RF Safe

On the SAR and field inhomogeneity of birdcage coils loaded with the human head.

Magnetic resonance in medicine · 1997 · Mixed · Low evidence

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MixedLow evidencePaper 4170

Using a finite-element method in a 2D birdcage-coil/2D human-head model at 64, 128, 171, and 256 MHz, SAR increased significantly with frequency for the same B1 field. For the same imaging method, SAR from a quadrature coil was significantly lower than from a…

Study snapshot

PublicationMagnetic resonance in medicine · 1997
Study typeengineering
Reported directionMixed
Evidence ratingLow

Abstract

Birdcage coils are widely used as a radiofrequency (RF) resonator in magnetic resonance imaging (MRI) because of their capability to produce a highly homogeneous B1 field over a large volume within the coil. When they are employed for high-frequency MRI, the interaction between the electromagnetic field and the object to be imaged deteriorates the B1-field homogeneity and increases the specific absorption rate (SAR) in the object. To investigate this problem, a finite-element method (FEM) is developed to analyze the SAR and the B1 field in a two-dimensional (2D) model of a birdcage coil loaded with a 2D model of a human head. The electric field, magnetic field, and SAR distributions are shown, and a comprehensive study is carried out for both linear and quadrature birdcage coils at 64, 128, 171, and 256 MHz. It is shown that to generate the same value of the B1 field, the SAR is increased significantly with the frequency, and for the same imaging method the SAR produced by a quadrature coil is significantly lower than that of a linear coil. It is also shown that the B1-field inhomogeneity is increased significantly with the frequency.

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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 directionmixed
PopulationModeled human head (2D model)
Sample sizeNot supplied
ExposureRF
Evidence strengthLowConfidence: 78% · Peer reviewed: yes

Main findings

Using a finite-element method in a 2D birdcage-coil/2D human-head model at 64, 128, 171, and 256 MHz, SAR increased significantly with frequency for the same B1 field. For the same imaging method, SAR from a quadrature coil was significantly lower than from a linear coil, and B1-field inhomogeneity increased significantly with frequency.

Outcomes measured

  • specific absorption rate (SAR)
  • B1-field homogeneity/inhomogeneity
  • electric field distribution
  • magnetic field distribution

Limitations

  • Finite-element analysis in a two-dimensional (2D) model rather than measurements in humans
  • Human head represented as a 2D model; generalizability to full 3D anatomy and clinical conditions not stated
View raw extracted JSON
{
  "study_type": "engineering",
  "exposure": {
    "band": "RF",
    "source": "MRI (birdcage coil)",
    "frequency_mhz": null,
    "sar_wkg": null,
    "duration": null
  },
  "population": "Modeled human head (2D model)",
  "sample_size": null,
  "outcomes": [
    "specific absorption rate (SAR)",
    "B1-field homogeneity/inhomogeneity",
    "electric field distribution",
    "magnetic field distribution"
  ],
  "main_findings": "Using a finite-element method in a 2D birdcage-coil/2D human-head model at 64, 128, 171, and 256 MHz, SAR increased significantly with frequency for the same B1 field. For the same imaging method, SAR from a quadrature coil was significantly lower than from a linear coil, and B1-field inhomogeneity increased significantly with frequency.",
  "effect_direction": "mixed",
  "limitations": [
    "Finite-element analysis in a two-dimensional (2D) model rather than measurements in humans",
    "Human head represented as a 2D model; generalizability to full 3D anatomy and clinical conditions not stated"
  ],
  "evidence_strength": "low",
  "confidence": 0.78,
  "peer_reviewed_likely": "yes",
  "keywords": [
    "MRI",
    "birdcage coil",
    "radiofrequency",
    "SAR",
    "B1 field",
    "field inhomogeneity",
    "finite-element method",
    "64 MHz",
    "128 MHz",
    "171 MHz",
    "256 MHz",
    "quadrature coil",
    "linear coil"
  ],
  "suggested_hubs": [
    {
      "slug": "mri-rf-sar",
      "weight": 0.9,
      "reason": "Study analyzes RF SAR and B1 inhomogeneity for MRI birdcage coils loaded with a human head model."
    }
  ]
}
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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.1910380615
PubMed: 9402197

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