Does a normal cardiac cmr with an rvef of 50% and no apparent thickening rule-out arvd? is the npv high for this disease on cmr?

No. Need T2-sat & Gd: The signs of ARVD on cardiac MRI (CMR) are fatty infiltration of the right ventricle, dilation, & contrast enhancement. You mentioned none of these. While the RVEF is respectable, the condition of ARVD (replacement of muscle tissue by fat) requires a T2-weighted "black blood" spin sequence (SSFP) with Gadolinium (Gd) contrast. NPV of CMR is high with this pulse sequence. Talk to your Cardiologist.