Multiple‐stepped Zeeman field offset method applied in acquiring enhanced resolution spin‐echo electron paramagnetic resonance images

Abstract

Purpose: Electron paramagnetic resonance ͑EPR͒ imaging techniques provide quantitative in vivo oxygen distribution images. Time-domain techniques including electron spin echo ͑ESE͒ 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 ͑B͒ 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 ͑OX063H͒ 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 T 2 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.

Frequency bandwidth extension by use of multiple Zeeman field offsets for electron spin-echo EPR oxygen imaging of large objects

“…17 Artifacts from frequency data truncation and noise amplification were shown to be minimized in this study as well. In this work, we describe how the multi-B technique can be applied to image objects with spatial dimensions comparable to those of human tumors.…”

“…17 Briefly, for each magnetic field gradient direction, or projection, the electron spin-echo detection is performed at multiple different Zeeman field offsets. The echo signal for each Zeeman field offset contains information from different regions of the projection in the frequency space.…”

“…In their overlap regions, the subprojections are combined in such a manner as to maximize the SNR at each frequency bin. 17,25 The result is the expansion of the effective imager AWF in frequency space, as illustrated in Fig. 2.…”

“…As explained by Epel et al 22 and Seifi et al, 17 for each image voxel, a least-squares exponential function was fitted to the five intensity values of each voxel obtained at five echo times. The exponential time constant was the estimated transverse magnetization relaxation time (T 2e ) within each image voxel.…”

Resonators for In Vivo Imaging: Practical Experience

“…The previous generation of resonators (Figure 5) used smaller 4 inch RF shields and resonators that could not be disassembled in the way shown in Figure 4 [38, 40]. The construction of the resonators was otherwise similar to split top resonators.…”