In clinical practice, brain SPECT imaging is often performed using fan beam or parallel beam collimators. The resolution of such SPECT systems is typically about 7-10 mm in the center of the field-of-view (FOV) and their sensitivity in the range of 2.10-4 - 4.5.10-4 cps/Bq, which is rather low. Compared with SPECT/CT, the combination of SPECT with MRI could provide superior soft tissue contrast and avoid radiation dose to obtain anatomical data. Therefore, we designed a SPECT insert for MRI based on high-resolution dSiPM detectors. We chose for a full-ring geometry using a multi-pinhole collimator to increase sensitivity and to allow for complete transverse sampling, without the need of rotation. We optimized the different parameters of the SPECT system to maximize the volume sensitivity for a target spatial resolution of 7.2 mm in the center of the brain. We obtained a sensitivity of 7.89e-04 cps/Bq which is 1.6 times the sensitivity of a triple head SPECT system with fan beam collimator with the same resolution and FOV. We also performed noiseless simulations of a cold rod and Defrise phantom and were able to reconstruct them without artifacts. In conclusion, we succeeded in designing a multi-pinhole collimator for brain SPECT imaging that is MRI compatible and has the potential of having improved sensitivity and resolution compared to clinical SPECT systems.