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.

RF H-field fluxtubes for safe and controlled hyperthermia.

A novel low RF frequency hyperthermia applicator is presented. This consists of a toroidal resonator on the conductive wall of which two radical treatment ports are cut away to generate an air gap. This exposes the body part to be treated to the RF magnetic field fluxtube of high density and uniformity that is circulating uninterrupted within the toroidal walls through the air gap. The configuration of the port-body assembly may be adjusted for steering the induced heating field over the target while maintaining the effectual energy coupling with the tissue. The results of the assessment of a 27 MHz high density fluxtube on cylindrical all-muscle and fat-muscle phantoms show that a purely inductive type of heating occurs, which is substantially localized inside the muscle tissue and a water bolus is unnecessary in the absence of stray E-fields. Heating field optimization is accomplished with fluxtube cross-section, pathway and air gap shaping. It is also shown that the superimposition of a uni-directional coherent field created by passive auxiliary electrodes to the fluxtube-induced solenoidal field produces a constructive interference and enhanced and more penetrating localized heating.

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.

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 which are working 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-section. This field is emerging from the waveguide active aperture and enters the tissue to be heated through an air gap on non-critical length. These devices work in a very large range of frequencies, and are extremely simple to manufacture even with a variety of cross-section size and shape, on account of their air-filled feature. This enables to obtain good heating field flexibility with improved penetration. Their operation is safe and practical also on irregular and curved tissue surfaces.

Low-frequency RF hyperthermia: IV--A 27 MHz hybrid applicator for localized deep tumor heating.

A 27 MHz dual-device applicator of novel design, which is aimed to heat noninvasively with improved safety tumor masses at depth, is proposed. A substantially localized temperature gain is obtained by superimposing two delocalized low RF frequency and phase-coherent current distributions, which are launched to constructively interfere over a limited region emcompassing the tumor volume. An hybrid applicator (HA) has been developed, integrated one capacitive and one inductive heating device, and has been assessed on a 20 cm diameter cylindrical fat-muscle phantom. The interference pattern is characterized by a deep broad SAR maximum and by the disappearance of the central null SAR value typical of single inductive devices. An 80% SAR useful therapeutic volume (UTV) of a near-cylindrical shape of about 800 cm3 is obtained with a penetration of 6-8 cm for the phantom surface, with a noncritical axial length of approximately 21 cm. The UTV may be somehow controlled in size and penetration. These results are obtained with the tissue-like medium surrounding the UTV heated uniformly and safely to a temperature pedestal below the therapeutic temperature with about half RF power values to each of the heating devices.

27 MHz conformal capacitive ring (CR) applicators for uniform hyperthermic/diathermic treatment of body segments with axial fields.

A family of 27 MHz heating devices has been developed and fundamental tests carried out. The devices consist in a pair of ring-like, flexible, capacitive electrodes, conformally wrapped round the tissue to be heated, for use in hyperthermia and rehabilitation treatment. These capacitive ring (CR) applicators produce an axial E-field which can uniformly heat the central portion of a 20 cm OD cylindrical phantom simulating fat and muscle tissues. This electrode-body configuration can be used to uniformly heat most human body segments with the highest symmetry congruence, the RF currents flowing parallel to the boundaries provided by the pseudo-axial symmetry of these segments. With respect to the rigid pads used in RF capacitive heating, the CR electrodes exhibit a larger working surface area. Moreover, they may be used with an interposed dielectric layer between the electrodes and the skin, to reduce edge effects. With respect to the axially symmetric radiators of higher frequency, they are applicable to larger cross-section body segments with increased penetration and without the need of an integral water bolus. The CR devices allow free access to most of the body surface for skin temperature measurement and conditioning, and for combined treatments. Moreover, the almost uniform heating pattern may represent a useful feature for standardizing treatment. The CR electrodes can easily be matched to the RF source even if they are low impedance devices, and they give rise to low-level stray fields, which can be taken care of by a local shield. These CR devices can be considered safe and practical and suitable for regional hyperthermic and rehabilitation treatment.

A new automatically controlled electric TAH.

Cardiovascular regulatory mechanisms are implemented by means of a very complex control system involving neural, humoral, and mechanical agents. The state of the art in TAH today reveals that we have passed a stage where we can be satisfied with a 6 or 9 mo survival. Development and implementation of a versatile control theory must be another milestone before a TAH can really serve the prospective patient population requiring such a device. This study reveals the vital role and merits as well as the feasibility of incorporating such a control theory into the device.