The effect of scan length on the reliability of resting-state fMRI connectivity estimates

Abstract

There has been an increasing use of functional magnetic resonance imaging (fMRI) by the neuroscience community to examine differences in functional connectivity between normal control groups and populations of interest. Understanding the reliability of these functional connections is essential to the study of neurological development and degenerate neuropathological conditions. To date, most research assessing the reliability with which resting-state functional connectivity characterizes the brain's functional networks has been on scans between 3 and 11 min in length. In our present study, we examine the test-retest reliability and similarity of resting-state functional connectivity for scans ranging in length from 3 to 27 min as well as for time series acquired during the same length of time but excluding half the time points via sampling every second image. Our results show that reliability and similarity can be greatly improved by increasing the scan lengths from 5 min up to 13 min, and that both the increase in the number of volumes as well as the increase in the length of time over which these volumes was acquired drove this increase in reliability. This improvement in reliability due to scan length is much greater for scans acquired during the same session. Gains in intersession reliability began to diminish after 9-12 min, while improvements in intrasession reliability plateaued around 12-16 min. Consequently, new techniques that improve reliability across sessions will be important for the interpretation of longitudinal fMRI studies.

Keywords: Functional connectivity; Reliability; Resting-state; Scan duration; Scan length; fMRI.

Fig. 1

The collection of nine 10-minute resting-state fMRI scans with three different resting conditions, the order of which was counterbalanced across both subjects and sessions, for a single subject.

Fig. 2

A schematic of the creation of scan lengths from the three resting conditions. Specifically, an example is given for the computation of functional connectivity between the left and right motor cortices for a scan length of 12 min. This process would be repeated for each of the different scan lengths.

Fig. 3

The intrasession and intersession intraclass correlation coefficients (ICCs), root-mean-square deviations (RMSDs), and Dice coefficients (DCs) are shown for the nine scan lengths. Bars represent means across specified group of connections (ICC) or subjects (RMSD and DC), and error bars are the standard deviations. The * denotes a Bonferroni-corrected p-value.

Fig. 4

The intrasession and intersession ICCs, RMSDs, and DCs are shown for the scan lengths of 3, 6, 12, and 24 min as well as scan lengths of 6, 12, and 24 min with half the number of time points obtained by sampling every second time point. The numerator indicates the amount of time (in minutes) necessary to acquire the number of time points (NT) that were taken from the total imaging duration (in minutes) given by the denominator. Bars represent means across the specified group of connections (ICC) or subjects (RMSD and DC), and error bars are the standard deviations. The * denotes a Bonferroni-corrected p-value. Note that the time series used in the computation of FC for this comparison were not temporally filtered.

Fig. 5

Mean number of connections with |z| > 0.3 and p < 0.05* over all scans are shown for the nine scan lengths. The * denotes a Bonferroni-corrected p-value.