Landmarking the brain for geometric morphometric analysis: An error study

Neuroanatomic phenotypes are often assessed using volumetric analysis. Although powerful and versatile, this approach is limited in that it is unable to quantify changes in shape, to describe how regions are interrelated, or to determine whether changes in size are global or local. Statistical shape analysis using coordinate data from biologically relevant landmarks is the preferred method for testing these aspects of phenotype. To date, approximately fifty landmarks have been used to study brain shape. Of the studies that have used landmark-based statistical shape analysis of the brain, most have not published protocols for landmark identification or the results of reliability studies on these landmarks. The primary aims of this study were two-fold: (1) to collaboratively develop detailed data collection protocols for a set of brain landmarks, and (2) to complete an intra- and inter-observer validation study of the set of landmarks. Detailed protocols were developed for 29 cortical and subcortical landmarks using a sample of 10 boys aged 12 years old. Average intra-observer error for the final set of landmarks was 1.9 mm with a range of 0.72 mm-5.6 mm. Average inter-observer error was 1.1 mm with a range of 0.40 mm-3.4 mm. This study successfully establishes landmark protocols with a minimal level of error that can be used by other researchers in the assessment of neuroanatomic phenotypes. © 2014 Chollet et al

Histogram of the intra-observer precision of each landmark.

<p>This histogram indicates the level of intra-observer precision associated with each landmark using the original (P1) and modified (P2) protocols. The error bar is equal to one standard deviation above and below the mean. Landmark numbers correspond with the landmark numbers in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone-0086005-t002" target="_blank">Table 2</a>.</p

Histogram of the inter-observer precision of each landmark.

<p>This histogram indicates the level of inter-observer precision associated with each landmark using the original (P1) and modified (P2) protocols. Landmark numbers correspond with the landmark numbers in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone-0086005-t002" target="_blank">Table 2</a>.</p

Average intra-observer error and inter-observer error measured for landmarks.

<p>In the landmark name, the backslash (/) indicates that the landmark is located at the intersection of the two sulci and s. is an abbreviation for sulcus. P1 is the imprecision (mm) for each landmark that was assessed in the first round of analysis. P2 is the imprecision (mm) for each landmark that was assessed in the second round of analysis using the modified protocols. The hyphen (-) indicates that the landmark was not reassessed in the second round of analysis because the error was less than 1.5 mm. ΔP is the difference between P2 and P1.</p

Landmarks and the associated error analyzed in this study.

<p>Left lateral view of a 3D reconstruction of the brain (anterior is to the left). Projected positions of landmarks are shown with numbers corresponding to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone-0086005-t002" target="_blank">Table 2</a>. Cortical surface landmarks are white with white wireframe; subcortical landmarks are purple with purple wireframe. The size of the pink ellipses around each landmark indicate the magnitude of average precision (error) at anatomic scale. Landmarks for which no ellipse is visible had average error less than the 1.5 mm radius of the landmark marker. Note that the greatest magnitudes of error were associated with cortical surface landmarks.</p

Brain landmarks tabulated from the literature.

<p>Landmarks were tabulated from published studies where the primary methodology was landmark-based shape analysis of the brain in order to determine each landmark's frequency of use. Column headings indicate the source of the landmarks: (1) Aldridge <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone.0086005-Aldridge1" target="_blank">[17]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone.0086005-Aldridge4" target="_blank">[49]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone.0086005-Aldridge5" target="_blank">[50]</a>. (2) DeQuardo <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone.0086005-DeQuardo1" target="_blank">[51]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone.0086005-DeQuardo2" target="_blank">[52]</a>. (3) Gharaibeh <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone.0086005-Gharaibeh1" target="_blank">[53]</a>. (4) Maudgil <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone.0086005-Maudgil1" target="_blank">[33]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone.0086005-Free1" target="_blank">[54]</a>. (5) Weinberg <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086005#pone.0086005-Weinberg1" target="_blank">[34]</a>.</p

Average intra-observer error by rater.

<p>In the landmark name, the backslash (/) indicates that the landmark is located at the intersection of the two sulci. P1 is the imprecision (mm) for each landmark in the first round of analysis. P2 is the imprecision (mm) for each landmark in the second round of analysis using the modified protocols. The hyphen (-) indicates that the landmark was not reassessed in the second round of analysis.</p