Single-point (constant-time) imaging in radiofrequency Fourier transform electron paramagnetic resonance

This study describes the use of the single-point imaging (SPI) modality, also known as constant-time imaging (CTI), in radiofrequency (RF) Fourier transform (FT) electron paramagnetic resonance (EPR). The SPI technique, commonly used for high-resolution solid-state nuclear magnetic resonance (NMR) imaging, has been successfully applied to 2D and 3D RF-FT-EPR imaging of phantoms containing narrow-line EPR spin probes. The SPI scheme is essentially a phase-encoding technique that operates by acquiring a single data point in the free induction decay (FID) after a fixed delay (phase-encoding time), following the pulsed RF excitation, in the presence of static magnetic field gradients. Since the phase-encoding time remains constant for a given image data set, the spectral information is automatically deconvolved, providing well-resolved pure spatial images. Therefore, images obtained using SPI are artifact-free and the resolution is not significantly limited by the line width, compared to the images obtained using the conventional filtered back-projection (FBP) scheme, suggesting that the SPI modality may have advantages for EPR imaging of large objects. In this work the advantages and limitations of SPI as compared to FBP are investigated by imaging suitable phantom objects. Although SPI takes longer to perform than the FBP method, optimization of the data collection scheme may increase the temporal resolution, rendering this technique suitable for in vivo studies. Spectral information can also be extracted from a series of SPI images that are generated as a function of the delay from the excitation pulse.