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Wellington, New Zealand

Teal P.D.,Victoria University of Wellington | Eccles C.,Magritek Ltd
Inverse Problems | Year: 2015

The two most successful methods of estimating the distribution of nuclear magnetic resonance relaxation times from two dimensional data are data compression followed by application of the Butler-Reeds-Dawson algorithm, and a primal-dual interior point method using preconditioned conjugate gradient. Both of these methods have previously been presented using a truncated singular value decomposition of matrices representing the exponential kernel. In this paper it is shown that other matrix factorizations are applicable to each of these algorithms, and that these illustrate the different fundamental principles behind the operation of the algorithms. These are the rank-revealing QR (RRQR) factorization and the LDL factorization with diagonal pivoting, also known as the Bunch-Kaufman-Parlett factorization. It is shown that both algorithms can be improved by adaptation of the truncation as the optimization process progresses, improving the accuracy as the optimal value is approached. A variation on the interior method viz, the use of barrier function instead of the primal-dual approach, is found to offer considerable improvement in terms of speed and reliability. A third type of algorithm, related to the algorithm known as Fast iterative shrinkage-thresholding algorithm, is applied to the problem. This method can be efficiently formulated without the use of a matrix decomposition. © 2015 IOP Publishing Ltd. Source


Zhen J.,Victoria University of Wellington | Gouws G.,Victoria University of Wellington | Dykstra R.,Victoria University of Wellington | Eccles C.,Magritek Ltd
Proceedings of the International Conference on Sensing Technology, ICST | Year: 2012

A 20 MHz Class D RF amplifier has been developed to work with an NMR MOUSE sensor. The prototype has been built on a 4-layer PCB with an area of 50 cm 2 and weighs less than 120 g; it outputs 100 W into a 50Ω load using a single 24 V DC supply. Even with the high switching power losses at that frequency, the amplifier is able to achieve 73% efficiency. Test results from the MOUSE sensor show the class D amplifier is operating well at 20 MHz with fast turn on and turn off times, producing constant amplitude pulses as short as 2 μs. and is able to run long CPMG experiments with hundreds of echoes. The results confirm that the Class D amplifier is a worthy replacement for the existing, market available Class A amplifier, which is running at 19% efficiency and weighs 480 g with a PCB area of 160 cm2. © 2012 IEEE. Source


Kittler W.C.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Galvosas P.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Hunter M.W.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Hunter M.W.,Magritek Ltd
Microporous and Mesoporous Materials | Year: 2014

Acquisition of displacement space with NMR, the conjugate of q-space, has many applications including diffusion, dispersion, velocimetry, tortuosity, and surface to volume ratio measurements in porous media and emulsions. Normally, acquisition of q-space data requires a series of scans over which current pulsed through a gradient coil producing a constant magnetic field gradient is varied to step through q-space. By replacing the constant magnetic field gradient with a second order field whose gradient strength varies in space, a range of gradient values are applied over the sample in a single experiment, and q-space encoded into real space. Using slice selection and a read gradient in conjunction with the second order field, a modified PGSE sequence has been developed which allows the parallel acquisition of q-space in a single-shot experiment for a homogeneous medium. A proof of concept is presented for the parallel acquisition of q-space under the diffusive process, allowing a single-shot, single-excitation, single observation time diffusion measurement to be made. In addition to this, the mapping between real space and q-space leads to a mapping between displacement space and time. This means that for this experiment, without the use of any Fourier transforms, the average propagator may be obtained simply by normalising the acquired echo and plotting it against displacement space. © 2014 Elsevier Inc. All rights reserved. Source


Kittler W.C.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Galvosas P.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Hunter M.W.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Hunter M.W.,Magritek Ltd
Journal of Magnetic Resonance | Year: 2014

A proof of concept is presented for the parallel acquisition of q-space under diffusion using a second order magnetic field. The second order field produces a gradient strength which varies in space, allowing a range of gradients to be applied in a single pulse, and q-space encoded into real space. With the use of a read gradient, the spatial information is regained from the NMR signal, and real space mapped onto q-space for a thin slice excitation volume. As the diffusion encoded image for a thin slice can be mapped onto q-space, and the average propagator is the inverse Fourier transform of the q-space data, it follows that the acquisition of the echo is a direct measurement of the average propagator. In the absence of a thin slice selection, the real space to q-space mapping is lost, but the ability to measure the diffusion coefficient retained with an increase in signal to noise. © 2014 Elsevier Inc. All rights reserved. Source


Kittler W.C.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Obruchkov S.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Galvosas P.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Hunter M.W.,MacDiarmid Institute for Advanced Materials and Nanotechnology | Hunter M.W.,Magritek Ltd
Journal of Magnetic Resonance | Year: 2014

Pulsed field gradient nuclear magnetic resonance provides a powerful tool for the measurement of particle diffusion and mobility. When these particles are contained in a porous medium, the diffusive process is influenced by the pore boundaries, and their effect on diffusion measurements provides information about the pore space. The acquisition of the apparent diffusion coefficient and its dependence on time, in the short time limit, reveals the surface to volume ratio of the porous medium, and in the long time limit, its tortuosity. With conventional pulsed field gradient techniques, processes where pore boundaries are evolving on the sub-second time scale cannot be resolved. Using pulsed second order magnetic fields in conjunction with one-dimensional imaging and the pulse sequence Difftrain, this paper presents a proof of concept for the first ever real time single-shot surface to volume NMR measurement. © 2014 Elsevier Inc. Source

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