Fig. 1.

Example of the placement of landmarks for manual tracing of the anterior limb of the internal capsule. The top row depicts a typical axial MRI showing the traced anterior limb of the internal capsule (ALIC) region-of-interest. The bottom row depicts the same MRI image with the automatic boundary-finding method, based on a sobel-gradient filter which allows for maximization of gray/white matter contrast for accurate placement of landmarks on the borders between the striatum and internal capsule. As shown in the middle column, a series of landmark points along the parallel walls of the caudate and putamen were placed to create a polygon containing the fibers of the ALIC. The right column shows the automated spline curve that was used to connect the landmark points and define the ALIC area. As in our prior striatal work (e.g., Buchsbaum et al., 2003), in order to trace the ALIC, we selected five slices based on the anatomy of the striatum with the most dorsal slice defined as the first slice in which gray matter belonging to the putamen was visible. The most ventral slice was defined as the last slice in which the caudate and putamen remained unmerged. The number of slices between the most dorsal and most ventral slices was divided by six to yield an increment accurate to two decimal places. This increment was added to the most ventral slice number five times with the result rounded to remove any decimal. This yielded five proportionally and equally spaced slices for tracing lying between the most dorsal and ventral levels of the ALIC. As discussed in Brickman et al. (2006), this set of axial slices is well suited for the examination of the white-matter tracts within the internal capsule. The ALIC was defined as the region bound medially by the caudate and laterally by the putamen. A sobel-gradient filter and magnified MRI image were used during manual tracing to allow for maximum gray/white matter contrast and accurate placement of landmark points. The ALIC medial boundary was traced along the wall of the caudate between its most lateral anterior corner and its most posterior corner. The lateral boundary of the ALIC was traced along the caudate-parallel wall of the putamen between its most anterior corner and its most medial corner. An automated spline curve was used to connect the landmark points and define the area of the ALIC. ALIC volume was determined for the four contiguous segments lying between the five slices that were traced. ALIC areas obtained from the five slices were used to extrapolate the volume of each of the four segments by the following formula: Segment volume = (A + B + sqrt(AB)) * (H/3) where H=distance between the slices bounding the segment, A=area of the more dorsal-lying slice, and B=area of the more ventral-lying slice (Fig. 2). We obtained absolute ALIC volume (in mm³) which was divided by total brain volume and then multiplied by 100. While ratio (relative) measures may not adequately correct for normal variation in intracranial volume, they are more commonly reported (Hazlett et al., 2008a). To determine inter-rater reliability for tracing the ALIC, two independent tracers (T.C. and J.E.) each traced both the left and right ALIC on the five dorsal/ventral slice levels for a subset of the sample comprising 10 randomly chosen subjects (5 healthy controls and 5 SPD participants). Next, whole left and right volume for the ALIC was compared between the two tracers. The intra-class correlation was 0.95 which is consistent with and in some cases higher than values reported in other studies examining inter-tracer reliability of tracing small structures (Spinks et al., 2002; Levitt et al., 2012).