Gel phantom for use in high-intensity focused ultrasound dosimetry

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.

A high magnetic resonance imaging (MRI) contrast agar/silica-based phantom for evaluating focused ultrasound (FUS) protocols

PURPOSE

Thermal ablation therapies require tissue mimicking phantoms for evaluating novel systems. Herein, an agar phantom exhibiting high magnetic resonance imaging (MRI) contrast to noise ratio (CNR) was developed for testing focused ultrasound (FUS) protocols.

METHODS

Four agar based phantoms (6 % w/v) were fabricated with varied silica concentrations (0, 2, 4, or 6 % w/v) and subjected to FUS inside a 3 T MRI. T2-Weighted Fast Spin Echo (T2-W FSE) images were acquired after sonications to assess the effect of varied silica on CNR of inflicted lesions. The highest CNR phantom was sonicated and its proton resonance frequency (PRF) coefficient, thermal dose denaturation threshold and ability to sustain good lesion CNR 0-44 min post exposures were assessed.

RESULTS

T2-W median lesion CNR between 1.5-453.5 was observed, exponentially increasing with increased silica concentration. High CNR was achieved with 4 % w/v silica, with the PRF coefficient of the phantom calculated at -0.00954 ppm/°C. The thermal dose denaturation threshold was revealed at 2 × 106 CEM43°C by comparing thermal dose maps with T2-W FSE lesion hyperenhancement. Progressive lesion CNR loss was observed, with CNR lost 28 min after sonications.

CONCLUSIONS

The proposed phantom possesses excellent T2-W contrast of inflicted lesions while exhibiting a tissue like PRF coefficient and can thus constitute an inexpensive reusable tool for validating FUS systems and protocols.

Spherical lesion formation in HIFU using robotic assistance for controlled focal point manipulation

We propose a robot-assisted method to generate spherical thermal lesions by high-intensity focused ultrasound (HIFU) ablation. Typically, HIFU-induced thermal lesions are cigar-shaped because the acoustic field in the focal area has a similar elongated shape. We assumed that HIFU irradiation with a certain range and pattern of motion could influence the heat transfer within the target area, allowing control over the lesion's shape and size. Based on the simulations of various motion models likely to induce a change of heat transfer pattern around the target area, we identified Gaussian random motion (GRM) as the optimal motion model for generating spherical thermal lesions. In the experiment, the GRM model was tested using a robotic arm on bovine serum albumin (BSA) gels. The transparency of the BSA gels allowed us to record the temporal changes in thermal lesions and compare them with those produced by conventional fixed-focus HIFU ablation. The experimental results showed that the average sphericity of the thermal lesions generated by the GRM model was 0.85, indicating that this method can produce nearly spherical lesions. In contrast, fixed-focus HIFU ablation resulted in elongated lesions with a sphericity of 0.33. As a result, focal motion using the GRM model enables HIFU ablation to generate spherical thermal lesions. We expect this approach to be applicable for non-invasive HIFU treatments of small, spherical tumors that can be detected at an early stage.

Revealing physical interactions of ultrasound waves with the body through photoelasticity imaging

Ultrasound is a ubiquitous technology in medicine for screening, diagnosis, and treatment of disease. The functionality and efficacy of different ultrasound modes relies strongly on our understanding of the physical interactions between ultrasound waves and biological tissue structures. This article reviews the use of photoelasticity imaging for investigating ultrasound fields and interactions. Physical interactions are described for different ultrasound technologies, including those using linear and nonlinear ultrasound waves, as well as shock waves. The use of optical modulation of light by ultrasound is presented for shadowgraphic and photoelastic techniques. Investigations into shock wave and burst wave lithotripsy using photoelastic methods are summarized, along with other endoscopic forms of lithotripsy. Photoelasticity in soft tissue surrogate materials is reviewed, and its deployment in investigating tissue-bubble interactions, generated ultrasound waves, and traumatic brain injury, are discussed. With the continued growth of medical ultrasound, photoelasticity imaging can play a role in elucidating the physical mechanisms leading to useful bioeffects of ultrasound for imaging and therapy.

Estimation of the Proton Resonance Frequency Coefficient in Agar-based Phantoms

AIM

Agar-based phantoms are popular in high intensity focused ultrasound (HIFU) studies, with magnetic resonance imaging (MRI) preferred for guidance since it provides temperature monitoring by proton resonance frequency (PRF) shift magnetic resonance (MR) thermometry. MR thermometry monitoring depends on several factors, thus, herein, the PRF coefficient of agar phantoms was estimated.

MATERIALS AND METHODS

Seven phantoms were developed with varied agar (2, 4, or 6% w/v) or constant agar (6% w/v) and varied silica concentrations (2, 4, 6, or 8% w/v) to assess the effect of the concentration on the PRF coefficient. Each phantom was sonicated using varied acoustical power for a 30 s duration in both a laboratory setting and inside a 3T MRI scanner. PRF coefficients were estimated through linear trends between phase shift acquired using gradient sequences and thermocouple-based temperatures changes.

RESULTS

Linear regression (R 2 = 0.9707-0.9991) demonstrated a proportional dependency of phase shift with temperature change, resulting in PRF coefficients between -0.00336 ± 0.00029 and -0.00934 ± 0.00050 ppm/°C for the various phantom recipes. Weak negative linear correlations of the PRF coefficient were observed with increased agar. With silica concentrations, the negative linear correlation was strong. For all phantoms, calibrated PRF coefficients resulted in 1.01-3.01-fold higher temperature changes compared to the values calculated using a literature PRF coefficient.

CONCLUSIONS

Phantoms developed with a 6% w/v agar concentration and doped with 0%-8% w/v silica best resemble tissue PRF coefficients and should be preferred in HIFU studies. The estimated PRF coefficients can result in enhanced MR thermometry monitoring and evaluation of HIFU protocols.

Optimizing targeting strategies for lithotripsy through in-vitro and in vivo studies with consideration of respiratory regularity

BACKGROUND

This work aimed to identify a method to achieve improved stone targeting and safety in shockwave lithotripsy by accounting for respiration.

METHODS

We set up an electromotive device simulating renal movement during respiration to place artificial stones within the phantom gel, measuring stone weight changes before and after shockwave exposure and the cavitation damage. We conducted clinical trials using respiratory masks and sensors to monitor and analyze patient respiration during shockwave lithotripsy.

RESULTS

The in vitro efficiency of lithotripsy was higher when adjusted for respiration than when respiration was not adjusted for. Slow respiration showed the best efficiency with higher hit rates when not adjusted for respiration. Cavitation damage was also lowest during slow respiration. The clinical study included 52 patients. Respiratory regularity was maintained above 90% in regular respiration. When respiration was regular, the lithotripsy rate was about 65.6%, which stayed at about 40% when respiration was irregular. During the lithotripsy, the participants experienced various events, such as sleep, taking off their masks, talking, movement, coughing, pain, nervousness, and hyperventilation. The generation of shockwaves based on respiratory regularity could reduce pain in patients.

CONCLUSION

These results suggest a more accurate lithotripsy should be performed according to respiratory regularity.

Tumor phantom model for MRI-guided focused ultrasound ablation studies

BACKGROUND

The persistent development of focused ultrasound (FUS) thermal therapy in the context of oncology creates the need for tissue-mimicking tumor phantom models for early-stage experimentation and evaluation of relevant systems and protocols.

PURPOSE

This study presents the development and evaluation of a tumor-bearing tissue phantom model for testing magnetic resonance imaging (MRI)-guided FUS (MRgFUS) ablation protocols and equipment based on MR thermometry.

METHODS

Normal tissue was mimicked by a pure agar gel, while the tumor simulator was differentiated from the surrounding material by including silicon dioxide. The phantom was characterized in terms of acoustic, thermal, and MRI properties. US, MRI, and computed tomography (CT) images of the phantom were acquired to assess the contrast between the two compartments. The phantom's response to thermal heating was investigated by performing high power sonications with a 2.4 MHz single element spherically focused ultrasonic transducer in a 3T MRI scanner.

RESULTS

The estimated phantom properties fall within the range of literature-reported values of soft tissues. The inclusion of silicon dioxide in the tumor material offered excellent tumor visualization in US, MRI, and CT. MR thermometry revealed temperature elevations in the phantom to ablation levels and clear evidence of larger heat accumulation within the tumor owing to the inclusion of silicon dioxide.

CONCLUSION

Overall, the study findings suggest that the proposed tumor phantom model constitutes a simple and inexpensive tool for preclinical MRgFUS ablation studies, and potentially other image-guided thermal ablation applications upon minimal modifications.

Enhancement of Boiling Histotripsy by Steering the Focus Axially During the Pulse Delivery

Boiling histotripsy (BH) is a pulsed high-intensity focused ultrasound (HIFU) method relying on the generation of high-amplitude shocks at the focus, localized enhanced shock-wave heating, and bubble activity driven by shocks to induce tissue liquefaction. BH uses sequences of 1-20 ms long pulses with shock fronts of over 60 MPa amplitude, initiates boiling at the focus of the HIFU transducer within each pulse, and the remainder shocks of the pulse then interact with the boiling vapor cavities. One effect of this interaction is the creation of a prefocal bubble cloud due to reflection of shocks from the initially generated mm-sized cavities: the shocks are inverted when reflected from a pressure-release cavity wall resulting in sufficient negative pressure to reach intrinsic cavitation threshold in front of the cavity. Secondary clouds then form due to shock-wave scattering from the first one. Formation of such prefocal bubble clouds has been known as one of the mechanisms of tissue liquefaction in BH. Here, a methodology is proposed to enlarge the axial dimension of this bubble cloud by steering the HIFU focus toward the transducer after the initiation of boiling until the end of each BH pulse and thus to accelerate treatment. A BH system comprising a 1.5 MHz 256-element phased array connected to a Verasonics V1 system was used. High-speed photography of BH sonications in transparent gels was performed to observe the extension of the bubble cloud resulting from shock reflections and scattering. Volumetric BH lesions were then generated in ex vivo tissue using the proposed approach. Results showed up to almost threefold increase of the tissue ablation rate with axial focus steering during the BH pulse delivery compared to standard BH.

Dual-frequency boiling histotripsy in an ex vivo bovine tendinopathy model

Histotripsy fractionates most soft tissues; however, healthy tendons have shown resistance to histotripsy fractionation. Prior work has shown that pre-heating tendons increases susceptibility to histotripsy fractionation; combining multiple driving frequencies may also allow successful fractionation of tendons. Here, we evaluate single- and dual-frequency histotripsy in four healthy and eight tendinopathic ex vivo bovine tendons. First, we evaluated single-frequency (1.07, 1.5, and 3.68 MHz) and dual-frequency (1.07 and 1.5 MHz or 1.5 and 3.68 MHz) bubble dynamics with high-speed photography in a tissue-mimicking phantom. Then, tendons were treated with histotripsy. Cavitation activity was monitored with a passive cavitation detector (PCD) and targeted areas were evaluated grossly and histologically. Results in tendinopathic tendons showed 1.5 MHz or 3.68 MHz single-frequency exposure caused focal disruption, whereas 1.5 and 3.68 MHz dual-frequency exposures caused fractionated holes; all treatments caused some thermal denaturation. Exposure to 1.07 MHz alone or combined with 1.5 MHz did not show fractionation in tendinopathic tendons. In healthy tendons, only thermal necrosis was observed for all tested exposures. PCD showed some differences in cavitation activity in tendinopathic tendons but did not predict successful fractionation. These results suggest that full histotripsy fractionation is possible using dual-frequency exposures in tendinopathic tendons.

Noninvasive mechanical destruction of liver tissue and tissue decellularisation by pressure-modulated shockwave histotripsy

Introduction

Boiling histotripsy (BH) is a promising High Intensity Focused Ultrasound (HIFU) technique that can be used to mechanically fractionate solid tumours at the HIFU focus noninvasively, promoting tumour immunity. Because of the occurrence of shock scattering phenomenon during BH process, the treatment accuracy of BH is, however, somewhat limited. To induce more localised and selective tissue destruction, the concept of pressure modulation has recently been proposed in our previous in vitro tissue phantom study. The aim of the present study was therefore to investigate whether this newly developed histotripsy approach termed pressure-modulated shockwave histotripsy (PSH) can be used to induce localised mechanical tissue fractionation in in vivo animal model.

Methods

In the present study, 8 Sprague Dawley rats underwent the PSH treatment and were sacrificed immediately after the exposure for morphological and histological analyses (paraffin embedded liver tissue sections were stained with H&E and MT). Partially exteriorised rat's left lateral liver lobe in vivo was exposed to a 2.0 MHz HIFU transducer with peak positive (P ₊) and negative (P _-) pressures of 89.1 MPa and -14.6 MPa, a pulse length of 5 to 34 ms, a pressure modulation time at 4 ms where P ₊ and P _- decreased to 29.9 MPa and - 9.6 MPa, a pulse repetition frequency of 1 Hz, a duty cycle of 1% and number of pulses of 1 to 20. Three lesions were produced on each animal. For comparison, the same exposure condition but no pressure modulation was also employed to create a number of lesions in the liver.

Results and Discussion

Experimental results showed that a partial mechanical destruction of liver tissue in the form of an oval in the absence of thermal damage was clearly observed at the HIFU focus after the PSH exposure. With a single pulse length of 7 ms, a PSH lesion created in the liver was measured to be a length of 1.04 ± 0.04 mm and a width of 0.87 ± 0.21 mm which was 2.37 times in length (p = 0.027) and 1.35 times in width (p = 0.1295) smaller than a lesion produced by no pressure modulation approach (e.g., BH). It was also observed that the length of a PSH lesion gradually grew towards the opposite direction to the HIFU source along the axial direction with the PSH pulse length, eventually leading to the generation of an elongated lesion in the liver. In addition, our experimental results demonstrated the feasibility of inducing partial decellularisation effect where liver tissue was partially destructed with intact extracellular matrix (i.e., intact fibrillar collagen) with the shortest PSH pulse length. Taken together, these results suggest that PSH could be used to induce a highly localised tissue fractionation with a desired degree of mechanical damage from complete tissue fractionation to tissue decellularisation through controlling the dynamics of boiling bubbles without inducing the shock scattering effect.

Modeling for Quantitative Analysis of Nakagami Imaging in Accurate Detection and Monitoring of Therapeutic Lesions by High-Intensity Focused Ultrasound

OBJECTIVE

Nakagami imaging is an appealing monitoring and evaluation technique for high-intensity focused ultrasound treatment when bubbles are present in ultrasound images. This study aimed to investigate the accuracy of thermal lesion detection using Nakagami imaging.

METHODS

Simulations were conducted to explore and quantify the influence of the bubbles and the subresolvable effect at the boundary of the thermal lesion on thermal lesion detection. The thermal ablation experiments were conducted in phantom and porcine liver ex vivo.

RESULTS

In the simulation, the estimated lateral and axial size of the thermal lesion in the Nakagami image was 4.91 and 4.79 mm, close to the actual size (5 × 5 mm). The simulation results indicated that the subresolvable region in high-intensity focused ultrasound treatment thermal ablation mainly happened at the boundary between bubbles and the untreated region and does not affect the accuracy of thermal lesion detection. The accurate detection of the thermal lesion using Nakagami imaging mainly depends on bubbles and thermal lesion characterization. Our thermal ablation experiments confirmed that Nakagami imaging has the ability to accurately identify thermal lesions from bubbles.

CONCLUSION

The subresolvable effect is helpful for thermal lesion identification, and precision is related to the Nakagami values chosen for boundary division in Nakagami imaging. Therefore, Nakagami imaging is a promising method for accurately evaluating thermal lesions. Further studies in vivo and in clinical settings will be needed to explore its potential applications.

Histotripsy Bubble Dynamics in Elastic, Anisotropic Tissue-Mimicking Phantoms

Elastic, anisotropic tissue such as tendon has proven resistant to mechanical fractionation by histotripsy, a subset of focused ultrasound that uses the creation, oscillation and collapse of cavitation bubbles to fractionate tissue. Our objective was to fabricate an optically transparent hydrogel that mimics tendon for evaluation of histotripsy bubble dynamics. Ex vivo bovine deep digital flexor tendons were obtained (n = 4), and varying formulations of polyacrylamide (PA), collagen and fibrin hydrogels (n = 3 each) were fabricated. Axial sound speeds were measured at 1 MHz for calculation of anisotropy. All samples were treated with a 1.5-MHz focused ultrasound transducer with 10-ms pulses repeated at 1 Hz (p+ = 127 MPa, p- = 35 MPa); treatments were monitored with passive cavitation imaging and high-speed photography. Dehydrated fibrin gels were found to be the most similar to tendon in cavitation emission energy (fibrin = 0.69 ± 0.24, tendon = 0.64 ± 0.19 [× 1010 V2]) and anisotropy (fibrin = 3.16 ± 1.12, tendon = 19.4). Bubble cloud area in dehydrated fibrin (0.79 ± 0.14 mm2) was significantly smaller than most other tested hydrogels. Finally, anisotropy was found to have moderately strong linear relationships with cavitation energy and bubble cloud size (r = -0.65 and -0.80, respectively). Dehydrated fibrin shows potential as a repeatable, transparent, tissue-mimicking hydrogel for evaluation of histotripsy bubble dynamics in elastic, anisotropic tissues.

A Cost-Effective Reusable Tissue Mimicking Phantom for High Intensity Focused Ultrasonic Liver Surgery

A polyacrylamide polysaccharide hydrogel (PASG) containing a nonionic surfactant of the polyoxyethylene nonylphenyl ethers series (NP14) has been adapted to the fabrication of a reusable cost-effective ultrasonic tissue-mimicking phantom for real-time visualization of the thermal lesions by high intensity focused ultrasound (HIFU) irradiation. The constructed NP14 (40% in w/v) PASG is optically transparent at room temperatures, and it turns out to be opaque white as heated over the clouding points of about 55 °C and returns to its original transparent state after cooling. The acoustic property of the proposed phantom is similar to those of human liver tissues, which includes the acoustic impedance of 1.68 Mrayls, the speed of sound of 1595 ± 5 m/s, the attenuation coefficient of 0.52 ± 0.05 dB cm^-1 (at 1 MHz), the backscatter coefficient of 0.21 ± 0.09 × 10^-3 sr^-1 cm^-1 (at 1 MHz), and the nonlinear parameter B/A of 6.4 ± 0.2. The NP14-PASG was tested to assess the characteristic information (sizes, shapes, and locations) of the thermal lesions visualized when exposed to typical HIFU fields (1.1 MHz, focal pressure up to 20.1 MPa, focal intensity 4075 W/cm²). The proposed NP14-PASG is expected to replace the existing costly BSA-PASG used for more effective testing of the performance of therapeutic ultrasonic devices based on thermal mechanisms.

The effects of elastic modulus and impurities on bubble nuclei available for acoustic cavitation in polyacrylamide hydrogels

Safety of biomedical ultrasound largely depends on controlling cavitation bubbles in vivo, yet bubble nuclei in biological tissues remain unexplored compared to water. This study evaluates the effects of elastic modulus (E) and impurities on bubble nuclei available for cavitation in tissue-mimicking polyacrylamide (PA) hydrogels. A 1.5 MHz focused ultrasound transducer with f# = 0.7 was used to induce cavitation in 17.5%, 20%, and 22.5% v/v PA hydrogels using 10-ms pulses with pressures up to peak negative pressure (p-) = 35 MPa. Cavitation was monitored at 0.075 ms through high-speed photography at 40 000 fps. At p- = 29 MPa for all hydrogels, cavitation occurred at random locations within the -6 dB focal area [9.4 × 1.2 mm (p-)]. Increasing p- to 35 MPa increased bubble location consistency and caused shock scattering in the E = 282 MPa hydrogels; as the E increased to 300 MPa, bubble location consistency decreased (p = 0.045). Adding calcium phosphate or cholesterol at 0.25% w/v or bovine serum albumin at 5% or 10% w/v in separate 17.5% PA as impurities decreased the cavitation threshold from p- = 13.2 MPa for unaltered PA to p- = 11.6 MPa, p- = 7.3 MPa, p- = 9.7 MPa, and p- = 7.5 MPa, respectively. These results suggest that both E and impurities affect the bubble nuclei available for cavitation in tissue-mimicking hydrogels.

Lead-Free HIFU Transducers

High-intensity focused ultrasound transducers operating at 4 MHz based on lead-free piezoceramics from the sodium bismuth titanate (NBT) family are described. First, the piezoelectric material (Pz12X) is evaluated from the standpoint of transducer design and its important characteristics, including temperature dependance of several parameters such as dielectric and mechanical coefficients. Then, the performance of six transducers of the same design is evaluated in terms of electro-acoustic efficiency and its dependency on the operating acoustic power level up to 30 W. Overall, the initial electro-acoustic efficiency of three independent transducers is approximately 50% at low acoustic power levels and slightly drops down to 42% as the input electric power reaches 10 W. This process is stable and fully reversible. Moreover, the stability of electro-acoustic efficiency over extended power burst cycling is studied using another two transducers up to 95 × 10³ power bursts of 250-ms duration and acoustic power of 10 W. This protocol is beyond the typical clinical use of similar devices in practice. No significant changes in electro-acoustic performance are noted. Additionally, the input electric power and the output acoustic power, together with the temperature of the piezoelectric component, are evaluated simultaneously over the period of one power burst. It is found that the maximum operating temperature over a high-input electric power burst of 600 J is below 60°C, which defines the operational limit for such devices, as the de-poling temperature of the lead-free material is around 85°C. It is found that the lead-free material from the NBT family is also a promising alternative to lead-based PZT-type materials in high-power therapeutic ultrasound.

Thermochromic Tissue-Mimicking Phantoms for Thermal Ablation Based on Polyacrylamide Gel

In recent years, thermal ablation has played an increasingly important role in treating various tumors in the clinic. A practical thermochromic phantom model can provide a favorable platform for clinical thermotherapy training of young physicians or calibration and optimization of thermal devices without risk to animals or human participants. To date, many tissue-mimicking thermal phantoms have been developed and are well liked, especially the polyacrylamide gel (PAG)-based phantoms. This review summarizes the PAG-based phantoms in the field of thermotherapy, details their advantages and disadvantages and provides a direction for further optimization. The relevant physical parameters (such as electrical, acoustic, and thermal properties) of these phantoms are also presented in this review, which can assist operators in a deeper understanding of these phantoms and selection of the proper recipes for phantom fabrication.

Strongly Focused HIFU Transducers With Simultaneous Optical Observation for Treatment of Skin at 20 MHz

High-intensity focused ultrasound (HIFU) transducers are proposed as a new treatment modality for dermatology. The shape and size of pressure fields generated by strongly focused transducers with an f-number equal to 0.75 operating at frequencies up to 20 MHz are analyzed analytically using the Lucas-Muir model and numerically with the wide-angle Khokhlov-Zabolotskaya-Kuznetsov method. The modeling results under quasi-linear conditions are compared against measurements performed in an acoustic tank with the aid of a fiberoptic hydrophone. The size of the focal zone expressed by their depth of focus and focal diameter is found to be directly controlled by their operating frequency and f-number. Devices manufactured for an operating frequency of 20 MHz are characterized by their 6 dB depth of focus of 490 μm and focal diameter of 80.6 µm. The devices are further studied at high power levels using a polyacrylic tissue-mimicking phantom. The devices are equipped with an optical observation system allowing simultaneous treatment and observation of the skin surface. In comparison to conventional cosmetic applications of HIFU, the devices analyzed are concluded to be ideal for treatment of precisely selected and confined layers of the human skin.

EGFR-Targeted Perfluorohexane Nanodroplets for Molecular Ultrasound Imaging

Perfluorocarbon nanodroplets offer an alternative to gaseous microbubbles as contrast agents for ultrasound imaging. They can be acoustically activated to induce a liquid-to-gas phase transition and provide contrast in ultrasound images. In this study, we demonstrate a new strategy to synthesize antibody-conjugated perfluorohexane nanodroplet (PFHnD-Ab) ultrasound contrast agents that target cells overexpressing the epidermal growth factor receptor (EGFR). The perfluorohexane nanodroplets (PFHnD) containing a lipophilic DiD fluorescent dye were synthesized using a phospholipid shell. Antibodies were conjugated to the surface through a hydrazide-aldehyde reaction. Cellular binding was confirmed using fluorescence microscopy; the DiD fluorescence signal of the PFHnD-Ab was 5.63× and 6× greater than the fluorescence signal in the case of non-targeted PFHnDs and the EGFR blocking control, respectively. Cells were imaged in tissue-mimicking phantoms using a custom ultrasound imaging setup consisting of a high-intensity focused ultrasound transducer and linear array imaging transducer. Cells with conjugated PFHnD-Abs exhibited a significantly higher (p < 0.001) increase in ultrasound amplitude compared to cells with non-targeted PFHnDs and cells exposed to free antibody before the addition of PFHnD-Abs. The developed nanodroplets show potential to augment the use of ultrasound in molecular imaging cancer diagnostics.

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

The thermal and mechanical effects induced in tissue by ultrasound can be exploited for therapeutic applications. Tissue-mimicking materials (TMMs), reflecting different soft tissue properties, are required for experimental evaluation of therapeutic potential. In the study described here, poly(vinyl alcohol) (PVA) hydrogels were characterized. Hydrogels prepared using different concentrations (5%-20% w/w) and molecular weights of PVA ± cellulose scatterers (2.5%-10% w/w) were characterized acoustically (sound speed, attenuation) as a function of temperature (25°C-45°C), thermally (thermal conductivity, specific heat capacity) and in terms of their cavitation thresholds. Results were compared with measurements in fresh sheep tissue (kidney, liver, spleen). Sound speed depended most strongly on PVA concentration, and attenuation, on cellulose content. For the range of formulations investigated, the PVA gel acoustic properties (sound speed: 1532 ± 17 to 1590 ± 9 m/s, attenuation coefficient: 0.08 ± 0.01 to 0.37 ± 0.02 dB/cm) fell within those measured in fresh tissue. Cavitation thresholds for 10% PVA hydrogels (50% occurrence: 4.1-5.4 MPa, 75% occurrence: 5.4-8.2 MPa) decreased with increasing cellulose content. In summary, PVA cellulose composite hydrogels may be suitable mimics of acoustic, cavitation and thermal properties of soft tissue for a number of therapeutic ultrasound applications.

MR relaxation times of agar-based tissue-mimicking phantoms

Agar gels were previously proven capable of accurately replicating the acoustical and thermal properties of real tissue and widely used for the construction of tissue-mimicking phantoms (TMPs) for focused ultrasound (FUS) applications. Given the current popularity of magnetic resonance-guided FUS (MRgFUS), we have investigated the MR relaxation times T1 and T2 of different mixtures of agar-based phantoms. Nine TMPs were constructed containing agar as the gelling agent and various concentrations of silicon dioxide and evaporated milk. An agar-based phantom doped with wood powder was also evaluated. A series of MR images were acquired in a 1.5 T scanner for T1 and T2 mapping. T2 was predominantly affected by varying agar concentrations. A trend toward decreasing T1 with an increasing concentration of evaporated milk was observed. The addition of silicon dioxide decreased both relaxation times of pure agar gels. The proposed phantoms have great potential for use with the continuously emerging MRgFUS technology. The MR relaxation times of several body tissues can be mimicked by adjusting the concentration of ingredients, thus enabling more accurate and realistic MRgFUS studies.

Robust and durable aberrative and absorptive phantom for therapeutic ultrasound applications

Phase aberration induced by soft tissue inhomogeneities often complicates high-intensity focused ultrasound (HIFU) therapies by distorting the field and, previously, we designed and fabricated a bilayer gel phantom to reproducibly mimic that effect. A surface pattern containing size scales relevant to inhomogeneities of a porcine body wall was introduced between gel materials with fat- and muscle-like acoustic properties-ballistic and polyvinyl alcohol gels. Here, the phantom design was refined to achieve relevant values of ultrasound absorption and scattering and make it more robust, facilitating frequent handling and use in various experimental arrangements. The fidelity of the interfacial surface of the fabricated phantom to the design was confirmed by three-dimensional ultrasound imaging. The HIFU field distortions-displacement of the focus, enlargement of the focal region, and reduction of focal pressure-produced by the phantom were characterized using hydrophone measurements with a 1.5 MHz 256-element HIFU array and found to be similar to those induced by an ex vivo porcine body wall. A phase correction approach was used to mitigate the aberration effect on nonlinear focal waveforms and enable boiling histotripsy treatments through the phantom or body wall. The refined phantom represents a practical tool to explore HIFU therapy systems capabilities.

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

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

Development and characterization of polyurethane-based tissue and blood mimicking materials for high intensity therapeutic ultrasound

A polyurethane-based tissue mimicking material (TMM) and blood mimicking material (BMM) for the acoustic and thermal characterization of high intensity therapeutic ultrasound (HITU) devices has been developed. Urethane powder and other chemicals were dispersed into either a high temperature hydrogel matrix (gellan gum) or degassed water to form the TMM and BMM, respectively. The ultrasonic properties of both TMM and BMM, including attenuation coefficient, speed of sound, acoustical impedance, and backscatter coefficient, were characterized at room temperature. The thermal conductivity and diffusivity, BMM viscosity, and TMM Young's modulus were also measured. Importantly, the attenuation coefficient has a nearly linear frequency dependence, as is the case for most soft tissues and blood at 37 °C. Their mean values are 0.61f^1.2dB cm^-1 (TMM) and 0.2f^1.1dB cm^-1 (BMM) based on measurements from 1 to 8 MHz using a time delay spectrometry (TDS) system. Most of the other relevant physical parameters are also close to the reported values of soft tissues and blood. These polyurethane-based TMM and BMM are appropriate for developing standardized dosimetry techniques, validating numerical models, and determining the safety and efficacy of HITU devices.

Numerical Study of Bubble Cloud and Thermal Lesion Evolution During Acoustic Droplet Vaporization Enhanced HIFU Treatment

Acoustic droplet vaporization (ADV) has been proven to enhance high intensity focused ultrasound (HIFU) thermal ablation of tumor. It has also been demonstrated that triggering droplets before HIFU exposure could be a potential way to control both the size and the shape of the thermal lesion. In this paper, a numerical model is proposed to predict the thermal lesion created in ADV enhanced HIFU treatment. Bubble oscillation was coupled into a viscoelastic medium in the model to more closely represent real applications in tissues. Several physical processes caused by continuous wave ultrasound and elevated temperature during the HIFU exposure were considered, including rectified diffusion, gas solubility variation with temperature in the medium, and boiling. Four droplet concentrations spanning two orders of magnitude were calculated. The bubble cloud formed from triggering of the droplets by the pulse wave ultrasound, along with the evolution of the shape and location of the bubble cloud and thermal lesion during the following continuous wave exposure was obtained. The increase of bubble void fraction caused by continuous wave exposure was found to be consistent with the experimental observation. With the increase of droplet concentration, the predicted bubble cloud shapes vary from tadpole to triangular and double triangular, while the thermal lesions move toward the transducer. The results show that the assumptions used in this model increased the accuracy of the results. This model may be used for parametrical study of ADV enhanced HIFU treatment and be further used for treatment planning and optimization in the future.

Effect of Perfluorocarbon Composition on Activation of Phase-Changing Ultrasound Contrast Agents

Background

While microbubble contrast agents (MCAs) are commonly used in ultrasound (US), they are inherently limited to vascular targets due to their size. Alternatively, phase‐changing nanodroplet contrast agents (PNCAs) can be delivered as nanoscale agents (i.e., small enough to extravasate), but when exposed to a US field of sufficient mechanical index (MI), they convert to MCAs, which can be visualized with high contrast using nonlinear US.

Purpose

To investigate the effect of perfluorocarbon (PFC) core composition and presence of cholesterol in particle coatings on stability and image contrast generated from acoustic activation of PNCAs using high‐frequency US suitable for clinical imaging.

Methods

PNCAs with varied core compositions (i.e., mixtures of perfluoropentane [C5] and/or perfluorohexane [C6]) and two coating formulations (i.e., with and without cholesterol) were characterized and investigated for thermal/temporal stability and postactivation, nonlinear US contrast in phantom and in vivo environments. Through hydrophone measurements and nonlinear numerical modeling, MI was estimated for pulse sequences used for PNCA activation.

Results

All PNCA compositions were characterized to have similar diameters (249–267 nm) and polydispersity (0.151–0.185) following fabrication. While PNCAs with majority C5 core composition showed higher levels of spontaneous signal (i.e., not due to US activation) in phantoms than C6‐majority PNCAs, all compositions were stable during imaging experiments. When activating PNCAs with a 12.3‐MHz US pulse (MI = 1.1), C6‐core particles with cholesterol‐free coatings (i.e., CF‐C6‐100 particles) generated a median contrast of 3.1, which was significantly higher (p < 0.001) than other formulations. Further, CF‐C6‐100 particles were activated in a murine model, generating US contrast ≥3.4.

Conclusion

C6‐core PNCAs can provide high‐contrast US imaging with minimal nonspecific activation in phantom and in vivo environments.

A robotic magnetic resonance-guided high-intensity focused ultrasound platform for neonatal neurosurgery: Assessment of targeting accuracy and precision in a brain phantom

BACKGROUND

Intraventricular Hemorrhage (IVH) is one of the most serious neurovascular complications resulting from premature birth. It can result in clotting of blood within the ventricles, which causes a buildup of cerebrospinal fluid that can lead to posthemorrhagic ventricular dilation and posthemorrhagic hydrocephalus. Currently, there are no direct treatments for these blood clots as the standard of care is invasive surgery to insert a shunt. Magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU) has been investigated as a non-invasive treatment to lyse blood clots. However, current MRgHIFU systems are not suitable in the context of treating IVH in neonates.

PURPOSE

We have developed a robotic MRgHIFU neurosurgical platform designed to treat the neonatal brain. This platform facilitates ergonomic patient positioning and directs treatment through their open anterior fontanelle while providing a larger treatment volume. The platform is based on an MR-compatible robot developed by our group. Further development of the platform has warranted investigation of its targeting ability to assess its feasibility in the neonatal brain. This study aimed to quantify the platform's targeting accuracy, precision, and repeatability using a brain phantom and clinical MRI system.

METHODS

A thermosensitive brain-mimicking phantom was developed to test the platform's targeting accuracy. Rectangular grid patterns were created with HIFU thermal energy "lesions" in the phantoms by targeting specific coordinate points. The intended target locations were demarcated by inserting carbon fibre rods through a targeting assessment template. Coordinates for the intended and actual targets were derived from T2-weighted MRI scans and the centroid distance between them was measured. Subsequently, the platform's targeting accuracy was quantified according to equations derived from ISO Standard 9283:1998.

RESULTS

HIFU ablation resulted in distinct thermal lesions within the thermosensitive phantoms, which appeared as discrete hypointense regions in T2-weighted MR scans. A total of 127 target points were included in the data analysis, which yielded a targeting accuracy of 0.6mm and targeting precision of 1.2mm.

CONCLUSIONS

The robotic MRgHIFU platform was shown to have a high degree of accuracy, precision, and repeatability. The results demonstrate the platform's functionality when targeting through simulated brain matter. These results serve as an initial verification of the platform targeting ability and showed promise towards the final application in a neonatal brain. This article is protected by copyright. All rights reserved.

MR relaxation properties of tissue-mimicking phantoms

High quality tissue-mimicking phantoms (TMPs) have a critical role in the preclinical testing of emerging modalities for diagnosis and therapy. TMPs capable of accurately mimicking real tissue in Magnetic Resonance guided Focused Ultrasound (MRgFUS) applications should be fabricated with precise T1 and T2 relaxation times. Given the current popularity of the MRgFUS technology, we herein performed a systematic review on the MR relaxation properties of different phantoms types. Polyacrylamide (PAA) and agar based phantoms were proven capable of accurately replicating critical thermal, acoustical, and MR relaxation properties of various body tissues. Although gelatin phantoms were also proven factional in this regard, they lack the capacity to withstand ablation temperatures, and thus, are only recommended for hyperthermia applications. Other gelling agents identified in the literature are Poly-vinyl alcohol (PVA), Polyvinyl Chloride (PVC), silicone, and TX-150/ TX-151; however, their efficacy in thermal studies is yet to be established. PAA gels are favorable in that they offer optical transparency enabling direct visualization of coagulative lesions. On the other hand, agar phantoms have lower preparation costs and were proven very promising for use with the MRgFUS technology, without the toxicity issues related to the preparation and storage of PAA materials. Remarkably, agar turned out to be the prominent modifier of the T2 relaxation time even for phantoms containing other types of gelling agents instead of agar. This review could be useful in manufacturing realistic MRgFUS phantoms while simultaneously indicating an opportunity for further research in the field with a particular focus on the MR behavior of agar-based TMPs.

Repeated Acoustic Vaporization of Perfluorohexane Nanodroplets for Contrast-Enhanced Ultrasound Imaging

Superheated perfluorocarbon nanodroplets are emerging ultrasound imaging contrast agents that boast biocompatible components, unique phase-change dynamics, and therapeutic loading capabilities. Upon exposure to a sufficiently high-intensity pulse of acoustic energy, the nanodroplet's perfluorocarbon core undergoes a liquid-to-gas phase change and becomes an echogenic microbubble, providing ultrasound contrast. The controllable activation leads to high-contrast images, while the small size of the nanodroplets promotes longer circulation times and better in vivo stability. One drawback, however, is that the nanodroplets can only be vaporized a single time, limiting their versatility. Recently, we and others have addressed this issue by using a perfluorohexane core, which has a boiling point above body temperature. Thus after vaporization, the microbubbles recondense back into their stable nanodroplet form. Previous work with perfluorohexane nanodroplets relied on optical activation via pulsed laser absorption of an encapsulated dye. This strategy limits the imaging depth and temporal resolution of the method. In this study, we overcome these limitations by demonstrating acoustic droplet vaporization with 1.1-MHz high-intensity focused ultrasound (HIFU). A short-duration, high-amplitude pulse of focused ultrasound provides a sufficiently strong peak negative pressure to initiate vaporization. A custom imaging sequence was developed to enable the synchronization of a HIFU transducer and a linear array imaging transducer. We show a visualization of repeated acoustic activation of perfluorohexane nanodroplets in polyacrylamide tissue-mimicking phantoms. We further demonstrate the detection of hundreds of vaporization events from individual nanodroplets with activation thresholds well below the tissue cavitation limit. Overall, this approach has the potential to result in reliable and repeatable contrast-enhanced ultrasound imaging at clinically relevant depths.

A Reusable Thermochromic Phantom for Testing High Intensity Focused Ultrasound Technologies

High Intensity Focused Ultrasound (HIFU) surgery is a promising technology for the treatment of several pathologies, including cancer. Testing is a fundamental step for verifying treatment efficacy and safety. Ex-vivo tissues represent the most common solution for replicating the properties of human tissues in the HIFU operative scenario. However, they constitute an avoidable waste of resources. Thus, tissue mimicking phantoms have been investigated as a more sustainable and reliable alternative. In this scenario, we proposed a reusable silicone-based thermochromic phantom. It is cost-effective and can be rapidly fabricated. The acoustic, mechanical, and thermal characterization of the phantom are reported. The phantom usability was evaluated with a HIFU robotic platform. 18 different working conditions were tested by varying both sonication power and duration. Temperature and simulated lesions' size were quantified for all testing conditions. An accordance between temperature and lesion dimension trend over time was found. The proposed phantom results a valid alternative to ex-vivo tissues, especially in the early stages of developing novel HIFU treatment paradigms.

Enhanced HIFU Theranostics with Dual-Frequency-Ring Focused Ultrasound and Activatable Perfluoropentane-Loaded Polymer Nanoparticles

High-intensity focused ultrasound (HIFU) has been widely used in tumor ablation in clinical settings. Meanwhile, there is great potential to increase the therapeutic efficiency of temporary cavitation due to enhanced thermal effects and combined mechanical effects from nonlinear vibration and collapse of the microbubbles. In this study, dual-frequency (1.1 and 5 MHz) HIFU was used to produce acoustic droplet vaporization (ADV) microbubbles from activatable perfluoropentane-loaded polymer nanoparticles (PFP@Polymer NPs), which increased the therapeutic outcome of the HIFU and helped realize tumor theranostics with ultrasound contrast imaging. Combined with PFP@Polymer NPs, dual-frequency HIFU changed the shape of the damage lesion and reduced the acoustic intensity threshold of thermal damage significantly, from 216.86 to 62.38 W/cm². It produced a nearly 20 °C temperature increase in half the irradiation time and exhibited a higher tumor inhibition rate (84.5% ± 3.4%) at a low acoustic intensity (1.1 MHz: 23.77 W/cm²; 5 MHz: 0.35 W/cm²) in vitro than the single-frequency HIFU (60.2% ± 11.9%). Moreover, compared with the traditional PFP@BSA NDs, PFP@Polymer NPs showed higher anti-tumor efficacy (81.13% vs. 69.34%; * p < 0.05) and better contrast-enhanced ultrasound (CEUS) imaging ability (gray value of 57.53 vs. 30.67; **** p < 0.0001), probably benefitting from its uniform and stable structure. It showed potential as a highly efficient tumor theranostics approach based on dual-frequency HIFU and activatable PFP@Polymer NPs.

Ultrasonic Attenuation of an Agar, Silicon Dioxide, and Evaporated Milk Gel Phantom

BACKGROUND

It has been demonstrated that agar-based gel phantoms can emulate the acoustic parameters of real tissues and are the most commonly used tissue-mimicking materials for high-intensity focused ultrasound applications. The following study presents ultrasonic attenuation measurements of agar-based phantoms with different concentrations of additives (percent of agar, silicon dioxide and evaporated milk) in an effort of matching the material's acoustic property as close as possible to human tissues.

METHODS

Nine different agar-based phantoms with various amounts of agar, silicon dioxide, and evaporated milk were prepared. Attenuation measurements of the samples were conducted using the through-transmission immersion techniques.

RESULTS

The ultrasonic attenuation coefficient of the agar-based phantoms varied in the range of 0.30-1.49 dB/cm-MHz. The attenuation was found to increase in proportion to the concentration of agar and evaporated milk. Silicon dioxide was found to significantly contribute to the attenuation coefficient up to 4% weight to volume (w/v) concentration.

CONCLUSION

The acoustic attenuation coefficient of agar-based phantoms can be adjusted according to the tissue of interest in the range of animal and human tissues by the proper selection of agar, silicon dioxide, and evaporated milk.

Microbubble-Enhanced Heating: Exploring the Effect of Microbubble Concentration and Pressure Amplitude on High-Intensity Focused Ultrasound Treatments

High-intensity focused ultrasound (HIFU) is a non-invasive tool that can be used for targeted thermal ablation treatments. Currently, HIFU is clinically approved for treatment of uterine fibroids, various cancers, and certain brain applications. However, for brain applications such as essential tremors, HIFU can only be used to treat limited areas confined to the center of the brain because of geometrical limitations (shape of the transducer and skull). A major obstacle to advancing this technology is the inability to treat non-central brain locations without causing damage to the skin and/or skull. Previous research has indicated that cavitation-induced bubbles or microbubble contrast agents can be used to enhance HIFU treatments by increasing ablation regions and shortening acoustic exposures at lower acoustic pressures. However, little research has been done to explore the interplay between microbubble concentration and pressure amplitude on HIFU treatments. We developed an in vitro experimental setup to study lesion formation at three different acoustic pressures and three microbubble concentrations. Real-time ultrasound imaging was integrated to monitor initial microbubble concentration and subsequent behavior during the HIFU treatments. Depending on the pressure used for the HIFU treatment, there was an optimal concentration of microbubbles that led to enhanced heating in the focal area. If the concentration of microbubbles was too high, the treatment was detrimentally affected because of non-linear attenuation by the pre-focal microbubbles. Additionally, the real-time ultrasound imaging provided a reliable method to monitor microbubble activity during the HIFU treatments, which is important for translation to in vivo HIFU applications with microbubbles.

AAPM Task Group 241: A medical physicist's guide to MRI-guided focused ultrasound body systems

Magnetic resonance-guided focused ultrasound (MRgFUS) is a completely non-invasive technology that has been approved by FDA to treat several diseases. This report, prepared by the American Association of Physicist in Medicine (AAPM) Task Group 241, provides background on MRgFUS technology with a focus on clinical body MRgFUS systems. The report addresses the issues of interest to the medical physics community, specific to the body MRgFUS system configuration, and provides recommendations on how to successfully implement and maintain a clinical MRgFUS program. The following sections describe the key features of typical MRgFUS systems and clinical workflow and provide key points and best practices for the medical physicist. Commonly used terms, metrics and physics are defined and sources of uncertainty that affect MRgFUS procedures are described. Finally, safety and quality assurance procedures are explained, the recommended role of the medical physicist in MRgFUS procedures is described, and regulatory requirements for planning clinical trials are detailed. Although this report is limited in scope to clinical body MRgFUS systems that are approved or currently undergoing clinical trials in the United States, much of the material presented is also applicable to systems designed for other applications.

Polyacrylamide hydrogel phantoms for performance evaluation of multispectral photoacoustic imaging systems

As photoacoustic imaging (PAI) begins to mature and undergo clinical translation, there is a need for well-validated, standardized performance test methods to support device development, quality control, and regulatory evaluation. Despite recent progress, current PAI phantoms may not adequately replicate tissue light and sound transport over the full range of optical wavelengths and acoustic frequencies employed by reported PAI devices. Here we introduce polyacrylamide (PAA) hydrogel as a candidate material for fabricating stable phantoms with well-characterized optical and acoustic properties that are biologically relevant over a broad range of system design parameters. We evaluated suitability of PAA phantoms for conducting image quality assessment of three PAI systems with substantially different operating parameters including two commercial systems and a custom system. Imaging results indicated that appropriately tuned PAA phantoms are useful tools for assessing and comparing PAI system image quality. These phantoms may also facilitate future standardization of performance test methodology.

A Prototype Therapy System for Boiling Histotripsy in Abdominal Targets Based on a 256-Element Spiral Array

Boiling histotripsy (BH) uses millisecond-long ultrasound (US) pulses with high-amplitude shocks to mechanically fractionate tissue with potential for real-time lesion monitoring by US imaging. For BH treatments of abdominal organs, a high-power multielement phased array system capable of electronic focus steering and aberration correction for body wall inhomogeneities is needed. In this work, a preclinical BH system was built comprising a custom 256-element 1.5-MHz phased array (Imasonic, Besançon, France) with a central opening for mounting an imaging probe. The array was electronically matched to a Verasonics research US system with a 1.2-kW external power source. Driving electronics and software of the system were modified to provide a pulse average acoustic power of 2.2 kW sustained for 10 ms with a 1-2-Hz repetition rate for delivering BH exposures. System performance was characterized by hydrophone measurements in water combined with nonlinear wave simulations based on the Westervelt equation. Fully developed shocks of 100-MPa amplitude are formed at the focus at 275-W acoustic power. Electronic steering capabilities of the array were evaluated for shock-producing conditions to determine power compensation strategies that equalize BH exposures at multiple focal locations across the planned treatment volume. The system was used to produce continuous volumetric BH lesions in ex vivo bovine liver with 1-mm focus spacing, 10-ms pulselength, five pulses/focus, and 1% duty cycle.

Characterization of a soft tissue-mimicking agar/wood powder material for MRgFUS applications

This study describes the development and characterization of an agar-based soft tissue-mimicking material (TMM) doped with wood powder destined for fabricating MRgFUS applications. The main objective of the following work was to investigate the suitability of wood powder as an inexpensive alternative in replacing other added materials that have been suggested in previous studies for controlling the ultrasonic properties of TMMs. The characterization procedure involved a series of experiments designed to estimate the acoustic (attenuation coefficient, absorption coefficient, propagation speed, and impedance), thermal (conductivity, diffusivity, specific heat capacity), and MR properties (T1 and T2 relaxation times) of the wood-powder doped material. The developed TMM (2% w/v agar and 4% w/v wood powder) as expected demonstrated compatibility with MRI scanner following images artifacts evaluation. The acoustic attenuation coefficient of the proposed material was measured over the frequency range of 1.1-3 MHz and found to be nearly proportional to frequency. The measured attenuation coefficient was 0.48 dB/cm at 1 MHz which was well within the range of soft tissue. Temperatures over 37 °C proved to increase marginally the attenuation coefficient. Following the transient thermoelectric method, the acoustic absorption coefficient was estimated at 0.34 dB/cm-MHz. The estimated propagation speed (1487 m/s) was within the range of soft tissue at room temperature, while it significantly increased with higher temperature. The material possessed an acoustic impedance of 1.58 MRayl which was found to be comparable to the corresponding value of muscle tissue. The thermal conductivity of the material was estimated at 0.51 W/m K. The measured relaxation times T1 (844 ms) and T2 (66 ms) were within the range of values found in the literature for soft tissue. The phantom was tested for its suitability for evaluating MRgFUS thermal protocols. High acoustic energy was applied, and temperature change was recorded using thermocouples and MR thermometry. MR thermal maps were acquired using single-shot Echo Planar Imaging (EPI) gradient echo sequence. The TMM matched adequately the acoustic and thermal properties of human tissues and through a series of experiments, it was proven that wood concentration enhances acoustic absorption. Experiments using MR thermometry demonstrated the usefulness of this phantom to evaluate ultrasonic thermal protocols by monitoring peak temperatures in real-time. Thermal lesions formed above a thermal dose were observed in high-resolution MR images and visually in dissections of the proposed TMM.

Influence of High-Intensity Focused Ultrasound (HIFU) Ablation on Arteries: Ex Vivo Studies

High-intensity focused ultrasound (HIFU) has been used to ablate solid tumors and cancers. Because of the hypervascular structure of the tumor and circulating blood inside it, the interaction between the HIFU burst and vessel is a critical issue in the clinical environment. Influences on lesion production and the potential of vessel rupture were investigated in this study for the efficiency and safety of clinical ablation. An extracted porcine artery was embedded in a transparent polyacrylamide gel phantom, with bovine serum albumin (BSA) as an indicator of the thermal lesion, and degassed water was driven through the artery sample. The HIFU focus was aligned to the anterior wall, middle of the artery, and posterior wall. After HIFU ablation, the produced lesion was photographically recorded, and then its size was quantified and compared with that in the gel phantom without artery. In addition, the bubble dynamics (i.e., generation, expansion, motion, and shrinkage of bubbles and their interaction with the artery) were captured using high-speed imaging. It was found that the presence of the artery resulted in a decrease in lesion size in both the axial and lateral directions. The characteristics of the lesion are dependent on the focus alignment. Acoustic and hydrodynamic cavitation play important roles in lesion production and interaction with the artery. Both thermal and mechanical effects were found on the surface of the artery wall after HIFU ablation. However, no vessel rupture was found in this ex vivo study.

Acoustical properties of 3D printed thermoplastics

With focused ultrasound (FUS) gaining popularity as a therapeutic modality for brain diseases, the need for skull phantoms that are suitable for evaluating FUS protocols is increasing. In the current study, the acoustical properties of several three-dimensional (3D) printed thermoplastic samples were evaluated to assess their suitability to mimic human skull and bone accurately. Samples were 3D printed using eight commercially available thermoplastic materials. The acoustic properties of the printed samples, including attenuation coefficient, speed of sound, and acoustic impedance, were investigated using transmission-through and pulse-echo techniques. The ultrasonic attenuation, estimated at a frequency of 1.1 MHz, varied from approximately 7 to 32 dB/cm. The frequency dependence of attenuation was described by a power law in the frequency range of 0.2-3.5 MHz, and the exponential index of frequency was found to vary from 1.30 to 2.24. The longitudinal velocity of 2.7 MHz sound waves was in the range of 1700-3050 m/s. The results demonstrate that thermoplastics could potentially be used for the 3D construction of high-quality skull phantoms.

Focused ultrasound phantom model for blood brain barrier disruption

In this paper a high intensity focused ultrasound (FUS) phantom model was developed, in order to be used in experiments for Blood Brain Barrier (BBB) disruption. The target was to create a phantom model that represents the disruption of the BBB during ultrasound application. An appropriate experimental setup was created bearing a single element transducer with diameter 50 mm and geometric focus 100 mm operating at 0.5 MHz. It included a set of tubes and a connector with multiple 0.4 mm openings, through which a suitable liquid is being circulated with a pump. The lesions were sealed with a thin homogenous layer of wax, preventing a liquid leakage. The system was tested successfully with FUS and a liquid leakage was achieved after FUS application. This set up is the first phantom model that has the potential to be utilized as a cost-effective solution for performing experiments for BBB disruption, without the need of using animal models.

A thermochromic tissue-mimicking phantom model for verification of ablation plans in thermal ablation

Background

Our study aims to develop a novel tissue-mimicking thermochromic with tumor model for visualization of thermal ablation and verification of ablation plans.

Methods

Polyacrylamide gel was mixed with thermochromic ink to produce a phantom model. A phantom model embedded in a tumor model was constructed and used to evaluate the ablation procedure. The phantom models were randomly divided into complete ablation group and incomplete ablation group. The ablation planning of the tumor was on the 3D US and performed on a phantom model. We guide the ablation procedures according to the ablation planning. The results measured in a gross specimen of the phantom model were compared with the expected results in ablation planning.

Results

The color of the model changes from cream white to magenta after heating. The mono-site ablation area is a spheroid after thermal ablation with a size of 3.0×1.8 cm at 60 W, 5 minutes, 3.5×2.5 cm at 60 W, 10 minutes, and 4.0×3.5 cm at 60 W, 15 minutes, respectively. According to the ablation planning, a total of 4 ablation points were needed to retrieve the complete ablation of a 3.0 cm tumor. The complete ablation and incomplete ablation were proved by a gross specimen of the phantom model as we expected.

Conclusions

A novel thermochromic tissue-mimicking phantom model with a spherical tumor model has been designed and developed. The ablation area can be visualized on this phantom model by the permanent color change. This phantom model can assess the ablation planning system's accuracy and train operators for ultrasound-guided thermal ablation.

Bilayer aberration-inducing gel phantom for high intensity focused ultrasound applications

Aberrations induced by soft tissue inhomogeneities often complicate high-intensity focused ultrasound (HIFU) therapies. In this work, a bilayer phantom made from polyvinyl alcohol hydrogel and ballistic gel was built to mimic alternating layers of water-based and lipid tissues characteristic of an abdominal body wall and to reproducibly distort HIFU fields. The density, sound speed, and attenuation coefficient of each material were measured using a homogeneous gel layer. A surface with random topographical features was designed as an interface between gel layers using a 2D Fourier spectrum approach and replicating different spatial scales of tissue inhomogeneities. Distortion of the field of a 256-element 1.5 MHz HIFU array by the phantom was characterized through hydrophone measurements for linear and nonlinear beam focusing and compared to the corresponding distortion induced by an ex vivo porcine body wall of the same thickness. Both spatial shift and widening of the focal lobe were observed, as well as dramatic reduction in focal pressures caused by aberrations. The results suggest that the phantom produced levels of aberration that are similar to a real body wall and can serve as a research tool for studying HIFU effects as well as for developing algorithms for aberration correction.

Viscoelastic characterization of HIFU ablation with shear wave by using K-space analysis combined with model-fitting correction method

The viscoelastic properties of tissues can reflect human physiological and pathological conditions. During and after the high-intensity focused ultrasound (HIFU) treatment, measuring the viscoelasticity of HIFU ablated tissue is important for therapy evaluation. Two-dimensional Fourier transform (2DFT) method has been reported to quantify elasticity and viscosity. However, a deviation is induced by under-sampling in practical application. This work proposes an approach based on the convolution theorem and model fitting to solve the finite spatial data problem. A model using the convolution theorem was constructed, and mean-square error (MSE) was calculated to determine the optimal fitting between the model and experimental data. For validation, HIFU therapeutic experiments were conducted in polyacrylamide-bovine serum (BAS) transparent tissue-mimicking phantoms. This approach was used to quantify the viscoelasticity of HIFU ablation and untreated phantoms. Acoustic-radiation-force (ARF) shear wave was generated by the same HIFU therapeutic transducer, and laser Doppler vibrometer (LDV) was used for the high-resolution measurement of shear wave signals. Results suggest that the shear elasticity and viscosity of untreated phantoms are generally smaller than those of HIFU ablation. Thus, the proposed method may be helpful for HIFU treatment monitoring.

Development and characterization of a tissue mimicking psyllium husk gelatin phantom for ultrasound and magnetic resonance imaging

Purpose: To develop and characterize a tissue-mimicking phantom that enables the direct comparison of magnetic resonance (MR) and ultrasound (US) imaging techniques useful for monitoring high-intensity focused ultrasound (HIFU) treatments. With no additions, gelatin phantoms produce little if any scattering required for US imaging. This study characterizes the MR and US image characteristics as a function of psyllium husk concentration, which was added to increase US scattering.Methods: Gelatin phantoms were constructed with varying concentrations of psyllium husk. The effects of psyllium husk concentration on US B-mode and MR imaging were evaluated at nine different concentrations. T1, T2, and T2* MR maps were acquired. Acoustic properties (attenuation and speed of sound) were measured at frequencies of 0.6, 1.0, 1.8, and 3.0 MHz using a through-transmission technique. Phantom elastic properties were evaluated for both time and temperature dependence.Results: Ultrasound image echogenicity increased with increasing psyllium husk concentration while quality of gradient-recalled echo MR images decreased with increasing concentration. For all phantoms, the measured speed of sound ranged between 1567-1569 m/s and the attenuation ranged between 0.42-0.44 dB/(cm·MHz). Measured T1 ranged from 974-1051 ms. The T2 and T2* values ranged from 97-108 ms and 48-88 ms, respectively, with both showing a decreasing trend with increased psyllium husk concentration. Phantom stiffness, measured using US shear-wave speed measurements, increased with age and decreased with increasing temperature.Conclusions: The presented dual-use tissue-mimicking phantom is easy to manufacture and can be used to compare and evaluate US-guided and MR-guided HIFU imaging protocols.

Histotripsy Liquefaction of Large Hematoma for Intracerebral Hemorrhage Using Millisecond-Length Ultrasound Pulse Groups Combined With Fundamental and Second Harmonic Superposition: A Preliminary Study

Intracerebral hemorrhage is a life-threatening acute cerebrovascular disease characterized by a 30-d mortality rate of 40% and substantial disability for those who survive. The objective of this study is to investigate the feasibility of histotripsy-mediated efficient and fine liquefaction of large-volume hematoma by utilizing a protocol of millisecond-length ultrasound pulse groups combined with fundamental and second harmonic superposition. Experiments were initially performed in an in vitro hematoma phantom, using a two-element confocal-annular array. Results showed that a single ellipsoid shape, histotripsy lesion with major dimensions of 10.8 ± 1.2 mm axially and 4.8 ± 0.2 mm laterally was successfully generated. Controllability of the lesion shape and size could be realized by modulating treatment parameters in single-spot experiments. Large-volume hematomas were efficiently and finely liquefied through multisonications via a treatment strategy under the relatively optimized treatment parameters. Liquefied contents were evacuated and analyzed using a particle sizing system. The size of the lysates for the most part ranged from 4-8 μm, with more than 99% of them being smaller than 10 μm. Experiments were then conducted in an optically transparent tissue phantom to explore the liquefaction mechanisms. The phantom was composed of polyacrylamide hydrogel, embedded with bovine serum albumin (BSA), and a thin phantom layer consisted of red blood cells in the BSA polyacrylamide gel was inlayed in the BSA gel phantom. The related mechanisms, such as the frequent boiling that occurred at multiple positions and the enhanced cavitation, revealed the quick development of the lesion in the phantom and the efficient liquefaction of the clot. These results indicated that the proposed histotripsy approach is feasible for the efficient, precise and fine liquefaction of large-volume hematoma and may be developed as a useful tool for intracerebral hemorrhage treatment.

Ferrogels Ultrasonography for Biomedical Applications

Ferrogels (FG) are magnetic composites that are widely used in the area of biomedical engineering and biosensing. In this work, ferrogels with different concentrations of magnetic nanoparticles (MNPs) were synthesized by the radical polymerization of acrylamide in stabilized aqueous ferrofluid. FG samples were prepared in various shapes that are suitable for different characterization techniques. Thin cylindrical samples were used to simulate the case of targeted drug delivery test through blood vessels. Samples of larger size that were in the shape of cylindrical plates were used for the evaluation of the FG applicability as substitutes for damaged structures, such as bone or cartilage tissues. Regardless of the shape of the samples and the conditions of their location, the boundaries of FG were confidently visualized over the entire range of concentrations of MNPs while using medical ultrasound. The amplitude of the reflected echo signal was higher for the higher concentration of MNPs in the gel. This result was not related to the influence of the MNPs on the intensity of the reflected echo signal directly, since the wavelength of the ultrasonic effect used is much larger than the particle size. Qualitative theoretical model for the understanding of the experimental results was proposed while taking into account the concept that at the acoustic oscillations of the hydrogel, the macromolecular net, and water in the gel porous structure experience the viscous Stocks-like interaction.

Delay multiply and sum beamforming method applied to enhance linear-array passive acoustic mapping of ultrasound cavitation

PURPOSE

Passive acoustic mapping (PAM) has been proposed as a means of monitoring ultrasound therapy, particularly nonthermal cavitation-mediated applications. In PAM, the most common beamforming algorithm is a delay, sum, and integrate (DSAI) approach. However, using DSAI leads to low-quality images for the case where a narrow-aperture receiving array such as a standard B-mode linear array is used. This study aims to propose an enhanced linear-array PAM algorithm based on delay, multiply, sum, and integrate (DMSAI).

METHODS

In the proposed algorithm, before summation, the delayed signals are combinatorially coupled and multiplied, which means that the beamformed output of the proposed algorithm is the spatial coherence of received acoustic emissions. We tested the performance of the proposed DMSAI using both simulated and experimental data and compared it with DSAI. The reconstructed cavitation images were evaluated quantitatively by using source location errors between the two algorithms, full width at half maximum (FWHM), size of point spread function (A₅₀ area), signal-to-noise ratio (SNR), and computational time.

RESULTS

The results of simulations and experiments for single cavitation source show that, by introducing DMSAI, the FWHM and the A₅₀ area are reduced and the SNR is improved compared with those obtained by DSAI. The simulation results for two symmetric or nonsymmetric cavitation sources and multiple cavitation sources show that DMSAI can significantly reduce the A₅₀ area and improve the SNR, therefore improving the detectability of multiple cavitation sources.

CONCLUSIONS

The results indicate that the proposed DMSAI algorithm outperforms the conventionally used DSAI algorithm. This work may have the potential of providing an appropriate method for ultrasound therapy monitoring.

Time and Frequency Characteristics of Cavitation Activity Enhanced by Flowing Phase-Shift Nanodroplets and Lipid-Shelled Microbubbles During Focused Ultrasound Exposures

This study investigated and compared the time and frequency characteristics of cavitation activity between phase-shift nanodroplets (NDs) and lipid-shelled microbubbles (MBs) exposed to focused ultrasound (FUS) under physiologically relevant flow conditions. Root-mean-square (RMS) of broadband noise, spectrograms of the passive cavitation detection signals and inertial cavitation doses (ICDs) were calculated during FUS at varying mean flow velocities and two different peak-rarefactional pressures. At a lower pressure of 0.94 MPa, the mean values of the RMS amplitudes versus time for the NDs showed an upward trend but slowed down as the mean flow velocity increased. For flowing NDs, the rate of growth in RMS amplitudes within 2-5 MHz decreased more obviously than those within 5-8 MHz. At a higher pressure of 1.07 MPa, the increase in RMS amplitudes was accelerated as the mean flow velocity increased from 0 to 10 cm/s and slowed down as the mean flow velocity reached 15 cm/s. The general downward trends of RMS amplitudes for the MBs were retarded as the mean flow velocity increased at both acoustic pressures of 0.94 MPa and 1.07 MPa. At 0.94 MPa, the mean ICD value for the NDs decreased from 57 to 36 as the mean flow velocity increased from 0 to 20 cm/s. At 1.07 MPa, the mean ICD value initially increased from 45 to 57 as the mean flow velocity increased from 0 to 10 cm/s and subsequently decreased to 43 as the mean flow velocity reached 20 cm/s. For the MBs, the mean ICD value increased with increasing mean flow velocity at both acoustic pressures. These results could aid in future investigations of cavitation-enhanced FUS with the flowing phase-shift NDs and encapsulated, gas-filled MBs for various applications.

Design, characterization and evaluation of a laser-guided focused ultrasound system for preclinical investigations

Background

The clinical applications of transcranial focused ultrasound continue to expand and include ablation as well as drug delivery applications in the brain, where treatments are typically guided by MRI. Although MRI-guided focused ultrasound systems are also preferred for many preclinical investigations, they are expensive to purchase and operate, and require the presence of a nearby imaging center. For many basic mechanistic studies, however, MRI is not required. The purpose of this study was to design, construct, characterize and evaluate a portable, custom, laser-guided focused ultrasound system for noninvasive, transcranial treatments in small rodents.

Methods

The system comprised an off-the-shelf focused ultrasound transducer and amplifier, with a custom cone fabricated for direct coupling of the transducer to the head region. A laser-guidance apparatus was constructed with a 3D stage for accurate positioning to 1 mm. Pressure field simulations were performed to demonstrate the effects of the coupling cone and the sealing membrane, as well as for determining the location of the focus and acoustic transmission across rat skulls over a range of sizes. Hydrophone measurements and exposures in hydrogels were used to assess the accuracy of the simulations. In vivo treatments were performed in rodents for opening the blood–brain barrier and to assess the performance and accuracy of the system. The effects of varying the acoustic pressure, microbubble dose and animal size were evaluated in terms of efficacy and safety of the treatments.

Results

The simulation results were validated by the hydrophone measurements and exposures in the hydrogels. The in vivo treatments demonstrated the ability of the system to open the blood–brain barrier. A higher acoustic pressure was required in larger-sized animals, as predicted by the simulations and transmission measurements. In a particular sized animal, the degree of blood–brain barrier opening, and the safety of the treatments were directly associated with the microbubble dose.

Conclusion

The focused ultrasound system that was developed was found to be a cost-effective alternative to MRI-guided systems as an investigational device that is capable of accurately providing noninvasive, transcranial treatments in rodents.

High-intensity focused ultrasound (HIFU) ablation by the frequency chirps: Enhanced thermal field and cavitation at the focus

High-intensity focused ultrasound (HIFU) has become popular in the noninvasive ablation of a variety of solid tumors and cancers with promising clinical outcomes. Its ablation efficiency should be improved for the reduced treatment duration, especially for a large target. The frequency chirps were proposed and investigated for the enhanced lesion production and bubble cavitation at the focus during HIFU ablation. First, a nonlinear wave model was used to simulate the acoustic field using different excitation strategies (at the constant frequency excitation, downward and upward frequency chirps) and subsequently, the bubble dynamics and cavitation-enhanced temperature elevation were calculated by the Gilmore and Bioheat equations, respectively. Then the temperature rises and the produced lesion in the gel phantom were measured by the thermocouple and recorded photographically, respectively. Bubble activities at the focus were measured by passive cavitation detection (PCD) to quantify the scattering and inertial cavitation levels using short-time Fourier-transform (STFT). Finally, the enhanced temperature elevation, lesion production, and bubble cavitation were further confirmed in the ex vivo tissue samples. It is found that the frequency sweeping time plays a more important role in the enhancement of HIFU-produced lesion in the gel phantom while the frequency sweeping range seems more critical in the tissue. Altogether, large frequency sweeping range in a short time is preferable, and the frequency sweeping direction has little influence on the lesion enhancement.