Sensitivity and uniformity improvement of phased array MR images using inductive coupling and RF detuning circuits
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RESEARCH ARTICLE
Sensitivity and uniformity improvement of phased array MR images using inductive coupling and RF detuning circuits Bu S. Park1 · Sunder S. Rajan2 · Brent McCright1 Received: 21 June 2019 / Revised: 26 December 2019 / Accepted: 9 January 2020 © This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2020
Abstract Objective To improve sensitivity and uniformity of MR images obtained using a phased array RF coil, an inductively coupled secondary resonator with RF detuning circuits at 300 MHz was designed. Materials and methods A secondary resonator having detuning circuits to turn off the resonator during the transmit mode was constructed. The secondary resonator was located at the opposite side of the four-channel phased array to improve sensitivity and uniformity of the acquired MR images. Numerical simulations along with phantom and in vivo experiments were conducted to evaluate the designed secondary resonator. Results The numerical simulation results of |B1+| in a transmit mode showed that magnetic field uniformity would be decreased with a secondary resonator having no detuning circuits because of unwanted interferences between the transmit birdcage coil and the secondary resonator. The standard deviation (SD) of |B1+| was decreased 57% with a secondary resonator containing detuning circuits. The sensitivity and uniformity of |B1−| in the receive mode using a four-channel phased array were improved with the secondary resonator. Phantom experiments using a uniform saline phantom had 20% improvement of the mean signal intensity and 50% decrease in the SD with the secondary resonator. Mice with excess adipose tissue were imaged to demonstrate the utility of the secondary resonator. Conclusion The designed secondary resonator having detuning circuits improved sensitivity and uniformity of mouse MR images acquired using the four-channel phased array. Keywords Resonator · Inductive coupling · Computational modeling · Detuning circuits
Introduction High-field magnetic resonance imaging (MRI) systems (> 3 T) are widely used in research, because the intrinsic signal-to-noise ratio (ISNR) in MRI is proportional to the square of static field magnitude [1]. However, high-field MRI also suffers from loss in radiofrequency (RF) field uniformity [1–4] resulting in decreased MR image uniformity and increased dissipated power [3, 5–7].
* Bu S. Park [email protected] 1
Division of Cellular and Gene Therapies (DCGT)/OTAT/ CBER, Food and Drug Administration, Silver Spring, MD 20993‑0002, USA
Division of Biomedical Physics (DBP)/OSEL/CDRH, Food and Drug Administration, Silver Spring, MD 20993‑0002, USA
2
The phased array receive only RF coil, or phased array, is commonly used in MRI to improve sensitivity during a receive mode [7]. Because the phased array is designed for receiving signal, a transmit RF coil is needed to make a uniform transmit RF magnetic field ( B1). Therefore, RF detuning circuits are needed in the phased array
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