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

Abstract

To measure tissue sodium concentrations in the human heart with (23)Na MRI using a surface coil, thereby eliminating the effects of inhomogeneous excitation by surface coils and minimizing T(1) and T(2) relaxation. We combined fully relaxed, very short-echo, (23)Na 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. (23)Na 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. AHP excitation yielded a homogeneous excitation flip angle and negligible losses compared to a 90 degrees hard pulse wherever the B1 field exceeded the adiabatic threshold, rendering this sequence suitable for applications that use surface coil excitation. An AHP (23)Na 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 micromol/g in the left ventricular (LV) free wall, septum, and adipose tissue, respectively, consistent with prior invasive measurements on biopsy and autopsy specimens. It is now possible to noninvasively quantify TSC in the human heart with surface coil (23)Na MRI.