Dependence of RF heating on SAR and implant position in a 1.5T MR system.

Abstract

PURPOSE: We evaluated radiofrequency (RF) heating of a humerus implant embedded in a gel phantom during magnetic resonance (MR) imaging for the specific absorption rate (SAR), angle between the implant and static magnetic field (B(0)), and position of the implant in the irradiation coil. METHODS: We embedded a stainless steel humerus implant 2 cm deep in tissue-equivalent loop and mass phantoms, placed it parallel to the static magnetic field of a 1.5T MR scanner, and recorded the temperatures of the implant surface with RF-transparent fiberoptic sensors. We measured rises in temperature at the tips of the implant by varying the SAR from 0.2 to 4.0 W/kg and evaluated RF heating of the implant for its angle to B(0) and its displacement along B(0) from the center of the RF irradiation coil. RESULTS: RF heating was similar for the loop and mass phantoms because the eddy current flows through the periphery of both. As the SAR increased, the temperature at the implant tip increased, and there was a linear relationship between the SAR and temperature rise. The values were 6.4 degrees C at 2.0 W/kg and 12.7 degrees C at 4.0 W/kg. Rise in temperature decreased steeply as the angle between the implant and B(0) surpassed 45 degrees . In addition, as the implant was displaced from the center of the RF coil to both ends, the rise in temperature decreased. CONCLUSION: The rise in temperature in deep tissue was estimated to be higher than 1.0 degrees C for SAR above 0.4 W/kg. RF heating was greatest when the implant was set parallel to B(0). In MR imaging of patients with implants, there is a risk of RF heating when the loop of the eddy current is formed inside the body.

AI evidence extraction

Main findings

In gel phantoms with a stainless steel humerus implant, temperature rise at the implant tip increased linearly with SAR (reported 6.4°C at 2.0 W/kg and 12.7°C at 4.0 W/kg). Heating decreased steeply when the implant angle relative to B0 exceeded 45°, and decreased when the implant was displaced from the center of the RF coil toward either end. The authors estimated deep-tissue temperature rise would exceed 1.0°C for SAR above 0.4 W/kg and noted a risk of RF heating in patients with implants when an eddy-current loop forms in the body.

Outcomes measured

• Temperature rise (RF heating) at implant tip/surface during MRI
• Dependence of heating on SAR
• Dependence of heating on implant angle relative to B0
• Dependence of heating on implant position within RF coil

Limitations

• Phantom (gel) study; not conducted in humans
• Implant type/material limited to a stainless steel humerus implant
• Frequency and other MRI sequence parameters not reported in abstract
• Sample size/number of measurements not reported in abstract

Suggested hubs

• mri-implants (0.9)
  Study evaluates RF heating of a metallic implant during 1.5T MRI as a function of SAR, orientation, and coil position.

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    "study_type": "exposure_assessment",
    "exposure": {
        "band": "RF",
        "source": "MRI",
        "frequency_mhz": null,
        "sar_wkg": null,
        "duration": null
    },
    "population": null,
    "sample_size": null,
    "outcomes": [
        "Temperature rise (RF heating) at implant tip/surface during MRI",
        "Dependence of heating on SAR",
        "Dependence of heating on implant angle relative to B0",
        "Dependence of heating on implant position within RF coil"
    ],
    "main_findings": "In gel phantoms with a stainless steel humerus implant, temperature rise at the implant tip increased linearly with SAR (reported 6.4°C at 2.0 W/kg and 12.7°C at 4.0 W/kg). Heating decreased steeply when the implant angle relative to B0 exceeded 45°, and decreased when the implant was displaced from the center of the RF coil toward either end. The authors estimated deep-tissue temperature rise would exceed 1.0°C for SAR above 0.4 W/kg and noted a risk of RF heating in patients with implants when an eddy-current loop forms in the body.",
    "effect_direction": "harm",
    "limitations": [
        "Phantom (gel) study; not conducted in humans",
        "Implant type/material limited to a stainless steel humerus implant",
        "Frequency and other MRI sequence parameters not reported in abstract",
        "Sample size/number of measurements not reported in abstract"
    ],
    "evidence_strength": "low",
    "confidence": 0.7800000000000000266453525910037569701671600341796875,
    "peer_reviewed_likely": "yes",
    "keywords": [
        "MRI",
        "radiofrequency heating",
        "specific absorption rate",
        "SAR",
        "implant",
        "humerus",
        "stainless steel",
        "phantom",
        "eddy current",
        "1.5T"
    ],
    "suggested_hubs": [
        {
            "slug": "mri-implants",
            "weight": 0.90000000000000002220446049250313080847263336181640625,
            "reason": "Study evaluates RF heating of a metallic implant during 1.5T MRI as a function of SAR, orientation, and coil position."
        }
    ]
}

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