Magnetic field distribution and signal decay in functional mri in very high fields (up to 9.4 T) using monte carlo diffusion modeling.

International journal of biomedical imaging · 2007 · Unclear · Insufficient evidence

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UnclearInsufficient evidencePaper 3420

Study snapshot

PublicationInternational journal of biomedical imaging · 2007

Study typeengineering

Reported directionUnclear

Evidence ratingInsufficient

Abstract

Extravascular signal decay rate R2 or R2 * as a function of blood oxygenation, geometry, and field strength was calculated using a Monte Carlo (MC) algorithm for a wider parameter range than hitherto by others. The relaxation rates of gradient-recalled-echo (GRE) and Hahn-spin-echo (HSE) imaging in the presence of blood vessels (ranging from capillaries to veins) have been computed for a wide range of field strengths up to 9.4T and 50% blood deoxygenation. The maximum HSE decay was found to be shifted to lower radii in higher compared to lower field strengths. For GRE, however, the relaxation rate was greatest for large vessels at any field strength. In addition, assessments of computational reliability have been carried out by investigating the influence of the time step, the Monte Carlo step procedure, boundary conditions, the number of angles between the vessel and the exterior field B0, the influence of neighboring vessels having the same orientation as the central vessel, and the number of proton spins. The results were compared with those obtained from a field distribution of the vessel computed by an analytic formula describing the field distribution of an ideal object (an infinitely long cylinder). It was found that the time step is not critical for values equal to or lower than 200 microseconds. The choice of the MC step procedure (three-dimensional Gaussian diffusion, constant one- or three-dimensional diffusion step) also failed to influence the results significantly; in contrast, the free boundary conditions, as well as taking too few angles into account, did introduce errors. Next neighbor vessels with the same orientation as the main vessel did not contribute significantly to signal decay. The total number of particles simulated was also found to play a minor role in computing R2/ R2 *.

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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

PopulationNot supplied

Sample sizeNot supplied

ExposureNot supplied

Evidence strengthInsufficientConfidence: 74% · Peer reviewed: yes

Main findings

Using Monte Carlo diffusion modeling, relaxation/signal decay rates (R2, R2*) were computed for GRE and HSE MRI across vessel sizes, blood oxygenation (up to 50% deoxygenation), and field strengths up to 9.4 T. Maximum HSE decay shifted to lower vessel radii at higher field strengths, while GRE relaxation rate was greatest for large vessels at any field strength. Computational reliability checks suggested time step (≤200 microseconds), Monte Carlo step procedure, neighboring vessels with same orientation, and number of simulated spins had minor influence, whereas free boundary conditions and too few vessel-field angles introduced errors.

Outcomes measured

Extravascular signal decay rate R2
Extravascular signal decay rate R2*
Relaxation rates in GRE and HSE imaging
Magnetic field distribution around blood vessels (model-based)

Limitations

Modeling/simulation study (Monte Carlo diffusion modeling) rather than empirical measurements
No human/animal participant data reported in the abstract
No exposure metrics beyond field strength (e.g., SAR) reported

View raw extracted JSON

{
  "study_type": "engineering",
  "exposure": {
    "band": null,
    "source": "MRI (static magnetic field B0)",
    "frequency_mhz": null,
    "sar_wkg": null,
    "duration": null
  },
  "population": null,
  "sample_size": null,
  "outcomes": [
    "Extravascular signal decay rate R2",
    "Extravascular signal decay rate R2*",
    "Relaxation rates in GRE and HSE imaging",
    "Magnetic field distribution around blood vessels (model-based)"
  ],
  "main_findings": "Using Monte Carlo diffusion modeling, relaxation/signal decay rates (R2, R2*) were computed for GRE and HSE MRI across vessel sizes, blood oxygenation (up to 50% deoxygenation), and field strengths up to 9.4 T. Maximum HSE decay shifted to lower vessel radii at higher field strengths, while GRE relaxation rate was greatest for large vessels at any field strength. Computational reliability checks suggested time step (≤200 microseconds), Monte Carlo step procedure, neighboring vessels with same orientation, and number of simulated spins had minor influence, whereas free boundary conditions and too few vessel-field angles introduced errors.",
  "effect_direction": "unclear",
  "limitations": [
    "Modeling/simulation study (Monte Carlo diffusion modeling) rather than empirical measurements",
    "No human/animal participant data reported in the abstract",
    "No exposure metrics beyond field strength (e.g., SAR) reported"
  ],
  "evidence_strength": "insufficient",
  "confidence": 0.74,
  "peer_reviewed_likely": "yes",
  "keywords": [
    "functional MRI",
    "high-field MRI",
    "9.4 T",
    "Monte Carlo",
    "diffusion modeling",
    "R2",
    "R2*",
    "GRE",
    "Hahn spin echo",
    "blood vessels",
    "oxygenation",
    "magnetic field distribution"
  ],
  "suggested_hubs": [
    {
      "slug": "who-icnirp",
      "weight": 0.2,
      "reason": "Involves high static magnetic fields (MRI), potentially relevant to exposure guideline context, but the abstract is primarily imaging physics/modeling rather than health effects."
    }
  ]
}

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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: 10.1155/2007/70309
PubMed: 18273394

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