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An Extensible MRI Simulator for PostProcessing Evaluation
, 1996
"... . An extensible objectoriented MRI simulation system is presented. This simulator uses firstprinciple modelling based on the Bloch equations to implement a discreteevent simulation of NMR signal production. A firstprinciple model of the image production process, incorporating noise and partial v ..."
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Cited by 81 (3 self)
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. An extensible objectoriented MRI simulation system is presented. This simulator uses firstprinciple modelling based on the Bloch equations to implement a discreteevent simulation of NMR signal production. A firstprinciple model of the image production process, incorporating noise and partial volume effects, has also been developed. This is used to generate realistic simulated MRI volumes, based on a labelled data set, for the evaluation of classification algorithms and other postprocessing routines. 1 Introduction One of the strengths of MRI is the versatility with which contrast can be obtained for images of soft tissue in the brain. However, while this ability makes MRI a flexible tool for clinical diagnosis, the multidimensional nature of the image parameter space often confounds the optimal selection of acquisition parameters. With the increasing need for postprocessing and quantitative analysis of medical images, it has become useful to evaluate the effects of imaging par...
The neural basis of the bloodoxygenleveldependent functional magnetic resonance imaging signal
 Philos Trans R Soc Lond B Biol Sci
, 2002
"... Magnetic resonance imaging (MRI) has rapidly become an important tool in clinical medicine and biological research. Its functional variant (functional magnetic resonance imaging; fMRI) is currently the most widely used method for brain mapping and studying the neural basis of human cognition. While ..."
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Cited by 57 (2 self)
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Magnetic resonance imaging (MRI) has rapidly become an important tool in clinical medicine and biological research. Its functional variant (functional magnetic resonance imaging; fMRI) is currently the most widely used method for brain mapping and studying the neural basis of human cognition. While the method is widespread, there is insuf � cient knowledge of the physiological basis of the fMRI signal to interpret the data con � dently with respect to neural activity. This paper reviews the basic principles of MRI and fMRI, and subsequently discusses in some detail the relationship between the bloodoxygenleveldependent (BOLD) fMRI signal and the neural activity elicited during sensory stimulation. To examine this relationship, we conducted the � rst simultaneous intracortical recordings of neural signals and BOLD responses. Depending on the temporal characteristics of the stimulus, a moderate to strong correlation was found between the neural activity measured with microelectrodes and the BOLD signal averaged over a small area around the microelectrode tips. However, the BOLD signal had signi � cantly higher variability than the neural activity, indicating that human fMRI combined with traditional statistical methods underestimates the reliability of the neuronal activity. To understand the relative contribution of several types of neuronal signals to the haemodynamic response, we compared local � eld potentials (LFPs), single and multiunit activity (MUA) with high spatiotemporal fMRI responses recorded simultaneously in monkey visual cortex. At recording sites characterized by transient responses, only the LFP signal was signi � cantly correlated with the haemodynamic response. Furthermore, the LFPs had the largest magnitude signal and linear systems analysis showed that the LFPs were better than the MUAs at predicting the fMRI responses. These � ndings, together with an analysis of the neural signals, indicate that the BOLD signal primarily measures the input and processing of neuronal information within a region and not the output signal transmitted to other brain regions.
An Introduction to Visualization of Diffusion Tensor Imaging and its Applications
 IN VISUALIZATION AND PROCESSING OF TENSOR FIELDS (2006), WEICKERT
, 2006
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Control of Inhomogeneous Ensembles
, 2006
"... In this thesis, we study a class of control problems which involves controlling a large number of dynamical systems with different values of parameters governing the system dynamics by using the same control signal. We call such problems control of inhomogeneous ensembles. The motivation for looking ..."
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Cited by 17 (6 self)
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In this thesis, we study a class of control problems which involves controlling a large number of dynamical systems with different values of parameters governing the system dynamics by using the same control signal. We call such problems control of inhomogeneous ensembles. The motivation for looking into these problems comes from the manipulation of an ensemble of nuclear spins in Nuclear Magnetic Resonance (NMR) spectroscopy and imaging with dispersion in natural frequencies and the strengths of the applied radio frequency (rf) field. A systematic study of these systems has immediate applications to broad areas of the control of systems in quantum and nano domains, such as coherent spectroscopy and quantum information processing. From the standpoint of mathematical control theory, the challenge is to simultaneously steer a continuum of systems between points of interest with the same control signal. This raises the intriguing question about ensemble controllability. We show that controllability of an ensemble can be understood by the study of the algebra of polynomials defined by the noncommuting vector fields governing the system dynamics. In practical magnetic resonance applications, this work leads to the design of a compensating
Quantum computing with trapped ions
, 2008
"... Quantum computers hold the promise to solve certain computational task much more efficiently than classical computers. We review the recent experimental advancements towards a quantum computer with trapped ions. In particular, various implementations of qubits, quantum gates and some key experiments ..."
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Cited by 15 (2 self)
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Quantum computers hold the promise to solve certain computational task much more efficiently than classical computers. We review the recent experimental advancements towards a quantum computer with trapped ions. In particular, various implementations of qubits, quantum gates and some key experiments are discussed. Furthermore, we review some implementations of quantum algorithms such as a deterministic teleportation of quantum information and an error correction scheme.
The Significance of the CNumerical Range and the Local CNumerical Range
 in Quantum Control and Quantum
"... This paper shows how Cnumericalrange related new strucures may arise from practical problems in quantum control—and vice versa, how an understanding of these structures helps to tackle hot topics in quantum information. We start out with an overview on the role of Cnumerical ranges in current res ..."
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Cited by 13 (2 self)
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This paper shows how Cnumericalrange related new strucures may arise from practical problems in quantum control—and vice versa, how an understanding of these structures helps to tackle hot topics in quantum information. We start out with an overview on the role of Cnumerical ranges in current research problems in quantum theory: the quantum mechanical task of maximising the projection of a point on the unitary orbit of an initial state onto a target state C relates to the Cnumerical radius of A via maximising the trace function tr{C † UAU †}. In quantum control of N qubits one may be interested (i) in having U ∈ SU(2 N) for the entire dynamics, or (ii) in restricting the dynamics to local operations on each qubit, i.e. to the Nfold tensor product SU(2) ⊗ SU(2) ⊗ · · · ⊗ SU(2). Interestingly, the latter then leads to a novel entity, the local Cnumerical range Wloc(C, A), whose intricate geometry is neither starshaped nor simply connected in contrast to the conventional Cnumerical range. This is shown in the accompanying paper on Relative CNumerical Ranges and Local CNumerical Ranges for Application in Quantum Computing [1]. We present novel applications of the Cnumerical range in quantum control assisted by gradient flows on the local unitary group: they serve as powerful tools (1) for deciding whether a quantum interaction can be inverted in time (in a sense generalising Hahn’s
S.: Classical computing in nuclear magnetic resonance
 IJUC
, 2010
"... As part of our longer term research objective of using the complex structure and dynamics of matter to perform non classical in materio computation, we show how to use NMR to perform classical computation. We describe three different approaches of using NMR to implement a single universal logic gate ..."
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Cited by 11 (7 self)
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As part of our longer term research objective of using the complex structure and dynamics of matter to perform non classical in materio computation, we show how to use NMR to perform classical computation. We describe three different approaches of using NMR to implement a single universal logic gate, and a circuit of these gates combined in parallel and in sequence that implement other logic gates, including various optimisations, and, in one case, a halfadder circuit. We then show how the three approaches are just specific instances of a more general approach set in a rich parameter space, and discuss how this parameter space might be exploited for more sophisticated computations. Key words: NMR, in materio computing, universal gates, trajectories 1
Mapping Complex Myoarchitecture in the Bovine Tongue with DiffusionSpectrum Magnetic Resonance Imaging
"... ABSTRACT The ability to resolve complex fiber populations in muscular tissues is important for relating tissue structure with mechanical function. To address this issue in the case of tongue, we employed diffusion spectrum imaging (DSI), an MRI method for determining threedimensional myoarchitectur ..."
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Cited by 7 (1 self)
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ABSTRACT The ability to resolve complex fiber populations in muscular tissues is important for relating tissue structure with mechanical function. To address this issue in the case of tongue, we employed diffusion spectrum imaging (DSI), an MRI method for determining threedimensional myoarchitecture where myofiber populations are variably aligned. By specifically varying gradient field strength, molecular displacement in a tissue can be determined by Fouriertransforming the echo intensity against gradient strength at fixed gradient pulse spacing. The displacement profiles are visualized by graphing threedimensional isocontour icons for each voxel, with the isocontour shape and size representing the magnitude and direction of the constituting fiber populations. To validate this method, we simulated a DSI experiment within the constraints of arbitrary crossing fibers, and determined that DSI accurately depicts the angular relationships between these fibers. Considering the fiber relationships in the whole bovine tongue, we compared the images obtained by DSI with those obtained by diffusion tensor imaging in an anterior slice of the lingual core, a region known to possess extensive fiber crossing. In contrast to diffusion tensor imaging, which depicts the anterior core solely as a region with low anisotropy due to the presence of mixedorientation fiber populations, DSI shows two distinct fiber populations, with an explicit orthogonal relationship to each other. In imaging the whole lingual tissue, we discerned arrays of crossing and noncrossing fibers involving the intrinsic and extrinsic muscles, which merged at regions of interface. We conclude that DSI has the capacity to determine threedimensional fiber orientation in structurally complex muscular tissues.