Accelerating the co-simulation method for the design of transmit array coils for MRI

Yılmaz

et al. 2021

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Purpose In simultaneous transmission and reception (STAR) MRI, along with the coupling of the excitation pulse to the received signal, noise, and undesired distortions (spurs) coming from the transmit chain also leak into the acquired signal and degrade image quality. Here, properties of this coupled noise and its relationship with the transmit amplifier gain, transmit chain noise density, isolation performance, and imaging bandwidth are analyzed. It is demonstrated that by utilizing a recently proposed STAR technique, the transmit noise can be reduced. The importance of achieving high isolation and careful selection of the corresponding parameters are demonstrated. Theory and Methods A cancellation algorithm, together with a vector modulator, is used for transmit‐receive isolation. The scanner is modeled as a pipeline of blocks to demonstrate the noise contribution from each block. With higher isolation, coupled transmit noise can be reduced to the point that the dominant noise source becomes acquisition noise, as in the case for pulsed MRI. Amplifiers with different gain and noise properties are used in the experiments to verify the derived noise‐transmit parameter relation. Results With the proposed technique, more than 80 dB isolation in the analog domain is achieved. The leakage noise and the spurs coupled from the transmit chain, are reduced. It is shown that the transmit gain plays the most critical role in determining sufficient isolation, whereas the amplifier noise figure does not contribute as much. Conclusion The transmit noise and the spurs in STAR imaging are analyzed and mitigated by using a vector modulator.

Section: Methodsmentioning

confidence: 99%

Section: Hardwarementioning

confidence: 99%

Analysis and mitigation of noise in simultaneous transmission and reception in MRI

Tasdelen

Yılmaz

et al. 2021

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“…In each loop, 5 distinct capacitor values, that is,

\bar{boldc} = false[\begin{matrix} c_{dr} & c_{dc} & c_{t} & c_{m} & c_{s} \end{matrix} false]

, mainly control the performance of the TxArray coils as free design parameters. The chosen structure for the dual‐row TxArray coil enables the coil to act like a single‐row degenerate birdcage TxArray coil 22,23,43,44,56‐59 when the mesh currents of the adjacent elements in the axial direction cancel each other in the mid‐ring segments of the coil. In the CP excitation mode, both rows are individually excited with identical power and linearly increasing phases, and the lower‐row channels are excited with a 180° phase shift relative to the upper‐row channels.…”

Section: Methodsmentioning

confidence: 99%

Eigenmode analysis of the scattering matrix for the design of MRI transmit array coils

Kazemivalipour

Atalar

2020

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To obtain efficient operation modes of transmit array (TxArray) coils using a general design technique based on the eigenmode analysis of the scattering matrix. Methods: We introduce the concept of modal reflected power and excitation eigenmodes, which are calculated as the eigenvalues and eigenvectors of S H S, where the superscript H denotes the Hermitian transpose. We formulate the normalized reflected power, which is the ratio of the total reflected power to the total incident power of TxArray coils for a given excitation signal as the weighted sum of the modal reflected power. By minimizing the modal reflected power of TxArray coils, we increase the excitation space with a low total reflection. The algorithm was tested on 4 dual-row TxArray coils with 8 to 32 channels. Results: By minimizing the modal reflected power, we designed an 8-element TxArray coil to have a low reflection for 7 out of 8 dimensions of the excitation space. Similarly, the minimization of the modal reflected power of a 16-element TxArray coil enabled us to enlarge the dimension of the excitation space by 50% compared with commonly employed design techniques. Moreover, we demonstrated that the low total reflected power for some critical excitation modes, such as the circularly polarized mode, can be achieved for all TxArray coils even with a high level of coupling. Conclusion: Eigenmode analysis is an efficient method that intuitively provides a quantitative and compact representation of the coil's power transmission capabilities. This method also provides insight into the excitation modes with low reflection.

Section: F I G U R Ementioning

confidence: 99%

“…growing in number and variety, [39][40][41][42][43][44][45][46][47][48][49] to cancel out the net-induced RF currents on multiple DBS electrodes simultaneously. Theoretically, this approach is not limited to 7 T and can be applied to any pTx coils at any field strength, including lower [39][40][41] and higher [42][43][44][45][46][47][48][49] field strengths. Yet, the implementation of this approach would not be possible using commonly used single-channel or even two-port-driven coils that are commercially available at lower field strengths (i.e., 1.5 T and 3 T).…”

Section: F I G U R Ementioning

confidence: 99%

Implant‐friendly MRI of deep brain stimulation electrodes at 7 T

DelaBarre

Zulkarnain

et al. 2023

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PurposeThe purpose of this study is to present a strategy to calculate the implant‐friendly (IF) excitation modes—which mitigate the RF heating at the contacts of deep brain stimulation (DBS) electrodes—of multichannel RF coils at 7 T.MethodsAn induced RF current on an implantable electrode generates a scattered magnetic field whose left‐handed circularly polarizing component () is approximated using a ‐mapping technique and subsequently used as a gauge for the electrode's induced current. Using this approach, the relative induced currents resulting from each channel of a multichannel RF coil on the DBS electrode were calculated. The IF modes of the corresponding multichannel coil were determined by calculating the null space of the relative induced currents. The proposed strategy was tested and validated for unilateral and bilateral commercial DBS electrodes (directional lead; Infinity DBS system, Abbott Laboratories) placed inside a uniform phantom by performing heating and imaging studies on a 7T MRI scanner using a 16‐channel transceive RF coil.ResultsNeither individual IF modes nor shim solutions obtained from IF modes induced significant temperature increase when used for a high‐power turbo spin‐echo sequence. In contrast, shimming with the scanner's toolbox (i.e., based on per‐channel fields) resulted in a more than 2°C temperature increase for the same amount of input power.ConclusionA strategy for calculating the IF modes of a multichannel RF coil is presented. This strategy was validated using a 16‐channel RF coil at 7 T for unilateral and bilateral commercial DBS electrodes inside a uniform phantom.