In vivo performance of a matrix-based quantitative ultrasound imaging device dedicated to calcaneus investigation

Quantitative ultrasound imaging monitoring progressive disuse osteopenia and mechanical stimulation mitigation in calcaneus region through a 90-day bed rest human study

Background

Osteoporosis parallels aging and functional mechanical unloading (e.g., space flight and bed rest), jeopardizing mineral density, microstructure, and integrity of bone and leading to an increased risk of fracture. A way to combat this deterioration is to harness the sensitivity of bone to mechanical signals.

Objective

This study evaluates the longitudinal effect of a dynamic mechanical loading through the heel on human bone in vivo during 90-day bed rest, monitored by quantitative ultrasound (QUS) imaging and dual-energy X-ray absorptiometry (DXA) in localized regions of interests, i.e., calcaneus.

Methods

A total of 29 bed rest individuals were evaluated (11 control and 18 treatment) with a brief (10-minute) daily low-intensity (0.3g), high-frequency (30Hz) dynamic mechanical stimulation countermeasure through vibrational inhibition bone erosion (VIBE). Both QUS and DXA detected longitudinal bone density and quality changes.

Results

Ultrasound velocity (UV) decreased in the control group and increased in the group treated with low-intensity loading. The UV increased by 1.9% and 1.6% at 60- and 90-day bed rest (p=0.01) in VIBE over control groups. A trend was found in broadband ultrasound attenuation (BUA), with a VIBE benefit of 1.8% at day 60 and 0.5% at day 90 in comparison with control (p=0.5). Bone mineral density (BMD) assessed by DXA decreased -4.50% for control individuals and -2.18% for VIBE individuals, showing a moderate effect of the mechanical intervention (p=0.19). Significant correlations between QUS and DXA were observed, with a combined BUA and UV vs. BMD: r²=0.70.

Conclusion

These results indicated that low-intensity, high-frequency loading has the potential to mitigate regional bone loss induced by long-term bed rest and that QUS imaging may be able to assess the subtle changes in bone alteration.

Translational potential of this article

Quantitative ultrasound has shown the efficacy of noninvasively assessing bone mass and structural properties in cadaver and isolated trabecular bone samples. While its ability in measuring in vivo bone quality and density is still unclear, a scanning confocal ultrasound imaging is developed and can perform an instant assessment for the subtle changes of such bone loss. This ultrasound imaging modality can potentially be used in the clinical assessment of bone mass. Moreover, physical stimulation has shown the ability to prevent bone loss induced by functional disuse and estrogen deficiency in animal models. However, its treatment capability is unclear. This study has shown that low-magnitude mechanical signals, introduced using low-intensity vibration (LIV), can mitigate regional bone loss caused by functional disuse. Thus localized mechanical treatment, and the quantitative ultrasound imaging have shown translational potential to noninvasively attenuate bone loss, and assess bone mass in the clinic, e.g., in an extreme condition such as long-term space mission, and long-term bedrest such as in case of spinal cord injury.

Exploration of trabecular bone nonlinear elasticity using time-of-flight modulation

Bone tissue contains microcracks that may affect its mechanical properties as well as the whole trabecular structure. The relationship between crack density and bone strength is nevertheless poorly understood. Linear ultrasound techniques being almost insensitive to the level of damage, we propose a method to measure acoustic non- linearity in trabecular bone using time-of-flight modulation (TOFM) measurements. Ultrasonic short bursts times-of- flight (TOF) are modulated as a result of nonlinear interaction with a low-frequency (LF) wave in the medium. TOF variations are directly related to elastic modulus variations. Classical and nonclassical nonlinear parameters beta, delta, and alpha can be derived from these measurements. The method was validated in materials with classical, quadratic, nonlinear elasticity. In dense trabecular bone region, TOFM related to classical, quadratic, nonlinear elasticity as a function of the LF pressure exhibits tension-compression asymmetry. The TOFM amplitude measured in dense areas of trabecular bone is almost one order of magnitude higher than in a low-density area, but the linear parameters show much smaller variations: 5% for ultrasound propagation velocity and 100% for broadband ultrasonic attenuation (BUA). In high-density trabecular bone regions, beta depends on the LF pressure amplitude and can reach 400 at 50 kPa.

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.

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 a matrix-based quantitative ultrasound imaging device (Beam scanner): reproducibility]

PURPOSE

A new matrix-based quantitative ultrasound (QUS) device was developed for the assessment of two parameters: BUA (Broadband Ultrasound Attenuation) and SOS (Speed Of Sound). This device, called Beam scanner, is a contact imaging device designed to evaluate the calcaneus. The aim of this preliminary study was to evaluate the reproducibility of results in a sample of patient with heterogeneous age distribution.

MATERIALS AND METHODS

Seventy-six subjects were included: 18 healthy young males, 18 healthy young women and 40 women aged over 50 years old, including 19 osteoporotic patients according to WHO criteria. Five measurements were made for each patient after foot repositioning. Short-term precision was estimated using the coefficient of variation (CV), standardized CV (SCV) and intra-class correlation coefficient (ICC).

RESULTS

SCV varied with the group of subjects between 2.0 and 4.3% for BUA, and between 3.1 and 4.5% for SOS. Mean values of BUA and SOS were statistically lower for osteoporotic women compared with healthy young women or healthy young males (p<0.001). For BUA, only SCV and ICC were better for women aged over 50 years old but without statistical difference.

CONCLUSION

This study shows that the heterogeneity of the studied sample population is not a significant factor when assessing precision. This new device has a precision similar to others QUS devices.

Power calculation for the likelihood ratio-test when comparing two dependent intraclass correlation coefficients

Comparing the reproducibility level of two devices with continuous outcome on a unique sample of subjects (each subject being assessed several times with both devices) comes down to compare two dependent intraclass correlation coefficients (ICCs). When planning such a reproducibility study, one has to specify both the number of subjects to be included and the number of replicates per subject associated to each device. We propose SAS and S-plus macros, which allow power calculations by implementing a simulation study where dependent ICCs are compared by means of a likelihood ratio-test.

2D arrays device for calcaneus bone transmission: an alternative technological solution using crossed beam forming

In the context of manned space flight with the European Space Agency, a quantitative ultrasound device for transmission imaging through the calcaneus bone has been developed. It includes two matrix transducers of 576 elements each in order to electronically perform the scanning and the focusing of the 500 kHz ultrasonic beam. This device called the BEAM scanner, provides two parametric images of attenuation (BUA, broadband ultrasonic attenuation) and velocity (SOS, speed of sound) of the investigated skeletal site. The cost and complexity of such a device has motivated the study of an alternative solution, less demanding in terms of technology, based on a crossed beam former [H. Ermert et al., A new concept for a real-time ultrasound transmission camera, in: IEEE Ultrasonics Symposium Proceedings, 2000, pp. 1611-1614]. It consists in forming two perpendicular cylindrically focused planes, one in emission, one in reception, instead of two spherically focused apertures. The crossing line of the two planes replaces the focused beam. The 2D beam forming technological challenge is moved to a 1D simpler and cheaper architecture. In this work the two solutions have been compared for in vivo measurements. Data sets have been acquired using all spatial combinations of emission and reception single elements of the matrix. Then signals have been processed using either the cylindrical or the spherical focussing mode. For cylindrical focussing, the increased level of the side lobes caused severe artefacts. Several apodization techniques have been implemented to reduce these artefacts, resulting in encouraging results. After a brief description of this new ultrasonic method for bone quantitative assessment, several reconstructed images using both processing schemes are presented. Corresponding statistical results obtained in 29 subjects are also provided.