UHF Coil & Array Design
Tamer S Ibrahim1

1University of Pittsburgh, United States

Synopsis

The clinical and research potential of MRI for whole-body applications at high (≥ 3 tesla) fields and of head applications at ultrahigh (UHF) (≥7 tesla) fields appears to be limitless. It is however limited by technical challenges. The most notable of these difficulties include 1) safety concerns regarding exceeding radiofrequency (RF) power deposition in tissue and 2) large image inhomogeneity/voids due to “undesired” RF field inhomogeneity across the anatomy. The main aim of this course is to explore UHF coil and array designs that aim at addressing these issues.

Introduction

The clinical and research potential of magnetic resonance imaging/spectroscopy (MRI/S) for whole-body applications at high (≥ 3 tesla) fields and of head applications at ultrahigh (UHF) (≥7 tesla) fields appears to be limitless. It is however limited by technical challenges. The most notable of these difficulties include 1) safety concerns regarding exceeding radiofrequency (RF) power deposition in tissue [1, 2] and 2) large image inhomogeneity/voids due to “undesired” RF field inhomogeneity [3, 4] across the anatomy. The main aim of this course is to explore UHF coil and array designs.

Background

Higher field strengths correspond to increased Larmor frequencies and therefore operational RF frequencies. A 7 Tesla for instance, coil/transmit array excites B1+ field at 298 MHz for proton imaging. At this frequency (wavelength is approximately 12-cm in tissue) and higher, the wavelengths of the electromagnetic waves produced by RF coils/arrays become smaller than the human head; or in other words, the human head becomes electrically “large”. Unlike the case at lower field strengths, the electromagnetic waves now have to “travel” significant electrical distances in the human head. As a result, the electromagnetic fields become non-uniform which will result in inhomogeneous B1+ and B1- fields in biological tissues as well as inhomogeneous electric fields and, therefore, localized SARs. Both which can have a devastating effect on the integrity of the images and on the safety of the patient.

RF Shimming

Variable phase/amplitude multi-port excitation or B1 shimming (in electromagnetic terms: phased array antenna excitation) is based on the fact that for high-frequency MRI operation and asymmetrical/inhomogeneous/irregularly-shaped loading (human head/body), integer multiples of phase-shifts and uniform amplitudes are not necessarily the ideal characteristics to impose on the voltages driving the transmit array in order to obtain a homogeneous transmit field. Furthermore, overall as well as localized RF field excitation in high field human MRI may be achieved with rather distinctive and non-obvious amplitudes/phases associated with the excitation voltages. B1+ shimming have generally aimed at reducing RF power deposition in tissue and homogenizing the RF field across the anatomy of interest.

RF Coils and Arrays

Since the advent of ultrahigh field MRI, distributed circuit resonators have shown good potential for use with human applications above 7 Tesla, as exemplified by the transmission line resonator [5], the transverse electromagnetic (TEM) resonator [6], and the free element resonator [7]. As magnet strength and the Larmor frequency increase, the length of the conductors used in head sized volume coils become a significant fraction of the operating wavelength. In this case, the coil struts develop self resonance which may degrade coil homogeneity and fill factor since the current is no longer uniform along each strut. As a result, the transmission line properties of the coil’s conductors become significant, invalidating lower field strength assumptions, which neglected them. In contrast to lumped element designs, distributed circuit resonators utilize and enhance the transmission line properties of conductors by using the intrinsic reactance of transmission line elements. Many designs of transmit coils and arrays have been used in exciting the RF field at various MRI field strengths. For instance transmit coils and arrays can be coupled or uncoupled. Decoupled coils and arrays tend to be more efficient and easy to operate while coupled transmit arrays and coil element tend to provide more B1 field coverage that is less sensitive to the subject being imaged.

Acknowledgements

National Institutes of Health

References

[1] J. T. Vaughan, M. Garwood, C. M. Collins, W. Liu, L. DelaBarre, G. Adriany, et al., "7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head images," Magn Reson Med, vol. 46, pp. 24-30, Jul 2001.

[2] T. S. Ibrahim and L. Tang, "Insight into RF power requirements and B1 field homogeneity for human MRI via rigorous FDTD approach," Journal of magnetic resonance imaging, vol. 25, pp. 1235-47, Jun 2007.

[3] T. S. Ibrahim, C. Mitchell, P. Schmalbrock, R. Lee, and D. W. Chakeres, "Electromagnetic perspective on the operation of RF coils at 1.5-11.7 Tesla," Magn Reson Med, vol. 54, pp. 683-90, Sep 2005.

[4] L. L. Wald, G. C. Wiggins, A. Potthast, C. J. Wiggins, and C. Triantafyllou, "Design considerations and coil comparisons for 7 T brain imaging," Applied Magnetic Resonance, vol. 29, pp. 19-37, 2005.

[5] P. K. Roschmann, "High-frequency coil system for magnetic resonance imaging apparatus," 4,746,866, 1988.

[6] J. T. Vaughan, H. P. Hetherington, J. O. Otu, J. W. Pan, and G. M. Pohost, "High-Frequency Volume Coils for Clinical Nmr Imaging and Spectroscopy," Magnetic Resonance in Medicine, vol. 32, pp. 206-218, Aug 1994.

[7] H. Wen, A. S. Chesnick, and R. S. Balaban, "The Design and Test of a New Volume Coil for High Field Imaging," Magn Reson Med, vol. 32, pp. 492-498, 1994.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)