We have developed a novel class of protein based contrast agents (ProCAs) for MRI by de novo design of Gd binding site(s) into a stable host protein with significantly improved MRI relaxivity at clinically used field strengths. We have recently discovered that modification of protein contrast agents leads to further increase of r1 and r2 relaxivities and a 100-fold improvement in in vivo dose efficiency in mouse models while reducing immunogenicity. Furthermore, we have designed several MRI contrast agents that specifically target EGFR, HER2/Neu, and GRPRs in tumor cells and xenograft mice models. These new MRI probes exhibit disease bio-marker dependent imaging contrast enhancement and desirable penetration of tumor tissue and the endothelial boundary, which is a significant improvement in clearance and targeting than previously reported albumin or antibody crosslinked with Gd-DTPA. our protein-based MRI-CAs provide improved MRI diagnostics, offering enhanced safety and sensitivity, expanded molecular imaging of biomarker changes, and numerous opportunities to target and identify specific disease tissues to guide the treatment of patients and minimize the unnecessary surgical/treatment procedures with associated cost and toxicity.