Discrimination of fractures by low-frequency axial transmission ultrasound in postmenopausal females

SUMMARY

In this cross-sectional study, 95 postmenopausal women, with and without fracture history, were measured by low-frequency axial transmission ultrasound. The measured ultrasound velocity discriminated the fractured subjects from the nonfractured ones equally or better than peripheral quantitative computed tomography (pQCT) and dual energy x-ray absorptiometry (DXA). These results suggest that low-frequency ultrasound is suitable for bone fragility assessment.

INTRODUCTION

Quantitative low-frequency axial transmission ultrasound is a promising modality for assessing mineral density and geometrical properties of long bones such as radius and tibia. The aim of the current study was to evaluate the ability of low-frequency axial transmission ultrasound to discriminate fractures retrospectively in postmenopausal women.

METHODS

A cross-sectional study involved 95 female subjects aged 45-88 years, whose fracture information was gathered retrospectively. The fracture group was defined as subjects with one or more low-/moderate-energy fractures. The radius and tibial shaft were measured with a custom-made ultrasonometer to assess the velocity of the low-frequency first-arriving signal (V (LF)). Site-matched pQCT was used to measure volumetric cortical and subcortical bone mineral density (sBMD), and cortical thickness (CTh). Areal BMD (aBMD) was measured using DXA for the whole body (WB), lumbar spine, and hip.

RESULTS

The majority (19/32; 59 %) of the fractures were in the upper limb. V (LF) in the radius, but not in the tibia, discriminated fractures with an age- and BMI-adjusted odds ratio (OR) of 2.06 (95 % CI 1.21-3.50, p < 0.01). In the radius, CTh and cortical BMD (CBMD) significantly discriminated fractures, as did the total, cortical, and sBMD in the tibia (adjusted OR 1.35-2.15, p < 0.05). Sensitivity and specificity were similar among all the measurements (area under the receiver operating characteristic curve 0.74-0.81, p < 0.001).

CONCLUSIONS

Low-frequency axial transmission ultrasound in the radius was able to discriminate fractured subjects from the nonfractured ones. This suggests that low-frequency axial transmission ultrasound has the potential to assess bone fragility in postmenopausal women.

Low-frequency axial ultrasound velocity correlates with bone mineral density and cortical thickness in the radius and tibia in pre- and postmenopausal women

Axial transmission velocity of a low-frequency first arriving signal (V (LF)) was assessed in the radius and tibia of 254 females, and compared to site-matched pQCT measurements. V (LF) best correlated with cortical BMD, but significantly also with subcortical BMD and cortical thickness. Correlations were strongest for the radius in postmenopausal females.

INTRODUCTION

Ultrasonic low-frequency (LF; 0.2-0.4 MHz) axial transmission, based on the first arriving signal (FAS), provides enhanced sensitivity to thickness and endosteal properties of cortical wall of the radius and tibia compared to using higher frequencies (e.g., 1 MHz). This improved sensitivity of the LF approach has not yet been clearly confirmed by an in vivo study on adult subjects. The aims of the present study were to evaluate the extent to which LF measurements reflect cortical thickness and bone mineral density, and to assess whether an individual LF measurement can provide a useful estimate for these bone properties.

METHODS

Velocity of the LF FAS (V (LF)) was assessed in the radius and tibia shaft by a new ultrasonometer (CV(RMS) = 0.5%) in a cross-sectional study involving 159 premenopausal (20-58 years) and 95 postmenopausal females (45-88 years). Site-matched volumetric total bone mineral density (BMD), cortical bone mineral density (CBMD), subcortical bone mineral density (ScBMD) and cortical thickness (CTh) were assessed using pQCT.

RESULTS

For the postmenopausal females, V (LF) correlated best with CBMD in the radius (R = 0.850, p < 0.001), but significantly also with ScBMD and CTh (R = 0.759 and R = 0.761, respectively; p < 0.001). Similar trends but weaker correlations were observed for the tibia and for the premenopausal women.

CONCLUSIONS

The LF assessment, with an optimal excitation frequency, thus provided good prediction of both cortical thickness and subcortical bone material properties. These results suggest that the LF approach does indeed have enhanced sensitivity for detecting osteoporotic changes that occur deep in the endosteal bone.

Low volumetric BMD is linked to upper-limb fracture in pubertal girls and persists into adulthood: a seven-year cohort study

The aetiology of increased incidence of fracture during puberty is unclear. This study aimed to determine whether low volumetric bone mineral density (vBMD) in the distal radius is associated with upper-limb fractures in growing girls, and whether any such vBMD deficit persists into adulthood. Fracture history from birth to 20 years was obtained and verified by medical records in 1034 Finnish girls aged 10-13 years. Bone density and geometry at distal radius, biomarkers and lifestyle/behavioural factors were assessed in a subset of 396 girls with a 7.5-year follow-up. We found that fracture incidence peaked during puberty (relative risk 3.1 at age of 8-14 years compared to outside this age window), and 38% of fractures were in the upper-limb. Compared to the non-fracture cohort, girls who sustained upper-limb fracture at ages 8-14 years had lower distal radial vBMD at baseline (258.9+/-37.5 vs. 287.5+/-34.1 mg/cm(3), p=0.001), 1-year (252.0+/-29.3 vs. 282.6+/-33.5 mg/cm(3), p=0.001), 2-year (258.9+/-32.2 vs. 289.9+/-40.1 mg/cm(3), p=0.003), and 7-year follow-ups (early adulthood, 307.6+/-35.9 vs. 343.6+/-40.9 mg/cm(3), p=0.002). There was a consistent trend towards larger bone cross-sectional area in the fracture cohort compared to non-fracture. In a logistic regression model, lower vBMD (p=0.001) was the only significant predictor of upper-limb fracture during the period of 8-14 years. Our results indicate that low BMD is an important factor underlying elevated upper-limb fracture risk during puberty, and that low BMD in pubertal girls with fracture persists into adulthood. Hence low vBMD during childhood is not a transient deficit. Methods to monitor vBMD and to maximise bone mineral accrual and reduce risks of falling in childhood should be developed.

Monitoring bone growth using quantitative ultrasound in comparison with DXA and pQCT

Quantitative ultrasound (QUS) is a safe, inexpensive, and nonradiation method for bone density assessment. QUS correlates with, and predicts fragility fractures comparable to, dual-energy X-ray absorptiometry (DXA)-derived bone mineral density (BMD) in postmenopausal women. However, its validity in monitoring bone growth in children is not well understood. Two hundred and fifty-eight 10-13 yr pubertal girls and 9 37-43 yr adults without diseases or history of medications known to affect bone metabolism were included in the 2-yr prospective study. Calcaneal broadband ultrasound attenuation (cBUA) was assessed using QUS-2 (Quidel, Santa Clara, CA), speed of sound of tibial shaft (tSOS) using Omnisense (Sunlight Technologies, Israel), apparent volumetric BMD (vBMD) of tibial shaft using peripheral quantitative computed tomography (pQCT; XCT2000, Stratec), and femoral neck (FN) and lumbar spine 2-4 (LS) areal BMD (aBMD) using DXA (Prodigy, GE). Over the 2 yr in girls, FN and LS aBMD showed the largest increases (17+/-8% and 20+/-8%, respectively), followed by tibial vBMD and cBUA (10+/-5% and 9+/-9%, respectively). There was no apparent change in tSOS (2+/-3%). The increase in FN and LS aBMD attenuated 48% and 40%, respectively, after adjustment of the change in body size. The change of cBUA correlated significantly with change in tibial vBMD and FN and LS aBMD (r=0.24-0.40). At the matched location, tSOS correlated only with cortical vBMD, not with cortical thickness, apparent vBMD, or bone size. The long-term reproducibility, assessed using the concordance correlation coefficient of young adults' pre-post measurements, was substantially lower in tSOS than cBUA, tibial vBMD, FN, and LS aBMD (0.65 vs 0.97, 0.95, 0.98, and 0.96; p<0.05). The transverse transmission method-derived calcaneal BUA, but not the axial transmission method-derived SOS, is comparable to DXA and pQCT for monitoring bone densitometric change in pubertal girls. The role of QUS in fracture-risk prediction in children and adolescents needs further investigation.

Ultrasonically determined thickness of long cortical bones: Three-dimensional simulations of in vitro experiments

It was reported in a previous study that simulated guided wave axial transmission velocities on two-dimensional (2D) numerically reproduced geometry of long bones predicted moderately real in vitro ultrasound data on the same bone samples. It was also shown that fitting of ultrasound velocity with simple analytical model yielded a precise estimate (UTh) for true cortical bone thickness. This current study expands the 2D bone model into three dimensions (3D). To this end, wave velocities and UTh were determined from experiments and from time-domain finite-difference simulations of wave propagation, both performed on a collection of 10 human radii (29 measurement sites). A 3D numerical bone model was developed with tuneable fixed material properties and individualized geometry based on X-ray computed tomography reconstructions of real bones. Simulated UTh data were in good accordance (root-mean-square error was 0.40 mm; r(2)=0.79, p<0.001) with true cortical thickness, and hence the measured phase velocity can be well estimated by using a simple analytical inversion model also in 3D. Prediction of in vitro data was improved significantly (by 10% units) and the upgraded bone model thus explained most of the variability (up to 95% when sites were carefully matched) observed in in vitro ultrasound data.

Ultrasonically determined thickness of long cortical bones: two-dimensional simulations of in vitro experiments

Previously it has been demonstrated that cortical bone thickness can be estimated from ultrasonic guided-wave measurements, in an axial transmission configuration, together with an appropriate analytical model. This study considers the impact of bone thickness variation within the measurement region on the ultrasonically determined thickness (UTh). To this end, wave velocities and UTh were determined from experiments and from time-domain finite-difference simulations of wave propagation, both performed on a set of ten human radius specimens (29 measurement sites). A two-dimensional numerical bone model was developed with tunable material properties and individualized geometry based on x-ray computed-tomography reconstructions of human radius. Cortical thickness (CTh) was determined from the latter. UTh data for simulations were indeed in a excellent accordance (root-mean-square error was 0.26 mm; r2=0.94, p<0.001) with average CTh within the measurement region. These results indicate that despite variations in cortical thickness along the propagation path, the measured phase velocity can be satisfactorily modeled by a simple analytical model (the A(0) plate mode in this case). Most of the variability (up to 85% when sites were carefully matched) observed in the in vitro ultrasound data was explained through simulations by variability in the cortical thickness alone.

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

BACKGROUND

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

METHODS

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

FINDINGS

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

INTERPRETATION

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

Assessment of the cortical bone thickness using ultrasonic guided waves: modelling and in vitro study

Determination of cortical bone thickness is warranted, e.g., for assessing the level of endosteal resorption in osteoporosis or other bone pathologies. We have shown previously that the velocity of the fundamental antisymmetric (or flexural) guided wave, measured for bone phantoms and bones in vitro, correlates with the cortical thickness significantly better than those by other axial ultrasound methods. In addition, we have introduced an inversion scheme based on guided wave theory, group velocity filtering and 2-D fast Fourier transform, for determination of cortical thickness from the measured velocity of guided waves. In this study, the method was validated for tubular structures by using numerical simulations and experimental measurements on tube samples. In addition, 40 fresh human radius specimens were measured. For tubes with a thin wall, plate theory could be used to determine the wall thickness with a precision of 4%. For tubes with a wall thicker than 1/5 of the outer radius, tube theory provided the wall thickness with similar accuracy. For the radius bone specimens, tube theory was used and the ultrasonically-determined cortical thickness was found to be U-Th = 2.47 mm +/- 0.66 mm. It correlated strongly (r(2) = 0.73, p < 0.001) with the average cortical thickness, C-Th = 2.68 +/- 0.53 mm, and the local cortical thickness (r(2) = 0.81, p < 0.001), measured using peripheral quantitative computed tomography. We can conclude that the guided-wave inversion scheme introduced here is a feasible method for assessing cortical bone thickness.

Measuring guided waves in long bones: modeling and experiments in free and immersed plates

Guided waves, consistent with the A0 Lamb mode, have previously been observed in bone phantoms and human long bones. Reported velocity measurements relied on line fitting of the observed wave fronts. Such an approach has limited ability to assess dispersion and is affected by interference by other wave modes. For a more robust identification of modes and determination of phase velocities, signal processing techniques using the fast Fourier transform (FFT) were investigated. The limitations of FFT because of spatial resolution were addressed to improve the precision of the measured modes. An inversion scheme was developed for determining the plate thickness from the measured velocity. Experiments were performed on free and immersed plates, mimicking bone without and with an overlying tissue. With group velocity filtering, modes could be identified reliably with precise phase velocities and thicknesses. These methods were essential for the immersed plates and they should lead to more reliable in vivo measurements.

Differential effects of sex hormones on peri- and endocortical bone surfaces in pubertal girls

CONTEXT

The role of sex steroids in bone growth in pubertal girls is not yet clear. Bone biomarkers are indicators of bone metabolic activity, but their value in predicting bone quality has not been studied in growing girls.

OBJECTIVE

This study examines the association of sex hormones and bone markers with bone geometry and density in pubertal girls.

DESIGN

The study was designed as a 2-yr longitudinal study in pubertal girls. Measurements were performed at baseline and at 1- and 2-yr follow-ups.

SETTING

The study was conducted in a university laboratory.

PARTICIPANTS

A total of 258 10- to 13-yr-old healthy girls at the baseline participated.

METHODS

Peripheral quantitative computed tomography was used to scan the left tibial shaft. Serum 17beta-estradiol (E2), testosterone (T), SHBG, osteocalcin (OC), bone-specific alkaline phosphatase, and tartrate-resistant acid phosphatase isoform 5b were assessed. Data were analyzed using hierarchical linear models with random effect.

RESULTS

E2 was a positive predictor for total bone mineral density (BMD), cortical thickness, and a negative predictor for endocortical circumference but had no predictive value for total bone cross-sectional area or periosteal circumference. T was a positive predictor for total cross-sectional area and periosteal circumference as well as endocortical circumference, and a negative predictor for total BMD. OC was negatively correlated with cortical BMD (R2 = 0.325; P < 0.001).

CONCLUSIONS

In pubertal girls, E2 and T have different influences on bone properties at the long bone shaft. The results suggest that, at the endocortical surface, E2 inhibits bone resorption during rapid growth, and later, after menarche, acts at higher concentrations to promote bone formation. At the periosteal surface, T promotes bone formation, whereas E2 does not affect it. In addition, OC might be used as a predictor of cortical BMD.

Effects of calcium, dairy product, and vitamin D supplementation on bone mass accrual and body composition in 10-12-y-old girls: a 2-y randomized trial

BACKGROUND

Little is known about the relative effectiveness of calcium supplementation from food or pills with or without vitamin D supplementation for bone mass accrual during the rapid growth period.

OBJECTIVE

The purpose was to examine the effects of both food-based and pill supplements of calcium and vitamin D on bone mass and body composition in girls aged 10-12 y.

DESIGN

This placebo-controlled intervention trial randomly assigned 195 healthy girls at Tanner stage I-II, aged 10-12 y, with dietary calcium intakes <900 mg/d to 1 of 4 groups: calcium (1000 mg) + vitamin D3 (200 IU), calcium (1000 mg), cheese (1000 mg calcium), and placebo. Primary outcomes were bone indexes of the hip, spine, and whole body by dual-energy X-ray absorptiometry and of the radius and tibia by peripheral quantitative computed tomography.

RESULTS

With the use of intention-to-treat or efficacy analysis, calcium supplementation with cheese resulted in a higher percentage change in cortical thickness of the tibia than did placebo, calcium, or calcium + vitamin D treatment (P = 0.01, 0.038, and 0.004, respectively) and in higher whole-body bone mineral density than did placebo treatment (P = 0.044) when compliance was >50%. With the use of a hierarchical linear model with random effects to control for growth velocity, these differences disappeared.

CONCLUSIONS

Increasing calcium intake by consuming cheese appears to be more beneficial for cortical bone mass accrual than the consumption of tablets containing a similar amount of calcium. Diverse patterns of growth velocity may mask the efficacy of supplementation in a short-term trial of children transiting through puberty.

Influence of physical activity and maturation status on bone mass and geometry in early pubertal girls

This study aimed to evaluate the influence of leisure-time physical activity on the development of bone mass and density in early pubertal girls. Scores of physical activity were obtained from 242 Finnish girls (10-12 years old within Tanner Stages I-II) using a questionnaire. Bone mass and density were assessed using different densitometric techniques. At Tanner Stage I, active girls had significantly higher bone mineral mass (BMC) and areal bone mineral density (aBMD) of the whole body and cortical volumetric BMD and thickness of the tibial shaft compared with sedentary girls (P<0.05). On the other hand, the active girls at Tanner Stage II showed significantly higher values only in BMC and aBMD at the lumbar spine (P=0.017 and P=0.007, respectively). These indicated that girls at Tanner Stage I with higher leisure-time physical activity level benefited more from physical activity in terms of their bone development than their less active counterparts. Our results provide evidence that the most beneficial time for physical exercise to exhibit its effect on bone development is in the earlier pubertal period for normal school children, but the positive effect on the lumbar spine is also demonstrated in Tanner Stage II.

Differences in estimates of change of bone accrual and body composition in children because of scan mode selection with the prodigy densitometer

Girls of age 10-13 yr with Tanner stage I-III maturation status (n = 155) were measured using the Prodigy (GE Lunar) densitometer. Bone area (BA), bone mineral content (BMC), and bone mineral density (BMD) were assessed for the whole body, lumbar spine, and proximal femur using the Thin (T) and Standard (S) scan modes at years 1 and 3 of the study. The differences obtained between the T and S mode at year 1 were 1-2% for the lumbar spine and proximal femur and 5-11% for the whole body. For those girls whose default mode changed from T at year 1 to S mode at year 3, the estimated gain in BA, BMC, and BMD was 3.4%, 7.6%, and 3.1% respectively, lower than that obtained when scanning with the T mode at both times for the whole body. Small changes in magnitude but large intersubject variability were noted in BA, BMC, and BMD of the lumbar spine and proximal femur when scanned with the default mode of T at year 1 and S at year 3 compared to T or S at both years. Errors of this size are comparable to the changes expected with longitudinal intervention studies and are, therefore, clinically relevant.

Thickness sensitivity of ultrasound velocity in long bone phantoms

One approach to bone disease diagnosis such as osteoporosis is to measure the velocity of ultrasound propagating axially along long bones. In this study, the variation in velocity as a function of radial position was assessed using two polyvinyl chloride (PVC) bone phantoms with cross-sectional geometry similar to the human tibia but differing in medullary cavity diameter. Two ultrasonometers were used: these were a commercial device operating at a relatively high frequency (HF) of 1.25 MHz and a prototype low frequency (LF) device operating at approximately 200 kHz. The LF measurements showed a larger variation with radial position, with changes in velocity of up to 20% occurring around the phantom compared with changes of only 4% at most for HF. The LF velocity correlated strongly with local thickness (r(2) = 0.81) but HF velocity did not. The results demonstrate that LF measurements have a greatly enhanced thickness sensitivity. Using LF, it may therefore be possible to assess bone thickness as a function of radial position and hence to determine the distribution of bone around the long axis.

Relationship of sex hormones to bone geometric properties and mineral density in early pubertal girls

This study aimed to evaluate the associations among serum 17beta-estradiol (E2), testosterone (T), sex hormone-binding globulin (SHBG), bone geometric properties, and mineral density in 248 healthy girls between the ages of 10 and 13 yr old. The left tibial shaft was measured by peripheral quantitative computed tomography (Stratec XCT-2000; Stratec Medizintechnik, GmbH, Pforzheim, Germany). The cortical bone and marrow cavity areas were expressed as proportions of the total tibial cross-sectional area (CSA). Cortical thickness and total volumetric bone mineral density (vBMD) were also determined. These tibial geometric and densitometric measures were correlated against the serum sex hormone concentrations after controlling for age and body size. The results showed that E2 was negatively associated with marrow cavity proportion (r = -0.19, P = 0.003) and positively associated with cortical proportion and thickness and with total vBMD (r = 0.26, P < 0.001; r = 0.25, P < 0.001; and r = 0.23, P < 0.001, respectively). However, T was not associated with these bone variables. On the other hand, SHBG was positively associated with marrow cavity proportion (r = 0.17, P = 0.007) and negatively associated with cortical proportion and thickness and with total vBMD (r = -0.14, P = 0.029; r = -0.16, P = 0.010; and r = -0.18, P = 0.005, respectively). Total bone CSA did not correlate with E2, T, or SHBG. These results suggest that E2 has a positive effect on bone geometric and densitometric development by suppressing bone turnover at the endocortical surface during the early pubertal period, that SHBG plays an opposite role to E2, and that T has no detectable effect.

Assessment of the tibia using ultrasonic guided waves in pubertal girls

The purpose of this study was to compare low frequency ultrasonic guided wave measurements with established ultrasound and bone density measurements in terms of their ability to characterize the tibia in pubertal girls. Subjects were 12-14-year-old girls ( n=106) who were participating in a calcium and vitamin D intervention study. A prototype low frequency pulse transmission device consisting of a uniaxial scanning mechanism and low frequency transducers orientated perpendicularly to the limb was used to measure two ultrasound velocities in the tibia. The first velocity, V1, was that of the first arriving signal, similar to that measured by existing commercial tibial ultrasound devices. The second velocity, V2, was that of a slower wave propagating at 1,500-2,000 m/s, which has been shown elsewhere to be consistent with the lowest order antisymmetric guided mode in the bone. In addition, commercial ultrasound devices (Omnisense, Sunlight Ltd.; QUS-2, Quidel Corp.) were used to measure the speed of sound (SOS) in the tibia and the radius and attenuation (BUA) in the calcaneus. Cortical bone cross-sectional area (CSA), mineral density (BMD) and cortical thickness (cTh) of the tibia were measured using pQCT, site-matched to the ultrasound measurements. Both V1 and V2 correlated significantly with cortical BMD and with cTh and CSA. On the other hand, tibial SOS correlated with BMD, but not with cTh and CSA. These results indicate that the prototype device using guided waves captures aspects of tibial cortical bone geometry in addition to bone density, thereby potentially offering increased diagnostic information compared to existing tibial ultrasound devices.

Effect of temperature on ultrasonic properties of the calcaneus in situ

To assess the dependence of calcaneal quantitative ultrasound (QUS) on foot temperature, a series of acoustic measurements were made in five cadaver feet in situ (all soft tissues retained) over a temperature range of 25 degrees C to 40 degrees C in steps of 5 degrees C. An implanted probe was used to measured temperature directly in the calcaneus itself. Ultrasound velocity decreased linearly with increasing temperature, with a mean thermal coefficient of -2.2 m/s/ degrees C. In contrast, broadband ultrasonic attenuation (BUA) increased with temperature with a mean thermal coefficient of +0.75 dB/MHz/ degrees C. We argue that the temperature trends in velocity are likely to be due to the influence of fat, present in the bone marrow and in the soft tissues, which has a negative thermal coefficient for acoustic velocity. The attenuation trends may arise, in part, from greater scattering losses inside the cancellous bone due to an increased acoustic impedance mismatch between trabeculae and fatty marrow at higher temperatures. These considerations suggest that the greatest temperature effects may be expected in patients with a high proportion of fat within the measured volume and/or low calcaneal bone density. Given the magnitude of the thermal coefficients observed, the clinical impact of temperature-related QUS errors is likely to be modest for diagnostic purposes but of greater significance in follow-up studies.

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.

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.