Design and first implementation of wireless square-shaped transmission line resonators in 1H MRI for small animal studies.

This work is dedicated to the development of a novel design for wireless transmission line resonators (TLRs). The TLRs are often considered as circular-shaped coils made up of two conductive circuits separated by a dielectric layer. We propose a square-shaped TLR design, wherein the coil has two square turns with two symmetrical gaps on each of the conductive layers, and the latter are rotated relative to each other by 90°. The calculation error of the resonant frequency of the square-shaped TLRs is no more than ∼3% of the measured value. The effectiveness of the square-shaped TLR design was evaluated in comparative 1H MRI studies to conventional wireless square loop of the same resonant frequency and with the same-sized inner square of the TLR. The Bruker birdcage was used as a transceiver and as inductively coupled with the wireless coils. We found that the performance of the square-shaped TLR and the square loop is comparable, but the B1+-field generated by the TLR has a wider distribution profile. It was reflected in rat brain studies, when some structures of rat head were not captured by the square loop. Comparative experiments with a standard circular-shaped TLR showed that a signal is predominantly concentrated inside the inner turn of the TLRs. The proposed TLR design can be a promising path to be explored, especially for scanning small objects of study, when the scan area is comparable to the size of the rigid lumped capacitors.

Computation of the resonance frequencies of the transmission line resonators used in MRI.

Stripline high frequency resonators or transmission line resonators (TLRs) manufactured as concentric RF coils at the opposite sides of a dielectric sheet serve as wireless self-resonant transmitters-receivers in MRI. Owing to their high quality factor relative to traditional RF coils composed of bulk inductor and capacitors, frequency selectivity of TLRs is high, making them promising elements for single-nucleus MRI. However, the computation of their resonance frequencies is cumbersome, and numerous mathematical mistakes and typos in publications lead to incorrect results. The present publication is the first to summarize the corrected formulas for computations and presents comparison of such computations to real measurements.