Interleaved 23Na/1H MRI of the human heart at 7 T using a combined 23Na/1H coil setup and 1H parallel transmission

Abstract

Purpose

To evaluate the feasibility of interleaved ²³Na/¹H cardiac MRI at 7 T using ¹H parallel transmission (pTx) pulses.

Methods

A combined setup consisting of a ²³Na volume coil and two ¹H transceiver arrays was employed and the transmit and receive characteristics were compared in vitro with those of the uncombined radiofrequency coils. Furthermore, the implemented interleaved ²³Na/¹H pTx sequence was validated in phantom measurements and applied to four healthy subjects. For the latter, three customized ¹H excitation pulses (universal and individual phase shims (UPS/IPS) and individual 4kT pulses (4kT)) were employed in the interleaved ²³Na/¹H pTx sequence and compared with the vendor-provided default cardiac phase shim (DPS).

Results

Combining both coils resulted in a reduction of the mean ²³Na transmit field (B₁⁺) efficiency and ²³Na signal-to-noise ratio by 18.9% and 15.4% for the combined setup, whereas the ¹H B₁⁺ efficiency was less influenced (−4.7%). Compared with single-nuclear acquisitions, interleaved dual-nuclear ²³Na/¹H MRI showed negligible influence on ²³Na and ¹H image quality. For all three customized ¹H pTx pulses the B₁⁺ homogeneity was improved (coefficients of variation: CV_UPS = 0.30, CV_IPS = 0.23, CV_4kT = 0.15) and no ¹H signal dropouts occurred compared with the vendor-provided default phase shim (CV_DPS = 0.37).

Conclusion

The incorporation of customized ¹H pTx pulses in an interleaved ²³Na/¹H sequence scheme was successfully demonstrated at 7 T and improvements of the ¹H B₁⁺ homogeneity within the heart were shown. Combining interleaved ²³Na/¹H MRI with ¹H pTx is an important tool to enable robust quantification of myocardial tissue sodium concentrations at 7 T within clinically acceptable acquisition times.