Measuring human cardiac tissue sodium concentrations using surface coils, adiabatic excitation, and twisted projection imaging with minimal T₂ losses

Purpose: To measure tissue sodium concentrations in the human heart with ²³Na MRI using a surface coil, thereby eliminating the effects of inhomogeneous excitation by surface coils and minimizing T₁ and T₂ relaxation. Materials and Methods: We combined fully relaxed, very short-echo, ²³Na twisted projection Imaging (TPI) with adiabatic half passage (AHP) excitation and external referencing on subjects and comparing with a concentration reference phantom scan to quantify TSC with surface coils. ²³Na signal losses during hard (square), composite, and tanh/tan amplitude/frequency-modulated AHP excitation pulses were analyzed over a wide range of RF field strengths and T_2short values. Results: AHP excitation yielded a homogeneous excitation flip single and negligible losses compared to a 90° hard pulse wherever the B1 field exceeded the adiabatic threshold, rendering this sequence suitable for applications that use surface coil excitation. An AHP ²³Na TPI sequence was used with a surface coil at 1.5 T to noninvasively quantify myocardial TSC In 10 normal volunteers. The mean TSC was 43 ± 4, 53 ± 12, and 17 ± 4 μmol/g In the left ventricular (LV) free wall, septum, and adipose tissue, respectively, consistent with prior invasive measurements on biopsy and autopsy specimens. Conclusion: It is now possible to noninvasively quantify TSC In the human heart with surface coil ²³Na MRI.