Heating Of Biological Tissues Research Articles

Lens applicator for localized microwave hyperthermia

AbstractHyperthermia is now recognized as an effective means for cancer therapy, and many reports on the irradiation of biological subjects by electromagnetic waves to produce dielectric heating and hyperthermia have been reported. Although it is desirable in hyperthermia selectively to heat the cancer tissue, it is difficult to estimate the heat distribution in the subject owing to insufficient analysis of the electromagnetic field distribution in the irradiated subject. At present it is not yet possible to control the temperature distribution in the biological subject. In electromagnetic wave hyperthermia, microwaves are effective from the viewpoint of efficiency or selective heating. On the other hand, it is difficult to heat a cancer in deep tissue because of reduced penetration of the microwaves into the subject. This paper shows experimentally and by theoretical analysis, using a microwave lens geometrically designed to produce a focused cylindrical wave as the applicator, that selective heating is possible for tissue 10 to 40 percent deeper than with planar wave. It is shown that the microwave lens applicator is effective for the heating of biological tissues.

New microstrip slot radiator for medical applications

A microstrip slot radiator for local heating of biological tissue operating at 2.45 GHz was designed and tested. The radiator is well matched within a relatively wide range of frequencies (v.s.w.r. < 1.5 for 2.45 ± 0.3 GHz) and provides energy deposition within an area of about 1.5 cm × 1 cm.

Heating of biological tissue in the induction field of VHF portable radio transmitters

The results of a research project on the heating of simulated human tissue in the induction field of portable radios at VHF are summarized. The investigation was initiated because measurements made with commercially available field probes indicated that, in some cases, apparent power levels higher than 10 mW/cm2are incident on the operator. Two phantom models have been built for RF heating tests. The first is a parallelepiped of simulated muscle material 26 in long, 9 in wide, and 6.5 in high, topped by a 0.5 in layer of fat and bone composition. The other phantom is a human-size head and shoulders. This is a 1/3-in thick shell of bone composition containing simulated brain material. The measurements of temperature increment due to radiation were performed with a digital thermometer having a sensitivity of 0.01°C. Temperature measurements on the parallelepipedal phantom have shown that the penetrating power densities in the simulated tissue are substantially lower than what could be expected from an incident plane wave with the same E-field intensity. The physical reason for this apparent discrepancy is that the strong fields of static nature emanating from a VHF helical antenna (commonly used with portable radios) are normally rather than tangentially directed to the surface of the phantom. These fields practically collapse at the air-body interface because of the high complex dielectric constant of human flesh. The results of the measurements performed on the head phantom have shown that a 6-W portable radio with a helical antenna held at 0.2 in from the operator's mouth causes very little heating of the simulated biological tissue (less than 0.1°C is highest temperature increase for one minute exposure). The maximum power density penetrating the dummy is less than 1 mW/cm2in the middle forehead. No detectable temperature increase is present in the immediate eye area. This is because in normal use, the eyes of the operator are exposed to the relatively low fields at the base of the antenna. A health hazard is present if the user places the tip of the antenna in the immediate vicinity of the eye (less than a 0.2-in distance) and then operates the transmitter. In this case, the possibility of damage is greatly reduced by a thick insulating cap at the tip of the antenna.