Fig. 1.

The PAG imaged at high resolution. The transaxial slice on the left shows the PAG from a functional scan at ultra-high field strength (7-T) and high resolution (0.75 mm isotropic). Scanning the PAG at lower resolutions prevents clear separation of the PAG from the aqueduct and surrounds. (A) The mean functional image for a single run at the 0.75-mm isotropic resolution used in this study shows the PAG crisply as indicated by the red arrow. (B) Downsampling the image to a resolution of 1.5 mm isotropic begins to blur the boundary between the PAG and its surrounds because of partial-volume effects. (C) Further downsampling the image to a resolution of 3 mm isotropic eliminates the ability to distinguish PAG from the aqueduct with any degree of confidence. (D–F) Smoothing with a standard 4-mm kernel further increases the partial-volume effects that blend signal from PAG with the aqueduct and surrounds, as shown for 0.75 mm isotropic (D), 1.5 mm isotropic (E), and 3 mm isotropic (F) resolutions. Most neuroimaging studies use a 3-mm isotropic resolution with a 4-mm or higher smoothing kernel. We addressed these issues by separating PAG voxels from the aqueduct before additional image processing (i.e., using the image shown in A) so that only voxels within the PAG are incorporated into later stages of analysis. The top of the transaxial image corresponds to the anterior portion of the head; the bottom corresponds to the posterior portion of the head.