In vivo multisite oximetry using EPR–NMR coimaging

Abstract

Coimaging employing electron paramagnetic resonance (EPR) imaging and MRI is used for rapid in vivo oximetry conducted simultaneously across multiple organs of a mouse. A recently developed hybrid EPR-NMR coimaging instrument is used for both EPR and NMR measurements. Oxygen sensitive particulate EPR probe is implanted in small localized pockets, called sites, across multiple regions of a live mouse. Three dimensional MRI is used to generate anatomic visualization, providing precise locations of implant sites. The pO2 values, one for every site, are then estimated from EPR measurements. To account for radio frequency (RF) phase inhomogeneities inside a large resonator carrying a lossy sample, a generalization of an existing EPR data model is proposed. Utilization of known spectral lineshape, sparse distribution, and known site locations reduce the EPR data collection by more than an order of magnitude over a conventional spectral-spatial imaging, enhancing the feasibility of in vivo EPR oximetry for clinically relevant models.

Uniform spinning sampling gradient electron paramagnetic resonance imaging

“…Since the Z component of the projections starts at zero and monotonically increases, the projections nearest the X‐Y plane get slightly uneven weighting. It is not possible to design a distribution of points which fulfills all of the following criteria simultaneously: a continuous first derivative of the magnetic gradients, uniform distribution on a hemisphere, and a uniform distribution in the X‐Y plane. Relaxing the first criterion would produce a distribution of points that is difficult to accurately realize on real magnetic gradients because there will be discontinuities after each rotation around the Z pole.…”

“…For example, Eqs. [1] and [3] can be evaluated for N ¼ 10,000 projections, and then the nonuniformity only occurs in the first 2.5% of projections in the acquisition. USS is well-defined for any number of projections, and data acquisitions with thousands of projections are easily realized with the spinning gradient acquisition, which is only limited by the sampling rate of the data acquisition cards and the gradient slew rate.…”

“…Both ESS and USS start in the X-Y plane and spiral upward toward the Z axis by definition in Eq. [1]. ESS takes the following form:…”

Spectral modeling for accelerated pH spectroscopy using EPR

“…Measure and process the in vivo data as if there were no frequency drift. Also, a small uncertainty in the location of the resonance peak, if present despite the proposed measures, can be tolerated by including it into the model as we have previously demonstrated for oximetry application [36]. …”

Multisite EPR oximetry from multiple quadrature harmonics

“…The application of the forward model described above requires the knowledge of site locations which are generally not known in advance except for special cases, such as NMR-EPR coimaging [26], where high resolution MRI can provide the location of EPR sites. Here, we adopt a two-stage approach with coherent and incoherent variations at stage 1.…”