An improved approach to detection of amplitude of low-frequency fluctuation (ALFF) for resting-state fMRI: fractional ALFF

Abstract

Most of the resting-state functional magnetic resonance imaging (fMRI) studies demonstrated the correlations between spatially distinct brain areas from the perspective of functional connectivity or functional integration. The functional connectivity approaches do not directly provide information of the amplitude of brain activity of each brain region within a network. Alternatively, an index named amplitude of low-frequency fluctuation (ALFF) of the resting-state fMRI signal has been suggested to reflect the intensity of regional spontaneous brain activity. However, it has been indicated that the ALFF is also sensitive to the physiological noise. The current study proposed a fractional ALFF (fALFF) approach, i.e., the ratio of power spectrum of low-frequency (0.01-0.08 Hz) to that of the entire frequency range and this approach was tested in two groups of resting-state fMRI data. The results showed that the brain areas within the default mode network including posterior cingulate cortex, precuneus, medial prefrontal cortex and bilateral inferior parietal lobule had significantly higher fALFF than the other brain areas. This pattern was consistent with previous neuroimaging results. The non-specific signal components in the cistern areas in resting-state fMRI were significantly suppressed, indicating that the fALFF approach improved the sensitivity and specificity in detecting spontaneous brain activities. Its mechanism and sensitivity to abnormal brain activity should be evaluated in the future studies.

Fig. 1

The results of one-sided one-sample t-tests of the original ALFF (a and c) and fALFF (b and d) approach, in the boy group (a and b) (n = 22, t > 6.0, p < 6.0 × 10⁻⁶, uncorrected) and adult group (c and d) (n = 14, t > 6.0, p < 4.5 × 10⁻⁵, uncorrected). The t maps were overlaid on the 3D images in the MRIcro software.

Fig. 2

Illustration of the improvement of ALFF approach. (a) The time series (without filtering) from a typical voxel in the suprasellar cistern (SC) (−1, −2, −18) and the posterior cingulate cortex (PCC) (−4, −56, 25) of an individual. (b) The power in the SC is higher than that in PCC at almost every frequency. (c) The ratio of the power at each frequency to the integrate power of the entire frequency range indicates that the power in the low-frequency range (0.01–0.08 Hz) is significantly suppressed in the SC.

Fig. 3

The Z scores (standardized ALFF and fALFF values) and the corresponding t values (one-sample t-test) of a few typical voxels selected from some areas in the boy group (a) and adult group (b). LV, lateral ventricle (5, −6, 21); SC, suprasellar cistern (−1, −2, −18); AC, ambient cistern (16, −14, −22); SF, Sylvian fissure (−41, 11, −6); MPFC, medial prefrontal cortex (−2, 38, 17); PCC, posterior cingulate cortex (−4, −56, 25).