New spectral-spatial imaging algorithm for full EPR spectra of multiline nitroxides and pH sensitive trityl radicals

Abstract

An algorithm is derived and demonstrated that reconstructs an EPR spectral-spatial image from projections with arbitrarily selected gradients. This approach permits imaging wide spectra without the use of the very large sweep widths and gradients that would be required for spectral-spatial imaging with filtered back projection reconstruction. Each projection is defined as the sum of contributions at the set of locations in the object. At each location gradients shift the spectra in the magnetic field domain, which is equivalent to a phase change in the Fourier-conjugate frequency domain. This permits solution of the problem in the frequency domain. The method was demonstrated for 2D images of phantoms consisting of (i) two tubes containing (14)N and (15)N nitroxide and (ii) two tubes containing a pH sensitive trityl radical at pH 7.0 and 7.2. In each case spectral slices through the image agree well with the full spectra obtained in the absence of gradient.

Keywords: Magnetic field gradients; Nitroxide; Rapid scan EPR; Spectral–spatial imaging; pH sensitive trityl.

Fig. 1

Schematic of the geometry for the imaging experiment and the spectral shift at location k.

Fig. 2

Geometry of the phantom.

Fig. 3

(a) 2D spectral-spatial image of phantom 1 consisting of ¹⁴N and ¹⁵N PDT. (b) Comparison of spectral slices (¹⁴N in green, ¹⁵N in blue) through the image with the zero-gradient spectrum (dashed red).

Fig. 4

(a) 2D spectral-spatial image of phantom 2 consisting of pH=7.0 and pH=7.2 aTAM₄ radicals. (b) Comparison of the spectral slice for pH=7.2 (blue) with the zero-gradient spectrum (dashed green). (c) Comparison of pH=7.0 slice (blue) with the zero-gradient spectrum (dashed green)