scholarly journals Feasibility of spiral fMRI based on an LTI gradient model

2019 ◽  
Author(s):  
Nadine N. Graedel ◽  
Lars Kasper ◽  
Maria Engel ◽  
Jennifer Nussbaum ◽  
Bertram J. Wilm ◽  
...  
Keyword(s):  
Image Quality ◽  
Impulse Response ◽  
Response Function ◽  
Linear Time ◽  
Impulse Response Function ◽  
Image Artifacts ◽  
List Type ◽  
Gradient Model ◽  
Time Calibration ◽  
Spatial Specificity

AbstractSpiral imaging is very well suited for functional MRI, however its use has been limited by the fact that artifacts caused by gradient imperfections and B0 inhomogeneity are more difficult to correct compared to EPI. Effective correction requires accurate knowledge of the traversed k-space trajectory. With the goal of making spiral fMRI more accessible, we have evaluated image reconstruction using trajectories predicted by the gradient impulse response function (GIRF), which can be determined in a one-time calibration step.GIRF-predicted reconstruction was tested for high-resolution (0.8 mm) fMRI at 7T. Image quality and functional results of the reconstructions using GIRF-prediction were compared to reconstructions using the delay-corrected nominal trajectory and concurrent field monitoring.The reconstructions using nominal spiral trajectories contain substantial artifacts and the activation maps contain misplaced activation. Image artifacts are substantially reduced when using the GIRF-predicted reconstruction, and the activation maps for the GIRF-predicted and monitored reconstructions largely overlap. The GIRF reconstruction provides a large increase in the spatial specificity of the activation compared to the nominal reconstruction.The GIRF-reconstruction generates image quality and fMRI results similar to using a concurrently monitored trajectory. The presented approach does not prolong or complicate the fMRI acquisition. Using GIRF-predicted trajectories has the potential to enable high-quality spiral fMRI in situations where concurrent trajectory monitoring is not available.HighlightsThis work investigates the feasibility of using a one-time system calibration to account for k-space trajectory deviations in spiral fMRI.This versatile calibration is based on a linear time-invariant gradient model, the gradient impulse response function (GIRF).We show that the image quality and the spatial specificity of the fMRI activation are substantially improved when using the GIRF-predicted trajectories.Basing reconstructions on nominal gradient inputs, on the other hand, induces image artifacts and misplaced fMRI activation.We demonstrate that system characterization via the GIRF can enable spiral fMRI in situations where concurrent trajectory monitoring is unavailable.

Image quality characterization of fine resolution RISAT-1 data using impulse response function

2018 ◽  
Vol 34 (6) ◽  
pp. 586-596
Author(s):  
Gautam Dadhich ◽  
Shweta Sharma ◽  
Mihir Rambhia ◽  
Aloke K. Mathur ◽  
P. R. Patel ◽  
...  
Keyword(s):  
Image Quality ◽  
Impulse Response ◽  
Response Function ◽  
Impulse Response Function ◽  
Fine Resolution

Transient Motions of Floating Bodies at Zero Forward Speed

1987 ◽  
Vol 31 (03) ◽  
pp. 164-176 ◽  
Author(s):  
Robert F. Beck ◽  
Stergios Liapis
Keyword(s):  
Integral Equations ◽  
Impulse Response ◽  
Response Function ◽  
Time Domain ◽  
Linear Time ◽  
Impulse Response Function ◽  
Time Domain Analysis ◽  
The Body ◽  
Floating Body ◽  
Forward Speed

Linear, time-domain analysis is used to solve the radiation problem for the forced motion of a floating body at zero forward speed. The velocity potential due to an impulsive velocity (a step change in displacement) is obtained by the solution of a pair of integral equations. The integral equations are solved numerically for bodies of arbitrary shape using discrete segments on the body surface. One of the equations must be solved by time stepping, but the kernel matrix is identical at each step and need only be inverted once. The Fourier transform of the impulse-response function gives the more conventional added-mass and damping in the frequency domain. The results for arbitrary motions may be found as a convolution of the impulse response function and the time derivatives of the motion. Comparisons are shown between the time-domain computations and published results for a sphere in heave, a sphere in sway, and a right circular cylinder in heave. Theoretical predictions and experimental results for the heave motion of a sphere released from an initial displacement are also given. In all cases the comparisons are excellent.

Fractional Vector-Order h-Realisation of the Impulse Response Function

2020 ◽  
Vol 14 (2) ◽  
pp. 108-113
Author(s):  
Ewa Pawłuszewicz
Keyword(s):  
Control Systems ◽  
Impulse Response ◽  
Response Function ◽  
Impulse Response Function ◽  
Linear Control ◽  
Linear Control Systems

AbstractThe problem of realisation of linear control systems with the h–difference of Caputo-, Riemann–Liouville- and Grünwald–Letnikov-type fractional vector-order operators is studied. The problem of existing minimal realisation is discussed.

scholarly journals Convolution-based time-domain simulation for fluidelastic instability in tube arrays

2021 ◽  
Author(s):  
Mingjie Zhang ◽  
Ole Øiseth
Keyword(s):  
Impulse Response ◽  
Response Function ◽  
Time Domain ◽  
Impulse Response Function ◽  
Time Domain Simulation ◽  
Unit Step ◽  
Tube Arrays ◽  
Unit Impulse ◽  
The Time Domain ◽  
Unit Impulse Response

AbstractA convolution-based numerical algorithm is presented for the time-domain analysis of fluidelastic instability in tube arrays, emphasizing in detail some key numerical issues involved in the time-domain simulation. The unit-step and unit-impulse response functions, as two elementary building blocks for the time-domain analysis, are interpreted systematically. An amplitude-dependent unit-step or unit-impulse response function is introduced to capture the main features of the nonlinear fluidelastic (FE) forces. Connections of these elementary functions with conventional frequency-domain unsteady FE force coefficients are discussed to facilitate the identification of model parameters. Due to the lack of a reliable method to directly identify the unit-step or unit-impulse response function, the response function is indirectly identified based on the unsteady FE force coefficients. However, the transient feature captured by the indirectly identified response function may not be consistent with the physical fluid-memory effects. A recursive function is derived for FE force simulation to reduce the computational cost of the convolution operation. Numerical examples of two tube arrays, containing both a single flexible tube and multiple flexible tubes, are provided to validate the fidelity of the time-domain simulation. It is proven that the present time-domain simulation can achieve the same level of accuracy as the frequency-domain simulation based on the unsteady FE force coefficients. The convolution-based time-domain simulation can be used to more accurately evaluate the integrity of tube arrays by considering various nonlinear effects and non-uniform flow conditions. However, the indirectly identified unit-step or unit-impulse response function may fail to capture the underlying discontinuity in the stability curve due to the prespecified expression for fluid-memory effects.

IMPROVING EXTRACTION OF IMPULSE RESPONSE FUNCTIONS USING STATIONARY WAVELET SHRINKAGE IN TERAHERTZ REFLECTION IMAGING

2010 ◽  
Vol 09 (04) ◽  
pp. 387-394 ◽  
Author(s):  
YANG CHEN ◽  
YIWEN SUN ◽  
EMMA PICKWELL-MACPHERSON
Keyword(s):  
Experimental Data ◽  
Impulse Response ◽  
Response Function ◽  
Impulse Response Function ◽  
Terahertz Imaging ◽  
Impulse Response Functions ◽  
Response Functions ◽  
Inverse Filtering ◽  
Wavelet Shrinkage ◽  
Gaussian Filtering

In terahertz imaging, deconvolution is often performed to extract the impulse response function of the sample of interest. The inverse filtering process amplifies the noise and in this paper we investigate how we can suppress the noise without over-smoothing and losing useful information. We propose a robust deconvolution process utilizing stationary wavelet shrinkage theory which shows significant improvement over other popular methods such as double Gaussian filtering. We demonstrate the success of our approach on experimental data of water and isopropanol.

Interest Rate–Economic Growth Nexus Under Currency Board Operations

2021 ◽  
Vol 8 (1) ◽  
pp. 13-24
Author(s):  
Martinianus Tshimologo Tibinyane ◽  
Teresia Kaulihowa
Keyword(s):  
Economic Growth ◽  
Interest Rate ◽  
Impulse Response ◽  
Response Function ◽  
Open Economy ◽  
Small Open Economy ◽  
Impulse Response Function ◽  
Policy Instrument ◽  
Variance Decomposition ◽  
Currency Board

This paper analyses the effect of the prime interest rate as a monetary policy instrument to stimulate economic growth in Namibia, a small open economy that is constrained by currency board operations. A Vector Autoregressive Model (VAR) was used for the period 1980–2019. The result shows that Namibia’s prime interest rate has no significant effect on economic growth. This finding remains robust and consistent when impulse response function and variance decomposition are employed. The impulse response function indicates a shock on the prime interest rate exhibits an inverse relationship. However, this effect is insignificant in both short and long-run scenarios. The variance decomposition indicates that the prime interest rate has a strongly exogenous impact, implying it has a weak influence on GDP growth. Policy implication indicates that small open economies under currency board operations need to identify different policy responses to circumvent external shocks and addresses their development needs.

Impulse-response function of splanchnic circulation with model-independent constraints: theory and experimental validation

2003 ◽  
Vol 285 (4) ◽  
pp. G671-G680 ◽  
Author(s):  
Ole L. Munk ◽  
Susanne Keiding ◽  
Ludvik Bass
Keyword(s):  
Portal Vein ◽  
Impulse Response ◽  
Response Function ◽  
Mean Transit Time ◽  
Impulse Response Function ◽  
Parameter Estimates ◽  
Time Activity ◽  
Time Activity Curve ◽  
Transit Times ◽  
Activity Curve

Modeling physiological processes using tracer kinetic methods requires knowledge of the time course of the tracer concentration in blood supplying the organ. For liver studies, however, inaccessibility of the portal vein makes direct measurement of the hepatic dual-input function impossible in humans. We want to develop a method to predict the portal venous time-activity curve from measurements of an arterial time-activity curve. An impulse-response function based on a continuous distribution of washout constants is developed and validated for the gut. Experiments with simultaneous blood sampling in aorta and portal vein were made in 13 anesthetized pigs following inhalation of intravascular [15O]CO or injections of diffusible 3- O-[11C]methylglucose (MG). The parameters of the impulse-response function have a physiological interpretation in terms of the distribution of washout constants and are mathematically equivalent to the mean transit time ( T̄) and standard deviation of transit times. The results include estimates of mean transit times from the aorta to the portal vein in pigs: T̄ = 0.35 ± 0.05 min for CO and 1.7 ± 0.1 min for MG. The prediction of the portal venous time-activity curve benefits from constraining the regression fits by parameters estimated independently. This is strong evidence for the physiological relevance of the impulse-response function, which includes asymptotically, and thereby justifies kinetically, a useful and simple power law. Similarity between our parameter estimates in pigs and parameter estimates in normal humans suggests that the proposed model can be adapted for use in humans.

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