98 research outputs found
Novel SAR reduction methods for magnetic resonance imaging
Ankara : The Department of Electrical and Electronics Engineering and the Institute of Engineering and Sciences of Bilkent University, 2011.Thesis (Ph. D.) -- Bilkent University, 2011.Includes bibliographical references leaves 86-88.In this thesis, novel methods are presented, which can be used to reduce the
heating of the human body due to radiofrequency fields in magnetic resonance
imaging (MRI). The proposed methods depend on the modification of the
electric field distribution for reducing the specific absorption rate (SAR). These
methods can be used to reduce the local SAR in the vicinity of metallic devices
and the whole-volume average SAR, as shown by electromagnetic field
simulations and phantom, animal and patient experiments. These results can
improve the safety of MRI scans performed on patients with metallic implants
and MRI-guided interventional procedures. Additionally, by reducing the whole
body average SAR, safer and faster MRI scans can be performed.Eryaman, YiğitcanPh.D
¹⁹F-MRI of inhaled perfluoropropane for quantitative imaging of pulmonary ventilation
PhD ThesisMRI of exogenous imaging agents offers a safely repeatable modality to assess regional
pulmonary ventilation. A small number of studies have validated the safety and potential
utility of 19F imaging of inhaled thermally polarised perfluoropropane. However, the relative
scarcity of signal in restrictive breath hold length acquisition times inhibits translation of this
technique to clinical application. This work presents methods used to maximise the attainable
image quality of inhaled perfluoropropane. Novel quantitative measures of ventilation and
perfusion have been investigated and discussed.
A preliminary healthy volunteer study was conducted to verify the efficacy of the imaging
technique and to assess perfluoropropane wash-in and wash-out rates. Quantitative
assessment of the suitability of four RF coil designs was performed, comparing power
efficiency with loading and signal homogeneity within the sensitive volume of each coil. The
3D spoiled gradient echo sequence was simulated, accounting for the power performance of
the chosen birdcage coil design, for calculation of acquisition parameter values required to
achieve the highest SNR in a fixed acquisition period for 19F-MRI of inhaled
perfluoropropane. Studies on resolution phantoms and healthy volunteers assessed the
performance of the optimised imaging protocol, in combination with a compressed sensing
technique that permitted up to three-fold acceleration.
Two novel lung-representative phantoms were fabricated and used to investigate the
behaviour of the MR properties of inhaled perfluoropropane with changing structural and
magnetic environments. Finally, a method for lengthening the T2* of inhaled
perfluoropropane by susceptibility matching the alveolar tissue to the inhaled gas by
intravenous injection of a highly paramagnetic contrast agent is presented. Initial development
work was conducted in phantoms and rodents before translation to healthy volunteers. This
technique offers the potential to concurrently acquire images reflecting both pulmonary
ventilation and perfusion
Radio Frequency Coils for Ultra-high Field Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) has become a powerful tool not only to analyze the anatomical structures of the human body non invasively but also to investigate brain activity with functional MRI. The promise of increase in signal to noise ratio and spectral resolution proportional to the main magnetic field strength motivated a few research laboratories to pursue even higher field strengths. The 9.4T whole body human scanner and the 16.4T animal scanner installed at the Max Planck Institute for Biological Cybernetics, Tuebingen were, for many years, the worlds strongest magnets in their respective categories. In addition to the strong magnets, radio frequency (RF) coils are also equally important in realising the benefits offered by the high field MRI scanners. The aim of this thesis work is to develop optimized RF coils and RF hardware for ultra-high high field MRI
Accelerated Cardiac Magnetic Resonance Imaging in the Mouse Using an Eight-Channel Array at 9.4 Tesla
MRI has become an important tool to noninvasively assess global and regional cardiac function, infarct size, or myocardial blood flow in surgically or genetically modified mouse models of human heart disease. Constraints on scan time due to sensitivity to general anesthesia in hemodynamically compromised mice frequently limit the number of parameters available in one imaging session. Parallel imaging techniques to reduce acquisition times require coil arrays, which are technically challenging to design at ultrahigh magnetic field strengths. This work validates the use of an eight-channel volume phased-array coil for cardiac MRI in mice at 9.4 T. Two- and three-dimensional sequences were combined with parallel imaging techniques and used to quantify global cardiac function, T1-relaxation times and infarct sizes. Furthermore, the rapid acquisition of functional cine-data allowed for the first time in mice measurement of left-ventricular peak filling and ejection rates under intravenous infusion of dobutamine. The results demonstrate that a threefold accelerated data acquisition is generally feasible without compromising the accuracy of the results. This strategy may eventually pave the way for routine, multiparametric phenotyping of mouse hearts in vivo within one imaging session of tolerable duration. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc
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