3D facial landmarks: Inter-operator variability of manual annotation

Abstract

BackgroundManual annotation of landmarks is a known source of variance, which exist in all fields of medical imaging, influencing the accuracy and interpretation of the results. However, the variability of human facial landmarks is only sparsely addressed in the current literature as opposed to e.g. the research fields of orthodontics and cephalometrics. We present a full facial 3D annotation procedure and a sparse set of manually annotated landmarks, in effort to reduce operator time and minimize the variance.MethodFacial scans from 36 voluntary unrelated blood donors from the Danish Blood Donor Study was randomly chosen. Six operators twice manually annotated 73 anatomical and pseudo-landmarks, using a three-step scheme producing a dense point correspondence map. We analyzed both the intra- and inter-operator variability, using mixed-model ANOVA. We then compared four sparse sets of landmarks in order to construct a dense correspondence map of the 3D scans with a minimum point variance.ResultsThe anatomical landmarks of the eye were associated with the lowest variance, particularly the center of the pupils. Whereas points of the jaw and eyebrows have the highest variation. We see marginal variability in regards to intra-operator and portraits. Using a sparse set of landmarks (n=14), that capture the whole face, the dense point mean variance was reduced from 1.92 to 0.54 mm.ConclusionThe inter-operator variability was primarily associated with particular landmarks, where more leniently landmarks had the highest variability. The variables embedded in the portray and the reliability of a trained operator did only have marginal influence on the variability. Further, using 14 of the annotated landmarks we were able to reduced the variability and create a dense correspondences mesh to capture all facial features.

Reproducibility and reliability of three-dimensional soft tissue landmark identification using three-dimensional stereophotogrammetry

“…13 These landmarks are relatively harder to determine visually, so require other methods such as palpation. 9 They are located on convex surfaces, and the operator should control their localization on the x-, y-, z-axes separately.…”

“…Since it is not possible to palpate soft tissue points on 3-D photo-graphs, the repeatability of the measurements of such soft tissue points-especially pogonion and zygion-is open to discussion. 10 There are several studies on repeatability and reliability of 3-D soft tissue facial landmarks, [8][9][10][11][12][13] but conflicting results have been reported in these studies and no consensus has been reached. The aim of this study was to evaluate the intraexaminer repeatability and interexaminer reproducibility of soft tissue landmarks on 3-D stereophotogrammetric images.…”

Integration of digital maxillary dental casts with 3D facial images in orthodontic patients:

“…Relevant research concluded that the reproducibility of either measurement through manual anthropometry or that on 3D photography was unsatisfactory due to distortion of soft tissues and illegibility of anatomical structures. 19,20 In this study, in order to reduce errors in landmark identification, a CBCT-based dentofacial image was chosen as reference standard. 21 Five landmarks were digitized on the original digital dental model before the registration procedure.…”

MeshMonk: Open-source large-scale intensive 3D phenotyping

“…However, manual landmarking is tedious to perform, difficult to standardize in practice, and prone to intra and inter-operator error 3–7 . Furthermore, sparse landmark configurations can only quantify form at defined landmarks that can be reliably identified and indicated by a human, and thus lack the resolution to fully characterize shape variation in between landmarks.…”