Reusable tissue-mimicking hydrogel phantoms for focused ultrasound ablation (original) (raw)
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IEEE Transactions on Biomedical Engineering, 2000
The present study aims to identify a new recipe for reusable tissue mimicking phantoms that allows the optical visualization of thermal lesions produced in various applications of therapeutic ultrasound where thermal mechanisms are important. The phantom was made of polyacrylamide hydrogel containing a nonionic surface-active agent (NiSAA) as a temperature-sensitive indicator. Threshold temperature above which a thermal lesion is regarded to be formed in the phantom is controlled by selecting an NiSAA. In the present study, three NiSAAs of polyoxyethylene alkyl ether series with nominal clouding points of 66 • C, 70 • C, and 80 • C were chosen. Test phantoms were prepared with polyacrylamide hydrogel, corn syrup and NiSAAs [5% (w/v)]. Key acoustic properties of the three NiSAA hydrogels were found to be similar to those of human liver. The phantoms were optically transparent at room temperature (25 • C) and became opaque after exceeding the clouding points. The transparency was recovered on cooling, although the system demonstrated hysteresis. The phantoms were tested both in their ability to provide visualization of thermal lesions produced by high-intensity focused ultrasound and also to examine any characteristic differences in the shape of the lesions formed at different threshold temperatures. The present study suggests that the NiSAA polyacrylamide hydrogel will be of a practical use in quality assurance in various applications of therapeutic ultrasound where thermal mechanisms are important.
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000
A tissue-mimicking material (TMM) for the acoustic and thermal characterization of high-intensity focused ultrasound (HIFU) devices has been developed. The material is a high-temperature hydrogel matrix (gellan gum) combined with different sizes of aluminum oxide particles and other chemicals. The ultrasonic properties (attenuation coefficient, speed of sound, acoustical impedance, and the thermal conductivity and diffusivity) were characterized as a function of temperature from 20 to 70°C. The backscatter coefficient and nonlinearity parameter B/A were measured at room temperature. Importantly, the attenuation coefficient has essentially linear frequency dependence, as is the case for most mammalian tissues at 37°C. The mean value is 0.64f 0.95 dB·cm −1 at 20°C, based on measurements from 2 to 8 MHz. Most of the other relevant physical parameters are also close to the reported values, although backscatter signals are low compared with typical human soft tissues. Repeatable and consistent temperature elevations of 40°C were produced under 20-s HIFU exposures in the TMM. This TMM is appropriate for developing standardized dosimetry techniques, validating numerical models, and determining the safety and efficacy of HIFU devices.
Contrast-agent-enhanced ultrasound thermal ablation
Ultrasound in Medicine & Biology, 2006
The small thermal lesions induced when using high-intensity focused ultrasound (HIFU) to ablate tumors results in long treatment duration. In this study, the effect of using ultrasound contrast agent (UCA, Definity ® ) to enhance the ultrasound (US) thermal effects and, thus to enlarge the lesion size, was studied in transparent tissue phantoms insonified by 1.85-MHz US with acoustical powers of 28.9 and 40.4 W. The experimental results show that the lesion size depended strongly on the electrical power and the concentration of UCA. UCA also reduced the power required to form a lesion of a certain size by about 30%. However, UCA moved the greatest heating position from the transducer focus, by 2.16 cm for 0.015% UCA at 40.4 W, and with lesions forming at the surface for UCA concentrations higher than 0.1%. An optimal result was obtained when using 0.001% UCA and 28.9-W US, which produced a lesion 12 times larger and an acceptable shift (less than half of the lesion length). UCA can effectively increase the size of the HIFU lesions, but lesion shift should be carefully considered while performing HIFU ablations. (
Journal of The Acoustical Society of America, 2006
The importance of nonlinear acoustic wave propagation and ultrasound-induced cavitation in the acceleration of thermal lesion production by high intensity focused ultrasound was investigated experimentally and theoretically in a transparent protein-containing gel. A numerical model that accounted for nonlinear acoustic propagation was used to simulate experimental conditions. Various exposure regimes with equal total ultrasound energy but variable peak acoustic pressure were studied for single lesions and lesion stripes obtained by moving the transducer. Static overpressure was applied to suppress cavitation. Strong enhancement of lesion production was observed for high amplitude waves and was supported by modeling. Through overpressure experiments it was shown that both nonlinear propagation and cavitation mechanisms participate in accelerating lesion inception and growth. Using B-mode ultrasound, cavitation was observed at normal ambient pressure as weakly enhanced echogenicity in the focal region, but was not detected with overpressure. Formation of tadpole-shaped lesions, shifted toward the transducer, was always observed to be due to boiling. Boiling bubbles were visible in the gel and were evident as strongly echogenic regions in B-mode images. These experiments indicate that nonlinear propagation and cavitation accelerate heating, but no lesion displacement or distortion was observed in the absence of boiling.
Langmuir : the ACS journal of surfaces and colloids, 2018
Poly(N-isopropylacrylamide) (pNIPAm) microgels (microgels) are colloidal particles that have been used extensively for biomedical applications. Typically, these particles are synthesized in the presence of an exogenous cross-linker, such as N,N'-methylenebis(acrylamide) (BIS); however, recent studies have demonstrated that pNIPAm microgels can be synthesized in the absence of an exogenous cross-linker, resulting in the formation of ultralow cross-linked (ULC) particles, which are highly deformable. Microgel deformability has been linked in certain cases to enhanced bioactivity when ULC microgels are used for the creation of biomimetic particles. We hypothesized that ultrasound stimulation of microgels would enhance particle deformation and that the degree of enhancement would negatively correlate with the degree of particle cross-linking. Here, we demonstrate in tissue-mimicking phantoms that using ultrasound insonification causes deformations of ULC microgel particles. Furtherm...
Physics Procedia, 2015
Although high intensity focused ultrasound beams (HIFU) have found rapid agreement in clinical environment as a tool for non invasive surgical ablation and controlled destruction of cancer cells, some aspects related to the interaction of ultrasonic waves with tissues, such as the conversion of acoustic energy into heat, are not thoroughly understood. In this work, innovative tissuemimicking materials (TMMs), based on Agar and zinc acetate, have been used to conduct investigations in order to determine a relation between the sample attenuation coefficient and its temperature increase measured in the focus region when exposed to an HIFU beam. An empirical relation has been deduced establishing useful basis for further processes of validations of numerical models to be adopted for customizing therapeutic treatments.
Journal of Therapeutic Ultrasound, 2013
Phase-shift nanoemulsions (PSNEs) provide cavitation sites when the perfluorocarbon (PFC) nanodroplets (ND) are vaporized to microbubbles by acoustic energy. Their presence lowers the power required to ablate tissue by high-intensity focused ultrasound (HIFU), potentially making it a safer option for a broader range of treatment sites. However, spatial control over the ablation region can be problematic when cavitation is used to enhance heating. This study explored relationships between vaporization, ablation, and the PSNE concentration in vitro to optimize the acoustic intensity and insonation time required for spatially controlled ablation enhancement using a PSNE that included a volatile PFC component. HIFU (continuous wave at 1 MHz; insonation times of 5, 10, 15, and 20 s; cool-down times of 2, 4, and 6 s; peak negative pressures of 2, 3, and 4 MPa) was applied to albumin-acrylamide gels containing PFC agents (1:1 mix of volatile decafluorobutane and more stable dodecafluoropentane at 10(5) to 10(8) PFC ND per milliliter) or agent-free controls. Vaporization fields (microbubble clouds) were imaged by conventional ultrasound, and ablation lesions were measured directly by calipers. Controlled ablation was defined as the production of 'cigar'-shaped lesions corresponding with the acoustic focal zone. This control was considered to be lost when ablation occurred in prefocal vaporization fields having a predominantly 'tadpole' or oblong shape. Changes in the vaporization field shape and location occurred on a continuum with increasing PSNE concentration and acoustic intensity. Working with the maximum concentration-intensity combinations resulting in controlled ablation demonstrated a dose-responsive relationship between insonation time and volumes of both the vaporization fields (approximately 20 to 240 mm(3)) and the ablation lesions (1 to 135 mm(3)) within them. HIFU ablation was enhanced by this PSNE and could be achieved using intensities ≤650 W/cm(2). Although the ablation lesions were located within much larger microbubble clouds, optimum insonation times and intensities could be selected to achieve an ablation lesion of desired size and location for a given PSNE concentration. This demonstration of controllable enhancement using a PSNE that contained a volatile PFC component is another step toward developing phase-shift nanotechnology as a potential clinical tool to improve HIFU.