Effectiveness of the Gaussian beam model in predicting SAR distributions from the lucite cone applicator.

The Gaussian beam model (GBM) has been shown to be a successful tool in the development of the current sheet applicator. As a result, the effectiveness of the GBM is investigated in single and dual array applications of the lucite cone applicator (LCA). The LCA is a modified water-filled waveguide applicator with an improved effective field size (EFS > 64 cm2, aperture 10 cm x 10 cm). The GB-source parameters were calculated from the emanating E-field of a single LCA. The SAR distribution from a single LCA was measured by E-field scanning and thermographic (TG) imaging, and compared with the GB-predicted SAR distribution. Deviations in the principal planes were found to be less than 5%. TG-measured and GB-predicted SAR distributions from three different dual LCA configurations were compared and evaluated. When water was used as intermedium between LCAs and phantom, a maximum SAR difference of 27% was calculated. In the absence of water as intermedium, this difference increased to 44%. These large deviations were only found in areas where the measured SAR distribution was disturbed due to antenna interactions. The average SAR differences with and without water as intermedium were 7% respectively 11%, indicating that the GBM can provide good qualitative information about the SAR distribution of dual LCA-arrays.

The use of a current sheet applicator array for superficial hyperthermia: incoherent versus coherent operation.

There are a number of potential advantages to be gained by using an array of applicators in hyperthermia treatments compared with single applicator systems. These advantages include the possibility of greater spatial control of power deposition and conformability to nonplanar sites. Arrays of applicators can be driven either coherently or incoherently. In the case of coherent operation, an added advantage is the ability to steer power deposition by varying the phases of the antennas. In this study, we investigated the relative merits of the two modes of operation when a 2 x 2 planar array of current sheet applicators is used. The effective field size (EFS) of the array was calculated using a Gaussian beam representation of the applicators on a layered model in which the fat layer had its thickness varied. Good agreement was obtained between the square of the electric field distribution (E2) and quantitative experimental results. It is shown that when the planar array is used with a fat layer greater than about 2 mm present, it should be driven incoherently as this results in a significantly larger EFS than that obtained when the array is driven coherently.

Modeling and observation of temperature changes in vivo using MRI.

Models have been developed for the signal changes occurring in human peripheral muscle after thermal stress for some methods of measuring temperature in vivo. These include the dependency of T1 on temperature, observed by both direct, and magnetization transfer, experiments, and the temperature sensitivity of the diffusion coefficient and the chemical shift of the water proton line. Results show the relative complexity of the T1-dependent models when there are significant variations in, possibly amongst other things, tissue perfusion, and how the predictions made using them match the behavior of the practical values quite well. Using changes in the diffusion coefficient appears less problematic, although there can still be significant errors in temperature calibrations. The problems with this approach are likely to be due to the consequences of anisotropy in tissue.

Further observations on the measurement of tissue T1 to monitor temperature in vivo by MRI.

Previous results for the measurement of T1 as a means of monitoring temperature have demonstrated a linear calibration with a coefficient of the order of 1.5%/degrees C. We have studied the changes in T1 further, and observe that changes in tissue perfusion are substantial contributors to the effects observed, and that the model of what is happening is complex and may not permit a simple temperature calibration.

Early experience of a commercial scanned focused ultrasound hyperthermia system.

Preliminary experiences of the Sonotherm 6500 scanned focused ultrasound (SFUS) system in the hyperthermic treatment of bulky tumours in breast, superficial sites and within the pelvis in 22 patients are reported. Tumour volumes ranged from 235 to 603 cm3 (breast), 105 to 209 cm3 (superficial sites) and 24 to 905 cm3 (pelvis). Temperature distributions in 58 evaluable treatments were analysed in terms of temporal peak and time-averaged temperatures (highest and lowest temperatures, percentage of sensors exceeding index temperatures, etc.) achieved within scanned volumes. Mean number of sensors implanted into tumour was 14 in breast and superficial tumours and approximately 11 in pelvic tumours. Mean time-averaged maximum and minimum temperatures for the best treatments (i.e. for each patient, that with the highest percentage of sensors recording time averaged temperatures greater than or equal to 42 degrees C) administered to patients with tumours in the breast and superficial sites were 44.6 +/- 1.7 degrees C and 39.7 +/- 1.1 degrees C, respectively and the mean number of sensors exceeding 42 degrees C was 58 +/- 19%. In the case of pelvic tumours these figures were 41.6 +/- 0.9 degrees C, 40.0 +/- 0.6 degrees C and 8 +/- 12%, respectively. Patient tolerance to treatments was, in general, good. Areas where technical improvement of the system is appropriate and further research and development are required are identified; these should lead to a better realization of the potential of the SFUS technique, particularly for pelvic tumours.

Control of specific absorption rate distribution using capacitive electrodes and inductive aperture-type applicators: implications for radiofrequency hyperthermia.

A method of controlling the specific absorption rate (SAR) distribution in radiofrequency hyperthermia is proposed. The superposition of the current density associated with capacitively coupled electrodes and that associated with H-field coupled inductive aperture-type applicators modifies the actual current distribution in the heating material. Using a two-dimensional finite element method, we have shown that "focusing" is possible such that the SAR at the center of a phantom can be adjusted to be approximately twice that in superficial regions, even though the wavelength is considerably greater than the dimensions of the phantom.

Treatment of menorrhagia by radiofrequency heating.

A new technique is described for the treatment of menorrhagia by heating the whole of the endometrial cavity of the uterus. A capacitively coupled probe at 27.12 MHz is inserted into the uterine cavity, which causes the basilis layer to be raised to approximately 50-55 degrees C whilst the rest of the pelvic contents remain at approximately normal (body) temperature. A major advantage of the method is that no special hysteroscopic skills are required, unlike the two other techniques currently used for endometrial ablation: the Nd-Yag laser or the hysteroresectoscopic loop. Also no toxic flushing/distension fluids are necessary, as are required for all hysteroscopic surgery. The method, however, does require the application of a large amount of RF power to the probe and so care must be taken to position the probe correctly in order to prevent any serious complications. Of 32 patients given a single treatment at a power level of 550 W for 20 min, the 'success rate' was 84% with 31% becoming amenhorrhoiec and 53% showing significant reduction in menstrual bleeding. However, retreatment is possible and by this means, combined with improved treatment techniques, an even higher success rate could be achieved. In two of the earlier patients treated at 550 W a fistula was produced at the anterior vaginal wall which had to be surgically repaired. The probe was subsequently modified, since when this problem has not recurred. This new treatment approach offers an alternative to hysterectomy in the treatment of menorrhagia and may offer a number of significant advantages over methods currently used for endometrial ablation.

A comparative study of RF-LCF and hot-source interstitial hyperthermia techniques.

Interstitial hyperthermia techniques have been developed by several groups for use in a number of sites, especially those for which brachytherapy is to be given. Three basic methods have been developed for interstitial hyperthermia, namely: local current fields, microwave antennas and various hot-source techniques. In this paper a numerical study comparing two of these techniques (local current fields and hot sources) is carried out to investigate whether any improvement can be gained by combining them. It is found that both methods are sensitive to inter-electrode spacing and to blood flow, and become less able to produce adequate temperature distributions as these parameters are increased. By controlling the temperature of the electrodes it is shown that more uniform temperature distributions can be obtained over those obtained when the localized current field technique is used alone.

Experimental and clinical studies with radiofrequency-induced thermal endometrial ablation for functional menorrhagia.

A method of ablating the endometrium has been introduced into clinical practice that uses radiofrequency electromagnetic energy to heat the endometrium, using a probe inserted through the cervix. Preliminary studies suggest that over 80% of patients treated will develop either amenorrhea or a significant reduction in flow. The advantages of radiofrequency endometrial ablation over laser ablation or resection are the avoidance of intravascular fluid absorption, simplicity (no special operative hysteroscopic skills are required), speed of operation, and reduced cost compared with the Nd:YAG laser. In this paper, we describe the experimental studies performed during development of this new technique.

Observation by MR imaging of in vivo temperature changes induced by radio frequency hyperthermia.

Volunteers have undergone radio frequency hyperthermia in a magnetic resonance (MR) imaging system to investigate which of several possible MR parameters would be most convenient and sensitive to use to observe in vivo temperature changes. Measurements were made for T1, T2, and perfusion and diffusion variations, although the number of sequences needed at each temperature meant that the number of data points obtainable was limited. However, in the temperature range studied (around 28-42 degrees C), changes in both T1 and the diffusion coefficient were observed that agreed quite well with those predicted theoretically (respectively around 1.3%/degrees C and 2.4/degrees C).

Preliminary studies of interstitial hyperthermia using hot water.

A hot water interstitial hyperthermia unit was used to heat normal tissue in the thighs of rabbits and pigs. A 4 x 4 array of metal needles or plastic tubes spaced at 10 or 14 mm was implanted. Temperature measurements were made using five-sensor thermocouple probes inserted parallel to the implanted needles or tubes. With a water temperature of 48 degrees C, tissue temperature within the implant exceeded 42.5 degrees C when tube spacing was 14 mm and reached 47 degrees C when the spacing was 10 mm. However, at the lower water temperature of 45.5 degrees C inter-tube spacing was more critical, since the tissue temperature was above 43.5 degrees C for a spacing of 10 mm but below 42.5 degrees C for a spacing of 14 mm. Temperatures observed in vivo tended to be higher than those predicted by computer simulations, in which blood flow was assumed to be greater than that of resting muscle i.e. approximately greater than 0.45 kg m-3 s-1. The results show that an interstitial system using hot water can be a simple and efficient method of inducing hyperthermia.

Treatment of functional menorrhagia by radiofrequency-induced thermal endometrial ablation.

42 patients were enrolled in a trial of radiofrequency-induced thermal endometrial ablation for the treatment of functional menorrhagia. The radiofrequency electromagnetic energy was delivered via a probe placed within the endometrial cavity. 10 patients received 330 kJ of energy, 10 received 445 kJ, and the other 22 received 660 kJ. 19 (87%) of those receiving the highest dose became amenorrheic or had a considerable reduction in menstrual flow. The procedure is simple and the heat induced in the endometrium does not penetrate much beyond the inner layers of the myometrium. There is no need for distension of the uterine cavity with flushing media.