Comparative study of temperature measurements inex vivoswine muscle and a tissue-mimicking material during high intensity focused ultrasound exposures

Parameter effects on arterial vessel sonicated by high-intensity focused ultrasound: an ex vivo vascular phantom study

Objective. This study is aimed to explore the effects of vascular and sonication parameters on ex vivo vessel sonicated by high-intensity focused ultrasound. Approach. The vascular phantom embedding the polyolefin tube or ex vivo vessel was sonicated. The vascular phantom with 1.6 and 3.2 mm tubes was sonicated at three acoustic powers (2.0, 3.5, 5.3 W). The occlusion level of post-sonication tubes was evaluated using ultrasound imaging. The vascular phantom with the ex vivo abdominal aorta of rabbit for three flow rates (0, 5, 10 cm s−1) was sonicated at two acoustic powers (3.5 and 5.3 W). Different distances between focus and posterior wall (2, 4, 6 mm) and cooling times (0 and 10 s) were also evaluated. The diameter of the sonicated vessel was measured by B-mode imaging and microscopic photography. Histological examination was performed for the sonicated vessels. Main results. For the 5 cm s−1 flow rate, the contraction index of vascular diameter (Dc) with 5.3 W and 10 s cooling time at 2 mm distance was 39 ± 9% (n = 9). With the same parameters except for 0 cm s−1 flow rate, the Dc was increased to 45 ± 7% (n = 4). At 3.5 W, the Dc with 5 cm s−1 flow rate was 23 ± 15% (n = 4). The distance and cooling time influenced the lesion along the vessel wall. Significance. This study has demonstrated the flow rate and acoustic power have the great impact on the vessel contraction. Besides, the larger lesion covering the vessel wall would promote the vessel contraction. And the in vivo validation is required in the future study.

A standard test phantom for the performance assessment of magnetic resonance guided high intensity focused ultrasound (MRgHIFU) thermal therapy devices

Purpose: Test objects for High Intensity Focused Ultrasound (HIFU) are required for the standardization and definition of treatment, Quality Assurance (QA), comparison of results between centers and calibration of devices. This study describes a HIFU test object which provides temperature measurement as a function of time, in a reference material compatible with Magnetic Resonance (MR) and ultrasound.Materials and methods: T-Type fine wire thermocouples were used as sensors and 5 correction methods for viscous heating artifacts were assessed. The phantom was tested in a MR-HIFU Philips Sonalleve device over a period of 12 months, demonstrating stability and validity to evaluate the performance of the device.Results: The study furnished useful information regarding the MR-HIFU sessions and highlighted potential limitations of the existing QA and monitoring methods. The importance of temperature monitoring along the whole acoustic path was demonstrated as MR Thermometry readings differed in the three MR plane views (coronal, sagittal, transverse), in particular when the focus was near a soft-tissue/bone interface, where there can be an MR signal loss with significant temperature and thermal dose underestimation (138% variation between the three plane views).Conclusions: The test object was easy to use and has potential as a valid tool for training, QA, research and development for MR guided HIFU and potentially ultrasound guided devices.

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.

Bayesian pharmacokinetic modeling of dynamic contrast-enhanced magnetic resonance imaging: validation and application

Tracer-kinetic analysis of dynamic contrast-enhanced magnetic resonance imaging data is commonly performed with the well-known Tofts model and nonlinear least squares (NLLS) regression. This approach yields point estimates of model parameters, uncertainty of these estimates can be assessed e.g. by an additional bootstrapping analysis. Here, we present a Bayesian probabilistic modeling approach for tracer-kinetic analysis with a Tofts model, which yields posterior probability distributions of perfusion parameters and therefore promises a robust and information-enriched alternative based on a framework of probability distributions. In this manuscript, we use the quantitative imaging biomarkers alliance (QIBA) Tofts phantom to evaluate the Bayesian tofts model (BTM) against a bootstrapped NLLS approach. Furthermore, we demonstrate how Bayesian posterior probability distributions can be employed to assess treatment response in a breast cancer DCE-MRI dataset using Cohen's d. Accuracy and precision of the BTM posterior distributions were validated and found to be in good agreement with the NLLS approaches, and assessment of therapy response with respect to uncertainty in parameter estimates was found to be excellent. In conclusion, the Bayesian modeling approach provides an elegant means to determine uncertainty via posterior distributions within a single step and provides honest information about changes in parameter estimates.

A closer look at ultrasonic attenuation and heating in a tissue-mimicking material

A well-characterized ultrasound tissue-mimicking material (TMM) can be important in determining the acoustic output and temperature rise from high intensity therapeutic ultrasound (HITU) devices and also in validating computer simulation models. A HITU TMM previously developed and characterized in our laboratory has been used in our acoustic and temperature measurements as well as modeled in our HITU simulation program. A discrepancy between thermal measurement and simulation, though, led us to further investigate the TMM properties. We found that the 2-parameter analytic fit commonly used to represent the attenuation of the TMM in the computer modeling was not adequate over the entire frequency range of interest, 1 MHz to 8 MHz in this study, indicating that we and others may have not been characterizing TMMs, and possibly tissue, optimally. By comparing measurements and simulations, we found that a 3-parameter analytic fit for attenuation gave a more accurate value for attenuation at 1 MHz and 2 MHz, and using that fit the temperature rise measurements in the TMM that agreed more closely with the simulation results.

Experimental study of beam distortion due to fiducial markers during salvage HIFU in the prostate

Background

Prostate cancer is frequently treated using external beam radiation therapy (EBRT). Prior to therapy, the prostate is commonly implanted with a small number of permanent fiducial markers used to monitor the position of the prostate during therapy. In the case of local cancer recurrence, high-intensity focused ultrasound (HIFU) provides a non-invasive salvage treatment option. However, the impact of the fiducial markers on HIFU treatment has not been thoroughly studied to date. The objective of this study was to experimentally investigate the effect of a single EBRT fiducial marker on the efficacy of HIFU treatment delivery using a tissue-mimicking material (TMM).

Methods

A TMM with the acoustic properties of the prostate was developed based on a polyacrylamide hydrogel containing bovine serum albumin. Each phantom was implanted with a cylindrical fiducial marker and then sonicated using a 3.3 MHz focused bowl HIFU transducer. Two sets of experiments were performed. In the first, a single lesion was created at different positions along either the anteroposterior or left-right axes relative to the marker. In the second, a larger ablation volume was created by raster scanning. The size and position of the ablated volume were assessed using a millimetre grid overlaid on the phantom.

Results

The impact of the marker on the position and size of the HIFU lesion was significant when the transducer focus was positioned within 7 mm anteriorly, 18 mm posteriorly or within 3 mm laterally of the marker. Beyond this, the generated lesion was not affected. When the focus was anterior to the marker, the lesion increased in size due to reflections. When the focus was posterior, the lesion decreased in size or was not present due to shadowing.

Conclusions

The presence of an EBRT fiducial marker may result in an undertreated region beyond the marker due to reduced energy arriving at the focus, and an overtreated region in front of the marker due to reflections. Depending on the position of the targeted regions and the distribution of the markers, both effects may be undesirable and reduce treatment efficacy. Further work is necessary to investigate whether these results indicate the necessity to reconsider patient selection and treatment planning for prostate salvage HIFU after failed EBRT.

Treatment of hepatic tumors by thermal versus mechanical effects of pulsed high intensity focused ultrasound in vivo

The purpose of this study is to comparatively assess the thermal versus mechanical effects of pulsed high intensity focused ultrasound (HIFU) treatment on hepatic tumors in vivo. Forty-five rabbits with hepatic VX2 tumors were randomly separated into three groups (15 animals per group) before HIFU ablation. The total HIFU energy (in situ) of 1250 J was used for each tumor for three groups. In groups I and II, animals were treated with 1 MHz pulsed ultrasound at 1 Hz pulsed repetition frequency (PRF), 0.5 duty cycle (0.5 s on and 0.5 s off) and10 s duration for one spot sonication. For group II, in addition to HIFU treatment, microbubbles (SonoVue, Bracco, Milan, Italy) were injected via vein before sonication acting as a synergist. In group III, animals were treated with 1 MHz pulsed ultrasound at 10 Hz PRF, 0.1 duty cycle (0.1 s on and 0.9 s off) and 10 s duration for one sonication. The total treatment spots were calculated according to the tumor volume. Tumors were examined with contrast-enhanced computed tomography (CECT) immediately prior to and post HIFU treatment. Histopathologic assessment was performed 3 h after treatment. Our study showed that all animals tolerated the HIFU treatment well. Our data showed that mechanical HIFU could lead to controlled injury in rabbit hepatic tumors with different histological changes in comparison to thermal HIFU with or without microbubbles.

The Feasibility of Thermal Imaging as a Future Portal Imaging Device for Therapeutic Ultrasound

This technical note describes a prototype thermally based portal imaging device that allows mapping of energy deposition on the surface of a tissue mimicking material in a focused ultrasound surgery (FUS) beam by using an infrared camera to measure the temperature change on that surface. The aim of the work is to explore the feasibility of designing and building a system suitable for rapid quality assurance (QA) for use with both ultrasound- and magnetic resonance (MR) imaging-guided clinical therapy ultrasound systems. The prototype was tested using an MR-guided Sonalleve FUS system (with the treatment couch outside the magnet bore). The system's effective thermal noise was 0.02°C, and temperature changes as low as 0.1°C were easily quantifiable. The advantages and drawbacks of thermal imaging for QA are presented through analysis of the results of an experimental session.

Design of a low power hybrid HIFU applicator for haemostasis based on acoustic propagation modelling

PURPOSE

The aim of this study was to design an applicator for haemostasis usage needing lower acoustic intensities (<880 W/cm(2)) than in previous devices intended for it, which is based on ultrasound propagation FEM modelling using a 2-MHz HIFU transducer.

MATERIALS AND METHODS

Acoustic field characterisation and numerical simulations in water were performed with and without the proposed applicator. Parameters such as form factor, ellipsoidal shape ratio, and Euclidean distance were used (among others) to compare simulated data with transducer measurements without applicator. A low density polyethylene cone was manufactured from geometries validated from acoustic field modelling. The hollow cone was filled with 10% polyacrylamide gel as a coupling medium with liver phantom or chicken liver. Focal temperature was measured with a thermocouple embedded in the phantom for 1-20 W driving powers for 120 s. Standing wave ratios (SWR) were used as coupling indexes. Ex vivo experimentation in chicken liver was made at 10-20 W.

RESULTS

Simulated acoustic patterns showed good concordance with measurements. Experimental focal distance was 20.72 ± 0.24 mm, while the simulated was 19.79 mm (≈4% error). SWR at low power were: 2.01 with transducer emitting in air, 1.53 at applicator tip, and 1.35 after phantom placement. Average SWR at high power was 1.31. Similarity of percentages for data comparison in focal plane was over 60%. Maximum temperature measured at focus was 88.7 °C with 20 W after 85 s.

CONCLUSIONS

Temperatures reached at focus suggest that this applicator has good efficiency, which notably reduces the power typically needed for haemostasis effect.

Head phantoms for transcranial focused ultrasound

PURPOSE

In the ongoing endeavor of fine-tuning, the clinical application of transcranial MR-guided focused ultrasound (tcMRgFUS), ex-vivo studies wlkiith whole human skulls are of great use in improving the underlying technology guiding the accurate and precise thermal ablation of clinically relevant targets in the human skull. Described here are the designs, methods for fabrication, and notes on utility of three different ultrasound phantoms to be used for brain focused ultrasound research.

METHODS

Three different models of phantoms are developed and tested to be accurate, repeatable experimental options to provide means to further this research. The three models are a cadaver, a gel-filled skull, and a head mold containing a skull and filled with gel that mimics the brain and the skin. Each was positioned in a clinical tcMRgFUS system and sonicated at 1100 W (acoustic) for 12 s at different locations. Maximum temperature rise as measured by MR thermometry was recorded and compared against clinical data for a similar neurosurgical target. Results are presented as heating efficiency in units (°C/kW/s) for direct comparison to available clinical data. The procedure for casting thermal phantom material is presented. The utility of each phantom model is discussed in the context of various tcMRgFUS research areas.

RESULTS

The cadaveric phantom model, gel-filled skull model, and full head phantom model had heating efficiencies of 5.3, 4.0, and 3.9 °C/(kW/s), respectively, compared to a sample clinical heating efficiency of 2.6 °C/(kW/s). In the seven research categories considered, the cadaveric phantom model was the most versatile, though less practical compared to the ex-vivo skull-based phantoms.

CONCLUSIONS

Casting thermal phantom material was shown to be an effective way to prepare tissue-mimicking material for the phantoms presented. The phantom models presented are all useful in tcMRgFUS research, though some are better suited to a limited subset of applications depending on the researchers needs.

Comparison between experimental and computational methods for the acoustic and thermal characterization of therapeutic ultrasound fields

For high intensity therapeutic ultrasound (HITU) devices, pre-clinical testing can include measurement of power, pressure/intensity and temperature distribution, acoustic and thermal simulations, and assessment of targeting accuracy and treatment monitoring. Relevant International Electrotechnical Commission documents recently have been published. However, technical challenges remain because of the often focused, large amplitude pressure fields encountered. Measurement and modeling issues include using hydrophones and radiation force balances at HITU power levels, validation of simulation models, and tissue-mimicking material (TMM) development for temperature measurements. To better understand these issues, a comparison study was undertaken between simulations and measurements of the HITU acoustic field distribution in water and TMM and temperature rise in TMM. For the specific conditions of this study, the following results were obtained. In water, the simulated values for p+ and p- were 3% lower and 10% higher, respectively, than those measured by hydrophone. In TMM, the simulated values for p+ and p- were 2% and 10% higher than those measured by hydrophone, respectively. The simulated spatial-peak temporal-average intensity values in water and TMM were greater than those obtained by hydrophone by 3%. Simulated and measured end-of-sonication temperatures agreed to within their respective uncertainties (coefficients of variation of approximately 20% and 10%, respectively).

Enhanced Heat Transfer and Thermal Dose Using Magnetic Nanoparticles During HIFU Thermal Ablation—An In-Vitro Study

Avoiding collateral damage to healthy tissues during the high intensity focused ultrasound (HIFU) ablation of malignant tumors is one of the major challenges for effective thermal therapy. Such collateral damage can originate out of the need for using higher acoustic powers to treat deep seated or highly vascularized tumors. The objective of this study is to assess the utility of using magnetic nanoparticles (mNPs) during HIFU procedures to locally enhance heating at low powers, thereby reducing the likelihood of collateral thermal damage and undesired destruction due to cavitation. Tissue phantoms with 0% (control), 1% and 3% mNPs concentrations by volume were fabricated. Each tissue phantom was embedded with four thermocouples (TCs) and sonicated using transducer acoustic powers of 5.15 W, 9.17 W, and 14.26 W. The temperature profiles during the heating and cooling periods were recorded for each embedded TC. The measured transient temperature profiles were used for thermal-dose calculations. The increase in the concentration of mNPs in the tissue phantoms, from 0% to 3%, resulted in the rise in the peak temperatures for all the TCs for each acoustic power. The thermal dose also increased with the rise in the concentration of mNPs in the tissue phantoms. For the highest applied acoustic power (14.26 W), the peak temperature at TC 1 (T1) in tissue phantoms with 1% and 3% mNPs concentrations increased (with respect to tissue phantom with 0% (control) mNPs concentration) by 1.59× and 2.09×, respectively. For an acoustic power of 14.26 W, the time required to achieve cellular necrosis as defined by a 240 equivalent min thermal dose was approximately 75 s in the absence of mNPs, 14 s for the 1% concentration, and 8 s for the 3% concentration. Magnetic nanoparticles have the potential to significantly reduce the time for HIFU thermal-ablation procedures. They can also decrease the likelihood of collateral damage by the propagating beam in HIFU procedures by reducing the intensity required to achieve cellular necrosis.

In vitro methods for evaluating therapeutic ultrasound exposures: present-day models and future innovations

Although preclinical experiments are ultimately required to evaluate new therapeutic ultrasound exposures and devices prior to clinical trials, in vitro experiments can play an important role in the developmental process. A variety of in vitro methods have been developed, where each of these has demonstrated their utility for various test purposes. These include inert tissue-mimicking phantoms, which can incorporate thermocouples or cells and ex vivo tissue. Cell-based methods have also been used, both in monolayer and suspension. More biologically relevant platforms have also shown utility, such as blood clots and collagen gels. Each of these methods possesses characteristics that are well suited for various well-defined investigative goals. None, however, incorporate all the properties of real tissues, which include a 3D environment and live cells that may be maintained long-term post-treatment. This review is intended to provide an overview of the existing application-specific in vitro methods available to therapeutic ultrasound investigators, highlighting their advantages and limitations. Additional reporting is presented on the exciting and emerging field of 3D biological scaffolds, employing methods and materials adapted from tissue engineering. This type of platform holds much promise for achieving more representative conditions of those found in vivo, especially important for the newest sphere of therapeutic applications, based on molecular changes that may be generated in response to non-destructive exposures.