Specific Absorption Rate Levels Research Articles

RF-related heating assessment of extracranial neurosurgical implants at 7 T

PurposeThe purpose was to evaluate radiofrequency (RF)-related heating of commonly used extracranial neurosurgical implants in 7-T magnetic resonance imaging (MRI). Materials and methodsExperiments were performed using a 7-T MR system equipped with a transmit/receive RF head coil. Four commonly used titanium neurosurgical implants were studied using a test procedure adapted from the American Society for Testing and Materials Standard F2182-11a. Implants (n=4) were tested with an MRI turbo spin echo pulse sequence designed to achieve maximum RF exposure [specific absorption rate (SAR) level=9.9W/kg], which was further validated by performing calorimetry. Maximum temperature increases near each implant's surface were measured using fiberoptic temperature probes in a gelled-saline-filled phantom that mimicked the conductive properties of soft tissue. Measurement results were compared to literature data for patient safety. ResultsThe highest achievable phantom averaged SAR was determined by calorimetry to be 2.0±0.1W/kg due to the highly conservative SAR estimation model used by this 7-T MR system. The maximum temperature increase at this SAR level was below 1.0°C for all extracranial neurosurgical implants that underwent testing. ConclusionThe findings indicated that RF-related heating under the conditions used in this investigation is not a significant safety concern for patients with the particular extracranial neurosurgical implants evaluated in this study.

4:02 PM Abstract No. 172 - Tungsten and nitinol braided endovascular catheters: an evaluation of heating in 1.5 t

Purpose Interventional MRI offers the ability to provide soft tissue contrast and functional information in the absence of ionizing radiation. However, performing endovascular procedures in MRI raises safety concerns due to heating from the metallic braid incorporated into catheters. Heating in tungsten and nitinol braided catheters was assessed in an in vitro model simulating a patient undergoing a scan at specific absorption rate (SAR) level of 4 W/kg. Materials and Methods Heating adjacent to the catheters was assessed with fluoroptic temperature probes placed at the device tip, 5cm, 10cm, and 20cm proximal to the tip in a polyacrylamide gel in the head portion of a phantom. A 2.8F tungsten braided catheter (Terumo Progreat, Somerset, NJ) and a 1.9F nitinol braided catheter (eV3 Echelon, Irvine, CA) were serially embedded in a laterally offset conformation. A 5F stainless steel braided catheter (Cook Beacon Tip Torcon Advantage, Bloomington, IN) was used as a positive control and a temperature sensor was placed in background gel, well away from devices. Endovascular devices were immersed at 98cm and exposed to 2-min of scanning followed by 8-min without scanning. The immersed length was shortened by 5cm and this cycle was repeated until the immersed length was 63cm. MR scanning was based on modified fast spin echo imaging on a 1.5T scanner. Results The tungsten braided catheter did not show heating. The maximal heating for the nitinol braided catheter occurred at 20cm proximal to the tip at an immersed length of 93cm. The maximal initial rate of heating was 4.00°C/min and the maximal temperature rise during a 2-min scan was 2.32°C. Heating varied along the catheter, with the most heating at 10cm and 20cm proximal to the tip. The maximal temperature rise during a 2-min scan was 0.33°C for the background and 2.65°C for the positive control. Conclusion The study provides information on the magnitude of focal heating induced in a head simulating a “worst-case” scenario of a patient undergoing a neurovascular procedure. Nitinol displayed substantial heating when compared to the tungsten device. This tungsten braided catheter showed potential safe use in interventional MRI without substantial SAR restrictions.

Whole‐body and local RF absorption in human models as a function of anatomy and position within 1.5T MR body coil

Radiofrequency energy deposition in magnetic resonance imaging must be limited to prevent excessive heating of the patient. Correlations of radiofrequency absorption with large-scale anatomical features (e.g., height) are investigated in this article. The specific absorption rate (SAR), as the pivotal parameter for quantifying absorbed radiofrequency, increases with the radial dimension of the patient and therefore with the large-scale anatomical properties. The absorbed energy in six human models has been modeled in different Z-positions (head to knees) within a 1.5T bodycoil. For a fixed B1+ incident field, the whole-body SAR can be up to 2.5 times higher (local SAR up to seven times) in obese adult models compared to children. If the exposure is normalized to 4 W/kg whole-body SAR, the local SAR can well-exceed the limits for local transmit coils and shows intersubject variations of up to a factor of three. The correlations between anatomy and induced local SAR are weak for normalized exposure, but strong for a fixed B1+ field, suggesting that anatomical properties could be used for fast SAR predictions. This study demonstrates that a representative virtual human population is indispensable for the investigation of local SAR levels.

English

  The genotoxic effects of 2.45 GHz microwave (MW) radiation on the testis and ovary of Sprague Dawley rats was investigated. The animals were exposed to varying levels of specific absorption rate (SAR) of 0 (control), 0.48, 0.95, 1.43, 1.91, 2.39, 2.90, 3.40, 3.80 and 4.30 Wkg-1, for 10 min. The induction of DNA damages was assessed using DNA direct amplification of length polymorphisms (DALP) and validated with single cell gel electrophoresis (SCGE) comet assay for same cells at SAR 2.39 Wkg-1. Potential damage at the organ level was assessed by histopathological study. The results show significant differences in the Olive moment and % DNA in the blood of the exposed animals when compared with the control (p < 0.05). Hyperchromasia was observed in the ovary of the animals exposed to MW radiation. Also, there was reduction in the number of germ cells and cell disorganization in the testis of exposed group with increasing SARs. These results suggest that MW radiation has the potential to affect both male and female fertility adversely.   Key words: 2.45 GHz microwave radiation, histopathology, DNA single strand break, ovary, testis.

Effect of microwave radiation on human EEG at two different levels of exposure

This study is aimed at evaluating the effect of microwave radiation on human brain bioelectric activity at different levels of exposure. For this purpose, 450 MHz microwave exposure modulated at 40 Hz frequency was applied to a group of 15 healthy volunteers at two different specific absorption rate (SAR) levels: a higher level of 0.303 W/kg (field strength 24.5 V/m) and a lower level of 0.003 W/kg (field strength 2.45 V/m). Ten exposure cycles (1 min off and 1 min on) at fixed SAR values were applied. A resting eyes-closed electroencephalogram (EEG) was continuously recorded. Results showed a statistically significant increase in the EEG power in the EEG beta2 (157%), beta1 (61%) and alpha (68%) frequency bands at the higher SAR level, and in the beta2 (39%) frequency band at the lower SAR level. Statistically significant changes were detected for six individual subjects in the EEG alpha band and four subjects in the beta1 and beta2 bands at the higher SAR level; three subjects were affected in the alpha, beta1 and beta2 bands at the lower SAR level. The study showed that decreasing the SAR 100 times reduced the related changes in the EEG three to six times and the number of affected subjects, but did not exclude the effect.

Multiparametric Cardiovascular Magnetic Resonance Assessment of Pacemaker-Induced Alterations in Ventricular Activation and Function

A 73-year-old woman with presyncope and frequent episodes of symptomatic 2:1 atrioventricular block underwent implantation of a dual-chamber MRI-conditional pacing system (Advisa MRI SureScan pacing system, Medtronic, Minneapolis, MN) to facilitate cardiovascular magnetic resonance (CMR) assessment of apparent isolated right ventricular (RV) hypertrophy seen on transthoracic echocardiography (Figure 1 and online-only Data Supplement Movies I and II). The ventricular lead was positioned in the outflow tract. This pacing system is approved, outside of the United States, for MRI without isocenter restriction, provided certain conditions are fulfilled, including a static magnetic field strength of 1.5 T, a maximum gradient slew rate ≤200 T/m per second, whole body average specific absorption rate levels <2.0 W/kg, and a minimum period of 6 weeks between implantation and scanning. Within the United States, the isocenter is presently required to be either superior to C1 or inferior to T12 vertebrae. Figure 1. Parasternal long-axis ( A ) and short-axis ( B ) echocardiogram images showing apparent right ventricular wall thickening. CMR assessment was performed in accordance with the stated conditions. The impact of pacing on the activation pattern and regional deformation of both ventricles, and aortic and pulmonic forward flow, were studied by imaging in unpaced and paced (VVO; 75 bpm) modes. On balanced steady-state free precession imaging marked susceptibility artifact led to almost nondiagnostic image quality (Figure 2A and online-only Data Supplement Movie III). Switching to a spoiled gradient echo sequence greatly improved image quality (Figure 2B and online-only Data Supplement Movie IV) and reduced the specific absorption rate. Figure 2. Marked susceptibility artifact from the right ventricular pacing lead is seen in the 3-chamber view acquired using balanced …

Electromagnetic fields at 2.45 GHz trigger changes in heat shock proteins 90 and 70 without altering apoptotic activity in rat thyroid gland

SummaryNon-ionizing radiation at 2.45 GHz may modify the expression of genes that codify heat shock proteins (HSP) in the thyroid gland. Using the enzyme-linked immunosorbent assay (ELISA) technique, we studied levels of HSP-90 and HSP-70. We also used hematoxilin eosin to look for evidence of lesions in the gland and applied the DAPI technique of fluorescence to search for evidence of chromatin condensation and nuclear fragmentation in the thyroid cells of adult female Sprague-Dawley rats. Fifty-four rats were individually exposed for 30 min to 2.45 GHz radiation in a Gigahertz transverse electromagnetic (GTEM) cell at different levels of non-thermal specific absorption rate (SAR), which was calculated using the finite difference time domain (FDTD) technique. Ninety minutes after radiation, HSP-90 and HSP-70 had decreased significantly (P<0.01) after applying a SAR of 0.046±1.10 W/Kg or 0.104±5.10−3 W/Kg. Twenty-four hours after radiation, HSP-90 had partially recovered and HSP-70 had recovered completely. There were few indications of lesions in the glandular structure and signs of apoptosis were negative in all radiated animals. The results suggest that acute sub-thermal radiation at 2.45 GHz may alter levels of cellular stress in rat thyroid gland without initially altering their anti-apoptotic capacity.

Specific absorption rate intersubject variability in 7T parallel transmit MRI of the head

Patient-specific radiofrequency shimming in high-field MRI strengthens the need for online, patient-specific specific absorption rate (SAR) monitoring. Numerical simulation is currently most effective for this purpose but may require a patient-specific dielectric model. To investigate whether a generic model may be combined with a safety factor to account for variation within the population, generic SAR behavior is studied for 7T MRI of the head. For six detailed head models, radiofrequency fields were simulated for an eight-channel parallel transmit array. SAR behavior is studied through comparison of the eigenvalues/eigenvectors of the local Q-matrices. Furthermore, numerical radiofrequency shimming experiments without and with SAR constraints were performed where SAR during optimization was evaluated on a generic model. In both cases, the ability of different generic models to predict actual SAR levels was evaluated. The largest eigenvalue distribution is comparable between models. Radiofrequency shimming without constraints improves the |B +1| homogeneity while the SAR increases substantially. Imposing constraints on SAR during optimization, estimating SAR on a generic model, was effective. A safety factor of 1.4 was found to be sufficient. Generic SAR behavior makes a generic head model a practical alternative to patient-specific models and allows effective |B +1| shimming with SAR constraints.

Differential effects of radio frequency magnetic fields in vitro and in vivo

Differential effects of RF magnetic fields in vitro and in vivoBackgroundThe present study investigates effects of radio frequency (RF) magnetic fields in the range 1–15 MHz on growth of cancer cells in vitro and in mice models. Most biomolecules have hyperfine splitting constants ranging from 0.1–10 MHz, which gives the basis for the chosen frequencies.MethodsFor in vitro experiments, fibrosarcoma cell line was treated 15 minutes a day with a combination of magnetic fields: 45 μT static fields in perpendicular and at 40 degrees to an RF signal. The exposures to an RF signal ranging from 5–10 MHz were conducted at a specific absorption rate (SAR) of 0.001, 0.01, 0.05 and 0.2 W/kg using continuous waves (CW). For in vivo experiments, mice with fibrosarcoma and pancreatic tumors were exposed continuously to low‐level static magnetic fields (0.5–2 μT) and a combination of 45 μT static fields in perpendicular to 10 MHz. All controls were exposed to the geomagnetic field (45 μT).ResultsA growth response curve as a function of SAR level is obtained indicating distinct effects for the SAR range used: inhibition of cellular proliferation is obtained for SAR level of 0.001 W/kg; SAR levels of 0.05 and 0.2 W/kg enhances proliferation in vitro. In vivo, 10 MHz magnetic fields inhibit tumor growth in mice as much as 35 percent on day 18 of exposure.ConclusionWe have shown that even a physically weak magnetic field can alter cellular growth. RF magnetic fields modulated proliferation of cancer cells both in vitro and in vivo. The exposure levels were below standard threshold levels for thermal effects due to electromagnetic radiation.

In vivo 31P MR spectroscopic imaging of the human prostate at 7 T: Safety and feasibility

(31)P MR spectroscopic imaging of the human prostate provides information about phosphorylated metabolites that could be used for prostate cancer characterization. The sensitivity of a magnetic field strength of 7 T might enable 3D (31)P MR spectroscopic imaging with relevant spatial resolution in a clinically acceptable measurement time. To this end, a (31)P endorectal coil was developed and combined with an eight-channel (1)H body-array coil to relate metabolic information to anatomical location. An extensive safety validation was performed to evaluate the specific absorption rate, the radiofrequency field distribution, and the temperature distribution of both coils. This validation consisted of detailed Finite Integration Technique simulations, confirmed by MR thermometry and B 1+ measurements in a phantom and in vivo temperature measurements. The safety studies demonstrated that the presence of the (31)P endorectal coil had no influence on the specific absorption rate levels and temperature distribution of the external eight-channel (1)H array coil. To stay within a 10 g averaged local specific absorption rate of 10 W/kg, a maximum time-averaged input power of 33 W for the (1)H array coil was allowed. For transmitting with the (31)P endorectal coil, our safety limit of less than 1°C temperature increase in vivo during a 15-min MR spectroscopic imaging experiment was reached at a time-averaged input power of 1.9 W. With this power setting, a second in vivo measurement was performed on a healthy volunteer. Using adiabatic excitation, 3D (31)P MR spectroscopic imaging produced spectra from the entire prostate in 18 min with a spatial resolution of 4 cm(3). The spectral resolution enabled the separate detection of phosphocholine, phosphoethanolamine, inorganic phosphate, and other metabolites that could play an important role in the characterization of prostate cancer.

In utero and early‐life exposure of rats to a Wi‐Fi signal: Screening of immune markers in sera and gestational outcome

An experimental approach was used to assess immunological biomarkers in the sera of young rats exposed in utero and postnatal to non-ionizing radiofrequency fields. Pregnant rats were exposed free-running, 2 h/day and 5 days/week to a 2.45 GHz Wi-Fi signal in a reverberation chamber at whole-body specific absorption rates (SAR) of 0, 0.08, 0.4, and 4 W/kg (with 10, 10, 12, and 9 rats, respectively), while cage control rats were kept in the animal facility (11 rats). Dams were exposed from days 6 to 21 of gestation and then three newborns per litter were further exposed from birth to day 35 postnatal. On day 35 after birth, all pups were sacrificed and sera collected. The screening of sera for antibodies directed against 15 different antigens related to damage and/or pathological markers was conducted using enzyme-linked immunosorbent assay (ELISA). No change in humoral response of young pups was observed, regardless of the types of biomarker and SAR levels. This study also provided some data on gestational outcome following in utero exposure to Wi-Fi signals. Mass evaluation of dams and pups and the number of pups per litter was monitored, and the genital tracts of young rats were observed for abnormalities by measuring anogenital distance. Under these experimental conditions, our observations suggest a lack of adverse effects of Wi-Fi exposure on delivery and general condition of the animals.

Radiation and absorption of waves emitted by a radial dipole in the presence of a layered dielectric sphere with a spherical screen

Considered is the problem of electromagnetic wave radiation and absorption in the presence of a two-layer lossy dielectric sphere partially covered with a metal screen and illuminated by an elementary electric dipole. To find the field, the authors expand it in terms of spherical functions and use the method of analytical regularisation to reduce the problem to a matrix equation with favourable features. This enables us to obtain the radiation and absorption characteristics in a wide range of parameter variations and compare the efficiencies of shielding by different screens of varying angular size. As the studied structure can be viewed as a model of a head phantom with a helmet, the presented analysis can be useful for a control of specific microwave absorption rate levels.

Statistical determination of whole-body average SARs in a 2 GHz whole-body exposure system for unrestrained pregnant and newborn rats

A 2 GHz whole-body exposure to rats over a multigeneration has been conducted as part of bio-effect research in Japan. In this study, the rats moved freely in the cage inside the exposure system. From observation of the activity of rats in the cage, we found that the rats do not stay in each position with uniform possibility. In order to determine the specific absorption rate (SAR) during the entire exposure period with high accuracy, we present a new approach to statistically determine the SAR level in an exposure system. First, we divided the rat cage in the exposure system into several small areas, and derived the fraction of time the rats spent in each small area based on the classification of the documentary photos of rat activity. Then, using the fraction of time spent in each small area as a weighting factor, we calculated the statistical characteristics of the whole-body average SAR for pregnant rats and young rats during the entire exposure period. As a result, this approach gave the statistical distribution as well as the corresponding mean value, median value and mode value for the whole-body SAR so that we can reasonably clarify the relationship between the exposure level and possible biological effect.

New low specific absorption rate (SAR) antenna design for mobile handset

We intend and design a new type of mobile handset antenna structure to reduce the specific absorption rate (SAR) in human head. The designed of low SAR handset antenna will be operating at 2.4 GHz frequency for wireless local area network (WLAN) by considering some parameters that determine the performance of the antenna. The authors carried out simulation by placing antenna at different position to determine position that are more suitable and effectively reduced the SAR level. The numerical results are compared and analyzed. The designed antenna operating at the range of frequency from 2373 to 2435 MHz with 2.6% bandwidth (BW) and the radiation pattern is omni-directional. The handset antenna design has achieved 1.483 W/kg for 1 gm SAR and 1.192 W/kg for 10 gm SAR which has been reduced for about 25.75% for 1 gm and 25.50% for 10 gm SAR value of the standard SAR level. Key words: Finite-difference time-domain (FDTD) method, patch antenna, specific absorption rate (SAR), symmetry.

The Effects of Single and Repeated Exposure to 2.45 GHz Radiofrequency Fields on c-Fos Protein Expression in the Paraventricular Nucleus of Rat Hypothalamus

This study investigated the effects of microwave radiation on the PVN of the hypothalamus, extracted from rat brains. Expression of c-Fos was used to study the pattern of cellular activation in rats exposed once or repeatedly (ten times in 2 weeks) to 2.45 GHz radiation in a GTEM cell. The power intensities used were 3 and 12 W and the Finite Difference Time Domain calculation was used to determine the specific absorption rate (SAR). High SAR triggered an increase of the c-Fos marker 90 min or 24 h after radiation, and low SAR resulted in c-Fos counts higher than in control rats after 24 h. Repeated irradiation at 3 W increased cellular activation of PVN by more than 100% compared to animals subjected to acute irradiation and to repeated non-radiated repeated session control animals. The results suggest that PVN is sensitive to 2.45 GHz microwave radiation at non-thermal SAR levels.

Health Council of the Netherlands: No need to change from SAR to time-temperature relation in electromagnetic fields exposure limits

The Health Council of the Netherlands (HCN) and other organisations hold the basic assumption that induced electric current and the generation and absorption of heat in biological material caused by radiofrequency electromagnetic fields are the only causal effects with possible adverse consequences for human health that have been scientifically established to date. Hence, the exposure guidelines for the 10 MHz–10 GHz frequency range are based on avoiding adverse effects of increased temperatures that may occur of the entire human body at a specific absorption rate (SAR) level above 4 W/kg.During the workshop on Thermal Aspects of Radio Frequency Exposure on 11–12 January 2010 in Gaithersburg, Maryland, USA, the question was raised whether there would be a practical advantage in shifting from expressing the exposure limits in SAR to expressing them in terms of a maximum allowable temperature increase. This would mean defining adverse time–temperature thresholds. In this paper, the HCN discusses the need for this, considering six points: consistency, applicability, quantification, causality, comprehensibility and acceptability.The HCN concludes that it seems unlikely that a change of dosimetric quantity will help us forward in the discussion on the scientific controversies regarding the existence or non-existence of non-thermal effects in humans following long duration, low intensity exposure to electromagnetic fields. Therefore, the HCN favours maintaining the current approach of basic restrictions and reference levels being expressed as SAR and in V/m or µT, respectively.

Low SAR planar antenna for multi standard cellular phones

In this paper the design of a multiband compact antenna for the integration into the new multi function mobile phones is presented. The antenna is matched to operate at GSM 920 MHz, WI-Fi 2.4 GHz and HiperLan 5.1 GHz standards with low SAR levels. Return loss coefficient and radiation pattern of this antenna are computed in free space as well as in the presence of head. The specific absorption rate (SAR) of the planar antenna is calculated and compared with that of the monopole antenna. To examine the performance of this antenna, a prototype was designed, fabricated and measured; the simulation analysis was performed using the HFSS software, good agreement with the simulation providing validation of the design procedure.

Cardiac magnetic resonance imaging at 1.5 T in patients with cardiac rhythm devices

Recent studies suggest that non-cardiac magnetic resonance imaging (MRI) scanning can be performed safely in selected cardiac rhythm device patients. However, little is known about the safety of performing specific cardiac MRI in this setting. We sought to determine the feasibility of cardiac MRI in patients with pacemakers (PMs) or implantable cardioverter-defibrillators (ICDs). Thirty-eight patients underwent a total of 39 (8 ICDs and 31 PM) cardiac MRI examinations at 1.5 T using usual protocols without specific absorption rate (SAR) restrictions. Nine PM-dependent patients were included. All devices were interrogated before and immediately after MRI. During the scan, pacing mode was programmed to asynchronous for PM-dependent patients whereas ICDs were programmed to a monitor-only mode. All devices were functioning appropriately after cardiac MRI. Comparison of device parameters obtained before and immediately after MRI revealed no significant changes in pacing threshold, lead impedance, battery status, or sensing signal amplitude. Neither clinical events nor patient complaints were reported. Significant imaging artefacts were present on 11 of 39 scans (28.2%). These artefacts were significantly more frequent in ICDs (8 of 8, 100%) vs. PMs (3 of 31, 9.7%) (P < 0.001). Diagnostic questions were answered in 92.3% of the cases, with just three pronounced artefacts preventing an adequate diagnosis in three ICD patients. Our results suggest that cardiac MRI may be performed safely in appropriately selected patients with close monitoring during the scan without limitation of peak SAR level using several precautionary measures. Image artefacts were more frequent in ICD patients.

Cálculo da SAR na cabeça produzida por telefones celulares dispostos assimetricamente

In this paper, the Specific Absorption Rate (SAR) evaluation in and nearby passive users when they are closed to mobile phones active users is made. With the help of CST two configurations were considered. The models are approximations of the human head by geoids geometries, with four interfaces. Each one represent the tissues present in the human head (skin, muscle, bone and brain). The electric field superposition phenomenon and SAR from different sources are observed. The calculated SAR levels are compared with the guidelines and published studies. Most of the SAR values obtained are below the exposure limits, while some ones are greater than those produced by only one source (mobile phone) as presented in published papers. As expected, these hot spots are due to the superposition of electric field. The obtained SAR levels in this study reached the maximum value of 73.6 mW/kg, a value that does not exceed the exposure limits of ANATEL.

Evaluation of specific absorption rate and temperature elevation in a multi-layered human head model exposed to radio frequency radiation using the finite-difference time domain method

In this study, using the finite–difference time domain (FDTD) method, specific absorption rate (SAR) distributions and temperature increase are evaluated in a multi-layered human head model exposed to the field radiated from cellular phones and wireless local area networks (WLANs) antennas. The bioheat equation with suitable boundary conditions is solved to find temperature elevation and exposure time effect. Ageing effect is figured out too. In addition, the case of using cellular phones in enclosed areas is studied. Moreover, the effect of distance between the source and the head model is investigated. The obtained results confirm the importance of performing a thermal analysis along with the dosimetric one. At the same levels of radiated power, SAR levels in the tissues are less than the safety limit recommendations, except in skin and cerebrospinal fluid (CSF) tissues. It is also found that the induced temperature elevation in the brain region, in all the examined conditions, never exceeds 0.4°C. This value is well below the threshold for the induction of adverse thermal effects to the neurons.