Differential phase contrast imaging has recently been demonstrated using both synchrotron and conventional xray sources with a grating interferometer. This approach offers the possibility of simultaneous CT reconstructions of both absorption and index of refraction from a single acquisition. This enables direct comparison of both types of reconstructed images under identical conditions. One of the most important performance metrics in CT imaging is that of contrast-to-noise ratio. These results measure the contrast-to-noise ratio for a grating interferometer-based differential phase contrast imaging system at a range of exposure levels and for several materials. For three of the four cases measured, the contrast-to-noise ratio of differential phase contrast CT images was superior to that of absorption CT images. The most dramatic improvement was noted in the contrast between PMMA and water, where the contrast-to-noise ratio increased from less than 1 in absorption CT images, to approximately 8 in the differential phase contrast CT images. Additionally, a breast tissue specimen containing a highly malignant carcinoma was scanned and reconstructed using both phase and absorption contrast reconstructions to illustrate the superior performance of the phase contrast imaging method.