pH-sensitive imaging by low-frequency EPR: a model study for biological applications

Abstract

The use of pH-sensitive nitroxides, in conjunction with low-frequency EPR, offers a unique opportunity for non-invasive assessment of pH values (in the range 0 to 14) in living animals. In the present study, we have investigated the potential use of pH-sensitive nitroxide free radicals in conjunction with EPR imaging techniques at low and very low frequencies (280 MHz-2.1 GHz). In particular, we have measured the hyperfine splitting (hfs) of a pH-sensitive probe at three different EPR frequencies: 280 MHz, 1.1 GHz and 2.1 GHz. We have also developed EPR imaging experiments with phantoms simulating in vivo conditions, using pH-sensitive probes at 280 MHz (spatial-spatial) and 1.1 GHz (spectral-spatial). Finally, we discuss the actual sensitivity/resolution limits of the EPR imaging techniques at low frequencies. Practical applications of this method in the biomedical field are suggested for the continuous and non-invasive localization of pH in vivo.

Electron Spin Relaxation Time Measurements Using Radiofrequency Longitudinally Detected ESR and Application in Oximetry

“…ESR techniques can detect unpaired electrons of free radicals directly (1,2) and perform an indirect assessment of pO 2 , microviscosity, and pH in vivo by means of measurements of certain spectral parameters such as electron relaxation times and hyperfine constants (3,4). These applications are of particular clinical interest in providing noninvasive, repetitive, rapid, sensitive, and quantitative measurements (5).…”

“…The longitudinal magnetization (M z ) oscillation magnitude, corresponding to expression [4], as a function of detection frequency, , together with the real and imaginary components, derived from expression [2], for…”

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

“…Electron paramagnetic resonance imaging (EPRI) has been successfully applied to biological systems to quantify physiologic parameters, most importantly tissue oxygenation based on spectral linewidth imaging [1][2][3][4][5][6]. One can extract information about the local fluid environment from the EPR spectrum, which is obtained for each spatial voxel in a 4-D spectral-spatial image [2,7].…”

Object dependent sweep width reduction with spectral–spatial EPR imaging

“…Useful information concerning local fluid environment is reported in the EPR spectra. One can design a variety of spin probes to be sensitive to specific aspects of physiology including temperature, pH, microviscosity, tissue oxygenation, and a distribution of exogenous paramagnetic species [1][2][3][4][5][6]. For an application of EPRI to in vivo physiology it is important to complete data acquisition in minimum time.…”