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We read with interest an article published recently in NeuroImage by Morey et al (2009). The paper is informative and timely as it tackles the issue of volume estimation of the hippocampus and amygdala which have been implicated in studies on the structural substrates of human brain gray matter development (Gogtay et al., 2006) and aging (Allen et al., 2005; Raz et al., 1997; Walhovd et al., 2005; Xue et al., 2008), cognition (Zimmerman et al., 2006) and disease (Bigler et al., 1997;Makris et al., 2002; McDonald et al., 2008; Plessen et al., 2006; Tae et al., 2008). The authors manually delineated the borders of the amygdala and hippocampus bilaterally on 20 healthy controls and compared the volumes obtained by two freely distributed segmentation packages (e.g. FreeSurfer and FSL-FIRST).
We believe the conclusion made by the authors in regards to the accuracy or “superiority” of one automated method over another automated approach has to be taken with scrutiny as the paper did not present some needed details to help interested readers.
First, Morey et al. (2009) did not provide basic relevant demographics such as gender, handedness and age range, mean and standard deviation of the 20 controls. Moreover, the authors did not report the sensitivity of the manual, FreeSurfer and FSL-FIRST normalized volumetry (volume per unit total brain volume) to important variables such as age. It is noteworthy that the dependence of hippocampus absolute or normalized volume on age across the healthy lifespan seems to be contradictory in published literature (Allen et al., 2005; Gogtay et al., 2006; Jernigan et al., 2005; Liu et al., 2003; Walhovd et al., 2005; Liu Zimmerman et al., 2008) and hence the paper could have documented and compared such age correlations. An automated method that provides systematically biased absolute values compared to a well-established gold standard may not necessarily be less sensitive to age (Walhovd et al., 2005) or to pathology effects (Tae et al., 2008).
Second, the authors did not consider two recent and relevant Neuroimage publications (Allen et al., 2008; Shattuck et al., 2008) that tabulated the hippocampus volumetry manually versus the values obtained using other methods. The absolute volume results obtained by Morey et al. (2009) via manual delineation were not tabulated for each of the 20 controls, and hence they could not be compared with published “manually-delineated” hippocampal volumetry literature nor can they be considered more accurate than what FreeSurfer or FSL provided. The absence of demographic data and manually delineated volumes thus renders it difficulty for a reader to gauge the extent to which the Morey et al. (2009) findings can be generalized with confidence.
Third, despite a sample of 20 healthy controls, scatter plots and degrees of freedom in the statistical analyses suggest a sample of 40. Therefore, right and left volume measurements were treated as independent measurements, which can be problematic given the high correlation between left and right volumetry of similar neuroanatomic structures. In such a scenario, the correlations and Bland-Altaman analysis between methods would be misleading. The information in the figures should have been presented on the mean values or even better separately or clearly marked so that linear model noise reflects only random effects.
Fourth, since there is supporting evidence that hippocampal volumes are right-ward asymmetric (Pedraza et al., 2004; Tae et al., 2008), a clear tabulation of the right and left volumes of all methods would have been preferred. To assist readers we compiled Table 1 from recent publications on the hippocampus using healthy controls. The Table shows clearly the rightward-asymmetry of the hippocampus, consistent with an earlier meta-analysis by Pedraza et al. (2004). The asymmetry of the hippocampus could have been used as an additional index to compare the three approaches (Tae et al., 2008).
Fifth, a key paper by Shattuck et al. (2008) -- not cited in Morey at al. (2009)-- may have provided important clues to why automated methods do differ compared to manually-delineated procedures. Since the same MRI data are used in both automated approaches, then differences between automated methods such as FSL-FIRST and FreeSurfer may result from sensitivity to data acquisition parameters (Bigler et al., 1997; Jack et al., 1995; Jovicich et al., 2009; Laakso et al., 1997), preprocessing stages, registration, spatial normalization (Allen et al., 2008), tissue segmentation details, and the use of specialized anatomical labeled atlases (Rodinov et al., 2009) with different spatial resolutions (Shattuck et al., 2008). The hippocampus and amygdale volumes in FSL and FreeSurfer atlases may not be identical as these are generated from different populations. It would have been interesting if Morey et al. (2009) had correlated the systematic bias with respect to the manual delineation (~ 30%) in the FreeSurfer-estimated hippocampus volume (see also Tae et al., 2008) with the volume of the hippocampus in the anatomical atlas used by FreeSurfer itself.
In conclusion, we suggest that manually-delineated hippocampal volumes need to be compared with published works after providing age, gender, handedness, and total brain volume. Without exploring sensitivity to variables such as age and gender on large samples of children and adults to account for development and aging trends, it remains premature at this stage to describe automated methods for tissue estimation as “Superior” over other methods.
National Institutes of Health-National Institute for Neurological Diseases and Stroke grant R01-NS052505-04 (K.M.H.).
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