Coupling efficiency of helical coil hyperthermia applications

Field analysis of a tape helix with embedded bio-media for microwave hyperthermia

A general field theory has been developed for the EM wave propagation on a tape helix with embedded bone, muscle, fat and skin co-axial biological layers in a multilayered dielectric environment. The electric and magnetic fields in every biological/dielectric layer is formulated in terms of complex modified Bessel functions and complex exponentials, i.e. in terms of complex radial and axial propagation constants for the azimuthally symmetric and higher order spatial harmonics. A complete dispersion relation is derived by substituting the field expressions into as many boundary conditions as there are unknown constants. The dispersion relation is numerically solved for the complex axial propagation constant and hence for the phase velocity and depth of penetration for n = 0 and 1 modes propagating along three dielectric loaded tape helices of different dimensions at several spot frequencies within the range of 300 MHz to 3.00 GHz. The results for variations in normalized phase velocity and depth of penetration with frequency for these modes separately and for the combined mode are presented. The patterns of specific absorption rate (SAR) across the cross-section of the dielectric loaded helices are also computed and presented. The variations with frequency, and cross-sectional dimension of the helix of the normalized phase velocity, axial depth and SAR across the cross-section of each of the helices are discussed for the modes considered. The theoretical results for the propagation constant obtained by using a tape helix model for the azimuthally symmetric mode propagating along a wire helix loaded with a phantom muscle sample of known dielectric constant are compared with experimental results and with those obtained theoretically employing a sheath helix model at 2.45 and 2.55 GHz. The use of a helix for human limb hyperthermia is also discussed.

The CDRH Helix-I: a physical evaluation

The use of a resonant helical coil with predominantly axial electric fields for regional hyperthermia in the abdomen and pelvis is addressed. The Helix-I applicator, which consists of a three-turn, 36-cm-long, oval-wound helical coil measuring 60 and 43 cm along its major and minor axes, respectively, is described, and specific absorption rate (SAR) measurements for the device are reported. Measurements of the E-field are also described. Specific absorption patterns for the Helix, determined by transient temperature measurements using a gel phantom, and by E(2)-field scans using a liquid phantom, are in general agreement. The general agreement of electric power intensity distribution, inside and outside the phantom, with corresponding SAR distributions obtained from transient temperature data, ensures reliability of the distribution patterns. The E(2) contours provide a clear picture of hot and cold spots as well as the nature of the general distribution.

Evaluation of a semi-cylindrical solenoid as an applicator for radio-frequency hyperthermia

A modified solenoid having a semi-circular cross section is evaluated as an applicator for radio-frequency heating of biological materials. The operating efficiency of this applicator and the distribution of the electric field established within a simulated biological sample are determined for a number of modes of excitation. It is shown that uniform heating to a substantial depth is possible at efficiencies in excess of 80 percent.

Future trends in heating technology of deep-seated tumors

Hyperthermia techniques and equipment have developed into a level of sophistication requiring special training and operator skills. Recent advances electromagnetic and ultrasound heating techniques are multi-applicator arrays, both external and invasive. These arrays are most effective when individual channel power control can be utilized to improve the heating pattern control during treatment. A preview of future capabilities shows that more selective tumor heating is possible with the advanced equipment. The role of computer pretreatment planning is a valuable aid in operator training as well as optimization of parameters. The current development of regional heating is in the deep focal heating by external focused arrays. The SIGMA 60 cylindrical dipole array deep steerable focal zone is described and test data presented showing the selective focusing obtained. A small invasive targeting E-field sensor is used to optimize phase and amplitude of the array to improve the selective heating in the target zone. These patented techniques and devices are expected to reduce systemic toxicity and increase tumor temperatures with lower power input. Future equipment development is predicted to include many types of hyperthermia devices and techniques. This trend suggests that systems must be designed to simplify the operator's tasks and improve the heating pattern controls. The computer power and control will become much more important in these future systems.