Computation of electromagnetic fields for high-frequency magnetic resonance imaging applications.

Physics in medicine and biology · 1996 · Unclear · Insufficient evidence

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UnclearInsufficient evidencePaper 4211

Study snapshot

PublicationPhysics in medicine and biology · 1996

Study typeengineering

Reported directionUnclear

Evidence ratingInsufficient

Abstract

A numerical method is presented to compute electromagnetic fields inside a 2 mm high resolution, anatomically detailed model of a human head for high-frequency magnetic resonance imaging (MRI) applications. The method uses the biconjugate gradient algorithm in combination with the fast Fourier transform to solve a matrix equation resulting from the discretization of an integrodifferential equation representing the original physical problem. Given the current distribution in an MRI coil, the method can compute both the electric field (thus the specific energy absorption rate (SAR)) and the magnetic field, also known as the B1 field. Results for the SAR and B1 field distribution, excited by a linear and a quadrature birdcage coil, are calculated and presented at 64 MHz, 128 MHz and 256 MHz, corresponding to the operating frequencies of the 1.5 T, 3 T and 6 T MRI systems. It is shown that compared with that at 64 MHz, the SAR at 128 MHz is increased by a factor over 5 and the SAR at 256 MHz is increased by a factor over 10, assuming the same current strength in the coil. Furthermore, compared with the linear excitation, the average SAR for the quadrature excitation is reduced by a factor over 2 and the maximum SAR is reduced by a factor over 3. It is also shown that the B1 field at high frequencies exhibits a strong inhomogeneity, which is attributed to dielectric resonance.

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AI evidence extraction

Structured fields below were generated from the paper’s abstract and metadata to make the evidence easier to inspect. Automated extraction may be incomplete or incorrect; verify important details against the original paper.

Study typeengineering

Effect directionunclear

PopulationAnatomically detailed model of a human head (2 mm resolution)

Sample sizeNot supplied

ExposureRF

Evidence strengthInsufficientConfidence: 78% · Peer reviewed: yes

Main findings

A numerical method is presented to compute electric fields (and SAR) and magnetic (B1) fields in a high-resolution anatomical human head model for MRI. SAR increased by over 5× at 128 MHz and over 10× at 256 MHz compared with 64 MHz (assuming the same coil current). Quadrature birdcage excitation reduced average SAR by over 2× and maximum SAR by over 3× compared with linear excitation; B1 fields at high frequencies showed strong inhomogeneity attributed to dielectric resonance.

Outcomes measured

Electric field
Specific energy absorption rate (SAR)
Magnetic field (B1 field) distribution
B1 field inhomogeneity (dielectric resonance)

Limitations

Results are based on numerical computation in an anatomical head model rather than measurements in humans.
Findings are reported assuming the same current strength in the coil across frequencies.

View raw extracted JSON

{
  "study_type": "engineering",
  "exposure": {
    "band": "RF",
    "source": "MRI coil (birdcage coil)",
    "frequency_mhz": null,
    "sar_wkg": null,
    "duration": null
  },
  "population": "Anatomically detailed model of a human head (2 mm resolution)",
  "sample_size": null,
  "outcomes": [
    "Electric field",
    "Specific energy absorption rate (SAR)",
    "Magnetic field (B1 field) distribution",
    "B1 field inhomogeneity (dielectric resonance)"
  ],
  "main_findings": "A numerical method is presented to compute electric fields (and SAR) and magnetic (B1) fields in a high-resolution anatomical human head model for MRI. SAR increased by over 5× at 128 MHz and over 10× at 256 MHz compared with 64 MHz (assuming the same coil current). Quadrature birdcage excitation reduced average SAR by over 2× and maximum SAR by over 3× compared with linear excitation; B1 fields at high frequencies showed strong inhomogeneity attributed to dielectric resonance.",
  "effect_direction": "unclear",
  "limitations": [
    "Results are based on numerical computation in an anatomical head model rather than measurements in humans.",
    "Findings are reported assuming the same current strength in the coil across frequencies."
  ],
  "evidence_strength": "insufficient",
  "confidence": 0.78,
  "peer_reviewed_likely": "yes",
  "keywords": [
    "MRI",
    "electromagnetic field computation",
    "human head model",
    "birdcage coil",
    "SAR",
    "B1 field",
    "64 MHz",
    "128 MHz",
    "256 MHz",
    "dielectric resonance",
    "fast Fourier transform",
    "biconjugate gradient algorithm"
  ],
  "suggested_hubs": []
}

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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: 10.1088/0031-9155/41/12/011
PubMed: 8971965

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