1. A computer implemented method of processing optical coherence tomography (OCT) image data, the OCT image data comprising a plurality of cross sectional images of a subject, each cross sectional image comprising a plurality of scan lines obtained by measuring the time delay of light reflected, in a depth direction, from optical interfaces within the subject, the method comprising: aligning the cross sectional images by determining relative translations for each cross sectional image in the depth direction and in a lateral direction, perpendicular to the depth direction, to form a set of aligned cross sectional images; dividing each aligned cross sectional image into a set of image patches across the lateral direction, each image patch comprising at least one scan line; aligning the image patches of the aligned cross sectional images by determining relative translations for each image patch in the depth direction to form a set of aligned image patches; forming an image matrix from the aligned image patches; and applying matrix completion to the image matrix to obtain a reduced speckle noise image of the subject.