High-resolution and semidynamic vessel wall imaging kinetics obtained from stable radical MRI in ex-vivo porcine aorta
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.