Browsing by Author "Tarakameh, Alireza Sadeghi"

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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
Novel techniques and innovative designs for the RF chain of magnetic resonance imaging scanners
(2020-07) Tarakameh, Alireza Sadeghi
In this dissertation, novel techniques and innovative designs are proposed to serve one of the main goals of MRI, which is achieving the best image quality by improving signal-to-noise ratio (SNR) while conﬁned to the patient’s safety restrictions determined by speciﬁc absorption rate (SAR) limits. This dissertation comprises ﬁve different contributions to the ﬁeld. First, the co-simulation method is accelerated using the equivalent circuit model of the intended coil and introduced as a compact novel technique for the fast design of transmit array (TxArray) coils. It is shown that the existing co-simulation method can be accelerated by more than 20-fold for different types of TxArray coils. In another study, a simple strategy is introduced for optimizing the topography of transmit elements to reduce the local SAR at ultra-high ﬁeld (UHF, B0 ≥ 7 T) MRI. It is shown that the peak local 10g-averaged SAR can be reduced more than 20% for different types of transmit elements at two different ﬁeld strengths (i.e., 7 T and 10.5 T). As another contribution, conﬁning to the safety limits, the ﬁrst in vivo human head images at 10.5 T is acquired. To ensure the safety of subjects, the EM simulation model of the radiofrequency (RF) coil is validated using an expanded validation workﬂow, then, safe RF power levels were calculated. In another study, a novel short dipole-like element with improved SAR performance, non-unform dielectric substrate (NODES) antenna, is introduced which provides an opportunity to design highly-dense TxArray coils for UHF-MRI. Eventually, a nine-channel transmit/receive coil is built and human cadaver spine images are acquired at 10.5 T. In the ﬁnal study, a new parameter, ultimate intrinsic SAR efﬁciency (UISARE), is introduced and calculated as the upper bound for SAR performance of the transmit coils at different ﬁeld strengths. Overall, this dissertation proposes novel techniques and innovative structures for designing the TxArray coils, which mainly contribute to enhancing image quality and patient safety by improving SNR and reducing SAR.