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

Abstract

Abstract Purpose To evaluate the feasibility of interleaved 23 Na/ 1 H cardiac MRI at 7 T using 1 H parallel transmission (pTx) pulses. Methods A combined setup consisting of a 23 Na volume coil and two 1 H 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 23 Na/ 1 H pTx sequence was validated in phantom measurements and applied to four healthy subjects. For the latter, three customized 1 H excitation pulses (universal and individual phase shims (UPS/IPS) and individual 4kT pulses (4kT)) were employed in the interleaved 23 Na/ 1 H pTx sequence and compared with the vendor‐provided default cardiac phase shim (DPS). Results Combining both coils resulted in a reduction of the mean 23 Na transmit field (B 1 + ) efficiency and 23 Na signal‐to‐noise ratio by 18.9% and 15.4% for the combined setup, whereas the 1 H B 1 + efficiency was less influenced (−4.7%). Compared with single‐nuclear acquisitions, interleaved dual‐nuclear 23 Na/ 1 H MRI showed negligible influence on 23 Na and 1 H image quality. For all three customized 1 H pTx pulses the B 1 + homogeneity was improved (coefficients of variation: CV UPS = 0.30, CV IPS = 0.23, CV 4kT = 0.15) and no 1 H signal dropouts occurred compared with the vendor‐provided default phase shim (CV DPS = 0.37). Conclusion The incorporation of customized 1 H pTx pulses in an interleaved 23 Na/ 1 H sequence scheme was successfully demonstrated at 7 T and improvements of the 1 H B 1 + homogeneity within the heart were shown. Combining interleaved 23 Na/ 1 H MRI with 1 H pTx is an important tool to enable robust quantification of myocardial tissue sodium concentrations at 7 T within clinically acceptable acquisition times.