This paper presents the basic design of a dual-mode readout system for MR-Compatible ultrahigh resolution SPECT/PET system, especially the design of a cathode waveform sampling and processing unit. This effort was motivated by the development of a stationary CdTe detector based MR-Compatible SPECT system. In our previous study, this CdTe detector shows around 3kev energy resolution around 140kev for SPECT imaging and 500μm spatial resolution with a prototype system inside a 3T Siemens MR scanner. With MR compatible rapid-prototyping made polyamide housing and proper compress air system cooling design, we expect this multi-head ring detectors can work well inside/outside MR scanner to serve as an ultrahigh resolution SPECT system. In addition, the ultrahigh resolution CdTe detectors used in the MRC-SPECT system also has the potential of serving as an ultrahigh resolution PET system. The original readout system designed for this detector only reads out anode pixels using four 2-D energy-resolved photon-counting (ERPC) ASICs. If we want to push this system into PET use, we would need to add another readout system that reads out and processes the cathode waveform. The added waveform sampling circuitry could provide a precise timing and depth-of-interaction (DOI), which are cortically needed for high resolution PET imaging. The cathode readout system consists of four low-noise pre-amplifiers connecting to individual cathodes of the detector and a fast sampling AD converter is used to digitize the signal waveforms (WFs) induced on the cathodes. Digitized data will be stored in a group of ring-buffers on a FPGA which also implements a lookup table for storing a series of pre-determined WF models. Precise interaction time, DOI and energy-deposition are achieved by comparing the sampled WF with the pre-stored WF models and finding the best fit. By integrating these two readout systems, we can choose this ring detection system to work either in MR/SPECT or MR/PET mode.