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Chapter 7
Genetic influences on scan quantity
To investigate genetic and environmental influences on scan quantity (number of scans completed), we calculated Pearson’s within-twin correlations for MZ and DZ twins and performed behavioral genetic analyses. Fisher r-to-z transformations showed that within-twin correlations for scan quantity were significantly stronger for MZ twins (rmz=.50, p<.001) than DZ twins (rdz=.14, p=.14), Z=3.21, p<.001. Behavioral genetic analyses revealed substantial influences of genetics (A=45%, 95% CI [18-56%]) and unique environment/measurement error (E=55%, 95% CI [44-68%]), with no influence of the shared environment (C=0%, CI: 0-22%).
MRI Quality
Scan quality
An overview of the number (and percentage) of scans with sufficient quality relative to the quantity of the scans is provided in Figure 3b. Of the 488 participants that started the MRI protocol, 385 participants (79%) had sufficient data in the first run. Sufficient MRI scan quality for task-based fMRI was defined as <3 mm (1 voxel) motion in all directions. The percentage sufficient data decreased over the first five task-based fMRI runs: 71% in the second run; 66% in the third run; 64% in the fourth run; and 55% in the fifth run. For the 3DT1 structural anatomy scans, 72% of the scans were classified as sufficient quality using a stringent threshold, and 88% was included using a lenient threshold (including scans coded as ‘Doubtful’). The percentage of DTI scans classified as sufficient quality was 92% using a stringent threshold and 96% using a lenient threshold (including ‘Doubtful’). The RS-fMRI data, which was the final run of the MRI session, showed the lowest scan quality, with 34% of the acquired data being of sufficient quality with a cut-off of <0.3 mm FD in > 20% of the volumes (Figure 3b). Using a more lenient cut-off of <0.5 mm FD in > 20% of the volumes, 52% of the acquired data would have been included. Inclusion based on <3 mm absolute displacement (similar to the threshold used for task-based fMRI data) resulted in 51% of sufficient RS fMRI data. Across all scans, we found a small positive association between percentage of the acquired data being of sufficient quality (using stringent thresholds) and age (r=.10, p=.03).
Scan quality over time
There was an increase in head motion over time, both framewise as well as absolute (x, y, and z-direction) displacement (Table 1). After excluding participants with insufficient data, head motion within the different task based and resting state fMRI runs was comparable (Table 1). To provide an overview of scan quality with respect to time, we calculated the percentage participants with sufficient quality data after 30, 45 and 60 minutes (for participants that completed the full scan protocol, n=433, 48% boys), see Figure 3c. The first 30
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