Thermal effects of focused ultrasound on the brain: determination with MR imaging.

PURPOSE: To determine the feasibility of the use of temperature-sensitive magnetic resonance (MR) imaging for the detection of local temperature elevations at the focus of a low-power ultrasound beam in the brain. MATERIALS AND METHODS: The brains in 28 rabbits were sonicated at acoustic power levels of 3.5-17.5 W. Four to five different locations were sonicated at different acoustic power levels in each rabbit. MR images were obtained 2 hours, 48 hours, 10 days, and 23 days after the sonications, depending on when the animals were sacrificed. Histologic evaluation of whole brain was performed. RESULTS: Forty of 43 (93%) of the lowest-power (3.5-W) sonications were visible on temperature-sensitive MR images and did not result in any short- or long-term histologic or MR imaging evidence of tissue damage. A contrast-to-noise ratio of approximately 6 and a temperature elevation of 7 degrees-8 degrees C were observed. CONCLUSION: Temperature elevations induced by means of focused ultrasound exposures that do not cause damage in the in vivo rabbit brain can be detected at temperature-sensitive MR imaging.

Histologic effects of high intensity pulsed ultrasound exposure with subharmonic emission in rabbit brain in vivo.

In this study, the threshold for subharmonic emission during in vivo sonication of rabbit brain was investigated. In addition, the histologic effects of pulsed sonication above this threshold were studied. Two spherically curved focused ultrasound transducers with a diameter of 80 mm and a radius of curvature of 70 mm were used in the sonications. The operating frequencies of the transducers were 0.936 and 1.72 MHz. The sonication duration was varied between 0.001 and 1 s and the repetition frequency between 0.1 and 5 Hz. The threshold for subharmonic emission at the frequency of 0.936 MHz was found to be approximately 2000 W cm-2 and 3600 W cm-2 for pulse durations of 1 s and 0.001 s, respectively. The threshold was approximately 1.5-fold as high at a frequency of 1.72 MHz. However, there was considerable variation from experiment to experiment. The multiple pulse experiments at a frequency of 1.72 MHz and an intensity of 7000 W cm-2 showed that the histologic effects ranged from no observable damage of the tissue, to blood-brain barrier breakage, to local haemorrhagia, to local destruction of the tissue, to gross hemorrhage resulting in the death of the animal. The severity of the tissue damage increased as the pulse duration, number of pulses and their repetition frequency increased. The results indicate that the end point of the tissue damage may be controlled by selecting the sonication parameters. Such control over tissue effects can have several different applications when brain disorders are treated.

Pulse duration and peak intensity during focused ultrasound surgery: theoretical and experimental effects in rabbit brain in vivo.

The goal of this study was to establish the exposure parameters that will generate predictable thermally induced lesions in brain. In addition, the accuracy of a theoretical model for prediction of the lesion size was tested. To do this, 160 adult rabbits were sonicated (frequency 0.936 and 1.72 MHz) and then sacrificed at various intervals after the sonications. The results showed that predictable thermal lesions could be induced if the exposure durations were between 0.5 and 2 s. Dimensions of the necrosed tissue volume were roughly predictable by the theoretical calculations based on purely thermal effects. Shorter sonications required higher intensities (above 3700 W cm-2 at 1.72 MHz) resulting in mechanical effects with extensive vascular damage. Lesion size varied more at longer exposures (5 and 10 s), perhaps due to the increased effect of tissue perfusion. As a conclusion, focused ultrasound can be used for destruction of tissues deep in brain without causing undesirable mechanical effects, if the exposure parameters are selected properly.

[Ultrasonic hyperthermia of the animal brain]. / Ul'trazvukovaia gipertermiia golovnogo mozga zhivotnykh.

The paper is concerned with the methods of ultrasonic hyperthermia of the cerebral tissues and the results of preliminary experiments on animals. Ultrasonic radiators made of piezoceramic plates were employed. Ultrasound frequency was 1.56 and 2.85 MHz. Tissue temperature was measured not less than at 14 points using single and "chain" thermocouples with 5 branches every 5 mm and thermistors. In spite of rather inhomogeneous acoustic fields of the ultrasonic radiators they can be used for obtaining homogeneous temperature distributions in tissues at depths up to 20 mm owing to high thermal conduction of tissues. The experiments have demonstrated a possibility of heating animal tissues up to hyperthermic temperatures by ultrasonic exposure of tissues both in cranial trepanation and through the intact skull.

[Apparatus and technical devices for ultrasonic hyperthermia]. / Apparatura i tekhnicheskie sredstva ul'trazvukovoi gipertermii.

The developed apparatus included ultrasonic generators operating at a frequency of 0.5-3 MHz, piezoceramic radiators of various design providing the heating of an object with convergent, divergent and plane ultrasonic waves, thermoprobes in the form of single or multiple thermocouples with the bends from 5 points at a 5 mm distance from one another, temperature meters and various auxiliaries. The results of measurements of the acoustical fields of the developed radiators were presented. The advantages and shortcomings of the ultrasonic method of tumor hyperthermia were discussed.

[Spreading depression in the cortex and subcortical structures of the brain of the rat induced by exposure to focused ultrasound]. / Rasprostraniuiushchaiasia depressiia v kore i podkorkovykh obrazovaniiakh golovnogo mozga krysy, vyzyvaemaia deistviem fokusirovannogo ul'trazvuka.

Steady potential shifts produced by focused ultrasound were recorded in the cerebral cortex, hippocampus, thalamus and caudate nucleus of the rat. Impulses of 50-100 ms duration were presented with frequency 5 and 10 Hz. Negative slow potential shifts gradually increased up to 3-7 mV during 10-30 s and were often followed by the spreading depression (SD) waves. In every analyzed structure the SD amplitude reached 20-30 mV; the SD duration in the cortex, caudate nucleus and thalamus was 30-40 s while in the hippocampus it was 80-120 s. Seizures initiated by the ultrasound influence proceded in some cases the SD. Threshold ultrasound doses were not effective for 5-7 min after every the SD wave but at the end of the refractory period they became effective again. So, local influence of the focused ultrasound may result in functional blockade of the brain structures due to the cortical and subcortical spreading depression.

[Characteristics of the formation of local destructive foci in brain tissues exposed to focused ultrasound]. / Nekotorye osobennosti formirovaniia lokal'nykh ochagov razrusheniia v tkaniakh golovnogo mozga pri deistvii fokusirovannogo ul'trazvuka.

Local lesions produced in the different brain structures by focused ultrasound were investigated. It was revealed that the blood flow volume velocity had a considerable influence on temperature distribution in the focal area and on the threshold doses and lesion dimensions. Calculation of the lesion diameters on the basis of a purely heat model shows rather good accordance with the experimental data for the large hemispherical cortex and thalamic nuclei with a sufficiently long radiation time. A correlation was found between the subharmonic component of cavitation noise and appearance of the cavities, ruptures, and local hemorrhages located mainly on the boundaries between different cerebral tissues.

[Use of focused ultrasound for local effects on deep brain structures]. / Primenenie fokusirovannogo ul'trazvuka dlia lokal'nogo vozdeistviia na glubokie struktury mozga.

The possibilities of the ultrasound technique for local destruction of certain brain areas, for reversible alteration of functional state of the c.n.s. structures and for study of structural-functional interrelationships in the brain are discussed.

[Method of local action by focussed ultrasound on deep brain structures in unrestrained unanesthetized animals]. / Metod lokal'nogo vozdeistviia fokusirovannym ul'trazvukom na gluboko raspolozhennye struktury mozga neobezdvizhennogo nenarkotizirovannogo zhivotnogo.

A method of local effect of focused ultrasound has been devised for blocking brain structures of an unrestrained experimental animal. The effect of reversible blocking of the visual tract following focused ultrasound was recorded.

[Reversible functional shutdown of the optic tract on exposure to focused ultrasound]. / Obratimoe funktsional'noe vykliuchenie zritel'nogo trakta pri deistvii fokusirovannym ul'trazvukom.

The possibility of reversible functional blockade of the optical tract and blockade of optical information conduction has been shown by means of undestructive doses of focused ultrasound. Electron microscopy of optic terminals in the superior colliculus in the period of restitution of the bioelectric activity of the optical tract has demonstrated that alterations in the ultrastructure of optic terminals were insignificant and reversible.

[Distribution of afferents from the association nuclei of the thalamus in the projection and association cortex in cats]. / Raspredelenie afferentov iz assotsiativnykh iader talamusa v proektsionnoi i assotsiativnoi kore bol'shikh polusharii mozga koshki.

Patterns of distribution of terminal degeneration in the parietal cortex (field 7) and in the occipital cortex (field 17) were studied after ultrasonic destruction of the pulvinar by the Fink-Heimer and electron microscopy methods. Degenerating fibers and their terminals were observed in the parietal cortex within all the layers; the greatest amount of degeneration was found in the III--V layers. In the occipital cortex the fibers from the pulvinar end predominantly in the IV layer. Degenerating axons end on the dendritic spines and thin dendritic branches both in the parietal and occipital cortex.

[Use of focused ultrasound for local destruction of different brain structures]. / Primenenie fokusirovannogo ul'trazvuka dlia lokal'nykh razrushenii razlichnykh struktur golovnogo mozga

It was demonstrated that destruction of the brain can be accomplished by different combinations of ultrasound intensity and duration of irradiation. By experimental means cavitational thresholds of brain tissues were determined and a calculation was made of increased temperature in the zone of focus in each separate regime of irradiation. The authors describe the foci of lesions where the main mechanism of destruction was only warmth, either only cavitation, either warmth and cavitation together. Irradiation of deep brain structures was not accompanied by changes of the cortical and subcortical structures, located above the focus of necrosis along the path of the ultrasound ray (light microscopy). The probability of a target hit into the given brain structure corresponded to the preciseness allowed in a stereotaxic operation.