8 research outputs found
Optimal density compensation factors for the reconstruction of the Fourier transform of bandlimited functions
An inverse nonequispaced fast Fourier transform (iNFFT) is a fast algorithm
to compute the Fourier coefficients of a trigonometric polynomial from
nonequispaced sampling data. However, various applications such as magnetic
resonance imaging (MRI) are concerned with the analogous problem for
bandlimited functions, i.e., the reconstruction of point evaluations of the
Fourier transform from given measurements of the bandlimited function. In this
paper, we review an approach yielding exact reconstruction for trigonometric
polynomials up to a certain degree, and extend this technique to the setting of
bandlimited functions. Here we especially focus on methods computing a diagonal
matrix of weights needed for sampling density compensation
Fast and direct inversion methods for the multivariate nonequispaced fast Fourier transform
The well-known discrete Fourier transform (DFT) can easily be generalized to
arbitrary nodes in the spatial domain. The fast procedure for this
generalization is referred to as nonequispaced fast Fourier transform (NFFT).
Various applications such as MRI, solution of PDEs, etc., are interested in the
inverse problem, i.e., computing Fourier coefficients from given nonequispaced
data. In this paper we survey different kinds of approaches to tackle this
problem. In contrast to iterative procedures, where multiple iteration steps
are needed for computing a solution, we focus especially on so-called direct
inversion methods. We review density compensation techniques and introduce a
new scheme that leads to an exact reconstruction for trigonometric polynomials.
In addition, we consider a matrix optimization approach using Frobenius norm
minimization to obtain an inverse NFFT
Nonuniform fast Fourier transforms with nonequispaced spatial and frequency data and fast sinc transforms
In this paper we study the nonuniform fast Fourier transform with
nonequispaced spatial and frequency data (NNFFT) and the fast sinc transform as
its application. The computation of NNFFT is mainly based on the nonuniform
fast Fourier transform with nonequispaced spatial nodes and equispaced
frequencies (NFFT). The NNFFT employs two compactly supported, continuous
window functions. For fixed nonharmonic bandwidth, it is shown that the error
of the NNFFT with two sinh-type window functions has an exponential decay with
respect to the truncation parameters of the used window functions. As an
important application of the NNFFT, we present the fast sinc transform. The
error of the fast sinc transform is estimated, too
Fast and direct inversion methods for the multivariate nonequispaced fast Fourier transform
The well-known discrete Fourier transform (DFT) can easily be generalized to arbitrary nodes in the spatial domain. The fast procedure for this generalization is referred to as nonequispaced fast Fourier transform (NFFT). Various applications such as MRI and solution of PDEs are interested in the inverse problem, i.e., computing Fourier coefficients from given nonequispaced data. In this article, we survey different kinds of approaches to tackle this problem. In contrast to iterative procedures, where multiple iteration steps are needed for computing a solution, we focus especially on so-called direct inversion methods. We review density compensation techniques and introduce a new scheme that leads to an exact reconstruction for trigonometric polynomials. In addition, we consider a matrix optimization approach using Frobenius norm minimization to obtain an inverse NFFT
Saccadic latency in hepatic encephalopathy: a pilot study
Hepatic encephalopathy is a common complication of cirrhosis. The degree of neuro-psychiatric impairment is highly variable and its clinical staging subjective. We investigated whether eye movement response timesāsaccadic latenciesācould serve as an indicator of encephalopathy. We studied the association between saccadic latency, liver function and paper- and pencil tests in 70 patients with cirrhosis and 31 patients after liver transplantation. The tests included the porto-systemic encephalopathy (PSE-) test, critical flicker frequency, MELD score and ammonia concentration. A normal range for saccades was established in 31 control subjects. Clinical and biochemical parameters of liver, blood, and kidney function were also determined. Median saccadic latencies were significantly longer in patients with liver cirrhosis when compared to patients after liver transplantation (244Ā ms vs. 278Ā ms pā<ā0.001). Both patient groups had prolonged saccadic latency when compared to an age matched control group (175Ā ms). The reciprocal of median saccadic latency (Ī¼) correlated with PSE tests, MELD score and critical flicker frequency. A significant correlation between the saccadic latency parameter early slope (ĻE) that represents the prevalence of early saccades and partial pressure of ammonia was also noted. Psychometric test performance, but not saccadic latency, correlated with blood urea and sodium concentrations. Saccadic latency represents an objective and quantitative parameter of hepatic encephalopathy. Unlike psychometric test performance, these ocular responses were unaffected by renal function and can be obtained clinically within a matter of minutes by non-trained personnel