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A thermal model and simulation program for determining the probability of exceeding thermal limits on surfaces of magnetic resonance imaging equipment / by Michael J. Bernico.

by Bernico, Michael J.

Diagnostic imaging -- Digital techniques

Heat -- Transmission -- Measurement

Monte Carlo method -- Computer programs.

Quality control -- Statistical methods

240 leaves : ill. ; 29 cm.

Advisor(s): Dr. Kishore Acharya, Dr. Lisa Milkowski, Dr. Vincent Canino.

Background and primer -- Project objectives -- Site selection process -- Site qualification process -- Automated data collection development -- Data extraction process -- Basic operating statistics -- Duty cycle analysis -- MRI procedure profiles -- Scrutinization of data -- Waveform simulations -- Justification of approach to simulation -- Scan and power calculations -- Development and verification of thermal model of system -- Development of simulation program -- Simulations -- Probability and confidence interval development -- Summary -- Conclusions -- References -- Appendix A) Program listings B) Idle time histograms C) PSD summary tables D) Class2 site PSD power summary tables E) Six Sigma scorecard spreadsheets F) Critical scan parameters.

The purpose of this project was to investigate a typical engineering problem by developing a thermal model for predicting the surface temperature for a subsystem (the patient opening) of a Magnetic Resonance Imaging (MRI) system. The inputs to the model were a randomized series of high current switching waveforms that were derived from a statistical analysis of the daily imaging sequences performed at an MRI imaging clinic. The purpose of the investigation was to understand if existing limits for a system safety parameter could be exceeded. The safety parameter that was examined is the average surface temperature of the patient bore of the MRI system.

The methodology that was employed for this project was the collection and statistical analysis of operational logs from several MRI sites, and the development of a Monte Carlo type simulation for the heat inputs generated from the operation of the MRI scanner. These inputs were then fed to a block simulation model, which in turn predicts the MRI bore temperature. For completeness, two simulations were run simultaneously to reflect both actual operating conditions (power input and intersequence delays), and worst-case conditions (power inputs without intersequence delays).

The end result of this simulation and analysis was the determination that, in the absence of other mitigating factors (heating due to parasitic currents in antenna structures), the MRI system under study, operating at specified ambient temperature conditions, can tolerate an increase of 1.5kW and 2.0kW average power before the risk of exceeding the thermal limits for the surface of the patient bore exceeds 1 failure per 10 years of operation. Ten years is the estimated life cycle of the MRI system. During that time, a scanner is expected to perform an average of approximately 365,000 scans.

In addition to this simulation and analysis, the model developed in this project will be utilized to evaluate future modifications to the MRI system. For example, determining the effects of increasing gradient RMS current limits, or evaluating the thermal effect of new imaging sequences prior to deployment to Customers. The use of this model will ensure patient safety, and compliance with regulatory agency guidelines.

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