Back projection reconstruction of spectroscopic NMR images from incomplete sets of projections

Continuous wave electron paramagnetic resonance imaging for in vivo mapping of spin distribution and spectral shape requires rapid data acquisition. A spectral-spatial imaging technique is presented that provides an order of magnitude reduction in acquisition time, compared to iterative tomographic reprojection. The proposed approach assumes that spectral shapes in the sample are wellapproximated by members from a parametric family of functions. A model is developed for the spectra measured with magnetic field modulation. Parameters defining the spin distribution and spectral shapes are then determined directly from the measurements using maximum a posteriori probability estimation. The approach does not suffer approximation error from limited sweep width of the main magnetic field and explicitly incorporates the variability in signal-to-noise ratio versus strength of magnetic field gradient. The processing technique is experimentally demonstrated on a one-dimensional phantom containing a nitroxide spin label with constant g-factor. Using an L-band EPR spectrometer, spectral shapes and spin distribution are accurately recovered from two projections and a spectral window which is comparable to the maximum linewidth of the sample.

Section: Resultsmentioning

confidence: 99%

Section: Comparison With Projection-reprojection Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A parametric approach to spectral–spatial EPR imaging

Som

Potter

Ahmad

et al. 2007

“…The relative magnitudes of field gradient and sweep width define a pseudo viewing angle for a projection measured in the spectral-spatial space. Introduced for NMR [25,26], continuous-wave (CW) spectral-spatial imaging was applied to EPR in two dimensions (one spectral, one spatial) [27][28][29]. In vivo 2D spectral-spatial EPR imaging was demonstrated at 250 MHz with curve fitting used as postprocessing to estimate spectral parameters [6].…”

Section: Epr Oximetry Techniquesmentioning

confidence: 99%

EPR oximetry in three spatial dimensions using sparse spin distribution

Som

Potter

Ahmad

et al. 2008

A method is presented to use continuous wave electron paramagnetic resonance imaging for rapid measurement of oxygen partial pressure in three spatial dimensions. A particulate paramagnetic probe is employed to create a sparse distribution of spins in a volume of interest. Information encoding location and spectral linewidth is collected by varying the spatial orientation and strength of an applied magnetic gradient field. Data processing exploits the spatial sparseness of spins to detect voxels with nonzero spin and to estimate the spectral linewidth for those voxels. The parsimonious representation of spin locations and linewidths permits an order of magnitude reduction in data acquisition time, compared to four-dimensional tomographic reconstruction using traditional spectral-spatial imaging. The proposed oximetry method is experimentally demonstrated for a lithium octa-n-butoxy naphthalocyanine (LiNc-BuO) probe using an L-band EPR spectrometer.

“…Although there are alternative reconstruction techniques [36] that can handle this problem seamlessly, the computation cost associated with such techniques especially for 4D render them impractical. For FBP, a few techniques such as projection-reprojection [37] have been proposed to handle this problem. Therefore, one simple solution would be to estimate the Fekete distribution over S 3 , remove the data points appearing in the "forbidden" region, and perform the reconstruction using projection-reprojection.…”

Section: Missing Angle Problem and Single-stage Reconstructionmentioning

confidence: 99%

Uniform distribution of projection data for improved reconstruction quality of 4D EPR imaging

Ahmad¹,

Vikram²,

Clymer³

et al. 2007

In continuous wave (CW) electron paramagnetic resonance imaging (EPRI), high quality of reconstruction in a limited acquisition time is a high priority. It has been shown for the case of 3D EPRI, that a uniform distribution of the projection data generally enhances reconstruction quality. In this work, we have suggested two data acquisition techniques for which the gradient orientations are more evenly distributed over the 4D acquisition space as compared to the existing methods. The first sampling technique is based on equal solid angle partitioning of 4D space, while the second technique is based on Fekete points estimation in 4D to generate a more uniform distribution of data. After acquisition, filtered backprojection (FBP) is applied to carryout the reconstruction in a single stage. The single-stage reconstruction improves the spatial resolution by eliminating the necessity of data interpolation in multi-stage reconstructions. For the proposed data distributions, the simulations and experimental results indicate a higher fidelity to the true object configuration. Using the uniform distribution, we expect about 50% reduction in the acquisition time over the traditional method of equal linear angle acquisition.