Browsing by Author "Takrimi, Manouchehr"

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Open Access
Broadband analysis of multiscale electromagnetic problems: Novel incomplete-leaf MLFMA for potential integral equations
(IEEE, 2021-06-24) Khalichi, Bahram; Ergül, Ö.; Takrimi, Manouchehr; Ertürk, Vakur B.
Recently introduced incomplete tree structures for the magnetic-field integral equation are modified and used in conjunction with the mixed-form multilevel fast multipole algorithm (MLFMA) to employ a novel broadband incomplete-leaf MLFMA (IL-MLFMA) to the solution of potential integral equations (PIEs) for scattering/radiation from multiscale open and closed surfaces. This population-based algorithm deploys a nonuniform clustering that enables to use deep levels safely and, when necessary, without compromising the accuracy resulting in an improved efficiency and a significant reduction for the memory requirements (order of magnitudes), while the error is controllable. The superiority of the algorithm is demonstrated in several canonical and real-life multiscale geometries.
Open Access
Broadband multilevel fast multipole algorithm for large-scale problems with nonuniform discretizations
(IEEE, 2016) Ergül, Ö.; Karaosmanoğlu, B.; Takrimi, Manouchehr; Ertürk, Vakur B.
We present a broadband implementation of the multilevel fast multipole algorithm (MLFMA) for fast and accurate solutions of multiscale problems involving highly nonuniform discretizations. Incomplete tree structures, which are based on population-based clustering with flexible leaf-level boxes at different levels, are used to handle extremely varying triangulation sizes on the same structures. Superior efficiency and accuracy of the developed implementation, in comparison to the standard and broadband MLFMA solvers employing conventional tree structures, are demonstrated on practical problems.
Open Access
A broadband multilevel fast multipole algorithm with incomplete-leaf tree structures for multiscale electromagnetic problems
(IEEE, 2016) Takrimi, Manouchehr; Ergül, Ö.; Ertürk, Vakur B.
An efficient, broadband, and accurate multilevel fast multipole algorithm (MLFMA) is proposed to solve a wide range of multiscale electromagnetic problems with orders of magnitude differences in the mesh sizes. Given a maximum RWG population threshold, only overcrowded boxes are recursively bisected into smaller ones, which leads to novel incomplete-leaf tree structures. Simulations reveal that, for surface discretizations possessing highly overmeshed local regions, the proposed method presents a more efficient and/or accurate results than the conventional MLFMA. The key feature of such a population-based clustering scenario is that the error is controllable, and hence, regardless of the number of levels, the efficiency can be optimized based on the population threshold. Numerical examples are provided to demonstrate the superior efficiency and accuracy of the proposed algorithm in comparison to the conventional MLFMA.
Open Access
Minimization of eddy power loss in the cryostat for a z-gradient array coil driven by an arbitrary pulse sequence: an electromagnetic approach
(Wiley, 2023-11-27) Takrimi, Manouchehr; Atalar, Ergin
Purpose This paper presents a novel computational approach to optimize gradient array performance for a given pulse sequence. Specifically, we propose an electromagnetic (EM) approach that minimizes eddy losses within the cryostat while maintaining key performance parameters such as field linearity, gradient strength, and imaging region's dimension and position. Methods High-resolution EM simulations on the cryostat's surface are deployed to compute the net EM fields generated by each element of a gradient array coil at different frequencies. The computed fields are stored and combined for each frequency to form a quadratic vector–matrix–vector computation. The overall time-average eddy power loss within the cryostat assembly for arbitrary pulse sequences is computed using frequency domain superposition. Results The proposed approach estimates and regulates eddy power losses within the cryostat assembly. When compared to the stray field minimization approach, it can achieve over twice the reduction in eddy power loss. The proposed approach eliminates the need to sample the stray fields on the cryostat surface, which the number and position of the samples would be challenging when designing tunable array coils with capabilities that disrupt field symmetries. Additionally, the loss calculation considers the entire cryostat assembly rather than just the inner cylindrical surface of the warm shield. Conclusion Our findings highlight the efficacy of an on-the-fly tuning method for the development of high-performance whole-body gradient array coils, effectively mitigating eddy losses within the cryostat and minimizing stray fields outside the coil assembly. This approach proves particularly advantageous for array coils with variable feeding currents.
Open Access
A modified equivalence principle for open surfaces
(IEEE, 2014) Takrimi, Manouchehr; Gürel, Levent
We introduce a new method for expanding equivalent surface current densities over open surfaces. The new method is based on the equivalence principle, which is theoretically used with closed surfaces. Utilizing low-order basis functions to compute the expansion coefficients of equivalent surface currents and then weighting these coefficients with an appropriate window function compensates for computational field errors originating from using truncated currents at the open boundaries. These modified coefficients can be used to reproduce the original electromagnetic fields inside a limited equivalent region. The simulations demonstrate that relative errors as low as 1-2.5% are achievable based on the specifics of the source and shape of the open surface. © 2014 IEEE.
Open Access
Windowed equivalence principle for open surfaces
(IEEE, 2013) Takrimi, Manouchehr; Gürel, Levent
We introduce a modified current expansion scheme over open surfaces based on the equivalence theorem, which employs closed surfaces, in principle. Weighting the expansion coefficients with a suitable window function compensates for the computed field errors that occur because of the open surfaces. Numerical simulations demonstrate that the equivalent surface currents expanded with low-ordered basis functions on an open surface and weighted by suitable functions can be used to obtain the correct electromagnetic fields in a limited volume near the surface.
Open Access
A z-gradient array coil with a dedicated active-shielded array coil for MRI
(John Wiley and Sons Inc., 2022-08-02) Takrimi, Manouchehr; Atalar, Ergin
Purpose: An array-based z-gradient coil with a set of programmable power amplifiers can outperform a conventional z-gradient coil and make it highly customizable with a broader range of tunable features. Methods: A dynamically adjustable imaging volume can be achieved using a pair of independent arrays and a modified optimization procedure based on analytic equations. Two modes of operation are provided: (a) standard mode that resembles a conventional coil; (b) advanced mode, where all performance parameters can be adjusted employing a controllable feeding mechanism. Commercial software is used to demonstrate the validity and feasibility of the proposed coil. Results: Primary and shield array diameters are 24 and 30 cm, both of which comprise 12 bundles of 10 turns copper wires. Maximum feeding voltage/current is 250 V/100 A for all array elements. Four distinct magnetic profiles are provided: (a) conventional profile with 140 mm diameter spherical region of interest, 120 mT/m gradient, and up to 4500 T/m/s slew rate; (b) profile of 200 mT/m, 70 mm region of interest, and up to 6900 T/m/s slew rate; (c) 60 mm axially shifted 70 mm region of interest with 120 mT/m strength and 3600 T/m/s slew rate; and (d) profile of 370 mT/m, 120 mm region of interest, and 3700 T/m/s slew rate when the active shield is reverse fed. Conclusion: By using an active-shielded gradient array coil, the magnetic field profile of the imaging volume can be adjusted dynamically, and it can provide new features and a wide range of field profiles for diverse applications in MRI.