Accuracy and precision of CT-based endocranial capacity estimations: a comparison with the conventional millet seed method and application to the Minatogawa 1 skull

Abstract

An increasing number of studies determine endocranial capacity (ECC) digitally using serial computed tomographic (CT) scan data. However, the multiple causes of errors inherent in such measurements and the resulting degree of accuracy have not yet been fully examined. In the present study, five observers estimated the ECCs of two modern human crania by segmentation of CT data (voxel size 0.380 mm) and by means of the conventional millet seed method. The ECC estimations were much more consistent in the CT-based (technical error, TEM = 1.4 cc) than the millet seed (TEM = 11.8 cc) methods. The estimated capacities also tended to be larger in the latter, suggesting possible systematic bias. Next, the causes and degree of error in the CT-based method were examined. Error due to size calibration of the CT images was no more than 3 cc (± 1.5 cc) per 1000 cc. Errors involved in delineating the endocranial cavity were evaluated as follows. First, we adopted a segmentation routine in which a single global threshold value was applied to most of the volume, and then supplemented locally by more appropriate values in the regions where the global value was insufficient. We then estimated the potential error introduced by choosing a global threshold value. We found this to be within 5 cc (per 1000 cc). This error range increased to 7.5 cc (per 1000 cc), when using a lower resolution data set (slice thickness 1.14 mm). Another source of interobserver error involves the blocking of foramina and canals, which we estimated to be less than 2 cc (per 1000 cc). Adding these various effects, our investigations indicate that ECC estimations based on high-resolution CT data are accurate to within ± 5 cc per 1000 cc. Finally, we reevaluated the ECC of the Minatogawa 1 skull to be 1335 cc (with a probable range of 1327-1343 ± 5 cc), considerably smaller than the originally reported value of 1390 cc obtained by the millet seed method.

Descriptions of the dental remains of <i>Homo floresiensis</i>

“…Prior to the scan, the system was calibrated with known size phantoms. Linear measurement error in a horizontal plane was less than 0.1% (Kubo et al, 2008). Original scans were taken at 130 kV and 0.20 or 0.17 mA with a 0.5 or 1 mm thick copper plate prefilter to lessen beam-hardening effects.…”

<i>Homo erectus</i> calvaria from Ngawi (Java) and its evolutionary implications

“…First, we virtually separated the fossil bone (with matrices) from the air by half-maximum-height thresholding between the CT values for bone and air (Kubo et al, 2008(Kubo et al, , 2011, and then removed the attached matrices from the endocranial surface using Amira software (FEI) (Figure 4a, b). These matrices, which account for 67 cm 3 , were distinguishable from the fossil bone by their higher intensities in the CT image.…”

Comparative analysis of the glabellar region morphology of the late Pleistocene Minatogawa crania: a three-dimensional approach

“…Critical chronological evaluations suggest a possible age range of ~15-20 kyr (see Matsu'ura and Kondo, 2011, for details). More recently, additional morphological investigations of the Minatogawa series have been initiated (Kubo et al, 2008(Kubo et al, , 2011Kaifu et al, 2011).…”