Myelin volume measurement based on rapid-simultaneous-relaxometry-imaging (RSRI) is useful for the clinic. However, the software that translates RSRI into the index map contains a potential weak point: the index is obtained using a pixelwise table-lookup that doesn't incorporate information from surrounding pixels. A novel deep-learning-based method was developed to overcome this problem. A myelin volume index based on magnetization-transfer saturation imaging was used as a reference for training and testing. The proposed method was evaluated by leave-one-out cross-validation using images from 20 healthy volunteers. Correlation with the reference was significantly higher for the proposed method.