High Specific Absorption Rate Research Articles

084 High field (7 Tesla) cardiovascular MRI: a feasibility and comparison study

<h3>Introduction</h3> High field strength cardiovascular MRI is attractive as a clinical research tool because of its higher signal to noise ratio and its potential for increased image resolution. Technical challenges include B0 and B1 field in-homogeneity, high field ECG artefacts and specific absorption rate (SAR) limits1. This study was designed to determine the practicality and reproducibility of anatomical and functional cardiac imaging at 7 T compared to 3 T and the gold-standard 1.5T. <h3>Methods</h3> Subjects (n=3) were scanned within 1 month in a MAGNETOM 7T, MAGNETOM Tim Trio (3T) and MAGNETOM Avanto (1.5 T) system all running VB15A and with equivalent gradient systems (Siemens Healthcare, Erlangen, Germany). At 7 T, a 16 channel transceive stripline array, independently powered by 16 RF amplifiers (16×1 kW), was tuned and matched to each subject. Electrocardiographic gating used the standard 3-lead Bluetooth based Vector Cardiogram system. At 1.5 T and 3 T standard clinical 12 channel receive arrays were used with the volume transmit coil. At 7 T, complex transmit B1 field distributions were optimised with B1 shimming for a tradeoff between transmit efficiency and homogeneity based on a rapid single breath-hold calibration scan. B0 Shimming was performed with the shim volume localized to the heart. To further optimise image quality, especially at 3 and 7 T a ‘frequency scout’ scan was performed to enable use of a B0 offset to avoid off-resonance artefacts. On each scanner a short axis stack of images was acquired using both FLASH and SSFP. Scans were performed by the same experienced operator and independently analysed using Argus post-processing software to measure cardiac volumes, mass and ejection fraction. <h3>Results</h3> Reliable ECG triggering was obtained in all subjects, however careful skin preparation and more adjustment with respect to lead placement was required at 7 T than at lower field strengths due to a more pronounced magnetohydrodynamic effect. Images acquired using SSFP at 7 T had consistent B0 related banding artefacts over the anterior and inferolateral walls despite local B0 shimming and the selection of optimal offset frequencies to shift the artefacts outside of the region of interest. FLASH images were quantitatively superior at higher field strengths (SNR: 1.5 T:99, 3 T:127, 7 T:355; CNR:1.5 T:52, 3 T:58, 7 T:205). Measurement of cardiac volumes, ejection and mass showed minimal variation between the 1.5 T, 3 T and 7 T field strengths for both FLASH and SSFP sequences (all ranges less than the clinical standard of 10%) (abstract 084 figure 1). <h3>Conclusions</h3> Using optimised sequences to minimise high field artefacts, assessment of cardiac volumes and function is practical and reproducible at 7 T. Measurement of cardiac volumes, ejection fraction and mass at 7 T with FLASH or SSFP are consistent with measurements made at 3 T and the clinical gold standard of 1.5 T.

Effects of Mn concentration on the ac magnetically induced heating characteristics of superparamagnetic MnxZn1−xFe2O4 nanoparticles for hyperthermia

The effects of Mn2+ cation concentration on the ac magnetically induced heating characteristics and the magnetic properties of superparamagnetic MnxZn1−xFe2O4 nanoparticles (SPNPs) were investigated to explore the biotechnical feasibility as a hyperthermia agent. Among the MnxZn1−xFe2O4 SPNPs, the Mn0.5Zn0.5Fe2O4 SPNP showed the highest ac magnetically induced heating temperature (ΔTac,mag), the highest specific absorption rate (SAR), and the highest biocompatibility. The higher out of phase susceptibility (χm″) value and the higher chemical stability systematically controlled by the replacement of Zn2+ cations by the Mn2+ cations on the A-site (tetrahedral site) are the primary physical reason for the promising biotechnical properties of Mn0.5Zn0.5Fe2O4 SPNP.

Research Highlights

With the increasing availability of 3 T MRI systems in the clinics, radiologists are getting acquainted with the different image sequences and its superiority over the images obtained by 1.5 T MRI. In comparison with 1.5 T MRI, 3 T MRI has prolonged T1 values, a higher specific absorption rate, increased susceptibility artifacts and magnetic field inhomogeneity. Therefore, it is necessary to optimize organ-specific image acquisition parameters and sequences. It is common practice to acquire postcontrast thin-slice 2D spin echo (SE) T1-weighted images (T1WI) for pituitary lesions. As mentioned by Kakite et al., owing to prolongation of T1 values for 3 T systems, postcontrast 2D-SE T1WI may not enable the correct demarcation or delineation of pituitary microadenoma from surrounding structures. Moreover, 3D gradient echo T1 sequences will have susceptible artifacts and high signal intensity from flowing blood, which may also make image interpretation difficult. The present study compared the image qualities in delineating the borders between pituitary gland and cavernous sinus, cavernous sinus and lesions, lesions and pituitary gland, and other surrounding anatomical structures. The study compared the image qualities in 33 consecutive patients with suspected sellar (pituitary fossa) lesions from May 2008 to March 2009. All patients underwent MRI by a clinical 3 T system (Signa Excite HD; GE healthcare) and postcontrast 3D fast-spoiled gradient echo (FSPGR) recalled acquisition in the steady state and 2D-SE T1WI were obtained. The following parameters were used to acquire the images: coronal 2D-SE (repetition time/echo time [TE]: 550–620/20–21 ms; flip angle [FA]: 90°; matrix: 512 × 256; one excitation; field of view: 21 cm; bandwidth: 122 Hz/pixel; slice thickness: 3 mm with 0.3 mm intersection gap); coronal 3D FSPGR T1WI (repetition time/TE: 9/2 ms; FA: 13°; matrix 512 × 256; one excitation; field of view: 21 cm; bandwidth: 122 Hz/pixel; slice thickness: 1.4 mm without intersection gap). The images of 3D FSPGR were reformatted to match SE T1WI (3 mm thickness and 0.3 mm gap). Approximately half of the patients had 3D FSPGR images obtained first and rest of the patients had 2D-SE T1WI obtained first, following intravenous administration of contrast agents. Two experienced neuroradiologists reviewed and evaluated the images independently for eight different points of interest: The border between the pituitary gland and the cavernous sinus The border between the cavernous sinus and the lesion The border between the pituitary gland and the lesion Visualization of the cranial nerves in the cavernous sinus Visualization of the optic nerve Susceptibility artifact Pulsation artefact Total image quality Except for the first point, 3D FSPGR images demonstrate a significantly higher quality and could demarcate different structures with confidence. The improved interpretations of 3D FSPGR images are due to a short TE, which reduces the pulsation artifacts, and thin-slice imaging, which reduces the partial volume effect; furthermore, FSPGR is less dependent on magnetic field inhomogeneity. However, FSPGR will have disadvantages, such as hyperintensity of the blood vessels, caused by short TE, and magnetic susceptibility effects. The study has the following limitations: optimal FA was not determined for sellar lesions, the authors used an FA that produces optimal contrast-to-noise ratio between gray and white matters; 2D-SE T1WI was acquired as a multislice sequence, which may have degraded the image quality; and 3D FSPGR acquisition was not isotropic. Reformation and multiplanar reconstruction from isotropic 3D images would provide better images, although acquisition time will increase.

ACOUSTIC PRESSURE WAVES INDUCED IN HUMAN HEADS BY RF PULSES FROM HIGH-FIELD MRI SCANNERS

The current evolution toward greater image resolution from magnetic resonance image (MRI) scanners has prompted the exploration of higher strength magnetic fields and use of higher levels of radio frequencies (RFs). Auditory perception of RF pulses by humans has been reported during MRI with head coils. It has shown that the mechanism of interaction for the auditory effect is caused by an RF pulse-induced thermoelastic pressure wave inside the head. We report a computational study of the intensity and frequency of thermoelastic pressure waves generated by RF pulses in the human head inside high-field MRI and clinical scanners. The U.S. Food and Drug Administration (U.S. FDA) guides limit the local specific absorption rate (SAR) in the body-including the head-to 8 W kg(-1). We present results as functions of SAR and show that for a given SAR the peak acoustic pressures generated in the anatomic head model were essentially the same at 64, 300, and 400 MHz (1.5, 7.0, and 9.4 T). Pressures generated in the anatomic head are comparable to the threshold pressure of 20 mPa for sound perception by humans at the cochlea for 4 W kg(-1). Moreover, results indicate that the peak acoustic pressure in the brain is only 2 to 3 times the auditory threshold at the U.S. FDA guideline of 8 W kg(-1). Even at a high SAR of 20 W kg(-1), where the acoustic pressure in the brain could be more than 7 times the auditory threshold, the sound pressure levels would not be more than 17 db above threshold of perception at the cochlea.

High Field Cardiac Magnetic Resonance Imaging – Current and Future Perspectives

Potential advantages of high field cardiac magnetic resonance imaging include superior signal-to-noise ratio, image contrast and spectral separation. These might be used to improve spatial and/or temporal resolution and imaging speed. However, high field imaging presents new challenges to the operator, including increased magnetic field heterogeneity, susceptibility, artefacts and higher specific absorption rate. Recently, studies have reported on the clinical utility of cardiac magnetic resonance imaging at 3 Tesla in the assessment of myocardial function, perfusion and viability, and coronary arteries. The results of these clinical studies are reviewed, as well as emerging applications in myocardial tagging and cardiac spectroscopy.

Absence of Short-Term Effects of UMTS Exposure on the Human Auditory System

The aim of this study, which was performed in the framework of the European project EMFnEAR, was to investigate the potential effects of Universal Mobile Telecommunications System (UMTS, also known as 3G) exposure at a high specific absorption rate (SAR) on the human auditory system. Participants were healthy young adults with no hearing or ear disorders. Auditory function was assessed immediately before and after exposure to radiofrequency (RF) radiation, and only the exposed ear was tested. Tests for the assessment of auditory function were hearing threshold level (HTL), distortion product otoacoustic emissions (DPOAE), contralateral suppression of transiently evoked otoacoustic emission (CAS effect on TEOAE), and auditory evoked potentials (AEP). The exposure consisted of speech at a typical conversational level delivered via an earphone to one ear, plus genuine or sham RF-radiation exposure obtained by an exposure system based on a patch antenna and controlled by software. Results from 73 participants did not show any consistent pattern of effects on the auditory system after a 20-min UMTS exposure at 1947 MHz at a maximum SAR over 1 g of 1.75 W/kg at a position equivalent to the cochlea. Analysis entailed a double-blind comparison of genuine and sham exposure. It is concluded that short-term UMTS exposure at this relatively high SAR does not cause measurable immediate effects on the human auditory system.

Radiofrequency Dosimetry in Subjects Implanted with Metallic Structures Undergoing MRI: a Numerical Study

A numerical study to investigate the effects of the exposure to radiofrequency (RF) field on biological tissues implanted with thin metallic structures has been carried out, using the finite difference time domain (FDTD) solution technique. The particular case of the RF used in the clinical 1.5 T magnetic resonance imaging (MRI) scanners has been studied. The results of the model show that the presence of a metallic wire yields to a significant increase in the local specific absorption rate (SAR) and in temperature around the implant. Present standards or guidelines do not specifically address this problem and use threshold levels and methods to define safe exposure conditions that cannot apply to reveal high SAR gradients, such as the ones generated by thin metallic implanted objects. In particular, an averaging mass of 10 g, typically used for SAR calculation to evaluate the effect of RF exposure of biological tissues, is too big if compared to the SAR gradient generated by thin metallic structures. The SAR estimation according to this method cannot be considered a reliable indicator to evaluate the potential damage due to the RF induced heating at the interface between the implant and biological tissues. Five wires of different lengths and thicknesses were simulated and the SAR at the wire tip was evaluated over 10g, 1g, 0.1g averaging mass: the SAR underestimation related to the 10g and 1g masses, compared to 0.1g mass, was greater than 90 % and 60 %, respectively. In conclusion, present standards cannot be applied to thin metallic implants. The amount of averaging mass should be chosen as a function of the dimension of the implanted object and of the SAR gradient that is generated around it. At the same time, the averaging volume has to be not too small to be just a single “hot spot”, not relevant for the evaluation of potential tissue damage due to RF exposure.

Imaging of the Musculoskeletal System In Vivo Using Ultra-high Field Magnetic Resonance at 7 T

Recently, great progress has been made in particularly in the imaging of cartilage and bone structure. Increased interest has focused on high-field (3 Tesla) imaging and more recently on ultra-high field (UHF) magnetic resonance imaging (MRI) at 7 T for in vivo imaging. Because the signal-to-noise ratio (SNR) scales linearly with field strength, a substantial increase in SNR is expected compared with lower field strengths. This gain in SNR can be used to increase spatial resolution or reduce imaging time. The goal of this review was to highlight recent developments and challenges in in vivo musculoskeletal (MSK) imaging using UHF-MRI at 7 T. One focus of this review is on the emerging methodology of quantitative MRI for the assessment of trabecular bone structure at the tibia, wrist, and knee. In particular for this application, susceptibility effects between the bone and bone marrow transitions that scale with field strength have to be considered. Another important MSK application is the characterization of knee cartilage morphology. The higher SNR provided by UHF-MRI is a potential advantage for visualizing, segmenting, and analyzing cartilage. Standard clinical MSK imaging relies heavily on T1, T2, and proton density weighted fast spin echo sequences. However, fast spin echo imaging has proven to be very challenging at higher fields because of very high specific absorption rates, using multiple pulses in a short time frame; thus the imaging protocols have to be adapted and gradient echo sequences may be more beneficial. Imaging of more central body parts such as the spine at 7 T is still in its infancy and dedicated coils have to be developed.

Studies of magnetite nanoparticles synthesized by thermal decomposition of iron (III) acetylacetonate in tri(ethylene glycol)

In this paper, water-soluble magnetite nanoparticles have been directly synthesized by thermal decomposition of iron (III) acetylacetonate, Fe(acac) 3 in tri(ethyleneglycol). Size and morphology of the nanoparticles are determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements while the crystal structure is identified using X-ray diffraction (XRD). Surface charge and surface coating of the nanoparticles are recognized using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectra (XPS) and zeta potential measurements. Magnetic properties are determined using vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID) measurements. The results show that as-prepared magnetite nanoparticles are relatively monodisperse, single crystalline and superparamagnetic in nature with the blocking temperature at around 100 K. The magnetite nanoparticles are found to be highly soluble in water due to steric and electrostatic interactions between the particles arising by the surface adsorbed tri(ethyleneglycol) molecules and associated positive charges, respectively. Cytotoxicity studies on human cervical (SiHa), mouse melanoma (B16F10) and mouse primary fibroblast cells demonstrate that up to a dose of 80 μg/ml, the magnetic nanoparticles are nontoxic to the cells. Specific absorption rate (SAR) value has been calculated to be 885 and 539 W/gm for samples with the iron concentration of 1 and 0.5 mg/ml, respectively. The high SAR value upon exposure to 20 MHz radiofrequency signifies the applicability of as-prepared magnetite nanoparticles for a feasible magnetic hyperthermia treatment.

A Miniature Spiral Diversity Antenna System With High Overall Gain Coverage and Low SAR

A miniature spiral diversity antenna system operating at 1.85 GHz is proposed. Two 10 times 6.25 mm antennas were placed at two corners of a printed circuit board at a height of 7 mm from the ground plane. Each antenna achieves nearly 7% bandwidth within 6 dB return loss. The mutual coupling between the two antennas is less than 10 dB. The proposed diversity antenna scheme exhibits high overall gain performance in all three principal planes compared to a single antenna solution. Computed specific absorption rate (SAR) data against a specific anthropomorphic mannequin (SAM) phantom and a hand model show that the diversity antenna system induces low SAR compared to a single antenna.

SAR Computation inside Fetus by RF Coil during MR Imaging Employing Realistic Numerical Pregnant Woman Model

This paper presents the computational electromagnetic dosimetry inside an anatomically based pregnant woman models exposed to electromagnetic wave during magnetic resonance imaging. The two types of pregnant woman models corresponding to early gestation and 26 weeks gestation were used for this study. The specific absorption rate (SAR) in and around a fetus were calculated by radiated electromagnetic wave from highpass and lowpass birdcage coil. Numerical calculation results showed that high SAR region is observed at the body in the vicinity of gaps of the coil, and is related to concentrated electric field in the gaps of human body such as armpit and thigh. Moreover, it has confirmed that the SAR in the fetus is less than International Electrotechnical Commission limit of 10W/kg, when whole-body average SARs are 2W/kg and 4W/kg, which are the normal operating mode and first level controlled operating mode, respectively.

The influence of magnetic and physiological behaviour on the effectiveness of iron oxide nanoparticles for hyperthermia

Magnetic nanoparticles are being developed for a wide range of biomedical applications. In particular, hyperthermia involves heating the magnetic nanoparticles through exposure to an alternating magnetic field. These materials offer the potential to selectively treat cancer by heating cancer tissue locally and at the cellular level. This may be a successful method if there are enough particles in a tumor possessing a sufficiently high specific absorption rate (SAR) to deposit heat quickly while minimizing thermal damage to surrounding tissue. High SAR magnetic nanoparticles have been developed and used in mouse models of cancer. The magnetic nanoparticles comprise iron oxide magnetic cores (mean core diameter of 50 nm) surrounded by a dextran layer shell for colloidal stability. In comparing two similar systems, the saturation magnetization is found to play a crucial role in determining the SAR, but is not the only factor of importance. (A difference in saturation magnetization of a factor of 1.5 yields a difference in SAR of a factor of 2.5 at 1080 Oe and 150 kHz.) Variations in the interactions due to differences in the dextran layer, as determined through neutron scattering, also play a role in the SAR. Once these nanoparticles are introduced into the tumor, their efficacy, with respect to tumor growth, is determined by the location of the nanoparticles within or near the tumor cells and the association of the nanoparticles with the delivered alternating magnetic field (AMF). This association (nanoparticle SAR and AMF) determines the amount of heat generated. In our setting, the heat generated and the time of heating (thermal dose) provides a tumor gross treatment response which correlates closely with that of conventional (non-nanoparticle) hyperthermia. This being said, it appears specific aspects of the nanoparticle hyperthermia cytopathology mechanism may be very different from that observed in conventional cancer treatment hyperthermia.

The influence of collective behavior on the magnetic and heating properties of iron oxide nanoparticles

Magnetic nanoparticles with a high specific absorption rate (SAR) have been developed and used in mouse models of cancer. The magnetic nanoparticles are comprised of predominantly iron oxide magnetic cores surrounded by a dextran layer for colloidal stability. The average diameter of a single particle (core plus dextran) is 92±14nm as measured by photon correlation spectroscopy. Small angle neutron scattering measurements under several H2O∕D2O contrast conditions and at varying nanoparticle concentrations have revealed three length scales: &amp;gt;10μm, several hundred nanometers, and tens of nanometers. The latter corresponds to the particle diameter; the several hundred nanometers corresponds to a hard sphere interaction radius of the core/shell nanoparticles; &amp;gt;10μm corresponds to the formation of long-range, many-particle structures held together by magnetic interactions and dextran. The long-range collective magnetic behavior appears to play a major role in enhancing the SAR. For samples having nominally equal concentrations and similar saturation magnetizations, the measured SAR is 1075W/(g of Fe) for tightly associated nanoparticles and 150W/(g of Fe) for very loosely associated nanoparticles at an applied field of 86kA∕m (1080Oe) and 150kHz.

Increased Levels of Numerical Chromosome Aberrations afterIn VitroExposure of Human Peripheral Blood Lymphocytes to Radiofrequency Electromagnetic Fields for 72 Hours

Mazor, R., Korenstein-Ilan, A., Barbul, A., Eshet, Y., Shahadi, A., Jerby, E. and Korenstein, R. Increased Levels of Numerical Chromosome Aberrations after In Vitro Exposure of Human Peripheral Blood Lymphocytes to Radiofrequency Electromagnetic Fields for 72 Hours. Radiat. Res. 169, 28-37 (2008). We investigated the effects of 72 h in vitro exposure of 10 human lymphocyte samples to radiofrequency electromagnetic fields (800 MHz, continuous wave) on genomic instability. The lymphyocytes were exposed in a specially designed waveguide resonator at specific absorption rates (SARs) of 2.9 and 4.1 W/kg in a temperature range of 36-37 degrees C. The induced aneuploidy of chromosomes 1, 10, 11 and 17 was determined by interphase FISH using semi-automated image analysis. We observed increased levels of aneuploidy depending on the chromosome studied as well as on the level of exposure. In chromosomes 1 and 10, there was increased aneuploidy at the higher SAR, while for chromosomes 11 and 17, the increases were observed only for the lower SAR. Multisomy (chromosomal gains) appeared to be the primary contributor to the increased aneuploidy. The effect of temperature on the level of aneuploidy was examined over the range of 33.5-40 degrees C for 72 h with no statistically significant difference in the level of aneuploidy compared to 37 degrees C. These findings suggest the possible existence of an athermal effect of RF radiation that causes increased levels of aneuploidy. These results contribute to the assessment of potential health risks after continuous chronic exposure to RF radiation at SARs close to the current levels set by ICNIRP guidelines.

2-D FDTD calculation of Specific Absorption Rate of hyperopic and myopic human eye: LabVIEW-based approach

This paper describes a Two-Dimensional (2-D) Finite Difference Time Domain (FDTD) calculation of Specific Absorption Rate (SAR) in human eye with visual disorder under LabVIEW environment. Common wireless communication frequencies of 900 MHz, 1800 MHz and 2450 MHz were used in this simulation. In myopic eye, the simulation results indicate that a somewhat higher SAR values are induced in the lens at 900 MHz and 1800 MHz compared to 2450 MHz. On the other hand, in hyperopic eye, the simulation results indicate higher SAR values are induced in the lens at 1800 MHz and 2450 MHz compared to 900 MHz.

Ornithine decarboxylase activity of L929 cells after exposure to continuous wave or 50 Hz modulated radiofrequency radiation—a replication study

A replication study with some extensions was made to confirm enhancement of ornithine decarboxylase (ODC) activity in murine L929 fibroblasts after radiofrequency (RF) field exposure reported in earlier studies. L929 cells purchased from two cell banks were exposed for 2, 8, or 24 h to continuous wave or DAMPS (burst modulated at 50 Hz, with 33% duty cycle) signals at specific absorption rate (SAR) levels of 2.5 or 6.0 W/kg. Exposures were carried out in Crawford and waveguide chambers, at frequencies 835 and 872 MHz, respectively. The results did not confirm findings of previous studies reporting increased ODC activity in RF-exposed cells. When Crawford cell exposure system was used, ODC activity was either not affected (in the case of 8 or 24 h exposures) or decreased after 2 h exposure at the highest SAR level (6 W/kg). The decrease was most pronounced when cooling with air flow was not used, and is most likely related to increased temperature. The minor methodological differences (use of antibiotics, increased sensitivity of ODC assay) are not likely to explain the inconsistency of the findings of the present and previous studies. Different results were obtained in experiments with the waveguide system that involves more efficient temperature control. In this exposure system, ODC activity was increased after 8 h exposure at 6 W/kg. Further studies are warranted to explore whether this finding reflects a true non-thermal effect. The present study did not provide evidence for modulation-specific effects reported in earlier studies.

Synthesis and antibody conjugation of magnetic nanoparticles with improved specific power absorption rates for alternating magnetic field cancer therapy

Bionized nanoferrite (BNF) particles with high specific power absorption rates were synthesized in the size range of 20–100 nm by high-pressure homogenization for targeted cancer therapy with alternating magnetic fields. Several strategies were used to conjugate antibodies to the BNF particles. These strategies were compared using an immunoassay to find optimal conditions to reach a high immunoreactivity of the final antibody–particle conjugate.

Magnetite ferrofluid with high specific absorption rate for application in hyperthermia

A magnetite ferrofluid coated by dextran with a high specific absorption rate (SAR) of 75 W/g in an AC field of 55 kHz and 200 Oe was prepared by the gel crystallization method with ultrasonic treatment. For comparison, uncoated magnetite particles with a mean diameter of 50 nm were also fabricated. Several possible mechanisms such as Brownian, Neel and diffusion relaxation processes on heating effects and their influence on SAR are discussed. Several factors which can increase the value of SAR were discussed, including dextran coating, ultrasonic treatment, proper particle size and the presence of defects and disorder in the particles.

Effects of continuous and intermittent exposure to RF fields with a wide range of SARs on cell growth, survival, and cell cycle distribution

To examine the biological effects of radio frequency (RF) electromagnetic fields in vitro, we have examined the fundamental cellular responses, such as cell growth, survival, and cell cycle distribution, following exposure to a wide range of specific absorption rates (SAR). Furthermore, we compared the effects of continuous and intermittent exposure at high SARs. An RF electromagnetic field exposure unit operating at a frequency of 2.45 GHz was used to expose cells to SARs from 0.05 to 1500 W/kg. When cells were exposed to a continuous RF field at SARs from 0.05 to 100 W/kg for 2 h, cellular growth rate, survival, and cell cycle distribution were not affected. At 200 W/kg, the cell growth rate was suppressed and cell survival decreased. When the cells were exposed to an intermittent RF field at 300 W/kg(pk), 900 W/kg(pk) and 1500 W/kg(pk) (100 W/kg(mean)), no significant differences were observed between these conditions and intermittent wave exposure at 100 W/kg. When cells were exposed to a SAR of 50 W/kg for 2 h, the temperature of the medium around cells rose to 39.1 degrees C, 100 W/kg exposure increased the temperature to 41.0 degrees C, and 200 W/kg exposure increased the temperature to 44.1 degrees C. Exposure to RF radiation results in heating of the medium, and the thermal effect depends on the mean SAR. Hence, these results suggest that the proliferation disorder is caused by the thermal effect.

Expression of the Proto-oncogeneFosafter Exposure to Radiofrequency Radiation Relevant to Wireless Communications

In this study the expression levels of the proto-oncogene Fos were measured after exposure to radiofrequency (RF) radiation at two relatively high specific absorption rates (SARs) of 5 and 10 W/kg for three types of modulated signals: 847.74 MHz code division multiple access (CDMA), 835.62 MHz frequency division multiple access (FDMA), and 836.55 MHz time division multiple access (TDMA). This work was undertaken to confirm a previous report by Goswami et al. (Radiat. Res. 151, 300-309, 1999) that CDMA and FDMA radiation caused small but statistically significant increases in Fos levels as cells entered plateau phase during exposure. No effects on Myc or Jun levels were observed in that study. Therefore, in the present study, analyses were restricted to Fos expression during the transition from exponential growth to plateau phase. Fos expression was measured using the real-time polymerase chain reaction (RT-PCR) technique. Serum-stimulated C3H 10T(1/2) cells were used as a positive control for Fos expression. Possible influences of final cell number or pH variability on Fos expression were evaluated. Expression of Fos mRNA in C3H 10T(1/2) cells was not significantly different from that found after sham exposure at either SAR level for any signal modulation. Therefore, the results of Goswami et al. could not be confirmed.