Assessment of the accuracy of an ultrasound elastography liver scanning system using a PVA-cryogel phantom with optimal acoustic and mechanical properties

Tissue-Mimicking Material Fabrication and Properties for Multiparametric Ultrasound Phantoms: A Systematic Review

Medical imaging has allowed for significant advancements in the field of ultrasound procedures over the years. However, each imaging modality exhibits distinct limitations that differently affect their accuracy. It is imperative to ensure the quality of each modality to identify and eliminate these limitations. To achieve this, a tissue-mimicking material (TMM) phantom is utilised for validation. This study aims to perform a systematic analysis of tissue-mimicking materials used for creating ultrasound phantoms. We reviewed 234 studies on the use of TMM phantoms in ultrasound that were published from 2013 to 2023 from two research databases. Our focus was on studies that discussed TMMs' properties and fabrication for ultrasound, elastography, and flow phantoms. The screening process led to the selection of 16 out of 234 studies to include in the analysis. The TMM ultrasound phantoms were categorised into three groups based on the solvent used; each group offers a broad range of physical properties. The water-based material most closely aligns with the properties of ultrasound. This study provides important information about the materials used for ultrasound phantoms. We also compared these materials to real human tissues and found that PVA matches most of the human tissues the best.

Mechanical properties of human hepatic tissues to develop liver-mimicking phantoms for medical applications

Using liver phantoms for mimicking human tissue in clinical training, disease diagnosis, and treatment planning is a common practice. The fabrication material of the liver phantom should exhibit mechanical properties similar to those of the real liver organ in the human body. This tissue-equivalent material is essential for qualitative and quantitative investigation of the liver mechanisms in producing nutrients, excretion of waste metabolites, and tissue deformity at mechanical stimulus. This paper reviews the mechanical properties of human hepatic tissues to develop liver-mimicking phantoms. These properties include viscosity, elasticity, acoustic impedance, sound speed, and attenuation. The advantages and disadvantages of the most common fabrication materials for developing liver tissue-mimicking phantoms are also highlighted. Such phantoms will give a better insight into the real tissue damage during the disease progression and preservation for transplantation. The liver tissue-mimicking phantom will raise the quality assurance of patient diagnostic and treatment precision and offer a definitive clinical trial data collection.

Systematic quantification of differences in shear wave elastography estimates between linear-elastic and viscoelastic material assumptionsa)

Quantitative, accurate, and standardized metrics are important for reliable shear wave elastography (SWE)-based biomarkers. For over two decades, the linear-elastic material assumption has been employed in SWE modes. In recent years, viscoelasticity estimation methods have been adopted in a few clinical systems. The current study aims to systematically quantify differences in SWE estimates obtained using linear-elastic and viscoelastic material assumptions. An acousto-mechanical simulation framework of acoustic radiation force impulse-based SWE was created to elucidate the effect of material viscosity and shear modulus on SWE estimates. Shear modulus estimates exhibited errors up to 72% when a numerical viscoelastic phantom was assessed as linearly elastic. Shear modulus estimates of polyvinyl alcohol phantoms between rheometry and SWE following the Kelvin-Voigt viscoelastic model assumptions were not significantly different. However, the percentage difference in shear modulus estimates between rheometry and SWE using the linear-elastic assumption was 50.1%-62.1%. In ex vivo liver, the percentage difference in shear modulus estimates between linear-elastic and viscoelastic methods was 76.1%. These findings provide a direct and systematic quantification of the potential error introduced when viscoelastic tissues are imaged with SWE following the linear-elastic assumption. This work emphasizes the need to utilize viscoelasticity estimation methods for developing robust quantitative imaging biomarkers.

Viscoelastic Characterization of Phantoms for Ultrasound Elastography Created Using Low- and High-Viscosity Poly(vinyl alcohol) with Ethylene Glycol as the Cryoprotectant

Ultrasound elastography enables noninvasive characterization of the tissue mechanical properties. Phantoms are widely used in ultrasound elastography for developing, testing, and validating imaging techniques. Creating phantoms with a range of viscoelastic properties relevant to human organs and pathological conditions remains an active area of research. Poly(vinyl alcohol) (PVA) cryogel phantoms offer a long shelf life, robustness, and convenient handling and storage. The goal of this study was to develop tunable phantoms using PVA with a clinically relevant range of viscoelastic properties. We combined low- and high-viscosity PVA to tune the viscoelastic properties of the phantom. Further, phantoms were created with an ethylene glycol-based cryoprotectant to determine whether it reduces the variability in the viscoelastic properties. Scanning electron microscopy (SEM) was performed to evaluate the differences in microstructure between phantoms. The density, longitudinal sound speed, and acoustic attenuation spectra (5-20 MHz) of the phantoms were measured. The phantoms were characterized using a shear wave viscoelastography approach assuming the Kelvin-Voigt model. Microstructural differences were revealed by SEM between phantoms with and without a cryoprotectant and with different PVA mixtures. The longitudinal sound speed and attenuation power-law fit exponent of the phantoms were within the clinical range (1510-1571 m/s and 1.23-1.38, respectively). The measured shear modulus (G) ranged from 3.3 to 17.7 kPa, and the viscosity (η) ranged from 2.6 to 7.3 Pa·s. The phantoms with the cryoprotectant were more homogeneous and had lower shear modulus and viscosity (G = 2.17 ± 0.2 kPa; η = 2.0 ± 0.05 Pa·s) than those without a cryoprotectant (G = 3.93 ± 0.7 kPa; η = 2.6 ± 0.14 Pa·s). Notably, phantoms with relatively constant viscosities and varying shear moduli were achieved by this method. These findings advance the development of well-characterized viscoelastic phantoms for use in elastography.

Anatomically realistic aortic dissection simulator as a potential training tool for point-of-care ultrasound

Aim

Simulators for aortic dissection diagnosis are limited by complex anatomy influencing the accuracy of point-of-care ultrasound for diagnosing aortic dissection. Therefore, this study aimed to create a healthy ascending aorta and class DeBakey, type II aortic dissection simulator as a potential point-of-care ultrasound training model.

Material and methods

3D mould simulators were created based on computed tomography images of one healthy and one DeBakey type II aortic dissection patient. In the next step, two polyvinyl alcohol-based and two silicone-based simulators were synthesised.

Results

The results of the scanning electron microscope assessment showed an aortic dissection simulator's surface with disorganised surface texture and higher root mean square (RMS or Rq) value than the healthy model of polyvinyl alcohol (RqAD = 20.28 > RqAAo = 10.26) and silicone (RqAD = 33.8 > RqAAo = 23.07). The ultrasound assessment of diameter aortic dissection showed higher than the healthy ascending aorta in polyvinyl alcohol (dAD = 28.2 mm > dAAo = 20.2 mm) and Si (dAD = 31.0 mm > dAAo = 22.4 mm), while the wall thickness of aortic dissection showed thinner than the healthy aorta in polyvinyl alcohol, which is comparable with the actual aorta measurement. The intimal flap of aortic dissection was able to replicate and showed a false lumen in the ultrasound images. The flap was measured quantitatively, indicating that the intimal flap was hyperechoic.

Conclusions

The simulators were able to replicate the surface morphology and echogenicity of the intimal flap, which is a linear hyperechoic area representing the separation of the aorta wall.

Laminar Biomaterial Composite of PVA Cryogel with Amnion as Potential Wound Dressing

Gel dressings, composed of polymers both natural and synthetic, are successfully used in the treatment of burn wounds. They protect the burn wound site against adverse external factors, ensure an adequate level of tissue hydration, have soothing and pain-relieving properties, and also support the healing process and reduce the risk of pathological scars. Another promising material that can be used in the wound-healing process is an amnion membrane. Due to its valuable properties such as protecting the body against bacterial infections and permeability to nutrition, it has found usage in different brands of medicine. In this work, we have combined the beneficial properties of hydrogels and amnion in order to make the laminar dressing that may serve for wound healing. For that purpose, the physically crosslinked cryogel of poly(vinyl alcohol) (PVA) was covered with an amnion membrane. Subsequently, gamma irradiation was performed, leading to the simultaneous internal crosslinking of the hydrogel, its permanent bonding with the amnion, and dressing sterilization. The physicochemical properties of the dressing including gel fraction, swelling, and hardness were studied. Biological tests such as the MTT assay, antimicrobial activity, and histopathological examination confirmed that the obtained material constituted a promising candidate for further, more in-depth studies aiming at wound dressing application.

Effects of Blood Pressure on Brain Tissue Pulsation Amplitude in a Phantom Model

OBJECTIVE

The precise mechanism and determinants of brain tissue pulsations (BTPs) are poorly understood, and the impact of blood pressure (BP) on BTPs is relatively unexplored. This study aimed to explore the relationship between BP parameters (mean arterial pressure [MAP] and pulse pressure [PP]) and BTP amplitude, using a transcranial tissue Doppler prototype.

METHODS

A phantom brain model generating arterial-induced BTPs was developed to observe BP changes in the absence of confounding variables and cerebral autoregulation feedback processes. A regression model was developed to investigate the relationship between bulk BTP amplitude and BP. The separate effects of PP and MAP were evaluated and quantified.

RESULTS

The regression model (R² = 0.978) revealed that bulk BTP amplitude measured from 27 gates significantly increased with PP but not with MAP. Every 1 mm Hg increase in PP resulted in a bulk BTP amplitude increase of 0.29 µm.

CONCLUSION

Increments in BP were significantly associated with increments in bulk BTP amplitude. Further work should aim to confirm the relationship between BP and BTPs in the presence of cerebral autoregulation and explore further physiological factors having an impact on BTP measurements, such as cerebral blood flow volume, tissue distensibility and intracranial pressure.

Characterizing Viscoelastic Polyvinyl Alcohol Phantoms for Ultrasound Elastography

Ultrasound phantoms mimic the acoustic and mechanical properties of native tissues. Polyvinyl alcohol (PVA) phantoms are used extensively as models for validating ultrasound elastography approaches. However, the viscous properties of PVA phantoms have not been investigated adequately. Glycerol is a viscous liquid that has been reported to increase the speed of sound of phantoms. This study aims to assess the acoustic and viscoelastic properties of PVA phantoms and PVA mixed with glycerol at varying concentrations. The phantoms were fabricated with 10% w/v PVA in water with varying concentrations of glycerol (10%, 15% and 20% v/v) and 2% w/v silicon carbide particles as acoustic scatterers. The phantoms were subjected to either one, two, or three 24-h freeze-thaw cycles. The longitudinal sound speeds of all PVA phantoms were measured, and ranged from 1529 to 1660 m/s. Attenuation spectroscopy was performed in the range of 5 to 20 MHz. The measured attenuation followed a power-law relationship with frequency, wherein the power-law fit constants and exponents ranged from 0.02 to 0.1 dB/cm/MHzⁿ and from 1.6 to 1.9, respectively. These results were in agreement with previous reports for soft tissues. Viscoelasticity of PVA phantoms was assessed using rheometry. The estimated values of shear modulus and viscosity using the Kelvin-Voigt and Kelvin-Voigt fractional derivative models were within the range of previously-reported tissue-mimicking phantoms and soft tissues. The number of freeze-thaw cycles were shown to alter the viscosity of PVA phantoms, even in the absence of glycerol. Scanning electron microscopy images of PVA phantoms without glycerol showed a porous hydrogel network, in contrast to those of PVA-glycerol phantoms with non-porous structure. Phantoms fabricated in this study possess tunable acoustic and viscoelastic properties within the range reported for healthy and diseased soft tissues. This study demonstrates that PVA phantoms can be manufactured with glycerol for applications in ultrasound elastography.

Tissue-Mimicking Materials for Ultrasound-Guided Needle Intervention Phantoms: A Comprehensive Review

Ultrasound-guided needle interventions are common procedures in medicine, and tissue-mimicking phantoms are widely used for simulation training to bridge the gap between theory and clinical practice in a controlled environment. This review assesses tissue-mimicking materials from 24 studies as candidates for a high-fidelity ultrasound phantom, including methods for evaluating relevant acoustic and mechanical properties and to what extent the reported materials mimic the superficial layers of biological tissue. Speed of sound, acoustic attenuation, Young's modulus, hardness, needle interaction forces, training efficiency and material limitations were systematically evaluated. Although gelatin and agar have the closest acoustic values to tissue, mechanical properties are limited, and strict storage protocols must be employed to counteract dehydration and microbial growth. Polyvinyl chloride (PVC) has superior mechanical properties and is a suitable alternative if durability is desired and some ultrasound realism to human tissue may be sacrificed. Polyvinyl alcohol (PVA), while also requiring hydration, performs well across all categories. Furthermore, we propose a framework for the evaluation of future ultrasound-guided needle intervention tissue phantoms to increase the fidelity of training programs and thereby improve clinical performance.

Fabrication and Characterization of Tissue-Mimicking Phantoms for Ultrasound-Guided Cannulation Training

Tissue-mimicking materials (TMMs) have been investigated and used for decades as imaging phantoms in various medical applications. They are designed and fabricated to replicate certain biological tissue characteristics, a process often dictated by the target application. Moreover, TMMs have been utilized in some medical procedural training requiring the use of imaging modalities. One potential application for TMMs is ultrasound-guided cannulation training. Cannulation is a procedure that requires a level of dexterity to gain vascular access using ultrasound guidance while avoiding complications like vessel laceration and bleeding. However, an ideal phantom for this application is yet to be developed. This work investigates the development and characterization of high-fidelity phantoms for cannulation training. The mechanical (shore hardness, elastic modulus, and needle-interaction forces) and acoustic (B-mode ultrasound scans) properties of candidate materials were quantitatively compared with biological tissue. The evaluated materials included ballistic gel, plasticized polyvinyl chloride (PVC), silicone, gelatin, agar, and polyvinyl alcohol (PVA)- cryogel. Mechanical testing demonstrated that each material could replicate the Shore hardness and elasticity characteristics of different biological tissues (skin, fat, and muscle), with PVA and PVC showing tunability by varying composition or fabrication processes. Shore hardness (OO-range) for PVA ranged between 6.3 ± 1.0 to 59.3 ± 2.6 and PVC from 4.8 ± 0.7 to 14.6 ± 0.8. Ultrasound scans of PVA were the closest to human scans, both qualitatively (based on experts' opinion) and quantitatively (based on pixel intensity measurements). Modified mixtures of PVA are found to best serve as high-fidelity cannulation phantoms. Alternatively, PVC can be used to avoid troublesome fabrication processes of PVA.

Evaluation of liver fibrosis in chronic hepatitis B patients with 2D shear wave elastography with propagation map guidance: a single-centre study

Background

The aims of this study were to evaluate liver fibrosis with two-dimensional (2D) shear wave elastography (SWE) in patients with chronic hepatitis B (CHB), to compare 2D-SWE with histopathology and to determine the change in liver stiffness values after antiviral therapy.

Material and methods

A total of 253 patients with CHB were included in this prospective study. 2D-SWE with propagation map guidance to measure liver stiffness, fibrosis-4 index (FIB-4) and aspartate aminotransferase to platelet ratio index (APRI) scoring and additional liver biopsy were performed in patients with CHB. Liver stiffness was measured again at 24 and 48 weeks in all patients. The Spearman rank correlation test was used to analyse the correlation between variables, and receiver operating curve analysis was used to evaluate the diagnostic performance in terms of fibrosis.

Results

Liver stiffness measurements made with 2D-SWE demonstrated a significant positive correlation with the fibrosis stage and FIB-4 score (r_s = 0.774 and 0.337, respectively, p < 0.001 for both). The area under the curve value for kPa for the prediction of significant fibrosis was 0.956 (95% CI_s) (0.920–0.991), and the optimal cut-off value was 8.2 kPa (sensitivity: 92.7% and specificity: 78.9%); these values were 0.978 (95% CI_s, 0.945–1.000) and 10.1 kPa (sensitivity: 92.9% and specificity: 96.4%) for the prediction of severe fibrosis. After antiviral treatment, a decrease in liver stiffness values measured by 2D-SWE was detected (mean kPa values at 0 and 48 weeks; 9.24 and 7.36, respectively, p < 0.001).

Conclusion

In conclusion, the measurement of liver stiffness with 2D-SWE has high diagnostic performance in the determination of hepatic fibrosis and can be used to evaluate the response to treatment in patients receiving antiviral therapy.

The dynamic viscoelastic characterisation and magnetic resonance imaging of poly(vinyl alcohol) cryogel: Identifying new attributes and opportunities

Poly(vinyl alcohol) (PVA) cryogel is a biocompatible, synthetic hydrogel, compatible with magnetic resonance (MR) imaging. It is widely used as a biomaterial in tissue scaffolds and mimics to test various diagnostic techniques. The aim of this study is to characterise the effect of varying PVA concentration, molecular weight (MW) and manufacturing protocol on the viscoelastic mechanical properties and MR T₂ relaxation time. Further to this MR imaging (MRI) was investigated as a method to quantify material homogeneity. Cylindrical samples of PVA, of varying MW, concentration and number of freeze thaw cycles (FTCs), were manufactured. Dynamic mechanical analysis was performed to evaluate the storage and loss moduli between frequencies of 0.5 and 10 Hz. MR T₂ relaxation maps were imaged using a 7 T MRI instrument. Storage and loss moduli were shown to increase with MW, concentration, or the number of FTCs; with storage modulus ranging from 55 kPa to 912 kPa and loss modulus ranging from 6 kPa to 103 kPa. MR T₂ relaxation time was shown to increase linearly with PVA concentration. The qualitative and quantitative heterogeneity of the PVA sample were identified through MR T₂ relaxation time maps. Excitingly, PVA demonstrated a composition-dependent casual correlation between the viscoelastic mechanical properties and MR T₂ relaxation time. In conclusion, this research thoroughly characterised the viscoelastic mechanical properties of PVA to support its extensive use as a biomaterial, and demonstrated the use of MRI to non-invasively identify sample heterogeneity and to predict the composition-dependent viscoelastic properties of PVA.

A Novel Elastography Phantom Prototype for Assessment of Ultrasound Elastography Imaging Performance

The aims of this study were firstly to manufacture and evaluate a novel elastography test phantom and secondly to assess the performance of an elastography system using this phantom. A novel Leicester-St. Thomas' Elastography Pipe (L-STEP) test phantom consisting of five soft polyvinyl acrylic-cryogel pipes of varying diameters (2-12 mm), embedded at 45° within an agar-based tissue-mimicking material was developed. A shear-wave elastography (SWE) scanner was used by two blinded operators to image and assess longitudinal sections of the pipes. Young's modulus estimates were dependent on the diameter of pipes and at superficial depths were greater than deeper depths (mean 98 kPa vs. 59 kPa) and had lower coefficients of variation (mean 21% vs. 53%). The penetration depth (maximum depth at which a SWE signal was obtained) increased with increasing pipe diameter. Penetration depth measurements had excellent inter- and intra-operator reproducibility (intra-class correlation coefficients >0.8) and coefficient of variation range of 2%-12%. A new metric, called the summative performance index, was defined as the sum of the ratios of the penetration depth/pipe diameter. The L-STEP phantom is suitable for assessing key aspects of elastography imaging performance: resolution, accuracy, reproducibility, depth dependence, sensitivity and our novel summative performance index.

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.

A Polyvinyl Alcohol-Based Thermochromic Material for Ultrasound Therapy Phantoms

Temperature estimation is a fundamental step in assessment of the efficacy of thermal therapy. A thermochromic material sensitive within the temperature range 52.5°C-75°C has been developed. The material is based on polyvinyl alcohol cryogel with the addition of a commercial thermochromic ink. It is simple to manufacture, low cost, non-toxic and versatile. The thermal response of the material was evaluated using multiple methods, including immersion in a temperature-controlled water bath, a temperature-controlled heated needle and high-intensity focused ultrasound (HIFU) sonication. Changes in colour were evaluated using both RGB (red, green, blue) maps and pixel intensities. Acoustic and thermal properties of the material were measured. Thermo-acoustic simulations were run with an open-source software, and results were compared with the HIFU experiments, showing good agreement. The material has good potential for the development of ultrasound therapy phantoms.

Multimodal Breast Phantoms for Microwave, Ultrasound, Mammography, Magnetic Resonance and Computed Tomography Imaging

The aim of this work was to develop multimodal anthropomorphic breast phantoms suitable for evaluating the imaging performance of a recently-introduced Microwave Imaging (MWI) technique in comparison to the established diagnostic imaging modalities of Magnetic Resonance Imaging (MRI), Ultrasound (US), mammography and Computed Tomography (CT). MWI is an emerging technique with significant potential to supplement established imaging techniques to improve diagnostic confidence for breast cancer detection. To date, numerical simulations have been used to assess the different MWI scanning and image reconstruction algorithms in current use, while only a few clinical trials have been conducted. To bridge the gap between the numerical simulation environment and a more realistic diagnostic scenario, anthropomorphic phantoms which mimic breast tissues in terms of their heterogeneity, anatomy, morphology, and mechanical and dielectric characteristics, may be used. Key in this regard is achieving realism in the imaging appearance of the different healthy and pathologic tissue types for each of the modalities, taking into consideration the differing imaging and contrast mechanisms for each modality. Suitable phantoms can thus be used by radiologists to correlate image findings between the emerging MWI technique and the more familiar images generated by the conventional modalities. Two phantoms were developed in this study, representing difficult-to-image and easy-to-image patients: the former contained a complex boundary between the mammary fat and fibroglandular tissues, extracted from real patient MRI datasets, while the latter contained a simpler and less morphologically accurate interface. Both phantoms were otherwise identical, with tissue-mimicking materials (TMMs) developed to mimic skin, subcutaneous fat, fibroglandular tissue, tumor and pectoral muscle. The phantoms’ construction used non-toxic materials, and they were inexpensive and relatively easy to manufacture. Both phantoms were scanned using conventional modalities (MRI, US, mammography and CT) and a recently introduced MWI radar detection procedure called in-coherent Multiple Signal Classification (I-MUSIC). Clinically realistic artifact-free images of the anthropomorphic breast phantoms were obtained using the conventional imaging techniques as well as the emerging technique of MWI.

Liver fibrosis imaging: A clinical review of ultrasound and magnetic resonance elastography

Liver fibrosis is a histological hallmark of most chronic liver diseases, which can progress to cirrhosis and liver failure, and predisposes to hepatocellular carcinoma. Accurate diagnosis of liver fibrosis is necessary for prognosis, risk stratification, and treatment decision-making. Liver biopsy, the reference standard for assessing liver fibrosis, is invasive, costly, and impractical for surveillance and treatment response monitoring. Elastography offers a noninvasive, objective, and quantitative alternative to liver biopsy. This article discusses the need for noninvasive assessment of liver fibrosis and reviews the comparative advantages and limitations of ultrasound and magnetic resonance elastography techniques with respect to their basic concepts, acquisition, processing, and diagnostic performance. Variations in clinical contexts of use and common pitfalls associated with each technique are considered. In addition, current challenges and future directions to improve the diagnostic accuracy and clinical utility of elastography techniques are discussed. Level of Evidence: 5 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2020;51:25-42.

Designing and validating a PVA liver phantom with respiratory motion for needle-based interventions

Purpose

The purpose is to design and validate an anthropomorphic polyvinyl alcohol (PVA) liver phantom with respiratory motion to simulate needle-based interventions. Such a system can, for example, be used as a validation tool for novel needles.

Methods

Image segmentations of CT scans of four patients during inspiration and expiration were used to measure liver and rib displacement. An anthropomorphic liver mold based on a CT scan was 3D printed and filled with 5% w/w PVA-to-water, undergoing two freeze–thaw cycles, in addition to a 3D-printed compliant rib cage. They were both held in place by a PVA abdominal phantom. A sinusoidal motion vector, based on the measured liver displacement, was applied to the liver phantom by means of a motion stage. Liver, rib cage and needle deflection were tracked by placing electromagnetic sensors on the phantom. Liver and rib cage phantom motion was validated by comparison with the CT images of the patients, whereas needle deflection was compared with the literature.

Results

CT analysis showed that from the state of expiration to inspiration, the livers moved predominantly toward the right (mean: 2 mm, range: − 11 to 11 mm), anterior (mean: 15 mm, range: 9–21 mm) and caudal (mean: 16 mm, range: 6–24 mm) direction. The mechatronic design of the liver phantom gives the freedom to set direction and amplitude of the motion and was able to mimic the direction of liver motion of one patient. Needle deflection inside the phantom increased from 1.6 to 3.8 mm from the initial expiration state to inspiration.

Conclusions

The developed liver phantom allows for applying different motion patterns and shapes/sizes and thus allows for patient-specific simulation of needle-based interventions. Moreover, it is able to mimic appropriate respiratory motion and needle deflection as observed in patients.

Electronic supplementary material

The online version of this article (10.1007/s11548-019-02029-6) contains supplementary material, which is available to authorized users.

Observations And Experiments For The Definition Of A New Robotic Device Dedicated To CT, CBCT And MRI-Guided Percutaneous Procedures

In this paper, we present the work achieved to define the robotic functionalities of interest for percutaneous procedures as performed in interventional radiology. Our contributions are twofold. First, a detailed task analysis is performed with workflow analysis of biopsies, one of the most frequent tasks, under three imaging modalities, namely CT, CBCT and MRI. Second, the functionalities of a robotic assistant are identified, and we analyze whether a single device can bring an added value during procedures in the three modalities while keeping the robotized workflow close to manual tasks, to minimize learning time and difficulty of use. Experimental analysis on CBCT is notably used to confirm the interest of the determined robotic functionalities.

Quantitative characterization of viscoelastic behavior in tissue-mimicking phantoms and ex vivo animal tissues

Viscoelasticity of soft tissue is often related to pathology, and therefore, has become an important diagnostic indicator in the clinical assessment of suspect tissue. Surgeons, particularly within head and neck subsites, typically use palpation techniques for intra-operative tumor detection. This detection method, however, is highly subjective and often fails to detect small or deep abnormalities. Vibroacoustography (VA) and similar methods have previously been used to distinguish tissue with high-contrast, but a firm understanding of the main contrast mechanism has yet to be verified. The contributions of tissue mechanical properties in VA images have been difficult to verify given the limited literature on viscoelastic properties of various normal and diseased tissue. This paper aims to investigate viscoelasticity theory and present a detailed description of viscoelastic experimental results obtained in tissue-mimicking phantoms (TMPs) and ex vivo tissues to verify the main contrast mechanism in VA and similar imaging modalities. A spherical-tip micro-indentation technique was employed with the Hertzian model to acquire absolute, quantitative, point measurements of the elastic modulus (E), long term shear modulus (η), and time constant (τ) in homogeneous TMPs and ex vivo tissue in rat liver and porcine liver and gallbladder. Viscoelastic differences observed between porcine liver and gallbladder tissue suggest that imaging modalities which utilize the mechanical properties of tissue as a primary contrast mechanism can potentially be used to quantitatively differentiate between proximate organs in a clinical setting. These results may facilitate more accurate tissue modeling and add information not currently available to the field of systems characterization and biomedical research.

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.

An examination of the elastic properties of tissue-mimicking phantoms using vibro-acoustography and a muscle motor system

Tissue hardness, often quantified in terms of elasticity, is an important differentiating criterion for pathological identity and is extensively used by surgeons for tumor localization. Delineation of malignant regions from benign regions is typically performed by visual inspection and palpation. Although practical, this method is highly subjective and does not provide quantitative metrics. We have previously reported on Vibro-Acoustography (VA) for tumor delineation. VA is unique in that it uses the specific, non-linear properties of tumor tissue in response to an amplitude modulated ultrasound beam to generate spatially resolved, high contrast maps of tissue. Although the lateral and axial resolutions (sub-millimeter and sub-centimeter, respectively) of VA have been extensively characterized, the relationship between static stiffness assessment (palpation) and dynamic stiffness characterization (VA) has not been explicitly established. Here we perform a correlative exploration of the static and dynamic properties of tissue-mimicking phantoms, specifically elasticity, using VA and a muscle motor system. Muscle motor systems, commonly used to probe the mechanical properties of materials, provide absolute, quantitative point measurements of the elastic modulus, analogous to Young's modulus, of a target. For phantoms of varying percent-by-weight concentrations, parallel VA and muscle motor studies conducted on 18 phantoms reveal a negative correlation (p < - 0.85) between mean signal amplitude levels observed with VA and calculated elastic modulus values from force vs. indentation depth curves. Comparison of these elasticity measurements may provide additional information to improve tissue modeling, system characterization, as well as offer valuable insights for in vivo applications, specifically surgical extirpation of tumors.

Anisotropic polyvinyl alcohol hydrogel phantom for shear wave elastography in fibrous biological soft tissue: a multimodality characterization

Shear wave elastography imaging techniques provide quantitative measurement of soft tissues elastic properties. Tendons, muscles and cerebral tissues are composed of fibers, which induce a strong anisotropic effect on the mechanical behavior. Currently, these tissues cannot be accurately represented by existing elastography phantoms. Recently, a novel approach for orthotropic hydrogel mimicking soft tissues has been developed (Millon et al 2006 J. Biomed. Mater. Res. B 305-11). The mechanical anisotropy is induced in a polyvinyl alcohol (PVA) cryogel by stretching the physical crosslinks of the polymeric chains while undergoing freeze/thaw cycles. In the present study we propose an original multimodality imaging characterization of this new transverse isotropic (TI) PVA hydrogel. Multiple properties were investigated using a large variety of techniques at different scales compared with an isotropic PVA hydrogel undergoing similar imaging and rheology protocols. The anisotropic mechanical (dynamic and static) properties were studied using supersonic shear wave imaging technique, full-field optical coherence tomography (FFOCT) strain imaging and classical linear rheometry using dynamic mechanical analysis. The anisotropic optical and ultrasonic spatial coherence properties were measured by FFOCT volumetric imaging and backscatter tensor imaging, respectively. Correlation of mechanical and optical properties demonstrates the complementarity of these techniques for the study of anisotropy on a multi-scale range as well as the potential of this TI phantom as fibrous tissue-mimicking phantom for shear wave elastographic applications.

Shear wave elastography assessment of carotid plaque stiffness: in vitro reproducibility study

This study assessed inter- and intra-observer reproducibility of shear wave elastography (SWE) measurements in vessel phantoms simulating soft and hard carotid plaque under steady and pulsatile flow conditions. Supersonic SWE was used to acquire cine-loop data and quantify Young's modulus in cryogel vessel phantoms. Data were acquired by two observers, each performing three repeat measurements. Mean Young's modulus was quantified within 2-mm regions of interest averaged across five frames and, depending on vessel model and observer, ranged from 28 to 240 kPa. The mean inter-frame coefficient of variation (CV) was 0.13 (range: 0.07-0.18) for observer 1 and 0.14 (range: 0.12-0.16) for observer 2, with mean intra-class correlation coefficients (ICCs) of 0.84 and 0.83, respectively. The mean inter-operator CV was 0.13 (range: 0.08-0.20), with a mean ICC of 0.76 (range: 0.69-0.82). Our findings indicate that SWE can quantify Young's modulus of carotid plaque phantoms with good reproducibility, even in the presence of pulsatile flow.

A framework for correcting brain retraction based on an eXtended Finite Element Method using a laser range scanner

BACKGROUND

Brain retraction causes great distortion that limits the accuracy of an image-guided neurosurgery system that uses preoperative images. Therefore, brain retraction correction is an important intraoperative clinical application.

METHODS

We used a linear elastic biomechanical model, which deforms based on the eXtended Finite Element Method (XFEM) within a framework for brain retraction correction. In particular, a laser range scanner was introduced to obtain a surface point cloud of the exposed surgical field including retractors inserted into the brain. A brain retraction surface tracking algorithm converted these point clouds into boundary conditions applied to XFEM modeling that drive brain deformation. To test the framework, we performed a brain phantom experiment involving the retraction of tissue. Pairs of the modified Hausdorff distance between Canny edges extracted from model-updated images, pre-retraction, and post-retraction CT images were compared to evaluate the morphological alignment of our framework. Furthermore, the measured displacements of beads embedded in the brain phantom and the predicted ones were compared to evaluate numerical performance.

RESULTS

The modified Hausdorff distance of 19 pairs of images decreased from 1.10 to 0.76 mm. The forecast error of 23 stainless steel beads in the phantom was between 0 and 1.73 mm (mean 1.19 mm). The correction accuracy varied between 52.8 and 100 % (mean 81.4 %).

CONCLUSIONS

The results demonstrate that the brain retraction compensation can be incorporated intraoperatively into the model-updating process in image-guided neurosurgery systems.

Regional cardiac motion and strain estimation in three-dimensional echocardiography: a validation study in thick-walled univentricular phantoms

Automatic quantification of regional left ventricular deformation in volumetric ultrasound data remains challenging. Many methods have been proposed to extract myocardial motion, including techniques using block matching, phase-based correlation, differential optical flow methods, and image registration. Our lab previously presented an approach based on elastic registration of subsequent volumes using a B-spline representation of the underlying transformation field. Encouraging results were obtained for the assessment of global left ventricular function, but a thorough validation on a regional level was still lacking. For this purpose, univentricular thick-walled cardiac phantoms were deformed in an experimental setup to locally assess strain accuracy against sonomicrometry as a reference method and to assess whether regions containing stiff inclusions could be detected. Our method showed good correlations against sonomicrometry: r(2) was 0.96, 0.92, and 0.84 for the radial (ε(RR)), longitudinal (ε(LL)), and circumferential (ε(CC)) strain, respectively. Absolute strain errors and strain drift were low for ε(LL) (absolute mean error: 2.42%, drift: -1.05%) and ε(CC) (error: 1.79%, drift: -1.33%) and slightly higher for ε(RR) (error: 3.37%, drift: 3.05%). The discriminative power of our methodology was adequate to resolve full transmural inclusions down to 17 mm in diameter, although the inclusion-to-surrounding tissue stiffness ratio was required to be at least 5:2 (absolute difference of 39.42 kPa). When the inclusion-to-surrounding tissue stiffness ratio was lowered to approximately 2:1 (absolute difference of 22.63 kPa), only larger inclusions down to 27 mm in diameter could still be identified. Radial strain was found not to be reliable in identifying dysfunctional regions.

Review of ultrasound elastography quality control and training test phantoms

While the rapid development of ultrasound elastography techniques in recent decades has sparked its prompt implementation in the clinical setting adding new diagnostic information to conventional imaging techniques, questions still remain as to its full potential and efficacy in the hospital environment. A limited number of technical studies have objectively assessed the full capabilities of the different elastography approaches, perhaps due, in part, to the scarcity of suitable tissue-mimicking materials (TMMs) and appropriately designed phantoms available. Few commercially available elastography phantoms possess the necessary test target characteristics or mechanical properties observed clinically, or indeed reflect the lesion-to-background elasticity ratio encountered during clinical scanning. Thus, while some phantoms may prove useful, they may not fully challenge the capabilities of the different elastography techniques, proving limited when it comes to quality control (QC) and/or training purposes. Although a variety of elastography TMMs, such as agar and gelatine dispersions, co-polymer in oil and poly(vinyl) alcohol cryogel, have been developed for specific research purposes, such work is yet to produce appropriately designed phantoms to adequately challenge the variety of current commercially available elastography applications. Accordingly, there is a clear need for the further development of elastography TMMs and phantoms to keep pace with the rapid developments in elastography technology, to ensure that the performance of these new diagnostic approaches are validated, and for clinical training purposes.

Optofluidic phantom mimicking optical properties of porcine livers

One strategy for assessing efficacy of a liver transplant is to monitor perfusion and oxygenation after transplantation. An implantable optical sensor is being developed to overcome inadequacies of current monitoring approaches. To facilitate sensor design while minimizing animal use, a polydimethylsiloxane (PDMS)-based liver phantom was developed to mimic the optical properties of porcine liver in the 630-1000 nm wavelength range and the anatomical geometry of liver parenchyma. Using soft lithography to construct microfluidic channels in pigmented elastomer enabled the 2D approximation of hexagonal liver lobules with 15mm sinusoidal channels, which will allow perfusion with blood-mimicking fluids to facilitate the development of the liver perfusion and oxygenation monitoring system.

Development of a vessel-mimicking material for use in anatomically realistic Doppler flow phantoms

Polyvinyl alcohol cryogel (PVA-C) is presented as a vessel-mimicking material for use in anatomically realistic Doppler flow phantoms. Three different batches of 10% wt PVA-C containing (i) PVA-C alone, (ii) PVA-C with antibacterial agent and (iii) PVA-C with silicon carbide particles were produced, each with 1-6 freeze-thaw cycles. The resulting PVA-C samples were characterized acoustically (over a range 2.65 to 10.5 MHz) and mechanically to determine the optimum mixture and preparation for mimicking the properties of healthy and diseased arteries found in vivo. This optimum mix was reached with the PVA-C with antibacterial agent sample, prepared after two freeze/thaw cycles, which achieved a speed of sound of 1538 ± 5 m s(-1) and a Young's elastic modulus of 79 ± 11 kPa. This material was used to make a range of anatomically realistic flow phantoms with varying degrees of stenoses, and subsequent flow experiments revealed that higher degrees of stenoses and higher velocities could be achieved without phantom rupturing compared with a phantom containing conventional wall-less vessels.