Charles A. Cain

Multiple-focus ultrasound phased-array pattern synthesis: optimal driving-signal distributions for hyperthermia

A method for computing array element amplitude and phase distributions for direct synthesis of multiple-focus field patterns using ultrasonic phased arrays is shown to be capable of producing desired field levels at a set of control points in the treatment volume. The complex pressure at any of these control points can be chosen to produce the desired power deposition at the point, including reducing the field level to avoid potential hot spots, thus providing a powerful tool for hyperthermia treatment planning. The method also allows the complex excitation vector to be weighted to reduce the dynamic range of the driving signals without disturbing the relative field levels at the control points, allowing near maximum power transfer from the array into the treatment volume.<<ETX>>

Concentric-Ring and Sector-Vortex Phased-Array Applicators for Ultrasound Hyperthermia

Concentric-ring phased arrays subdivided into sectors (radial slices) can, with appropriate phasing, produce power absorption patterns useful for hyperthermia cancer therapy. The ability of a concentric-ring array to move a focal region along the central axis of the transducer is well known. Less well known is the ability of such an array to produce variable diameter annular (or ring) focal regions. Such focal rings can be effective in heating some tumors if directed around the tumor periphery. These focal rings have been produced in the past by fixed annular focus lenses, or effectively by mechanical scanning of point focus ultrasonic transducers. Production of these focal rings by a concentric-ring phased array has the advantage of allowing the focal ring diameter and focal length to be easily changed and scanned by phasing providing much greater heating flexibility. However, under some conditions such arrays produce very large secondary focus effects along the central axis of the amay. Concentric-ring arrays can also provide only patterns of circular symmetry. These problems can be partially solved by dividing the disk of the array into sectors. By appropriate phasing of the sectors, the intensity along the central axis can be greatly reduced. Moreover, appropriate phasing of the rings and sectors can produce patterns that are circularly asymmetric. By controlling these asymmetries, nonspherical tumors can be heated more optimally. Power absorption patterns in Iossy media for this class of applicators are analyzed numerically allowing a quantitative evaluation of both advantages and limitations of this approach. A thermal model based on the bioheat equation is also used to predict temperature distributions in volumes where important thermal parameters, particularly blood flow, are varied.

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.<<ETX>>

Focused Array Hyperthermia Applicator: Theory and Experiment

The design and analysis of a focused linear array at 2450 MHz for microwave hyperthermia research is described. The array, which was submerged in deionized water to reduce its size and to provide a better impedance match to a high dielectric medium representative of human tissues, consisted of four titanium dioxide loaded horn antennas with apertures of 2.0 x 1.4 cm and a feed network with weighted phase shifts, Power radiation pattern measurements were made in planes ranging from 7.6 to 10.2 cm from the array to determine the focusing characteristics and beam spot size. Due to high attenuation in the medium, planes beyond 10.2 cm were not considered. The half-power beamwidth (HPBW) measured at the focal point was approximately 1.3 cm. The measured patterns were found to be in close agreement with theoretical predictions.

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.<<ETX>>

An annular focus ultrasonic lens for local hyperthermia treatment of small tumors

A system for applying local hyperthermia employing ultrasonic transducers with annular focusing lenses gave better temperature uniformity in small fibrosarcomas than conventional unfocused transducers. The lenses were designed for ultrasonic frequencies of 3, 4, 5 and 9 MHz for tumors approximately 6 mm dia. Tests in degassed water indicated that the focusing lenses concentrated approx. 80% of the ultrasonic energy into an annular-shaped focus of 6 mm dia. located at a distance of 3 cm in front of the lens. These transducers were used to heat subcutaneous PARA-7 fibrosarcomas in hamsters. Steady-state temperature measurements indicated temperature gradients within tumors of 0.5-1.0 degree C when transducers employing annular-focused lenses were used, versus gradients of 2.0-2.3 degree C with 10 mm dia. unfocused transducers.

Acoustical evaluation of a prototype sector-vortex phased-array applicator

A prototype sector-vortex phased-array applicator for ultrasound hyperthermia was constructed and acoustically evaluated. The array transducer consists of special lead-titanate ceramic elements of 16 sectors and two tracks attached on a element is driven by a complementary pair of power MOSFETs at 750 kHz. An annular focal field approximated by the Mth order Bessel function is theoretically predicted to be formed when the array elements are driven with a phase distribution that rotates M (<or=8) times per rotation along a track. A 3-D acoustic field measurement system was constructed using a multichannel membrane hydrophone, and fields from the prototype transducer were measured at various drive modes in water. The measured multidimensional field patterns agree well with theoretical predictions at each mode. Focal acoustic power from the transducer was also measured at various drive modes by deflection of a plate located in the focal region. It is demonstrated that the applicator is capable of producing focal acoustic power at the level required for hyperthermia.<<ETX>>

A field conjugation method for direct synthesis of hyperthermia phases-array heating patterns

A phased-array field conjugation method is investigated as a means for synthesizing directly many ultrasound field patterns useful for tumor heating. For virtually any ultrasound phased array, the method permits the computation of element driving amplitude and phase distributions appropriate for synthesizing directly diffuse heating patterns without the need for mechanical or electrical scanning. Moreover, the proposed method offers the possibility of creating simultaneously, at different sites, more than one focus, which can then be scanned electronically. This attractive feature eliminates the need for operating at high spatial-peak temporal-peak focal intensities, a potential problem associated with conventional scanning, while achieving a desired heating pattern. The method is applied to two different applicator configurations: a concentric-ring and a square n*n array. Computer simulations of different heating patterns, synthesized using the field conjugation method, are presented. Important practical design parameters, such as the size and number of the array elements, are discussed.<<ETX>>

A Tapered Phased Array Ultrasound Transducer for Hyperthermia Treatment

Abstracl-An ultrasonic tapered phased array transducer, consisting of a linear phased array employing elements with a tapered thickness, was developed to study the feasibility of its use for hyperthermic treatment of deep-seated tumors. The cylindrical focal region is generated and steered in two dimensions by controlling the phases of the driving signals on each element and moved in the third dimension by changing the driving frequency. Experimentally obtained field intensity distributions are in good agreement with theoretical predictions. Acoustical power output measurements indicate that tapered phased arrays are capable of providing the intensities necessary for producing therapeutic temperatures in tumors.

Theoretical Analysis of Acoustic Signal Generation in Materials Irradiated with Microwave Energy

Stress gradients generated by thermal expansion, electrostriction, and radiation pressure are sources of elastic waves in microwave irradiated materials. A theoretical analysis taking into account induced volume and surface forces due to these interaction mechanisms is presented. Complete solutions of the dynamical equations for the one-dimensional special case are given for different boundary conditions. The closed-form solutions were found to consist of both a stationary part, whose effect is important only in the immediate region of the incident electromagnetic wave, and a traveling part which propagates through the elastic material. Expressions for the Fourier transforms of these solutions are also given. To quantify these results, pressure and displacement waveforms in microwave irradiated physiological saline were computed. Thermal expansion was considerably more effective than either electrostriction or radiation pressure in converting electromagnetic energy to acoustic energy.