A cylindrical-section ultrasound phased-array applicator for hyperthermia cancer therapy

A phased-array applicator geometry for deep localized hyperthermia is presented. The array consists of rectangular transducer elements forming a section of a cylinder that conforms to the body portals in the abdominal and pelvic regions. Focusing and scanning properties of the cylindrical-section array are investigated in homogeneous lossy media using appropriate computer simulations. The characteristic focus of this array is shown to be spatially limited in both transverse and longitudinal directions with intensity gain values suitable for deep hyperthermia applications. The ability of the cylindrical-section phased array to generate multiple foci using the field conjugation method is examined. The effect of the grating lobes on the power deposition pattern of the scanned field is shown to be minimal. Steady-state temperature distributions are simulated using a three-dimensional thermal model of the normal tissue layers surrounding a tumor of typical volume. The advantages and the limitations of this array configuration are discussed.     

Experimental evaluation of a prototype cylindrical section ultrasound hyperthermia phased-array applicator

A prototype 64-element, 75 degrees cylindrical-section ultrasonic phased-array hyperthermia applicator has been designed and constructed. The ability of this applicator to focus ultrasonic energy at its geometric focus is verified in a water medium. The array is then driven by excitation vectors obtained using the pseudoinverse pattern synthesis method to generate shifted-focus and multiple-focus field patterns. Experimental results of single and multiple-focus patterns at 500 kHz are given and are in good agreement with theory. The results indicate that the main beam in single-focus patterns is generally insensitive to errors in the phases and amplitudes of the particle velocities of the array elements. The effect of such errors is largely exhibited in the sidelobes which, for all practical purposes, remain at levels acceptable for hyperthermia. This is true for both the geometric focus and shifted foci.     

Computationally efficient algorithms for control of ultrasound phased-array hyperthermia applicators based on a pseudoinverse method

Computationally efficient incremental algorithms for application of the pseudoinverse method for ultrasound phased-array field pattern synthesis are discussed and shown to significantly reduce computing effort when more control points should be added to existing subsets of points previously manipulated. These algorithms avoid calculation of inverse matrices, a time-consuming job for microcomputers when the dimensions of the matrices are large. This technique can be used to recursively modify the heating patterns of a phased-array hyperthermia applicator in the clinic.     

NxN square-element ultrasound phased-array applicator: simulated temperature distributions associated with directly synthesized heating patterns

Computer simulations to demonstrate the possibility of heating small tumors by appropriately driving the electronic control circuitry of an nxn square-element ultrasonic phased array are conducted. The synthesis method consists of simultaneously focusing the ultrasonic beam at different points uniformly distributed along the tumor periphery and, hence, involves no scanning. It is demonstrated that by combining the multiple focusing feature with a new field phasing concept, typical undesired hot spots can be eliminated. The tissue thermal response to this heating modality is investigated by iteratively solving the three-dimensional steady-state bioheat equation. Temperature distributions associated with different directly synthesized power deposition patterns are simulated and discussed.     

Effect of phase errors on field patterns generated by an ultrasound phased-array hyperthermia applicator

The effect of phase quantization errors and Gaussian distributed random phase errors on field patterns synthesized by a rectangular ultrasound phased array hyperthermia applicator is studied. The parameters defined show that, over the range of four-bit to one-bit quantization, the simulated, patterns degrade with increasing phase errors. However, the overall shape and position of the foci remain unchanged. Simulation results show that, even with one-bit phase quantization, or with a Gaussian distributed random phase error of standard deviation of about 52 degrees , the applicator can still produce useful, if not particularly clean, power deposition patterns. Thus, the power deposition patterns are remarkably stable tolerating quite large phase error levels. This suggests that the power deposition patterns inside the body may be relatively insensitive to phase errors due to tissue inhomogeneities.     

The concentric-ring phased-array hyperthermia applicator: problems associated with directly synthesized annular heating patterns

The authors investigate the synthesis of different size annular patterns and examine the relative intensities produced in the ultrasonic field proximal and distal to the focal plane. Computer simulations using a concentric-ring array are presented and discussed. The size and position of the synthesized annular patterns are varied by adjusting the phase and amplitude of the driving signal to each array element. It is demonstrated that annular patterns need to be larger than a limiting size (R(s)) to avoid excessive energy accumulation along the array axis. Analysis and computer simulations demonstrate that smaller size annual patterns (r/=R(s)) show that while therapeutic temperatures are reached at the periphery, the tumor center temperature cannot be evenly raised.     

Field conjugate acoustic lenses for ultrasound hyperthermia

Multipoint foci have been synthesized by applying the pseudoinverse field conjugation method to a single ultrasonic transducer coupled to a polystyrene lens. The lens design is based on phased array calculations are then fabricated on a computer-controlled milling machine. The measured beam patterns from the lenses agree closely with the beam patterns predicted by theory for the equivalent phased arrays. Temperature distributions from thermal modeling and those measured in tissue equivalent phantoms show that the lens system is capable of generating strongly localized, controlled temperature fields for hyperthermia.     

Annular phased-array high-intensity focused ultrasound device for image-guided therapy of uterine fibroids

An ultrasound (US), image-guided high-intensity focused ultrasound (HIFU) device was developed for noninvasive ablation of uterine fibroids. The HIFU device was an annular phased array, with a focal depth range of 30-60 mm, a natural focus of 50 mm, and a resonant frequency of 3 MHz. The in-house control software was developed to operate the HIFU electronics drive system for inducing tissue coagulation at different distances from the array. A novel imaging algorithm was developed to minimize the HIFU-induced noise in the US images. The device was able to produce lesions in bovine serum albumin-embedded polyacrylamide gels and excised pig liver. The lesions could be seen on the US images as hyperechoic regions. Depths ranging from 30 to 60 mm were sonicated at acoustic intensities of 4100 and 6100 W/cm2 for 15 s each, with the latter producing average lesion volumes at least 63% larger than the former. Tissue sonication patterns that began distal to the transducer produced longer lesions than those that began proximally. The variation in lesion dimensions indicates the possible development of HIFU protocols that increase HIFU throughput and shorten tumor treatment times.     

Ultrasonic phased-array scanner with digital echo synthesis for Doppler echocardiography

For the purpose of the quantitative assessment of subtle disease processes in the cardiovascular system an electronically steered sector scanner that combines echographic imaging and Doppler blood velocity measurements has been developed. The integrated operation of a fast Fourier transform (FFT) Doppler signal processor for the simultaneous blood velocity evaluation of 64 individual gates is among the specific design goals. The instrument incorporates an unusually high degree of digital signal processing, which allows for high integration density, easy manufacturing and high reliability in future designs. The complex Doppler spectra are determined for each of the 64 Doppler gates in real time, and the subsequent computation of the first moment provides a reliable estimate of the mean blood flow velocities at the respective locations. The instantaneous velocity profile along the Doppler beam is displayed together with the calculated volume flow rate and a range-selected complete frequency spectrum. Results of both in vitro and in vivo tests indicate that in the future, a higher degree of digital signal processing could be implemented in complex ultrasonic systems.     

Treatment planning for hyperthermia with ultrasound phased arrays

Treatment planning for ultrasound phased arrays suggests a strategy for hyperthermia therapy which satisfies therapeutic conditions at the target and spares other sensitive anatomical structures. To predict both desirable and harmful interactions between ultrasound and important structures such as the tumor, bones, and air pockets, a hyperthermia treatment planning system has been developed for ultrasound phased arrays. This collection of treatment planning routines consists of geometric and thermal optimization procedures specific to ultrasound phased arrays, where geometric treatment planning, combined with thermal treatment planning and three-dimensional visualization, provides essential information for the optimization of individual patient treatments. A patient image data set for cancer of the prostate, a difficult target situated in the midst of multiple pelvic bone obstructions, illustrates the geometric treatment planning algorithm and other tools for treatment analysis. The results indicate that the analysis of complex three-dimensional relationships between the applicator, anatomical structures, and incident fields provides an important means of predicting treatment limiting conditions, thereby allowing the hyperthermia applicator to electronically adapt to individual patients and specific sites     

The sector-vortex phased array: acoustic field synthesis for hyperthermia

A sector-vortex phased array capable of generating directly annular-shape foci is analyzed. By driving the sectors of the array with signals whose phase rotates M times around the circular track, annular foci with the same acoustic-signal-phase rotation are produced in the geometrical focal plane of the array. Because of this phase modulation around the focal annuli and the resulting high-spatial-frequency content, the produced acoustic fields are free from secondary foci both behind and in front of the focal plane. The diameter of the focal annuli can be increased by increasing the mode number M. By providing the array with multiple tracks, it is possible to get larger focal annuli than with a single track with the same mode number M. It is also possible to achieve some control of the power deposition patterns in the depth direction in this way. Using a dual track sector-vortex array with practical ranges of the aperture size and number of elements, acoustic power deposition patterns capable of heating the peripheral and central regions of a nonsuperficial tumor a few centimeters in diameter are obtained by computer simulation.     

Convolution-kernel-based optimal trade-off filters for optical pattern recognition

An architecture for the implementation of optical pattern recognition is proposed that makes use of convolution-kernel-based optimal trade-off filters to allow for an increased speed of operation and filter storage capability. The derivation of these new convolution-kernel-based optimal trade-off filters is presented, and their noise robustness and discrimination capabilities are discussed.     

Noninvasive thermometry assisted by a dual-function ultrasound transducer for mild hyperthermia

Mild hyperthermia is increasingly important for the activation of temperature-sensitive drug delivery vehicles. Noninvasive ultrasound thermometry based on a 2-D speckle tracking algorithm was examined in this study. Here, a commercial ultrasound scanner, a customized co-linear array transducer, and a controlling PC system were used to generate mild hyperthermia. Because the co-linear array transducer is capable of both therapy and imaging at widely separated frequencies, RF image frames were acquired during therapeutic insonation and then exported for off-line analysis. For in vivo studies in a mouse model, before temperature estimation, motion correction was applied between a reference RF frame and subsequent RF frames. Both in vitro and in vivo experiments were examined; in the in vitro and in vivo studies, the average temperature error had a standard deviation of 0.7°C and 0.8°C, respectively. The application of motion correction improved the accuracy of temperature estimation, where the error range was 1.9 to 4.5°C without correction compared with 1.1 to 1.0°C following correction. This study demonstrates the feasibility of combining therapy and monitoring using a commercial system. In the future, real-time temperature estimation will be incorporated into this system.     

Generating realistic laminate fiber angle distributions for optimal variable stiffness laminates

The advent of advanced fiber placement technology has made it possible, through the use of fiber steering, to exploit the anisotropic properties of composite materials to a larger extent than was previously possible. Spatial variation of stiffness can be induced by steering composite fibers in curvilinear paths to give beneficial load and stiffness distribution patterns. Buckling of composite panels is one area where fiber steering has been proven to be very effective. Fiber angles and predefined fiber angle variations are used in most of the research on fiber steered composites reported in the literature, however, from an optimization point of view it is attractive to design such variable stiffness (VS) structures in terms of lamination parameters (LPs). This results in a two-step design approach. In the first step a VS composite is designed in terms of LPs, and in the second step the LPs are converted into fiber angle distributions for each layer in the laminate. A methodology is proposed to convert a known LP distribution for a VS composite laminate into a realistic design in terms of fiber angles, with minimum loss of structural performance, whilst satisfying a constraint on in-plane fiber angle curvature. The proposed conversion process is formulated as an optimization problem and can be used for any number of equi-thickness plies. The methodology was tested by converting a known optimal LP design for a sample structure, a square plate under bi-axial compression into a fiber angle design. The effect of the in-plane curvature constraint, the number of layers in the laminate, and the choice of objective function for the conversion process were studied for a balanced symmetric lay-up.     

The optimal confidence probability for compositions of error component distributions for measurement results

It is shown that a fiducial probability of 0.85 is optimal or to a certain lesser extent the interval 0.75–0.90 for a composition of a certain form for the distributions of the residual systematic error and the random error in the total error of measurement results, when that error is approximated by simple or accurate expressions within the ranges respectively of ±1.5 and ±3%. __________ Translated from Izmeritel’naya Tekhnika, No. 5, pp. 26–31, May, 2007.     

Self-calibration of large phased-array antennas for radar

Self-calibration of a phased antenna array is required when distortions in the array are not known a priori and cannot be measured, or when turbulence in the propagation medium perturbs the radiation field. Two types of self-calibration procedures are discussed that have proved successful in experimental high-resolution two-dimensional microwave radar imaging. Each extracts information from the backscattered reradiation field to deduce a compensating weight vector for the phased array antenna. The first depends upon the presence of a strong reflector in the field of view of the transmitter. The second calibrates the array from correlation estimates of wavefront samples in the array. The basic algorithm in each group is described, along with two more sophisticated algorithms in the latter class. Two-dimensional radar images of airplanes are shown with resolution comparable to human vision. The performance of each algorithm and comparisons between them are illustrated by these images of targets and by simulation experiments.©1993 John Wiley & Sons Inc     

A point-matching method for array pattern synthesis

In this paper, a newly developed point-matching method is presented to obtain a set of excitation coefficients of a linear array that generates a desired radiation pattern with arbitrarily suppressed sidelobe levels. This method can be used for linear arrays with nonuniform spacing and nonisotropic elements. The design examples presented show that the point-matching method is both effective and efficient