Got some more understanding on SoS referring IMAGE RECONSTRUCTION FROM PHASED-ARRAY MRI DATA BASED ON MULTICHANNEL BLIND DECONVOLUTION which states
"In SoS, the image is reconstructed from multiple images acquired with phased array coils by taking the square root of the sum of the absolute squares of the images acquired at each channel. The SoS method is based on the assumption that the sum of the absolute squares of all sensitivity functions are spatially uniform such that the spatially varying sensitivity weighting is removed. However, this assumption is usually violated with surface coils, which causes the reconstructed image to be dark at locations further away from all coils."