FIG. 1.

Schematic drawing of theory of several EPRI modalities. (A) Spatial imaging using a back projection image reconstruction. The projection data are collected using rotating but a constant magnitude of field gradient. (B) Spectral–spatial 2D imaging in frequency domain. The projection data are collected using unidirection but incrementing magnitude of field gradient. G, magnitude of field gradient (Gauss/cm); ΔH and ΔL, spectral and spatial window width of the pseudo spectral–spatial matrix; SW, sweep width; α, viewing angle on the pseudo spectral–spatial matrix; H, magnetic field; L, spatial length. SW is varied depending on the α. Rotating field gradient direction achieves 3D or 4D imaging. (C) Spectral–spatial imaging in time domain. Data are collected using incrementing field gradient strengths and a constant time window (sweep width in frequency domain). Fourier transformation along G axis gives a spatial profile. With combinations of two or three orthogonal field gradient set, 3D or 4D imaging is available. EPRI, electron paramagnetic resonance imaging.