1. A non-invasive method for measuring blood flow in principal vessels of a heart of a subject, the method comprising: illuminating a region of interest in the heart with a coherent light source, wherein the coherent light source has a wavelength of from about 600 nm to about 1100 nm; sequentially acquiring at least two speckle images of the region of interest in the heart during a fixed time period, wherein sequentially acquiring the at least two speckle images comprises acquiring the at least two speckle images in synchronization with motion of the heart of the subject; and electronically processing the at least two acquired speckle images based on the temporal variation of the pixel intensities in the at least two acquired speckle images to generate a laser speckle contrast imaging (LSCI) image and determine spatial distribution of blood flow speed in the principal vessels and quantify perfusion distribution in tissue in the region of interest in the heart from the LSCI image.