EPR imaging in biological applications: Towards microtomography

Abstract

EPR imaging and microtomography promises to be a powerful tool for the EPR spectroscopist, but the expansion of this technique was hindered by the lack of commercial apparatus. A simple modification of an existing commercial system that can reach a resolution of the order of 75 pm with a gradient of 150 mT an-' is described. This resolution can he increased by using larger field gradients. In many biological applications this resolution is signscant. As an example, the distribution of a paramaguetic probe in an amphibious egg cell is presented that permits one to distinguish the nucleus of the cell. A summary of the mathematical techniques which enable one to account for the presence of a hyperfine structure in most EPR paramagnetic probes and to increase resolution is also presented.

Bioimaging of Nitric Oxide

“…EPR imaging is considered to be the most effective technique available for noninvasive observation of the spatial distribution of free radicals. , In vivo EPR imagings were first performed using an exogenous, synthetic, and stable free radical which was locally administered to small animals. Early in vivo direct EPR spin-trapping detection of bioradicals was difficult, − and subsequent improvement of low-frequency EPR instrumentation increased the feasibility of successful in vivo EPR imaging of bioradicals . The NO adducts of the Fe−DTC traps are very stable in biological media, and the first in vivo EPR detection and imaging of endogenous NO was successfully accomplished …”

“…In section II, we will review the progress and current status of in vitro and in vivo detection and imaging of NO in biological systems by EPR and related techniques. For more information on EPR spectrometry for various bioradicals, several general reviews and books are available. − ,,,− …”

“…Berliner et al reported the first EPR imaging of a biologically relevant sample as a one-dimensional (1-D) image of a celery stem soaked in a solution of a stable nitroxide radical, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-yloxyl (TEMPOL) . Since then, many in vivo EPR imaging experiments have been successfully carried out in various localized regions of small animals and have shed light on the spatial distribution and pharmacokinetics of exogenously supplied stable radicals such as TEMPOL, 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-yloxyl (carbamoyl-PROXYL), and their analogues. − Furthermore, the oxygen induces the broadening of EPR signals of stable nitroxides and the other paramagnetic agents. Therefore, the application of in vivo EPR spectroscopy to the measurement of concentration or partial pressure of oxygen in tissues has been extensively developed as a promising approach to oxymetry. − This technique has a potential for noninvasive measurement of tissue oxygen status.…”

First observation of very long-lasting emissive polarization in EPR spectra of Fremy's salt when reduced by ascorbic acid. An unusual aspect of the radical pair mechanism

“…Fremy's salt is easily detected by EPR spectroscopy and in solution it shows the typical three line of the nitroxide spectrum. It has been used in many studies on biological systems such as in EPR imaging, 3 in evaluating the radical scavenging activity 4 and in EPR kinetic studies. 1,5,6 In biological systems, nitroxides are reduced by various biological agents, 7 but the reaction is known to be relatively slow compared to that mediated by ascorbic acid.…”

Spatially uniform sampling in 4-D EPR spectral-spatial imaging

“…The extremely short relaxation time of the electron spin system makes the continuous wave (CW) EPRI technique accessible for most applications. Filtered back-projection (FBP) is commonly used for image reconstruction in CW EPRI [8][9][10][11][12].…”