Generating MRI-based pseudo-CT images, providing electron density information for dose calculation, is the first step towards MRI-based radiation therapy treatment planning. Existing methods either require prior knowledge of a CT-MR atlas or require acquisition of multiple scans. In this study, we demonstrated the feasibility of producing pseudo-CT images by using a single multi-gradient-echo sequence. This method takes advantage of tissue-specific relaxation properties of MRI signal to provided segmentation of bone, air and other anatomical structures. Since all images are generated from a single scan and are naturally co-registered, this method is fast and avoids registration errors.