Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI), which exploits saturation transfer induced proton exchange and its corresponding, indirect loss of water signals, has been shown to provide a novel contrast mechanism [1]. However, in CEST MRI, multiple acquisition of imaging data with varying saturation frequencies (z-spectrum) is typically needed, which prohibitively prolongs imaging time and spatial coverage is correspondingly limited. To tackle these problems, in this work we propose whole-brain spiral CEST encoding with spectral and spatial B₀ correction that employs RF-segmented, uneven irradiation [2] to quickly acquire multi-slice data.

Whole-brain Spiral CEST Encoding with RF-Segmented uneven irradiation: We proposed RF-segmented, uneven irradition that a long primary RF saturation pulse generates the steady state CEST contrast, and repetitive short secondary RF saturation pulse maintains the CEST contrast. A timing diagram of the proposed spiral-CEST encoding is shown in Fig. 1. In the proposed fast spiral encoding acqusition, CEST contrast is maintained by secondary short RF saturation pulse. Gaussian-shaped RF pulses applied with the amplitude of 0.7uT (50ms). The primary RF irradiation time was 3 sec (T_s1) and secondary RF irradiation time was 0.5 sec (T_s2), while single shot spiral acquisition [3] was employed in the step of CEST encoding. Each z-spectrum consisted of 21 offsets -5ppm to 5ppm with an increment of 0.5ppm. Other parameters used: T_I = 50ms, TE = 4ms, flip angle = 60°, matrix size = 64×64, FOV = 200×200mm², we acquired thirty slices for slices thickness = 3mm and slice interval = 1.5mm. To investigate the effectiveness of the proposed method against conventional echo-planar-imaging (EPI) CEST MRI, we also acquire single shot EPI with unevenly segmented method. The parameters were, T_I = 60ms, TE = 20ms, T_s1/T_s2 = 3s/0.5s, and same protocol of other parameters. EPI-CEST is implemented with dicom images. All images were acquired at 3.0T (Magnetom Trio, Siemens Medical Solutions, Erlangen, Germany).

Spatial and Spectral B₀ Correction: In order to compensate spiral trajectory errors, which result from gradient hardware imperfection and eddy-currents, actual k-space trajectory were measured and corrected [4]. Then, convolution-interpolated followed by inverse Fourier transform were performed to produce spatiotemporal images in the x-z dimension that are potentially blurred due to the magnetic field inhomogeneities. In order to correct spectral B₀ shift, we applied for WASSR B₀ correction method [5]. The field map estimated from the analysis of the z-spectrum, in each image correction of spatial blurring was performed using multi-frequency interpolation (MFI) method [6]. With correction of image blurring in the spatial dimension of images, the initial field map was refined using WASSR, and spectral B₀ correction was performed to eliminate data inconsistency over the spectral dimension that potentially occurred in the previous step. To investigate the effectiveness of the proposed method, CEST maps were generated using no B₀ correction, spectral B₀ correction, and spectral and spatial B₀ correction.

Fig. 2 shows selected slice CEST-weighted images and MTR maps. It is noted that the first and second columns show severe blurring and distortion artifact due to the field inhomogeneities especially edge of brain both without B₀ correction and with spectral B₀ correction. Third column shows spectral and spatial B₀ corrected CEST-weighted images and MTR maps, which shows improvement in spatial homogeneity in particular region of edge. First and Second rows represent S(-3.5ppm) and S(3.5ppm) images and third row shows MTR maps. The results of that the field inhomogeneities leading to artifact in CEST analysis can be corrected using proposed method. Fig. 3 represents that compare EPI-CEST to spiral-CEST with spatial B₀ corrected multislice images. It shows that EPI-CEST more distortion artifact in front of brain. Fig. 4 represents that MTR maps compare EPI-CEST to spiral-CEST. Proposed spiral-CEST with spectral and spatial B₀ correction of field inhomogeneities is better able to assess CEST analysis without distortion artifacts. We successfully demonstrated that the proposed spiral-CEST encoding is highly effective in the generating proton exchange induced whole-brain imaging contrast within clinically reasonable scan time.