Imaging of Cracks and Weak Spots in Steel and Aluminium Plate Rolls

Because of several - sometimes extreme - complications caused by cracks and weak spots in a metal industry roll, it is of great importance to detect these defects in time. Measurements on rolls with artificial- and natural defects have been performed. An imaging operator is introduced, which uses the measurement data to depict the correct locations of the scatterers in the roll. Even when almost 95% of the original measurement data is discarded, the defects in the roll can still be detected. This thesis shows how these imperfections cause deviations in the eddy current measurement setup and presents how the deviations can be used to locate these defects in the steel. The roll is modeled as a conductive half space after which the inhomogeneous Helmholtz equation will be solved to find the electromagnetic fields inside the steel. Defects are modeled as small spheroids with respect to the wavelength, which makes it possible to find the fields inside these scatterers. Furthermore, an equation is found which relates deviations in receiver signals to the electric- and magnetic fields and contrasts in the roll. The quasi-static approach is used to simplify this equation, after which results are shown for different defects and antenna configurations. Similar outcomes are obtained when the measurement data is compared to the theory.Circuits and SystemsTelecommunicationsElectrical Engineering, Mathematics and Computer Scienc

Efficient computational methods in Magnetic Resonance Imaging: From optimal dielectric pad design to effective preconditioned imaging techniques

This dissertation describes how to design dielectric pads that can be used to increase image quality in Magnetic Resonance Imaging, and how to accelerate image reconstruction times using a preconditioner.Image quality is limited by the signal to noise ratio of a scan. This ratio is increased for higher static magnetic field strengths and therefore there is great interest in high-field MRI. The wavelength of the transmitted magnetic RF field decreases for higher field strengths, and it becomes comparable to the dimensions of the human body. Consequently, RF interference patterns are encountered which can severely degrade image quality because of a low transmit efficiency or because of inhomogeneities in the field distribution. Dielectric pads can be used to improve this distribution as the pads tailor the field by inducing a secondary magnetic field due to its high permittivity. Typically, the pads are placed tangential to the body and in the vicinity of the region of interest. The exact location, dimensions, and constitution of the pad need to be carefully determined, however, and depend on the application and the MR configuration. Normally, parametric design studies are carried out using electromagnetic field solvers to find a suitable pad, but this is a very time consuming process which can last hours to days. In contrast with these design studies, we present methods to efficiently model and design the dielectric pads using reduced order modeling and optimization techniques. Subsequently, we have created a design tool to bridge the gap between the advanced design methods and the practical application by the MR community. Now, pads can be designed for any 7T neuroimaging and 3T body imaging application within minutes.In the second part of the thesis a preconditioner is designed for parallel imaging (PI) and compressed sensing (CS) reconstructions. MRI acquisition times can be strongly reduced by using PI and CS techniques by acquiring less data than prescribed by the Nyquist criterion to fully reconstruct the anatomic image; this is beneficial for patient's comfort and for minimizing the risk of patient's movement. Although acquisition times are reduced, the reconstruction times are increased significantly. The reconstruction times can be reduced when a preconditioner is used. In this thesis, we construct such a preconditioner for the frequently used iterative Split Bregman framework. We have tested the performance in a conjugate gradient framework, and show that for different coil configurations, undersampling patterns, and anatomies, a five-fold acceleration can be obtained for solving the linear system part of Split Bregman.Microwave Sensing, Signals & System

High-Permittivity Pad Design for Dielectric Shimming in Magnetic Resonance Imaging Using Projection-Based Model Reduction and a Nonlinear Optimization Scheme

Inhomogeneities in the transmit radio frequency magnetic field ( {\text{B}}-{1}^{+} ) reduce the quality of magnetic resonance (MR) images. This quality can be improved by using high-permittivity pads that tailor the {\text{B}}-{1}^{+} fields. The design of an optimal pad is application-specific and not straightforward and would therefore benefit from a systematic optimization approach. In this paper, we propose such a method to efficiently design dielectric pads. To this end, a projection-based model order reduction technique is used that significantly decreases the dimension of the design problem. Subsequently, the resulting reduced-order model is incorporated in an optimization method in which a desired field in a region of interest can be set. The method is validated by designing a pad for imaging the cerebellum at 7 T. The optimal pad that is found is used in an MR measurement to demonstrate its effectiveness in improving the image quality.Accepted author manuscriptMicrowave Sensing, Signals & SystemsCircuits and System

A simulation study on the effect of optimized high permittivity materials on fetal imaging at 3T

PURPOSE: One of the main concerns in fetal MRI is the radiofrequency power that is absorbed both by the mother and the fetus. Passive shimming using high permittivity materials in the form of "dielectric pads" has previously been shown to increase the formula presented efficiency and homogeneity in different applications, while reducing the specific absorption rate (SAR). In this work, we study the effect of optimized dielectric pads for 3 pregnant models. METHODS: Pregnant models in the 3rd, 7th, and 9th months of gestation were used for simulations in a birdcage coil at 3T. Dielectric pads were optimized regions of interest (ROI) using previously developed methods for formula presented efficiency and homogeneity and were designed for 2 ROIs: the entire fetus and the brain of the fetus. The SAR was evaluated in terms of the whole-body SAR, average SAR in the fetus and amniotic fluid, and maximum 10 g-averaged SAR in the mother, fetus, and amniotic fluid. RESULTS: The optimized dielectric pads increased the transmit efficiency up to 55% and increased the formula presented homogeneity in almost every tested configuration. The formula presented -normalized whole-body SAR was reduced by more than 31% for all body models. The formula presented -normalized local SAR was reduced in most scenarios by up to 62%. CONCLUSION: Simulations have shown that optimized high permittivity pads can reduce SAR in pregnant subjects at the 3rd, 7th, and 9th month of gestation, while improving the transmit field homogeneity in the fetus. However, significantly more work is required to demonstrate that fetal imaging is safe under standard operating conditions.Microwave Sensing, Signals & SystemsCircuits and System

Accelerating implant RF safety assessment using a low-rank inverse update method

Purpose: Patients who have medical metallic implants, e.g. orthopaedic implants and pacemakers, often cannot undergo an MRI exam. One of the largest risks is tissue heating due to the radio frequency (RF) fields. The RF safety assessment of implants is computationally demanding. This is due to the large dimensions of the transmit coil compared to the very detailed geometry of an implant. Methods: In this work, we explore a faster computational method for the RF safety assessment of implants that exploits the small geometry. The method requires the RF field without an implant as a basis and calculates the perturbation that the implant induces. The inputs for this method are the incident fields and a library matrix that contains the RF field response of every edge an implant can occupy. Through a low-rank inverse update, using the Sherman–Woodbury–Morrison matrix identity, the EM response of arbitrary implants can be computed within seconds. We compare the solution from full-wave simulations with the results from the presented method, for two implant geometries. Results: From the comparison, we found that the resulting electric and magnetic fields are numerically equivalent (maximum error of 1.35%). However, the computation was between 171 to 2478 times faster than the corresponding GPU accelerated full-wave simulation. Conclusions: The presented method enables for rapid and efficient evaluation of the RF fields near implants and might enable situation-specific scanning conditions.Microwave Sensing, Signals & SystemsCircuits and System