Bai Jingfeng

Unequal heating duration to reduce the treatment time in high-intensity focused ultrasound therapy

Cooling periods are usually intervened between two consecutive sonications to avoid near-field heating during high-intensity focused ultrasound (HIFU) therapy, which leads to a long treatment time. A possible strategy to shorten the treatment time is to eliminate the cooling intervals. An unequal heating duration method without the cooling time is proposed to reduce the treatment time during HIFU. The starting focal point in the target region is set with the longest duration. Two unequal periods with a long-short pattern are prepared for two sequential focal points begun with that starting point; with the two points, the previous one is heated for a long duration while the latter is heated for a short duration. The durations for the rest focal points are set by analogy. If the distance between a point and its previous heated point is more than double focal spacings, it is set as a new starting point with a long heating period. Computer simulations and ex vivo experiments with the focus scanned by a 90-element spherical phased array had been carried out. An H-shaped lesion consisting of 17 single lesions about 2 mm in diameter was generated with a 2-mm focal spacing using the proposed method, and average time per point was reduced by 65% and 76% respectively in simulation and experiment. Moreover, in simulation 2 separate prefocal lesions (≪ 3 mm in diameter) were generated 2 mm proximal of the focal plane while no lesion was generated 5 mm proximal of the focal plane. The results demonstrated the proposed method was able to effectively shorten the treatment time by setting unequal heating durations for consecutive focal points and sparing the cooling interval through sonications. The results also indicated that it was promising to alleviate nontarget surrounding tissue damage in HIFU treatments by optimizing heating duration distributions of focal points.

A Comparison of the Pathologically Transmural and Electrical Isolation of Atrial Tissues after Once and Three Times of Bipolar Radiofrequency Ablation

A finite element model was developed to simulate the ablation results. The optimal parameters (30 w, 15 s) were obtained by changing the input energy (20 w, 25 w, 30 w, 35 w, 40 w) with different operating times (15 s, 20 s, 25 s, 30 s). Then, in vitro and in vivo experiments were performed based on the results from FEM simulation. In the in vitro experiments, 240 pieces of strips of tissues were equally divided into group A and group B, and group A was ablated once by bipolar RFA system while group B was ablated three times. After stained with Mason trichrome, the tissue was examined under the microscope to validate whether a pathologically transmural ablation was successfully achieved. In the in vivo experiments, 20 pigs were equally divided into group C and group D. Left pulmonary veins in Group C and left atrial appendages ablated once while the right pulmonary veins and atrial appendage were ablated three times, and left atrial were ablated once while their right atrial ablated three times in group D. The electrophysiological mapping was utilized to validate the electro physiologically transmural lesion. And after fed for 4 weeks, the second electrophysiological mapping was utilized to confirm the long-term electrical isolation. Pathologically transmural lesion was achieved in 51.67% and 98.33% of the atrial tissue in Group A and B respectively. Total immediate electrical isolation rate was 61.67% for once ablation, while it was 100% for three times of ablation. And the total long-term electrical isolation rate was 31.67% for once ablation, while it was 85% for three times of ablation. The results demonstrate that the three times of ablation can improve the rate of pathologically transmural and electrical isolation significantly.

The ultrasound fields estimation using uncooled infrared system

The ultrasound fields through a certain thickness of tissue are a critical parameter in the high intensity focused ultrasound (HIFU) treatment. Although the ultrasonic fields in tissue are difficult to measure, the temperature elevation induced by ultrasound can be obtained to estimate the ultrasonic intensity. In this work, the ex-vivo experiments were conducted in porcine muscle with the thickness of 10-30 mm. The distance from the transducer to the surface of muscle can be varied. The temperature distribution in tissue-air surface can be obtained from an uncooled infrared system (spatial and thermal resolutions are 0.59 mm and 0.08 °C). According the Snells law, the intensity at interface is about four times of the incident wave when ultrasound wave propagates from tissue to air, due to the total internal reflection. Then the intensity distribution of incident wave can be estimated by temperature distribution in tissue-air surface. The experimental temperature was consistent with that of simulations. ...

A method of estimating ultrasound fields at full power using infrared and hydrophone system

The acoustic characterization of ultrasound fields is not only important for the safety and efficiency of clinical treatment, but also for therapy planning. In this paper, a method to estimating the ultrasound fields at full power using hydrophone and uncooled infrared system was developed. The method includes four steps. The amplitude absorption coefficient of the medium can be measured through the first two steps. Through the second and third steps, we can estimate the ratio of absorbers heat capacity per unit volume to absorbers ultrasonic amplitude absorption coefficient. This ratio will be used to in step four, in order to estimate the intensity of HIFU at high driving voltage. This method made it is possible to determine the intensity of HIFU driving with high voltage, and there were no need to know the thermal and acoustic parameters of acoustic absorber.

Sub‐volume Heating Strategy To Shorten Treatment Time In Ultrasound Surgery

Treating tumors or other potentially large targets for thermal ablation can be a very time‐consuming task in ultrasound surgery. Tumors are conventionally treated by pointwise scanning over the tumor region using predetermined focal points. During scanning, cooling times are needed between two consecutive focal points to diffuse thermal buildup in the near field, which can prevent undesired temperature rises in healthy tissue. It is possible to accelerate the treatment while ensuring the safety of healthy tissue by reallocation of the heating and cooling times for individual focus points. In this study, we proposed a sub‐volume heating strategy by reducing the number of cooling periods, to shorten the treatment time. The target volume is divided into several equally sized sub‐volumes, each of which is covered by the same number of focal points. The sub‐volume is sonicated, followed by a constant cooling period (the number of cooling periods = the number of sub‐volumes). The proposed strategy was evaluated...

A Method of Estimating Pressure and Intensity Distributions of Multielement Phased Array High Intensity Focused Ultrasonic Field at Full Power Using a Needle Hydrophone

The pressure and intensity distribution of high intensity focused ultrasound (HIFU) fields at full power are critical for predicting heating patterns and ensuring safety of the therapy. With the limitations of maximum pressure at the hydrophone and damage from cavitation or thermal effects, it is hard to measure pressure and intensity directly when HIFU is at full power. HIFU‐phased arrays are usually composed of large numbers of small elements and the sound power radiated from some of them at full power is measureable using a hydrophone, we grouped them based on the limitation of maximum permissible pressure at the hydrophone and the characteristics of the element arrangement in the array. Then sound field measurement of the group was carried out at full power level. Using the acoustic coherence principle, the pressure and intensity distribution of the array at full power level can be calculated from corresponding values from the groups. With this method, computer simulations and sound field measurement ...