Broadband Acoustic Measurement of an Agar-Based Tissue-Mimicking-Material: A Longitudinal Study

Commercially available ultrasound quality assurance test phantoms rely on the long-term acoustic stability of the tissue-mimicking-material (TMM). Measurement of the acoustic properties of the TMM can be technically challenging, and it is important to ensure its stability. The standard technique is to film-wrap samples of TMM and to measure the acoustic properties in a water bath. In this study, a modified technique was proposed whereby the samples of TMM are measured in a preserving fluid that is intended to maintain their characteristics. The acoustic properties were evaluated using a broadband pulse-echo substitution technique over the frequency range 4.5–50 MHz at 0, 6 and 12 months using both techniques. For both techniques, the measured mean values for the speed of sound and attenuation were very similar and within the International Electrotechnical Commission-recommended value. However, the results obtained using the proposed modified technique exhibited greater stability over the 1-y period compared with the results acquired using the standard technique.     

Reference Characterisation of Sound Speed and Attenuation of the IEC Agar-Based Tissue-Mimicking Material Up to a Frequency of 60 MHz

To support the development of clinical applications of high-frequency ultrasound, appropriate tissue-mimicking materials (TMMs) are required whose acoustic properties have been measured using validated techniques. This paper describes the characterisation of the sound speed (phase velocity) and attenuation coefficient of the International Electrotechnical Commission (IEC) agar-based TMM over the frequency range 1 to 60 MHz. Measurements implemented a broadband through-transmission substitution immersion technique over two overlapping frequency ranges, with co-axially aligned 50 MHz centre-frequency transducers employed for characterisation above 15 MHz. In keeping with usual practice employed within the technical literature, thin acoustic windows (membranes) made of 12-μm-thick Mylar protected the TMM from water damage. Various important sources of uncertainty that could compromise measurement accuracy have been identified and evaluated through a combination of experimental studies and modelling. These include TMM sample thickness, measured both manually and acoustically, and the influence of interfacial losses that, even for thin protective membranes, are significant at the frequencies of interest. In agreement with previous reports, the attenuation coefficient of the IEC TMM exhibited non-linear frequency dependence, particularly above 20 MHz, yielding a value of 0.93 ± 0.04 dB cm⁻¹ MHz⁻¹ at 60 MHz, derived at 21 ± 0.5°C. For the first time, phase velocity, measured with an estimated uncertainty of ±3.1 m s⁻¹, has been found to be dispersive over this extended frequency range, increasing from 1541 m s⁻¹ at 1 MHz to 1547 m s⁻¹ at 60 MHz. This work will help standardise acoustic property measurements, and establishes a reference measurement capability for TMMs underpinning clinical applications at elevated frequencies.     

The acoustic properties, centered on 20 MHZ, of an IEC agar-based tissue-mimicking material and its temperature, frequency and age dependence

The purpose of this study was to characterize the ultrasonic properties of agar-based tissue-mimicking materials (TMMs) at ultrasound frequencies centered around 20 MHz. The TMM acoustic properties measured are the amplitude attenuation coefficient alpha (dB cm(-1)MHz(-1)), the speed of sound (ms(-1)) and the backscattered power spectral density (distribution of power per unit frequency normalized to the total received power) characteristics of spectral slope (dB MHz(-1)), y-axis intercept (dB) and reflected power (dB). The acoustic properties are measured over a temperature range of 22 to 37 degrees C. An intercomparison of results between two independent ultrasound measurement laboratories is also presented. A longitudinal study of the acoustic properties over a period of two years is also detailed, and the effect of water immersion on the acoustic properties of TMM is measured. In addition, the physical parameters of mass density rho (kg m(-3)) and specific heat capacity C (J kg(-1) K(-1)) are included. The measurement techniques used were based on the substitution technique using both broadband and narrowband pulses centered on 20 MHz. Both the attenuation coefficient and speed of sound (both group and phase) showed good agreement with the expected values of 0.5 dB cm(-1) MHz(-1) and 1540 ms(-1), respectively, with average values over the three-year period of 0.49 dBcm(-1)MHz1 (SD +/- 0.05) and 1540.9 ms(-1) (SD +/- 8.7). These results also showed agreement between the two independent measurement laboratories. Speed of sound and attenuation coefficient were shown to change with temperature with rates of + 2.1 m s(-1) degrees C(-1) and -0.005 dB cm(-1) MHz(-1) degrees C(-1), respectively. Attenuation changed linearly with frequency at the high frequency range of 17 to 23 MHz, and speed of sound was found to be independent of frequency in this range. The spectral slope of relative backscattered power for the material increased with frequency at typically 1.5 dB MHz(-1). This compared favorably with theoretical spectral slope values, calculated for a variety of scatterer sizes, albeit at a lower frequency range. It is also noticed that, on extrapolation back to lower frequencies, the backscatter is comparable with that measured at 7 MHz. Overall, this non-commercial agar-based TMM is shown to perform as expected at the higher frequency range of 17 to 23 MHz and is seen to retain its acoustic properties of attenuation and speed of sound over a three-year period.     

Acoustic Properties of Small Animal Soft Tissue in the Frequency Range 12–32 MHz

Quality assurance phantoms are made of tissue-mimicking materials (TMMs) the acoustic properties of which mimic those of soft tissue. However, the acoustic properties of many soft tissue types have not been measured at ultrasonic frequencies >9?MHz. With the increasing use of high-frequency ultrasound for both clinical and pre-clinical applications, it is of increasing interest to ensure that TMMs accurately reflect the acoustic properties of soft tissue at these higher frequencies. In this study, the acoustic properties of ex vivo brain, liver and kidney samples from 50 mice were assessed in the frequency range 12–32?MHz. Measurements were performed within 6?min of euthanasia in a phosphate-buffered saline solution maintained at 37.2?±?0.2?°C. The measured mean values for the speed of sound for all organs were found to be higher than the International Electrotechnical Commission guideline recommended value for TMMs. The attenuation coefficients measured for brain, liver and kidney samples were compared with the results of previous studies at lower frequencies. Only the measured kidney attenuation coefficient was found to be in good agreement with the International Electrotechnical Commission guideline. The information provided in this study can be used as a baseline on which to manufacture a TMM suitable for high-frequency applications.     

Development of a Tissue-Mimicking Phantom of the Brain for Ultrasonic Studies

Constructing tissue-mimicking phantoms of the brain for ultrasonic studies is complicated by the low backscatter coefficient of brain tissue, causing difficulties in simultaneously matching the backscatter and attenuation properties. In this work, we report on the development of a polyvinyl alcohol-based tissue-mimicking phantom with properties approaching those of human brain tissue. Polyvinyl alcohol was selected as the base material for the phantom as its properties can be varied by freeze–thaw cycling, variations in concentration and the addition of scattering inclusions, allowing some independent control of backscatter and attenuation. The ultrasonic properties (including speed of sound, attenuation and backscatter) were optimized using these methods with talc powder as an additive. It was determined that the ultrasonic properties of the phantom produced in this study are best matched to brain tissue in the frequency range 1–3 MHz, indicating its utility for laboratory ultrasonic studies in this frequency range.     

Novel tissue mimicking materials for high frequency breast ultrasound phantoms

The development and acoustical characterisation of a range of novel agar-based tissue mimicking material (TMMs) for use in clinically relevant, quality assurance (QA) and anthropomorphic breast phantoms are presented. The novel agar-based TMMs described in this study are based on a comprehensive, systematic variation of the ingredients in the International Electrotechnical Commission (IEC) TMM. A novel, solid fat-mimicking material was also developed and acoustically characterised. Acoustical characterisation was carried out using an in-house scanning acoustic macroscope at low (7.5 MHz) and high frequencies (20 MHz), using the pulse-echo insertion technique. The speeds of sound range from 1490 to 1570 m. s⁻¹, attenuation coefficients range from 0.1 to 0.9 dB. cm^?1. MHz⁻¹ and relative backscatter ranges from 0 to -20 dB. It was determined that tissues can be mimicked in terms of independently controllable speeds of sound and attenuation coefficients. These properties make these novel TMMs suitable for use in clinically relevant QA and anthropomorphic phantoms and would potentially be useful for other high frequency applications such as intravascular and small animal imaging. (E-mail: Louise.cannon@dit.ie)     

An anthropomorphic tissue-mimicking phantom of the oesophagus for endoscopic ultrasound

This study details the design and construct of an anthropomorphic phantom of the oesophagus suitable for use with endoscopic ultrasound (EUS) and 3-D volume measurements. The phantom was constructed using agar-based tissue-mimicking material (TMM) of different acoustical properties to simulate various anatomical and pathologic features. The acoustical properties were measured with a scanning acoustical macroscope. An Olympus GF-UM200 echo-endoscope and digital position measurement arm were used to scan the phantom at 7.5 and 12 MHz. Comparative dimensional measurements were performed on the phantom via 2-D and 3-D EUS. TMM attenuation varied between 0.1 and 0.5 dB/cm.MHz. Backscatter power, relative to normal TMM, was from 0 to -12.2 dB, with an average speed of sound of 1537 +/- 1.9 m/s. Measurements of objects within the phantom by 2-D and 3-D EUS had mean errors of 8% and 2.2%, respectively. The construction of the anthropomorphic EUS phantom facilitated EUS training and research and development studies.     

Spectral characterization and attenuation measurements in ultrasound

Various means of characterizing ultrasonic attenuation in tissue are reviewed. A simple method for estimating frequency-dependent attenuation via measurement of the zero crossing density of the signal is presented and validated. Both the effects of the frequency dependence of scatter and stochastic variability of the measurement are considered and discussed. Results of measurements made in phantoms, animals and humans are presented and compared to the theoretical model. The technique is shown to be technically feasible.     

Apparatus for ultrasound tissue characterization of excised specimens

Apparatus is described for measurement of sound speed and ultrasound attenuation coefficients by the substitution technique in the frequency range 3 to 8 MHz. Phase-cancellation artifacts leading to overestimation of attenuation coefficients are avoided by use of an acoustoelectric transducer. Specimens confined by polystyrene windows can be interrogated by focused ultrasound beams at selected locations spaced on a grid of 3 x 3 mm voxels. Pulse time of flight is measured with an accuracy of 30 ns, yielding sound speeds accurate to +/- 6.7 m/s, for samples 10 mm thick. Uncertainties in measured insertion losses range from 0.1 dB in low-loss (10 dB) specimens to 0.5 dB in high-loss (25 dB) specimens.     

Reference Phantom Method for Ultrasonic Imaging of Thin Dynamic Constructs

Quantitative ultrasound has a great potential for the non-destructive evaluation of tissue engineered constructs, where the local attenuation and the integrated backscatter coefficient (IBC) can be used for monitoring the development of biological processes. The local determination of both parameters can be achieved using the reference phantom method (RPM). However, its accuracy can be affected when evaluating constructs of evolving sound speed, attenuation and thickness, for example, when evaluating biodegradable hydrogels developing neocartilage. To assess the feasibility of using the RPM under such dynamic conditions while employing a 50-MHz transducer, we conducted a series of experiments on 3-mm-thick acellular hydrogels laden with microspheres. The ultrasonic evaluation procedure used was validated by detecting and compensating for large attenuation variations occurring in the construct, up to 20-fold with respect to the reference phantom, with estimations errors below 1%. We found that sound speed mismatch does not affect the local attenuation estimation, but causes a strong diffraction effect by reducing the backscatter intensity. Such intensity reduction was compensated by determining the IBC percentage change (IBC_Δ) as function of sound speed mismatch with respect to the reference phantom (ΔSS), with the equation IBC_Δ = (0.63 ± 0.07) ΔSS + (8.54 ± 0.76) 10^–3 ΔSS². The investigated ΔSS interval was up to 120 m/s and using two different concentrations of microspheres, with estimation errors below 7% relative to the construct's actual IBC. Finally, we found that the spectral difference method is sufficient to measure within a few millimetres in depth mismatch, and when combining with sound speed mismatch, we found negligible additional effects. These results pave the way for the use of a generic reference phantom for the evaluation of thin dynamic constructs, thus simplifying the need for using different phantoms depending on the construct's properties.     

Cardiac Tissue-Mimicking Ballistic Gel Phantom for Ultrasound Imaging in Clinical and Research Applications

Ballistic gel was investigated as a tissue-mimicking material in an anthropomorphic cardiac phantom for ultrasound imaging. The gel was tested for its acoustic properties and its compatibility with conventional plastics molding techniques. Speed of sound and attenuation were evaluated in the range 2–12 MHz. The speed of sound was 1537 ± 39 m/s, close to typical values for cardiac tissue (∼1576 m/s). The attenuation coefficient was 1.07 dB/cm·MHz, within the range of values previously reported for cardiac tissue (0.81–1.81 dB/cm·MHz). A cardiac model based on human anatomy was developed using established image segmentation processes and conventional plastic molding techniques. Key anatomic features were observed, captured and identified in the model using an intracardiac ultrasound imaging system. These favorable results along with the material's durability and processes that allow for repetitive production of detailed whole-heart models at low cost are promising. There are numerous applications for geometrically complex phantoms in research, training, device development and clinical use.     

Characterizing acoustic attenuation of homogeneous media using focused impulsive acoustic radiation force

A new method to characterize a material's attenuation using acoustic radiation force is proposed. Comparison of displacement magnitudes generated in a homogeneous material by acoustic radiation force excitations can be used to estimate the material's attenuation when the excitations are applied over a range of focal depths while maintaining a constant lateral focal configuration. Acoustic attenuations are related to the inverse of the excitation focal depth that yields the greatest focal zone displacement for this protocol. Experimental studies in calibrated tissue-mimicking phantoms are presented to demonstrate the feasibility of this method. Attenuations ranging from 0.3-1.5 dB/cm/MHz were characterized over excitation focal depths ranging from 5-30 mm, with an accuracy of 0.1 +/- 0.15 dB/cm/MHz. As currently implemented, this method is limited to characterizing materials that have homogeneous material properties and acoustic attenuations. This method for characterizing acoustic attenuation can be performed using conventional diagnostic scanners without any additional hardware and could also be performed concurrently with acoustic radiation force-based imaging modalities to generate images of mechanical properties and attenuation that are spatially co-registered with B-mode images.     

Evaluation of lens hardness in cataract surgery using high-frequency ultrasonic parameters in vitro

Ultrasonic parameters of sound velocity and frequency-dependent attenuation ranging from 25 to 45 MHz were measured for the purpose of evaluating the hardness of lenses in cataract surgery (phacoemulsification). Measurements were performed with a 35-MHz ultrasonic transducer on porcine lenses in which artificially cataracts were induced. The hardness of the cataractous lens was also evaluated by mechanical measurement of its elastic properties. The results indicated that the ultrasonic attenuation coefficients in normal porcine lenses were approximately 4.49 +/- 0.05 (mean +/- SD) and 6.32 +/- 0.04 dB/mm at 30 and 40 MHz, respectively. The development progression of the cataracts resulted in the attenuation coefficient increasing linearly to 7.36 +/- 0.25 and 11.1 +/- 0.92 dB/mm, respectively, corresponding to an increase of Young's modulus from 2.6 to 101.2 kPa. The sound velocity concomitantly increased from 1639.8 +/- 4.2 to 1735.6 +/- 10.4 m/s. Evaluation of the relationship between the phacoemulsification energy level and ultrasonic parameters in vitro by surgeons revealed that both the attenuation coefficient and sound velocity were linearly correlated with the phacoemulsification energy (r = 0.941 and 0.915, respectively). These results showed that measuring high-frequency ultrasonic parameters provides surgeons with good capability and reproducibility for selecting the optimal energy level for phacoemulsification.     

Acoustic and Elastic Properties of Glycerol in Oil-Based Gel Phantoms

Phantoms are important tools for image quality control and medical training. Many phantom materials have been proposed for ultrasound; most of them use water as the solvent, but these materials have disadvantages such as dehydration and low temporal stability if not properly stored. To overcome these difficulties, copolymer-in-oil gel was proposed as an inert and stable material; however, speed of sound for these materials is still lower than what is described for most biological tissues. Here, we propose the glycerol dispersion in oil-based gels to modify the acoustic and elastic properties of copolymer-in-oil phantoms. We manufactured copolymer-in-oil gels using styrene-ethylene/butylene-styrene (SEBS) in concentrations 8%–15%. We used 2 types of mineral oils with different viscosities. Glycerol was added in a volume fraction 0%–30% of the total amount of liquid. The acoustic (i.e., speed of sound, attenuation and backscattering) and the mechanical (i.e., density and Young's modulus) properties of the samples were within the range of values observed for soft tissues. The acoustic parameters of the samples were dependent on oil viscosity and glycerol concentration. The speed of sound ranged 1423 m/s – 1502 m/s, while the acoustic attenuation and the ultrasonic backscattering increased by adding glycerol. The density and the Young's moduli were less affected by the presence of glycerol. We conclude that glycerol can be used to control the acoustic parameters of copolymer-in-oil gels. Additionally, it opens the possibility of incorporating other oil-insoluble substances to control further properties of the phantom.     

Acoustic Assessment of a Konjac–Carrageenan Tissue-Mimicking Material at 5–60 MHz

The acoustic properties of a robust tissue-mimicking material based on konjac–carrageenan at ultrasound frequencies in the range 5–60 MHz are described. Acoustic properties were characterized using two methods: a broadband reflection substitution technique using a commercially available preclinical ultrasound scanner (Vevo 770, FUJIFILM VisualSonics, Toronto, ON, Canada), and a dedicated high-frequency ultrasound facility developed at the National Physical Laboratory (NPL, Teddington, UK), which employed a broadband through-transmission substitution technique. The mean speed of sound across the measured frequencies was found to be 1551.7 ± 12.7 and 1547.7 ± 3.3 m s^?1, respectively. The attenuation exhibited a non-linear dependence on frequency, f (MHz), in the form of a polynomial function: 0.009787f² + 0.2671f and 0.01024f² + 0.3639f, respectively. The characterization of this tissue-mimicking material will provide reference data for designing phantoms for preclinical systems, which may, in certain applications such as flow phantoms, require a physically more robust tissue-mimicking material than is currently available.     

Acoustic characterization of tissue-mimicking materials for ultrasound perfusion imaging research

Materials with well-characterized acoustic properties are of great interest for the development of tissue-mimicking phantoms with designed (micro)vasculature networks. These represent a useful means for controlled in-vitro experiments to validate perfusion imaging methods such as Doppler and contrast-enhanced ultrasound (CEUS) imaging. In this work, acoustic properties of seven tissue-mimicking phantom materials at different concentrations of their compounds and five phantom case materials are characterized and compared at room temperature. The goal of this research is to determine the most suitable phantom and case material for ultrasound perfusion imaging experiments. The measurements show a wide range in speed of sound varying from 1057 to 1616 m/s, acoustic impedance varying from 1.09 to 1.71 × 10⁶ kg/m²s, and attenuation coefficients varying from 0.1 to 22.18 dB/cm at frequencies varying from 1 MHz to 6 MHz for different phantom materials. The nonlinearity parameter B/A varies from 6.1 to 12.3 for most phantom materials. This work also reports the speed of sound, acoustic impedance and attenuation coefficient for case materials. According to our results, polyacrylamide (PAA) and polymethylpentene (TPX) are the optimal materials for phantoms and their cases, respectively. To demonstrate the performance of the optimal materials, we performed power Doppler ultrasound imaging of a perfusable phantom, and CEUS imaging of that phantom and a perfusion system. The obtained results can assist researchers in the selection of the most suited materials for in-vitro studies with ultrasound imaging.     

Tissue-mimicking materials for teaching sonographers and evaluation of their specifications after three years

In this paper, an algorithm is introduced for making breast phantoms and abnormal masses (cyst, fibroadenoma and cancer). The evaluation of ultrasonic properties (sound velocity and amplitude attenuation coefficient) of these normal breast phantoms and abnormal masses and their ultrasonic images shows that they are stable over 3 years. The measurement and evaluation results of ultrasonic images of our phantoms show that they behave like human breast tissues. Practicing with these phantoms for developing fine-needle aspiration (FNA) skills in sonographers shows that these phantoms reduce the time required for successful FNA. The ultrasonic phantoms have different ultrasonic applications, including ultrasonic medical imaging, the quality control of ultrasound (US) instruments, processing technique evaluation and US training; moreover, they are stable in room temperature and can keep their specifications for more than 3 years. (E-mail: Mokhtarm@modares.ac.ir)     

The speed of sound and attenuation of an IEC agar-based tissue-mimicking material for high frequency ultrasound applications

This study characterized the acoustic properties of an International Electromechanical Commission (IEC) agar-based tissue mimicking material (TMM) at ultrasound frequencies in the range 10–47 MHz. A broadband reflection substitution technique was employed using two independent systems at 21°C ± 1°C. Using a commercially available preclinical ultrasound scanner and a scanning acoustic macroscope, the measured speeds of sound were 1547.4 ± 1.4 m?s⁻¹ and 1548.0 ± 6.1 m?s⁻¹, respectively, and were approximately constant over the frequency range. The measured attenuation (dB?cm⁻¹) was found to vary with frequency f (MHz) as 0.40f + 0.0076f². Using this polynomial equation and extrapolating to lower frequencies give values comparable to those published at lower frequencies and can estimate the attenuation of this TMM in the frequency range up to 47 MHz. This characterisation enhances understanding in the use of this TMM as a tissue equivalent material for high frequency ultrasound applications.     

The ultrasonic properties of gastric cancer tissues obtained with a scanning acoustic microscope system.

A newly developed scanning acoustic microscope (SAM) system operating in the frequency range of 100-200 MHz has been employed to measure the attenuation and the sound speed of formalin-fixed specimens of five different types of gastric cancer. Signet-ring cell carcinoma specimens exhibit attenuation constant and sound speed values significantly lower than other types of gastric cancer tissues. Tubular adenocarcinoma specimens exhibit a trend toward higher attenuation and sound speed values as the cell type became differentiated. Our measurements and observations suggest that the ultrasonic properties are influenced by cellular arrangement, intercellular junction and intracellular chemical components.     

Acoustic properties of lesions generated with an ultrasound therapy system

Methods for quantitative imaging of ultrasound propagation properties were applied to the examination of the acoustic appearance of lesions generated by high intensity focused ultrasound in excised pig livers. Single lesions, about 10 mm maximum diameter by 30 mm long, were created in each of six liver specimens. Two dimensional images (32 by 32 points) of sound speed, mean attenuation coefficient (as a function of frequency in the range 3 to 8.5 MHz) and mean backscattering coefficient (5 to 8 MHz) were obtained in 7 mm thick sections of tissue, cut to include a cross-section through the lesion. Images of these properties, presented alongside surface photographs of the samples, provided a qualitative demonstration that attenuation coefficient was the most useful and backscattering coefficient was the least useful acoustic parameter for visualizing such lesions. Quantitatively the data demonstrated significant increases in attenuation coefficient and sound speed in lesioned liver relative to normal, whereas backscattering was shown not to change in a significant manner except when undissolved gas is the mechanism for increased acoustic scattering. Samples where gas was not fully removed following lesion production gave significant increases in backscattering at the lesion centre, but the shape and size of regions of high backscattering coefficient corresponded poorly with the shape and size of the lesions, unlike attenuation and sound speed for which such correspondence was good.