Tissue similarity maps (TSMs): a new means of mapping vascular behavior and calculating relative blood volume in perfusion weighted imaging.

This study introduces a new processing means that uses the original signal (rather than contrast agent concentration) from dynamic susceptibility contrast (DSC) perfusion weighted imaging (PWI) to calculate a relative cerebral blood volume map and a tissue similarity map (TSM). Ten healthy volunteers and eight multiple sclerosis (MS) patients were studied using high resolution PWI. The TSM is found by choosing a reference region in one slice and comparing its signal in a mean squared error sense to the signal from every pixel in all images throughout the brain. The TSMs provide a means to determine which tissues have similar flow characteristics with high contrast and signal-to-noise ratios. The effective blood volume measured from this approach is nearly identical to that from conventional relative cerebral blood volume (rCBV) maps but with better signal-to-noise. Of interest is the fact that choosing one MS lesion as the reference tissue appears to be enough to find nearly all lesions throughout the brain. That is, these lesions all behave the same from a vascular point of view. The TSM results are robust within and across slices properly nulling the same type of tissue throughout the brain for a given reference region. TSM derived rCBV agrees well with the conventional derived rCBV using contrast agent concentration. TSM may provide a new means to study similarities between blood flow patterns in tissue in the brain and in better diagnosing vascular differences between tissues and lesions.