Temperature and SAR measurement errors in the evaluation of metallic linear structures heating during MRI using fluoroptic probes.

The purpose of this work is to evaluate the error associated with temperature and SAR measurements using fluoroptic temperature probes on pacemaker (PM) leads during magnetic resonance imaging (MRI). We performed temperature measurements on pacemaker leads, excited with a 25, 64, and 128 MHz current. The PM lead tip heating was measured with a fluoroptic thermometer (Luxtron, Model 3100, USA). Different contact configurations between the pigmented portion of the temperature probe and the PM lead tip were investigated to find the contact position minimizing the temperature and SAR underestimation. A computer model was used to estimate the error made by fluoroptic probes in temperature and SAR measurement. The transversal contact of the pigmented portion of the temperature probe and the PM lead tip minimizes the underestimation for temperature and SAR. This contact position also has the lowest temperature and SAR error. For other contact positions, the maximum temperature error can be as high as -45%, whereas the maximum SAR error can be as high as -54%. MRI heating evaluations with temperature probes should use a contact position minimizing the maximum error, need to be accompanied by a thorough uncertainty budget and the temperature and SAR errors should be specified.

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.

Cellular phone interference testing of implantable cardiac defibrillators in vitro.

An in vitro study was undertaken to investigate the potential for cellular telephones to interfere with representative models of presently used ICDs. Digital cellular phones (DCPs) generate strong, amplitude modulated fields with pulse repetition rates near the physiological range sensed by the ICD as an arrhythmia. DCPs with Time Division Multiple Access (TDMA) pulsed amplitude modulation caused the most pronounced effect--high voltage firing or inhibition of pacing output of the ICDs. This electromagnetic interference (EMI) occurred only when the phones were within 2.3-5.8 cm of the ICD pulse generator that was submerged 0.5 cm in 0.18% saline. ICD performance always reverted to baseline when the cellular phones were removed from the immediate proximity of the ICD. Three models of ICDs were subjected to EMI susceptibility testing using two types of digital phones and one analog cellular phone, each operating at their respective maximum output power. EMI was observed in varying degrees from all DCPs. Inhibition of pacer output occurred in one ICD, and high voltage firing occurred in the two other ICDs, when a TDMA-11 Hz DCP was placed within 2.3 cm of the ICD. For the ICD that was most sensitive to delivering unintended therapy, inhibition followed by firing occurred at distances up to 5.8 cm. When a TDMA-50 Hz phone was placed at the minimum test distance of 2.3 cm, inhibition followed by firing was observed in one of the ICDs. EMI occurred most frequently when the lower portion of the monopole antenna of the cellular phone was placed over the ICD header.

Specific absorption rates and induced current distributions in an anatomically based human model for plane-wave exposures.

We have previously reported local, layer-averaged, and whole-body-averaged specific absorption rates and induced currents for a 5,628-cell anatomically based model of a human for plane-wave exposures 20-100 MHz (Chen and Gandhi 1989). Using a higher resolution, 45,024-cell model of the human body, calculations have now been extended to 915 MHz using the finite-difference time-domain method. Because of the higher resolution of the model, it has been possible to calculate specific absorption rates for various organs (brain, eyes, heart, lungs, liver, kidneys, and intestines) and for various parts of the body (head, neck, torso, legs, and arms) as a function of frequency in the band 100-915 MHz. Consistent with some of the experimental data in the literature, the highest part-body-averaged specific absorption rate for the head and neck region (as well as for the eyes and brain) occurs at 200 MHz for the isolated condition and at 150 MHz for the grounded condition of the model. Also observed is an increasing specific absorption rate for the eyes for frequencies above 350 MHz due to the superficial nature of power deposition at increasing frequencies.

United States radiation safety and regulatory considerations for radiofrequency hyperthermia systems.

The control of Radiofrequency (RF) radiation (including microwave radiation) that is emitted by therapeutic medical devices is the responsibility of the Food and Drug Administration's (FDA) Bureau of Radiological Health (BRH). Several studies of RF emissions from various shortwave (27 MHz) and microwave (2450 MHz) diathermy devices have been conducted by the Electromagnetics Branch of the Bureau's Division of Electronic Products. BRH studies have led to a proposed standard for microwave diathermy devices operating above 900 MHz. Shortwave diathermy devices used in physical therapy situations have been found to produce relatively high levels of unintended exposures (sometimes exceeding present U.S. exposure standards) to device operators and to the nonprescribed tissues of the patient. BRH is initiating further studies to ascertain the need for controls to be placed on these shortwave devices to ensure safety and medical effectiveness. Radiation safety standards, which presently exist in the United States, allow much higher unintended human exposures than do the standards existing in the several eastern European countries. A trend to lower permissible exposures to 5 mW/cm2 or even 1 mW/cm2 is under way in the U.S. The various provisions of FDA's Medical Device regulations apply to investigational as well as commercially-marketed RF/microwave devices and require both safety and medical effectiveness aspects of performance to be addressed by their manufacturer. A set of microwave radiation safety considerations has been developed by BRH for newly emerging cancer therapy protocols which utilize microwave hyperthermia devices.