SAR Distribution Research Articles

Temperature data and specific absorption rates in pelvic tumours: predictive factors and correlations

The system BSD 2000 has been in clinical use for regional hyperthermia for more than 10 years. Several technical details of this hyperthermia system, as well as the results of clinical studies employing this system have been investigated. The intention of this paper is to investigate the correlation between technical efficiency or feasibility of hyperthermia with the BSD 2000, in terms of power densities and temperatures depending upon parameters such as tumour histology, tumour location, patient age, patient sex, and patient cross section. The possible conclusions of predictive factors derived from the above correlations were closely scrutinized. Data acquired from 772 treatment sessions of 190 patients with pelvic tumours, mainly sarcomas and carcinomas of the rectum, cervix, prostate and anus, have been evaluated. For every session, index temperatures T 90 (temperature attained at 90% of tumour related measurement points), cumulative minutes for T90 > Tref, tumour related power density (SAR: specific absorption rate, in W/kg) and the effective perfusion Weff (inml/100gmin) were calculated. Temperatures were measured either invasively or endoluminally. The statistics software SPSS was employed subsequently for univariate, as well as multivariate analyses. The results exhibit that index temperatures mainly depend on the power density SAR and the hyperthermia induced effective perfusion. The total power P (in 100W) and, complementarily, the relative power density ||SAR|| (= SAR/P) seem to have lesser influence. Clear differences between the tumour entities were established regarding their index temperatures and temperature distributions. SAR, Weff and P were correlated with several anatomical, biological and clinical factors. Sessions rendering low index temperatures and SAR values also revealed decreased individual tolerance to the treatment. This clearly displays that powerinduced side effects define the limits of the efficiency of regional hyperthermia. Equivalent relationships and correlations are derived from intratumoural and endoluminal thermometry. Individual limitations of regional hyperthermia caused by anatomical, biological and clinical factors are liable to be difficult to overcome with the rather restricted potentials of the BSD 2000 system to control the SAR distribution.

A Precise Measurement of the SAR Distribution in the Human Head Model Using the Thermographic Method

A Precise Measurement of the SAR Distribution in the Human Head Model Using the Thermographic Method

High-resolution SAR modelling for regional hyperthermia: testingquasistatic zooming at 10 MHz

Present-day regional hyperthermia treatment planning systems are limited tocentimetre resolution. To obtain CT-resolution SAR distributions, a methodcalled quasistatic zooming has been developed: using the centimetre-resolutionE⃗-field distribution and the CT-resolution tomogram, the CT-resolution SARdistribution is obtained. For a low frequency of 10 MHz this method has beenvalidated sucessfully using CT-resolution SAR computations.It appears that these CT-resolution SAR distributions are completely differentfrom centimetre-resolution SAR distributions, indicating the necessity forhigh-resolution SAR modelling. Using the presented zooming technique, reliableCT-resolution SAR modelling is now possible with relatively short computationtimes. So far, the zooming method has only been validated for low frequencies,but clinically relevant frequencies appear to be possible.

A brain‐equivalent solid phantom and its application to SAR estimation by the thermographic method

Recently, with the rapid proliferation of mobile communication devices, the effects of electromagnetic waves on the human body have drawn attention. Although these effects depend significantly on frequency, the thermal effects due to absorption of the electromagnetic energy are considered predominant at microwave frequencies. In general, as the guideline for the thermal effects, the power per unit mass absorbed in the biological body, namely, the specific absorption rate (SAR [W/kg]), is used. Cellular phones are employed in close proximity to the head where vital functions of the body are concentrated. Even if the output is weak, there is the possibility that the power absorption may become locally excessive. Therefore, it is extremely important from a safety point of view to establish an experimental evaluation procedure regarding the SAR for the human head. In this paper, the characteristics of thermography are recognized by means of which the surface SAR can be measured, and thus the SAR distribution in an arbitrarily shaped medium. Using the biologically equivalent phantom developed in the authors' laboratory for improving the conventional phantom used for the same purpose, a fundamental investigation is carried out as to how accurately the SAR distribution can be measured. © 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(12): 24–34, 2000

Application of finite difference time domain method for the design of birdcage RF head coils using multi-port excitations

A three-dimensional finite difference time domain model was developed where the high pass birdcage coil and the imaged object are analysed as a single unit. A study was performed comparing linear, conventional quadrature, and four-port excitation at 64 MHz and 200 MHz for different coil loadings, namely muscle phantoms and an anatomically detailed human head model. A phase array concept was utilized to excite the birdcage coil in four ports. Two phase conditions were analyzed, the simple fixed phase and the variable phase. At 200 MHz, compared to the conventional quadrature drive, the four-port drive reduces the effects of the tissue-coil interactions leading to more uniform currents on the coil legs and consequently to a better B 1 field homogeneity. Also at 200 MHz, driving the coil in four ports provides an SAR distribution with peak values that are significantly less than those with linear or quadrature excitations.

Mix-mode technique of feeding arrays of dipole microwave antennae

A 4-element array of coherently driven dipole microwave antennae produces a hot spot in the central region of the implanted volume and cold spots in the peripheral regions of the implant. Conversely, an incoherently driven array of antennae predominantly heats peripheral (along the antennae) regions of the implant. These two modes of feeding the antennae are complementary in a sense that the cold zones obtained with the coherently driven antennae coincide with the hot zones obtained with the incoherently driven antennae and vice versa. The SAR distributions resulting from mixing these modes of feeding (mixed-mode technique) were studied theoretically and experimentally. A theoretical model that allowed calculation of SAR distributions of a 4-element array of microwave antennae fed coherently, incoherently or using a mixed-mode technique was developed in this work. The goal of the theoretical study was to determine the proper mix of the coherent and incoherent modes of feeding the antennae such that the adequately (enclosed within a 50% isoSAR surface) heated volume was maximized. In experimental studies, the antennae were driven in a cyclical manner with a duty cycle equal to the weight of the coherent mode in the mix. The duty cycle was defined as the ratio of the time the antennae were driven coherently to the total duration of the cycle. To facilitate the periodical change from the coherent to incoherent feeding, a special electromechanical switch was developed. This switch allowed a wide range of variation of the duty cycle and cycle period. Theoretical and experimental studies have demonstrated that, if the relative weight of the coherent feeding in the mix-mode technique was 30% (duty cycle=0.3), the adequately heated volume was significantly larger and the SAR distribution was more uniform than those obtained with either the coherent or incoherent mode of feeding.

A brain-equivalent solid phantom and its application to SAR estimation by the thermographic method

Recently, with the rapid proliferation of mobile communication devices, the effects of electromagnetic waves on the human body have drawn attention. Although these effects depend significantly on frequency, the thermal effects due to absorption of the electromagnetic energy are considered predominant at microwave frequencies. In general, as the guideline for the thermal effects, the power per unit mass absorbed in the biological body, namely, the specific absorption rate (SAR [W/kg]), is used. Cellular phones are employed in close proximity to the head where vital functions of the body are concentrated. Even if the output is weak, there is the possibility that the power absorption may become locally excessive. Therefore, it is extremely important from a safety point of view to establish an experimental evaluation procedure regarding the SAR for the human head. In this paper, the characteristics of thermography are recognized by means of which the surface SAR can be measured, and thus the SAR distribution in an arbitrarily shaped medium. Using the biologically equivalent phantom developed in the authors' laboratory for improving the conventional phantom used for the same purpose, a fundamental investigation is carried out as to how accurately the SAR distribution can be measured. © 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(12): 24–34, 2000

Resonance effects in applicator water boluses and their influence on SAR distribution patterns

Spurious electromagnetic oscillations in the water layer of applicator water boluses are investigated. Two oscillation types volume oscillations (VO) and surface standing wave oscillations (SWO) are considered. Conditions leading to their excitation in water boluses, commonly used in clinical practice for local hyperthermia, are examined. It was found that the thickness d of the water layer is the main critical parameter which determines the possibility of spurious oscillation modes excitation. Analytical equations are derived, allowing calculations of critical dc and resonance dr values for a given water bolus, and determine the approximate structure of electrical field strength distribution at its radiating plane, taking account of spurious oscillations. Calculated dc and dr values are in satisfactory quantitative agreement with experimental data, whereas experimentally observed complicated SAR distribution perturbations correlate qualitatively with the calculations in terms of volume and/or surface standing wave oscillations excited within the water bolus.

Comparison of averaging procedures for SAR distributions at 900 and 1800 MHz

The authors studied the effect of the position of the averaging volume on the averaged specific-absorption-rate values at the typical European cellular telephone frequencies around 900 and 1800 MHz. The Council of the European Union recommends a 10-g contiguous volume, as the averaging volume. A simple cubic tissue can be used provided that the calculated dosimetric quantities have conservative values relative to the exposure guidelines of the Council of the European Union. Large differences can occur depending on the averaging procedure. The situation is definitely most critical at 1800 MHz.

Effectiveness of FDTD in predicting SAR distributions from the lucite cone applicator

The benefits of using superficial hyperthermia together with radiotherapy has long been proven for recurrent breast carcinomas. The lucite cone applicator has been introduced by some hospital hyperthermia units for superficial treatments. It is characterized by a large effective field size. The modeling techniques used in the past for the study of this, as well as other applicators used for superficial hyperthermia, have failed to address some treatment parameters, such as the dimensions of the waterbolus, which are significant for clinical practice. In this paper, the finite-difference time-domain (FDTD) method is used for modeling the applicators. The numerical results are compared with thermographic measurements. The agreement between predicted and measured specific-absorption-rate distributions is very good. The use of the FDTD method is expected to promote the study of treatment specific factors and help improve future treatment quality.

A study of the SAR distribution of a coaxial-slot antenna for interstitial microwave hyperthermia by the FDTD method

One of the treatments for cancer is hyperthermia, whereby the cancer cells are heated and killed by making use of the difference in thermal sensitivity of the cancer cells and normal cells. The authors have performed an analysis of the coaxial-slot antenna as a heating apparatus for microwave hyperthermia by means of a simulation by the method of moments. In this method, it was not possible to accurately evaluate the thermal profile on the surface of a lossy material, which is extremely important for evaluation of the heating characteristics of the antenna. Also, since the analysis model used assumed that the present antenna is inserted into a lossy material (muscle), it was not possible to evaluate the heating characteristics in multilayered media in an actual human body configuration. In the present paper, the FDTD analysis is introduced for the analysis of the coaxial-slot antenna. First, we ascertain what shape is appropriate for the analysis model. Using this information, the heating profile on the surface of a lossy material is obtained. Further, the heating characteristics when the present antenna is inserted into a multilayered medium simulating the human body are quantitatively determined. © 2000 Scripta Technica, Electron Comm Jpn Pt 1, 84(3): 48–55, 2001

Dielectric-loaded coaxial-slot antenna for interstitial microwave hyperthermia: longitudinal control of heating patterns

In this paper, the microwave interstitial antenna with the dielectric load in part near the tip is introduced to realize the tip-heating and to improve the dependence of the heating patterns on the insertion depth. Numerical simulations using the Finite Difference Time Domain (FDTD) method have been conducted at the frequency of 915MHz for four different configurations of the coaxial-slot antenna inserted into a catheter: the media between the antenna and the catheter are (a) no, (b) a thin air layer, (c) a thin dielectric layer, and (d) a thin air layer and a dielectric load in part near the tip. The diameter of the antenna including the catheter is sufficiently small for minimally invasive therapy. Comparison of the SARs for the four configurations makes it clear that the dielectric-loaded antenna can realize the best tip-heating and suppress the hot spot near the surface of the human body. Dependence of the SAR distributions on the insertion depth of the antenna has also been examined. It is found from the investigation that the dielectric-loaded antenna has little dependence on the insertion depth.

Characterisation of Energy Deposition in the Head from Cellular Phones

This paper is an extension of previous work on the evaluation of power deposition in the head from hand-held transceivers deployed at 900 and 1800 MHz. A realistic model of the head was produced by segmenting a close-grained set of serial MRI slices from one subject. A generic transceiver was considered to be a λ/4 monopole on a metal box. The purpose of that study was to find the maximum SAR values averaged over I g and 10 g for comparison with protection standards based on thermal considerations. The present work investigates the distribution of energy deposition in the head with particular reference to putative sites of cancer induction. Various modifications of the original model had to be made to perform this study including the introduction of the paired parotid, submaxillary and sublingual glands. The SAR distribution in the head at 900 and 1800 MHz has been calculated for various positions of the transceiver. The importance of attenuation through the head and its effect on the bilateral distribution of SAR in paired organs such as the salivary glands, ears and tear ducts is noted.

Technical aspects of microwave thermotherapy

We describe our new technical results dealing with microwave thermotherapy (hyperthermia) in cancer treatment, see Refs. [S.B. Field, C. Franconi (Eds.), Physics and technology of hyperthermia, NATO Seminar Proceedings, Urbino, Italy, 1986; J. Hand, J.R. James (Eds.), Physical Techniques in Clinical Hyperthermia, Wiley, New York, 1986; J. Vrba, M. Lapeš, Microwave Applicators for Medical Purposes, CTU Press, 1996, in Czech; J. Vrba, C. Franconi, M. Lapeš, Theoretical limits for the penetration depth of the intracavitary applicators, International Journal of Hyperthermia, 12:6 (1996) 737–742; C. Franconi, J. Vrba, F. Montecchia, 27 MHz hybrid evanescent-mode applicators with flexible heating field for deep and safe subcutaneous hyperthermia, International Journal of Hyperthermia, 9:5 (1993) 655–6741.]. Our research interest is to develop applicators for deep local heating and for intracavitary cancer and/or prostate treatment as well. Further, a system for 3D SAR distribution measurements in water phantom is explained. Basic evaluation of clinical results is given.

Catheter microwave ablation therapy for cardiac arrhythmias

This article describes three microwave catheter antennas for percutaneous cardiac ablation. A particular design feature of these antennas is that there is no reflected microwave current from the antenna flowing up the transmission line. Thus, it minimizes heating of the coaxial cable. The power reflection coefficients are very low (4% or less) in phantom equivalent materials. These antennas can also serve as bipolar electrodes for sensing endocardiac electrograms. Our studies in dogs, during both cardiopulmonary bypass and closed-chest operations via the femoral vein, have shown microwave energy greater than 200 joules (J) delivered to the heart through a split-tip dipole catheter antenna can produce irreversible block of the heart rhythms. This energy was achieved either by increasing the delivered power from 20 to 40 watts or by increasing the treatment duration from 7 to 11 s (210 to 330 J per application). It produced an endocardium temperature of about 65 degrees C. We found that the percutaneous, transcatheter microwave system is capable of inducing AV blocks consistently in dogs using the flexible, curved tip, split-tip catheter antenna. In addition, our studies have shown that the width and height of SAR distributions for cap-choke and split-tip catheter antennas are similar for the same antenna length. The cap-slot design had a much longer SAR distribution compared to the others. Moreover, a longer (4 mm) split-tip antenna can also induce larger lesions. These results suggest that it could be possible to ablate a ventricular tachycardia focus using the 4 mm split-tip as well as the cap-slot microwave catheter antennas.

Localized current field hyperthermia in carcinoma of the cervix: 3-D computer simulation of SAR distribution

Software was developed for the 3D simulation of SAR distribution for a 500kHz localized current field hyperthermia system to be used in patients with carcinoma of the cervix. This hyperthermia system was specifically designed for use with a modified Fletcher-Suit intracavitary applicator. It consists of software modules for data input, tetrahedral grid generation and a numerical calculation of SAR distribution using an adaptive, multilevel finite element code. The AVS (Advanced Visual System, Inc.) system was used for the visual presentation of the results. A quasi-static approach was employed for the determination of SAR distribution. Results of the performed numerical tests were presented and they showed an important, clinically relevant ability to obtain a selective power deposition. This selective power deposition depended on the applicator geometry, i.e. the distance between the components of a Fletcher-Suit applicator and their relative position and the use of different modes of excitation.

Quantitative evaluation of arrays of Lucite cone applicators in flat layered phantoms using Gaussian-beam-predicted and thermographically measured SAR distributions

SAR distributions from four different E-field-orientated arrays of incoherently driven Lucite cone applicators (LCAs) were investigated. The LCAs operated at 433 MHz with an aperture of cm each. Two techniques were used to obtain SAR distributions in flat layered phantoms: Gaussian beam (GB) predictions and thermographical (TG) imaging. The GB predictions showed that the effective field size of the different array configurations varied by up to 3%. The TG-measured SAR distribution showed significant deviations from the GB-predicted SAR distributions (maximum 34.6%). The difference between GB-predicted and TG-measured SAR levels (averaged per 10% GB-predicted SAR intervals) equalled less than 11.3% for the parallel E-field orientated array and respectively 15.1% for the clockwise-orientated array. When antennae in the clockwise-orientated array were more widely spread (array aperture cm) in order to diminish their mutual interactions, these differences decreased to 12.4%. However, the overall difference within the 50% SAR or higher range decreased from 14% to 9%. The results lead us to conclude that LCAs can be used clinically and their antenna interactions are not considered to be a problem under clinical conditions.

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 × 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 theprincipal 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 of27% xwas 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 hat the GBM can provide good qualitative information about the SAR distribution of dual LCA-arrays.

A 433 MHz Lucite Cone waveguide applicator for superficial hyperthermia

The effective field size (EFS, SAR ≥ 50% of the maximum SAR at 1 cm depth) of a conventional 433 MHz water filled waveguide applicator (32 cm2, aperture area 100 cm2) has been increased by: (1) replacement of the two diverging brass side walls which are parallel to the direction of the electric field by Lucite walls; and (2) Placement of a heterogeneous permittivity in the centre of the aperture. SAR distributions were measured at several depths in layered fat-muscle phantoms. With Lucite side walls the SAR distribution becomes wider in the H-plane of the aperture, resulting in a circular SAR distribution. In this situation the EFS is 67 cm2. Additional insertion of a PVC cone with a top angle of 15d` at the centre of the aperture increases the EFS to 91 ± 6 cm2 for a waterbolus of 18 × 18 × 1 cm3. The experiments also demonstrated that the resulting EFS is affected by the waterbolus size and shape. Calorimetric measurements showed that the efficiency of the improved applicator is comparable to the efficiency of the conventional water filled waveguide applicator, 50 and 56% respectively. The modifications reported provide a simple and inexpensive means to increase the EFS and can be easily implemented in water filled waveguide applicators.

Effect of practical layered dielectric loads on SAR patterns from dual concentric conductor microstrip antennas

Radiation patterns of 2 and 4 cm square Dual Concentric Conductor (DCC) microstrip antennas were studied theoretically with Finite Difference Time Domain (FDTD) analysis and compared with experimental measurements of power deposition (SAR) in layered lossy dielectric loads. Single and array configurations were investigated with 915 MHz excitation applied across either one, two or four sides, or four corners of the square apertures. FDTD simulations were carried out for realistic models of a muscle tissue load coupled to the DCC antennas with a 5 mm thick bolus of either distilled water or low loss Silicone Oil. This study characterizes the effect on SAR of adding three additional thin dielectric layers which are necessary for clinical use of the applicator. These layers consist of a 0.1 mm thick dielectric coating on the array surface to provide electrical isolation of DCC apertures, and 0.15 mm thick plastic layers above and below the bolus to contain the liquid. Experimental measurements of SAR in a plane 1 cm deep in muscle phantom agree well with theoretical FDTD simulations in the multi-layered tissue models. These studies reveal significant changes in SAR for applicator configurations involving low dielectric constant (εr) layers on either side of a high εr water bolus layer. Prominent changes include a broadening and centring of the SAR under each aperture as well as increased SAR penetration in muscle. No significant differences are noted between the simple and complete load configurations for the low εr Silicone Oil bolus. Both theoretical and measured data demonstrate relatively uniform SAR distributions with 50% of maximum SAR extending to the perimeter of single and multi-aperture array configurations of DCC applicators when using a thin 5 mm water or Silicone Oil bolus.