Jan Vrba

Time-reversal focusing in microwave hyperthermia for deep-seated tumors

A fast beam-forming method for hyperthermia treatment of deep-seated tumors is described and verified. The approach is based on the time-reversal characteristics of Maxwell equations. The basic principle of the method is coupling of the electromagnetic modeling of the system with the actual application. In this modeling the wavefront of the source is propagated through a patient-specific model from a virtual antenna placed in the tumor of the model. The simulated radiated field is then captured using a computer model of the surrounding antenna system. The acquired amplitudes and phases are then used in the real antenna system. The effectiveness of this procedure is demonstrated by calculating the power absorption distribution using FDTD electromagnetic simulations of a realistic 2D breast model as well as a 2D neck model. Several design parameters, i.e. number of antennas, operating frequency and dimensions, have been evaluated by performance indicators. The promising results suggest that the development of this technique is pursued further.

High-frequency electric field and radiation characteristics of cellular microtubule network

Microtubules are important structures in the cytoskeleton, which organizes the cell. Since microtubules are electrically polar, certain microtubule normal vibration modes efficiently generate oscillating electric field. This oscillating field may be important for the intracellular organization and intercellular interaction. There are experiments which indicate electrodynamic activity of variety of cells in the frequency region from kHz to GHz, expecting the microtubules to be the source of this activity. In this paper, results from the calculation of intensity of electric field and of radiated electromagnetic power from the whole cellular microtubule network are presented. The subunits of microtubule (tubulin heterodimers) are approximated by elementary electric dipoles. Mechanical oscillation of microtubule is represented by the spatial function which modulates the dipole moment of subunits. The field around oscillating microtubules is calculated as a vector superposition of contributions from all modulated elementary electric dipoles which comprise the cellular microtubule network. The electromagnetic radiation and field characteristics of the whole cellular microtubule network have not been theoretically analyzed before. For the perspective experimental studies, the results indicate that macroscopic detection system (antenna) is not suitable for measurement of cellular electrodynamic activity in the radiofrequency region since the radiation rate from single cells is very low (lower than 10⁻²⁰ W). Low noise nanoscopic detection methods with high spatial resolution which enable measurement in the cell vicinity are desirable in order to measure cellular electrodynamic activity reliably.

Planar Circuit Analysis of Microstrip Radial Stub

Microstrip radial line stubs are analyzed using a planar circuit technique and characterized for design purposes. Experiments performed on various structures are in excellent agreement with the theory.

Evanescent-mode applicators (EMA) for superficial and subcutaneous hyperthermia

Evanescent-mode waveguide aperture applicators are proposed for hyperthermic treatments of superficial and subcutaneous tissues. They consist of air-filled waveguide segments that work below the cutoff frequency and therefore support only evanescent transverse modes. These are excited by radiators of suitable symmetry and configuration to produce modal heating fields of selected cross-sections. This field emerges from the waveguide active aperture and enters the tissue to be heated through an air gap. These devices work in a very large range of frequencies and are extremely simple to manufacture, even with a variety of cross-section sizes and shapes, because of their air-filled feature. This enables good heating field flexibility with improved penetration to be obtained. Their operation is safe and practical also on irregular and curved tissue surfaces.<<ETX>>

Operator Independent Focused High Frequency ISM Band for Fat Reduction: Porcine Model

Selective fat reduction has been clearly shown for various methods and energy modalities including cryolipolysis and high intensity focused thermal ultrasound. Mathematical modeling of focused high frequency of the EM spectrum has indicated that selective heating of fat is possible using wavelengths not previous explored. The purpose of this study was to demonstrate in the porcine model that selective heating of fat is possible with a non‐contact, operator independent device.

Evaluation of a patch antenna applicator for time reversal hyperthemia

Purpose: To describe the design, analysis and evaluation of a new antenna array system for microwave hyperthermia. The proposed antenna array was evaluated by the focusing method based on the time-reversal principle. Materials and methods: Power absorption distributions in a cylindrical homogeneous and inhomogeneous phantom were calculated for the frequency range 500–900 MHz. Two set-ups with 12 and 16 antennas were analysed by comparing the changes in focusing areas enclosed by the 50%, 75% and 90% iso-SAR contours. For a more quantitative evaluation of the results the average power absorption ratio and remaining tissue maximum index were calculated. Results: The sharpest focusing area in the centre of the phantom, 151 mm2 (9 × 20 mm) (90% iso-SAR), was obtained by using 16 antennas at frequency 900 MHz. The largest focusing area of 280 mm2 (13 × 24 mm) (90% iso-SAR) was obtained by using 16 antennas at 500 MHz. The SAR focus was steered in the desired radial direction obtaining a 43 mm2 90% iso-SAR focus-width in a semi-three-dimensional neck phantom. The results showed qualitative agreement between three dimensions (3D) and two dimensions (2D) for the performance indicators. Conclusions: The conducted study confirms the feasibility of the time-reversal-based focusing methods for microwave hyperthermia. The proposed system shows promise and is suitable for further development in the treatment of head and neck tumours, and extremities application.

Theoretical limits for the penetration depth of intracavitary applicators

We present an approximated analytical model for calculating the attenuation features of the wavefront power density, or SAR, for linear radiators inside a coaxial cylindrical cavity in both non-lossy and lossy tissues. The results are evidencing the determinant role of the cavity radius in affecting the SAR radial decay and the associated penetration depth. A further explicit finding is that the upper limit for the penetration depth in endocavitary radiative heating is equal to the cavity radius, a limit of general validity which holds in both lossy and non-lossy media for any radius value, and is not affected by the approximated nature of the model. Thus, a simple exponential equation allows a straightforward predictive evaluation of both the penetration depth intrinsic upper limit and the approximate penetration depth values, with only the knowledge of the cavity radius and the operating frequency required, without the need to refer to time-consuming electromagnetic field calculations.

27 MHz hybrid evanescent-mode applicators (HEMA) with flexible heating field for deep and safe subcutaneous hyperthermia

A new class of low-frequency electromagnetic applicators for hyperthermic treatment of superficial and subcutaneous tissues is described. These applicators employ an air-filled waveguide segment which is operating below the cut-off frequency, the evanescent modes of which are energized by suitable exciters to produce model field components. Direct radiators are integrated into the waveguide to generate additional direct field components. All field components may be combined in different power level ratio, phase, and orientation, to provide a composite heating field exhibiting a large variety of field sizes, shapes, and penetration features. The composite field emerging from the waveguide aperture propagates within the tissue to be heated through a non-critical air-gap. These versatile heating devices appear of potential interest to heat a variety of deep and localized subcutaneous tissues to therapeutic temperatures without injury to access fat layers of substantial thickness.

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. Lapes, Microwave Applicators for Medical Purposes, CTU Press, 1996, in Czech; J. Vrba, C. Franconi, M. Lapes, 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) 655-674.]. 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.

Microwave Thermotherapy in Cancer Treatment: Evaluation of Homogeneity of SAR Distribution

Medical applications of microwaves (i.e., a possibility to use microwave energy and/or microwave technique and technology for therapeutical purposes) are a quite new and very rapidly developing fleld. Microwave thermotherapy is being used in medicine for cancer treatment and treatment of some other diseases since early eighties. This paper is a contribution to a theory of phase array applicators to be used for a microwave thermotherapy (microwave hyperthermia) in a cancer treatment. It deals with a study and theoretical evaluation of homogeneity of SAR distribution in cylindrical agar phantom for several difierent values of its radius. Discussed SAR distribution is in our case created by simulations of EM fleld exposure done by aid of four microwave stripline type TEM mode applicators of the same type.