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Longitudinal tensor-based morphometry in healthy aging
Laura D Reyes1,2, Neda Sadeghi2, François M Lalonde3, and Carlo Pierpaoli2

1Henry M. Jackson Foundation, Bethesda, MD, United States, 2NIBIB, National Institutes of Health, Bethesda, MD, United States, 3NIMH, National Institutes of Health, Bethesda, MD, United States

Synopsis

Previous studies of healthy elderly populations combined a longitudinal design with tensor-based morphometry (TBM) and found significant gray matter (GM) atrophy over short time periods. We examined a separate healthy elderly population using a different method to determine if previous results are biologically driven, and investigated the relationship between GM and cognition. We also detected significant GM atrophy, but did not find a link between GM, age, and cognition. Our longitudinal TBM approach is sensitive to subtle, short-term GM changes, but further investigation is necessary to examine the effect of different methodological approaches on the relationship between GM and cognition.

Introduction

A longitudinal study design can reduce between-subject variance and sample heterogeneity, allowing for brain structure changes to be assessed at the individual level.1,2 A longitudinal design used with tensor-based morphometry (TBM)3 can identify subtle volume changes over time. A few studies of healthy elderly populations combined these methods to examine brain volume changes over short time periods, and detected global, frontal, and temporal gray matter (GM) atrophy.4-11 One objective of the current study is to examine a separate healthy elderly population using different methods for acquisition, processing, and analysis to replicate these previous findings and determine if they are biologically rather than methodologically driven. Another objective is to investigate the relationship between brain volume changes and cognition.

Methods

T1-weighted MPRAGE images from 72 healthy elderly adults aged 66-95 from Rush Alzheimer's Disease Memory and Aging Project were acquired on a 3T Siemens Magnetom TrioTim MRI scanner at a two-year interval.12 For each individual, we performed TBM by registering the second collected T1-weighted image to the first using ANTs SyN registration.13 We computed the natural log of the Jacobian (lnJ) of the deformation field (combined affine and highly deformable) between the two time points to obtain the volume change (lnJTime). Desikan atlas and global GM ROIs were generated for each individual in native space using Freesurfer.14-25 Median lnJTime values were calculated for global GM and each ROI. Left and right ROIs were combined since their median lnJTime values were not significantly different.

Cognitive change was assessed using scores from 16 tests administered at both time-points and was calculated as the difference between the two scores. Raw scores were used from five tests: category fluency, line orientation, number comparison, progressive matrices, symbol digits modalities. The remaining tests consisted of different versions (immediate and delayed word list, East Boston, and logical memory; Stroop word-reading and color-naming; digit span forward, backward, and ordering) that were each highly correlated (r>0.7). Composite scores were computed by calculating the z-score for each test and taking the mean.

One-sample t-tests were performed to test if lnJTime was significantly different than zero. If lnJTime was significantly different than zero, we performed a linear model between lnJTime and age. For cognitive tests, we performed one-sample t-tests to assess if the score changes were significantly different than zero. For tests showing a significant change, we performed linear models to test for a relationship between score change and lnJTime. Statistical tests performed on the cognitive tests and ROI comparisons were corrected for multiple comparisons using false discovery rate (FDR) to obtain the q-value. Analyses were performed using ANTsR and R.26-27

Results

We detected significant global GM atrophy of -0.6727% over two years (Figure 1A). GM atrophy was not significantly related to age or cognitive decline since test scores remained stable over two years (Figures 1B-C,2). The line orientation test showed a significant difference from zero (p<0.01), but this result did not survive multiple comparisons correction (q=0.09; Figure 2), and was not significantly related to GM atrophy (Figure 1C). The ROI analysis showed significant atrophy throughout the cortex (Figure 3A-B), particularly in temporal GM (Figure 3B), but no regional relationship between GM atrophy and age (Figure 4). GM atrophy in inferior parietal and parahippocampal ROIs was significantly associated with line orientation score change (p<0.02), but this did not survive multiple comparisons correction (q=0.228; Figure 5).

Discussion

These results support previous findings of detectable GM atrophy over a two-year interval that is most evident in temporal GM. Global GM and ROI atrophy was within the range of 0.5-1% per year shown in prior studies (Figure 3B) even with differences in acquisition, processing, and analytical methods.28 A previous analysis of brain volume change and cognitive trajectories showed that global and temporal GM atrophy was strongly associated with cognitive decline in a sample including both healthy elderly and cognitively impaired individuals.10 The current study did not replicate these findings since we did not detect any age effects or relationship between atrophy and cognition. However, differences in sample composition and analytical model may account for differences in these results.

Conclusion

The longitudinal TBM approach from the current study is sensitive to subtle changes in the cortex over a short two-year period and is capable of detecting patterns of GM atrophy that have been identified in previous studies using different samples and methods. In contrast to previous studies, we found that GM atrophy is not related to cognitive change in a healthy elderly population. Future work will further investigate how GM atrophy may be affected by different processing and analytical methods and will examine how these different approaches can affect the relationship between GM atrophy and cognition.

Acknowledgements

No acknowledgement found.

References

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Figures

Results from one-sample t tests for change in score on each cognitive test. Only one t test, line orientation, had a significant uncorrected p value (p = 0.0091), but this did not survive multiple comparisons correction (q = 0.0906).

Gray matter results showing (A) distribution ln values, (B) lnJTime values by age at first scan, and (C) line orientation score change by lnJTime value. Blue lines indicate lnJTime values of zero for A and B and a score change of zero for C. In A, lnJTime values are significantly less than zero, indicating overall gray matter atrophy in the sample. The gray matter atrophy also has no relationship with age (B) or change in line orientation test score (C).

Distributions of lnJTime values by ROI (A) and t test results (B). In A, blue lines indicate lnJTime values of zero for each ROI. In B, ROIs are ranked by the magnitude of the lnJTime value. All of the ROIs showed atrophy over the two year period, and 23/25 ROIs were significantly less than zero (q ≤ 0.004). The largest magnitude of atrophy was in ROIs from the temporal lobe including the middle temporal, parahippocampal, temporal pole, fusiform, and inferior temporal cortex.

ROI lnJTime values by age at first scan. Blue lines indicate lnJTime values of zero for each ROI. There was no relationship between age and lnJTime values in any of the ROIs.

Change in line orientation score by lnJTime values in all ROIs. Blue lines indicate a line orientation score change value of zero for each ROI. Only two ROIs, inferior parietal and parahippocampal cortex, showed a relationship with line orientation score change (p < 0.02), but this relationship did not survive multiple comparisons correction (q = 0.228). Nonetheless, these two regions are involved in visuospatial processing, which is the primary skill tested by the line orientation test.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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