Bassen H, Zaidi T. Parameters Affecting Worst-Case Gradient-Field Heating of Passive Conductive Implants.
J Magn Reson Imaging 2022;
56:1197-1204. [PMID:
35778374 DOI:
10.1002/jmri.28321]
[Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2021] [Revised: 06/11/2022] [Accepted: 06/13/2022] [Indexed: 11/08/2022] Open
Abstract
BACKGROUND
Testing MRI gradient-induced heating of implanted medical devices is required by regulatory organizations and others. A gradient heating test of the ISO 10974 Technical Specification (TS) for active implants was adopted for this study of passive hip implants. All but one previous study of hip implants used nonuniform gradient exposure fields in clinical scanners and reported heating of less than 5 °C. This present study adapted methods of the TS, addressing the unmet need for identifying worst-case heating via exposures to uniform gradient fields.
PURPOSE
To identify gradient-field parameters affecting maximum heating in vitro for a hip implant and a cylindrical titanium disk.
STUDY TYPE
Computational simulations and experimental validation of induced heating.
PHANTOM
Tissue-simulating gel.
FIELD STRENGTH
42 T/s RMS, sinusoidal, continuous B fields with high spatial uniformity ASSESSMENT: Hip implant heating at 1-10 kHz, via computational modeling, validated by limited point measurements. Experimental measurements of exposures of an implant at 42 T/s for 4, 6, and 9 kHz, analyzed at 50, 100, and 150 seconds.
STATISTICAL TESTS
One sample student's t-test to assess difference between computational and experimental results. Experimental vs. computational results were not significantly different (p < 0.05).
RESULTS
Maximum simulated temperature rise (10-minute exposure) was 10 °C at 1 kHz and 0.66 °C at 10 kHz. The ratio of the rise for 21 T/s vs. 42 T/s RMS was 4, after stabilizing at 50 seconds (dB/dt ratio squared).
DATA CONCLUSIONS
Heating of an implant is proportional to the frequency of the B field and the implant's cross-sectional area and is greater for a thickness on the order of its skin depth. Testing with lower values of dB/dt RMS with lower cost amplifiers enables prediction of heating at higher values for dB/dt squared (per ISO TS) with identical frequency components and waveforms, once thermal equilibrium occurs.
EVIDENCE LEVEL
1 TECHNICAL EFFICACY: Stage 1.
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