Rosemary A. Renaut

Higher-order finite-difference schemes for electromagnetic radiation, scattering, and penetration .2. Applications

For pt.1 see ibid., vol.44, no.1, p.134-42 (2002). Higher-order schemes for the finite-difference time-domain (FDTD) method - in particular, a second-order-in-time, fourth-order-in-space method, FDTD(2,4) - are applied to a number of problems. The problems include array analysis, cavity resonances, antenna coupling, and shielding effectiveness case studies. The latter includes a simplified model of a commercial airliner, with a personal electronic device operating in the vicinity of the aircraft. The FDTD computations are also compared to measured data for this case. Incorporating PEC and other types of material boundaries into higher-order FDTD is problematic; a hybrid approach using the standard FDTD method in the proximity of the boundary is proposed, and shown to perform well.

Clustering huge data sets for parametric PET imaging.

A new preprocessing clustering technique for quantification of kinetic PET data is presented. A two-stage clustering process, which combines a precluster and a classic hierarchical cluster analysis, provides data which are clustered according to a distance measure between time activity curves (TACs). The resulting clustered mean TACs can be used directly for estimation of kinetic parameters at the cluster level, or to span a vector space that is used for subsequent estimation of voxel level kinetics. The introduction of preclustering significantly reduces the overall time for clustering of multiframe kinetic data. The efficiency and superiority of the preclustering scheme combined with thresholding is validated by comparison of the results for clustering both with and without preclustering for FDG-PET brain data of 13 healthy subjects.

Higher order absorbing boundary conditions for the finite-difference time-domain method

Higher-order absorbing boundary conditions are introduced and implemented in a finite-difference time-domain (FDTD) computer code. Reflections caused by the absorbing boundary conditions are examined. For the case of a point source radiating in a finite computational domain, it is shown that the error decreases as the order of approximation of the absorbing boundary condition increases. Fifth-order approximation reduces the normalized reflections to less than 0.2%, whereas the widely used second-order approximation produces about 3% reflections. A method for easy implementation of any order approximation is also presented. >

An automated algorithm for the computation of brain volume change from sequential MRIs using an iterative principal component analysis and its evaluation for the assessment of whole-brain atrophy rates in patients with probable Alzheimer's disease

This article introduces an automated method for the computation of changes in brain volume from sequential magnetic resonance images (MRIs) using an iterative principal component analysis (IPCA) and demonstrates its ability to characterize whole-brain atrophy rates in patients with Alzheimers disease (AD). The IPCA considers the voxel intensity pairs from coregistered MRIs and identifies those pairs a sufficiently large distance away from the iteratively determined PCA major axis. Analyses of simulated and real MRI data support the underlying assumption of a linear relationship in paired voxel intensities, identify an outlier distance threshold that optimizes the trade-off between sensitivity and specificity in the detection of small volume changes while accounting for global intensity changes, and demonstrate an ability to detect changes as small as 0.04% of brain volume without confounding effects of between-scan shifts in voxel intensity. In eight patients with probable AD and eight age-matched normal control subjects, the IPCA was comparable to the established but partly manual digital subtraction (DS) method in characterizing annual rates of whole-brain atrophy: resulting rates were correlated (Spearman rank correlation = 0.94, P < 0.0005) and comparable in distinguishing probable AD from normal aging (IPCA-detected atrophy rates: 2.17 +/- 0.52% per year in the patients vs. 0.41 +/- 0.22% per year in the controls [Wilcoxon-Mann-Whitney test P = 7.8 x 10(-4)]; DS-detected atrophy rates: 3.51 +/- 1.31% per year in the patients vs. 0.48 +/- 0.29% per year in the controls [P = 7.8 x 10(-4)]). The IPCA could be used in tracking the progression of AD, evaluating the disease-modifying effects of putative treatments, and investigating the course of other normal and pathological changes in brain morphology.

Improved Total Variation-Type Regularization Using Higher Order Edge Detectors

We present a novel deconvolution approach for accurately restoring piecewise smooth signals from blurred data. There are two separate stages. The first stage uses higher order total variation (TV) restorations to obtain an estimate of the location of jump discontinuities from the blurred data. In the second stage the estimated jump locations are used to determine the local orders of a variable order TV restoration. The method replaces the first order derivative approximation used in standard TV by a variable order derivative operator. Smooth segments as well as jump discontinuities are restored, while the staircase effect typical for standard first order TV regularization is avoided. Compared to first order TV, signal restorations are more accurate representations of the true signal, as measured in a relative

Improving tissue segmentation of human brain MRI through preprocessing by the Gegenbauer reconstruction method

l^2

Characterization of the image-derived carotid artery input function using independent component analysis for the quantitation of [18F] fluorodeoxyglucose positron emission tomography images*

-norm. The method can also be used to obtain an accurate estimation of the locations and sizes of the true jump discontinuities. The approach is independent of the algorithm used for the standard TV problem and is, consequently, readily incorporated into existing TV restoration codes.

HIRF penetration and PED coupling analysis for scaled fuselage models using a hybrid subgrid FDTD(2,2)/FDTD(2,4) method

The Gegenbauer image reconstruction method, previously shown to improve the quality of magnetic resonance images, is utilized in this study as a segmentation preprocessing step. It is demonstrated that, for all simulated and real magnetic resonance images used in this study, the Gegenbauer reconstruction method improves the accuracy of segmentation. Although it is more desirable to use the k-space data for the Gegenbauer reconstruction method, only information acquired from MR images is necessary for the reconstruction, making the procedure completely self-contained and viable for all human brain segmentation algorithms.

A Newton root-finding algorithm for estimating the regularization parameter for solving ill-conditioned least squares problems

We previously developed a noninvasive technique for the quantification of fluorodeoxyglucose (FDG) positron emission tomography (PET) images using an image-derived input function obtained from a manually drawn carotid artery region. Here, we investigate the use of independent component analysis (ICA) for more objective identification of the carotid artery and surrounding tissue regions. Using FDG PET data from 22 subjects, ICA was applied to an easily defined cubical region including the carotid artery and neighboring tissue. Carotid artery and tissue time activity curves and three venous samples were used to generate spillover and partial volume-corrected input functions and to calculate the parametric images of the cerebral metabolic rate for glucose (CMRgl). Different from a blood-sampling-free ICA approach, the results from our ICA approach are numerically well matched to those based on the arterial blood sampled input function. In fact, the ICA-derived input functions and CMRgl measurements were not only highly correlated (correlation coefficients >0.99) to, but also highly comparable (regression slopes between 0.92 and 1.09), with those generated using arterial blood sampling. Moreover, the reliability of the ICA-derived input function remained high despite variations in the location and size of the cubical region. The ICA procedure makes it possible to quantify FDG PET images in an objective and reproducible manner.

Absorbing boundary conditions, difference operators, and stability

A hybrid method of subgrid FDTD(2,2) with FDTD(2,4) is presented. Both the standard FDTD(2,2) as well as the hybrid technique are applied to shielding effectiveness analysis of a scaled model of a Boeing 757. Also, analysis of EMI generated by personal electronic devices is performed on the same scaled fuselage model.