Excessive presence of reactive oxygen species (ROS) induces oxidative stress and has been implicated in cardiovascular disease, neurological disorders, and cancer¹. This creates the need for non-invasive imaging methods for early and specific ROS detection before significant disease progression and permit early effective treatments.We developed a ROS-sensitive T1-shortening (contrast) agent for use with T1 mapping MRI. This agent consists of a mononuclear Mn(II) complex with the redox-active ligand N,N’-bis(2,5-dihydroxybenzyl)-N,N’-bis(2-pyridinylmethyl)-1,2-ethanediamine² (H4qtp2). We applied this H4qtp2 agent with cardiac MRI (CMR) T1 mapping in doxorubicin (Dox)-treated Sprague Dawley rats, where it is established that Dox-treatment induces congestive cardiac failure³. All rats in this study were treated under the guidelines of our institutional animal care and use committee. Two groups of rats as 1) normal Controls (n=3) and 2) Dox-treated (15 mg/kg, n=6) received CMR cine (for LV function) and CMR T1 mapping in a 7T human-size scanner (Siemens, Erlangen, Germany) with a birdcage small animal RF coil (Rapid, Columbus, OH). Customized in-house rodent CMR sequences were developed for this study. Fig 1 shows the sequence timing as an ECG-triggered, non-selective inversion Look-Locker, with an initial fixed TI at 10 ms followed by 9 more TI points spaced at the cardiac RR period (160-190 ms) for a total 10 TI points. Outer loop TR was 2400-2800 ms. This timing allowed for acquisition of 2 k-space lines per Look-Locker TI readout sample. Dox rats received CMR after 8 days of Dox-treatment. All rats first completed dark-blood cine to quantify LV function, then pre-H4qtp2 T1 mapping, then post-H4qtp2 T1 mapping 15-20 minutes after intravenous injection of H4qtp2 (10 mg/kg). As an extra T1 mapping test, the control group received intraperitoneal injected Gd-DO3A-butrol (Gadovist, 0.2 mmol/kg) followed by a 3rd T1 mapping scan. All CMR image T1 curve-fitting and statistical analysis for each group was performed on custom Matlab programs (Mathworks, Natick, MA) where regions of interest included the segmented LV myocardium (Myo) and chest wall skeletal muscle (SM) . Two-tailed t-tests were run between groups where p<0.05 was considered statistically significant. All statistical results are presented as group mean ± standard deviation.CMR cine functional analysis indicated the control group LV ejection fraction (LVEF) was 60.5±2.5 % and the Dox-treated group LVEF reduced to 50±2.8% (cine data not shown). Fig 2 presents example T1 maps and Fig 3 presents statistical summary graphs that show the Control group had no significant change of T1 in either myocardium (1.01±0.01 sec) or skeletal muscle (1.09±0.01 sec) after H4qtp2 injection. The Control group only showed a significant T1 change in both myocardium (0.76±0.02 sec) and skeletal muscle (0.92±0.03 sec) after Gd-DO3A-butrol injection. This gadolinium test confirmed the T1-mapping sequence was working properly. Likewise the Dox group pre-H4qtp2 T1 for myocardium (1.02±0.02 sec) and skeletal muscle (1.06±0.01 sec) was statistically similar to Control group pre-H4qtp2 T1 for myocardium (0.99±0.03 sec) and skeletal muscle (1.05±0.05 sec). Importantly, the Dox group post-H4qtp2 myocardium T1 (0.90±0.02 sec) was significantly reduced from the pre-H4qtp2 myocardium T1(1.02±0.02 sec) while the post-H4qtp2 skeletal muscle T1 remained statistically unchanged. Overall, these results show in the Dox-treated group only myocardium is under oxidative stress while skeletal muscle remained unaffected.This specialized application CMR T1-mapping with the H4qtp2 agent in Dox-treated rats demonstrated successful and specific detection of cardiac oxidative stress. The ability for the H4qtp2 agent to shorten the T1 in only the Dox-treated myocardium highlights its specificity to ROS presence. Our summary review of all the T1 maps indicated the T1 change in myocardium was global and not regional. This can be important since the quantitative nature of T1 mapping allows such global detection while conventional T1-weighted contrast MRI is better suited for visualizing regional T1 variation.In vivo T1 mapping cardiac MRI combined with a ROS-sensitive T1-shortening agent can specifically detect phamacologically-induced cardiac oxidative stress. The findings from this study warrant further investigation into expanded applications of H4qtp2 with T1 mapping in other oxidative stress disease models.