Selecting the Reference Image for Registration of CEST Series

Background: To compare different reference images selected for registration among chemical exchange saturation transfer (CEST) series. Materials and Methods: Five normal volunteers and eight brain tumor patients were studied on a 3 Tesla scanner. Image registration was performed by choosing each of the acquired CEST saturation or unsaturation dynamic images as the reference. CEST images at 3.5ppm (amide proton transfer, APT) were computed for each motion-corrected data set after main magnetic field inhomogeneity correction. A uniformity index was defined to quantify the efficacy of image registration using different reference images. Joint histograms and the structural similarity index (SSIM) were used to analyze the intrinsic image similarity between various dynamic images. Results: Image registration increased the average uniformity index by 18% if the 3.5ppm saturated image was selected as the reference image. However, registering to the unsaturated dynamic image reduced the uniformity index by 13% on average. The joint histogram analysis showed that the saturated dynamic images were highly similar (SSIM=0.89 +/- 0.01), and were considerably different from the unsaturated dynamic image (SSIM=0.58 +/- 0.03). Conclusion: The selection of the 3.5ppm dynamic image as the reference image generated the highest uniformity index for APT imaging though other saturated images were equally suited as reference images.