Comparison of overall brain volume and midsagittal corpus callosum surface area as obtained from NMR scans and direct anatomical measures: a within-subject study on autopsy brains
Introduction
Even a cursory review of the brain size literature reveals that there are large differences in the estimate of ‘average brain size’ in Homo sapiens. Much of the variability relates to sampling procedures, but there is also a substantial discrepancy between studies that examine relatively similar samples [19]. Overall brain size has numerous important theoretical implications, both in the evolutionary context [1], [14] and in the context of models of brain function. For example, it is not irrelevant for models of brain function whether the cortical surface area is the generally accepted average figure of 2400–2500 cm2 with some 100 billion estimated neurons [18], [22] or whether the surface area and neuron count is very much less than that [8], [15]. For humans, the canonical value of brain weight in the literature appears to be in the range of 1400–1450 g but numerous studies yield far lower weights [19].
This note reports an attempt to test the comparability of brain volume and areal estimates based on NMR imaging with those based on post mortem anatomy. It thus follows a time-honored tradition of attempts to correlate brain volume or weight measures for different methods. One of the earliest such studies was done by Davis [9] who used cranial measures obtained by the Viennese physician Weisbach to calculate estimated brain weights, which were then compared with the fresh brain weights obtained by Weisbach. In our study, the object was to see whether the NMR-derived values correlate well enough with autopsy measures to encourage full use of the potential advantages offered by NMR technology. In obtaining brain measures, NMR procedures offer better experimental control than is available for autopsy samples, because subject age, sex, health status, body parameters, mental status and any other variable of interest can be selected deliberately, and a priori. In addition, NMR studies offer the opportunity to conduct within- subject longitudinal studies. However, NMR-derived values of brain volume or other measures depend on a reconstruction of numerical data. Thus, the validity of the final outcome depends on the algorithms employed, which differ across different laboratories. It comes as no surprise that rather different volumes are given for brain volume by different researchers [3], [4], [7], [10], [19], [20], [21], and the differences may be due to different methods of reconstructing NMR images from the numerical data. In the above studies there is no independent way to validate the accuracy of brain volume measures because only within subject design using different methods can achieve this. The present study offers an independent validation of NMR-derived measures by comparing autopsy brains which were analyzed both with traditional morphometric or displacement methods with NMR scans and reconstructions. In our study, we sidestep the problems that would have arisen if a reverse order of methods had been used, that is, taking NMR scans prior to death and comparing these to post mortem material. While that would also have been a within-subject design, there can be changes in brain volume at death, notably through edema [2], [23], which make comparisons between NMR and autopsy measures problematic.
Section snippets
Subjects
Ten brains (eight males and two females, average age 80.3 years, SD=5 years) were studied. For the purposes of this study, the advanced age of the subjects is not likely to be of importance.
Direct volume determination
Meninges, and major blood vessels were removed. The brains were turned and shaken for about 30 s to eliminate some of the adhering water, with lateral tilting of the brain and care was taken to allow as much ventricular water as possible to drain via the openings provided by the removal of the pituitary
Results and discussion
Table 1 shows the various estimates of weight and volume. It can be seen that there is a very considerable reduction in substance once the meninges are stripped and the CSF is taken into account, and that the brain volumes are relatively low. The most relevant series, and the one which is demographically similar to the one from which the present set of brains was drawn from is the one described by Zilles [24], and the low brain volumes are in keeping with the volumes obtained for individuals
Conclusion
This study was prompted by the question as to how direct brain measures correlate with NMR-derived estimates. For the purposes of general studies, in which large numbers of brains are compared, the NMR based estimates appear adequate, with the proviso that such estimates are based on thin rather than thick sections. For thick sections, the overestimate of brain tissue volume is significant both in statistical terms and in terms of the effect size. However, while the correspondences between
Acknowledgments
The authors would like to thank Kai Lutz and Christoph Klein for their help in data preparation. The first author was supported by NSERC Grant A 7054, the second author was supported by DFG JA 737/4-1 and JA 737/5-1 and the third author supported by DFG KZ (SFB194/A6).
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