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Introduction: Excessive production of Reactive Oxygen Species (ROS) leads to homeostatic breakdown followed b y inammation and cell injury. Increased local ROS level is considered a marker of early stage of atherosclerosis. Nitroxides can react with ROS with conversion from paramagnetic to diamagnetic species and, thus, changing local T1-contrast accessible by MR-imaging. The redox-sensitive properties were explored in organs mostly in spectroscopic based studies due to relatively short t ime frame available before its reduction by endogenous ROS. M RI of vessel wall using nitroxide radicals might offer a non-invasive method of analyzing both underlying anatomical structure and pathophysiological changes caused by ROS-production in vasculature. Purpose: We established a protocol for high spatial and temporal resolution dynamic MRI of porcine aorta ex-vivo to visualize: 1) The distribution and diffusion of TEMPOL inside the vessel wall; 2) Kinetics of generated T1-contrast due to the conversion of TEMPOL to hydroxyamide stimulated by exogenously applied ascorbic acid to model of ROS overproduction. Methods: MRI measurements of porcine aorta were performed on a 7T Bruker preclinical MRI system using a TX/RX 1H-cryoprobe. Excised aortic tissue was kept in isotonic saline solution and MRI scans were performed at 2, 24, 48 and 72 hours post excision. 5mm thick rings of aorta were prepared just before the treatment with low (10 or 30mM) dose of TEMPOL. Subsequent treatment of TEMPOL-perfused probes by 5 to 20mM ascorbic acid demonstrated the possibility of visualizing change of the redox of TEMPOL inside the vascular wall. The incubation times were 180 sec and 30 sec at 37∘C for TEMPOL and ascorbic acid, respectively. The subsequent measurements were performed at ambient temperature. Protocols for maximizing spatial and temporal resolution were optimized by adjustment parameters of T1-weighted gradient (GRE) and spin-echo (RARE) sequences depending on time passed after excision and tissue treatment. Results: Fig. 1a shows an untreated aortic ring with native T1 tissue contrast and with 100mm in-plane spatial resolution. Exposure to TEMPOL initially results in rapid accumulation on the intima and adventitia (Fig. 1b) and concentration-gradient driven diffusion through all aortic layers. Subsequent treatment with ascorbic acid results in conversion of TEMPOL to diamagnetic hydroxyamide clearly observed dynamically by significant T1-contrast reduction in images as ascorbic acid diffuses through the vascular tissue layers (Fig 1c). Conclusion: High-resolution imaging protocol with 100mm spatial and 30 sec per image temporal resolution was established in ex-vivo porcine aorta using GRE and RARE pulse sequences. Successful proof-of-principle imaging of stable nitroxides as a T1 redox-sensitive contrast agent in porcine aorta provides the method for future in-vivo ROS imaging in vascular t issue. Organizational support by David Lohr is appreciated.

Introduction: In vivo diffusion tensor cardiac magnetic resonance (DT-CMR) has recently been used to characterize myocardial microstructure dynamics during left ventricular wall thickening, showing distinct, abnormal sheetlet function in HCM and DCM patients. However, the necessary trade-off between the number of averages, diffusion weighted directions, and image resolution in clinically used parameter settings have been shown to lead to poor precision of sheetlet and helix angles (HA) of±31∘ and±14∘ respectively. MRI at higher field strength such as 7T may provide higher signal-to-noise ratio (SNR), allowing for improvement of spatial and angular resolution, image quality as well as consistency of diffusion metrics in clinically applied parameter settings. Purpose: To assess feasibility of DT-CMR at 7T and compare SNR and diffusion metrics in spin echo based diffusion tensor acquisitions of the unfixed, ex vivo porcine heart at 7T and 3T. Methods: Measurements were performed on 7T and 3T whole body MRI systems using 1Tx/32Rx head coils. Porcine hearts were imaged in physiological saline solution, unfixed, at room temperature within 10 hours after cardiac arrest. MRI with spatial resolution of 1.3x1.3x1.3 mm3 was performed obtaining 25 and 8 cDTI datasets at 3T and 7T as a reference. Scans at 7T were repeated using parallel imaging acceleration (GRAPPA) factors R = 2, R = 3, R = 4. Apparent diffusion coefficient (ADC), fractional anisotropy (FA), HA and secondary eigenvector angle (E2A) were compared within segmentation according to the American Heart Association (AHA). Results: DTI data was successfully acquired for all parameter settings used. SNR values in b0 images of scans at 3T and 7T was 34±3 and 45±4 for identical scans. On average DTI analysis was performed for 6±2 slices of apical cap, 8±1 apical, 8±1 mid-cavity, and 8±1 basal slices. Maximal and minimal deviations of ADC [10-3mm2/s], FA, and |E2A| [∘] within the AHA segments were used to define a bias range for 7T acquisitions relative to the 3T reference. The smallest bias was found for scans using GRAPPA factor R = 2 (Figure 1). Derived metrics from scans using GRAPPA factor R = 3 showed significant differences in FA in mid-cavity and basal parts. No significant differences were found for scans using GRAPPA factor R = 2. Conclusion: Feasibility of DTI acquisitions of ex vivo porcine hearts was demonstrated for spin echo based acquisitions with spatial resolution of 1.3x1.3x1.3 mm3 via comparison with a reference data set measured at 3 T. Derived metrics were statistically similar to the 3T reference data using GRAPPA factors R = 2 and R = 3. The relative SNR gain in 7T acquisitions may benefit clinically used parameter settings for DT-CMR allowing optimization of the trade-off between averages, diffusion directions, and image resolution, especially, when using stimulated echo approaches, which can utilize the increased T1 at higher field strengths.