D.J. Watmough

Errors in temperature measurement by thermocouple probes during ultrasound induced hyperthermia

A major problem in the use of localised hyperthermia for treatment of malignant tumours is to obtain an accurate measurement of the temperature in the tissue being treated. Thermocouple probes have generally been employed for measuring temperature elevation during ultrasound irradiation. However, when small objects such as thermocouples are in an ultrasound field in a medium such as tissue, viscous forces acting between the object and the tissue will cause an additional local rise in temperature (Fry & Fry, 1954a, b; Dunn, 1962; Hynynen et al, 1982). This will produce an error in any measurement of tissue temperature with invasive probes. The magnitude of the temperature elevation resulting from shear viscosity has been measured for 50 μm thermocouples in tumour tissue.

Design of ultrasonic transducers for local hyperthermia

Abstract Computer simulations of the interdependence of different pairs of the parameters of spherically focusing ultrasonic transducers were performed, taking the ultrasonic attenuation in biological tissue into consideration. The maximum distance and gain of the last intensity maximum were calculated with different frequencies between 0.3 and 5 MHz and frequencies around 1 MHz were found to be most useful for hyperthermia. The relationships between the distance of the last axial maximum and the radius of curvature, as well as between the intensity gain and radius of curvature with different diameters of ultrasonic transducers operating with a frequency of 1 MHz were calculated. Also the effect of thermal conduction on the temperature distribution was discussed.

The biophysical effects of therapeutic ultrasound on HeLa cells.

Abstract Microscopic examination of HeLa cells sonicated by 750 kHz ultrasound at therapeutic intensity levels showed various forms of damage. These included instances in which plasmalemmal, nuclear, mitochondrial and endoplasmic reticulum membranes were the sites of damage. In addition an abnormal distribution of electron-dense particles was found within the cytoplasm. Changes in cell volume distribution curves were consistent with the interpretation that sonic irradiation of cell suspensions caused some destruction at spatial peak intensities above about 0.7 W cm −2 with an increase in subcellular debris. These experiments also support the view that below the threshold for collapse cavitation, ultrasonically-induced degassing within the culture medium and/or the cells contributes in part to the observed damage. Evidence for the existence of microstreaming in the culture medium is also described.

A new motor-driven surgical probe and its in vitro comparison with the cavitron ultrasonic surgical aspirator

The Cavitron Ultrasonic Surgical Aspirator (CUSA) has been applied in neurosurgery for several years, but its mode of action is not yet clear and its efficiency at removing soft tissue has not been quantified. We describe here how we have measured the rate of removing soft tissue per unit time, taking ox-liver tissue as the test material. A motor-driven vibrator/aspirator has been developed in our laboratory. It has permitted us to examine the effect of varying independently frequency, amplitude of the vibration, and suction pressure on the removal rate. The results of this investigation show that beyond a certain tip acceleration amplitude (about 100 g) the removal rate does not increase significantly. Also the removal rate is more or less independent of vibration amplitude within the range between 300 micron and 1 mm. Our in vitro experiments with the new probe show that a tip acceleration of about 100 g is enough to remove ox-liver tissue and then the rate of removal is comparable to that obtained with the CUSA operating at maximum vibration amplitude. Analysis of the particle size of the debris collected from CUSA and from our motor-driven device shows that the particle size distribution is similar over the range of 0.5 micron less than d less than 250 micron.

Ultrasound-induced damage of veins in pig ears, as revealed by scanning electron microscopy

Pig ear veins have been treated in situ with ultrasound at a frequency of 750 kHz and intensity of 1.5 W cm-2 (spatial average) during which the temperature in the surrounding tissues rose to 52-54 degrees C. The veins were examined by scanning electron microscopy. Gaps developed between the endothelial cells, which showed many fine perforations in their membranes. Extensive blood clots were observed in which erythrocytes had become more spherical and showed damaged membranes. Effects on membranes were also found with HeLa cells heated to 50 degrees C, and have been previously described by others in heat-treated blood vessels.

Evidence that ultrasonically-induced microbubbles carry a negative electrical charge

When ultrasound, propagating in a cationic dye solution, is incident on a porous material such as paper, the diffraction pattern of the sound field is recorded by virtue of variations of dye concentration retained on the paper surface. By examining such patterns formed with ‘Methylene Blue’ dye and comparing with those obtained with ‘Direct Sky Blue’ dye, it is concluded that the mechanism of pattern formation requires the presence of an electric field to be induced ‘close to the paper surface.’ Using light directed at grazing incidence to illuminate the sonicated paper surface it is possible to demonstrate and record photographically a mapping of microscopic bubbles caused by sonication which has, broadly, the geometric features of a diffraction pattern. Theoretical arguments are adduced to show that such bubbles should generate a significant electric field close to the paper. In order to confirm that this field will alter dye deposition on paper, electrodes were affixed to the underside of the same paper as used to make the dye patterns. When an externally generated potential (1 kV) was applied, the Methylene Blue and Sky Blue dyes were deposited on the paper close to the negative and positive electrodes respectively. We therefore conclude that acoustically-induced gas bubbles carry a negative electric charge and for micrometre-sized bubbles we estimate the field to be about 7 × 105 V m−1. There is no a priori reason to expect acoustically excited bubbles to have the same charge as that on comparatively stable bubbles mechanically introduced into water. These findings raise again the question of whether phenomena such as sonoluminescence are caused by microscopic electrical discharges and whether inter-bubble forces are adequately described by current theories.

Possible explanation for the unexpected absence of gross biological damage to membranes of cells insonated in suspension and in surface culture in chambers exposed to standing and progressive wave fields

Quoted values of cavitation thresholds reported in the literature vary over several orders of magnitude. This paper describes an investigation of the threshold in a chamber with acoustically transparent windows, situated around the last axial maximum of a 0.75 MHz standing wave field. The chamber, the transducer and the reflector are submerged in a tank containing distilled water which also fills the chamber. The formation of bubbles outside the chamber, both above and below it, occurred at lower intensities than those needed to generate bubbles in the chamber. This unexpected finding led to a theoretical study of streaming patterns in progressive and standing wave fields. These showed that there is a region around the last axial maximum in which there is no gradient of energy density. Therefore if this region is enclosed by an acoustically transparent chamber, no significant bulk streaming occurs within it. We speculate on how this lack of streaming raises the cavitation threshold. The cavitation phenomenon is also examined by Doppler ultrasound of frequency 8 MHz. This confirms the occurrence of increased intensity thresholds within the chamber. It also shows that the time between filling the chamber and the start of sonication strongly influences the magnitude of the cavitation threshold. We expect that the effects described here may have consequences for sonication of cells in suspension culture when the samples are held in chambers situated around the last axial maximum of an ultrasound beam from a plane transducer.

Observations of ultrasound-induced effects in the fish xiphophorous maculatus

Tails of the fish Xiphophorous maculatus have been studied by transmission light microscopy during irradiation with continuous wave ultrasound (frequencies 0.78-3 MHz, spatial average intensities 0.01-3 Wcm-2). Blood flow started to increase a few minutes after the start of an irradiation, reaching a maximum after 5-10 min. Periodic variations in blood flow rate were often seen, and the response varied considerably among individual specimens. Acoustic microstreaming, which resulted in rapid rotation of clusters of cells, was observed in blood vessels adjacent to cartilaginous rods in the tail. The threshold average spatial intensities to initiate this were a few hundred mW cm-2 at 0.78, 1.5 and 3 MHz. The microstreaming resembled that occurring around ultrasonically stimulated microbubbles, but no evidence of any association was found. Microbubbles, possibly originating in water, were sometimes seen in the tissue of the fish following treatment with ultrasound.

Local hyperthermia induced by focussed and overlapping ultrasonic fields—An in vivo demonstration

Axial temperature distributions were measured in living and post mortem porcine tissues during sonication with plane, focussed and overlapping ultrasonic fields. With the focussed field it was always possible to induce the temperature maxima at depths up to 50 mm, although the actual temperatures achieved varied from animal to animal. The plane 0.75 MHz transducer produced a maximum temperature close to the skin surface. With 7 overlapping plane fields a relatively uniform temperature distribution was produced in a large tissue volume. The blood perfusion in tissue has a significant effect not only on the magnitude of the temperature increase, but also on the temperature distribution.

The effects of some physical factors on the production of hyperthermia by ultrasound in neoplastic tissues

SummaryA one-dimensional and a three-dimensional computer model have been built in order to study the importance of blood flow and ultrasonic absorption in tissues during local hyperthermia. The decreased blood flow in the interior of certain tumours and possibly the increased ultrasonic absorption of the malignant tissue in some cases may cause selectively higher temperatures inside the tumours though the heat input is the same as in the surrounding tissues. Also, the vasodilation of blood vessels in normal tissues as a response to heat causes a therapeutically useful temperature difference. These blood flow differences can lead to enhanced effects during sonication to produce hyperthermia in the tumour. The inhomogenity of blood flow in the tumour causes a non-uniform temperature distribution leaving the well-perfused cells in the advancing front at a much lower temperature than the cells in the necrotic centre. Thus, the combination of local hyperthermia with radio-and chemotherapy seems to offer the most attractive means of destroying malignant tissue.