Computed SAR and thermal elevation in a 0.25-mm 2-D model of the human eye and head in response to an implanted retinal stimulator. II. Results

Lazzi, Gianluca; DeMarco, S.C.; Liu, Wentai; Weiland, James D.; Humayun, Mark S.

doi:10.1109/tap.2003.816394

Cited by 50 publications

(18 citation statements)

References 7 publications

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“…3), the electromagnetic fields were highly localized near the chip(s). Note that this method is similar to that utilized in 8,30 which dealt with 2-D calculations of the retina and provided very good experimental verifications. 30 Additionally, the radiated power was not included in this calculation, as this quantity was tested and found to be several orders of magnitude less than the absorbed power.…”

Section: The Electromagnetic Modelmentioning

confidence: 87%

“…Then, heat diffusion between the head and the environment, as well as basal metabolic heating effects within the head, were allowed to continue until the temperatures in the brain leveled to steady-state. 30 (Note that all references to steadystate in this work satisfy at least dT/ dt = 6.5 Â 10 )6°C /s for 30 s) Temperatures at this equilibrium condition in the head were near 37°C except for regions near the outer boundary, where diffusive effects lead to cooling of approximately 0.25°C. Thus, all temperature changes reported are with respect to this equilibrium state, rather than a constant 37°C.…”

Section: The Electromagnetic Modelmentioning

confidence: 90%

“…4,8,30 The initial body temperature, T b , was set to 37°C. 8 Also, utilizing the fact that the transfer of heat between the domain and the surrounding environment is proportional to temperature difference between the surface and the environment, the following boundary condition was applied:…”

Section: The Electromagnetic Modelmentioning

confidence: 99%

“…6,7,10,12,[17][18][19][20]23,27,33,35,39 In addition, studies have been conducted on the effects of implantable chips placed on the retina, cardiac muscle, and other structures within the body. 4,8,15,16,26,30,34,37 However, much less has been done on the consequences of wirelessly powering devices within the skull 14,31 . In this work, a 2-D numerical model with approximations to 3D was developed and implemented to provide an analysis of the SAR values and temperature changes in the cortical tissue surrounding the implanted and electromagnetically-excited BMI chip(s).…”

Section: Introductionmentioning

confidence: 99%

“…5(plots describing temperature elevations within neural tissue due to power dissipation in the chip(s). The first author utilized such approach in measuring temperature rises due to external coils during MRI experiment at 340 MHz 30. …”

mentioning

confidence: 99%

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Electromagnetic Power Absorption and Temperature Changes due to Brain Machine Interface Operation

2007

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To fully understand neural function, chronic neural recordings must be made simultaneously from 10s or 100s of neurons. To accomplish this goal, several groups are developing brain machine interfaces. For these devices to be viable for chronic human use, it is likely that they will need to be operated and powered externally via a radiofrequency (RF) source. However, RF exposure can result in tissue heating and is regulated by the FDA/FCC. This paper provides an initial estimate of the amount of tissue heating and specific absorption rate (SAR) associated with the operation of a brain-machine interface (BMI). The operation of a brain machine interface was evaluated in an 18-tissue anatomically detailed human head mesh using simulations of electromagnetics and bio-heat phenomena. The simulations were conducted with a single chip, as well as with eight chips, placed on the surface of the human brain and each powered at four frequencies (13.6 MHz, 1.0 GHz, 2.4 GHz, and 5.8 GHz). The simulated chips consist of a wire antenna on a silicon chip covered by a Teflon dura patch. SAR values were calculated using the finite-difference time-domain method and used to predict peak temperature changes caused by electromagnetic absorption in the head using two-dimensional bio-heat equation. Results due to SAR alone show increased heating at higher frequencies, with a peak temperature change at 5.8 GHz of approximately 0.018 degrees C in the single-chip configuration and 0.06 degrees C in the eight-chip configuration with 10 mW of power absorption (in the human head) per chip. In addition, temperature elevations due to power dissipation in the chip(s) were studied. Results show that for the neural tissue, maximum temperature rises of 3.34 degrees C in the single-chip configuration and 7.72 degrees C in the eight-chip configuration were observed for 10 mW dissipation in each chip. Finally, the maximum power dissipation allowable in each chip before a 1.0 degrees C temperature increase (most stringent standards as denoted in the FDA guidelines) is exceeded in the head was simulated and found to be 2.92 mW in the single-chip configuration and 1.25 mW in the eight-chip configuration. As thermal heating due to SAR was insignificant, this study suggests that wireless electromagnetics, i.e., RF may be a viable option for powering, and communicating with brain machine interfaces for clinical applications.

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Section: The Electromagnetic Modelmentioning

confidence: 87%

Section: The Electromagnetic Modelmentioning

confidence: 90%

Section: The Electromagnetic Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Electromagnetic Power Absorption and Temperature Changes due to Brain Machine Interface Operation

2007

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A numerical evaluation of SAR distribution and temperature changes around a metallic plate in the head of a RF exposed worker

McIntosh

Anderson²,

McKenzie

2005

Bioelectromagnetics

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The 1998 International Commission for Non-Ionising Radiation (ICNIRP) Guidelines for human exposure to radiofrequency (RF) fields contain a recommendation to assess the potential impact of metallic implants in workers exposed up to the allowable occupational field limits. This study provides an example of how numerical electromagnetic (EM) and thermal modelling can be used to determine whether scattered RF fields around metallic implants in workers exposed to allowable occupational ambient field limits will comply with the recommendations of relevant standards and guidelines. A case study is performed for plane wave exposures of a 50 mm diameter titanium cranioplasty plate, implanted around 5-6 mm under the surface of the forehead. The level of exposures was set to the ambient power flux density limits for occupational exposures specified in the 1998 ICNIRP guidelines and the current 1999 IEEE C95.1 standard over the frequency range 100-3000 MHz. Two distinct peak responses were observed. There was a resonant response for the whole implant at 200-300 MHz where the maximum dimension of the implant is around a third of the wavelength of the RF exposure. This, however, resulted in relatively low peak specific energy absorption rate (SAR) levels around the implant at the exposure limits. Between 2100-2800 MHz, a second SAR concentrating mechanism of constructive interference of the wave reflected back and forth between the air-scalp interface and the scalp-plate interface resulted in higher peak SARs that were within the allowable limits for the ICNIRP exposures, but not for the IEEE C95.1 exposures. Moreover, the IEEE peak SAR limits were also exceeded, to a lesser degree, even when the implant was not present. However, thermal modelling indicated that the peak SAR concentrations around the implant did not result in any peak temperature rise above 1 degrees C for occupational exposures recommended in the ICNIRP guidelines, and hence would not pose any significant health risk.

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Studies of RF shimming techniques with minimization of RF power deposition and their associated temperature changes

Tang¹,

Hue²,

Ibrahim³

2011

Concepts Magn Reson

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In ultrahigh field (UHF), human magnetic resonance imaging (MRI) concerns related to the homogeneity of the italicB1+ field [the radiofrequency (RF) magnetic field component responsible for the excitation of the spins] and the local/average specific absorption rate (SAR) are highly evident. In this work, through RF shimming techniques, a full-wave electromagnetic model that treats a coupled-RF coil and the load (an 18-tissue anatomically detailed human head model) as a single system is utilized to simultaneously (1) improve the homogeneity of italicB1+ field in various regions of interest across the volume of the human head and (2) minimize the total RF power deposition at 7 and 9.4 T. The numerical results illustrate that the italicB1+ field homogeneity (evaluated by the coefficient of variance) can be greatly improved in 3D slabs that vary in orientations and sizes, in the brain, and in the entire head volume without increasing the total RF power deposition in the head to exceed that obtained with quadrature excitation. The RF shimming simulation results are experimentally validated (by performing RF shimming without experimental B1 measurements) on a head-sized phantom using a 7-T human MRI scanner equipped with a transmit array excitation system. The SAR and associated temperature changes under quadrature and RF shimming excitation conditions are calculated and compared.

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Computed SAR and thermal elevation in a 0.25-mm 2-D model of the human eye and head in response to an implanted retinal stimulator. II. Results

Cited by 50 publications

References 7 publications

Electromagnetic Power Absorption and Temperature Changes due to Brain Machine Interface Operation

Electromagnetic Power Absorption and Temperature Changes due to Brain Machine Interface Operation

A numerical evaluation of SAR distribution and temperature changes around a metallic plate in the head of a RF exposed worker

Studies of RF shimming techniques with minimization of RF power deposition and their associated temperature changes

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