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Optimizing the gradient ring of a low-field MRI scanner

Meijer, Angeline (author)

This report describes the study on the optimization of the magnetic field generated by a low-field magnetic resonance imaging modality. In the optimization the Genetic Algorithm and a quasi-Newton Algorithm are used.

bachelor thesis 2019

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Model based image reconstruction for low-field hand-held MRI: On imaging using field geometry and sample translations

Oppelaar, Pieter (author)

In this report, the conversion from spin-echo signals, obtained with a low-field hand-held MRI scanner that was designed and built at the Leiden University, to images of the proton density within the sample is considered. This scanner does not make use of switchable gradient coils, but instead relies solely on the natural inhomogeneity of the...

bachelor thesis 2021

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Effects of the pre-absorption of superabsorbent polymers and the water-cement ratio of paste on the adsorption-desorption of superabsorbent polymers: an NMR study

Yang, Jingbin (author), Sun, Zhenping (author), Yanliang, Ji (author), Li, Biyun (author), Pang, Min (author)

As an internal curing material, superabsorbent polymers (SAPs) can effectively reduce the risk of volume shrinkage of cement-based materials. By preparing the cement paste mixed with SAPs, the amount of pre-absorbed water and the water-cement ratio of paste on the water absorption-release behavior in the fresh cement paste were studied by using...

conference paper 2021

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A Dictionary Learning Approach for Joint Reconstruction and Denoising in Low Field Magnetic Resonance Imaging

Ahishakiye, Emmanuel (author), van Gijzen, M.B. (author), Shan, X. (author), Tumwiine, Julius (author), Obungoloch, Johnes (author)

Currently, many children with hydrocephalus in East Africa and other resource-constrained countries do not have access to Magnetic Resonance Imaging (MRI) scanners, the preferred imaging tool during the disease administration and treatment. Conventional MRI scanners are costly to buy and manage, which limits their utilization in low-income...

conference paper 2021

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Description of a low-field MRI scanner based on permanent magnets

de Leeuw den Bouter, M.L. (author), Gecmen, Dilan (author), Meijer, Angeline (author), de Gans, D.H. (author), Middelplaats, L.N.M. (author), Remis, R.F. (author), van Gijzen, M.B. (author)

More than 6,000 infants develop hydrocephalus in East Africa every year. Magnetic Resonance Imaging is the preferred technique to diagnose hydrocephalus. In countries such as Uganda, MRI is unaffordable at even major referral hospitals. In order to provide a sustainable diagnostic tool we are developing an inexpensive and easy-to-use MRI...

conference paper 2020

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A Dictionary Learning Approach for Noise-Robust Image Reconstruction in Low-Field Magnetic Resonance Imaging

Ahishakiye, Emmanuel (author), van Gijzen, M.B. (author), Tumwiine, Julius (author), Obungoloch, Johnes (author)

Objective: Image denoising has been considered as a separate procedure from image reconstruction which could otherwise be combined with acquisition and reconstruction. This paper discusses a joint image reconstruction and denoising algorithm in low-field MRI using a dictionary learning approach. Method: Our proposed algorithm uses a two-level...

conference paper 2020

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Electrical transport in monolayers of phthalocyanine molecular wires and AFM imaging of a single wire bridging two electrodes

Tans, S.J. (author), Miedema, R. (author), Geerligs, L.J. (author), Dekker, C. (author), Wu, J. (author), Wegner, G. (author)

journal article 1997

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Deflated preconditioned Conjugate Gradient methods for noise filtering of low-field MR images

Shan, X. (author), van Gijzen, M.B. (author)

We study efficient implicit methods to denoise low-field MR images using a nonlinear diffusion operator as a regularizer. This problem can be formulated as solving a nonlinear reaction–diffusion equation. After discretization, a lagged-diffusion approach is used which requires a linear system solve in every nonlinear iteration. The choice of...

journal article 2022

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On-site construction of a point-of-care low-field MRI system in Africa

Obungoloch, Johnes (author), Muhumuza, Ivan (author), Teeuwisse, Wouter (author), Harper, Joshua (author), Etoku, Ivan (author), Asiimwe, Robert (author), Tusiime, Patricia (author), Gombya, George (author), van Gijzen, M.B. (author)

Purpose: To describe the construction and testing of a portable point-of-care low-field MRI system on site in Africa. Methods: All of the components to assemble a 50 mT Halbach magnet-based system, together with the necessary tools, were air-freighted from the Netherlands to Uganda. The construction steps included individual magnet sorting,...

journal article 2023

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Low-field magnetic resonance imaging using multiplicative regularization

de Leeuw den Bouter, M.L. (author), van Gijzen, M.B. (author), Remis, R.F. (author)

In this paper we present a magnetic resonance imaging (MRI) technique that is based on multiplicative regularization. Instead of adding a regularizing objective function to a data fidelity term, we multiply by such a regularizing function. By following this approach, no regularization parameter needs to be determined for each new data set...

journal article 2021

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Design, Characterisation and Performance of an Improved Portable and Sustainable Low-Field MRI System

de Vos, B. (author), Parsa, Javad (author), Abdulrazaq, Zaynab (author), Teeuwisse, Wouter M. (author), Van Speybroeck, Camille D.E. (author), de Gans, D.H. (author), Remis, R.F. (author), O’Reilly, Tom (author), Webb, A. (author)

Low-field permanent magnet-based MRI systems are finding increasing use in portable, sustainable and point-of-care applications. In order to maximize performance while minimizing cost many components of such a system should ideally be designed specifically for low frequency operation. In this paper we describe recent developments in...

journal article 2021

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Inversion of incomplete spectral data using support information with an application to magnetic resonance imaging

de Leeuw den Bouter, M.L. (author), van den Berg, P.M. (author), Remis, R.F. (author)

In this paper we discuss an imaging method when the object has known support and its spatial Fourier transform is only known on a certain k-space undersampled pattern. The simple conjugate gradient least squares algorithm applied to the corresponding truncated Fourier transform equation produces reconstructions that are basically of a similar...

journal article 2021

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Gradient Coil Design and Realization for a Halbach-Based MRI System

De Vos, Bart (author), Fuchs, P.S. (author), O'Reilly, Thomas (author), Webb, A. (author), Remis, R.F. (author)

In this article, we design and construct gradient coils for a Halbach permanent-magnet array magnetic resonance (MR) scanner. The target field method, which is widely applied for the case of axial static magnetic fields, has been developed for a transverse static magnetic field as produced by a Halbach permanent-magnet array. Using this...

journal article 2020

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Adaptive-size dictionary learning using information theoretic criteria for image reconstruction from undersampled k-space data in low field magnetic resonance imaging

Ahishakiye, Emmanuel (author), van Gijzen, M.B. (author), Tumwiine, Julius (author), Obungoloch, Johnes (author)

Background: Magnetic resonance imaging (MRI) is a safe non-invasive and nonionizing medical imaging modality that is used to visualize the structure of human anatomy. Conventional (high-field) MRI scanners are very expensive to purchase, operate and maintain, which limit their use in many developing countries. This study is part of a project...

journal article 2020

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In vivo 3D brain and extremity MRI at 50 mT using a permanent magnet Halbach array

O’Reilly, Thomas (author), Teeuwisse, Wouter M. (author), de Gans, D.H. (author), Koolstra, Kirsten (author), Webb, A. (author)

Purpose: To design a low-cost, portable permanent magnet-based MRI system capable of obtaining in vivo MR images within a reasonable scan time. Methods: A discretized Halbach permanent magnet array with a clear bore diameter of 27 cm was designed for operation at 50 mT. Custom-built gradient coils, RF coil, gradient amplifiers, and RF...

journal article 2021

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Conjugate gradient variants for ℓ _p -regularized image reconstruction in low-field MRI

de Leeuw den Bouter, M.L. (author), van Gijzen, M.B. (author), Remis, R.F. (author)

We consider the MRI physics in a low-field MRI scanner, in which permanent magnets are used to generate a magnetic field in the millitesla range. A model describing the relationship between measured signal and image is derived, resulting in an ill-posed inverse problem. In order to solve it, a regularization penalty is added to the least...

journal article 2019

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Characterization of concomitant gradient fields and their effects on image distortions using a low-field point-of-care Halbach-based MRI system

de Vos, B. (author), Remis, R.F. (author), Webb, A. (author)

Purpose: Concomitant gradient fields have been extensively studied at clinical field strengths. However, their effects have not yet been modeled for low-field point-of-care (POC) systems. The purpose of this work is to characterize the effects associated with concomitant fields for POC Halbach-array-based systems. Methods: The concomitant...

journal article 2023

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An integrated target field framework for point-of-care halbach array low-field MRI system design

de Vos, B. (author), Remis, R.F. (author), Webb, A. (author)

Objective: Low-cost low-field point-of-care MRI systems are used in many different applications. System design has correspondingly different requirements in terms of imaging field-of-view, spatial resolution and magnetic field strength. In this work an iterative framework has been created to design a cylindrical Halbach-based magnet along...

journal article 2023

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Segmented RF shield design to minimize eddy currents for low-field Halbach MRI systems

de Vos, B. (author), Remis, R.F. (author), Webb, A. (author)

MRI systems have a thin conducting layer placed between the gradient and RF coils, this acts as a shield at the RF-frequency, minimizing noise coupled into the experiment, and decreasing the coupling between the RF and gradient coils. Ideally, this layer should be transparent to the gradient fields to reduce eddy currents. In this work the...

journal article 2024

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Segmentation techniques for noisy MRI scans

Slepova, Ksenia (author)

Every year, 180000 new cases of hydrocephalus are diagnosed among infants in Sub-Saharan Africa. Unfortunately, more than two-thirds of the population in this region lacks access to essential medical imaging technologies, such as magnetic resonance imaging (MRI). To address this issue, a collaborative effort between the TU Delft, Leiden...

master thesis 2023