Ultrasound With Cancellous Bone: A Review

Ultrasound is now a clinically accepted modality in the management of osteoporosis. The most common commercial clinical devices assess fracture risk from measurements of attenuation and sound speed in cancellous bone. This review discusses fundamental mechanisms underlying the interaction between ultrasound and cancellous bone. Because of its two-phase structure (mineralized trabecular network embedded in soft tissue-marrow), its anisotropy, and its inhomogeneity, cancellous bone is more difficult to characterize than most soft tissues. Experimental data for the dependencies of attenuation, sound speed, dispersion, and scattering on ultrasound frequency, bone mineral density, composition, microstructure, and mechanical properties are presented. The relative roles of absorption, scattering, and phase cancellation in determining attenuation measurements in vitro and in vivo are delineated. Common speed of sound metrics, which entail measurements of transit times of pulse leading edges (to avoid multipath interference), are greatly influenced by attenuation, dispersion, and system properties, including center frequency and bandwidth. However, a theoretical model has been shown to be effective for correction for these confounding factors in vitro and in vivo. Theoretical and phantom models are presented to elucidate why cancellous bone exhibits negative dispersion, unlike soft tissue, which exhibits positive dispersion. Signal processing methods are presented for separating "fast" and "slow" waves (predicted by poroelasticity theory and supported in cancellous bone) even when the two waves overlap in time and frequency domains. Models to explain dependencies of scattering on frequency and mean trabecular thickness are presented and compared with measurements. Anisotropy, the effect of the fluid filler medium (marrow in vivo or water in vitro), phantoms, computational modeling of ultrasound propagation, acoustic microscopy, and nonlinear properties in cancellous bone are also discussed.

Velocity dispersion and backscatter in marrow-filled and water-filled trabecular bone samples in vitro

The phase velocity and the backscatter coefficient were measured in 28 bovine femoral trabecular bone samples filled with marrow and water in vitro from 0.2 to 0.6 MHz. The phase velocities decreased approximately linearly with increasing frequency and the average dispersion rate of -34 ms^-1MHz^-1 in the marrow-filled samples was higher than that of -42 ms^-1MHz^-1 in the water-filled samples. The backscatter coefficients exhibited nonlinear, monotonically increasing dependences on the frequency and the average value of the exponent n = 2.92 (frequency dependence) in the marrow-filled samples was higher than the value of n = 2.79 in the water-filled samples.

Relationships among ultrasonic and mechanical properties of cancellous bone in human calcaneus in vitro

Clinical bone sonometers applied at the calcaneus measure broadband ultrasound attenuation and speed of sound. However, the relation of ultrasound measurements to bone strength is not well-characterized. Addressing this issue, we assessed the extent to which ultrasonic measurements convey in vitro mechanical properties in 25 human calcaneal cancellous bone specimens (approximately 2×4×2cm). Normalized broadband ultrasound attenuation, speed of sound, and broadband ultrasound backscatter were measured with 500kHz transducers. To assess mechanical properties, non-linear finite element analysis, based on micro-computed tomography images (34-micron cubic voxel), was used to estimate apparent elastic modulus, overall specimen stiffness, and apparent yield stress, with models typically having approximately 25-30 million elements. We found that ultrasound parameters were correlated with mechanical properties with R=0.70-0.82 (p<0.001). Multiple regression analysis indicated that ultrasound measurements provide additional information regarding mechanical properties beyond that provided by bone quantity alone (p≤0.05). Adding ultrasound variables to linear regression models based on bone quantity improved adjusted squared correlation coefficients from 0.65 to 0.77 (stiffness), 0.76 to 0.81 (apparent modulus), and 0.67 to 0.73 (yield stress). These results indicate that ultrasound can provide complementary (to bone quantity) information regarding mechanical behavior of cancellous bone.

Nonlinear attenuation and dispersion in human calcaneus in vitro: statistical validation and relationships to microarchitecture

Through-transmission measurements were performed on 30 human calcaneus samples in vitro. Nonlinear attenuation and dispersion measurements were investigated by estimating 95% confidence intervals of coefficients of polynomial expansions of log magnitude and phase of transmission coefficients. Bone mineral density (BMD) was measured with dual x-ray absorptiometry. Microarchitecture was measured with microcomputed tomography. Statistically significant nonlinear attenuation and nonzero dispersion were confirmed for a clinical bandwidth of 300-750 kHz in 40%-43% of bone samples. The mean linear coefficient for attenuation was 10.3 dB/cm MHz [95% confidence interval (CI): 9.0-11.6 dB/cm MHz]. The mean quadratic coefficient for attenuation was 1.6 dB/cm MHz(2) (95% CI: 0.4-2.8 dB/cm MHz(2)). Nonlinear attenuation provided little information regarding BMD or microarchitecture. The quadratic coefficient for phase (which is related to dispersion) showed moderate correlations with BMD (r = -0.65; 95% CI: -0.82 to -0.36), bone surface-to-volume ratio (r = 0.47; 95% CI: 0.12-0.72) and trabecular thickness (r = -0.40; 95% CI: -0.67 to -0.03). Dispersion was proportional to bone volume fraction raised to an exponent of 2.1 ± 0.2, which is similar to the value for parallel nylon-wire phantoms (2.4 ± 0.2) and supports a multiple-scattering model for dispersion.

Introduction to Through-Transmission Alveolar Ultrasonography (TAU) in Dental Medicine

Through-transmission alveolar ultrasonography (TAU) is a novel imaging modality in dental medicine. A brief introduction to through-transmission ultrasonography (TTU) is followed by a description of the first commercially available TAU device, the Cavitat CAV 4000 (Cavitat Medical Technologies, Inc., Alba, TX). Recent associations between systemic osteoporosis, oral osteoporosis, periodontal diseases, and cardiovascular diseases underline the importance of early detection and treatment of oral cancellous bone pathologies associated with low bone density (LBD), such as regional ischemic osteoporosis, chronic nonsuppurative osteomyelitis, bone marrow edema, and cavitational ischemic osteonecrosis (osteocavitation). While the impact of osteoporosis on maxillofacial bones is acknowledged, there is a lack of reliable prevalence rate, and the National Institutes of Health (NIH) recommend that more attention should be paid to skeletal health, especially in persons with conditions known to be associated with secondary osteoporosis. TAU, a safe and effective imaging modality, can be a valuable tool in research as well as for the clinical assessment of alveolar cancellous bone pathologies associated with LBD and ischemia.

Ultrasonic properties in marrow-filled and water-filled bovine femoral trabecular bones in vitro

Phase velocity and normalized broadband ultrasound attenuation (nBUA) were measured in 22 marrow-filled and water-filled bovine femoral trabecular bone samples. Replacement of marrow by water led to a significant increase in the mean phase velocity of 47 ± 12 m/s (+3.1%), but a decrease in the mean nBUA of 10.4 ± 2.9 dB/cm/MHz (-38.9%). All the ultrasonic properties in the marrow-filled and water-filled samples exhibited significant negative Pearson's correlation coefficients of r = -0.87 to -0.92 with porosity. High correlations were also observed between pairs of the ultrasonic properties, with r = 0.85 to 0.93.

Relationships of quantitative ultrasound parameters with cancellous bone microstructure in human calcaneus in vitro

Ultrasound parameters (attenuation, phase velocity, and backscatter), bone mineral density (BMD), and microarchitectural features were measured on 29 human cancellous calcaneus samples in vitro. Regression analysis was performed to predict ultrasound parameters from BMD and microarchitectural features. The best univariate predictors of the ultrasound parameters were the indexes of bone quantity: BMD and bone volume fraction (BV/TV). The most predictive univariate models for attenuation, phase velocity, and backscatter coefficient yielded adjusted squared correlation coefficients of 0.69-0.73. Multiple regression models yielded adjusted correlation coefficients of 0.74-0.83. Therefore attenuation, phase velocity, and backscatter are primarily determined by bone quantity, but multiple regression models based on bone quantity plus microarchitectural features achieve slightly better predictive performance than models based on bone quantity alone.

Properties of ultrasonic waves in bovine bone marrow

We investigated the properties of ultrasonic waves in bovine bone marrow. Six bone marrow samples were obtained from different parts (proximal, middle and distal) of the shafts of two bovine femora without destruction. The measured frequency range was 3 to 10 MHz, and the temperature range was 15 to 40°C. Both wave velocity and attenuation coefficient in bone marrow always decreased as temperature increased. The velocity ranged from 1400 to 1610 m/s and attenuation coefficient ranged from 4 to 16 dB/cm. Wave velocities in bone marrow were similar to those in water, whereas the temperature dependences were different, and the wave attenuation coefficients were much higher than those in water. The dependence of velocity on temperature changed slightly around 23-24°C, where a transition from soft gel to oily liquid occurred. The transition temperature was confirmed by differential scanning calorimetry (DSC). Below this transition temperature, positive velocity dispersion was observed.

Frequency dependence of average phase shift from human calcaneus in vitro

If dispersion in a medium is weak and approximately linear with frequency (over the experimental band of frequencies), then it can be shown that the constant term in a polynomial representation of phase shift as a function of frequency can produce errors in measurements of phase-velocity differences in through-transmission, substitution experiments. A method for suppressing the effects of the constant phase shift in the context of the single-wave-model was tested on measurements from 30 cancellous human calcaneus samples in vitro. Without adjustment for constant phase shifts, the estimated phase velocity at 500 kHz was 1516+/-6 m/s (mean+/-standard error), and the estimated dispersion was -24+/-4 m/s MHz (mean+/-standard error). With adjustment for constant phase shifts, the estimated mean velocity decreased by 4-9 m/s, and the estimated magnitude of mean dispersion decreased by 50%-100%. The average correlation coefficient between the measured attenuation coefficient and frequency was 0.997+/-0.0026 (mean+/-standard deviation), suggesting that the signal for each sample was dominated by one wave. A single-wave, linearly dispersive model conformed to measured complex transfer functions from the 30 cancellous-bone samples with an average root-mean-square error of 1.9%+/-1.0%.

Influence of the filling fluid on frequency-dependent velocity and attenuation in cancellous bones between 0.35 and 2.5 MHz

The paper is focused on experiments on human cancellous bones filled with different fluids with the goal of evaluating their contribution to velocity dispersion, absorption, and scattering mechanisms. The specimens were measured first filled with marrow and subsequently, after marrow removal, with water and alcohol. No significant influence of the fluids was evidenced on the attenuation coefficient. Given the absence of impact of viscosity of the saturating fluid, the authors hypothesized that the source of attenuation is associated with viscoelastic absorption in the solid trabeculae and with scattering. Alteration of scattering obtained by changing the acoustic impedance mismatch between the fluid (alcohol vs water) and the trabeculae was reflected neither in the attenuation nor in its slope. This led the authors to suggest that longitudinal-to-shear scattering together with absorption in the solid phase are candidates as main sources for the attenuation. The differences in velocity values indicate that the elastic properties of the fluid are main determinants of the phase velocity. This finding is particularly significant in the context of /in vivo/ measurements, because it demonstrates that the subject-dependent properties of marrow may partly explain the inter-subject variability of speed of sound values.

Effect of bone marrow on acoustic properties of trabecular bone--3D finite difference modeling study

The composition of bone marrow is influenced by many factors, such as age and diseases. The present numerical study investigates the contribution of marrow on the acoustic measurements of trabecular bone. Cylindrical bone samples (n = 11), extracted from three anatomical sites of human cadaver knees, were imaged with a high-resolution microtomography (microCT). Three-dimensional finite difference time domain (FDTD) models (Wave 3000 Pro 2.2, Cyberlogic Inc., NY, USA) were created using the segmented microCT images of each sample. First, we evaluated the effect of voxel size on the computer resource requirements, morphological parameters and acoustic simulations. Second, the effect of bone marrow on ultrasonic measurements was assessed. The simulations were repeated with two voxel sizes before and after substitution of bone marrow (i.e., fat) with water. The voxel size of the FDTD mesh controlled the fine structure of the modeled calcified matrix and significantly affected the simulation results. However, present simulations showed that the effect of bone marrow on ultrasound parameters can be reliably simulated with the applied voxel sizes of 72 and 90 microm. Ultrasound attenuation and speed were found (p < 0.01) to decrease and increase, respectively, when bone marrow was substituted with water. Moreover, reflection from the surface of the sample increased (p < 0.01) and backscatter from internal structures decreased (p < 0.01) after removal of marrow. The effect of bone marrow on the acoustic properties was stronger in samples with low bone volume fraction. The present results indicate that the amount and quality of bone marrow significantly influence the acoustic properties of trabecular bone. Possible interindividual differences in the composition of bone marrow may increase uncertainty in clinical ultrasound diagnostics of osteoporosis. Importantly, the effect is most significant in osteoporotic low-density bone.

Mechanisms for attenuation in cancellous-bone-mimicking phantoms

Broadband ultrasound attenuation (BUA) in cancellous bone is useful for prediction of osteoporotic fracture risk, but its causes are not well understood. To investigate attenuation mechanisms, 9 cancellous-bone-mimicking phantoms containing nylon filaments (simulating bone trabeculae) embedded within soft-tissue-mimicking fluid (simulating marrow) were interrogated. The measurements of frequency-dependent attenuation coefficient had 3 separable components: 1) a linear (with frequency) component attributable to absorption in the soft-tissue-mimicking fluid, 2) a quasilinear (with frequency) component, which may include absorption in and longitudinal-shear mode conversion by the nylon filaments, and 3) a nonlinear (with frequency) component, which may be attributable to longitudinal-longitudinal scattering by the nylon filaments. The slope of total linear (with frequency) attenuation coefficient (sum of components #1 and #2) versus frequency was found to increase linearly with volume fraction, consistent with reported measurements on cancellous bone. Backscatter coefficient measurements in the 9 phantoms supported the claim that the nonlinear (with frequency) component of attenuation coefficient (component #3) was closely associated with longitudinal-longitudinal scattering. This work represents the first experimental separation of these 3 components of attenuation in cancellous bone-mimicking phantoms.

Ultrasonic scattering from cancellous bone: a review

This paper reviews theory, measurements, and computer simulations of scattering from cancellous bone reported by many laboratories. Three theoretical models (binary mixture, Faran cylinder, and weak scattering) for scattering from cancellous bone have demonstrated some consistency with measurements of backscatter. Backscatter is moderately correlated with bone mineral density in human calcaneus in vitro (r(2) = 0.66 - 0.68). Backscatter varies approximately as frequency cubed and trabecular thickness cubed in human calcaneus and femur in vitro. Backscatter from human calcaneus and bovine tibia exhibits substantial anisotropy. So far, backscatter has demonstrated only modest clinical utility. Computer simulation models have helped to elucidate mechanisms underlying scattering from cancellous bones.

A method for improved standardization of in vivo calcaneal time-domain speed-of-sound measurements

Although calcaneal speed of sound (SOS) is an effective predictor of osteoporotic fracture risk, clinical SOS measurements exhibit a high degree of inter-system variability. Calcaneal SOS is usually computed from time-of-flight measurements of broadband ultrasound pulses that propagate through the foot. In order to minimize the effects of multi-path interference, many investigators measure time-of-flight from markers near the leading edge of the pulse. The calcaneus is a highly attenuating, highly inhomogeneous bone that distorts propagating ultrasound pulses via frequency-dependent attenuation, reverberation, dispersion, multiple scattering, and refraction. This pulse distortion can produce errors in leading-edge transit-time marker-based SOS measurements. In this paper, an equation to predict dependence of time-domain SOS measurements on system parameters (center frequency and bandwidth), transit-time marker location, and bone properties (attenuation coefficient and thickness) is validated with through-transmission measurements in a bone-mimicking phantom and in 73 women in vivo, using a clinical bone sonometer. In order to test the utility of the formula for suppressing system dependence of SOS measurements, a wideband laboratory data acquisition system was used to make a second set of through-transmission measurements on the phantom. The compensation formula reduced system-dependent leading-edge transit-time marker-based SOS measurements in the phantom from 41 m/s to 5 m/s and reduced average transit-time marker-related SOS variability in 73 women from 40 m/s to 10 m/s. The compensation formula can be used to improve standardization in bone sonometry.

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.

Acoustic properties of trabecular bone--relationships to tissue composition

In osteoporosis, changes in tissue composition and structure reduce bone strength and expose it to fractures. The current primary diagnostic technique, i.e., dual energy X-ray absorptiometry, measures areal bone mineral density (BMD) but provides no direct information on trabecular structure or organic composition. Although still poorly characterized, ultrasound techniques may bring about information on bone composition and structure. In this study, relationships of 2.25-MHz ultrasound speed, attenuation, reflection and backscattering with composition of human trabecular bone (n=26) were characterized experimentally, as well as by using numerical analyses. We also determined composition of the trabecular sample (fat and water content, bone volume fraction) and that of the calcified matrix (mineral, proteoglycan and collagen content of trabeculae). In experimental analyses, bone volume fraction and mineral content of the calcified matrix were the only determinants of BMD. Further, bone volume fraction served as the strongest determinant of ultrasound parameters (r=0.51-0.87). In numerical simulations, density and mechanical properties of the calcified matrix systematically affected ultrasound speed, attenuation, reflection and backscattering. However, partial correlation coefficients revealed only low associations(|r|<or=0.4) between the composition of calcified matrix and ultrasound parameters in experimental measurements. To conclude, the content and structure of calcified matrix, rather than its composition, affect more significantly acoustic properties of healthy trabecular bone.

Influence of overlying soft tissues on trabecular bone acoustic measurement at various ultrasound frequencies

Ultrasound (US) has been introduced as a promising tool for osteoporosis diagnostics. However, soft tissues overlying the bones affect reliability of the ultrasound (US) techniques. In this in vitro study, the effect of soft tissues on bone US measurements was investigated numerically and experimentally. Particularly, the dependence of the error induced by soft tissues on the applied US frequency (0.3 to 6.7 MHz) was addressed. For these aims, human trabecular bone samples (n = 25) were measured using acoustic, dual energy x-ray absorptiometry (DXA) and mechanical techniques. US attenuation, speed, reflection and backscattering were determined from the through-transmission and pulse-echo measurements. Numerical correction, based on the inclusion of acoustic characteristics of specific soft tissue components, i.e., adipose and lean tissues, was derived for the analysis of experimental measurements. Values of US parameters, interrelationships between the US parameters and mechanical properties, as well as the errors induced by the soft tissues, were significantly dependent on the US frequency. The errors induced by the soft tissues on the US measurement were typically reduced by approximately 50% after introduction of the numerical correction technique. Thereby, the acoustic prediction of mechanical properties of trabecular bone was also improved. We conclude that the numerical correction of the contribution of overlying soft tissues on acoustic measurements can reduce uncertainties related to in vivo US measurements.

The temperature dependence of the speed of sound in bovine bone marrow at 750 kHz

We present values for the speed of sound (SOS) in bovine bone marrow as a function of temperature between 17 degrees C and 44 degrees C. The measurements were made using a time-of-flight approach on a volume of roughly 10 mL, at 750 kHz. The equipment was validated using both distilled water and castor oil. The results show a linear response with SOS changing from 1456.23 ms(-1) at 17 degrees C to 1342.40 ms(-1) at 44 degrees C. The mean value at 37 degrees C was (1371.91 ms(-1)). The temperature coefficient of the SOS was found to be -4.21 +/- 0.19 ms(-1) degrees C(-1). This was well fitted to a least squares model with R2 = 0.88.

In vitro acoustic waves propagation in human and bovine cancellous bone

The acoustic behavior of cancellous bone with regard to its complex poroelastic nature has been investigated. The existence of two longitudinal modes of propagation is demonstrated in both bovine and human cancellous bone. Failure to take into account the presence of these two waves may result in inaccurate material characterization.

INTRODUCTION

Acoustic wave propagation is now a commonly used nondestructive method for cancellous bone characterization. However, wave propagation in this material may be affected by fluid-solid interactions inherent to its poroelastic nature, resulting in two different longitudinal waves. This phenomenon has been demonstrated in previous studies and is in agreement with Biot's theory. The purpose of this paper is to extend these findings to human trabecular bone and to thoroughly investigate these two waves.

MATERIALS AND METHODS

Sixty human and 14 bovine cancellous bone cubic specimens were tested in vitro in three different directions using an immersion acoustic transmission method. Original procedures were developed to quantify both velocity and attenuation characteristics of each wave. In term of attenuation, a modified broadband ultrasound attenuation (BUA), describing the rate of change of the frequency-dependent attenuation, was defined for each wave (FDUA).

RESULTS

Both waves were identified in most of the specimens. The fast wave velocities demonstrated a negative linear correlation with porosity (1500-2300 m/s, R2 = 0.44, p < 10(-3)), whereas the slow wave velocities exhibited two different behaviors: (1) a first set of data clearly dependent on porosity showing a positive linear correlation (1150-1450 m/s, R2 = 0.26, p < 10(-3)) and (2) a second group independent on porosity. The fast wave FDUA (20-140 dB/cmMHz) showed a parabolic behavior and reached a maximum for 75% porosity (second degree relationship R2 = 0.41,p < 10(-3)), whereas a positive linear behavior was observed for the slow wave FDUA (15-40 dB/cmMHz; R2 = 0.15, p < 10(-2)).

CONCLUSIONS

Existence of two wave propagation modes were demonstrated in human cancellous bone. Our data suggest that, in some cases, the amplitude of the slow wave is much larger than the amplitude of the fast wave. For this reason, care should be taken when using measurement systems that incorporate simple threshold detection because the fast wave could remain undetected. Moreover, failure to consider the presence of these two waves could result in an inaccurate quantification of cancellous bone physical properties.

The dependence of ultrasonic backscatter on trabecular thickness in human calcaneus: theoretical and experimental results

Trabecular thickness within cancellous bone is an important determinant of osteoporotic fracture risk. Noninvasive assessment of trabecular thickness potentially could yield useful diagnostic information. Faran's theory of elastic scattering from a cylindrical object immersed in a fluid has been used to predict the dependence of ultrasonic backscatter on trabecular thickness. The theory predicts that, in the range of morphological and material properties expected for trabecular bone, the backscatter coefficient at 500 kHz should be approximately proportional to trabecular thickness to the power of 2.9. Experimental measurements of backscatter coefficient were performed on 43 human calcaneus samples in vitro. Mean trabecular thicknesses on the 43 samples were assessed using micro computed tomography (CT). A power law fit to the data showed that the backscatter coefficient empirically varied as trabecular thickness to the 2.8 power. The 95% confidence interval for this exponent was 1.7 to 3.9. The square of the correlation coefficient for the linear regression to the log transformed data was 0.40. This suggests that 40% of variations in backscatter may be attributed to variations in trabecular thickness. These results reinforce previous studies that offered validation for the Faran cylinder model for prediction of scattering properties of cancellous bone, and provide added evidence for the potential diagnostic utility of the backscatter measurement.

Estimation of critical and viscous frequencies for Biot theory in cancellous bone

The use of Biot theory for modelling ultrasonic wave propagation in porous media involves the definition of a "critical frequency" above which both fast and slow compressional waves will, in principle, propagate. Critical frequencies have been evaluated for healthy and osteoporotic cancellous bone filled with water or marrow, using data from the literature. The range of pore sizes in bone gives rise to a critical frequency band rather than a single critical frequency, the mean of which is lower for osteoporotic bone than normal bone. However, the critical frequency is a theoretical concept and previous researchers considered a more realistic "viscous frequency" above which both fast and slow waves may be experimentally observed. Viscous frequencies in bone are found to be several orders of magnitude greater than calculated critical frequencies. Whereas two waves may well be observed at all ultrasonic frequencies for water-filled cancellous bone at 20 degrees C, it is probable megahertz frequencies would be needed for observation of two waves in vivo.

Bone marrow influences quantitative ultrasound measurements in human cancellous bone

Quantitative ultrasound (QUS) transmission and backscatter measurements were made in 46 human cancellous bone specimens from the calcaneus. All QUS measurements were made at 35 degrees C, initially with marrow filling the pores and then repeated after substituting water for marrow. Bone mineral density (BMD) was determined using x-ray absorptiometry. Marrow significantly decreased ultrasound (US) velocity, but increased attenuation, attenuation slope and backscatter (p < 0.001 for all) compared to the water-saturated state. The impact of marrow on QUS measurements was greater at lower BMD values (p < 0.05), and was greater in women than in men (p < 0.05). QUS measurements in marrow-saturated specimens correlated less strongly with BMD than did corresponding measurements in water-saturated specimens (p < 0.05), consistent with interspecimen marrow heterogeneity. These data indicate that the potential impact of marrow should be considered when interpreting clinical QUS measurements. Understanding and exploiting these effects could lead to novel approaches for ultrasonic characterisation of both bone and marrow.

Fundamental precision limitations for measurements of frequency dependence of backscatter: applications in tissue-mimicking phantoms and trabecular bone

Various models for ultrasonic scattering from trabecular bone have been proposed. They may be evaluated to a certain extent by comparison with experimental measurements. In order to appreciate limitations of these comparisons, it is important to understand measurement precision. In this article, an approach proposed by Lizzi and co-workers is adapted to model precision of estimates of frequency-dependent backscatter for scattering targets (such as trabecular bone) that contain many scatterers per resolution cell. This approach predicts uncertainties in backscatter due to the random nature of the interference of echoes from individual scatterers as they are summed at the receiver. The model is validated in experiments on a soft-tissue-mimicking phantom and on 24 human calcaneus samples interrogated in vitro. It is found that while random interference effects only partially explain measured variations in the magnitude of backscatter, they are virtually entirely responsible for observed variations in the frequency dependence (exponent of a power law fit) of backscatter.

Relationships of ultrasonic backscatter with ultrasonic attenuation, sound speed and bone mineral density in human calcaneus

Ultrasonic attenuation and sound speed have been investigated in trabecular bone by numerous authors. Ultrasonic backscatter has received much less attention. To investigate relationships among these three ultrasonic parameters and bone mineral density (BMD), 30 defatted human calcanei were investigated in vitro. Normalized broadband ultrasonic attenuation (nBUA), sound speed (SOS), and logarithm of ultrasonic backscatter coefficient (LBC) were measured. Bone mineral density was assessed using single-beam dual energy x-ray absorptiometry (DEXA). The correlation coefficients of least squares linear regressions of the three individual ultrasound (US) parameters with BMD were 0.84 (nBUA), 0.84 (SOS) and 0.79 (LBC). The 95% confidence intervals for the correlation coefficients were 0. 69-0.92 (nBUA), 0.68-0.92 (SOS) and 0.60-0.90 (LBC). The correlations among pairs of US variables ranged from 0.63-0.79. Variations in nBUA accounted for r(2) = 62% of the variations in LBC. Variations in SOS accounted for r(2) = 40% of the variations in LBC. These results suggest that ultrasonic backscattering properties may contain substantial information not already contained in nBUA and SOS. A multiple regression model including all three US variables was somewhat more predictive of BMD than a model including only nBUA and SOS.

Quantitative ultrasound and dual X-ray absorptiometry measurements of the calcaneus in patients on maintenance hemodialysis

It has been suggested that quantitative ultrasound measurements (QUS), which reflect mainly bone density, could be influenced by bone micro-architecture. The aim of the study was to assess whether the relationship of QUS to dual X-ray absorptiometry (DXA) would reflect abnormalities of bone structure observed in renal osteodystrophy. QUS and bone mineral density of the calcaneus (BMDc) were measured by DXA in 30 patients on maintenance hemodialysis and 34 age- and gender-matched controls. QUS parameters and BMDc were significantly lower in hemodialysis patients than in controls (speed of sound [SOS] and broadband ultrasound attenuation [BUA], p = 0. 030; stiffness, p = 0.003; BMDc, p = 0.006). Bone measurements were not correlated with serum parathyroid hormone (PTH). The regression lines of SOS, BUA, and stiffness to BMDc were not significantly different from that of the controls. When dividing the patients into two subgroups according to their median PTH (203 pg/mL), the slopes of the regression lines of BUA to BMDc were significantly different between these two subgroups (p = 0.052). The slope of the subgroup with PTH </= 203 pg/mL was significantly different from that of the controls (p = 0.030). In conclusion, QUS of the calcaneus can be used for bone assessment in patients on maintenance hemodialysis. The differences in the slopes of patients with a less pronounced degree of hyperparathyroidism compared with patients with a higher degree of hyperparathyroidism and to controls suggest that BUA of the calcaneus contains information on bone complementary to DXA measured at the same site. The clinical relevance of this finding is presently unclear.

Anisotropy of ultrasonic backscatter and attenuation from human calcaneus: implications for relative roles of absorption and scattering in determining attenuation

Although bone sonometry has been demonstrated to be useful in the diagnosis of osteoporosis, much remains to be learned about the processes governing the interactions between ultrasound and bone. In order to investigate these processes, ultrasonic attenuation and backscatter in two orientations were measured in 43 human calcaneal specimens in vitro at 500 kHz. In the mediolateral (ML) orientation, the ultrasound propagation direction is approximately perpendicular to the trabecular axes. In the anteroposterior (AP) orientation, a wide range of angles between the ultrasound propagation direction and trabecular axes is encountered. Average attenuation slope was 18% greater while average backscatter coefficient was 50% lower in the AP orientation compared with the ML orientation. Backscatter coefficient in both orientations approximately conformed to a cubic dependence on frequency, consistent with a previously reported model. These results support the idea that absorption is a greater component of attenuation than scattering in human calcaneal trabecular bone.

Measurements of phase velocity and group velocity in human calcaneus

Ultrasonic velocity in calcaneus correlates highly with bone mineral density, which is a good predictor of osteoporotic fracture risk. Several commercial bone sonometers perform a velocity measurement based on the transit time of a broadband pulse to assess skeletal status. This approach is somewhat problematic, however, because several authors have reported ambiguities in measurements in calcaneus. Phase velocity is an alternative that may be less dependent on device spectral characteristics. In addition, dispersion (the frequency-dependence of phase velocity) is a fundamental property worth investigating to increase understanding of interaction between ultrasound and bone. To compare two group-velocity measurement methods and one phase-velocity measurement method, a polycarbonate sample (for method validation) and 24 human calcanei were investigated in vitro. Phase velocity in calcaneus at 500 kHz was 1511 m/s +/- 30 m/s (mean +/- standard deviation). Average phase velocity decreased approximately linearly with frequency (-18 m/s MHz). The two group velocity measurements were comparable and tended to be slightly lower than phase velocity. The magnitude of dispersion showed little correlation with bone mineral density.

Theoretical study of convergent ultrasound hyperthermia for treating bone tumors

This study investigates the optimal external parameters for using an ultrasound applicator for treating bone tumors. This system utilized spherically arranged applicators such as scanned focused ultrasound, and spherically focused multielement applicators. The power deposition pattern is modeled as geometric gain with exponential attenuation. The specific absorption rate ratio (SARR) criteria have been used to determine the proper heating domain of ultrasound driving frequency and therapeutic tumor diameter. The results demonstrate that the optimal driving frequency depends on tumor depth, ultrasound absorption of bone marrow, and diameter of bone, but it is independent of the acoustic window area and SARR. The treatable diameter of bone tumor increased when the absorption ratio of bone marrow to tumor, acoustic window of surface skin, and diameter of bone were elevated. However, the treatable diameter of bone tumor decreased when muscle thickness, SARR of bone tumor site to the surface skin, bone marrow, and bone declined. To deliver the ultrasound energy into the tumor site and to avoid the potential damage to the normal tissue as much as possible, the specific absorption rate (SAR) in the bone tumor site has to be three times higher than that in the surface skin, tumor/marrow, and marrow/bone interfaces. The temperature distributions can verify the SARR criteria in this model. This study provides the information for choosing the optimal operating frequency of the ultrasound transducer and the acoustic window on the skin surface, and for designing the ultrasound applicator for clinical implementation.

The relationship between ultrasonic backscatter and bone mineral density in human calcaneus

Backscatter and attenuation coefficients were measured from 24 human calcanei in vitro. The logarithm of the backscatter coefficient at 500 kHz showed moderate correlations with bone mineral density (r=0.81, 95% confidence interval: 0.59-0.91) and attenuation (r=0.79, 95% CI: 0.56-0.91). These results suggest that backscatter measurements may be useful in the diagnosis of osteoporosis.

Frequency dependence of ultrasonic backscatter from human trabecular bone: theory and experiment

A model describing the frequency dependence of backscatter coefficient from trabecular bone is presented. Scattering is assumed to originate from the surfaces of trabeculae, which are modeled as long thin cylinders with radii small compared with the ultrasonic wavelength. Experimental ultrasonic measurements at 500 kHz, 1 MHz, and 2.25 MHz from a wire target and from trabecular bone samples from human calcaneus in vitro are reported. In both cases, measurements are in good agreement with theory. For mediolateral insonification of calcaneus at low frequencies, including the typical diagnostic range (near 500 kHz), backscatter coefficient is proportional to frequency cubed. At higher frequencies, the frequency response flattens out. The data also suggest that at diagnostic frequencies, multiple scattering effects on the average are relatively small for the samples investigated. Finally, at diagnostic frequencies, the data suggest that absorption is likely to be a larger component of attenuation than scattering.

Impact of weight in obese subjects on bone speed of sound

RATIONALE AND OBJECTIVES

The authors determined the effect of obesity on measurements of amplitude-dependent speed of bone ultrasound (Ad-SOS [m/sec]) and compared them to the total body bone mineral content (TBBMC/g).

METHODS

A total of 25 women were studied (mean age 41.8 +/- 10.2 years). In all the subjects, body mass index (BMI) exceeded 30 kg/m2 (range, 31.12-47.47 kg/m2); mean body weight was 104 +/- 17 kg. Ad-SOS was measured at the proximal phalanges and TBBMC in whole body with dual-energy x-ray absorptiometry.

RESULTS

Correlation study (Fisher's r to z) showed that Ad-SOS correlated negatively with weight (r = -0.85, P < 0.0001) and with TBBMC (r = -0.71, P < 0.0001). The correlation between TBBMC and weight was r = 0.76, P < 0.0001. Body fat percentage correlated partially with TBBMC (r = 0.40, P < 0.05) and negatively with Ad-SOS (r = -0.75, P < 0.0001). When the correlation test was adjusted for weight (partial correlation), the correlation between Ad-SOS and TBBMC was not significant (r = -0.21, P = NS), and the correlation between Ad-SOS and weight continued to be inversely significant (r = -0.67, P < 0.0001).

CONCLUSIONS

The results showed a clearly negative effect of weight on Ad-SOS measurements and indicated the limitations of this technique when employed in overweight and obese patients. Broad-band ultrasound attenuation and speed of sound, two commonly measured variables in bone ultrasound studies, may be differently affected by soft tissue.

Ultrasonic propagation in cancellous bone: a new stratified model

The theoretical modeling of ultrasonic propagation in cancellous bone is pertinent to improving the ultrasonic diagnosis of osteoporosis. First, this paper reviews applications of Biot's theory to this problem. Next, a new approach is presented, based on an idealization of cancellous bone as a periodic array of bone-marrow layers. Schoenberg's theory is applied to this model to predict wave properties. Bovine bone samples were tested in vitro using pulses centered at 1 MHz over various angles relative to the orientated cancellous structure. Two longitudinal modes (fast and slow waves) were observed for propagation parallel to the structure, but only one was observed for propagation normal to the structure. Angular-dependence of velocities was examined, and the fast wave was found to be strongly anisotropic. These results gave qualitative agreement with predictions of Schoenberg's theory. Although this new model is a simplification of the cancellous architecture, it has potential for future research.

Computational methods for ultrasonic bone assessment

Ultrasound has been proposed as a means to noninvasively assess bone and, particularly, bone strength and fracture risk. Although there has been some success in this application, there is still much that is unknown regarding the propagation of ultrasound through bone. Because strength and fracture risk are a function of both bone mineral density and architectural structure, this study was carried out to examine how architecture and density interact in ultrasound propagation. Due to the difficulties inherent in obtaining fresh bone specimens and associated architectural and density features, simulation methods were used to explore the interactions of ultrasound with bone. A sample of calcaneal trabecular bone was scanned with micro-CT and subjected to morphological image processing (erosions and dilations) operations to obtain a total of 15 three-dimensional (3-D) data sets. Fifteen two-dimensional (2-D) slices obtained from the 3-D data sets were then analyzed to evaluate their respective architectures and densities. The architecture was characterized through the fabric feature, and the density was represented in terms of the bone volume fraction. Computer simulations of ultrasonic propagation through each of the 15 2-D bone slices were carried out, and the ultrasonic velocity and mean frequency of the received waveforms were evaluated. Results demonstrate that ultrasound propagation is affected by both density and architecture, although there was not a simple linear correlation between the relative degree of structural anisotropy with the ultrasound measurements. This study elucidates further aspects of propagation of ultrasound through bone, and demonstrates as well as the power of computational methods for ultrasound research in general and tissue and bone characterization in particular.

Calcaneus as a site for the assessment of bone mass

The calcaneus is a skeletal site frequently used for monitoring bone loss after spaceflight, because it is sensitive to microgravity-induced bone mineral loss and reflects the degree of demineralization in the vertebra and the femoral neck. In this article, methods for assessing the calcaneus are reviewed, and their potential applications and limitations as the monitoring site for bone loss in weightlessness are discussed. Currently, single or dual energy X-ray absorptiometry appears to be most sensitive for monitoring bone mineral loss in weightlessness. The results of recent studies suggest two- to threefold longer follow-up times required for ultrasound techniques. However, ultrasound devices can be designed to be portable, making them attractive for inflight use, and ultrasound techniques are expected to provide information related to bone quality. Additional investigations that assess new ultrasound techniques would be important to determine and utilize the full potential of this technology for monitoring bone loss in weightlessness.

Effect of bovine bone constituents on broadband ultrasound attenuation measurements

Broadband ultrasound attenuation (BUA) has been found to correlate positively with bone mineral density (BMD) measured by dual-energy X-ray absorptiometry. However, because there is a significant amount of unexplained variation in this correlation, it has been suggested that BUA might also provide information about bone structure. The purpose of this study was to determine the contribution of bone mineral and organic matrix to BUA and BMD measurements. The influence of sample length on both BUA and BMD was also investigated by normalizing these measurements to length. BUA (Walker Souix, 575+) and BMD (Lunar DPX) values were obtained on bicortical cores removed from 12 bovine femoral necks. BMD and BUA measurements were repeated on the samples after: (1) mechanical removal of the cortices; (2) defatting using a 2:1 chloroform:methanol solution; and (3) decalcifying using formic acid. The data demonstrate that the cortical component of the bone contributes significantly to BMD. We found that 41.7% of the normalized BMD reflect cortical bone. Defatting the samples did not affect BUA. Decalcifying the trabecular bone while maintaining an intact collagenous structure significantly reduces BUA by 89% and BMD by 96% compared to the whole core samples. Normalizing BMD and BUA to sample length, in cases where large variation is present, does influence the correlation between the variables. We conclude that BUA is influenced mainly by the presence of bone mineral, whereas the presence of the organic matrix contributes very little to BUA.

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.