Purpose: Electron paramagnetic resonance (EPR) imaging techniques provide quantitative in vivo oxygen distribution images. Time-domain techniques including electron spin echo (ESE) imaging have been under study in recent years for their robustness and promising new features. One of the limitations of ESE imaging addressed here is the finite acquisition frequency bandwidth, which imposes limits on applied magnetic field gradients and the resulting image spatial resolution. In order to improve the image spatial resolution, we have extended the effective frequency bandwidth of the imaging system by acquiring projections at multiple Zeeman magnetic field offsets and combining them to restore complete projections obtained with more uniform frequency response, resulting in higher quality images.
Methods: In multiple-stepped magnetic field or multi-B scheme, every projection of the three dimensional object is acquired at different main or Zeeman magnetic field (B) offset values. The data from field offset steps are combined, normalizing to the imaging system frequency acquisition window function, a sensitivity profile, to restore the complete projection. A multipurpose pulse EPR imager and phantoms containing the same type of spin probe (OX063H) used in routine animal imaging were also used in this study.
Results: Using the multi-B method, we were able to acquire images of our phantoms with enhanced spatial resolution compared to the conventional ESE approach. Compared to standard single-B ESE images, the T2 resolutions of multi-B images were superior using a high spatial-resolution regime. Image artifacts present in high-gradient single-B ESE images are also substantially reduced using in the multi-B scheme.
Conclusions: The multi-B method is less susceptible to instrumental limitations for larger gradient fields and acquiring images with higher spatial resolution better overall quality, without the need to alter the existing pulse ESE image acquisition hardware.