Development and validation of a magneto-hydrodynamic solver for blood flow analysis.

The objective of this study was to develop a numerical solver to calculate the magneto-hydrodynamic (MHD) signal produced by a moving conductive liquid, i.e. blood flow in the great vessels of the heart, in a static magnetic field. We believe that this MHD signal is able to non-invasively characterize cardiac blood flow in order to supplement the present non-invasive techniques for the assessment of heart failure conditions. The MHD signal can be recorded on the electrocardiogram (ECG) while the subject is exposed to a strong static magnetic field. The MHD signal can only be measured indirectly as a combination of the heart's electrical signal and the MHD signal. The MHD signal itself is caused by induced electrical currents in the blood due to the moving of the blood in the magnetic field. To characterize and eventually optimize MHD measurements, we developed a MHD solver based on a finite element code. This code was validated against literature, experimental and analytical data. The validation of the MHD solver shows good agreement with all three reference values. Future studies will include the calculation of the MHD signals for anatomical models. We will vary the orientation of the static magnetic field to determine an optimized location for the measurement of the MHD blood flow signal.

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.

MRI-induced heating of selected thin wire metallic implants-- laboratory and computational studies-- findings and new questions raised.

We performed experiments and computer modeling of heating of a cardiovascular stent and a straight, thin wire by RF fields in a 1.5 T MRI birdcage coil at 64 MHz. We used ASTM F2182-02a standard and normalized results to 4 W/kg whole body average. We used a rectangular saline-gel filled phantom and a coiled, double stent (Intracoil by ev3 Inc) 11 cm long. The stent had thin electrical insulation except for bare ends (simulating drug eluting coating). The stent and phantom were placed close to the wall of the RF Coil and had approximately 0.5 degrees C initial temperature rise at the ends (local SAR = 320 W/kg). We exposed a wire (24.1 cm, 0.5 mm diameter) with 0.5 mm insulation and saw an 8.6 degrees C temperature rise (local SAR = 5,680 W/kg) at the bare ends. All heating was within 1 mm3 of the ends, so the position of our fiber optic temperature probe was critical for repeatability. Our computational study used finite difference time domain software with a thermodynamics solver. We modeled a coiled bare-wire stent as a spiral with a rectangular cross section and found a maximum increase of 0.05 degrees C induced at the tips for plane wave exposures. A maximum local SAR of up to 200 W/kg occurred in a volume of only 8 x 10(-3) mm. We developed improved computational exposure sources-- optimized birdcage coils and quasi-MRI fields that may eliminate the need to model an RF coil. We learned that local (point) SAR (initial linear temperature rise) is the most reliable indicator of the maximum heating of an implant. Local SAR depends greatly on implant length, insulation and shape, and position in the MRI coil. Accurate heating must be measured with sensors or software having millimeter resolution. Many commercially available fiber optic temperature probes do meet this requirement.

MRI induced heating of pacemaker leads: effect of temperature probe positioning and pacemaker placement on lead tip heating and local SAR.

The radio frequency field used in magnetic resonance imaging (MRI) procedures leads to temperature and local absorption rate (SAR) increase for patients with implanted pacemakers (PM). In this work a methodological approach for temperature and SAR measurements using fluoroptic probes is presented. Experimental measures show how the position of temperature probes affects the temperature and SAR value measured at the lead tip. The transversal contact between the active portion of the probe and the lead tip is the configuration associated with the highest values for temperature and SAR, whereas other configurations may lead to an underestimation close to 11% and 70% for temperature and SAR, respectively. In addition measurements were performed on a human-shaped phantom inside a real MRI system, in order to investigate the effect of the PM placement and of the lead geometry on heating and local SAR.

IEEE Committee on Man and Radiation (COMAR) Technical Information Statement "exposure of medical personnel to electromagnetic fields from open magnetic resonance imaging systems".

Open magnetic resonance imaging (MRI) systems enable performing image-guided medical procedures for long periods of time very close to, or inside, the patient imaging area. Medical personnel can be exposed to relatively high static, gradient, and radiofrequency fields compared to most other MRI systems. The Committee on Man and Radiation of the Institute of Electrical and Electronics Engineers calculated or used existing data on magnetic flux densities and field strengths in or near the patient area to assess occupational exposure levels. Potential exposures to each field type were analyzed and compared to relevant values specified in international exposure limits including those of the Institute of Electrical and Electronics Engineers and the International Commission on Nonionizing Radiation Protection. Exposures of the head or torso of a worker to gradient fields near the center of the patient-imaging area can exceed most exposure limits even for times less than a second. Exposures to radiofrequency fields can exceed limits if sustained exposures (minutes or more) occur to parts of the body. Static magnetic fields used by present Open MRI systems are below exposure limits of all of the standards that address these fields. Overall results of this study suggest that manufacturers and others who program or operate Open MRI systems should take care to ensure that operating parameters produce exposures that comply with the relevant exposure limits. Also, since field levels fall off rapidly with increasing distance, user practices may be implemented that reduce exposures significantly.

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.

Radio frequency electromagnetic exposure: tutorial review on experimental dosimetry.

Radio frequency (RF) dosimetry is the quantification of the magnitude and distribution of absorbed electromagnetic energy within biological objects that are exposed to RF fields. At RF, the dosimetric quantity, which is called the specific absorption rate (SAR), is defined as the rate at which energy is absorbed per unit mass. The SAR is determined not only by the incident electromagnetic waves but also by the electrical and geometric characteristics of the irradiated subject and nearby objects. It is related to the internal electric field strength (E) as well as to the electric conductivity and the density of tissues; therefore, it is a suitable dosimetric parameter, even when a mechanism is determined to be "athermal." SAR distributions are usually determined from measurements in human models, in animal tissues, or from calculations. This tutorial describes experimental techniques that are used commonly to determine SAR distributions along with the SAR limitations and unresolved problems. The methods discussed to obtain point, planar, or whole-body averaged SARs include the use of small E-field probes or measurement of initial rate of temperature rise in an irradiated object.

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.

ELF in vitro exposure systems for inducing uniform electric and magnetic fields in cell culture media.

Many in vitro experiments on the biological effects of extremely low frequency (ELF) electromagnetic fields utilize a uniform external magnetic flux density (B) to expose biological materials. A significant number of researchers do not measure or estimate the resulting electric field strength (E) or current density (J) in the sample medium. The magnitude and spatial distribution of the induced E field are highly dependent on the sample geometry and its relative orientation with respect to the magnetic field. We have studied the E fields induced in several of the most frequently used laboratory culture dishes and flasks under various exposure conditions. Measurements and calculations of the E field distributions in the aqueous sample volume in the containers were performed, and a set of simple, quantitative tables was developed. These tables allow a biological researcher to determine, in a straightforward fashion, the magnitudes and distributions of the electric fields that are induced in the aqueous sample when it is subjected to a uniform, sinusoidal magnetic field of known strength and frequency. In addition, we present a novel exposure technique based on a standard organ culture dish containing two circular, concentric annular rings. Exposure of the organ culture dish to a uniform magnetic field induces different average electric fields in the liquid medium in the inner and outer rings. Results of experiments with this system, which were reported in a separate paper, have shown the dominant role of the magnetically induced E field in producing specific biological effects on cells, in vitro. These results emphasize the need to report data about the induced E field in ELF in-vitro studies, involving magnetic field exposures. Our data tables on E and J in standard containers provide simple means to enable determination of these parameters.

CDRH RF phantom for hyperthermia systems evaluations.

The National Cancer Institute (NCI) sponsored clinical evaluations of investigational 'regional' hyperthermia systems at four clinical institutions. To support this project, the Center for Devices and Radiological Health (CDRH) developed a series of test instruments to evaluate the magnitude and repeatability of the induced heating by radiofrequency (RF) systems. Data from three institutions using the same model hyperthermia system have been analyzed. After heating, the average temperature from measurements taken at several points in the test phantom at each institution agree within +/- 0.002 degrees C. These differences are about equal to the measurement uncertainty. Thus, this technique can be used for preclinical evaluation and quality control of the total system operation. After one of the institutions relocated its hyperthermia system, a subsequent set of data showed inconsistencies compared to their earlier data. Investigation traced this to cable loss and power meter interference. From the analysis of the data from the three institutions, the utility of the CDRH RF phantom for hyperthermia systems evaluation is demonstrated.

In vitro studies of microwave-induced cataract. II. Comparison of damage observed for continuous wave and pulsed microwaves.

Depth of damage caused by pulsed (PU) and continuous wave (CW) microwaves was estimated by scanning electron microscopy in rat lenses fixed immediately, after irradiation in vitro in circulating thermostatically controlled buffered saline. Pulses of 10 microseconds width and 24 kW peak power were delivered to the lens at different repetition rates in order to permit the same total energy to be delivered during 6, 20 or 60 min of irradiation at specific absorption rate (SAR) values of 0, 5.75, 11.5, 23, 69, 231 and 750 mW g-1; total energy [power (pow) x time] deposited in the lens was 0, 0.23, 0.46, 1.38, 4.6, and 15 W min g-1. Damage (granular degeneration of cells at the lens equator) was measured at the apex of penetration of the degeneration. The depth of degeneration (dep) of Pu or CW was compared either: (1) by a one-way analysis of variance (ANOVA) for the CW data alone and the 11 combinations of (pow x time); or (2) by using two alternative models to fit the data, to permit experimental distinguishment between: (a) reciprocal effects of pow x time; and (b) separate effects. Using the ANOVA analysis, the Pu mode of irradiation resulted in more damage at the same average power for every combination tested except one (23 mW g-1, 6 min). Although the separate-effects models explained more of the variation in depth of damage, the reciprocal effects model may provide an adequate fit for practical purposes and has the advantage of greater simplicity. For both models, the pulsed irradiation mode produced 4.7 times the depth of damage caused by CW irradiation. These results are discussed in relation to previous Pu-CW comparisons. It is proposed that this additional damage at the same average power is caused by thermoelastic expansion (TEE).

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.

An EM radiation safety controller.

A safety control system has been developed for use in high power RF/microwave radiation exposure facilities. The system features Fail-Safe RF detectors, visible "RF ON" indicators, door-status sensors and digital logic to maintain safe operating conditions in spite of human errors or unsafe equipment malfunctions.