Evaluating template bias when synthesizing population averages

Establishing the average shape and spatial variability for a set of similar anatomical objects is important for detecting and discriminating morphological differences between populations. This may be done using deformable templates to synthesize a 3D CT/MRI image of the average anatomy from a set of CT/MRI images collected from a population of similar anatomical objects. This paper investigates the error associated with the choice of template selected from the population used to synthesize the average population shape. Population averages were synthesized for a population of five infant skulls with sagittal synostosis and a population of six normal adult brains using a consistent linear-elastic image registration algorithm. Each data set from the populations was used as the template to synthesize a population average. This resulted in five different population averages for the skull population and six different population averages for the brain population. The displacement variance distance from a skull within the population to the other skulls in the population ranged from 5.5 to 9.9 mm(2) while the displacement variance distance from the synthesized average skulls to the population ranged from 2.2 to 2.7 mm(2). The displacement variance distance from a brain within the population to the other brains in the population ranged from 9.3 to 14.2 mm(2) while the displacement variance distance from the synthesized average brains to the population ranged from 3.2 to 3.6 mm(2). These results suggest that there was no significant difference between the choice of template with respect to the shape of the synthesized average data set for these two populations.