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... Show moreMRI 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 design of such a shield, specifically for low-field point-of-care Halbach based MRI devices, is discussed. A segmented double layer shield is designed and constructed based on eddy current simulations. Subsequently, the performance of the improved shield is compared to a reference shield by measuring the eddy current decay times as well as using noise measurements. A maximum reduction factor of 2.9 in the eddy current decay time is observed. The segmented shield couples in an equivalent amount of noise when compared to the unsegmented reference shield. Turbo spin echo images of a phantom and the brain of a healthy volunteer show improvements in terms of blurring using the segmented shield. Show less
PurposeConcomitant 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... Show morePurposeConcomitant 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.MethodsThe concomitant fields associated with a cylindrical gradient coils designed for a transverse B0$$ {B}_0 $$ and a signal model including the tilting effect of the effective magnetic field are derived. The formalism is used to simulate and predict concomitant field related distortions. A 46-mT Halbach-array-based system with a maximum gradient strength of 15 mT/m is used to verify the model using two-dimensional spin-echo sequences.ResultsThe simulations and experimental results are in good agreement with the derived equations. The fundamental characteristics of the concomitant field equations are different to conventional MRI systems: Image distortions occur primarily in the transverse directions and a cross-term only exists when applying transverse gradient pulses simultaneously.ConclusionThe level of image warping in the frequency encoding direction is insignificant for the POC systems discussed here. However, when trying to achieve short echo-times by using strong phase encoding and readout-dephasing gradients, the combination can result in image warping and blurring which should be accounted for in image interpretation. Show less
ObjectiveLow-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... Show moreObjectiveLow-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 with integrated gradient and RF coils that most efficiently fulfil a set of user-specified imaging requirements.MethodsFor efficient integration, target field methods are used for each of the main hardware components. These have not been used previously in magnet design, and a new mathematical model was derived accordingly. These methods result in a framework which can design an entire low-field MRI system within minutes using standard computing hardware.ResultsTwo distinct point-of-care systems are designed using the described framework, one for neuroimaging and the other for extremity imaging. Input parameters are taken from literature and the resulting systems are discussed in detail.DiscussionThe framework allows the designer to optimize the different hardware components with respect to the desired imaging parameters taking into account the interdependencies between these components and thus give insight into the influence of the design choices. Show less
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... Show moreLow-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 constructing and characterising a low-field portable MRI system for in vivo imaging at 50 mT. These developments include the design of i) high-linearity gradient coils using a modified volume-based target field approach, ii) phased-array receive coils, and iii) a battery-operated three-axis gradient amplifier for improved portability and sustainability. In addition, we report performance characterisation of the RF amplifier, the gradient amplifier, eddy currents from the gradient coils, and describe a quality control protocol for the overall system. Show less
Vos, B. de; Fuchs, P.; O'Reilly, T.; Webb, A.; Remis, R. 2020
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... Show moreIn 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 method, current densities for three gradient directions are obtained and subsequently verified using a commercial magneto-static solver. Stream functions are used to turn the surface current densities into wire patterns for constructing the gradient coils. The measured fields are in good agreement with simulations and their prescribed target fields. 3-D images have been acquired using the constructed gradient coils with a very low degree of geometric distortion. Show less
Rijken, J.A.; Vos, B. de; Leemans, C.R.; Zwezerijnen, G.J.C. ben; Graaf, P. de; Hensen, E.F.; ... ; Symersky, P. 2020
Management of functional intrathoracic sympathetic paragangliomas in succinate dehydrogenase subunit D (SDHD) mutation carriers is challenging, and there is no uniform guideline for treatment to... Show moreManagement of functional intrathoracic sympathetic paragangliomas in succinate dehydrogenase subunit D (SDHD) mutation carriers is challenging, and there is no uniform guideline for treatment to date. The risks of potential malignant behavior and long-term cardiovascular morbidity have to be weighed against the risks of treatment complications. We report the multidisciplinary and shared decision-making approach that resulted in successful surgical removal of 3 paragangliomas in a SDHD mutation carrier. (C) 2020 by The Society of Thoracic Surgeons Show less
Rijken, J.A.; Vos, B. de; Hest, L.P. van; Dreijerink, K.M.A.; Heijer, M. den; Wisselink, W.; ... ; Leemans, C.R. 2019
Executive Summary In June 2016, High Representative Mogherini presented the EU’s new Global Strategy on Foreign and Security Policy (EUGS) to the European Council. With the Strategy now... Show moreExecutive Summary In June 2016, High Representative Mogherini presented the EU’s new Global Strategy on Foreign and Security Policy (EUGS) to the European Council. With the Strategy now finalized, attention needs to turn to its implementation in an environment mired by crises both within Europe and the wider world. In September 2016, The Hague Institute for Global Justice and Europe House—the European Parliament Information Office and the Representation of the European Commission in The Netherlands—organized an expert meeting and a public panel discussion, which inform the present document as a first appraisal of the Global Strategy. Focusing on three areas of particular salience in EU foreign policy—the EU as a security actor, developing rules-based global governance in new areas, and the ‘joined-up’ approach in pursuing the UN’s Sustainable Development Goals (SDGs)— the following recommendations for the implementation of the EUGS can be made: The EU as a security actor The EU needs a pragmatic and flexible approach in order to solve the crises around Europe and to improve its credibility in the short and long term, for example by using ad hoc coalitions. Moreover, a possible withdrawal of the United Kingdom from the EU could open up political space for deepened defense cooperation. The EU would need to find ways to make use of this political space in order to generate political will in the capitals for deepening defense cooperation. More broadly speaking, the EU should devise plans to connect with its citizens as a security actor and communicate the message that EU defense cooperation tangibly benefits the security of all citizens. Developing rules-based global governance Using the area of cyber governance as an example given its cross-cutting importance, the EU should fully embrace the role of ‘agenda-shaper, connector, coordinator and facilitator within a networked web of players’ by investing in multistakeholder initiatives and, together with the Member States, showing coordinated political leadership in this area. The EU, furthermore, needs to take its internal normative innovations (such as the “right to be forgotten”) to the global stage, where they can serve as inspiration to other actors. It should also use regional approaches and coalitions of like-minded countries as building blocks for working towards a global consensus. In addition, the EU should bolster its credibility as a cyber power by capacity-building, both within the EU Member States and third countries, to fight criminal activities and strengthen cooperation between law enforcement agencies. Capacity-building as part of a ‘joined-up approach’ in the case of the SDGs For SDGs to be progressively realized through ‘joined-up’ EU action, they need to be translated and concretized into measurable goals, which should be pursued through already existing policies and strategies and be taken into account during the framing of new ones. Moreover, the EU should invest, in tandem with the Member States, in communicating to the public that the SDGs are a global commitment with implications, both positive and negative, in the daily lives of citizens. For the EU, the most important next step is to translate the EUGS into prioritized and coherent sub-strategies with a view to maintaining the SDGs as a central element of the follow-up of the EUGS. The Global Strategy will remain the core guidance for EU external action for years to come. However, the period until the first yearly progress report in June 2017 will be crucial for establishing the traction and first concrete results produced by the Strategy. As the calendars of the EU institutions and Member State policymakers are filling up with more milestones for its implementation, existing sectorspecific strategies will be updated and new ones developed. Implementing the Global Strategy will be a momentous endeavor by any standard, both for the EU and its Member States. How well they will work together, use their resources, build political momentum and voice their common message will ultimately determine the role of the EU in the world. Show less
