A multi-element interstitial ultrasound applicator for the thermal therapy of brain tumors

Surgical Management of Brain Tumors with Focused Ultrasound

Focused ultrasound is a novel technique for the treatment of aggressive brain tumors that uses both mechanical and thermal mechanisms. This non-invasive technique can allow for both the thermal ablation of inoperable tumors and the delivery of chemotherapy and immunotherapy while minimizing the risk of infection and shortening the time to recovery. With recent advances, focused ultrasound has been increasingly effective for larger tumors without the need for a craniotomy and can be used with minimal surrounding soft tissue damage. Treatment efficacy is dependent on multiple variables, including blood-brain barrier permeability, patient anatomical features, and tumor-specific features. Currently, many clinical trials are currently underway for the treatment of non-neoplastic cranial pathologies and other non-cranial malignancies. In this article, we review the current state of surgical management of brain tumors using focused ultrasound.

The histotripsy spectrum: differences and similarities in techniques and instrumentation

Since its inception about two decades ago, histotripsy - a non-thermal mechanical tissue ablation technique - has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.

Evaluation of the dual-frequency transducer for controlling thermal ablation morphology using frequency shift keying signal

PURPOSE

The catheter-based ultrasound (CBUS) can reach the target tissue directly and achieve rapid treatment. The frequency shift keying (FSK) signal is proposed to regulate and evaluate tumor ablation by a miniaturized dual-frequency transducer.

METHODS

A dual-frequency transducer prototype (3 × 7 × 0.4 mm) was designed and fabricated for the CBUS applicator (OD: 3.8 mm) based on the fundamental frequency of 5.21 MHz and the third harmonic frequency of 16.88 MHz. Then, the acoustic fields and temperature field distributions using the FSK signals (with 0, 25, 50, 75, and 100% third harmonic frequency duty ratios) were simulated by finite element analysis. Finally, tissue ablation and temperature monitoring were performed in phantom and ex vivo tissue, respectively.

RESULTS

At the same input electrical power (20 W), the output acoustic power of the fundamental frequency of the transducer was 10.03 W (electroacoustic efficiencies: 50.1%), and that of the third harmonic frequency was 6.19 W (30.6%). As the third harmonic frequency duty ratios increased, the shape of thermal lesions varied from strip to droplet in simulated and phantom experimental results. The same trend was observed in ex vivo tests.

CONCLUSION

Dual-frequency transducers excited by the FSK signal can control the morphology of lesions.

SIGNIFICANCE

The acoustic power deposition of CBUS was optimized to achieve precise ablation.

Ultrasound Ablation in Neurosurgery: Current Clinical Applications and Future Perspectives

The concept of focusing high-intensity ultrasound beams for the purpose of cerebral ablation has interested neurosurgeons for more than 70 yr. However, the need for a craniectomy or a cranial acoustic window hindered the clinical diffusion of this technique. Recent technological advances, including the development of phased-array transducers and magnetic resonance imaging technology, have rekindled the interest in ultrasound for ablative brain surgery and have led to the development of the transcranial magnetic resonance-guided focused ultrasound (MRgFUS) thermal ablation procedure. In the last decade, this method has become increasingly popular, and its clinical applications are broadening. Despite the demonstrated efficacy of MRgFUS, transcranial thermal ablation using ultrasound is limited in that it can target exclusively the central region of the brain where the multiple acoustic beams are most optimally focused. On the contrary, lesioning of the cortex, the superficial subcortical areas, and regions close to the skull base is not possible with the limited treatment envelope of current phased-array transducers. Therefore, new ultrasound ablative techniques, which are not based on thermal mechanisms, have been developed and tested in experimental settings. This review describes the mechanisms by which these novel, nonthermal ablative techniques are based and also presents the current clinical applications of MRgFUS thermal ablation.

Miniaturized Intracavitary Forward-Looking Ultrasound Transducer for Tissue Ablation

OBJECTIVE

This paper aims to develop a miniaturized forward-looking ultrasound transducer for intracavitary tissue ablation, which can be used through an endoscopic device. The internal ultrasound (US) delivery is capable of directly interacting with the target tumor, resolving adverse issues of currently available US devices, such as unintended tissue damage and insufficient delivery of acoustic power.

METHODS

To transmit a high acoustic pressure from a small aperture (<3 mm), a double layer transducer (1.3 MHz) was designed and fabricated based on numerical simulations. The electric impedance and the acoustic pressure of the actual device was characterized with an impedance analyzer and a hydrophone. Ex vivo tissue ablation tests and temperature monitoring were then conducted with porcine livers.

RESULTS

The acoustic intensity of the transducer was 37.1 W/cm² under 250 V_pp and 20% duty cycle. The tissue temperature was elevated to 51.8 °C with a 67 Hz pulse-repetition frequency. The temperature profile in the tissue indicated that ultrasound energy was effectively absorbed inside the tissue. During a 5-min sonification, an approximate tissue volume of 2.5 × 2.5 × 1.0 mm³ was ablated, resulting in an irreversible lesion.

CONCLUSION

This miniaturized US transducer is a promising medical option for the precise tissue ablation, which can reduce the risk of unintended tissue damage found in noninvasive US treatments.

SIGNIFICANCE

Having a small aperture (2 mm), the intracavitary device is capable of ablating a bio tissue in 5 min with a relatively low electric power (<17 W).

Robotic Assisted MRI-Guided Interventional Interstitial MR-Guided Focused Ultrasound Ablation in a Swine Model

BACKGROUND

Ablative lesions are current treatments for epilepsy and brain tumors. Interstitial magnetic resonance (MR) guided focused ultrasound (iMRgFUS) may be an alternate ablation technique which limits thermal tissue charring as compared to laser therapy (LITT) and can produce larger ablation patterns nearer the surface than transcranial MR guided focused ultrasound (tcMRgFUS).

OBJECTIVE

To describe our experience with interstitial focused ultrasound (iFUS) ablations in swine, using MR-guided robotically assisted (MRgRA) delivery.

METHODS

In an initial 3 animals, we optimized the workflow of the robot in the MR suite and made modifications to the robotic arm to allow range of motion. Then, 6 farm pigs (4 acute, 2 survival) underwent 7 iMRgFUS ablations using MRgRA. We altered dosing to explore differences between thermal dosing in brain as compared to other tissues. Imaging was compared to gross examination.

RESULTS

Our work culminated in adjustments to the MRgRA, iMRgFUS probes, and dosing, culminating in 2 survival surgeries; swine had ablations with no neurological sequelae at 2 wk postprocedure. Immediately following iMRgFUS therapy, diffusion-weighted imaging, and T1 weighted MR were accurate reflections of the ablation volume. T2 and fluid-attenuated inversion-recovery (FLAIR) images were accurate reflections of ablation volume 1-wk postprocedure.

CONCLUSION

We successfully performed MRgRA iFUS ablation in swine and found intraoperative and postoperative imaging to correlate with histological examination. These data are useful to validate our system and to guide imaging follow-up for thermal ablation lesions in brain tissue from our therapy, tcMRgFUS, and LITT.

Applications of Focused Ultrasound in Cerebrovascular Diseases and Brain Tumors

Oncology and cerebrovascular disease constitute two of the most common diseases afflicting the central nervous system. Standard of treatment of these pathologies is based on multidisciplinary approaches encompassing combination of interventional procedures such as open and endovascular surgeries, drugs (chemotherapies, anti-coagulants, anti-platelet therapies, thrombolytics), and radiation therapies. In this context, therapeutic ultrasound could represent a novel diagnostic/therapeutic in the armamentarium of the surgeon to treat these diseases. Ultrasound relies on mechanical energy to induce numerous physical and biological effects. The application of this technology in neurology has been limited due to the challenges with penetrating the skull, thus limiting a prompt translation as has been seen in treating pathologies in other organs, such as breast and abdomen. Thanks to pivotal adjuncts such as multiconvergent transducers, magnetic resonance imaging (MRI) guidance, MRI thermometry, implantable transducers, and acoustic windows, focused ultrasound (FUS) is ready for prime-time applications in oncology and cerebrovascular neurology. In this review, we analyze the evolution of FUS from the beginning in 1950s to current state-of-the-art. We provide an overall picture of actual and future applications of FUS in oncology and cerebrovascular neurology reporting for each application the principal existing evidences.

Magnetic resonance-guided interstitial high-intensity focused ultrasound for brain tumor ablation

Currently, treatment of brain tumors is limited to resection, chemotherapy, and radiotherapy. Thermal ablation has been recently explored. High-intensity focused ultrasound (HIFU) is being explored as an alternative. Specifically, the authors propose delivering HIFU internally to the tumor with an MRI-guided robotic assistant (MRgRA). The advantage of the authors' interstitial device over external MRI-guided HIFU (MRgHIFU) is that it allows for conformal, precise ablation and concurrent tissue sampling. The authors describe their workflow for MRgRA HIFU delivery.

Capacitive Micromachined Ultrasound Transducers for Interstitial High-Intensity Ultrasound Therapies

Capacitive micromachined ultrasound transducers (CMUTs) exhibit several potential advantages over conventional piezo technologies for use in therapeutic ultrasound (US) devices, including ease of miniaturization and integration with electronics, broad bandwidth (>several megahertz), and compatibility with magnetic resonance imaging (MRI). In this paper, the electroacoustic performance of CMUTs designed for interstitial high-intensity contact US (HICU) applications was evaluated and the feasibility of generating US-induced heating and thermal destruction of biological tissues was studied. One-dimensional CMUT linear arrays as well as a prism-shaped 2-D array composed of multiple 1-D linear arrays mounted on a cylindrical catheter were fabricated. The electromechanical and acoustic characteristics of the CMUTs were first studied at low intensity. Then, the acoustic output during continuous wave (CW) driving was studied while varying the bias voltage ( V_DC ) and driving voltage ( V_AC ). US heating was performed in tissue-mimicking gel phantoms under infrared (IR) or MR-thermometry monitoring. Acoustic intensities compatible with thermal ablation were obtained by driving the CMUTs in the collapse-snapback operation mode ( [Formula: see text]). Hysteresis in the acoustic output was observed with varying V_DC . IR- and MR-thermometry monitoring showed directional US-induced heating patterns in tissue-mimicking phantoms (frequency: 6-8 MHz and exposure time: 60-240 s) extending over 1.5-cm depth from the CMUT surface. Irreversible thermal damage was produced in turkey breast tissue samples ( [Formula: see text]). Multidirectional US-induced heating was also achieved in 3-D with the CMUT catheter. These studies demonstrate that CMUTs can be integrated into HICU devices and be used for heating and destruction of tissue under MR guidance.

Influence of geometric and material properties on artifacts generated by interventional MRI devices: Relevance to PRF-shift thermometry

PURPOSE

Magnetic resonance imaging (MRI) is capable of providing valuable real-time feedback during medical procedures, partly due to the excellent soft-tissue contrast available. Several technical hurdles still exist to seamless integration of medical devices with MRI due to incompatibility of most conventional devices with this imaging modality. In this study, the effect of local perturbations in the magnetic field caused by the magnetization of medical devices was examined using finite element analysis modeling. As an example, the influence of the geometric and material characteristics of a transurethral high-intensity ultrasound applicator on temperature measurements using proton resonance frequency (PRF)-shift thermometry was investigated.

METHODS

The effect of local perturbations in the magnetic field, caused by the magnetization of medical device components, was examined using finite element analysis modeling. The thermometry artifact generated by a transurethral ultrasound applicator was simulated, and these results were validated against analytic models and scans of an applicator in a phantom. Several parameters were then varied to identify which most strongly impacted the level of simulated thermometry artifact, which varies as the applicator moves over the course of an ablative high-intensity ultrasound treatment.

RESULTS

Key design parameters identified as having a strong influence on the magnitude of thermometry artifact included the susceptibility of materials and their volume. The location of components was also important, particularly when positioned to maximize symmetry of the device. Finally, the location of component edges and the inclination of the device relative to the magnetic field were also found to be important factors.

CONCLUSIONS

Previous design strategies to minimize thermometry artifact were validated, and novel design strategies were identified that substantially reduce PRF-shift thermometry artifacts for a variety of device orientations. These new strategies are being incorporated into the next generation of applicators. The general strategy described in this study can be applied to the design of other interventional devices intended for use with MRI.

Catheter-based ultrasound technology for image-guided thermal therapy: current technology and applications

Catheter-based ultrasound (CBUS) is applied to deliver minimally invasive thermal therapy to solid cancer tumours, benign tissue growth, vascular disease, and tissue remodelling. Compared to other energy modalities used in catheter-based surgical interventions, unique features of ultrasound result in conformable and precise energy delivery with high selectivity, fast treatment times, and larger treatment volumes. We present a concise review of CBUS technology being currently utilized in animal and clinical studies or being developed for future applications. CBUS devices have been categorised into interstitial, endoluminal and endovascular/cardiac applications. Basic applicator designs, site-specific evaluations and possible treatment applications have been discussed in brief. Particular emphasis has been given to ablation studies that incorporate image guidance for applicator placement, therapy monitoring, feedback control, and post-procedure assessment. Examples of devices included here span the entire spectrum of the development cycle from preliminary simulation-based design studies to implementation in clinical investigations. The use of CBUS under image guidance has the potential for significantly improving precision and applicability of thermal therapy delivery.