Browsing by Subject "Co-simulation"

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Open Access
Accelerating the co-simulation method for the design of transmit array coils for MRI
(Springer, 2020) Sadeghi‑Tarakameh, Alireza; Kazemivalipour, Ehsan; Gündoğdu, Umut; Erdoğan, Serhat; Atalar, Ergin
Objective: Accelerating the co-simulation method for the design of transmit array (TxArray) coils is studied using equivalent circuit models. Materials and methods: Although the co-simulation method dramatically reduces the complexity of the design of TxArray coils, finding the optimum solution is not trivial since there exist many local minima in the optimization problem. We propose to utilize an equivalent circuit model of the TxArray coil to obtain a proper initial guess for the optimization process of the co-simulation method. To prove the concept, six different TxArray coils (i.e., three degenerate birdcage coils (DBC), two dual-row head coils, and one elliptical body TxArray coil) with two different loading strategies (cylindrical phantom and human head/body model) at 3 T field strength are investigated theoretically; as an example study, an eight-channel head-DBC is constructed using the obtained values. Results: This approach accelerates the design process more than 20-fold for the coils that are investigated in this manuscript. Conclusion: A fast and accurate method for tuning and decoupling of a TxArray coil can be achieved using its equivalent circuit model combined with the co-simulation method.
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.