The effects of RF coils and SAR supervision strategies for clinically applicable nonselective parallel-transmit pulses at 7 T.

Magnetic resonance in medicine · 2023 · Mixed · Low evidence

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MixedLow evidencePaper 5809

Study snapshot

PublicationMagnetic resonance in medicine · 2023

Study typeengineering

Reported directionMixed

Evidence ratingLow

Abstract

PURPOSE: To investigate the effects of using different parallel-transmit (pTx) head coils and specific absorption rate (SAR) supervision strategies on pTx pulse design for ultrahigh-field MRI using a 3D-MPRAGE sequence. METHODS: The PTx universal pulses (UPs) and fast online-customized (FOCUS) pulses were designed with pre-acquired data sets (B , B maps, specific absorption rate [SAR] supervision data) from two different 8 transmit/32 receive head coils on two 7T whole-body MR systems. For one coil, the SAR supervision model consisted of per-channel RF power limits. In the other coil, SAR estimations were done with both per-channel RF power limits as well as virtual observation points (VOPs) derived from electromagnetic field (EMF) simulations using three virtual human body models at three different positions. All pulses were made for nonselective excitation and inversion and evaluated on 132 B , B , and SAR supervision datasets obtained with one coil and 12 from the other. At both sites, 3 subjects were examined using MPRAGE sequences that used UP/FOCUS pulses generated for both coils. RESULTS: For some subjects, the UPs underperformed when simulated on a different coil from which they were derived, whereas FOCUS pulses still showed acceptable performance in that case. FOCUS inversion pulses outperformed adiabatic pulses when scaled to the same local SAR level. For the self-built coil, the use of VOPs showed reliable overestimation compared with the ground-truth EMF simulations, predicting about 52% lower local SAR for inversion pulses compared with per-channel power limits. CONCLUSION: FOCUS inversion pulses offer a low-SAR alternative to adiabatic pulses and benefit from using EMF-based VOPs for SAR estimation.

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AI evidence extraction

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Study typeengineering

Effect directionmixed

PopulationHuman subjects undergoing 7T MRI (MPRAGE)

Sample size6

ExposureNot supplied

Evidence strengthLowConfidence: 74% · Peer reviewed: yes

Main findings

Universal pulses sometimes underperformed when simulated on a different coil than the one used to derive them, while FOCUS pulses showed acceptable performance in that case. FOCUS inversion pulses outperformed adiabatic pulses when scaled to the same local SAR level. For one coil, VOP-based SAR supervision reliably overestimated compared with ground-truth EMF simulations and predicted about 52% lower local SAR for inversion pulses than per-channel power limits.

Outcomes measured

pTx pulse performance (universal pulses vs fast online-customized pulses)
Local SAR estimation/supervision strategy performance (per-channel RF power limits vs VOPs)
Comparison of inversion pulse types (FOCUS vs adiabatic) at matched local SAR

Limitations

Frequency of the RF exposure not stated in the abstract
Only 3 subjects examined at each of two sites (total n=6)
Results depend on two specific 8Tx/32Rx head coils and their SAR supervision models
VOPs derived from EMF simulations using three virtual human body models at three positions (limited modeling scenarios)

View raw extracted JSON

{
  "study_type": "engineering",
  "exposure": {
    "band": null,
    "source": "MRI RF coils (parallel-transmit head coils) at 7T",
    "frequency_mhz": null,
    "sar_wkg": null,
    "duration": null
  },
  "population": "Human subjects undergoing 7T MRI (MPRAGE)",
  "sample_size": 6,
  "outcomes": [
    "pTx pulse performance (universal pulses vs fast online-customized pulses)",
    "Local SAR estimation/supervision strategy performance (per-channel RF power limits vs VOPs)",
    "Comparison of inversion pulse types (FOCUS vs adiabatic) at matched local SAR"
  ],
  "main_findings": "Universal pulses sometimes underperformed when simulated on a different coil than the one used to derive them, while FOCUS pulses showed acceptable performance in that case. FOCUS inversion pulses outperformed adiabatic pulses when scaled to the same local SAR level. For one coil, VOP-based SAR supervision reliably overestimated compared with ground-truth EMF simulations and predicted about 52% lower local SAR for inversion pulses than per-channel power limits.",
  "effect_direction": "mixed",
  "limitations": [
    "Frequency of the RF exposure not stated in the abstract",
    "Only 3 subjects examined at each of two sites (total n=6)",
    "Results depend on two specific 8Tx/32Rx head coils and their SAR supervision models",
    "VOPs derived from EMF simulations using three virtual human body models at three positions (limited modeling scenarios)"
  ],
  "evidence_strength": "low",
  "confidence": 0.74,
  "peer_reviewed_likely": "yes",
  "keywords": [
    "7T MRI",
    "parallel transmit",
    "pTx",
    "RF coil",
    "specific absorption rate",
    "SAR supervision",
    "virtual observation points",
    "VOP",
    "EMF simulations",
    "MPRAGE",
    "universal pulses",
    "FOCUS pulses",
    "adiabatic pulses"
  ],
  "suggested_hubs": [
    {
      "slug": "who-icnirp",
      "weight": 0.2,
      "reason": "Mentions SAR supervision/estimation approaches relevant to RF exposure safety in MRI, but not a policy paper."
    }
  ]
}

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AI-extracted fields are generated from the abstract and metadata and may be incomplete or incorrect. This material is for informational purposes and is not medical advice.

Identifiers and source

DOI: 10.1002/mrm.29569
PubMed: 36622945

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