Jingfeng Bai

Focus Shift and Phase Correction in Soft Tissues During Focused Ultrasound Surgery

During the treatment of soft tissue tumors using focused ultrasound surgery (FUS), the focus can shift away from the desired point due to tissue inhomogeneity. In this paper, a numerical method to calculate the focus shift in multiple-layered tissues and a faster phase-correction method to restore the focus were developed. Data from the simulations showed that the focus shifted about 2 mm along the transducer axis in multiple-layered soft tissues. After phase correction, the focus was restored at the desired point. The ex vivo experiments were conducted to verify the simulations, and the results agreed well with those of the simulations. An empirical formula was obtained to estimate the focus shift in a two-layered water-tissue model and was verified by numerical calculations. Moreover, the focus shift in multiple-layered tissues can be summed by the shifts in the component of each layer of tissues. The factors affecting the focus shift were studied. The focus shift varied linearly with the tissue acoustic speed and tissue thickness, whereas it slightly changed with transducer F number (radius of curvature/diameter). Overall, the findings of this study can help in the development of a better treatment plan for FUS in soft tissues.

High-intensity focused ultrasound with large scale spherical phased array for the ablation of deep tumors

Under some circumstances surgical resection is feasible in a low percentage for the treatment of deep tumors. Nevertheless, high-intensity focused ultrasound (HIFU) is beginning to offer a potential noninvasive alternative to conventional therapies for the treatment of deep tumors. In our previous study, a large scale spherical HIFU-phased array was developed to ablate deep tumors. In the current study, taking into account the required focal depth and maximum acoustic power output, 90 identical circular PZT-8 elements (diameter=1.4 cm and frequency=1 MHz) were mounted on a spherical shell with a radius of curvature of 18 cm and a diameter of 21 cm. With the developed array, computer simulations and ex vivo experiments were carried out. The simulation results theoretically demonstrate the ability of the array to focus and steer in the specified volume (a 2 cm×2 cm×3 cm volume) at the focal depth of 15 to 18 cm. Ex vivo experiment results also verify the capability of the developed array to ablate deep target tissue by either moving single focal point or generating multiple foci simultaneously.

A Study of Heating Duration and Scanning Path in Focused Ultrasound Surgery

A conventional method of avoiding normal tissue overheating during focused ultrasound surgery (FUS) is to apply equal heating duration for each sonication in between cooling intervals. However, this method is time-consuming and expensive. A novel method with unequal heating duration in different scanning paths without cooling intervals was investigated in this paper. This method was compared with the conventional method through the ablation of a 10 × 10 cm2 target. The simulation results indicated that the new method was able to reduce treatment time by more than 50%, with higher than 95% coverage index, and more uniform thermal dose distribution. The method was further verified through the ablation of a circular lesion. Ex vivo experiments were also performed to confirm the simulation results. Both simulation and experimental results have proven that the new method can increase heating efficiency, and are a promising new approach in FUS.

Effects of different parameters in the fast scanning method for HIFU treatment

PURPOSE High-intensity focused ultrasound is a promising method for the noninvasive treatment of benign and malignant tumors. This study analyzes the effects of scanning path, applied power, and geometric characteristics of the transducer on ablation using fast scanning method, a new scanning method that uses high-intensity focused ultrasound at different blood perfusion levels. METHODS Two transducers, six scanning paths, and three focal patterns were used to examine the ablation results of the fast scanning method using power densities from 1.35 × 10(7) W∕m(3) to 4.5 × 10(7) W∕m(3) and blood perfusion rates from 2 × 10(-3) ml∕ml∕s to 16 × 10(-3) ml∕ml∕s. The Pennes equation was solved using the finite-difference time-domain method to simulate the heating procedure. RESULTS Based on the results of the fast-scanning method, the different scanning paths exhibited small effect on the total treatment time supported by both simulation and least-square fit. Similar-sized lesions can result from the five different repeated paths, whereas a random path may lead to relative large fluctuations in ablation volume because of asymmetry of the lesions. Higher power levels increase the lesion volume and decrease the treatment time required for ablating a target area using the fast scanning method, whereas increased blood perfusion has the opposite effect on ablation volume and treatment time. A symmetric lesion can be produced through fast scanning method using a 65-element and a 90-element transducer. However, lesion production using the same operation scheme differs between the two transducers. CONCLUSIONS Unlike traditional scanning methods, fast scanning method produces a planned lesion regardless of scanning path, as long as the path consists of repeated subsequences. This attribute makes fast scanning method an easy-operation scheme that produces relatively large symmetric lesions in homogeneous tissues. Applied power is the most important factor; however, high blood perfusion levels can limit or even hinder the full ablation of the target area. Therefore, tissue perfusion and transducer type should be given special attention to ensure the success and safety of ablation treatment.

Approximate Analytical Solution for Tissue Temperature Distribution in High-Intensity Focused Ultrasound

An approximate analytical solution for tissue temperature distribution in high-intensity focused ultrasound (HIFU) is obtained in this work. Based on variable separation method (VSM), the solution is performed to solve the simplified bioheat transfer equation (BHTE). Lack of enough boundary conditions; the parameters in this analytic solution are estimated using the temperature data calculated by finite element method (FEM) with nonlinear least square algorithm. The absolute root mean square error (RMSE) of temperature is 0.21℃ and 0.22℃ for two strategies, respectively. Feasibility and accuracy of the analytical solution are verified by comparison with FEM for transient temperature response in a longer time domain. The average deviation of temperature is about 0.35℃. So this analytic solution is suitable to calculate spatiotemporal temperature distribution in HIFU treatment, and it is quite useful to the treatment planning.

The calibration of targeting errors for an ultrasound-guided high-intensity focused ultrasound system

Accurate targeting is one indispensable feature of image-guided high-intensity focused ultrasound (HIFU) systems for treatment safety and efficacy. In our previously developed ultrasound-guided phased-array HIFU system, a rotatable imaging probe was mounted into the central hole of applicator for targeting and monitoring. Two-dimensional image sequence of different imaging planes can be obtained by rotation of the probe. The misalignment between the spots predetermined in the image and the spots sonicated in the tissue can result in the ablation of normal tissue outside the targeting volume, and thus targeting error is unavoidable. An acrylic plate internally placing two flat-head bolts was constructed to measure and calibrate the targeting error. The imaging planes were switched from −90° to 90° with a 30° step, and the targeting errors were measured 12 times for each of these planes before and after calibration. The targeting errors in other imaging planes could be estimated by linear interpolation using the measured errors in the nearest two imaging planes. The coordinates of the spots to be sonicated were corrected in consideration of the targeting errors in the selected imaging plane. After calibration, the mean targeting errors were reduced to 0.30∼0.68 mm from 0.86∼1.74 mm. Besides, in the ex vivo experiment the needle-thermocouple tip was used as the target which could be identified in the image. The temperature rise measured by the thermocouple during sonication was in accordance with the theoretical result. In conclusion, the calibration of targeting errors is effective for our system, and the targeting accuracy is also capable to ensure safe sonication.

Experimental evaluation of blocking effect on emboli in the aorta ascendens by using phased array ultrasound

Emboli generated in cardiac surgery can result in many postoperative neurological complications. There are many methods to reduce the amount of emboli or to block them. Nevertheless, many of methods have limitation. In this study, ultrasonic radiation force generated by phased array ultrasound is used to block emboli and the blocking effect is evaluated. We do vivo experiment on eight pigs (40kg). After opening thorax, we put ultrasound transducer between ascending aorta and brachiocephalic artery. The man-made emboli are injected in the left auricle 8 times on each pig, 4 times with ultrasound transducers off and 4 times with ultrasound transducers on. When emboli flow to ascending aorta, ultrasonic radiation force will act on emboli and block some of them from flowing into brachiocephalic artery. And emboli flowing into the right common carotid artery would be detected by PHILIPS CX50. At the end of the experiment, the sonicated tissue is cut for histopathology test. The results of vivo experiment demonstrate the feasibility and effect of blocking in the evaluation.

The calibration of targeting for an ultrasound-guided high-intensity focused ultrasound system

Accurate targeting is essential to ensure safety for high-intensity focused ultrasound (HIFU) treatment. Recently we developed an ultrasonography-guided high-intensity focused ultrasound (USgHIFU) system, in which the imaging probe is mounted into the central hole of spherical HIFU phased-array transducer for targeting and monitoring. An acrylic plate with two flat-head bolts is used for the assessment of the targeting accuracy. Imaging planes with the angles from −90° to 90° with an interval of 30° were calibrated and respective targeting errors were measured. And the targeting errors of non-calibrated imaging planes were derived from the average of the nearest two calibrated planes. The coordinates in HIFU sonication were corrected by the error in the specific imaging plane. With the proposed calibration approach, the targeting error has been reduced to less than 0.5 mm. In vitro experimental result also shows that the temperature rise of the targeting focus in the image is similar to the result of previous work. In conclusion, our USgHIFU system is capable of accurate sonication.

Dual-focus scanning in volumetric HIFU ablation: Preliminary simulation study

Volumetric HIFU ablation has been implemented in clinical practice to create large thermal lesion while reducing treatment durations. However, it still takes several hours to cover the whole target using volumetric ablation. Multiple focus intensity patterns have been proved beneficial to create larger lesions. The feasibility of dual-focus scanning in volumetric ablation has been evaluated. Simulations of dual-focus scanning and single-focus scanning were performed with the treatment cells of 4, 8, 12 and 16 mm in diameter. Comparison of both scanning approaches indicates that the dual-focus scanning saves treatment time by 20% and needs lower peak acoustic pressure, therefore the potential cavitation could be avoided. Although the maximum near-field temperatures are slightly higher than single-focus scanning, they are acceptable in HIFU treatment.

Experimental evaluation of targeting accuracy of a B-mode ultrasound-guided phased-array focused ultrasound system using a thermocouple array

Image-guide high-intensity focused ultrasound (HIFU) has been widely used in the treatment of uterine fibroids. However, before HIFU system is ready for use, the targeting accuracy should be known for the safety of important abdominal organs surrounding the target. Researchers have used magnetic resonance (MR) images or computed tomography (CT) to evaluate the targeting accuracy of current MR-guided HIFU systems, whereas these methods cannot be directly used in the ultrasound-guided HIFU (USgHIFU) systems. In this study, a thermocouple array is used in combination with the in-house built acrylic plate to evaluate the targeting accuracy of the existing 112-channel phased-array USgHIFU system. The results of ex vivo experiment demonstrate the feasibility and the efficacy of the thermocouple array in the evaluation, and the targeting accuracy of the USgHIFU system is about 1mm in the focal plane. Thus, this accuracy is adequate for the treatment of uterine fibroids.