Characterization of Acoustic, Cavitation, and Thermal Properties of Poly(vinyl alcohol) Hydrogels for Use as Therapeutic Ultrasound Tissue Mimics

Overview of Therapeutic Ultrasound Applications and Safety Considerations: 2024 Update

A 2012 review of therapeutic ultrasound was published to educate researchers and physicians on potential applications and concerns for unintended bioeffects (doi: 10.7863/jum.2012.31.4.623). This review serves as an update to the parent article, highlighting advances in therapeutic ultrasound over the past 12 years. In addition to general mechanisms for bioeffects produced by therapeutic ultrasound, current applications, and the pre-clinical and clinical stages are outlined. An overview is provided for image guidance methods to monitor and assess treatment progress. Finally, other topics relevant for the translation of therapeutic ultrasound are discussed, including computational modeling, tissue-mimicking phantoms, and quality assurance protocols.

Evaluating the effect of tissue stimulation at different frequencies on breast lesion classification based on nonlinear features using a novel radio frequency time series approach

Objective

Radio Frequency Time Series (RF TS) is a cutting-edge ultrasound approach in tissue typing. The RF TS does not provide dynamic insights into the propagation medium; when the tissue and probe are fixed. We previously proposed the innovative RFTSDP method in which the RF data are recorded while stimulating the tissue. Applying stimulation can unveil the mechanical characteristics of the tissue in RF echo.

Materials and methods

In this study, an apparatus was developed to induce vibrations at different frequencies to the medium. Data were collected from four PVA phantoms simulating the nonlinear behaviors of healthy, fibroadenoma, cyst, and cancerous breast tissues. Raw focused, raw, and beamformed ultrafast data were collected under conditions of no stimulation, constant force, and various vibrational stimulations using the Supersonic Imagine Aixplorer clinical/research ultrasound imaging system. Time domain (TD), spectral, and nonlinear features were extracted from each RF TS. Support Vector Machine (SVM), Random Forest, and Decision Tree algorithms were employed for classification.

Results

The optimal outcome was achieved using the SVM classifier considering 19 features extracted from beamformed ultrafast data recorded while applying vibration at the frequency of 65 Hz. The classification accuracy, specificity, and precision were 98.44 ± 0.20 %, 99.49 ± 0.01 %, and 98.53 ± 0.04 %, respectively. Applying RFTSDP, a notable 24.45 % improvement in accuracy was observed compared to the case of fixed probe assessing the recorded raw focused data.

Conclusions

External vibration at an appropriate frequency, as applied in RFTSDP, incorporates beneficial information about the medium and its dynamic characteristics into the RF TS, which can improve tissue characterization.

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.

High-quality Agar and Polyacrylamide Tumor-mimicking Phantom Models for Magnetic Resonance-guided Focused Ultrasound Applications

Background

Tissue-mimicking phantoms (TMPs) have been used extensively in clinical and nonclinical settings to simulate the thermal effects of focus ultrasound (FUS) technology in real tissue or organs. With recent technological developments in the FUS technology and its monitoring/guided techniques such as ultrasound-guided FUS and magnetic resonance-guided FUS (MRgFUS) the need for TMPs are more important than ever to ensure the safety of the patients before being treated with FUS for a variety of diseases (e.g., cancer or neurological). The purpose of this study was to prepare a tumor-mimicking phantom (TUMP) model that can simulate competently a tumor that is surrounded by healthy tissue.

Methods

The TUMP models were prepared using polyacrylamide (PAA) and agar solutions enriched with MR contrast agents (silicon dioxide and glycerol), and the thermosensitive component bovine serum albumin (BSA) that can alter its physical properties once thermal change is detected, therefore offering real-time visualization of the applied FUS ablation in the TUMPs models. To establish if these TUMPs are good candidates to be used in thermoablation, their thermal properties were characterized with a custom-made FUS system in the laboratory and a magnetic resonance imaging (MRI) setup with MR-thermometry. The BSA protein's coagulation temperature was adjusted at 55°C by setting the pH of the PAA solution to 4.5, therefore simulating the necrosis temperature of the tissue.

Results

The experiments carried out showed that the TUMP models prepared by PAA can change color from transparent to cream-white due to the BSA protein coagulation caused by the thermal stress applied. The TUMP models offered a good MRI contrast between the TMPs and the TUMPs including real-time visualization of the ablation area due to the BSA protein coagulation. Furthermore, the T2-weighted MR images obtained showed a significant change in T2 when the BSA protein is thermally coagulated. MR thermometry maps demonstrated that the suggested TUMP models may successfully imitate a tumor that is present in soft tissue.

Conclusion

The TUMP models developed in this study have numerous uses in the testing and calibration of FUS equipment including the simulation and validation of thermal therapy treatment plans with FUS or MRgFUS in oncology applications.

Getting hot in here! Comparison of Holmium vs. thulium laser in an anatomic hydrogel kidney model

As laser technology has advanced, high-power lasers have become increasingly common. The Holmium: yttrium-aluminum-garnet (Ho:YAG) laser has long been accepted as the standard for laser lithotripsy. The thulium fiber laser (TFL) has recently been established as a viable option. The aim of this study is to evaluate thermal dose and temperature for the Ho:YAG laser to the TFL at four different laser settings while varying energy, frequency, operator duty cycle (ODC). Utilizing high-fidelity, 3D-printed hydrogel models of a pelvicalyceal collecting system (PCS) with a synthetic BegoStone implanted in the renal pelvis, laser lithotripsy was performed with the Ho:YAG laser or TFL. At a standard power (40W) and irrigation (17.9 ml/min), we evaluated four different laser settings with ODC variations with different time-on intervals. Temperature was measured at two separate locations. In general, the TFL yielded greater cumulative thermal doses than the Ho:YAG laser. Thermal dose and temperature were typically greater at the stone when compared away from the stone. Regarding the TFL, there was no general trend if fragmentation or dusting settings yielded greater thermal doses or temperatures. The TFL generated greater temperatures and thermal doses in general than the Ho:YAG laser with Moses technology. Temperatures and thermal doses were greater closer to the laser fiber tip. It is inconclusive as to whether fragmentation or dusting settings elicit greater thermal loads for the TFL. Energy, frequency, ODC, and laser-on time significantly impact thermal loads during ureteroscopic laser lithotripsy, independent of power.

WATTS happening? Evaluation of thermal dose during holmium laser lithotripsy in a high-fidelity anatomic model

PURPOSE

To evaluate the thermal profiles of the holmium laser at different laser parameters at different locations in an in vitro anatomic pelvicalyceal collecting system (PCS) model. Laser lithotripsy is the cornerstone of treatment for urolithiasis. With the prevalence of high-powered lasers, stone ablation efficiency has become more pronounced. Patient safety remains paramount during surgery. It is well recognized that the heat generated from laser lithotripsy has the potential to cause thermal tissue damage.

METHODS

Utilizing high-fidelity, 3D printed hydrogel models of a PCS with a synthetic BegoStone implanted in the renal pelvis, laser lithotripsy was performed with the Moses 2.0 holmium laser. At a standard power (40 W) and irrigation pressure (100 cm H2O), we evaluated operator duty cycle (ODC) variations with different time-on intervals at four different laser settings. Temperature was measured at two separate locations-at the stone and away from the stone.

RESULTS

Temperatures were highest closest to the laser tip with a decrease away from the laser. Fluid temperatures increased with longer laser-on times and higher ODCs. Thermal doses were greater with increased ODCs and the threshold for thermal injury was reached for ODCs of 75% and 100%.

CONCLUSION

Temperature generation and thermal dose delivered are greatest closer to the tip of the laser fiber and are not dependent on power alone. Significant temperature differences were noted between four laser settings at a standardized power (40 W). Temperatures can be influenced by a variety of factors, such as laser-on time, operator duty cycle, and location in the PCS.

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.

Evaluation of physical properties and image of polyvinyl chloride as breast tissue equivalence for dual-modality (mammography and ultrasound)

TMP is gradually becoming a fundamental element for quality assurance and control in ionizing and non-ionizing radiation imaging modalities as well as in the development of different techniques. This study aimed to evaluate and obtain polyvinyl chloride tissue mimicking material for dual-modality breast phantoms in mammography and ultrasound. Breast tissue equivalence was evaluated based on X-ray attenuation properties, speed of sound, attenuation, and acoustic impedance. There are six samples of PVC-plasticizer material with variations of PVC concentration and additives. The evaluation of X-ray attenuation was carried out using mammography from 23 to 35 kV, while the acoustic properties were assessed with mode A ultrasound and a transducer frequency of 5 MHz. A breast phantom was created from TMP material with tissue equivalence and was then evaluated using mammography as well as ultrasound to analyze its image quality. The results showed that samples A (PVC 5%, DOP 95%), B (PVC 7%, DOP 93%), C (PVC 10%, DOP 90%), E (PVC 7%, DOP 90%, graphite 3%), and F (PVC 7%, DOP 90%, silicone oil 3%) have the closest equivalent to the ACR breast phantom material with a different range of 0.01-1.39 in the 23-35 kV range. Based on the evaluation of the acoustic properties of ultrasound, A had high similarity to fat tissue with a difference of 0.03 (dB cm- 1 MHz- 1) and 0.07 (106 kg m- 2 s- 1), while B was close to the glandular tissue with a difference of 9.2 m s- 1. Multilayer breast phantom images' results showed gray levels in mammography and ultrasound modalities. Therefore, this study succeeded in establishing TMP material for mammography and ultrasound. It can also be used for simple quality assurance and control programs.

Development of Ultrasound Phantom Made of Transparent Material: Feasibility of Optical Particle Image Velocimetry

OBJECTIVE

The need for ultrasound flow phantoms to validate ultrasound systems requires the development of materials that can clearly visualize the flow inside for measurement purposes.

METHODS

A transparent ultrasound flow phantom material composed of poly(vinyl alcohol) hydrogel (PVA-H) with dimethyl sulfoxide (DMSO) and water solution manufactured using the freezing method and mixed with quartz glass powder to exhibit scattering effects is proposed. To achieve transparency of the hydrogel phantom, the refractive index (RI) was changed to match that of the glass by modifying the PVA concentration and the ratio of DMSO to water in the solvent. The feasibility of optical particle image velocimetry (PIV) was verified by comparing an acrylic rectangular cross-section channel with a rigid wall. After the feasibility tests, an ultrasound flow phantom was fabricated to conduct ultrasound B-mode visualization and Doppler-PIV comparison.

DISCUSSION

The results revealed that the PIV measured through PVA-H material exhibited 0.8% error in the measured maximum velocity compared with PIV through the acrylic material. B-mode images are similar to real tissue visualization with a limitation of a higher sound velocity, when compared with human tissue, of 1792 m/s. Doppler measurement of the phantom revealed approximately 120% and 19% overestimation of maximum and mean velocities, respectively, compared with those from PIV.

CONCLUSION

The proposed material possesses the advantage of the single-phantom ability to improve the ultrasound flow phantom for validation of flow.

Holographic Focused Ultrasound Hyperthermia System for Uniform Simultaneous Thermal Exposure of Multiple Tumor Spheroids

Hyperthermia is currently used to treat cancer due to its ability to radio- and chemo-sensitize and to stimulate the immune response. While ultrasound is non-ionizing and can induce hyperthermia deep within the body non-invasively, achieving uniform and volumetric hyperthermia is challenging. This work presents a novel focused ultrasound hyperthermia system based on 3D-printed acoustic holograms combined with a high-intensity focused ultrasound (HIFU) transducer to produce a uniform iso-thermal dose in multiple targets. The system is designed with the aim of treating several 3D cell aggregates contained in an International Electrotechnical Commission (IEC) tissue-mimicking phantom with multiple wells, each holding a single tumor spheroid, with real-time temperature and thermal dose monitoring. System performance was validated using acoustic and thermal methods, ultimately yielding thermal doses in three wells that differed by less than 4%. The system was tested in vitro for delivery of thermal doses of 0-120 cumulative equivalent minutes at 43 °C (CEM43) to spheroids of U87-MG glioma cells. The effects of ultrasound-induced heating on the growth of these spheroids were compared with heating using a polymerase chain reaction (PCR) thermocycler. Results showed that exposing U87-MG spheroids to an ultrasound-induced thermal dose of 120 CEM43 shrank them by 15% and decreased their growth and metabolic activity more than seen in those exposed to a thermocycler-induced heating. This low-cost approach of modifying a HIFU transducer to deliver ultrasound hyperthermia opens new avenues for accurately controlling thermal dose delivery to complex therapeutic targets using tailored acoustic holograms. Spheroid data show that thermal and non-thermal mechanisms are implicated in the response of cancer cells to non-ablative ultrasound heating.

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.

Ultrasound Responsive Smart Implantable Hydrogels for Targeted Delivery of Drugs: Reviewing Current Practices

Over the last two decades, the process of delivering therapeutic drugs to a patient with a controlled release profile has been a significant focus of drug delivery research. Scientists have given tremendous attention to ultrasound-responsive hydrogels for several decades. These smart nanosystems are more applicable than other stimuli-responsive drug delivery vehicles (ie UV-, pH- and thermal-, responsive materials) because they enable more efficient targeted treatment via relatively non-invasive means. Ultrasound (US) is capable of safely transporting energy through opaque and complex media with minimal loss of energy. It is capable of being localized to smaller regions and coupled to systems operating at various time scales. However, the properties enabling the US to propagate effectively in materials also make it very difficult to transform acoustic energy into other forms that may be used. Recent research from a variety of domains has attempted to deal with this issue, proving that ultrasonic effects can be used to control chemical and physical systems with remarkable specificity. By obviating the need for multiple intravenous injections, implantable US responsive hydrogel systems can enhance the quality of life for patients who undergo treatment with a varied dosage regimen. Ideally, the ease of self-dosing in these systems would lead to increased patient compliance with a particular therapy as well. However, excessive literature has been reported based on implanted US responsive hydrogel in various fields, but there is no comprehensive review article showing the strategies to control drug delivery profile. So, this review was aimed at discussing the current strategies for controlling and targeting drug delivery profiles using implantable hydrogel systems.