A dynamic time-domain model with improved reverse bias modeling performance suitable of modeling high speed switching in MRI coils with PIN diodes is presented. This model is easily integrated with previous forward bias charge control models and now allows full modeling of high speed, high frequency PIN diodes in time domain simulators. The model is verified with experimental data and good agreement was obtained. The model is then used in determining the dynamic switching characteristics of MRI coils.
Positive-intrinsic-negative (PIN) silicon diodes are commonly used in magnetic resonance imaging (MRI) coils to perform active or passive blocking and detuning, or to disable circuit functions. However, diode packages with large magnetic moments are known to cause image artifacts in high field MRI systems. In this study, diode packages with low magnetic moment were designed by compensating components of ferromagnetic nickel and paramagnetic tungsten with diamagnetic silver. The new diodes have an initial positive susceptibility up to fields of 1 T and a negative susceptibility from 1 to 7 T. Their magnetic moments are one to two orders of magnitude smaller than those of standard diodes; moments as small as 20 nJ/T at 7 T were achieved.
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