The design and test of a new volume coil for high field imaging

Abstract

A major problem in the development of high field (>100 MHz) large volume (>6000 cm 3 ) MR coils is the interaction of the coil with the subject as well as the radiation loss to the environment. To reduce subject perturbation of the coil resonance modes, a volume coil that uses an array of freely rotating resonant elements radially mounted between two concentric cylinders was designed for operation at 170 MHz. Substantial electromagnetic energy is stored in the resonant elements outside the sample region without compromising the efficiency of the overall coil. This stored energy reduces the effect of the subject on the circuit and maintains a high Q, facilitating the tuning and matching of the coil. The unloaded O of the coil is 680; when loaded with a head, it was 129. The ratio of 5.3 of the unloaded to loaded Q supports the notion that the efficiency of the coil was maintained in comparison with previous designs. The power requirement and signal-to-noise performance are significantly improved. The coil is tuned by a mechanism that imparts the same degree of rotation on all of the elements simultaneously, varying their degree of mutual coupling and preserving the overall coil symmetry. A thin radiofrequency shield is an integral part of the coil to reduce the radiation effect, which is a significant loss mechanism at high fields. MR images were collected at 4T using this coil design with high sensitivity and B 1 homogeneity.

Sodium and T1ρ MRI for molecular and diagnostic imaging of articular cartilage

“…Further, the recent proliferation of 7 T whole-body MRI scanners in clinical research centers could have a significant impact on sodium MRI and its potential for clinical use. Since SNR scales as B 7=2 0 (158)(159)(160)(161) and the lack of B 1 penetration and B 0 susceptibility are issues that plague proton imaging, sodium MRI can be particularly advantageous at higher fields. Further, unlike proton T 1 , which increases with field, as the T 1 of sodium is predominantly due to quadrupolar interaction, it may not change appreciably at higher field.…”

Ultrahigh field magnetic resonance imaging and spectroscopy

“…SNR, however, becomes rather complex when high magnetic fields (hence high frequencies) are considered with lossy biologic samples such as the human body and the human head. The relationship between SNR and resonance frequency, , or equivalently field strength has been examined for biologic samples in numerous studies [3][4][5][6][7][8][9], predicting increases with field strength. At high frequencies such as 170 MHz ( 1 H frequency for 4 Tesla) and above, SNR must be considered as a function of location within a sample and for particular sample geometries.…”

“…Today, functional images with subcentimeter resolution of the entire human brain can be generated in single subjects and in data acquisition times of several minutes using 1. 5 Tesla MRI scanners that are often employed in hospitals for clinical diagnosis. However, there have been advantages in using significantly higher magnetic fields such as 4 Tesla, and recently 7 Tesla in humans, and 9.…”

In‐depth study of the electromagnetics of ultrahigh‐field MRI

“…Since the advent of UHF MRI, distributed-circuit resonators have shown good potential for use with human applications above 7 T, as exemplified by the transmission line resonator of Roschmann (25), the transverse electromagnetic (TEM) resonator of Vaughan et al (26) and the free element resonator of Wen et al (27). As magnet strength and the Larmor frequency increase, the length of the conductors (struts) used in head-sized volume coils become a significant fraction of the operating wavelength.…”