title: On evaluating brain tissue classifiers without a ground truth.
abstract: In this paper, we present a set of techniques for the evaluation of brain tissue classifiers on a large data set of MR images of the head. Due to the difficulty of establishing a gold standard for this type of data, we focus our attention on methods which do not require a ground truth, but instead rely on a common agreement principle. Three different techniques are presented: the Williams' index, a measure of common agreement; STAPLE, an Expectation Maximization algorithm which simultaneously estimates performance parameters and constructs an estimated reference standard; and Multidimensional Scaling, a visualization technique to explore similarity data. We apply these different evaluation methodologies to a set of eleven different segmentation algorithms on forty MR images. We then validate our evaluation pipeline by building a ground truth based on human expert tracings. The evaluations with and without a ground truth are compared. Our findings show that comparing classifiers without a gold standard can provide a lot of interesting information. In particular, outliers can be easily detected, strongly consistent or highly variable techniques can be readily discriminated, and the overall similarity between different techniques can be assessed. On the other hand, we also find that some information present in the expert segmentations is not captured by the automatic classifiers, suggesting that common agreement alone may not be sufficient for a precise performance evaluation of brain tissue classifiers.
authors.affiliation: Psychiatry Neuroimaging Laboratory, Department of Psychiatry, Brigham and Women's Hospital, Boston, MA, USA. sylvain@bwh.harvard.edu
authors.forename: Sylvain
authors.initials: S
authors.lastname: Bouix
authors.forename: Marcos
authors.initials: M
authors.lastname: Martin-Fernandez
authors.forename: Lida
authors.initials: L
authors.lastname: Ungar
authors.forename: Motoaki
authors.initials: M
authors.lastname: Nakamura
authors.forename: Min-Seong
authors.initials: MS
authors.lastname: Koo
authors.forename: Robert W
authors.initials: RW
authors.lastname: McCarley
authors.forename: Martha E
authors.initials: ME
authors.lastname: Shenton
mesh_terms: D000465:Algorithms; D000704:Analysis of Variance; D001185:Artificial Intelligence; D005260:Female; D005625:Frontal Lobe / Q000473:pathology*; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D012015:Reference Standards; D012680:Sensitivity and Specificity; D012984:Software; D013702:Temporal Lobe / Q000473:pathology*