Sodium MRI in human heart: a review

Abstract

This paper offers a critical review of the properties, methods and potential clinical application of sodium ((23)Na) MRI in human heart. Because the tissue sodium concentration (TSC) in heart is about ~40 µmol/g wet weight, and the (23)Na gyromagnetic ratio and sensitivity are respectively about one-quarter and one-11th of that of hydrogen ((1)H), the signal-to-noise ratio of (23)Na MRI in the heart is about one-6000th of that of conventional cardiac (1)H MRI. In addition, as a quadrupolar nucleus, (23)Na exhibits ultra-short and multi-component relaxation behavior (T1 ~ 30 ms; T2 ~ 0.5-4 ms and 12-20 ms), which requires fast, specialized, ultra-short echo-time MRI sequences, especially for quantifying TSC. Cardiac (23)Na MRI studies from 1.5 to 7 T measure a volume-weighted sum of intra- and extra-cellular components present at cytosolic concentrations of 10-15 mM and 135-150 mM in healthy tissue, respectively, at a spatial resolution of about 0.1-1 ml in 10 min or so. Currently, intra- and extra-cellular sodium cannot be unambiguously resolved without the use of potentially toxic shift reagents. Nevertheless, increases in TSC attributable to an influx of intra-cellular sodium and/or increased extra-cellular volume have been demonstrated in human myocardial infarction consistent with prior animal studies, and arguably might also be seen in future studies of ischemia and cardiomyopathies--especially those involving defects in sodium transport. While technical implementation remains a hurdle, a central question for clinical use is whether cardiac (23)Na MRI can deliver useful information unobtainable by other more convenient methods, including (1)H MRI.

Keywords: MRI; T1; T2; heart; myocardial infarction; quantification; sodium; total sodium content; ultra-short echo time.

Figure 1

(a) Partial k-space sampling scheme for twisted projection imaging (TPI). The maximum k forms a sphere of radius k_max, and the projections lie on a surface of a cone whose twist is adjusted to maintain critical sample density (56). (b) The TPI pulse sequence with adiabatic half passage excitation. The hard pulse of the original TPI sequence (56) is replaced by an amplitude (amp.) and phase modulated AHP pulse, but the gradient waveforms, G_x, G_y, and G_z, are the same, and depicted for one of the projections (21). (c) Simulated point spread function for T_2f =2ms and T_2s =15ms signal components excited by a TPI sequence with TE=0.36ms, a 31.3 kHz bandwidth, 12 ms readout, 22cm FOV, 44×44×44 array size, and Δx =5mm nominal isotropic spatial resolution (58). The T_2s component has a full-width at half-maximum (fwhm) of about 1.5Δx (7.5 mm), while the fwhm of the T_2f component is 2.4Δx (12.0 mm). (Adapted from Refs. 21,56, 58).

Figure 2

Custom-built coils for human cardiac ²³Na MRI at 1.5 Tesla (16.9 MHz). (a) Square 25×25 cm three-turn transmit/receive surface coil used for quantitative cardiac TSC studies employing adiabatic excitation (21). The coil has 3 embedded vials of saline gel to serve as sensitivity references, and a mount permitting exchange with a conventional cardiac ¹H coil for cardiac MRI (21,22). (b) A 4-channel ²³Na phased-array constructed from 15-cm, 3-turns coils for increased inductance at the lower ²³Na frequency (46). The detectors were fabricated on 0.25-mm thick flexible printed circuit board, with a separate 40-cm square single-turn transmit coil for excitation.

Figure 3

(a) Conventional ¹H trans-axial image, and (b) corresponding single slice ²³Na MRI of the human heart acquired with the phased-array in Fig. 2(b) using a slice-selective gradient refocused echo (GRE) sequence in 4s, and (c) 200s (array size, 32 × 32; resolution, 1 × 1 × 4 cm³; TR/TE= 33/5 ms; from Ref. 46).

Figure 4

(a) TSC values in infarcted and remote myocardium in 20 MI patients, and in adjacent tissue in 11 of the patients. Vertical bars denote means ±SD. TSC is significantly increased in MI vs remote tissue (P <0.001). (b) Trans-axial gated ¹H fast spin-echo image, and (c) corresponding ²³Na image from a 3D TPI data set (TR/TE=85/0.4 ms; isotropic spatial resolution =6 mm) from a 61-year-old man with a remote septal MI (arrow). Color scale is proportional to TSC and includes B₁ correction; LV=left ventricle. (Adapted from Ref. 22).