Browsing by Subject "SAR reduction"

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Open Access
Design of a birdcage-like radio frequency transmit array coil for the magnetic resonance imaging using equivalent circuit model
(2016-05) Tarakameh, Alireza Sadeghi
One of the conditions to have a good magnetic resonance (MR) image is applying a homogeneous radio-frequency (RF) excitation (magnetic field) with effciently high intensity to the region of interest. However, there are some limitations such as specific absorption rate (SAR) which is not allowed to exceed some standard levels. Since SAR level directly depends on the electric field and the electric field is coupled to the magnetic field, there is a trade-off between high-intensity RF-excitation and low SAR level. Moreover, in conventional RF coils (birdcage) for the MRI, the magnetic field profile is almost constant so that its intensity is pretty high at the center of the coil and decreases toward the coil. In such a coil, it is not possible to aim an off-center small region of interest and make the homogeneity concentrated at that region. Transmit array (Tx-array) coils provide high controllability on both electric and magnetic field, so, they would be good solutions for all of these issues, although, they come across the effciency problem at the center when the same performance of a conventional RF coil is required. This problem has been already handled using a birdcage-like Tx-array coil, however, there are some diffculties to design and tune such a coil. In this thesis, we proposed a novel design method for birdcage-like Tx-array coil; an eight-channel birdcage-like Tx-array coil is designed using the equivalent lumped-element circuit model. This design profits controllability feature of an array and high transmit effciency of a birdcage coil at the center, simultaneously. A capacitive decoupling method is utilized in order to get rid of reactive interactions between channels of the array. Then, an optimization (the steepest-descent method) with constraints based on minimizing the electric field and smoothing the magnetic field is applied to the voltage-excitations of the Tx-array coil. The proposed decoupling method provides 15dB matching for each channel and higher than 12dB decoupling between adjacent channels and at least 19dB for nonadjacent channels. This Tx-array coil provides only 3% less effciency versus the birdcage coil at the center of the coil, while, at the regions close to the surface of the phantom we achieved more than 72% better effciency in comparison to the birdcage coil. Furthermore, we demonstrated that the Tx-array is capable to produce a homogeneous magnetic field at an arbitrary (off-center) region of interest. This adjustment can be performed for the electric field as well such that the electric field and so the SAR can be minimized locally. Consequently, the proposed configuration of the Tx-array coil provides an ef- ficient excitation while capability of local RF shimming and local electric-fieldreduction can be achieved.
Open Access
A novel verse optimal RF pulse design method for parallel transmission in magnetic resonance imaging
(2009) Bayındır, Haldun Özgür
A novel radio frequency (RF) pulse design method for magnetic resonance imaging (MRI) and an improvement to an existing method that reduces specific absorption rate (SAR) in MRI are presented. The new RF pulse design method, variable rate selective excitation optimal RF pulse design method for parallel transmission (VERSEp), is developed for parallel transmission and aim of the method is to design RF pulses with lowest SAR after SAR reduction with variable rate selective excitation (VERSE) method. This is achieved by modifying the SAR optimal RF pulse deisgn method for parallel transmission. Performance of the VERSEp method is tested by comparing VERSE-SAR reduced SAR of the RF pulses designed with SAR optimal RF pulse design method and VERSE-SAR reduced SAR of the RF pulses designed using VERSEp. In the simulations, SAR reductions up to 47% are obtained. Different aspects of VERSEp are also shown with simulations. The second contribution of this work is an improvement made to an existing constrained VERSE-SAR reduction method. The existing VERSE-SAR reduction method uses a peak RF constaint for SAR reduction. In this work, peak square root power constraint is used instead of peak RF constraint in the VERSE-SAR reduction method. In the simulation results, the SAR of the RF pulses designed using the improved method were compared with SAR of the RF pulses designed using the method before improvement. SAR reductions up to 50% are obtained by using peak square root power constrained SAR reduction instead of peak RF constrained SAR reduction.