Hsu Chang

Feasibility of transrib focused ultrasound thermal ablation for liver tumors using a spherically curved 2D array: a numerical study.

The use of focused ultrasound thermal ablation to treat hepatocarcinoma and other liver tumors produces promising clinical results. However, one of the major drawbacks is the high absorption of ultrasonic energy by the rib, making partial rib removal necessary in many cases. This study numerically investigated the feasibility of using a spherical ultrasound phased array for transrib liver-tumor thermal ablation. An independently array-element activitation scheme, which switches off the transducer elements obstructed by the ribs based on feedback anatomical medical imaging, was proposed to reduce the rib-overheating problem. The numerical results showed that the proposed treatment planning strategy can effectively reduce the specific energy absorbed by the rib while maintaining the energy at the target position, which both reduces the rib-overheating problem and increases the possibility of treating a target lesion under an intact rib. The analysis also demonstrated that the target position and the ultrasound frequency play key roles in the treatment. Patients with diverse characteristics were also tested to show the generality of the proposed strategy. The proposed treatment planning strategy also provides useful information for evaluating the treatment effectiveness prior to clinically performing transrib ultrasound liver-tumor thermal ablation.

Split-focused ultrasound transducer with multidirectional heating for breast tumor thermal surgery

This study investigated the feasibility of using a split-focused ultrasound transducer to perform thermal surgery on breast tumors, based on a multidirectional heating scheme. The transducer is a square section of a sphere with a radius of 10 cm. The transducer was tilted such that its acoustic beam was 45 degrees relative to the rib surface, and its focal zone was arranged by a shift of 6 mm away from the center of the planning target volume. The multifocus switching technique was employed to enhance the heating efficiency. When a single transducer was used, the transducer sonicated from a certain position for a given duration, and then rotated sequentially to continue the heating. Computer simulations and in vitro phantom experiments have been studied for this heating system. Both simulation and experimental results demonstrated that the system based on a multidirectional heating scheme is capable of generating a proper thermal lesion within 8 min. Meanwhile, from the simulation results, the rib heating was effectively alleviated by tilting the transducer to induce the total reflection at the muscle/bone interface. While using multiple ultrasound transducers, an appropriate arrangement was designed to have the same configuration of acoustic beams as is used for a single-transducer strategy. The simulation results from the four-transducer strategy indicated that the heating results could be further improved. This study revealed that it is very promising to have an appropriate arrangement of a single split-focused ultrasound transducer with mechanical rotation, or to have multiple split-focused transducers that use multidirectional heating for breast tumor thermal therapy.

Design and characterization of dual-curvature 1.5-dimensional high-intensity focused ultrasound phased-array transducer

A dual-curvature focused ultrasound phased-array transducer with a symmetric control has been developed for noninvasive ablative treatment of tumors. The 1.5-D array was constructed in-house and the electro-acoustic conversion efficiency was measured to be approximately 65%. In vitro experiments demonstrated that the array uses 256 independent elements to achieve 2-D wide-range high-intensity electronic focusing.

DESIGN AND FABRICATION OF HIGH-INTENSITY FOCUSED ULTRASOUND PHASED ARRAY FOR LIVER TUMOR THERAPY

Noninvasive surgery of the liver tumors has been carried out by using the high-intensity focused ultrasound (HIFU). However, the liver tumor can be moved by the human respirations and heartbeats, which may cause the ablation and damage of normal tissues during the sonications of HIFU. The purpose of this study was to design and fabricate a cylindrical HIFU phased array transducer for treating the moving liver tumor efficiently. The total number of the element was 512 but only 256 channels were required since the elements along the elevation direction were connected in pairs with respect to the central line of the array. Field II software was used to simulate the acoustic field, and a formula for predicting the spatial averaged intensity at focus was developed based on the practical factors. The results of the simulations showed that the cylindrical HIFU phased array in water had a dynamic focusing range from 145 to 175 mm in the depth direction and a steering range from -15 to 15 mm in azimuthal direction with respect to the center of the array. After the dissipation of cables and the attenuation of various media, the designed array could still generate the intensity at focus up to 1095 W/cm2 when the input electrical power was approximately 410 W. The prototype of the array was fabricated and the preliminary test was completed. The testing results showed that each element of the array prototype can work well.

P2G-2 High Intensity Focused Ultrasound Thermal Therapy for Liver Tumor with Respiration Motion

High intensity focused ultrasound thermal therapy for liver tumor may have a risk of normal tissue damage due to the respiratory motion. The goal of this study is to investigate the thermal dose pattern by using one/two 80-elements linear phased array for liver tumor thermal therapy. The two phased array transducers con-focus at the desired treatment area which is moving due to the respiration. A simple, finite state model of respiratory motion pattern is used to simulate the regular and irregular respiration. An energy deposition algorithm is employed to predict the thermal lesion location. Temperature and thermal dose distributions showed that a better heating can be obtained when the transducer is perpendicular to the respiratory motion direction. The experiment results showed that the thermal lesion location was coincident with that predicted by the energy deposition algorithm. The study revealed that the thermal dose and temperature patterns are highly affected by the respiratory motion and the energy deposition algorithm can be used for the window gating strategy as the high intensity focused ultrasound employed in liver tumor thermal therapy

BEAM STEERING AND FOCUSING ABILITY OF A CONTACT ULTRASOUND TRANSDUCER FOR TRANSSKULL BRAIN DISEASE THERAPY

This paper was to examine the steering and focusing ability of a contact hemispherical ultrasound transducer (80 mm radius of curvature, 160 mm diameter) for transskull brain diseases therapy without skull-specific aberration correction. This work employs a simulation program to investigate the effect of ultrasound transducer parameters on the steering and focusing ability for transskull therapy. The acoustic pressure distribution and the grating lobes in tissue were used to determine the steering and focusing ability of this transducer for a set of given conditions. Simulation results demonstrated that this hemispherical phased array transducer with low frequencies can steer a high-pressure focal zone in a large range within the brain. The peak and size of the high-pressure focal zone mainly depend on ultrasound frequency and the steering distance of the focal zone. By comparing the peak pressures between the focal zone and the grating lobe, 0.1 MHz transducer performed the desired results for large ranges (140 mm x-y direction and 138 mm z direction) of beam steering. The results reveal the feasibility of using a hemispherical phased array transducer with beam steering method at low frequency for brain diseases therapy within almost full range of the brain without performing a craniectomy.

Design and fabrication of a wide-aperture HIFU annular array transducer for the treatment of deep-seated tumors

In HIFU treatment applications, the annular array transducer is a feasible solution for the clinical/engineering requirements which are as follows: ablation of tumors deep inside body, electronic dynamic focusing in the depth direction, simple configuration/operation, and lower cost due to fewer elements/channels of amplifier. A 12 cm‐diameter, 12 cm‐radius‐of‐curvature annular array transducer has been developed in this study. The pseudo‐inverse method was adopted to calculate the desired phase of each element for focusing, and the Rayleigh‐Summerfield integral was used to obtain the ultrasonic pressure field. In the simulation, the operating frequency was 0.9 MHz, and the acoustic medium was water. A piece of 1–3 piezocomposite was fabricated using the dice and fill technique for the pilot test. The dimension of the sample was 4×2 cm, and it was thermally shaped using a spherical mold of 12 cm in radius. The results of the simulation showed that the focus could not be moved electronically in the depth d...

Development of 1.5D cylindrical HIFU phased array

High intensity focused ultrasound, HIFU has been used to non-invasively treat human tumors, such as uterine fibroids, prostate cancer, breast cancer, liver tumor, and brain tumor. However, the tumor in some organs can be moved by human breathing and heart beat, which may cause the ablation and damage of normal tissues during the sonication of HIFU. The purpose of this study was to develop a HIFU phased array for tracking the moving tumors. In the initial design, the array had a center frequency of 1.0 MHz and 512 elements that allowed symmetric control. PZT-4 1-3 composites were formed via the ldquodice and fillrdquo technology and then shaped into a cylindrical structure. The results simulated by Field II demonstrated that the array in water had a dynamic focusing range from 145 mm to 175 mm in Depth and a steering range from -15 mm to 15 mm in azimuthal direction with respect to the center of the array. A prototype of the array was fabricated. The aperture of the array was 15 cm by 12 cm and the radius of curvature was 15 cm. The impedance of each channel (a pair of elements) was measured and the average was 1500 Ohm. Surface acoustic intensity up to 12.6 W/cm2 was applied to one piece of cylindrical PZT4-composite consisting of 22 elements for 50 minutes, and the temperature of the composite was not observed to rise appreciably. A 256-channel amplifier was utilized to drive the array without impedance matching in water and the water spring was observed at the natural focus of the array when the driving signal to each channel was zero-phase at 0.9 MHz. The phase control and impedance matching circuits for each channel are being designed.