Evaluation of specific absorption rate and heating in children exposed to a 7T MRI head coil

Shaihan Malik; Jeffrey Hand; David Carmichael; Jo Hajnal

doi:10.1002/mrm.29283

Evaluation of specific absorption rate and heating in children exposed to a 7T MRI head coil

Shaihan Malik, Jeffrey Hand, David Carmichael, Jo Hajnal

Research output: Contribution to journal › Article › peer-review

5 Citations (Scopus)

Abstract

Purpose: To evaluate specific absorption rate (SAR) and temperature distributions resulting from pediatric exposure to a 7T head coil. Methods: Exposure from a 297-MHz birdcage head transmit coil (CP mode single-channel transmission) was simulated in several child models (ages 3–14, mass 13.9–50.4 kg) and one adult, using time-domain electromagnetic and thermal solvers. Position variability, age-related changes in dielectric properties, and differences in thermoregulation were also considered. Results: Age-adjusted dielectric properties had little effect in this population. Head average SAR (hdSAR) was the limiting factor for all models centered in the coil. The value of hdSAR (normalized to net power) was found to decrease linearly with increasing mass (R ² = 0.86); no equivalent relationship for peak-spatial 10g averaged SAR (psSAR _10g) was identified. Relatively small (< 10%) variability was observed in hdSAR for position shifts of ±25 mm in each orthogonal direction when normalized to net power; accounting for (Formula presented.) efficiency can lead to much larger variability. Position sensitivity of psSAR _10g was greater, but in most cases hdSAR remained the limiting quantity. For thermal simulations, if blood temperature is fixed (i.e., asserting good thermoregulation), maximum temperatures are compliant with International Electrotechnical Commission limits during 60-min exposure at the SAR limit. Introducing variable blood temperature leads to core temperature changes proportional to whole-body averaged SAR, exceeding guideline limits for all child models. Conclusions: Children experienced higher SAR than adults for the 297-MHz head transmit coil examined in this work. Thermal simulations suggest that core temperature changes could occur in smaller subjects, although experimental data are needed for validation.

Original language	English
Pages (from-to)	1434-1449
Number of pages	16
Journal	Magnetic resonance in medicine
Volume	88
Issue number	3
DOIs	https://doi.org/10.1002/mrm.29283
Publication status	Published - Sept 2022

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10.1002/mrm.29283Licence: CC BY

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@article{125d97e10d734b22af5779f65090ecf0,

title = "Evaluation of specific absorption rate and heating in children exposed to a 7T MRI head coil",

abstract = "Purpose: To evaluate specific absorption rate (SAR) and temperature distributions resulting from pediatric exposure to a 7T head coil. Methods: Exposure from a 297-MHz birdcage head transmit coil (CP mode single-channel transmission) was simulated in several child models (ages 3–14, mass 13.9–50.4 kg) and one adult, using time-domain electromagnetic and thermal solvers. Position variability, age-related changes in dielectric properties, and differences in thermoregulation were also considered. Results: Age-adjusted dielectric properties had little effect in this population. Head average SAR (hdSAR) was the limiting factor for all models centered in the coil. The value of hdSAR (normalized to net power) was found to decrease linearly with increasing mass (R 2 = 0.86); no equivalent relationship for peak-spatial 10g averaged SAR (psSAR 10g) was identified. Relatively small (< 10%) variability was observed in hdSAR for position shifts of ±25 mm in each orthogonal direction when normalized to net power; accounting for (Formula presented.) efficiency can lead to much larger variability. Position sensitivity of psSAR 10g was greater, but in most cases hdSAR remained the limiting quantity. For thermal simulations, if blood temperature is fixed (i.e., asserting good thermoregulation), maximum temperatures are compliant with International Electrotechnical Commission limits during 60-min exposure at the SAR limit. Introducing variable blood temperature leads to core temperature changes proportional to whole-body averaged SAR, exceeding guideline limits for all child models. Conclusions: Children experienced higher SAR than adults for the 297-MHz head transmit coil examined in this work. Thermal simulations suggest that core temperature changes could occur in smaller subjects, although experimental data are needed for validation. ",

author = "Shaihan Malik and Jeffrey Hand and David Carmichael and Jo Hajnal",

note = "Funding Information: information European Research Council, 319456; Great Ormond Street Hospital Charity, V4419; National Institute for Health Research (NIHR), Wellcome Trust, WT 203148/Z/16/ZSupported by ERC grant agreement no. 319456 (dHCP project); GOSHCC Sparks Grant (V4419); the Wellcome EPSRC Center for Medical Engineering at King's College London (WT 203148/Z/16/Z); and by the National Institute for Health Research Biomedical Research Center based at Guy's and St. Thomas' NHS Foundation Trust and King's College London. The views expressed are those of the authors and not necessarily those of the NHS, the National Institute for Health Research, or the Department of Health. We acknowledge the receipt of scientific licenses for Sim4Life from ZMT (www.zurichmedtech.com). This research was funded in whole, or in part, by the Wellcome Trust (WT 203148/Z/16/Z). For the purpose of open access, the author has applied a CC BY public copyright license to any author-accepted manuscript version arising from this submission. Funding Information: This research was funded in whole, or in part, by the Wellcome Trust (WT 203148/Z/16/Z). For the purpose of open access, the author has applied a CC BY public copyright license to any author‐accepted manuscript version arising from this submission. Funding Information: Supported by ERC grant agreement no. 319456 (dHCP project); GOSHCC Sparks Grant (V4419); the Wellcome EPSRC Center for Medical Engineering at King's College London (WT 203148/Z/16/Z); and by the National Institute for Health Research Biomedical Research Center based at Guy's and St. Thomas' NHS Foundation Trust and King's College London. The views expressed are those of the authors and not necessarily those of the NHS, the National Institute for Health Research, or the Department of Health. We acknowledge the receipt of scientific licenses for from ZMT ( www.zurichmedtech.com ). Sim4Life Publisher Copyright: {\textcopyright} 2022 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.",

year = "2022",

month = sep,

doi = "10.1002/mrm.29283",

language = "English",

volume = "88",

pages = "1434--1449",

journal = "Magnetic resonance in medicine",

issn = "0740-3194",

publisher = "WILEY-BLACKWELL",

number = "3",

}

TY - JOUR

T1 - Evaluation of specific absorption rate and heating in children exposed to a 7T MRI head coil

AU - Malik, Shaihan

AU - Hand, Jeffrey

AU - Carmichael, David

AU - Hajnal, Jo

N1 - Funding Information: information European Research Council, 319456; Great Ormond Street Hospital Charity, V4419; National Institute for Health Research (NIHR), Wellcome Trust, WT 203148/Z/16/ZSupported by ERC grant agreement no. 319456 (dHCP project); GOSHCC Sparks Grant (V4419); the Wellcome EPSRC Center for Medical Engineering at King's College London (WT 203148/Z/16/Z); and by the National Institute for Health Research Biomedical Research Center based at Guy's and St. Thomas' NHS Foundation Trust and King's College London. The views expressed are those of the authors and not necessarily those of the NHS, the National Institute for Health Research, or the Department of Health. We acknowledge the receipt of scientific licenses for Sim4Life from ZMT (www.zurichmedtech.com). This research was funded in whole, or in part, by the Wellcome Trust (WT 203148/Z/16/Z). For the purpose of open access, the author has applied a CC BY public copyright license to any author-accepted manuscript version arising from this submission. Funding Information: This research was funded in whole, or in part, by the Wellcome Trust (WT 203148/Z/16/Z). For the purpose of open access, the author has applied a CC BY public copyright license to any author‐accepted manuscript version arising from this submission. Funding Information: Supported by ERC grant agreement no. 319456 (dHCP project); GOSHCC Sparks Grant (V4419); the Wellcome EPSRC Center for Medical Engineering at King's College London (WT 203148/Z/16/Z); and by the National Institute for Health Research Biomedical Research Center based at Guy's and St. Thomas' NHS Foundation Trust and King's College London. The views expressed are those of the authors and not necessarily those of the NHS, the National Institute for Health Research, or the Department of Health. We acknowledge the receipt of scientific licenses for from ZMT ( www.zurichmedtech.com ). Sim4Life Publisher Copyright: © 2022 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.

PY - 2022/9

Y1 - 2022/9

N2 - Purpose: To evaluate specific absorption rate (SAR) and temperature distributions resulting from pediatric exposure to a 7T head coil. Methods: Exposure from a 297-MHz birdcage head transmit coil (CP mode single-channel transmission) was simulated in several child models (ages 3–14, mass 13.9–50.4 kg) and one adult, using time-domain electromagnetic and thermal solvers. Position variability, age-related changes in dielectric properties, and differences in thermoregulation were also considered. Results: Age-adjusted dielectric properties had little effect in this population. Head average SAR (hdSAR) was the limiting factor for all models centered in the coil. The value of hdSAR (normalized to net power) was found to decrease linearly with increasing mass (R 2 = 0.86); no equivalent relationship for peak-spatial 10g averaged SAR (psSAR 10g) was identified. Relatively small (< 10%) variability was observed in hdSAR for position shifts of ±25 mm in each orthogonal direction when normalized to net power; accounting for (Formula presented.) efficiency can lead to much larger variability. Position sensitivity of psSAR 10g was greater, but in most cases hdSAR remained the limiting quantity. For thermal simulations, if blood temperature is fixed (i.e., asserting good thermoregulation), maximum temperatures are compliant with International Electrotechnical Commission limits during 60-min exposure at the SAR limit. Introducing variable blood temperature leads to core temperature changes proportional to whole-body averaged SAR, exceeding guideline limits for all child models. Conclusions: Children experienced higher SAR than adults for the 297-MHz head transmit coil examined in this work. Thermal simulations suggest that core temperature changes could occur in smaller subjects, although experimental data are needed for validation.

AB - Purpose: To evaluate specific absorption rate (SAR) and temperature distributions resulting from pediatric exposure to a 7T head coil. Methods: Exposure from a 297-MHz birdcage head transmit coil (CP mode single-channel transmission) was simulated in several child models (ages 3–14, mass 13.9–50.4 kg) and one adult, using time-domain electromagnetic and thermal solvers. Position variability, age-related changes in dielectric properties, and differences in thermoregulation were also considered. Results: Age-adjusted dielectric properties had little effect in this population. Head average SAR (hdSAR) was the limiting factor for all models centered in the coil. The value of hdSAR (normalized to net power) was found to decrease linearly with increasing mass (R 2 = 0.86); no equivalent relationship for peak-spatial 10g averaged SAR (psSAR 10g) was identified. Relatively small (< 10%) variability was observed in hdSAR for position shifts of ±25 mm in each orthogonal direction when normalized to net power; accounting for (Formula presented.) efficiency can lead to much larger variability. Position sensitivity of psSAR 10g was greater, but in most cases hdSAR remained the limiting quantity. For thermal simulations, if blood temperature is fixed (i.e., asserting good thermoregulation), maximum temperatures are compliant with International Electrotechnical Commission limits during 60-min exposure at the SAR limit. Introducing variable blood temperature leads to core temperature changes proportional to whole-body averaged SAR, exceeding guideline limits for all child models. Conclusions: Children experienced higher SAR than adults for the 297-MHz head transmit coil examined in this work. Thermal simulations suggest that core temperature changes could occur in smaller subjects, although experimental data are needed for validation.

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U2 - 10.1002/mrm.29283

DO - 10.1002/mrm.29283

M3 - Article

SN - 0740-3194

VL - 88

SP - 1434

EP - 1449

JO - Magnetic resonance in medicine

JF - Magnetic resonance in medicine

IS - 3

ER -

Evaluation of specific absorption rate and heating in children exposed to a 7T MRI head coil

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