Endoluminal ultrasound applicators for MR-guided thermal ablation of pancreatic tumors: Preliminary design and evaluation in a porcine pancreas model

An endoluminal cylindrical sectored-ring ultrasound phased-array applicator for minimally-invasive therapeutic ultrasound

BACKGROUND

The size of catheter-based ultrasound devices for delivering ultrasound energy to deep-seated tumors is constrained by the access pathway which limits their therapeutic capabilities.

PURPOSE

To devise and investigate a deployable applicator suitable for minimally-invasive delivery of therapeutic ultrasound, consisting of a 2D cylindrical sectored-ring ultrasound phased array, integrated within an expandable paraboloid-shaped balloon-based reflector. The balloon can be collapsed for compact delivery and expanded close to the target position to mimic a larger-diameter concentric-ring sector-vortex array for enhanced dynamic control of focal depth and volume.

METHODS

Acoustic and biothermal simulations were employed in 3D generalized homogeneous and patient-specific heterogeneous models, for three-phased array transducers with 32, 64, and 128 elements, composed of sectored 4, 8, and 16 tubular ring transducers, respectively. The applicator performance was characterized as a function of array configuration, focal depth, phasing modes, and balloon reflector geometry. A 16-element proof-of-concept phased array applicator assembly, consisting of four tubular transducers each divided into four sectors, was fabricated, and characterized with hydrophone measurements along and across the axis, and ablations in ex vivo tissue.

RESULTS

Simulation results indicated that transducer arrays (1.5 MHz, 9 mm OD × 20 mm long), balloon sizes (41-50 mm expanded diameter, 20-60 mm focal depth), phasing mode (0-4) and sonication duration (30 s) can produce spatially localized acoustic intensity focal patterns (focal length: 3-22 mm, focal width: 0.7-8.7 mm) and ablative thermal lesions (width: 2.7-16 mm, length: 6-46 mm) in pancreatic tissue across a 10-90 mm focal depth range. Patient-specific studies indicated that 0.1, 0.46, and 1.2 cm³ volume of tumor can be ablated in the body of the pancreas for 120 s sonications using a single axial focus (Mode 0), or four, and eight simultaneous foci in a toroidal pattern (Mode 2 and 4, respectively). Hydrophone measurements demonstrated good agreement with simulation. Experiments in which chicken meat was thermally ablated indicated that volumetric ablation can be produced using single or multiple foci.

CONCLUSIONS

The results of this study demonstrated the feasibility of a novel compact ultrasound applicator design capable of focusing, deep penetration, electronic steering, and volumetric thermal ablation. The proposed applicator can be used for compact endoluminal or laparoscopic delivery of localized ultrasound energy to deep-seated targets.

Deployable ultrasound applicators for endoluminal delivery of volumetric hyperthermia

PURPOSE

To investigate the design of an endoluminal deployable ultrasound applicator for delivering volumetric hyperthermia to deep tissue sites as a possible adjunct to radiation and chemotherapy.

METHOD

This study considers an ultrasound applicator consisting of two tubular transducers situated at the end of a catheter assembly, encased within a distensible conical shaped balloon-based reflector that redirects acoustic energy distally into the tissue. The applicator assembly can be inserted endoluminally or laparoscopically in a compact form and expanded after delivery to the target site. Comprehensive acoustic and biothermal simulations and parametric studies were employed in generalized 3D and patient-specific pancreatic head and body tumor models to characterize the acoustic performance and evaluate heating capabilities of the applicator by investigating the device at a range of operating frequencies, tissue acoustic and thermal properties, transducer configurations, power modulation, applicator positioning, and by analyzing the resultant 40, 41, and 43 °C isothermal volumes and penetration depth of the heating volume. Intensity distributions and volumetric temperature contours were calculated to define moderate hyperthermia boundaries.

RESULTS

Parametric studies demonstrated the frequency selection to control volume and depth of therapeutic heating from 62 to 22 cm³ and 4 to 2.6 cm as frequency ranges from 1 MHz to 4.7 MHz, respectively. Width of the heating profile tracks closely with the aperture. Water cooling within the reflector balloon was effective in controlling temperature to 37 °C maximum within the luminal wall. Patient-specific studies indicated that applicators with extended OD in the range of 3.6-6.2 cm with 0.5-1 cm long and 1 cm OD transducers can heat volumes of 1.1-7 cm³, 3-26 cm³, and 3.3-37.4 cm³ of pancreatic body and head tumors above 43, 41, and 40 °C, respectively.

CONCLUSION

In silico studies demonstrated the feasibility of combining endoluminal ultrasound with an integrated expandable balloon reflector for delivering volumetric hyperthermia in regions adjacent to body lumens and cavities.

Contactless Thermometry by MRI and MRS: Advanced Methods for Thermotherapy and Biomaterials

Control of temperature variation is of primordial importance in particular areas of biomedicine. In this context, medical treatments such as hyperthermia and cryotherapy, and also the development and use of hydrogel-based biomaterials, are of particular concern. To enable accurate temperature measurement without perturbing or even destroying the biological tissue or material to be monitored, contactless thermometry methods are preferred. Among these, the most suitable are based on magnetic resonance imaging and spectroscopy (MRI, MRS). Here, we address the latest developments in this field as well as their current and anticipated practical applications. We highlight recent progress aimed at rendering MR thermometry faster and more reproducible, versatile, and sophisticated and provide our perspective on how these new techniques broaden the range of applications in medical treatments and biomaterial development by enabling insight into finer details of thermal behavior. Thus, these methods facilitate optimization of clinical and industrial heating and cooling protocols.

Endobronchial high-intensity ultrasound for thermal therapy of pulmonary malignancies: simulations with patient-specific lung models

Objective: This study investigates the feasibility of endobronchial ultrasound applicators for thermal ablation of lung tumors using acoustic and biothermal simulations.Methods: Endobronchial ultrasound applicators with planar (10 mm width) or tubular transducers (6 mm outer diameter (OD)) encapsulated by expandable coupling balloons (10 mm OD) are considered for treating tumors from within major airways; smaller catheter-based applicators with tubular transducers (1.7-4 mm OD) and coupling balloons (2.5-5 mm OD) are considered within deep lung airways. Parametric studies were applied to evaluate transducer configurations, tumor size and location, effects of acoustic reflection and absorption at tumor-lung parenchyma interfaces, and the utility of lung flooding for enhancing accessibility. Patient-specific anatomical lung models, with various geometries and locations of tumors, were developed for further evaluation of device performance and treatment strategies. Temperature and thermal dose distributions were calculated and reported.Results: Large endobronchial applicators with planar or tubular transducers (3-7 MHz, 5 min) can thermally ablate tumors attached to major bronchi at up to 3 cm depth, where reflection and attenuation of normal lung localize tumor heating; with lung flooding, endobronchial applicators can ablate ∼2 cm diameter tumors with up to ∼2 cm separation from the bronchial wall, without significant heating of intervening tissue. Smaller catheter-based tubular applicators can ablate tumors up to 2-3 cm in diameter from deep lung airways (5-9 MHz, 5 min).Conclusion: Simulations demonstrate the feasibility of endobronchial ultrasound applicators to deliver thermal coagulation of 2-3 cm diameter tumors adjacent to or accessible from major and deep lung airways.

Deployable cylindrical phased-array applicator mimicking a concentric-ring configuration for minimally-invasive delivery of therapeutic ultrasound

A novel design for a deployable catheter-based ultrasound applicator for endoluminal and laparoscopic intervention is introduced. By combining a 1D cylindrical ring phased array with an expandable paraboloid or conical-shaped balloon-based reflector, the applicator can be controllably collapsed for compact delivery and deployed to mimic a forward-firing larger diameter concentric ring array with tight focusing and electronic steering capabilities in depth. Comprehensive acoustic and biothermal parametric studies were employed to characterize the capabilities of the applicator design as a function of transducer dimensions, phased array configuration, and balloon reflector geometry. Modeling results indicate that practical balloon sizes (43-57 mm expanded diameter), transducer array configurations (e.g. 1.5 MHz, 10 mm OD  ×  20 mm length, 8 or 16 array elements), and sonication durations (30 s) are capable of producing spatially-localized acoustic intensity focal patterns and ablative thermal lesions (width: 2.8-4.8 mm; length: 5.3-40.1 mm) in generalized soft tissue across a 5-100 mm depth range. Larger focal intensity gain magnitudes and narrower focal dimensions are attainable using paraboloid-shaped balloon reflectors with natural geometric focal depths of 25-55 mm, whereas conical-shaped reflectors (angled 45-55°) produce broader foci and extend electronic steering range in depth. A proof-of-concept phased array applicator assembly was fabricated and characterized using hydrophone and radiation force balance measurements and demonstrated good agreement with simulation. The results of this study suggest that combining small diameter cylindrical phased arrays with expandable balloon reflectors can enhance minimally invasive ultrasound-based intervention by augmenting achievable focal gains and penetration depths with dynamic adjustment of treatment depth.

Thermal ablation of pancreatic cancer: A systematic literature review of clinical practice and pre-clinical studies

PURPOSE

Pancreatic cancer is a challenging malignancy with low treatment option and poor life expectancy. Thermal ablation techniques were proposed as alternative treatment options, especially in advanced stages and for unfit-for-surgery patients. This systematic review describes the thermal ablative techniques -i.e., Laser (LA), Radiofrequency (RFA), Microwave (MWA) Ablation, High-Intensity Focused Ultrasound (HIFU) and cryoablation- available for pancreatic cancer treatment. Additionally, an analysis of the efficacy, complication rate and overall survival for each technique is conducted.

MATERIAL AND METHODS

This review collects the ex vivo, preclinical and clinical studies presenting the use of thermal techniques in the pancreatic cancer treatment, searched up to March 2018 in PubMed and Medline. Abstracts, letters-to-the-editor, expert opinions, reviews and non-English language manuscripts were excluded.

RESULTS

Sixty-five papers were included. For the ex vivo and preclinical studies, there are: 12 records for LA, 8 for RFA, 0 for MWA, 6 for HIFU, 1 for cryoablation and 3 for hybrid techniques. For clinical studies, 1 paper for LA, 14 for RFA, 1 for MWA, 17 for HIFU, 1 for cryoablation and 1 for hybrid techniques.

CONCLUSIONS

Important technological advances are presented in ex vivo and preclinical studies, as the real-time thermometry, nanotechnology and hybrid techniques to enhance the thermal outcome. Conversely, a lack of standardization in the clinical employment of the procedures emerged, leading to contrasting results on the safety and feasibility of some analyzed techniques. Uniform conclusions on the safety and feasibility of these techniques for pancreatic cancer will require further structured investigation.

Integration of deployable fluid lenses and reflectors with endoluminal therapeutic ultrasound applicators: Preliminary investigations of enhanced penetration depth and focal gain

PURPOSE

Catheter-based ultrasound applicators can generate thermal ablation of tissues adjacent to body lumens, but have limited focusing and penetration capabilities due to the small profile of integrated transducers required for the applicator to traverse anatomical passages. This study investigates a design for an endoluminal or laparoscopic ultrasound applicator with deployable acoustic reflector and fluid lens components, which can be expanded after device delivery to increase the effective acoustic aperture and allow for deeper and dynamically adjustable target depths. Acoustic and biothermal theoretical studies, along with benchtop proof-of-concept measurements, were performed to investigate the proposed design.

METHODS

The design schema consists of an array of tubular transducer(s) situated at the end of a catheter assembly, surrounded by an expandable water-filled conical balloon with a secondary reflective compartment that redirects acoustic energy distally through a plano-convex fluid lens. By controlling the lens fluid volume, the convex surface can be altered to adjust the focal length or collapsed for device insertion or removal. Acoustic output of the expanded applicator assembly was modeled using the rectangular radiator method and secondary sources, accounting for reflection and refraction at interfaces. Parametric studies of transducer radius (1-5 mm), height (3-25 mm), frequency (1.5-3 MHz), expanded balloon diameter (10-50 mm), lens focal length (10-100 mm), lens fluid (silicone oil, perfluorocarbon), and tissue attenuation (0-10 Np/m/MHz) on beam distributions and focal gain were performed. A proof-of-concept applicator assembly was fabricated and characterized using hydrophone-based intensity profile measurements. Biothermal simulations of endoluminal ablation in liver and pancreatic tissue were performed for target depths between 2 and 10 cm.

RESULTS

Simulations indicate that focal gain and penetration depth scale with the expanded reflector-lens balloon diameter, with greater achievable performance using perfluorocarbon lens fluid. Simulations of a 50 mm balloon OD, 10 mm transducer outer diameter (OD), 1.5 MHz assembly in water resulted in maximum intensity gain of ~170 (focal dimensions: ~12 mm length × 1.4 mm width) at ~5 cm focal depth and focal gains above 100 between 24 and 84 mm depths. A smaller (10 mm balloon OD, 4 mm transducer OD, 1.5 MHz) configuration produced a maximum gain of 6 at 9 mm depth. Compared to a conventional applicator with a fixed spherically focused transducer of 12 mm diameter, focal gain was enhanced at depths beyond 20 mm for assembly configurations with balloon diameters ≥ 20 mm. Hydrophone characterizations of the experimental assembly (31 mm reflector/lens diameter, 4.75 mm transducer radius, 1.7 MHz) illustrated focusing at variable depths between 10-70 mm with a maximum gain of ~60 and demonstrated agreement with theoretical simulations. Biothermal simulations (30 s sonication, 75 °C maximum) indicate that investigated applicator assembly configurations, at 30 mm and 50 mm balloon diameters, could create localized ellipsoidal thermal lesions increasing in size from 10 to 55 mm length × 3-6 mm width in liver tissue as target depth increased from 2 to 10 cm.

CONCLUSIONS

Preliminary theoretical and experimental analysis demonstrates that combining endoluminal ultrasound with an expandable acoustic reflector and fluid lens assembly can significantly enhance acoustic focal gain and penetration from inherently smaller diameter catheter-based applicators.