L. R. Gavrilov

Application of focused ultrasound for the stimulation of neural structures

The feasibility of the use of focused ultrasound for stimulation of the superficial and deep-seated receptor structures of humans and animals are presented in this review article. Applications of this method in physiology, for research into somatosensory and hearing perception, and also in clinical medicine for the diagnosis of neurological, dermatological and hearing disorders involving changes in perception of sensations different from normal, are discussed. It is proposed that the main effective factor of focused ultrasound as a stimulus of neural structures is a mechanical one. Such a mechanical effect could produce a change in membrane potential resulting in the stimulation of neural structures, which is related to the origin of tactile, thermal and hearing sensations. The direct action of sign-altering ultrasonic oscillations during the use of comparatively long ultrasound stimuli could possibly be the main effective factor for the induction of pain sensations and can also change the thresholds of other sensations (thermal, hearing and so forth).

A theoretical assessment of the relative performance of spherical phased arrays for ultrasound surgery

Computer modeling of spherical-section phased arrays for ultrasound surgery (tissue ablation) is described. The influence on performance of the number of circular elements (68 to 1024), their diameter (2.5 to 10 mm), frequency (1 to 2 MHz), and degree of sparseness in the array is investigated for elements distributed randomly or in square, annular, and hexagonal patterns on a spherical shell (radius of curvature, 120 mm). Criteria for evaluating the quality of the intensity distributions obtained when focusing the arrays both on and away from their center of curvature, and in both single focus and simultaneous multiple foci modes, are proposed. Of the arrays studied, the most favorable performance, for both modes, is predicted for 256 5-mm diameter, randomly distributed elements. For the single focus mode, this performed better than regular arrays of 255 to 1024 elements and, for the case of nine simultaneous foci produced on a coplanar 3/spl times/3 grid with 4-mm spacing, better than square, hexagonal, or annular distributed arrays with a comparable number of elements. Randomization improved performance by suppressing grating lobes significantly. For single focus mode, a several-fold decrease in the number of elements could be made without degrading the quality of the intensity distribution.

Focusing of high-intensity ultrasound through the rib cage using a therapeutic random phased array.

A method for focusing high-intensity ultrasound (HIFU) through a rib cage that aims to minimize heating of the ribs while maintaining high intensities at the focus (or foci) was proposed and tested theoretically and experimentally. Two approaches, one based on geometric acoustics and the other accounting for diffraction effects associated with propagation through the rib cage, were investigated theoretically for idealized source conditions. It is shown that for an idealized radiator, the diffraction approach provides a 23% gain in peak intensity and results in significantly less power losses on the ribs (1% vs. 7.5% of the irradiated power) compared with the geometric one. A 2-D 1-MHz phased array with 254 randomly distributed elements, tissue-mimicking phantoms and samples of porcine rib cages are used in experiments; the geometric approach is used to configure how the array is driven. Intensity distributions are measured in the plane of the ribs and in the focal plane using an infrared camera. Theoretical and experimental results show that it is possible to provide adequate focusing through the ribs without overheating them for a single focus and several foci, including steering at +/- 10-15 mm off and +/- 20 mm along the array axis. Focus splitting caused by the periodic spatial structure of ribs is demonstrated both in simulations and experiments; the parameters of splitting are quantified. The ability to produce thermal lesions with a split focal pattern in ex vivo porcine tissue placed beyond the rib phantom is also demonstrated. The results suggest that the method is potentially useful for clinical applications of HIFU, for which the rib cage lies between the transducer(s) and the targeted tissue.

A random phased array device for delivery of high intensity focused ultrasound

Randomized phased arrays can offer electronic steering of a single focus and simultaneous multiple foci concomitant with low levels of secondary maxima and are potentially useful as sources of high intensity focused ultrasound (HIFU). This work describes laboratory testing of a 1 MHz random phased array consisting of 254 elements on a spherical shell of radius of curvature 130 mm and diameter 170 mm. Acoustic output power and efficiency are measured for a range of input electrical powers, and field distributions for various single- and multiple-focus conditions are evaluated by a novel technique using an infrared camera to provide rapid imaging of temperature changes on the surface of an absorbing target. Experimental results show that the array can steer a single focus laterally to at least +/-15 mm off axis and axially to more than +/-15 mm from the centre of curvature of the array and patterns of four and five simultaneous foci +/-10 mm laterally and axially whilst maintaining low intensity levels in secondary maxima away from the targeted area in good agreement with linear theoretical predictions. Experiments in which pork meat was thermally ablated indicate that contiguous lesions several cm(3) in volume can be produced using the patterns of multiple foci.

ULTRASONIC HYDROPHONE BASED ON SHORT IN-FIBER BRAGG GRATINGS

We investigate the feasibility of using in-fiber Bragg gratings for measuring acoustic fields in the megahertz range. We found that the acoustic coupling from the ultrasonic field to the grating leads to the formation of standing waves in the fiber. Because of these standing waves, the system response is complex and, as we show, the grating does not act as an effective probe. However, significant improvement in its performance can be gained by use of short gratings coupled with an appropriate desensitization of the fiber. A noise-limited pressure resolution of approximately 4.5 x 10(-3) atm/ radicalHz was found.

A study of reception with the use of focused ultrasound. I. Effects on the skin and deep receptor structures in man

The possibility of use of focused ultrasound (focused beam of high-frequency mechanical waves) for stimulation of nerve structures was investigated. The stimulation of human hand resulted in various sensations: tactile, temperature, pain etc. The corresponding thresholds were determined and characteristic features of ultrasonically induced sensations were studied. The modality of temperature sensation (warmth-cold) was found to depend on the environmental temperature. The character of pain was dependent upon the type of tissue stimulated. Effective factors in ultrasonic stimulation are discussed.

Use of focused ultrasound for stimulation of nerve structures

Pulsed focused ultrasound can stimulate the receptor and conductive nerve structures of humans and animals as well as the neurons of the central nervous system of invertebrates. The possibility of a wide practical use of this method in medicine and physiology is considered. For example, the stimulating ability of focused ultrasound is applied to the diagnosis of neurological diseases, to the study of skin and tissue sensitivity in man, to the diagnosis of hearing disorders, and to the introduction of auditory information to the deaf with certain forms of hearing pathology. The factors that affect focused ultrasound as a stimulus for the irritation of nerve structures are discussed.

In-fibre Bragg gratings for ultrasonic medical applications

We investigate the feasibility of using in-fibre Bragg gratings to measure ultrasonic fields for medical applications. Two signal processing schemes for interrogating the gratings are described. Preliminary results for each scheme (one a homodyne approach, the other a heterodyne one) give noise-limited pressure resolutions of and atm respectively, each within a 1 Hz bandwidth. The second scheme, however, gives a more stable response.

The Effect of Focused Ultrasound on the Skin and Deep Nerve Structures of Man and Animal

Publisher Summary This chapter discusses a variety of nerve structures that can be excited with focused ultrasound (US). Stimulation of the human arm with short stimuli results in various sensations similar to natural stimulation; with the center of the focal region within the skin, the stimuli of increasing intensity produced sensations of touch, tickling, warmth, cold, itching, and pain. When the center of the focal region is projected into the deep structures of the arm, only the pain sensations appear, their character depending upon the type of the tissue (soft tissues, bone, and joints). Stimulation of the same spots could result either in cold or in warmth sensations. Electrophysiological experiments exhibit that single mechanoreceptors—Pacinian corpuscles—and receptor structures of the frogs labyrinth can be excited by the focused US. Tactile sensations are because of excitation of the skin mechanoreceptors, whereas the temperature and pain sensations may involve the direct excitation of the nerve fibers. The comparative investigation of parameters to characterize the US propagation in the medium exhibit the main active factor in US stimulation is of a mechanical nature, that is, medium displacement in the focal region. With certain parameters of the stimulus, the local heating of the tissue at the place of stimulation is also involved.

A method of reducing grating lobes associated with an ultrasound linear phased array intended for transrectal thermotherapy

Some practical aspects of planar linear ultrasound phased arrays for transrectal thermotherapy of prostate diseases are discussed. Several regimens for driving the array are investigated and spatial distributions of ultrasound intensities are measured in water and compared with computer simulations. Practical recommendations for suppressing grating lobes based on the use of subsets of elements and de-activation of several elements in the array are given. Treatment safety could be increased by adopting these measures since the relative intensities and power in grating lobes and other secondary intensity peaks are decreased, as is the overall ultrasound energy introduced into the body without significant reduction in the maximum power at the focus.