Paramagnetic nitroxyl radicals have been proposed as a redox sensitive MR contrast agent. A time-sequence of the T1-weighted MR images at 4.7 T showed a readily visible enhancement of image intensity in mice due to paramagnetic nitroxyl contrast agent and then sequentially showed gradual decrement of the image enhancement level. The dose 1.5 umol/g b.w. of the carbamoyl-PROXYL, which was a nitroxyl contrast agent used in the previous report, was enough lower than its maximum tolerated dose (MTD) level for mice. However, the dose 1.5 umol/g b.w. is near to the MTD of TEMPOL for mice, which is reported as 275 mg/kg b.w. (1.6 umol/g b.w.). In addition the dose 1.5 umol/g b.w. is lager than dose levels of common medicines (< 1 umol/g b.w.). Although the dose of the nitroxyl contrast agent used in future clinical trial should be decreased, the relatively small T1-relaxivity of the nitroxyl radicals (= 0.2 mM-1s-1) makes difficult to detect low concentration levels. Higher magnetic field, such as 7 T, may increase the percentage of signal enhancement and the signal to noise ratio (SNR), while most clinical machines were worked at lower magnetic field, such as 3 T or 1.5 T. To obtain enough detectable T1-image enhancements quantitatively using lower dose, modification of T1-relaxivity of the nitroxyl based contrast agents was tested. In this paper, the several nitroxyl contrast agents which have different number of spins in the molecule were chemically synthesized. The relaxivity of those nitroxyl contrast agents were measured. The suitable concentration range gave a linear relation with the image enhancement was observed for each nitroxyl contrast agent. The availability of the multi-spin nitroxyl contrast agent for MR-based redox imaging was described. The T1-relaxivity of nitroxyl contrast agents were increased depending on the number of spins on the molecule. The suitable linear relation between image enhancement and concentration of the contrast agent was obtained for a nitroxyl contrast agent in this paper.SFRBM\u27s 13th Annual Meetin