No. They have different strengste and weaknesses. Roughly, CT is for bones and rough details of soft tissues, MRI for detailed views on soft tissue in patients without metal implants. More nuanced views at http://www.ct-scan-info.com/mrivsctscan.html, http://www.diffen.com/difference/CT_Scan_vs_MRI or http://blog.radiology.ucsf.edu/neuroradiology/exploring-the-...

For the discussion at hand, http://en.wikipedia.org/wiki/Magnetic_resonance_imaging#MRI_... has some discussion.