is derived by a convolution of the arterial input function and the tissue irf multiplied by the tissue blood flow this is the case where there is no delay or dispersion so that the selected aif a t from a major artery is taken to be the same as the aif t t directly feeding the tissue in practice however it is difficult to measure the arterial input function at the input to different tissues of interest this is due to the practicalities that arteries directly feeding the tissues are usually small in size and subject to a substantial partial voluming effect in the case of major vessel disease such as acute stroke or carotid artery stenosis the aif selected from a major artery is often associated with a delay and dispersion before it reaches the abnormal tissue of interest as an example reference is made to fig3 where the arterial input function is measured in artery 8 resulting in the graph of fig3 a it can be seen from the graph that there is a time t a taken from injection for the contrast agent to arrive at the point where the arterial input function is measured in artery 8 it results in a narrow u2018 pulse u2019 having a large amplitude then in fig3 c there is shown the arterial input function if measured at the tissue 6 input artery designated by 60 it can be seen that the graph has dispersed somewhat or broadened as well as involving a time delay t d in traversing the smaller artery 62 where a vessel disease such as stroke or stenosis may occur other normal small arteries supplying.