Development and Optimisation of Radial Techniques for Sodium Magnetic Resonance Imaging

The goal of this work was to provide techniques and hardware for 23Na MRI of the human brain, heart and muscle in a clinical scanner at 1.5T. For this purpose, radiofrequency (RF) coils were developed and a transmit/receive switch was adapted to 16.84 MHz. A 3D radial gradient echo (GRE) sequence was implemented, with a minimum echo time TEmin = 0.07ms for 1H and 0.2ms for 23Na, allowing to detect both the short (T2= 0.5ms) and the long (T2= 12-25ms) components of the 23Na NMR signal for total 23Na content evaluation. A gridding reconstruction algorithm with a Kaiser-Bessel window and a rho filter was implemented and optimised for both 23Na and 1H MRI. At an acquisition time Tacq= 10min and a nominal resolution Dx= 4mm, the signal-to-noise ratio SNR of the in-vivo 23Na 3D radial images was twofold higher than in standard cartesian GRE MRI (TEmin= 2ms). In the radial images blurring due to T2 signal decay during data acquisition was observed, reducing the resolution by approximately a factor of two. Both cartesian and radial GRE methods were compared at 1.5T and 4T. An SNR increase of SNR(4T)/SNR(1.5T) ~4 was measured in-vivo. In brain tumour patients, a 20% 23Na MRI signal increase in the tumour region was detected. A contrast-to-noise ratio CNR = 23% between healthy tissue and tumour achieved with the 3D radial was 20% higher than the CNR of the cartesian sequence. In patients with a 23Na channel disfunction, a 23Na MRI signal increase of ~10% was measured. This method allows for the detection of changes in the intracellular 23Na concentration and may provide a new tool to non-invasively evaluate tissue vitality