Measurement of MR Gradient Effects in CZT Detectors Used in a SPECT/MRI System

The first stage of testing towards the further development of a full-ring SPECT/MR system has been evaluated. Performance of the cadmium zinc telluride (CZT) detectors were found to be unaffected by the static magnetic field, however they were found to react unreliably during simultaneous imaging when the MR sequence repetition time (TR) was less than 400ms and were influenced significantly by the imaging gradients. In an effort to better understand this phenomenon, component testing was performed by subjecting the CZT detector modules and electronics to various gradient sequences in a stand-alone gradient testing apparatus. We exposed the modules to 31 unique gradient sequence shapes of varying power and duration, typical of those used in clinical MR devices while simultaneously operating the CZT modules in event readout mode. Results indicate that certain detector pixels were preferentially affected (typically around the periphery of the detector array), leading to induced currents in the detector material and significant increases in erroneous event rates during the gradient ramp-up and ramp-down phases. The shape of the gradient waveform was also seen to have an effect on the detector performance with higher slew rates leading to impaired performance. Hardware disabling of the most severely affected pixels led to some reduction in erroneous background events, however, it also resulted in reduced detector resolution and sensitivity. Subsequent work led to the development of a gradient triggering circuit to momentarily suspend CZT event detection during the MR gradient ramp up and ramp down intervals. Subsequent testing of this circuit in a clinical 3T MR was performed. When gradient triggering was enabled, erroneous event data was reduced to near zero levels and resultant images show no structural differences when compared to control acquisitions. During gradient application, the CZT modules are suspended, thus yielding a 60% drop in sensitivity due to the additional deadtime of the triggering. The produced MR image showed a reduction in SNR of 92%. Co-registration of the MR and gamma camera images was successfully performed, showing both the structural and activity makeup of the phantom.