Paul S. Ruggera

Development of a Family of RF Helical Coil Applicators Which Produce Transversely Uniform Axially Distributed Heating in Cylindrical Fat-Muscle Phantoms

Uniform transverse heating, without excessive surface heating, in phantoms which simulate human limbs or trunks has long been sought after as a precursor for predictable hyperthermia treatment of deep-seated cancerous tissues. A family of helical coils developed at the National Center for Devices and Radiological Health has induced transversely uniform axially distributed heating in the muscle portion of simulated fat-muscle, cylindrical arm- and thigh-sized phantoms without excessively heating the simulated fat. The final design parameters for the coils were derived using different sized coil/phantom combinations and electric (E)-and magnetic (H)-field strength measurement scans. Transverse heating patterns, in the form of thermographic pictures, will be the principal format for presentation of the experimental data in this paper. Independent nonperturbing thermometry data are also included to enhance the accuracy of the thermographic results obtained.

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 1CD 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 TDMA11 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.

Rapid and uniform electromagnetic heating of aqueous cryoprotectant solutions from cryogenic temperatures

Devitrification (ice formation during warming) is one of the primary obstacles to successful organ vitrification (solidification without ice formation). The only feasible approach to overcoming either devitrification or its damaging effects in a large organ appears at present to be the use of some form of electromagnetic heating (EH) to achieve the required high heating rates. One complication of EH in this application is the need for warming within a steel pressure vessel. We have previously reported that resonant radiofrequency (RF) helical coils provide very uniform heating at ambient temperatures and low heating rates and can be modified for coaxial power transmission, which is necessary if only one cable is to penetrate through the wall of the pressure vessel. We now report our initial studies using a modified helical coil, high RF input power, and cryogenic aqueous cryoprotectant solutions [60% (w/v) solution of 4.37 M dimethylsulfoxide and 4.37 M acetamide in water and 50% (w/w) 1,2-propanediol]. We also describe the electronic equipment required for this type of research. Temperatures were monitored during high-power conditions with Luxtron fiberoptic probes. Thermometry was complicated by the use of catheters needed for probe insertion and guidance. The highest heating rates we observed using catheters occurred at temperatures ranging from about -70 to -40 degrees C, the temperature zone where devitrification usually appears in unstable solutions during slow warming. We find that in this range we can achieve measured heating rates of approximately 300 degrees C/min in 30- to 130-ml samples using 200 to 700 W of RF power without overheating the sample at any point. However, energy conservation calculations imply that our measured peak heating rates may be considerably higher than the true heating rates occurring in the bulk of our solutions. We were able to estimate the overall true heating rates, obtaining an average value of about 20 degrees C/min/100 W/100 ml, which implies a heating efficiency close to 100%. It appears that it should be possible to warm vitrified rabbit kidneys rapidly enough under high-pressure conditions to protect them from devitrification.

Medical device EMI: FDA analysis of incident reports, and recent concerns for security systems and wireless medical telemetry

FDA has evaluated reports of medical device malfunctions caused by electromagnetic interference (EMI), performed device testing, and developed standardized test procedures. Over 500 incident reports are suspected to be attributable to EMI affecting cardiac devices. More than 80 of these reports involve cardiac and other medical device interactions with electronic security systems. EMI presents a risk to patient safety and medical device effectiveness that is likely to continue as the use of electromagnetic energy in the medical device environment increases (e.g., cell phones, security systems). Developments can reduce these risks, such as the allocation of dedicated frequency bands for the new wireless medical telemetry service (WMTS) designed to protect transmissions of patient vital signs from interference by other intentional transmitters.

Magnetic fields associated with a nuclear magnetic resonance medical imaging system

Measurements were made of magnetic and electric field levels in and around a nuclear magnetic resonance imaging system undergoing a clinical trial. Magnetic field levels ranged from 0.04 tesla (T) in the imaging volume down to about 0.0006 T at the end of the patient table. The peak radio-frequency magnetic field level was 15 amperes per meter (A/m) in the imaging volume, while the rms value was 4.6 A/m. The specific absorption rate resulting from the radio-frequency magnetic field was calculated to be no more than 0.017 watts per kilogram (W/kg). The radio-frequency electric field was detectable only within a few centimeters of the coil assembly, and does not significantly contribute to the specific absorption rate. These exposure levels were much lower than existing guidelines for clinical NMR procedures.

Wireless Coexistence and EMC of Bluetooth and 802.11b Devices in Controlled Laboratory Settings.

This paper presents experimental testing that has been performed on wireless communication devices as victims of electromagnetic interference (EMI). Wireless victims included universal serial bus (USB) network adapters and personal digital assistants (PDAs) equipped with IEEE 802.11b and Bluetooth technologies. The experimental data in this paper was gathered in an anechoic chamber and a gigahertz transverse electromagnetic (GTEM) cell to ensure reliable and repeatable results. This testing includes: Electromagnetic compatibility (EMC) testing performed in accordance with IEC 60601-1-2, an in-band sweep of EMC testing, and coexistence testing. The tests in this study show that a Bluetooth communication was able to coexist with other Bluetooth devices with no decrease in throughput and no communication breakdowns. However, testing revealed a significant decrease in throughput and increase in communication breakdowns when an 802.11b source is near an 802.11b victim. In a hospital setting decreased throughput and communication breakdowns can cause wireless medical devices to fail. It is therefore vital to have an understanding of the effect EMI can have on wireless communication devices.

Changes in the susceptibility of a medical device resulting from connection to a full-size model of a human

The Radiofrequency (RF) susceptibility of an apnea monitor was evaluated while connected to a saline-filled, full-size model of an adult human. Experimental measurements were performed at an outdoor electromagnetic compatibility (EMC) test facility. The RF susceptibility of the apnea monitor increased by approximately 8 to 15 dB when its fully-extended patientmonitoring leads and electrodes were connected to the model, rather than being terminated with a resistor. This increase in electric field strength susceptibility of 2.5 to 5.6 times occurred in the 63 to 100 MHz range. This frequency range includes the resonant frequency of the model, and of actual adult humans. This finding has clinical significance due to the widespread presence of high power FM radio and VHF television transmitters in the U.S. and other countries.

Automated radiofrequency electromagnetic interference testing of apnea monitors using an open area test site

Medical devices that electronically sense physiological functions may be affected by external radiofrequency (RF) radiation from sources such as television and radio stations. When RF disrupts normal device function, it is referred to as electromagnetic interference (EMI). The Center for Devices and Radiological Health (CDRH) has been testing infant apnea monitors for susceptibility to EMI. Laboratory testing and user site surveys were conducted that confirmed susceptibility to EMI in infant apnea monitors. The field strength value above which device performance was compromised will be referred to as the devices threshold. An automated procedure has now been developed which enables us to obtain threshold EMI field-strength versus frequency plots and identify EMI regions by frequency band In addition, precise measurements at an outdoor open area test site have found that the ≃300-cm (≃10-ft) patient leads, when fully extended, act as significant receivers of RF signals below 50 MHz. Thresholds, averaged over the 10–54 MHz frequency range, were 0.22 V/m with minimums reaching 0.05 V/m for four of the nine monitors tested when 100% amplitude modulation at 0.5 Hz was imposed on the RF carrier. In the FM band (88–108 MHz) the thresholds averaged 0.88 V/m, with one model reaching a minimum of 0.08 V/m. These results were similar to our earlier laboratory data which were taken with the patient and power supply leads not fully extended in the RF exposure Held, indicating that testing with full lead length exposures may not be necessary at higher frequencies.

Tests for Radio Frequency Interference to Medical Telemetry Operating in the Private Land Mobile Radio Service (PLMRS) from Newly Allocated PLMRS Channels

Abstract Radio frequency interference (RFI) is a well-known risk for wireless medical telemetry, particularly for older telemetry systems still operating in the Private Land Mobile Radio Service (PLMRS) frequencies between 450 and 470 MHz. Testing was performed with medical telemetry systems at two local hospitals using the older 25 kHz wide, 460 to 470 MHz channel PLMRS telemetry to assess the RFI potential with transmissions using newly allocated 12.5 kHz and 6.25 kHz wide channels. Significant interference and loss of telemetry was observed on the telemetry monitors when the new channels both overlapped with and were adjacent to the medical telemetry channels. This work demonstrates the vulnerability of older medical telemetry using the radio frequencies between 460 and 470 MHz to new PLMRS transmitters that will operate in these frequencies after December 31, 2005. Telemetry users are urged to assess their equipment RFI vulnerabilities, particularly in the PLMRS frequency band, and migrate to frequenc...

An approach for rapidly rewarming cryogenically preserved organs

Wire-length resonant coils, which are based on equating a wavelength of RF with the wire length used in winding a coil, were previously developed for use in hyperthermia treatment for cancer therapy. Design modifications, coupled with a series of experiments, established their potential use for rapidly and uniformly rewarming canines following hypothermic cardiac surgery. The authors detail a new design which incorporates a single-end coaxial input and RF shielding, allowing it to be used outside of a screen room environment. Data from early experiments to investigate this systems usefulness in rewarming cryogenically preserved donor organs are presented and discussed.<<ETX>>