**1. Introduction**

Magnetic resonance spectroscopic and imaging techniques are methods of choice for *in vivo* applications in animals and humans due to sufficient depth of microwave penetration in living tissues. Nuclear magnetic resonance, NMR, and its imaging modality, MRI, have found numerous biomedical and clinical applications but still suffer from limited functional resolution. Low intrinsic NMR sensitivity and overlap of the various endogenous NMR signals limit functional *in vivo* NMR and MRI applications beyond anatomical resolution. Electron paramagnetic resonance, EPR, has the advantage over NMR in functional specificity due to the absence of endogenous EPR signals but requires stable *in vivo* exogenous paramagnetic probes. Nitroxyl radicals, NRs, represent the most diverse class of stable organic radicals varying in stability, spectral properties and functionality which have been successfully used in numerous EPR spectroscopic and imaging applications. Synthesis of stable organic NRs (Lebedev & Kayanovskii, 1959; Neiman et al., 1962), with an unpaired electron localized at a sterically protected NO group ( <sup>0</sup> 0.6; 0.4 *<sup>N</sup>* ), revolutionized numerous areas of EPR applications. Half a century of continuous progress in nitroxide chemistry has resulted in the design of specific NRs for spin labeling (Berliner, 1998), sitedirected spin labeling (Hubbell et al., 2000), EPR oximetry (Halpern et al., 1994; Swartz, 2004), pH (Khramtsov et al., 1982, 2005), thiols (Khramtsov et al., 1989, 1997), redox (Swartz et al., 2007) and NO measurements (Akaike et al., 1993; Joseph et al., 1993; Woldman et al., 1994). Over the past decade low-field L-band (1.2 GHz) EPR spectrometers and imagers have become commercially available allowing for functional EPR measurements in isolated organs (Komarov et al., 2012) and small animals such as mice (Kuppusamy et al., 2002; Bobko et al., 2012). Moreover, continuous waves (CW) EPR (Halpern et al., 1994; He et al., 2002) and pulsed EPR (Yasui et al., 2010) instruments with lower radio frequencies (RF) down to 250 MHz have been constructed allowing for spectral-spatial functional imaging in

© 2012 Khramtsov, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Khramtsov, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

larger animals, and potentially in humans. Recently, a functional proton-electron doubleresonance imaging (PEDRI) approach (Khramtsov et al., 2010; Efimova et al., 2011) based on MRI detection with EPR excitation of paramagnetic probes at pre-selected EPR fields/frequencies, has been developed. The latter approach inherits high resolution, fast image acquisition and the availability of anatomical information from MRI techniques. This chapter overviews the recent applications of nitroxide probes for functional spectroscopy and imaging of living tissues.
