Focused ultrasound offers a non-invasive way of depositing acoustic energy deep into the body, which can be harnessed for a broad spectrum of therapeutic purposes, including tissue ablation, the targeting of therapeutic agents, and stem cell delivery. Phased array transducers enable electronic control over the beam geometry and direction, and can be tailored to provide optimal energy deposition patterns for a given therapeutic application. Their use in combination with modern medical imaging for therapy guidance allows precise targeting, online monitoring, and post-treatment evaluation of the ultrasound-mediated bioeffects. In the past there have been some technical obstacles hindering the construction of large aperture, high-power, densely-populated phased arrays and, as a result, they have not been fully exploited for therapy delivery to date. However, recent research has made the construction of such arrays feasible, and it is expected that their continued development will both greatly improve the safety and efficacy of existing ultrasound therapies as well as enable treatments that are not currently possible with existing technology. This review will summarize the basic principles, current statures, and future potential of image-guided ultrasound phased arrays for therapy.

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Image-guided ultrasound phased arrays are a disruptive technology for non-invasive therapy

A CT scanner yields CT numbers which are proportional to the fractional difference in effective local electron density of the subject material with respect to that of calibration material. A homogeneous water-equivalent material is used as a calibration phantom under isothermal conditions. Any temperature variation (spatial or temporal) in the subject material subsequently scanned, will generate a CT-number shift in the CT image because of density changes due to thermal expansion. The potential use of the thermally generated CT-number shift in noninvasive thermometry during cancer hyperthermia was studied in vitro in samples of water and muscle tissue. The reproducibility of the area-averaged CT-number measurement on our EMI-7070 scanner was found to depend strongly on the time interval between successive counterclockwise scans and on the size of the region of interest used for area averaging. A linear relationship was found between the CT number and water density in the water temperature range from 10 to 55 degrees C. In the hyperthermia temperature range (36-50 degrees C), the relationship between the CT number and temperature itself is linear, with a CT-number thermal shift of about 0.4 and 0.45 HU/degrees C for water and muscle tissue, respectively. The achievable temperature discrimination is a fraction of degree C at spatial resolutions of the order of a centimeter.

Noninvasive thermometry with a clinical x-ray ct scanner

With a change in phased-array configuration from one dimension to two, the electrical impedance of the array elements is substantially increased because of their decreased width (w)-to-thickness (t) ratio. The most common way to compensate for this impedance increase is to employ electrical matching circuits at a high cost of fabrication complexity and effort. In this paper, we introduce a multilayer lateral-mode coupling method for phased-array construction. The direct comparison showed that the electrical impedance of a single-layer transducer driven in thickness mode is 1/(n2(1/(w/t))2) times that of an n-layer lateral mode transducer. A large reduction of the electrical impedance showed the impact and benefit of the lateral-mode coupling method. A one-dimensional linear 32-element 770-kHz imaging array and a 42-element 1.45-MHz high-intensity focused ultrasound (HIFU) phased array were fabricated. The averaged electrical impedances of each element were measured to be 58 Ω at the maximum phase angle of -1.2° for the imaging array and 105 Ω at 0° for the HIFU array. The imaging array had a center frequency of 770 kHz with an averaged -6-dB bandwidth of approximately 52%. For the HIFU array, the averaged maximum surface acoustic intensity was measured to be 32.8 W/cm2 before failure.

Large improvement of the electrical impedance of imaging and high-intensity focused ultrasound (HIFU) phased arrays using multilayer piezoelectric ceramics coupled in lateral mode

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.

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

Cooling time between sequential sonications in the conventional heating method for high-intensity focused ultrasound (HIFU) leads to long treatment time. In order to reduce treatment time, an unequal heating duration method, which means heating in long and short durations alternately without cooling time, is introduced in this paper. Three treatment plans, which are conventional method, unequal heating duration method in linear scanning and unequal heating duration method in spiral scanning, are compared through ablating the same area in simulation. To evaluate the treatment results, the finite element method (FEM) is employed to perform transient thermal analysis and thermal doses are calculated. With the unequal heating duration method, treatment time is reduced by more than 50 percent compared with the conventional method while the target necrosis rate is more than 97 percent. Simulation results show that the unequal heating duration method is effective and promising in HIFU.

Treatment Planning of Scanning Time and Path for Phased High-Intensity Focused Ultrasound Surgery

Currently, most phased-array systems intended for therapy are one-dimensional (1-D) and use between 5 and 200 elements, with a few two-dimensional (2-D) systems using several hundred elements. The move toward lambda/2 interelement spacing, which provides complete 3-D beam steering, would require a large number of closely spaced elements (0.15 mm to 3 mm). A solution to the resulting problem of cost and cable assembly size, which this study examines, is to quantize the phases available at the array input. By connecting elements with similar phases to a single wire, a significant reduction in the number of incoming lines can be achieved while maintaining focusing and beam steering capability. This study has explored the feasibility of such an approach using computer simulations and experiments with a test circuit driving a 100-element linear array. Simulation results demonstrated that adequate focusing can be obtained with only four phase signals without large increases in the grating lobes or the dimensions of the focus. Experiments showed that the method can be implemented in practice, and adequate focusing can be achieved with four phase signals with a reduction of 20% in the peak pressure amplitude squared when compared with the infinite-phase resolution case. Results indicate that the use of this technique would make it possible to drive more than 10,000 elements with 33 input lines. The implementation of this method could have a large impact on ultrasound therapy and diagnostic devices

A novel phase assignment protocol and driving system for a high-density focused ultrasound array

Currently, most phased array systems intended for therapy are 1-dimensional and employ between 5 and 200 elements, with a few 2-dimensional systems using several hundred elements. The move towards lambda/2 inter-element spacing, which provides complete 3D beam steering, would require a large number of closely spaced elements (0.15 mm to 3 mm). The solution to the resulting problem of cost and cable assembly size, which this study examines, is to quantize the phases available at the array input. By connecting elements with similar phases to a single wire, a significant reduction in the number of incoming lines can be achieved, while maintaining focusing and beam steering capability. This study has explored the feasibility of such an approach using computer simulations and experiments with a test circuit driving a 100-element linear array. The simulation results demonstrated that adequate focusing can be obtained with only four phase signals, without large increases in the grating lobes or the dimensions of the focus. The experiments showed that the method can be implemented in practice and adequate focusing can be achieved with four phase signals with a reduction of 20% in the peak pressure amplitude squared when compared with the infinite phase resolution case. The results indicate that the use of this technique would make it possible to drive more than 10000 elements with 33 input lines. The implementation of this method could have a large impact on ultrasound therapy and diagnostic devices

1C-3 A Design for a High-Density Focused Ultrasound Array Addressing and Driving System

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A novel phase assignment protocol and driving system for a high-density focused ultrasound array

1C-3 A Design for a High-Density Focused Ultrasound Array Addressing and Driving System