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Purpose: This article explores the feasibility of using coupled electromagnetic and thermodynamic simulations to improve planning and control of hyperthermia treatments for cancer. The study investigates the usefulness of preplanning to improve heat localisation in tumour targets in treatments monitored with PRFS-based magnetic resonance thermal imaging (MRTI)., Methods: Heating capabilities of a cylindrical radiofrequency (RF) mini-annular phased array (MAPA) applicator were investigated with electromagnetic and thermal simulations of SAR in homogeneous phantom models and two human leg sarcomas. High frequency structure simulator (HFSS) (Ansoft) was used for electromagnetic simulations and SAR patterns were coupled into EPhysics (Ansoft) for thermal modelling with temperature-dependent variable perfusion. Simulations were accelerated by integrating tumour-specific anatomy into a pre-gridded whole body tissue model. To validate this treatment planning approach, simulations were compared with MR thermal images in both homogenous phantoms and heterogeneous tumours., Results: SAR simulations demonstrated excellent agreement with temperature rise distributions obtained with MR thermal imaging in homogeneous phantoms and clinical treatments of large soft-tissue sarcomas. The results demonstrate feasibility of preplanning appropriate relative phases of antennas for localising heat in tumour., Conclusions: Advances in the accuracy of computer simulation and non-invasive thermometry via MR thermal imaging have provided powerful new tools for optimisation of clinical hyperthermia treatments. Simulations agree well with MR thermal images in both homogeneous tissue models and patients with lower leg tumours. This work demonstrates that better quality hyperthermia treatments should be possible when simplified hybrid model simulations are performed routinely as part of the clinical pretreatment plan.

The design and performance of annular antenna arrays with reflectors is presented. Arrays with three shapes of reflectors are analysed and simulated. These include the corner reflector, the circular reflector and the elliptical reflector. Power-density distributions within the annular arrays with and without reflectors are obtained by using the FDTD method. Also, the image theory method is used to verify the FDTD results in one case. By comparing the power-density distribution pattern of all four of the array designs (three with different reflectors, one without reflector), it is readily seen in each case that the array with reflectors yields better power-efficiency than the array without reflectors and the elliptical reflector yields the best performance. Comparisons of each array are made using 4, 6 and 8 antennae in the annular array. By using the optimized results of the elliptical reflector, the requirement for the input-power level to each antenna is greatly reduced.

This paper explores the behaviour of the primary and secondary hot spots in homogeneous and two-dimensional inhomogeneous medium. Circular arrays are considered with a radius of up to five wavelengths. The number of sources and their positions in the array are varied, and the influence of these variations on the primary and secondary hot spots is observed. It is found that the primary hot spot reaches its final shape with the addition of a very small number of sources to the array. An increase in the number of sources results in a reduction of the normalized magnitude of the secondary hot spots, but the size of the normalized primary hot spot remains the same. An upper limit of sources in the array exists after which no further reduction of the secondary hot spots is observed. The finite-difference time-domain method (FDTD) is used to obtain the electric-field distribution in the inhomogeneous medium. A genetic algorithm is then applied to find the optimal positions of the antennae in the array.

This paper develops and presents an application of Huygen's principle herein referred to as the equivalent uniformly illuminated aperture method (EUAM). This method is used to predict the E-field patterns of aperture antennas in both the near- and far-field regions. In applying this method the radiating aperture is mathematically replaced by an array of smaller, uniformly illuminated aperture sources. Thus the true aperture field is approximated in a step-wise manner by the individual sub-apertures that are uniformly illuminated by the field at the point on the aperture where the sub-aperture is located. This method is suitable for analysing any antenna if its aperture fields are known. Comparisons are made between this method, an equivalent dipole method, and experimental measurements. These calculations and measurements show that the EUAM is more accurate (especially in the near-field region), is much simpler to implement, and requires less execution time than the equivalent dipole method.

Temperature measurement using magnetic resonance imaging (MRI) of water self-diffusion is investigated. Diffusion images and derived temperatures are obtained in polyacrylamide gel phantom. The temperatures measured from MRI are compared with those from temperature probes to verify their accuracy. In general, the difference between temperatures determined from MRI diffusion images over 0.3 cm3 regions of interest and from temperature probes were 0.2 degrees C. It is concluded that current MRI technology allows noninvasive temperature tomography that is comparable with invasive thermometry with respect to temperature accuracy, has spatial and time resolutions that would be useful in hyperthermic oncology.

Measurements made on the interstitial microwave antennas used for hyperthermia cancer therapy indicate that the heating patterns vary with the insertion depths (defined as the distance from the antenna tip to air-tissue interface). The antennas are made of thin coaxial cables with a radiation gap or gaps on the outer conductor. The antennas are inserted into small polypropylene catheters implanted in the tumour volume. This type of antenna may be simulated as an asymmetric dipole with one arm being the tip section consisting of the expanded extension of the inner conductor, and the other arm being the section of the outer conductor from the gap to the insertion point (air-tissue interface). We use four of the antennas to form a 2 cm x 2 cm array. The antennas are positioned on the corners of a 2 cm square. Measurements on both single antennas and multi-antenna arrays show that the maximum heating is not stationary with position along the antenna when the depth of insertion is changed. This paper investigates the theoretical prediction of the changes in heating patterns of interstitial microwave antennas at different insertion depths. Each of the antennas in the array is simulated as an asymmetric dipole. The SAR (specific absorption rate) is computed by using the insulated dipole theory. The temperature distribution in absence of perfusion is obtained through a thermal simulation routine to convert the SAR pattern into the temperature pattern. Excellent qualitative agreement is found between the theoretical heating pattern and the measured pattern in a non-perfused phantom on a 2 cm x 2 antenna array. Since the insertion depths of the interstitial antennas are different from patient to patient, it is recommended that simulation of the heating be done before treatments, to confirm the delivery of power to the target region.