A phantom for quantitative ultrasound of trabecular bone

Improved Transcranial Plane-Wave Imaging With Learned Speed-of-Sound Maps

Although transcranial ultrasound plane-wave imaging (PWI) has promising clinical application prospects, studies have shown that variable speed-of-sound (SoS) would seriously damage the quality of ultrasound images. The mismatch between the conventional constant velocity assumption and the actual SoS distribution leads to the general blurring of ultrasound images. The optimization scheme for reconstructing transcranial ultrasound image is often solved using iterative methods like full-waveform inversion. These iterative methods are computationally expensive and based on prior magnetic resonance imaging (MRI) or computed tomography (CT) information. In contrast, the multi-stencils fast marching (MSFM) method can produce accurate time travel maps for the skull with heterogeneous acoustic speed. In this study, we first propose a convolutional neural network (CNN) to predict SoS maps of the skull from PWI channel data. Then, use these maps to correct the travel time to reduce transcranial aberration. To validate the performance of the proposed method, numerical, phantom and intact human skull studies were conducted using a linear array transducer (L11-5v, 128 elements, pitch = 0.3 mm). Numerical simulations demonstrate that for point targets, the lateral resolution of MSFM-restored images increased by 65%, and the center position shift decreased by 89%. For the cyst targets, the eccentricity of the fitting ellipse decreased by 75%, and the center position shift decreased by 58%. In the phantom study, the lateral resolution of MSFM-restored images was increased by 49%, and the position shift was reduced by 1.72 mm. This pipeline, termed AutoSoS, thus shows the potential to correct distortions in real-time transcranial ultrasound imaging, as demonstrated by experiments on the intact human skull.

Angle-dependent light scattering in tissue phantoms for the case of thin bone layers with predominant forward scattering

The cochlea forms a key element of the human auditory system in the temporal bone. Damage to the cochlea continues to produce significant impairment for sensory reception of environmental stimuli. To improve this impairment, the optical cochlear implant forms a new research approach. A prerequisite for this method is to understand how light propagation, as well as scattering, reflection, and absorption, takes place within the cochlea. We offer a method to study the light distribution in the human cochlea through phantom materials which have the objective to mimic the optical behavior of bone and Monte-Carlo simulations. The calculation of an angular distribution after scattering requires a phase function. Often approximate functions like Henyey-Greenstein, two-term Henyey-Greenstein or Legendre polynomial decompositions are used as phase function. An alternative is to exactly calculate a Mie distribution for each scattering event. This method provides a better fit to the data measured in this work.

Investigation of silk as a phantom material for ultrasound and photoacoustic imaging

Comprehensive characterization of biomedical imaging systems require phantoms that are easy to fabricate and can mimic human tissue. Additionally, with the arrival of engineered tissues, it is key to develop phantoms that can mimic bioengineered samples. In ultrasound and photoacoustic imaging, water-soluble phantom materials such as gelatin undergo rapid degradation while polymer-based materials such as polyvinyl alcohol are not conducive for generating bioengineered tissues that can incorporate cells. Here we propose silk protein-based hydrogels as an ultrasound and photoacoustic phantom material that has potential to provide a 3D environment for long-term sustainable cell growth. Common acoustic, optical, and biomechanical properties such as ultrasound attenuation, reduced scattering coefficient, and Young's modulus were measured. The results indicate that silk acoustically mimics many tissue types while exhibiting similar reduced optical scattering in the wavelength range of 400-1200 nm. Furthermore, silk-based materials can be stored long-term with no change in acoustic and optical properties, and hence can be utilized to assess the performance of ultrasound and photoacoustic systems.

Development of Tough Hydrogel Phantoms to Mimic Fibrous Tissue for Focused Ultrasound Therapies

Tissue-mimicking gels provide a cost-effective medium to optimize histotripsy treatment parameters with immediate feedback. Agarose and polyacrylamide gels are often used to evaluate treatment outcomes as they mimic the acoustic properties and stiffness of a variety of soft tissues, but they do not exhibit high toughness, a characteristic of fibrous connective tissue. To mimic pathologic fibrous tissue found in benign prostate hyperplasia (BPH) and other diseases that are potentially treatable with histotripsy, an optically transparent hydrogel with high toughness was developed that is a hybrid of polyacrylamide and alginate. The stiffness was established using shear wave elastography (SWE) and indentometry techniques and was found to be representative of human BPH ex vivo prostate tissue. Different phantom compositions and excised ex vivo BPH tissue samples were treated with a 700-kHz histotripsy transducer at different pulse repetition frequencies. Post-treatment, the hybrid gels and the tissue samples exhibited differential reduction in stiffness as measured by SWE. On B-mode ultrasound, partially treated areas were present as hyperechoic zones and fully liquified areas as hypoechoic zones. Phase contrast microscopy of the gel samples revealed liquefaction in regions consistent with the target lesion dimensions and correlated to findings identified in tissue samples via histology. The dose required to achieve liquefaction in the hybrid gel was similar to what has been observed in ex vivo tissue and greater than that of agarose of comparable or higher Young's modulus by a factor >10. These results indicate that the developed hydrogels closely mimic elasticities found in BPH prostate ex vivo tissue and have a similar response to histotripsy treatment, thus making them a useful cost-effective alternative for developing and evaluating different treatment protocols.

Measurement of Ultrasound Parameters of Bovine Cancellous Bone as a Function of Frequency for a Range of Porosities via Through-Transmission Ultrasonic Spectroscopy

The relationship between ultrasonic parameters (attenuation coefficients and velocity) and bone porosity in bovine cancellous bone is explored to understand the possibility of fracture risk diagnosis associated with osteoporosis by applying ultrasound. In vitro measurements of ultrasonic parameters on twenty-one bovine cancellous bone samples from tibia were conducted, using ultrasonic spectroscopy in the through-transmission mode. Transducers of three different center frequencies were used to cover a wide diagnostic frequency range between 1.0–7.8 MHz. The nonlinear relationship of porosity and normalized attenuation coefficient (nATTN) and normalized broadband attenuation coefficient (nBUA) were well described by a third-order polynomial fit, whereas porosity and the phase velocity (UV) were found to be negatively correlated with the linear correlation coefficients of −0.93, −0.89 and −0.83 at 2.25, 5.00 and 7.50 MHz, respectively. The results imply that the ultrasound parameters attain maximum values for the bone sample with the lowest porosity, and then decrease for samples with greater porosity for the range of porosities in our samples for all frequencies. Spatial variation in the ultrasound parameters was found to be caused by non-uniform pore size distribution, which was examined at five different locations within the same bone specimen. However, it did not affect the relationship of ultrasound parameters and porosity at these frequencies.

Criteria for the design of tissue-mimicking phantoms for the standardization of biophotonic instrumentation

A lack of accepted standards and standardized phantoms suitable for the technical validation of biophotonic instrumentation hinders the reliability and reproducibility of its experimental outputs. In this Perspective, we discuss general criteria for the design of tissue-mimicking biophotonic phantoms, and use these criteria and state-of-the-art developments to critically review the literature on phantom materials and on the fabrication of phantoms. By focusing on representative examples of standardization in diffuse optical imaging and spectroscopy, fluorescence-guided surgery and photoacoustic imaging, we identify unmet needs in the development of phantoms and a set of criteria (leveraging characterization, collaboration, communication and commitment) for the standardization of biophotonic instrumentation.

Recycled windshield glass as new material for producing ultrasonic phantoms of cortical bone-healing stages

The quantitative ultrasound technique was used to evaluate bone-mimicking phantoms; however, these phantoms do not mimic the intermediate stages of cortical bone healing. We propose using windshield glass as an original material to produce phantoms that mimic the characteristics of three different stages of cortical bone healing. This material was processed via a route that included breaking, grinding, compacting, drying, and sintering in four temperature groups: 625 °C, 645 °C, 657 °C, and 663 °C. The parameters evaluated were the ultrasonic longitudinal phase velocity (cL), corrected (αc) ultrasonic attenuation coefficient, and bulk density (ρs). The results showed that the mean values ofcL,αc,andρsvaried from 2, 398 to 4, 406 m·s^-1, 3 to 10 dB·cm^-1, and 1, 563 to 2, 089 kg·m^-3, respectively. The phantoms exhibited properties comparable with the three stages of cortical bone healing and can be employed in diagnostic and therapeutic studies using ultrasound.

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.

Mild hyperthermia by MR-guided focused ultrasound in an ex vivo model of osteolytic bone tumour: optimization of the spatio-temporal control of the delivered temperature

BACKGROUND

Magnetic resonance guided focused ultrasound was suggested for the induction of deep localized hyperthermia adjuvant to radiation- or chemotherapy. In this study we are aiming to validate an experimental model for the induction of uniform temperature elevation in osteolytic bone tumours, using the natural acoustic window provided by the cortical breakthrough.

MATERIALS AND METHODS

Experiments were conducted on ex vivo lamb shank by mimicking osteolytic bone tumours. The cortical breakthrough was exploited to induce hyperthermia inside the medullar cavity by delivering acoustic energy from a phased array HIFU transducer. MR thermometry data was acquired intra-operatory using the proton resonance frequency shift (PRFS) method. Active temperature control was achieved via a closed-loop predictive controller set at 6 °C above the baseline. Several beam geometries with respect to the cortical breakthrough were investigated. Numerical simulations were used to further explain the observed phenomena. Thermal safety of bone heating was assessed by cross-correlating MR thermometry data with the measurements from a fluoroptic temperature sensor inserted in the cortical bone.

RESULTS

Numerical simulations and MR thermometry confirmed the feasibility of spatio-temporal uniform hyperthermia (± 0.5 °C) inside the medullar cavity using a fixed focal point sonication. This result was obtained by the combination of several factors: an optimal positioning of the focal spot in the plane of the cortical breakthrough, the direct absorption of the HIFU beam at the focal spot, the "acoustic oven effect" yielded by the beam interaction with the bone, and a predictive temperature controller. The fluoroptical sensor data revealed no heating risks for the bone and adjacent tissues and were in good agreement with the PRFS thermometry from measurable voxels adjacent to the periosteum.

CONCLUSION

To our knowledge, this is the first study demonstrating the feasibility of MR-guided focused ultrasound hyperthermia inside the medullar cavity of bones affected by osteolytic tumours. Our results are considered a promising step for combining adjuvant mild hyperthermia to external beam radiation therapy for sustained pain relief in patients with symptomatic bone metastases.

Various 3D printed materials mimic bone ultrasonographically: 3D printed models of the equine cervical articular process joints as a simulator for ultrasound guided intra-articular injections

Introduction

In the equine racehorse industry, reduced athletic performance due to joint injury and lameness has been extensively reviewed. Intra-articular injections of glucocorticoids are routinely used to relieve pain and inflammation associated with osteoarthritis. Intra-articular injections of pharmaceutical agents require practice for precise needle placement and to minimize complications. Training on simulators or models is a viable alternative for developing these technical skills. The purpose of this study was to compare the qualitative ultrasonographic characteristics of three-dimensional (3D) printed models of equine cervical articular process joints to that of a dissected equine cervical spine (gold standard).

Methods

A randomized complete block design study was conducted in which a total of thirteen cervical articular process joint models were printed using several materials, printers, and printing technologies. Ultrasound video clips with the models immersed in water were recorded. Two board certified veterinary radiologists and three veterinary radiology residents reviewed the videos and responded to a survey assessing and comparing the ultrasonographic characteristics of the 3D printed models to those of the gold standard.

Results

Six 3D printed models had ultrasonographic characteristics similar to the gold standard. These six models were (material, printer, printing technology): nylon PA 12, EOS Formiga P100, selective laser sintering (P = 0.99); Onyx nylon with chopped carbon fiber, Markforged Onyx Two, fused deposition modeling (P = 0.48); polycarbonate, Ultimaker 3, fused deposition modeling (P = 0.28); gypsum, ProJet CJP 660 Pro, ColorJet Printing (P = 0.28); polylactic acid, Prusa I3, fused deposition modeling (P = 0.23); and high temperature V1 resin, Form 2, stereolithography (P = 0.22).

Conclusion

When assessed in water, it is possible to replicate the qualitative ultrasonographic characteristics of bone using three dimensional printed models made by combining different materials, printing technologies, and printers. However, not all models share similar qualitative ultrasonographic characteristics with bone. We suggest that the aforementioned six models be used as proxy for simulating bones or joints for use with ultrasound. In order to replicate the resistance and acoustic window provided by soft tissues, further work testing the ability of these models to withstand embedding in material such as ballistic gelatin is required.

Characterization of a polymer, open-cell rigid foam that simulates the ultrasonic properties of cancellous bone

Materials that simulate the ultrasonic properties of tissues are used widely for clinical and research purposes. However, relatively few materials are known to simulate the ultrasonic properties of cancellous bone. The goal of the present study was to investigate the suitability of using a polymer, open-cell rigid foam (OCRF) produced by Sawbones^®. Measurements were performed on OCRF specimens with four different densities. Ultrasonic speed of sound and normalized broadband ultrasonic attenuation were measured with a 0.5 MHz transducer. Three backscatter parameters were measured with a 5 MHz transducer: apparent integrated backscatter, frequency slope of apparent backscatter, and normalized mean of the backscatter difference. X-ray micro-computed tomography was used to measure the microstructural characteristics of the OCRF specimens. The trabecular thickness and relative bone volume of the OCRF specimens were similar to those of human cancellous bone, but the trabecular separation was greater. In most cases, the ultrasonic properties of the OCRF specimens were similar to values reported in the literature for cancellous bone, including dependence on density. In addition, the OCRF specimens exhibited an ultrasonic anisotropy similar to that reported for cancellous bone.

Sensitivity of simulated transcranial ultrasound fields to acoustic medium property maps

High intensity transcranial focused ultrasound is an FDA approved treatment for essential tremor, while low-intensity applications such as neurostimulation and opening the blood brain barrier are under active research. Simulations of transcranial ultrasound propagation are used both for focusing through the skull, and predicting intracranial fields. Maps of the skull acoustic properties are necessary for accurate simulations, and can be derived from medical images using a variety of methods. The skull maps range from segmented, homogeneous models, to fully heterogeneous models derived from medical image intensity. In the present work, the impact of uncertainties in the skull properties is examined using a model of transcranial propagation from a single element focused transducer. The impact of changes in bone layer geometry and the sound speed, density, and acoustic absorption values is quantified through a numerical sensitivity analysis. Sound speed is shown to be the most influential acoustic property, and must be defined with less than 4% error to obtain acceptable accuracy in simulated focus pressure, position, and volume. Changes in the skull thickness of as little as 0.1 mm can cause an error in peak intracranial pressure of greater than 5%, while smoothing with a 1 [Formula: see text] kernel to imitate the effect of obtaining skull maps from low resolution images causes an increase of over 50% in peak pressure. The numerical results are confirmed experimentally through comparison with sonications made through 3D printed and resin cast skull bone phantoms.

Stable phantom materials for ultrasound and optical imaging

Phantoms mimicking the specific properties of biological tissues are essential to fully characterize medical devices. Water-based materials are commonly used to manufacture phantoms for ultrasound and optical imaging techniques. However, these materials have disadvantages, such as easy degradation and low temporal stability. In this study, we propose an oil-based new tissue-mimicking material for ultrasound and optical imaging, with the advantage of presenting low temporal degradation. A styrene-ethylene/butylene-styrene (SEBS) copolymer in mineral oil samples was made varying the SEBS concentration between 5%-15%, and low-density polyethylene (LDPE) between 0%-9%. Acoustic properties, such as the speed of sound and the attenuation coefficient, were obtained using frequencies ranging from 1-10 MHz, and were consistent with that of soft tissues. These properties were controlled varying SEBS and LDPE concentration. To characterize the optical properties of the samples, the diffuse reflectance and transmittance were measured. Scattering and absorption coefficients ranging from 400 nm-1200 nm were calculated for each compound. SEBS gels are a translucent material presenting low optical absorption and scattering coefficients in the visible region of the spectrum, but the presence of LDPE increased the turbidity. Adding LDPE increased the absorption and scattering of the phantom materials. Ultrasound and photoacoustic images of a heterogeneous phantom made of LDPE/SEBS containing a spherical inclusion were obtained. Annatto dye was added to the inclusion to enhance the optical absorbance. The results suggest that copolymer gels are promising for ultrasound and optical imaging, making them also potentially useful for photoacoustic imaging.

Ultrasound temporal-spatial phase-interference in complex composite media; a comparison of experimental measurement and simulation prediction

The propagation of ultrasound through solid:liquid complex composite media such as cancellous bone suffers from a lack of a comprehensive understanding of the dependence upon density and structure. Assuming that a propagating ultrasound wave may be considered as an array of parallel sonic rays, we may determine the transit time of each by the relative proportion of the two constituents. A transit time spectrum (TTS) describes the proportion of sonic rays having a particular transit time between the minimum (tmin) and maximum (tmax) values; representing, for example, entire bone tissue and marrow respectively in the case of cancellous bone. Langton has proposed that the primary ultrasound attenuation mechanism in such media is phase-interference. The phase-interference of two or more ultrasound pulses detected at a phase-sensitive transducer has both temporal and spatial components. The temporal component is primarily dependent upon the transit time difference (dt) between the pulses and the propagating pulse-length (PL). The spatial component is primarily dependent upon the lateral separation (ds) of the detectedpulses of differing transit time and the lateral dimension of the ultrasound receive transducer aperture (dL). The aim of the paper was to explore these temporal and spatial dependencies through a comparison of experimental measurement and computer simulation in solid:liquid models of varying temporal and spatial complexity. Transmission measurements at nominal ultrasound frequencies of 1MHz and 5MHz were performed, thereby investigating the dependency upon period. The results demonstrated an overall agreement between experimental measurement and computer simulation of 87±16% and 85±12% for temporal and spatial components respectively. It is envisaged that a comprehensive understanding of ultrasound propagation through complex structures such as cancellous bone could provide an improved non-invasive tool for osteoporosis assessment.

Experimental observation of ultrasound fast and slow waves through three-dimensional printed trabecular bone phantoms

In this paper, ultrasound measurements of 1:1 scale three-dimensional (3D) printed trabecular bone phantoms are reported. The micro-structure of a trabecular horse bone sample was obtained via synchrotron x-ray microtomography, converted to a 3D binary data set, and successfully 3D-printed at scale 1:1. Ultrasound through-transmission experiments were also performed through a highly anisotropic version of this structure, obtained by elongating the digitized structure prior to 3D printing. As in real anisotropic trabecular bone, both the fast and slow waves were observed. This illustrates the potential of stereolithography and the relevance of such bone phantoms for the study of ultrasound propagation in bone.

Development of an anthropomorphic shoulder phantom model that simulates bony anatomy for sonographic measurement of the acromiohumeral distance

The purpose of this project was to create a sonographic phantom model of the shoulder that was accurate in bone configuration. Its main purpose was for operator training to measure the acromiohumeral distance. A computerized 3-dimensional model of the superior half of the humerus and scapula was rendered and 3-dimensionally printed. The bone model was embedded in a gelatin compound and set in a shoulder-shaped mold. The materials used had speeds of sound that were well matched to soft tissue and epiphyseal bone. The model was specifically effective in simulating the acromiohumeral distance because of its accurate bone geometry.

Phase velocity and attenuation predictions of waves in cancellous bone using an iterative effective medium approximation

The quantitative determination of wave dispersion and attenuation in bone is an open research area as the factors responsible for ultrasound absorption and scattering in composite biological tissues have not been completely explained. In this study, we use the iterative effective medium approximation (IEMA) proposed in [1] so as to calculate phase velocity and attenuation in media with properties similar to those of cancellous bones. Calculations are performed for a frequency range of 0.4-0.8 MHz and for different inclusions' volume concentrations and sizes. Our numerical results are compared with previous experimental findings so as to assess the effectiveness of IEMA. It was made clear that attenuation and phase velocity estimations could provide supplementary information for cancellous bone characterization.

Three-dimensional printing (3DP) of neonatal head phantom for ultrasound: thermocouple embedding and simulation of bone

A neonatal head phantom, comprising of an ellipsoidal geometry and including a circular aperture for simulating the fontanel was designed and fabricated, in order to allow an objective assessment of thermal rise in tissues during trans-cranial ultrasonic scanning of pre-term neonates. The precise position of a series of thermocouples was determined on the basis of finite-element analysis, which identified crucial target points for the thermal monitoring within the phantom geometry. Three-Dimensional Printing (3DP) was employed for the manufacture of the skull phantom, which was subsequently filled with dedicated brain-mimic material. A novel 3DP material combination was found to be able to mimic the acoustic properties of neonatal skull bone. Similarly, variations of a standard recipe for tissue mimic were examined, until one was found to mimic the brain of an infant. A specific strategy was successfully pursued to embed a thermocouple within the 3DP skull phantom during the manufacturing process. An in-process machine vision system was used to assess the correct position of the deposited thermocouple inside the fabricated skull phantom. An external silicone-made skin-like covering completed the phantom and was manufactured through a Direct Rapid Tooling (DRT) technique.

Measurements of ultrasonic phase velocities and attenuation of slow waves in cellular aluminum foams as cancellous bone-mimicking phantoms

The water-saturated aluminum foams with an open network of interconnected ligaments were investigated by ultrasonic transmission technique for the suitability as cancellous bone-mimicking phantoms. The phase velocities and attenuation of nine samples covering three pores per inch (5, 10, and 20 PPI) and three aluminum volume fractions (5, 8, and 12% AVF) were measured over a frequency range of 0.7-1.3 MHz. The ligament thickness and pore sizes of the phantoms and low-density human cancellous bones are similar. A strong slow wave and a weak fast wave are observed for all samples while the latter is not visible without significant amplification (100x). This study reports the characteristics of slow wave, whose speeds are less than the sound speed of the saturating water and decrease mildly with AVF and PPI with an average 1469 m/s. Seven out of nine samples show positive dispersion and the rest show minor negative dispersion. Attenuation increases with AVF, PPI, and frequency except for the 20 PPI samples, which exhibit non-increasing attenuation level with fluctuations due to scattering. The phase velocities agree with Biot's porous medium theory. The RMSE is 16.0 m/s (1%) at n = 1.5. Below and above this value, the RMSE decreases mildly and rises sharply, respectively.

The 25th Anniversary of BUA for the Assessment of Osteoporosis: Time for a New Paradigm?

The measurement of broadband ultrasonic attenuation (BUA) in cancellous bone at the calcaneus for the assessment of osteoporosis was first described within this journal 25 years ago. It was recognized in 2006 by Universities UK as being one of the ‘100 discoveries and developments in UK Universities that have changed the world’ over the past 50 years. In 2008, the UK's Department of Health also recognized BUA assessment of osteoporosis in a publication highlighting 11 projects that have contributed to ‘60 years of NHS research benefiting patients’. The BUA technique has been extensively clinically validated and is utilized worldwide, with at least seven commercial systems currently providing calcaneal BUA measurement. However, there is still no fundamental understanding of the dependence of BUA upon the material and structural properties of cancellous bone. This review aims to provide an ‘engineering in medicine’ perspective and proposes a new paradigm based upon phase cancellation due to variation in propagation transit time across the receive transducer face to explain the non-linear relationship between BUA and bone volume fraction in cancellous bone.

A review of tissue substitutes for ultrasound imaging

The characterization and calibration of ultrasound imaging systems requires tissue-mimicking phantoms with known acoustic properties, dimensions and internal features. Tissue phantoms are available commercially for a range of medical applications. However, commercial phantoms may not be suitable in ultrasound system design or for evaluation of novel imaging techniques. It is often desirable to have the ability to tailor acoustic properties and phantom configurations for specific applications. A multitude of tissue-mimicking materials and phantoms are described in the literature that have been created using a variety of materials and preparation techniques and that have modeled a range of biological systems. This paper reviews ultrasound tissue-mimicking materials and phantom fabrication techniques that have been developed over the past four decades, and describes the benefits and disadvantages of the processes. Both soft tissue and hard tissue substitutes are explored.

The dependencies of phase velocity and dispersion on volume fraction in cancellous-bone-mimicking phantoms

Frequency-dependent phase velocity was measured in eight cancellous-bone-mimicking phantoms consisting of suspensions of randomly oriented nylon filaments (simulating trabeculae) in a soft-tissue-mimicking medium (simulating marrow). Trabecular thicknesses ranged from 152 to 356 mum. Volume fractions of nylon filament material ranged from 0% to 10%. Phase velocity varied approximately linearly with frequency over the range from 300 to 700 kHz. The increase in phase velocity (compared with phase velocity in a phantom containing no filaments) at 500 kHz was approximately proportional to volume fraction occupied by nylon filaments. The derivative of phase velocity with respect to frequency was negative and exhibited nonlinear, monotonically decreasing dependence on volume fraction. The dependencies of phase velocity and its derivative on volume fraction in these phantoms were similar to those reported in previous studies on (1) human cancellous bone and (2) phantoms consisting of parallel nylon wires immersed in water.

The measurement of broadband ultrasonic attenuation in cancellous bone--a review of the science and technology

The measurement of broadband ultrasonic attenuation (BUA) in cancellous bone at the calcaneus was first described in 1984. The assessment of osteoporosis by BUA has recently been recognized by Universities UK, within its EurekaUK book, as being one of the "100 discoveries and developments in UK Universities that have changed the world" over the past 50 years, covering the whole academic spectrum from the arts and humanities to science and technology. Indeed, BUA technique has been clinically validated and is utilized worldwide, with at least seven commercial systems providing calcaneal BUA measurement. However, a fundamental understanding of the dependence of BUA upon the material and structural properties of cancellous bone is still lacking. This review aims to provide a science- and technology-orientated perspective on the application of BUA to the medical disease of osteoporosis.

Tissue mimicking materials for dental ultrasound

While acoustic tissue mimicking materials have been explored for a variety of soft and hard biological tissues, no dental hard tissue mimicking materials have been characterized. Tooth phantoms are necessary to better understand acoustic phenomenology within the tooth environment and to accelerate the advancement of dental ultrasound imaging systems. In this study, soda lime glass and dental composite were explored as surrogates for human enamel and dentin, respectively, in terms of compressional velocity, attenuation, and acoustic impedance. The results suggest that a tooth phantom consisting of glass and composite can effectively mimic the acoustic behavior of a natural human tooth.

Phase velocity and normalized broadband ultrasonic attenuation in Polyacetal cuboid bone-mimicking phantoms

The variations of phase velocity and normalized broadband ultrasonic attenuation (nBUA) with porosity were investigated in Polyacetal cuboid bone-mimicking phantoms with circular cylindrical pores running normal to the surface along the three orthogonal axes. The frequency-dependent phase velocity and attenuation coefficient in the phantoms with porosities from 0% to 65.9% were measured from 0.65 to 1.10 MHz. The results showed that the phase velocity at 880 kHz decreased linearly with porosity, whereas the nBUA increased linearly with porosity. This study provides a useful insight into the relationships between ultrasonic properties and porosity in bone at porosities lower than 70%.

The dependencies of phase velocity and dispersion on trabecular thickness and spacing in trabecular bone-mimicking phantoms

Frequency-dependent phase velocity was measured in trabecular-bone-mimicking phantoms consisting of two-dimensional arrays of parallel nylon wires (simulating trabeculae) with thicknesses ranging from 152 to 305 microm and spacings ranging from 700 to 1000 microm. Phase velocity varied approximately linearly with frequency over the range from 400 to 750 kHz. Dispersion was characterized by the slope of a linear least-squares regression fit to phase velocity versus frequency data. The increase in phase velocity (compared with that in water) at 500 kHz was approximately proportional to the (1) square of trabecular thickness, (2) inverse square of trabecular spacing, and (3) volume fraction occupied by nylon wires. The first derivative of phase velocity with respect to frequency was negative and exhibited nonlinear, monotonically decreasing dependencies on trabecular thickness and volume fraction. The dependencies of phase velocity and its first derivative on volume fraction in the phantoms were consistent with those reported in trabecular bone.

Use of multiple acoustic wave modes for assessment of long bones: model study

Multiple acoustic wave mode method has been proposed as a new modality in axial bone QUS. The new method is based on measurement of ultrasound velocity at different ratio of wavelength to the bone thickness, and taking into account both bulk and guided waves. It allows assessment of changes in both the material properties related to porosity and mineralization as well as the cortical thickness influenced by resorption from inner layers, which are equally important in diagnostics of osteoporosis and other bone osteopenia. Developed method was validated in model studies using a dual-frequency (100 and 500 kHz) ultrasound device. Three types of bone phantoms for long bones were developed and tested: (1) tubular specimens from polymer materials to model combined changes of material stiffness and cortical wall thickness; (2) layered specimens to model porosity in compact bone progressing from endosteum towards periosteum; (3) animal bone specimens with both cortical and trabecular components. Observed changes of the ultrasound velocity of guided waves at 100 kHz followed gradual changes in the thickness of the intact cortical layer. On the other hand, the bulk velocity at 500 kHz remained nearly constant at the different cortical layer thickness but was affected by the material stiffness. Similar trends were observed in phantoms and in fragments of animal bones.

Acoustic wave propagation in bovine cancellous bone: application of the Modified Biot-Attenborough model

Acoustic wave propagation in bovine cancellous bone is experimentally and theoretically investigated in the frequency range of 0.5-1 MHz. The phase velocity, attenuation coefficient, and broadband ultrasonic attenuation (BUA) of bovine cancellous bone are measured as functions of frequency and porosity. For theoretical estimation, the Modified Biot-Attenborough (MBA) model is employed with three new phenomenological parameters: the boundary condition, phase velocity, and impedance parameters. The MBA model is based on the idealization of cancellous bone as a nonrigid porous medium with circular cylindrical pores oriented normal to the surface. It is experimentally observed that the phase velocity is approximately nondispersive and the attenuation coefficient linearly increases with frequency. The MBA model predicts a slightly negative dispersion of phase velocity linearly with frequency and the nonlinear relationships of attenuation and BUA with porosity. The experimental results are in good agreement with the theoretical results estimated with the MBA model. It is expected that the MBA model can be usefully employed in the field of clinical bone assessment for the diagnosis of osteoporosis.

The cooling effect of liquid flow on the focussed ultrasound-induced heating in a simulated foetal brain

There is a need to investigate the thermal effects of diagnostic ultrasound (US) to assist the development of appropriate safety guidelines for obstetric use. The cooling effect of a single liquid flow channel was measured in a model of human foetal brain and skull bone heated by a focussed beam of simulated pulsed spectral Doppler US. Insonation conditions were 5.7 micros pulses, repeated at 8 kHz from a focussed transducer operating with a centre frequency of 3.5 MHz, producing a beam of -6 dB diameter of 3.1 mm at the focus and power outputs of up to 255 +/- 5 mW. Brain perfusion was simulated by allowing distilled water to flow at various rates in a 2 mm diameter wall-less channel in the brain soft tissue phantom material. This study established that the cooling effect of the flowing water; 1. was independent of the acoustic source power, 2. was more effective close to the flow channel, for example, there was a marked cooling at a distance of 1 mm and negligible cooling at a distance of 3 mm from the channel; and 3. initially increased at low flow rates, but further increase above normal perfusion had very little effect.

Ultrasound-induced heating in a foetal skull bone phantom and its dependence on beam width and perfusion

The cooling effect of single and multiple perfusing channels has been measured in a model of human foetal skull bone heated by wide and narrow beams of simulated pulsed spectral Doppler ultrasound (US). A focussed transducer operating with a centre frequency of 3.5 MHz, that emitted pulses of 5.7 micros duration with a repetition frequency of 8 kHz, was used. This produced a beam of power 100 +/- 2 mW with -6 dB diameters of 3.1 mm and 7.8 mm at 9 cm and 6 cm, respectively, from the transducer face. Arterial perfusion was simulated by allowing distilled water to flow in a large single channel or a grid of fine channels near the heated bone target. This study has established that: 1. perfusion-induced cooling is significantly enhanced when the bone phantom is heated by a wide rather than a narrow beam; 2. irrespective of the US beam width, a grid of small channels is more effective in cooling a heated bone target than a single larger diameter channel with the same volume flow rate; 3. the measured temperature rise and rate of temperature rise support the prediction of inverse proportionality to the US beam width; and 4. the perfusion time constants determined in our phantom model are 2 to 30 times larger than that assumed for the thermal index (TIB) algorithm.

Influence of bone tissue density and elasticity on ultrasound propagation: an in vitro study

Ultrasound (US) waves are mechanical vibrations that are applied to a material--bone tissue--in order to study its properties, that is, density, elasticity, and structure. In this study we evaluated in which way density and elasticity of the spongy bone influenced the transmission of 1.25 MHz US pulses. Twelve cylindrical specimens (diameter, 8 mm; height, 5 mm) excised from phalanxes of pig were decalcified with 0.5 M EDTA for different times (0, 2, and 5 days). During these periods, the samples underwent the following investigations: US transmission, density, and elasticity measurements. To assess the homogeneity of decalcification, the cross-sections of some samples were microradiographed. A detailed analysis of the US signal received was performed using velocity, Fourier analysis, and some parameters typical of signal processing technique. A good correlation was found between US velocity and density (r2 = 0.70); a lower correlation was found between velocity and elasticity (r2 = 0.59). If density and elasticity are considered simultaneously, the correlation with the US velocity improves significantly (r2 = 0.84). Fourier analysis enabled us to observe a shift of the main frequency toward lower values as the decalcification process advanced. We also observed that in the regressions weighted for density, US velocity correlated poorly with elasticity (r2 = 0.16), whereas signal processing parameters maintain a good correlation with elasticity (ultrasound peak amplitude [UPA], r2 = 0.48; slope, r2 = 0.62). In this study, it has been observed that when using a signal processing technique to analyze US pulses, it is possible to identify some parameters that are related in different ways to density and to elastic properties of bone. Our results show the potentiality of US technique to separate information on bone density and elasticity that X-ray-based densitometric methods do not provide.

Factors influencing the speed of sound through the proximal phalanges

The amplitude-dependent speed of sound (AD-SOS) in the proximal phalanges is reported to be sensitive to osteoporotic changes. We investigated the influence of bone thickness and cortical thickness on AD-SOS. Phantoms made of Perspex were designed to simulate different bone width (11-16 mm) and cortical thickness (3-7.5 mm). The phantoms were designed with two opposing flat and cylindrical surfaces. The effect of cortical thickness was examined by drilling holes (simulating the medullary canal) of different diameters (1-7 mm) in the middle of the Perspex cylinders. The effect of sample thickness was investigated on solid Perspex phantoms of varied lengths. The standardized precision errors of AD-SOS measurement in vivo and in vitro on volunteers and phantoms were 2.8 and 0.9%, respectively. AD-SOS was influenced by the bone width, cortical thickness, and location along the phalanx. A decrease in either cortical width or cortical thickness resulted in a decrease in AD-SOS. The effect is dependent on whether the contact surface is curved or flat. It is possible that a curved surface has a focusing effect on the wave through the porous core, whereas for a flat surface, the path of the waves might not pass through the center. When cortical thickness and bone width were expressed as a ratio, there was a linear relationship between this ratio and AD-SOS through the phantoms. AD-SOS was independent of thickness for samples greater than 11 mm.

A model for ultrasonic scattering in cancellous bone based on velocity fluctuations in a binary mixture

A scattering model based on velocity fluctuations in a binary mixture (marrow fat and cortical matrix) was used to estimate the ultrasonic attenuation in cancellous bone as a function of volume fraction. The calculation of velocity fluctuations alone seems to be suitable for the qualitative estimation of attenuation. The predicted values of the attenuation were of the same order of magnitude as experimentally determined values from the literature. This agreement was achieved with only a small number of variables (the velocities of the two components and the scatterer size) in the model, representing a major advantage compared with other theories. Hence the suggested approach appears to be a good starting point for further theoretical investigations using scattering theories. However, this has to be accompanied by accurate ultrasonic and microstructural measurements.

An evaluation of the reproducibility and responsiveness of four 'state-of-the-art' ultrasonic heel bone measurement systems using phantoms

This paper evaluates four modern ultrasonic heel bone scanners: the Osteometer DTU-one, Hologic Sahara, CUBA Clinical and Lunar Achilles+. Six phantoms, ranging in porosity from 50% to 83%, were used to evaluate the range of values of broadband ultrasonic attenuation (BUA), speed of sound (SOS) and Stiffness/quantitative ultrasound index (where available) from each machine. Differences in inter-system variability between the lowest and highest porosity phantoms up to a factor of 3.8 were demonstrated. The reproducibility of each system was measured using a single phantom and gave values of 0.03-0.15% for SOS, 0.39-1.6% for BUA and 0.73-1.11% for Stiffness. This contrasted with values of range normalized standard deviation (CX) of 0.2-1.19% for SOS, 0.71-1.86% for BUA and 0.83-1.12% for Stiffness when the output range of the measurement is taken into account. Measures of all quantities differed between machines and care should be taken when expressing and comparing results from different systems.

Development of a cancellous bone structural model by stereolithography for ultrasound characterisation of the calcaneus

A novel method for the development of a user-defined structural model simulating cancellous bone of the human calcaneus is described using stereolithography (SL). The digital image of a cancellous bone section was modified by skeletonisation and dilation to produce a structural model of uniform wall thickness, determined by the resolution of the stereolithography system. Six SL models were produced using the same data file. The SL models were assessed using the McCue CUBAclinical ultrasound bone densitometer. The broadband ultrasound attenuation (BUA) and velocity (VOS) values obtained were commensurate with the commercial phantom provided with the CUBAclinical system. The intra- and inter-sample variability for the six SL models were similar at 5% for BUA and 2.5% for VOS. Stereolithography offers the potential to firstly, simulate perforation and thinning of cancellous bone associated with osteoporosis, and secondly, to evaluate the dependence of ultrasonic and mechanical parameters upon cancellous bone structure.

Reference data for ultrasonic bone measurement: variation with age in 2087 Caucasian women aged 16-93 years

Data from the measurement of broadband ultrasonic attenuation (BUA) and speed of sound (SOS), using the Lunar Achilles ultrasonic densitometer, were collected for Caucasian women from five centres in the United Kingdom (Leeds, London, Nottingham, Lincoln and Sheffield). After correcting for machine variability at each site, the data were combined into a central reference database comprising 2087 women aged 16-93 years. The data are presented in 5-year bands and show a mean fall of 0.36% per year for BUA and 0.08% per year for SOS in the 60 years following the attainment of peak bone mass. This fall in BUA compares with that observed using dual energy X-ray absorptiometry studies of the lumbar spine and femoral neck of 0.32% per year and 0.44% per year, respectively, for the age range 25-65 years.

The influence of porosity and pore size on the ultrasonic properties of bone investigated using a phantom material

Ultrasonic propagation in bone has been investigated using the Leeds Ultrasonic Bone Phantom Material. Phantoms were produced with different porosities in the range of 45-83% and pore sizes of 1.3 and 0.6 mm. The phase velocity at 600 kHz was found to follow a second-order polynomial as a function of porosity. Phase velocity values between 1545 and 2211 m s-1 were measured and found to be largely independent of pore size for a given porosity. The slope of the phase velocity as a function of frequency (dispersion) decreases with increasing porosity. The values obtained from samples having different pore sizes were also similar. The attenuation coefficient and normalized broadband ultrasonic attenuation (nBUA) reached a maximum at about 50%. The normalized attenuation ranged from 6 to 25 dB cm-1 over the porosity range available and consistently showed higher values for the larger pore size. Similarly, the nBUA values were found to be between 14 and 53 dB MHz-1 cm-1, with the values for the larger pore size being roughly 10 dB MHz-1 cm-1 greater than those for the smaller pore size. These findings demonstrate that the Leeds phantom can be used to investigate the effect of structural changes in bone and to aid the understanding of quantitative ultrasound. The results support the assumption that the velocity in trabecular bone is not dependent on pore size but is influenced by the mechanical properties of the bone's constituents and the overall framework, whereas the attenuation and BUA are also influenced by structure.

The non-linear relationship between BUA and porosity in cancellous bone

There is growing interest in assessing the clinical value of ultrasound in the prediction and management of osteoporosis. However, the mechanism of ultrasound propagation in cancellous bone is not well understood. The Biot theory is one approach to modelling the interaction of sound waves with cancellous structure, and porosity is one of its input parameters. In this paper we report the relationship between broadband ultrasonic attenuation (BUA) corrected for specimen thickness (nBUA) and porosity in a porous Perspex cancellous bone mimic, a stereolithography cancellous bone mimic and in natural human and bovine tissue. nBUA and porosity have a non-linear parabolic relationship. The maximum nBUA value (nBUAmax) occurs at approximately 30% porosity in the Perspex mimic, approximately 70% in the stereolithography mimic and approximately 75% in natural cancellous bone. We discuss the effect of structure on the form of the nBUA-porosity relationship.

The nonlinear transition period of broadband ultrasound attenuation as bone density varies

The purpose of this study was to determine whether a transition period occurs between cortical and cancellous bone in the relationship between ultrasound parameters [broadband ultrasound attenuation (BUA) and ultrasonic velocity] and density. Twenty-two cancellous bone discs wee obtained from proximal bovine tibiae. Also included were three samples of human vertebral cancellous bone from an elderly female and four samples of bovine cortical bone. Ultrasonic velocity did not show any transition period as density varied from cancellous to cortical bone. Ultrasonic velocity exhibited a definite linear dependence on density over the entire range examined. However, BUA has shown a transition period as density varied. Although BUA increased linearly with density for a low density cancellous bone tested (below 0.64 g cm-3), the dependence of BUA on density is nonlinear with a downwardly inflected parabola shape when covering a wide density range (0.130-0.913 g cm-3) of cancellous bone. When one includes cortical bone, the parabola tends to level off in a slow exponential decay. This nonlinear dependence may help to understand the characteristics of BUA measurement.

The measurement of the velocity of ultrasound in fixed trabecular bone using broadband pulses and single-frequency tone bursts

The velocity of ultrasound in a series of 10 fixed os calces was measured using both short pulses and 750 kHz tonebursts. The values obtained from the pulse measurements differed from the toneburst values by up to 16% depending on the selection of the zero-crossing point used as a reference in the pulse measurements. It is demonstrated that the discrepancy between the values is itself a function of the frequency-dependent attenuation in the propagating medium and this is confirmed by theoretical modelling. The toneburst results are also compared with measurements using a cross-correlation technique, and these show a close agreement.

Ultrasonic measurement: an evaluation of three heel bone scanners compared with a bench-top system

Three commercial ultrasound bone scanners designed for os calcis measurements (Lunar Achilles, C.U.B.A. "Research" and UBA 575) were compared using the Leeds Ultrasonic Bone Phantoms. The porosity of the phantoms ranged from 50% to 83% with velocities between 1490 and 1621 m s-1 and broadband ultrasound attenuation (BUA) values in the range 46-115 dB MHz-1. The three devices tested were able to discriminate porosity differences of at least 3%, although the values obtained for the propagation parameters varied widely. Velocity differences of up to 38 m s-1 and BUA variations of up to 33 dB MHz-1 were found, although a relationship was identified between the velocity and BUA measurements. In some cases, the variation can be attributed to differences in the measurement technique adopted, although there also seem to be detailed differences in the definition of the parameters themselves. The variation between different devices from the same manufacturer (Lunar) was also studied. Measurements taken from five devices showed variation in velocity values of up to 25 m s-1 (SD 10.8 m s-1) and in BUA values of up to 11 dB MHz-1 (SD 4.3 dB MHz-1). We conclude that the variation both between manufacturers and between nominally identical machines may be of clinical significance. Both users and manufacturers need to consider urgently the introduction of quality standards and consensus definition of terms and techniques. The fact that all machines studied have been superseded commercially does not invalidate these conclusions, since many of the devices tested remain in clinical use and there is no evidence of fundamental change in manufacturers' procedures.