EPR imaging from projections: errors due to misalignment of projection centres and their rectification by a novel acquisition modality

Continuous wave and pulsed wave electron paramagnetic resonance imaging (EP RI) makes use of classical methods of acquisition of projections. Acquisiti on/reconstruction techniques, such as spin-echo, gradient-echo, etc, cannot be applied to EPRI because they would require very short switching times f or the gradient coils. Due to the use of the polar acquisition technique, i t is necessary to define a centre of rotation about which the measured proj ections are rotated during the reconstruction process. This centre represen ts the point at which the field gradient coils must produce zero magnetic f ield. Due to the presence of a magnetic field control system that serves to compensate for field variations, principally due to heating, some interfer ence can occur in the control system between the main magnetic field and th e magnetic field produced by the gradient coils. The effect changes as the orientation changes. This results in a shift of the centres of the projecti ons as a function of the variation of magnetic field produced by the gradie nt coils on the control Hall probe. If this condition is present, some arte facts can appear on the reconstructed image. This effect is irrelevant when EPR is used for imaging of paramagnetic probes whose linewidths are of the order of 10(-4) T, while it can be significant in the case of linewidths o f the order of 10(-5) T or lower or when EPR is used in microimaging applic ations (i.e. for high values of magnetic field gradient). We describe the e ffects that misalignments of the projections have on the reconstructed imag es. We present a useful method for estimating the real position of the cent re and correcting the measured projections before the application of the re construction algorithm. Moreover, we demonstrate the functioning of our tec hnique by presenting some examples of EPR reconstruction collected by an X- band EPR imaging apparatus.