Douglas R. Daum

Design and evaluation of a feedback based phased array system for ultrasound surgery

A driving system has been designed for phased array ultrasound applicators. The system is designed to-operate in the bandwidth 1.2 to 1.8 MHz, with independent channel power control up to 60 W (8 bit resolution) for each array element. To reduce power variation between elements, the system utilizes switching regulators in a feedback loop to automatically adjust the DC supply of a class D/E power converter. This feedback reduces the RF electrical power variation from 20% to 1% into a 16 element array. DC-to-RF efficiencies close to 70% for all power levels eliminates the need for large heat sinks. In addition to power control, each channel may be phase shifted 360/spl deg/ with a minimum of 8 bit resolution. To ensure proper operation while driving ultrasound arrays with varying element sizes, each RF driving channel implements phase feedback such that proper phase of the driving signal is produced either at the amplifier output before the matching circuitry or after the matching circuitry at the transducer face. This feedback has been experimentally shown to increase the focal intensities by 20 to 25% of two tested phased arrays without array calibration using a hydrophone.

In vivo demonstration of noninvasive thermal surgery of the liver and kidney using an ultrasonic phased array

A 256-element, continuous-wave ultrasonic phased array has been used to thermally coagulate deep-seated liver and kidney tissue. The array elements were formed on a 1-3 piezocomposite bowl with a 10-cm radius of curvature and 12-cm diameter. The 0.65 x 0.65 cm2 projection elements were driven at 1.1 MHz by a custom-built amplifier system. A series of in vivo porcine experiments demonstrated the ability to coagulate liver and kidney tissue using the large-scale phased array. The temperature response of the treatment was guided and monitored using magnetic resonance (MR) images. Focal lesion volumes greater than 0.5 cm3 in kidney and 2 cm3 in liver were formed from a single 20-s sonication.

Thermal dose optimization via temporal switching in ultrasound surgery

Temporal switching has been simulated and implemented in vivo experiments as a method to optimize thermal dose in ultrasound surgery. By optimizing the thermal dose over a tissue volume, the peak temperature is decreased, less average power is expended, and overall treatment time is shortened. To test this hypothesis, a 16 element, spherically sectioned array has been constructed for application in ultrasound surgery guided by magnetic resonance imaging. A simulation study for the array was performed to determine an optimal treatment from a set of multiple focus fields. These fields were generated using the mode scanning technique with power levels determined numerically using a direct weighted gradient search in the attempt to create an optimally uniform thermal dose over a 0.6/spl times/0.6/spl times/1.0 cm/sup 3/ tissue volume. Comparisons of the switched fields and a static multiple focus field indicate that the switching technique can lower power requirements and decrease treatment time by 20%. More importantly, the peak temperature of the sonication was lowered 13/spl deg/C, thus decreasing the possibility of cavitation. The simulated results of the 16 element array were then experimentally tested using MRI to noninvasively monitor temperature elevations and predict lesion size in rabbit thigh muscle in vivo. In addition, the results show that the switching technique can be less sensitive to tissue inhomogeneities than static field sonication while creating contiguous necrosis regions at equal average powers.

Feasibility of using ultrasound phased arrays for MRI monitored noninvasive surgery

The purpose of this paper was to evaluate the in vivo feasibility of using phased arrays for MRI guided ultrasound surgery. Two different array concepts were investigated: a spherically curved concentric ring array to move the focus along the central axis and a spherically curved 16 square element array to make the focus larger. Rabbit thigh muscles were exposed in vivo in a 1.5 T MRI scanner to evaluate the array performance. The results showed that both of the arrays performed as expected, and the focus could be moved and enlarged. In addition, adequate power could be delivered from the arrays to necrose in vivo muscle tissue in 10 s. This study was the first implementation of phased arrays for MRI guided ultrasound surgery. The results demonstrate that phased arrays have significant potential for noninvasive tissue coagulation.

Theoretical design of a spherically sectioned phased array for ultrasound surgery of the liver

GOALnThe theoretical explanation of the limits of an array transducer to coagulate large tissues volumes.nnnMETHODSnA theoretical model is used to illustrate the focal limitations of a spherically sectioned array designed for the treatment of deep seated tissue, e.g. liver. The design optimizes the acoustic dose as a function of the focal depth and available acoustic aperture with the goal of coagulating large volumes in a single sonication period. A quantitative measure of the possible region of focal necrosis is modeled as a function of array parameters with the limiting criteria being near field heating and patient pain.nnnRESULTSnAcoustic simulations show that the maximum distance to produce continuous necrosis between foci in a multiple focus pattern and in a temporally multiplexed pattern is approximately 50% larger than the distance needed between sequential foci.nnnCONCLUSIONnMultiple focus patterns or rapidly scanned single foci are significantly advantageous to sequential sonications of a single focus transducer.

MR-guided noninvasive thermal coagulation of in-vivo liver tissue using ultrasonic phased array

Magnetic resonance (MR) imaging was used to guide and monitor the thermal tissue coagulation of in vivo porcine tissue using a 256 element ultrasonic phased array. The array could coagulate tissue volumes greater than 2 cm3 in liver and 0.5 cm3 in kidney using a single 20 second sonication. This sonication used multiple focus fields which were temporally cycled to heat large tissue volumes simultaneously. Estimates of the coagulated tissue using a thermal dose threshold compare well with T2-weighted images of post sonication lesions. The overlapping large focal volumes could aid in the treatment of large tumor volumes which require multiple overlapping sonications. The ability of MR to detect the presence and undesirable thermal increases at acoustic obstacle such as cartilaginous and bony ribs is demonstrated. This could have a significant impact on the ability to monitor thermal treatments of the liver and other organs which are acoustically blocked.

Phased array development for noninvasive focused ultrasound therapy of brain and liver

In order to perform noninvasive ultrasound therapy of brain and liver, several phased arrays have been developed in our laboratory. A 256 element array and a 449 element array have been constructed, both operate at 1.1 MHz. We have obtained lesions in pig livers through the abdominal wall using the 256 element array. Additionally, we have designed a hemisphere array to operate at 625 kHz for transskull brain therapies, and numerical results are presented when the hemisphere transducer is divided into 228 elements.

Experimental verification of the advantage of a 256-element ultrasonic phased array for thermal coagulation of deep seated tissue

A 256 element ultrasonic phased array was constructed from thermal treatment of deep seated tissue. The 1.1 MHz array had a 10 cm radius of curvature and a 12 cm diameter. The elements formed a planar projection grid of 0.65 X 0.65 cm2 such that the focal range of the array was approximately +/- 1 cm from the natural focus of the array both in the focal plane and +/- 2 cm along the array axis. The array was driven with phased continuous wave signals to both shift individual foci and to create multiple focus patterns. The goals of this study were to demonstrate that an array with many elements has the ability to coagulate large volumes of deep seated tissue in a single sonication and to experimentally compare the in vivo thermal measurements of large focal volume sonications to those predicted in a simple simulation model. It was found that the array could coagulate thigh muscle volumes of 3 - 5 cm3 in a twenty second sonication.

Ultrasound phased arrays for noninvasive surgery and therapy

Practical high power phased arrays have been developed and tested in our laboratory. The results show that they offer significant advantages over conventional single focused ultrasound transducers for noninvasive ultrasound surgery. These advantages include but are not limited to: compensation of phase distortion induced by overlying tissues, potential for inducing optimal energy delivery patterns, and generation of large focal spots for tumor coagulation. This paper will describe some of our results with the therapy phased array.

MRI monitoring and control of focused ultrasound surgery

The clinical and animal tests of focused ultrasound surgery have shown significant variation between the effectiveness of the same power sonications. Therefore, it has become important to seek methods to monitor the in vivo ultrasound exposures. One of the most promising methods is the use of magnetic resonance imaging (MRI) to detect the temperature elevation. In this paper the calibration and accuracy of MRI thermometry will be presented. The in vivo mapped temperature distributions allow the thermal exposure of the tissue to be estimated. These estimates have been compared with the tissue coagulation produced in muscle and brain tissues in vivo. The results show that the lesion boundary is predicted with the resolution of the MRI voxels. This has significant clinical value since the MRI thermometry can be used to control the treatment on‐line. These experiments allowed comparison of the accuracy of the theoretical models for predicting the temperature elevation and tissue necrosis in vivo. Similarly, t...