An Adjustable-Length Dipole Using Forced-Current Excitation for 7T MR

Abstract

Ultrahigh field imaging of the body and the spine is challenging due to the large field-of-view (FOV) required. It is especially difficult for RF transmission due to its requirement on both the length and the depth of the ${\rm{B}}_{1}^{{\rm + }}$ field. One solution is to use a long dipole to provide continuous current distribution. The drawback is the natural falloff of the ${\rm{B}}_{1}$ field toward the ends of the dipole, therefore the ${\rm{B}}_{1}^{{\rm + }}$ per unit square root of maximum specific absorption rate ${\rm{(B}}_{1}^{{\rm + }}{\rm{/ \surd SAR}}_{{\rm{max}}})$ performance is particularly poor toward the end of the dipole. In this study, a segmented element design using forced-current excitation and a switching circuit is presented. The design provides long FOV when desired and allows flexible FOV switching and power distribution without additional power amplifiers. Different element types and arrangements were explored and a segmented dipole design was chosen as the best design. The segmented dipole was implemented and tested on the bench and with a phantom on a 7T whole body scanner. The switchable mode dipole enabled a large FOV in the long mode and improved ${\rm{B}}_{1}^{{\rm + }}{\rm{/ \surd SAR}}_{{\rm{max}}}$ efficiency in a smaller FOV in the short mode.

Multiple slot modules for high field magnetic resonance imaging array coils

“…This provides a clean area outside the modules for placement of a receive coil. Theoretically, slots driven by forced current excitation could then be detuned by shorting the feed gap in the manner described by Cui et al 29 In future design, slot configuration, the number of modules, number of slots within each module, separating distance between the slots, and the length of each slot element are some of the variables that might be optimized to achieve better performance. Additional improvements include designing a controllable feed that allows for independent phase and amplitude control to facilitate B 1 shimming and implementing a selective feed that allows exciting only the region of interest.…”

“…The spacing between the centers of the slots was 7 cm along the y‐axis. Using forced current excitation, each slot was fed at the center through a quarter wavelength twinax cable (RG108A/U) connected to a common‐voltage feed point 29,47,48 and a semi‐balanced matching and tuning network. High voltage non‐magnetic variable trimmer capacitors (SGNMNC1206, Sprague Goodman Electronics) allowed for matching and tuning the modules.…”

“…With the intention of improving the performance of multi‐channel coils at high field MRI, several research groups are investigating coil elements such as dipoles 29–35 . Dipoles as multi‐channel coil elements show much promise when applied to high field MRI 34,36,37 .…”

8-channel Tx dipole and 20-channel Rx loop coil array for MRI of the cervical spinal cord at 7 Tesla

“…Local transmit coils can reduce this difficulty by placing the coil in close proximity to the subject. This has been accomplished with two-channel coils 7,[35][36][37] and array coils 2,5,8,20,[38][39][40] . To additionally leverage the capability of array coils to improve transmit uniformity through RF shimming or spatially selective pulses, an 8-channel transverse electromagnetic (TEM) coil 17 and a 16-channel meandered stripline coil 41 were developed for torso imaging, including the lower spinal cord.…”

“…As such, dipoles have been used as the building block for transmit coils intended for head imaging [49][50][51][52] and body imaging 15,53 . For spine imaging, dipoles have been implemented to improve transmit efficiency 7,12,39 , SAR efficiency 19,54 , and to increase the potential field-of-view 36 .…”