1,721,006 research outputs found
Joint spectral-polarimetric analysis of accelerated hot star wind transient
No full textTransient stellar mass loss has already been studied as an inverse problem by Brown and Wood (1994) and by Calvini, Bertero and Brown (1995), in the hypothesis of a uniform flow speed nu (r)= nu 0. Here we consider a generalization of this inverse problem to an accelerated wind profile based on an empirical form for nu (r) with an adjustable acceleration parameter c0. We deduce the two generalized convolution equations linking the time ( tau ) evolution of the equatorial mass loss rate m( tau ) and the asphericity 'shape function' u( tau ) to the observed polarization and absorption line strength variations in time. In order to perform the regularized inversion of these equations, we introduce an iterative algorithm which allows us to take into account the positivity constraint on the solutions. This method is tested on simulated data for various choices of hypothetical, m( tau ) and a( tau ) and the results are compared with those provided by Fourier deconvolution, for several values of the parameter c0. It is found that the iterative algorithm is more efficient than the Fourier one and that, for both techniques, recoveries are less good for finite c0 (accelerating wind) than for c0 to infinity (impulsive acceleration at the stellar surface and then steady wind speed) due to slower sampling of the important inner-wind region
Fusion of the MR image to SPECT with possible correction for partial volume effect.
No full textABSTRACT—Low spatial resolution and the related partial volume effects limit the diagnostic potential of brain single photon emission computed tomography (SPECT) imaging. As a possible remedy for this problem we propose a technique for the fusion of SPECT and MR images, which requires for a given patient the SPECT data and the T1-weighted MR image. Basically, after the reconstruction and coregistration steps, the high-frequency part of the MR, which would be unrecoverable by the set SPECT acquisition system +
reconstruction algorithm, is extracted and added to the SPECT image. The tuning of the weight of the MR on the resulting fused image can be performed very quickly, any iterative reconstruction algorithm can be used and, in the case that the SPECT projections are not available, the proposed technique can also be applied directly to the SPECT image, provided that the performance of the scanner is known. The procedure has the potential of increasing the diagnostic value of a SPECT image. Even in the locations of SPECT-MR mismatch it does not significantly affect quantitation over regions of interest (ROIs) whose dimensions are decidedly larger than the SPECT resolution distance. On the other hand, appreciable corrections for partial volume effects are expected in the locations where the contrast in the structural MR matches the corresponding contrast in functional activity
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