8 research outputs found
Analytic Expressions for the Ultimate Intrinsic Signal-to-Noise Ratio and Ultimate Intrinsic Specific Absorption Rate in MRI
Cataloged from PDF version of article.The ultimate intrinsic signal-to-noise ratio is the highest possible
signal-to-noise ratio, and the ultimate intrinsic specific
absorption rate provides the lowest limit of the specific absorption
rate for a given flip angle distribution. Analytic expressions
for ultimate intrinsic signal-to-noise ratio and ultimate intrinsic
specific absorption rate are obtained for arbitrary sample
geometries. These expressions are valid when the distance
between the point of interest and the sample surface is smaller
than the wavelength, and the sample is homogeneous. The
dependence on the sample permittivity, conductivity, temperature,
size, and the static magnetic field strength is given in
analytic form, which enables the easy evaluation of the change in
signal-to-noise ratio and specific absorption rate when the sample
is scaled in size or when any of its geometrical or electrical
parameters is altered. Furthermore, it is shown that signal-tonoise
ratio and specific absorption rate are independent of the
permeability of the sample. As a practical case and a solution
example, a uniform, circular cylindrically shaped sample is studied.
Magn Reson Med 66:846–858, 2011. © 2011 Wiley-Liss,
In
Specific Absorption Rate Reduction Using Nonlinear Gradient Fields
Cataloged from PDF version of article.The specific absorption rate is used as one of the main safety
parameters in magnetic resonance imaging. The performance
of imaging sequences is frequently hampered by the limitations
imposed on the specific absorption rate that increase in severity
at higher field strengths. The most well-known approach to
reducing the specific absorption rate is presumably the variable
rate selective excitation technique, which modifies the gradient
waveforms in time. In this article, an alternative approach is introduced
that uses gradient fields with nonlinear variations in space
to reduce the specific absorption rate. The effect of such gradient
fields on the relationship between the desired excitation
profile and the corresponding radiofrequency pulse is shown. The
feasibility of the method is demonstrated using three examples
of radiofrequency pulse design. Finally, the proposed method
is compared with and combined with the variable rate selective
excitation technique. Magn Reson Med 70:537–546, 2013.
© 2012 Wiley Periodicals, In
A synthesis-based approach to compressive multi-contrast magnetic resonance imaging
In this study, we deal with the problem of image reconstruction from compressive measurements of multi-contrast magnetic resonance imaging (MRI). We propose a synthesis based approach for image reconstruction to better exploit mutual information across contrasts, while retaining individual features of each contrast image. For fast recovery, we propose an augmented Lagrangian based algorithm, using Alternating Direction Method of Multipliers (ADMM). We then compare the proposed algorithm to the state-of-the-art Compressive Sensing-MRI algorithms, and show that the proposed method results in better quality images in shorter computation time. © 2017 IEEE
A simple analytical expression for the gradient induced potential on active implants during MRI
During magnetic resonance imaging, there is an interaction between the time-varying magnetic fields and the active implantable medical devices (AIMD). In this study, in order to express the nature of this interaction, simplified analytical expressions for the electric fields induced by time-varying magnetic fields are derived inside a homogeneous cylindrical volume. With these analytical expressions, the gradient induced potential on the electrodes of the AIMD can be approximately calculated if the position of the lead inside the body is known. By utilizing the fact that gradient coils produce linear magnetic field in a volume of interest, the simplified closed form electric field expressions are defined. Using these simplified expressions, the induced potential on an implant electrode has been computed approximately for various lead positions on a cylindrical phantom and verified by comparing with the measured potentials for these sample conditions. In addition, the validity of the method was tested with isolated frog leg stimulation experiments. As a result, these simplified expressions may help in assessing the gradient-induced stimulation risk to the patients with implants. © 1964-2012 IEEE
Magnetic resonance imaging during a pandemic: recommendations by the ISMRM Safety Committee
The COVID-19 pandemic highlighted the challenges delivering face-to-face patient care across healthcare systems. In particular the COVID-19 pandemic challenged the imaging community to provide timely access to essential diagnostic imaging modalities while ensuring appropriate safeguards were in place for both patients and personnel. With increasing vaccine availability and greater prevalence of vaccination in communities worldwide we are finally emerging on the other side of the COVID-19 pandemic. As we learned from our institutional and healthcare system responses to the pandemic, maintaining timely access to MR imaging is essential. Radiologists and other imaging providers partnered with their referring providers to ensure that timely access to advanced MR imaging was maintained. On behalf of the International Magnetic Resonance in Medicine (ISMRM) Safety Committee, this white paper is intended to serve as a guide for radiology departments, imaging centers, and other imaging specialists who perform MR imaging to refer to as we prepare for the next pandemic. Lessons learned including strategies to triage and prioritize MR imaging research during a pandemic are discussed. Level of Evidence 5 Technical Efficacy Stage 5Radiolog
A synthesis-based approach to compressive multi-contrast magnetic resonance imaging
In this study, we deal with the problem of image reconstruction from compressive measurements of multi-contrast magnetic resonance imaging (MRI). We propose a synthesis based approach for image reconstruction to better exploit mutual information across contrasts, while retaining individual features of each contrast image. For fast recovery, we propose an augmented Lagrangian based algorithm, using Alternating Direction Method of Multipliers (ADMM). We then compare the proposed algorithm to the state-of-the-art Compressive Sensing-MRI algorithms, and show that the proposed method results in better quality images in shorter computation time. © 2017 IEEE
Compressed Multi-Contrast Magnetic Resonance Image reconstruction using Augmented Lagrangian Method [Sikistirilmis Çoklu-Kontrast Manyetik Rezonans Görüntülerinin Genisletilmis Lagrange Metodu ile Gerikazanimi]
In this paper, a Multi-Channel/Multi-Contrast image reconstruction algorithm is proposed. The method, which is based on the Augmented Lagrangian Method uses joint convex objective functions to utilize the mutual information in the data from multiple channels to improve reconstruction quality. For this purpose, color total variation and group sparsity are used. To evaluate the performance of the method, the algorithm is compared in terms of convergence speed and image quality using Magnetic Resonance Imaging data to FCSA-MT [1], an alternative approach on reconstructing multi-contrast MRI data. © 2016 IEEE
A simple analytical expression for the gradient induced potential on active implants during MRI
Abstract:
During magnetic resonance imaging, there is an interaction between the time-varying magnetic fields and the active implantable medical devices (AIMD). In this study, in order to express the nature of this interaction, simplified analytical expressions for the electric fields induced by time-varying magnetic fields are derived inside a homogeneous cylindrical volume. With these analytical expressions, the gradient induced potential on the electrodes of the AIMD can be approximately calculated if the position of the lead inside the body is known. By utilizing the fact that gradient coils produce linear magnetic field in a volume of interest, the simplified closed form electric field expressions are defined. Using these simplified expressions, the induced potential on an implant electrode has been computed approximately for various lead positions on a cylindrical phantom and verified by comparing with the measured potentials for these sample conditions. In addition, the validity of the method was tested with isolated frog leg stimulation experiments. As a result, these simplified expressions may help in assessing the gradient-induced stimulation risk to the patients with implants