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Abstract

Purpose/Introduction: Sodium imaging benefits greatly from the possibility to perform MRI at ultra-high field strength due to the high signal-to-noise ratio (SNR). Here we demonstrate the feasibility to obtain high-resolution tissue sodium concentration (TSC) maps at 9.4 T in combination with correction methods for B0 and B1 inhomogeneity. Subjects and Methods: All the measurements were performed on a healthy human volunteer on a 9.4 T whole-body MRI scanner. A 16-rung birdcage coil was used for the excitation and reception of the sodium signal. The latter was sampled with an acquisition weighted stack of spirals [1]. The repetition time was set to 150ms in order to allow for a full recovery of the magnetization. Further sequence parameters were: echo time 0.3ms, rectangular RF pulse, flip angle (FA) 90°, resolution 1x1x5mm3, field-of-view 240x240mm2, partitions 44, averages 6, acquisition time 30min. Two images with a nominal FA of 45° and 90° and a resolution of 5x5x5mm3 were acquired to calculate a FA map with the double-angle method [2]. The measurement time was 5min for each image. A B1 correction of the sodium density weighted images was performed based on these FA maps. A proton patch antenna [3] placed behind the sodium coil was used for shimming and the acquisition of B0 maps, which served as the basis for a posterior off-resonance correction with a conjugate phase reconstruction [4]. Results: The birdcage coil achieved a homogenous excitation of the sodium nuclei as can be seen in Figure 1a. The corresponding B0 map is shown in Figure 1b. The range of measured off-resonance for the sodium nuclei is about 200 Hz. The effectiveness of the B0 correction can be appreciated in Figure 1c,d. The B0 and B1 corrected TSC maps are shown in Figure 2. The measured values for CSF and brain matter are comparable to values found in the literature [5]. Discussion/Conclusion: This initial study demonstrates that high resolution field-corrected quantitative sodium images can be obtained at 9.4 T. The long scanning time could be reduced up to a certain degree without losing SNR by adding a tightly fitting receive array to the birdcage coil [6]. More measurements are required to assess the potential of sodium imaging at ultra-high field.