Design and construction of an optimized transmit/receive hybrid birdcage resonator to improve full body images of medium-sized animals in 7T scanner.

Nibardo Lopez Rios,Mathieu Dehaes,Konstantinos Papoutsis,Frédéric Lesage,Philippe Pouliot,Julien Cohen-Adad,Nikola Stikov,Alexandru Foias,Yong Fan

doi:10.1371/journal.pone.0192035

Abstract

The purpose of this work was to develop an optimized transmit/receive birdcage coil to extend the possibilities of a 7T preclinical MRI system to conduct improved full body imaging in medium-sized animals, such as large New Zealand rabbits. The coil was designed by combining calculation and electromagnetic simulation tools. The construction was based on precise mechanical design and careful building practice. A 16-leg, 20 cm long, 16 cm inner diameter, shielded quadrature hybrid structure was selected. Coil parameters were assessed on the bench and images were acquired on phantoms and rabbits. The results were compared to simulations and data obtained with an available commercial coil. An inexpensive assembly with an increase of 2 cm in useful inner diameter and 50 Ω matching with larger animals was achieved. A reduction in radiofrequency (RF) power demand of 31.8%, an improvement in image uniformity of 18.5 percentage points and an increase in signal-to-noise ratio of up to 42.2% were revealed by phantom image acquisitions, which was confirmed by in vivo studies. In conclusion, the proposed coil extended the possibilities of a preclinical 7T system as it improved image studies in relatively large animals by reducing the RF power demand, and increasing image uniformity and signal-to-noise ratio. Shorter scans and time under anesthesia or reduced RF exposure, resulting in better images and lower animal health risk during in vivo experiments, were achieved.

Highlights

Rabbits are medium-sized animals of importance for cardiac research due to some similarity of their hearts to those of humans and their lower cost compared to larger animals [1]
The locally installed Agilent (Varian) 7T scanner (Santa Clara, California, US) has a 21 cm bore, which is reduced to 14 cm by the largest available radiofrequency (RF) coil: a quadrature birdcage made by the same manufacturer for a loading range that includes rabbits
The quality factor of the parallel plate capacitors was 454 at 300 MHz, which results in an equivalent series resistance (ESR) of 0.194 O

Summary

Introduction

Rabbits are medium-sized animals of importance for cardiac research due to some similarity of their hearts to those of humans and their lower cost compared to larger animals [1]. The locally installed Agilent (Varian) 7T scanner (Santa Clara, California, US) has a 21 cm bore, which is reduced to 14 cm by the largest available radiofrequency (RF) coil: a quadrature birdcage made by the same manufacturer for a loading range that includes rabbits This type of commercial volume coil is usually optimized to fit the average sample of a selected loading range, but they can be impractical when more space is required to accommodate larger animals, such as obese rabbits of more than 4 kg used for cardiac research. This work presents the development of an optimized transmit/receive birdcage with extended space, increased image uniformity and higher signal-to-noise ratio (SNR) These improvements expanded the application range of the aforementioned scanner to mediumsized animals, such as large New Zealand rabbits, which cannot be optimally studied with the commercial coil

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