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A major limitation of the BOLD fMRI (also see Section 2.1) is that the BOLD signal should be interpreted in a relative sense, as the difference of brain signals between different conditions. The value from fMRI data per se cannot be directly referred to the actual neural activity. Some factors other than neural activity, e.g. CBF or vessel volume, may influence the BOLD signal. Perfusion MRI, in contrast, assesses the delivery of cerebral blood and provides a quantitative measure that can be linked to the actual state of blood perfusion by the unit ml/100 g/min for the volume of blood passing 100 g of tissue within one minute (Jenkinson and Chappell 2018). The basic concept of perfusion MRI is to label part of the blood flow and detect the labelled marker after a fixed time delay. Then, the change of the labelled content against time can be quantified. In arterial spinning labelling (ASL), water molecules are used as an intrinsic marker. In ASL‐MRI, labelling is achieved by altering the magnetic properties of the hydrogen nuclei (i.e. their spinning behaviour) using different radiofrequency. Because changes in CBF can be a critical characteristic of neurological disorders, perfusion MRI has become an important tool for diagnosing neurodegenerative disorders, tumours and migraines (Telischak et al. 2015).