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MRI scan quantity and quality in childhood
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Discussion
To address questions on quality of MRI scans in developmental samples we provided an overview of scan quantity and scan quality in a large developmental twin sample (N=512 7-9-year-olds). Overall, scan quantity was high and 88% of the children completed all runs. We report a drop in the number of runs completed after approximately 45 minutes of scan time, which is comparable with prior findings in this age range (Engelhardt et al., 2017). Scan quality decreased with increasing scan time, consistent with previous studies that reported an increase in head motion over time (Centeno et al., 2016; Engelhardt et al., 2017; Fassbender et al., 2017b).
Genetic influences on scan quantity and quality
As a complement to the growing literature on familial similarities in head motion (Couvy-Duchesne et al., 2014; Engelhardt et al., 2017), we also investigated genetic and environmental influences on scan quantity and scan quality. . Behavioral genetic modeling showed substantial to strong heritability estimates (45-46%) for both scan quantity (number of runs completed) and scan quality (percentage of scans included). Whether or not a scan was included was based on often used, but arbitrary cut-off of head motion (task fMRI: <3 mm absolute head displacement; structural scans: manual ratings; RS-fMRI: <20% volumes with >0.3 mm framewise displacement). Therefore, we additionally estimated genetic influences of MRI scan quality on a more sophisticated and continuous measure of scan quality, i.e., the quantitative measures of head motion for all fMRI runs (framewise and absolute displacement in mm) including all scanned participants. Head motion over fMRI runs was stable (α=.77-84) and within-twin correlations were higher in MZ than DZ twins. Similar findings were previously reported by Engelhardt et al. (2017), showing familial similarity of pediatric framewise head displacement in RS-fMRI. To provide direct estimates of the percentage of variation explained by genetics and (shared and unique) environment, we used behavioral genetic analyses. These analyses revealed that head motion in fMRI runs was substantially influenced by genetics, with heritability estimates ranging from 29-65%, consistent with heritability estimates in adults (Couvy-Duchesne et al., 2014). Thus, both the overall measure of scan quality (percentage of scans included), as well as the more sophisticated measure of scan quality in fMRI runs (framewise- and absolute displacement) showed substantial influence of genetics. Together, these findings show evidence for genetic contributions to head motion, highlighting the need for careful control of motion related artifacts (Caballero- Gaudes and Reynolds, 2017; Power, 2017), specifically for studies in domains where genetic effects might play a strong role, such as in the case of psychiatric disorders that have a genetic basis (Hyman, 2000).
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