In vitro assessment of tissue heating near metallic medical implants by exposure to pulsed radio frequency diathermy.

A patient with bilateral implanted neurostimulators suffered significant brain tissue damage, and subsequently died, following diathermy treatment to hasten recovery from teeth extraction. Subsequent MRI examinations showed acute deterioration of the tissue near the deep brain stimulator (DBS) lead's electrodes which was attributed to excessive tissue heating induced by the diathermy treatment. Though not published in the open literature, a second incident was reported for a patient with implanted neurostimulators for the treatment of Parkinson's disease. During a diathermy treatment for severe kyphosis, the patient had a sudden change in mental status and neurological deficits. The diathermy was implicated in causing damage to the patient's brain tissue. To investigate if diathermy induced excessive heating was possible with other types of implantable lead systems, or metallic implants in general, we conducted a series of in vitro laboratory tests. We obtained a diathermy unit and also assembled a controllable laboratory exposure system. Specific absorption rate (SAR) measurements were performed using fibre optic thermometry in proximity to the implants to determine the rate of temperature rise using typical diathermy treatment power levels. Comparisons were made of the SAR measurements for a spinal cord stimulator (SCS) lead, a pacemaker lead and three types of bone prosthesis (screws, rods and a plate). Findings indicate that temperature changes of 2.54 and 4.88 degrees C s(-1) with corresponding SAR values of 9129 and 17,563 W kg(-1) near the SCS and pacemaker electrodes are significantly higher than those found in the proximity of the other metallic implants which ranged from 0.04 to 0.69 degrees C s(-1) (129 to 2471 W kg(-1)). Since the DBS leads that were implanted in the reported human incidents have one-half the electrode surface area of the tested SCS lead, these results imply that tissue heating at rates at least equal to or up to twice as much as those reported here for the SCS lead could occur for the DBS leads.

Failure of transcutaneous electrical nerve stimulation in the conventional and burst modes to alter digital skin temperature.

The purpose of this study was to determine if transcutaneous electrical nerve stimulation (TENS), when applied using stimulation settings that might be used in clinical practice, produces alterations in skin temperature. Stimulation in the conventional and burst modes was applied to the skin of the forearm overlying the median nerve in a healthy human subject. Skin temperature was measured at the index finger, little finger, and on the volar surface of the forearm in the stimulated limb. TENS applied for 20 minutes in the conventional mode at an intensity sufficient to produce a perceptible though not uncomfortable sensation and a slight muscle contraction of the forearm musculature failed to alter skin temperature at any of the three measurement sites. Stimulation using the same pulse characteristics delivered in the burst mode failed to alter skin temperature of the fingers but produced a significant increase in skin temperature at the forearm. The increase in skin temperature at the forearm persisted for four minutes following the cessation of stimulation after which skin temperature returned to prestimulation levels. We conclude that TENS, as applied in this study, does not influence skin temperature in the peripheral distribution of the nerve stimulated, and hypothesize that the rise in skin temperature at the forearm was a result of increased blood flow in the forearm muscles produced by the stimulation-induced contraction of those muscles.

Automated microwave oven survey systems: evaluation criteria and procedures.

Criteria and evaluation procedures have been established to assess the measurement accuracy of Automated Microwave Oven Leakage Survey Systems. These criteria and procedures are based upon analytical characterizations of the oven leakage fields, measurement data and statistical analysis and experience gained from evaluation of hand-held oven survey instruments. Results of our evaluation of a typical survey system are presented.

The performance of a new 915-MHz direct contact applicator with reduced leakage.

The technical feasibility of commercially developing a safe and effective direct contact diathermy applicator operating at the industrial, scientific, medical (ISM) frequency of 915 MHz is demonstrated. The basic design consists of a circular waveguide which is internally loaded with two orthogonal pairs of forward ridges to obtain circular polarization and two rear ridges with a probe to excite the guide. Two prototype designs are considered: the small applicator (15 cm diameter) has one annular choke covered with a 2.5-cm thick microwave absorber, and the large applicator (25 cm diameter) has two additional concentric chokes to limit leakage radiation. The performance of the applicators was evaluated in terms of the requirements of a ORH microwave diathermy test protocol to control stray radiation and deliver a thermally effective absorbed dose rate to simulated muscle tissue of a phantom with a 1-cm or 2-cm fat layer. The net power required to deliver a thermally effective 235-W/kg specific absorption rate (SAR) to such a planar phantom was determined. For this net power, leakage levels considerably less than 5 mW/cm2 (at 5 cm from applicator-phantom boundary) were obtained for the applicators in direct contact with the phantom. If a small spacing (1 cm) between these applicators and planar phantoms is introduced, the net power required to deliver an effective SAR to a phantom and the associated leakage can become excessive. For the small applicator, the required net power for inducing an SAR of 235 W/kg in muscle tissue of a 1-cm fat layer phantom is about 330 W and the leakage is about 120 mW/cm2. For a 2-cm fat layer phantom, these values are somewhat higher. For the large applicator, using a 1-cm fat layer phantom, the values are about 200 W and about 17 mW/cm2. Again, for a 2-cm fat layer phantom, these values are somewhat higher.

An evaluation of microwave diathermy applicators using free space electric field mapping.

An electric field mapping study is described which was designed and executed to study the performance characteristics of various prototype and currently used microwave diathermy applicators operating at 2450 MHZ. A miniature diode detector/dipole probe, developed by the Bureau of Radiological Health, was used to map the 'near' electric fields in the aperture plane of each applicator. From the field mapping results, parameters for use in the evaluation of the radiation safety of various applicators are developed. A thermographic camera was used to determine heating characteristics of the applicators in simulated biological material. The thermal effectiveness of these applicators was also investigated by determining their Specific Absorption Rate, a recently developed parameter. The superiority of a specially designed prototype applicator (in terms of uniformity and effectiveness of energy deposition, and minimal leakage radiation) compared with currently used applicators is demonstrated.