Efficient 23 Na triple-quantum signal imaging on clinical scanners: Cartesian imaging of single and triple-quantum 23 Na (CRISTINA)

Abstract

Purpose: To capture the multiquantum coherence (MQC) ²³ Na signal. Different phase-cycling options and sequences are compared in a unified theoretical layout, and a novel sequence is developed.

Methods: An open source simulation overview is provided with graphical explanations to facilitate MQC understanding and access to techniques. Biases such as B₀ inhomogeneity and stimulated echo signal were simulated for 4 different phase-cycling options previously described. Considerations for efficiency and accuracy lead to the implementation of a 2D Cartesian single and triple quantum imaging of sodium (CRISTINA) sequence employing two 6-step cycles in combination with a multi-echo readout. CRISTINA was compared to simultaneous single-quantum and triple-quantum-filtered MRI of sodium (SISTINA) under strong static magnetic gradient. CRISTINA capabilities were assessed on 8 × 60 mL, 0% to 5% agarose phantom with 50 to 154 mM ²³ Na concentration at 7 T. CRISTINA was demonstrated subsequently in vivo in the brain.

Results: Simulation of B₀ inhomogeneity showed severe signal dropout, which can lead to erroneous MQC measurement. Stimulated echo signal was highest at the time of triple-quantum coherences signal maximum. However, stimulated echo signal is separated by Fourier Transform as an offset and did not interfere with MQC signals. The multi-echo readout enabled capturing both single-quantum coherences and triple-quantum coherences signal evolution at once. Signal combination of 2 phase-cycles with a corresponding B₀ map was found to recover the signal optimally. Experimental results confirm and complement the simulations.

Conclusion: Considerations for efficient MQC measurements, most importantly avoiding B₀ signal loss, led to the design of CRISTINA. CRISTINA captures triple-quantum coherences and single-quantum coherences signal evolution to provide complete sodium signal characterization including $T_2^{\ast }$ fast, $T_2^{\ast }$ slow, MQC amplitudes, and sodium concentration.

Keywords: triple-quantum sodium imaging; 23Na MRI; coherence transfer; multiple-quantum coherences.