This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Signal loss in the course of an imaging experiment was determined earlier this year to be caused by resonant frequency shift of the ESRM probe dielectric resonator with temperature increase due to power dissipation the gradient coils while acquiring the image. The frequency shift of the at 15GHz is approximately ~10MHz/? C, giving, for a resonator Q factor of ~500 and bandwidth of ~30MHz, a required temperature stability of ?1.5 ? C. When very intense current pulses are applied to the co-located X, Y and Z gradient coils, the coil temperatures can increase dramatically in a short time, raising the temperature of the resonator within them and causing loss of the echo signal by a factor of 3 or more. For sub-micron imaging without ancillary cooling, the signal averaging repetition rate must be reduced to 1KHz or less to avoid heating problems due to high peak gradient currents. With proper cooling, however, the same experiment can be run at 20KHz, a significant reduction of collection time for the requisite signal averaging. To address this heat removal need, we constructed a cooling system that directs cold, dry gas through the imaging probe, with the gas flow controlled by a process controller (Omega, Inc.) that monitors the resonator temperature. Another controller/alarm unit monitors a thermocouple reporting gradient coil temperature, with both software threshold and backup hardware inhibit of the gradient driver current pulses in case the coil set temperature exceeds the alarm value. With this temperature control addition, we have determined that the ESRM imaging experiments can be reliably performed at resonator temperature setpoints from -15C to 20 C .
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