Application of continuous‐wave EPR spectral‐spatial image reconstruction techniques for in vivo oxymetry: Comparison of projection reconstruction and constant‐time modalities

Abstract

In this study we report the application of continuous-wave (CW) electron paramagnetic resonance (EPR) constant-time spectral spatial imaging (CTSSI) for in vivo oxymetry. 2D and 3D SSI studies of a phantom and live mice were carried out using projection reconstruction (PR) and constant-time (CT) modalities using a CW-EPR spectrometer/imager operating at 300 MHz frequency. Distortion of line shape, which is inherent in the PR method, was minimized by the CTSSI modality. It was also found that CTSSI offers improved noise reduction, restores a smoother line shape, and gives high convergence of estimated values. Spatial resolution was also improved by CTSSI, although fundamental spectral line-width broadening was observed. Although additional corrections are required for accurate estimations of spectral line width, CTSSI was able to demonstrate distinct differences in oxygen tension between a tumor and the normal legs of a C3H mouse. The PR method, on the other hand, was unable to make such a distinction unequivocally with the triarylmethyl spin probes. CTSSI promises to be a more suitable method for quantitative in vivo oxymetric studies using radiofrequency EPR imaging (EPRI). Magn Reson Med 50:865-874,

Assessment of tumor oxygenation by electron paramagnetic resonance: principles and applications

“…119 The recent development of triarylmethyl radicals (Fig. 4) has significantly expanded the potential for using oxygen-sensitive free radicals in EPR, [156][157][158] EPR imaging 128,159,160 and related techniques such as dynamic nuclear polarization. [161][162][163] In human blood, the stability of triarylmethyl radicals varies from a half-life of a few hours to one of more than 24 h depending on the particular structure of the compound.…”

“…129 Spectral-spatial EPR imaging encodes both the spatial distribution of the spin probe and the spectral information, which allows mapping of molecular oxygen. 159,160,[191][192][193][194][195][196][197][198] For this purpose, the use of soluble EPR materials such as trityl radicals is more convenient as they can diffuse in the whole tissue. Compared with one-site and multisite EPR spectroscopy, there is a significant gain in terms of spatial information.…”

“…In several studies, measurements of tumor oxygenation using EPR have been carried out as a proof of principle in order to show that a specific method or a newly developed paramagnetic material was applicable for this purpose. 119,183,160,214,221,222 In other studies, EPR oximetry was compared with other methods that provide direct or indirect measurements of tumor oxygenation: comparison with polarographic electrodes, 163,223 the distribution of nitroimidazoles, 83 the BOLD effect in MRI, 107,159,224 and pO 2 recordings using OxyLite 1 . [66][67][68] More interestingly, the power of EPR oximetry to measure oxygen from the same site over long periods of time offers the possibility of developing novel approaches to modifying tumor oxygenation, and thus optimizing anti-cancer treatments.…”

“…Breathing carbogen (possibly combined with nicotinamide administration) is now applied succesfully in the clinic together with irradiation. 102 The increase in pO 2 in tumors induced by breathing carbogen was shown in several tumor lines using EPR spectroscopy, 107,176,183,224 EPR imaging 146,160,159 or PEDRI. 163 Oxygen images were obtained in tumors before and after breathing carbogen by the group at the NCI using dynamic nuclear polarization and the extraction of T 2 of trityl radicals.…”

Cardiac applications of EPR imaging

“…3,6 During the last few years significant progress has been made to enable the performance of EPR imaging of biological samples with improved image modality, resolution and quality to obtain useful information. [9][10][11][12][13][14][15][16][17][18] Over the last decade we have focused on developing instrumentation optimized for measurements of free radicals in isolated beating hearts, and in recent several years we have extended our efforts to develop hardware and software to enable spatial and spectral-spatial imaging of free radicals in the heart. 9,[19][20][21][22][23][24][25] This review provides a summary of the development and application of the imaging techniques with examples including imaging of stable free radical labels, oxygen concentrations and nitric oxide generation in the normal or ischemic heart.…”