In vivo decay rates of a
nitroxyl contrast agent were estimated by a MR redox imaging (MRRI) technique and compared with the decay rates obtained by the electron paramagnetic resonance spectroscopy (EPRS) and imaging (EPRI). MRRI is a dynamic imaging technique employing T1-weighted pulse sequence, which can visualise a
nitroxyl-induced enhancement of signal intensity by T1-weighted contrast. EPR techniques can directly measure the paramagnetic
nitroxyl radical. Both the
squamous cell carcinoma (SCC) tumour-bearing and normal legs of a female C3H mouse were scanned by T1-weighted SPGR sequence at 4.7 T with the
nitroxyl radical, carbamoyl-proxyl (
CmP), as the
contrast agent. Similarly, the time course of
CmP in normal muscle and tumour tissues was obtained using a 700-MHz EPR spectrometer with a surface coil. The time course imaging of
CmP was also performed by 300 MHz CW EPR imager. EPRS and EPRI gave slower decay rates of
CmP compared to the MRRI. Relatively slow decay rate at peripheral region of the tumour tissues, which was found in the image obtained by MRRI, may contribute to the slower decay rates observed by EPRS and/or the EPRI measurements. To reliably determine the tissue redox status from the reduction rates of nitroxyls such as
CmP, heterogenic structure in the tumour tissue must be considered. The high spatial and temporal resolution of T1-weighted MRI and the T1-enhancing capabilities of nitroxyls support the use of this method to map tissue redox status which can be a useful
biomarker to guide appropriate treatments based on the tumour microenvironment.