Elsevier

Ultrasonics

Volume 54, Issue 5, July 2014, Pages 1162-1169
Ultrasonics

Multi-frequency axial transmission bone ultrasonometer

https://doi.org/10.1016/j.ultras.2013.09.025Get rights and content

Highlights

  • We describe a novel technology of multi-frequency axial transmission of cortical bones.

  • Broadband parameters of guided and bulk waves are sensitive to cortical thickness and porosity.

  • Structure and function of a new multi-frequency bone ultrasonometer are described.

  • Bench testing showed ability to detect changes of thickness gradients and porosity in the cortex.

  • Waveform profiles along bones at different frequencies are specific to osteoporosis manifestations.

Abstract

The last decade has seen a surge in the development of axial transmission QUS (Quantitative UltraSound) technologies for the assessment of long bones using various modes of acoustic waves. The condition of cortical bones and the development of osteoporosis are determined by numerous mechanical, micro-structural, and geometrical or macro-structural bone properties like hardness, porosity and cortical thickness. Such complex manifestations of osteoporosis require the evaluation of multiple parameters with different sensitivities to the various properties of bone that are affected by the disease. This objective may be achieved by using a multi-frequency ultrasonic examination The ratio of the acoustic wavelength to the cortical thickness can be changed by varying the frequency of the ultrasonic pulse propagating through the long bone that results in the change in composition of the induced wave comprised of a set of numerous modes of guided, longitudinal, and surface acoustic waves. The multi-frequency axial transmission QUS method developed at Artann Laboratories (Trenton, NJ) is implemented in the Bone Ultrasonic Scanner (BUSS). In the current version of the BUSS, a train of ultrasonic pulses with 60, 100, 400, 800, and 1200 kHz frequencies is used. The developed technology was tested on a variety of bone phantoms simulating normal, osteopenic, and osteoporotic bones. The results of this study confirm the feasibility of the multi-frequency approach for the assessment of the processes leading to osteoporosis.

Introduction

The diagnosis of osteoporosis has been improved by the development of new quantitative methods of skeletal assessment; however, the advanced methods like quantitative computed tomography are technically challenging and not widely accepted [1]. Thus, the development of a portable and affordable technique for the assessment of bone quality in osteoporosis with improved ability for prediction of fracture risk, monitoring of treatments, and identifying the populations at risk is important. Low bone material density (BMD) as diagnosed by dual-energy X-ray absorption (DXA) is only one of the contributing factors to skeletal fragility [2]. Fracture risk is also increased by the reduction of bone mass, and by the micro-architectural deterioration of the bone tissue [3]. Bone ultrasonometry, or quantitative ultrasound (QUS), is a simple, safe, and cost-effective technique that is sensitive to the mechanical and structural features of the bone, and has potential to become an effective complement or alternative to DXA [4], [5]. Only heel QUS devices are currently acknowledged as decision making tools for osteoporosis [6]. These devices apply through-transmission ultrasound to the spongeous bone of the calcaneus and measure the ultrasound velocity or “speed-of-sound” (SOS) as well as the broadband ultrasound attenuation (BUA).

Axial transmission ultrasonometers, or axial transmission QUS devices, work with the compact tissue of the accessible long bones, such as the tibia and the radius [7]. The probes are placed along the body part over the examined bone and radiate ultrasound waves into the cortical layer through the surface layer of soft tissues. Commercial axial transmission ultrasonometers typically measure the ultrasound velocity at fixed frequencies close to 1 MHz [8], [9]. This velocity corresponds to the propagation of longitudinal ultrasonic waves. It varies from 3.5 to 4.2 km/s and is mainly sensitive to the variations in the elastic modulus of the compact bone. The longitudinal waves mostly propagate in the surface layer of the cortex, near the periosteum, and their propagation velocities are weakly sensitive to the progression of porosity away from the endosteum. The uncertainty of these measurements is increased by the variations of the mineralization degree and the intracortical porosity. This limitation has led to the criticism that axial transmission ultrasonometers have a low specificity to osteoporosis manifestations and a weak sensitivity to the disease’s incipience [10], [11].

Novel approaches to the evaluation of cortical bones with respect to osteoporosis have been explored in recent years. One of these approaches applies through-transmission ultrasound directly to the diagnostically important sites, the hip and the forearm, that are composed mainly of cortical bone. A scanner for the femoral neck has also been developed [12]. It depicts the bone inside the body and measures the SOS and the BUA in a way similar to the heel QUS. Another device has been proposed and constructed to evaluate the ultrasound propagation through the arm radius bone at a single location by through-transmission [13]. The device is based on an array of 64 transducers and measures the SOS in the bone by calculating the difference in the transmit times between ultrasonic pulses propagating through the bone and the soft tissues, and the ultrasonic pulses propagating through the soft tissues alone. A promising approach based on ultrasonic backscattering from the cortical sites has also been proposed [14]. The values of the integral backscatter in the radius and the tibia correlate with the BMD in the femoral neck.

Axial transmission QUS using surface transmission of ultrasonic waves has the advantages of easy unilateral access to the measurement sites and the compactness of the ultrasonic hardware. Dzenis et al. first performed a detailed investigation of the topographical patterns of ultrasound velocity in long bones at low frequencies (around 100 kHz) using point-contact transducers on a short acoustic base in the 1980s [15], [16]. They used an ultrasonic system originally developed for non-destructive testing of construction materials. The topographical variation of acoustic properties of long bones, particularly the velocity of slowly propagating flexural waves, was shown to be highly sensitive to the conditions of the bones [17], [18], [19].

The last decade has seen a major surge in the development of axial transmission QUS technologies based on the use of guided acoustic waves having the wavelengths close to or exceeding the cortical thickness [20], [21], [22], [23], [24], [25], [26], [27]. Numerical modeling techniques and physical models have been used to study propagation of the various modes of acoustic waves in bones under different boundary conditions. The relationships between the velocities of various acoustic wave propagation modes and the elastic properties of the bone as well as the cortical thickness have been investigated. The phase and group velocities of guided waves have a characteristic nonlinear dependence on the ratio of the bone thickness to the ultrasonic wavelength, a phenomenon known as geometrical dispersion [28]. The velocities of guided waves have been demonstrated to be considerably more sensitive to the manifestations of osteoporosis than the velocities of longitudinal waves [20], [29]. The high correlation of guided wave velocities to the cortical thickness (CTh) has been shown for the distal radius in vitro. The velocity was determined for the intensity maxima of the received signals using different methods of processing, such as singular value decomposition [30] and two-dimensional spectral analysis based on group velocity filtering and the fast Fourier transform [23].

The condition of cortical bones is determined by numerous mechanical, micro-structural, and geometrical or macro-structural properties like hardness, porosity, and cortical thickness. There are significant limitations to osteoporosis assessment when only one frequency is used to probe a single anatomical location. The complex manifestations of osteoporosis require the evaluation of multiple ultrasonic parameters with different sensitivities to the various properties of bone that are affected by the disease. Aged bones may demonstrate little or no decrease in mineralization [31] which is the main feature affecting the SOS, a key measurable of conventional QUS. Aged bones may also demonstrate hypermineralized areas [32], adding a confounding factor to increased porosity and thus reducing the efficiency of osteoporosis detection by conventional axial transmission QUS.

The main hypothesis of the presented study is that complex nature of the processes leading to osteoporosis requires a multi-frequency ultrasonic examination, which has a potential of differential sensitivities to the various bone properties affected by aging and disease. The ratio of the acoustic wavelength to the cortical thickness can be changed by varying the frequency of the ultrasonic pulse propagating through the long bone. As a result, the composition of the propagating wave packet comprised of various acoustic wave modes changes. This change is the basis for using a multi-parameter classifier in further clinical studies [33].

Section snippets

Design and function of the multi-frequency Bone UltraSonic Scanner (BUSS)

The multi-frequency axial transmission QUS method developed at Artann Laboratories is implemented in the Bone Ultrasonic Scanner (BUSS). The BUSS consists of the two major components: a hand-held ultrasonic probe and a portable electronics unit. The electronics unit includes a computer with an integrated ultrasonic data acquisition board. The device has a small footprint and may be placed on the physician’s table or a movable cart. A general view and block diagram of the BUSS are depicted in

The BUSS broadband probe

The BUSS probe (Fig. 2) provides an ergonomic handgrip that allows an operator to comfortably take the required multiple measurements along the subject’s bone. It acquires ultrasound signals that propagate through the tibia by surface transmission from the emitter to the receiver. The probe is positioned in the medial surface of the tibia, wherein the transducers’ base is oriented along the longitudinal axis of the bone. Typically, the contact pressure is within the range of 2–3 N. Its adequacy

Multi-frequency operation of the BUSS

The main innovation implemented in the BUSS is the use of the multi-frequency mode for bone assessment. It is necessary to make measurements at a larger set of frequencies to separate the contribution of the different modes of the acoustic waves in the received signals and to evaluate their propagation parameters.

Fig. 3 illustrates the approach implemented in the BUSS. Each measurement along the bone is conducted using a train of short ultrasonic pulses with different carrier frequencies. The

Bench testing of BUSS

The BUSS bench testing on models was intended to demonstrate the feasibility and effectiveness of the multi-frequency principle for the assessment of the cortical bones in osteoporosis. Bone resorption in osteoporosis starts from the endosteal surface, bordering the medullar cavity, and eventually leads to the thinning of the cortex and the trabecularization of the inner cortical layer. Demineralization and hypermineralization of the bone affect the mechanical properties and consequently the

Conclusion

The results of this study confirm the feasibility of the multi-frequency approach for the assessment of the processes leading to osteoporosis. The multi-frequency approach allows one to derive a plurality of ultrasonic parameters with differential sensitivity to the various bone properties affected by aging and disease from the ultrasound axial transmission data obtained in a wide range of frequencies from 60 to 1200 kHz.

Acknowledgements

The authors gratefully acknowledge Aleksandr Pasechnik for his contribution in the review and editing of the manuscript. The research reported in this publication was supported by the National Institute on Aging of the National Institutes of Health under Award Number R44AG017400. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

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