A Simple, but Variable Transformer-Matched Resonance Circuit for High Resolution Sodium-23 Magnetic Resonance Imaging...

A novel variable transformer-matched circuit was designed, developed, and tested for Sodium-23 Magnetic Resonance Imaging (MRI) of the rat brain. Sodium-23 MRI signal is approximately 20000 times lower than commonly used hydrogen-1 MRI signal and therefore highly optimized resonance detectors promise to maximize the possible image resolution. Physiologically, sodium-23 nuclei accumulate in brain tissue with impaired viability, for instance after a vascular blockage of an artery supplying the brain. Developing highly optimized radio-frequency circuits for MRI is challenging as the large and strong static magnets only provide limited access to adjust circuits inside the scanner. However, adjustment is needed to compensate for various loss mechanisms due to circuit and tissue interaction. Common circuits use capacitive matching through commercially available trimmer capacitors. Alternatively, transformer-matching offers further reduced losses while it is challenging to mechanically vary the mutual inductance in practice. Here we propose to use a novel mechanical mechanism that translates a rotation of a rod from outside the scanner to a longitudinal shift of the inductive coupling coil relative to the tuned MRI resonance coil. By doing so the coupling factor between the two inductors can be stably varied and hence optimal 50Ω matching can be achieved. Superior Signal-to-Noise-Ratio of ~60 in ventricles and ~20 in tissue was recorded in volume elements of 0.5 x 0.5 x 2mm nominal resolution recorded in 5minutes using a gradient echo sequence. Most notably, the non-invasive measurement of the sodium-23 signal showed an increased intensity in ischemic tissue which may provide a promising measure to monitor cell viability in the future. We conclude that sodium-23 MRI at 9.4T benefits from an optimized resonant circuit for signal detection providing a promising tool to study metabolic and ion concentration related changes after stroke non-invasively with sufficient spatial and temporal resolution.Clinical Relevance- This report presents superior image resolution for sodium-23 Magnetic Resonance Imaging of a rat stroke model that may enable to monitor tissue viability faster and with higher resolution aiding development of new treatment strategies.