Ischemic
In an ischemic stroke, blood supply to part of the brain is decreased, leading to dysfunction of the brain tissue in that area. There are four reasons why this might happen:

Thrombosis (obstruction of a blood vessel by a blood clot forming locally)
Embolism (obstruction due to an embolus from elsewhere in the body),
Systemic hypoperfusion (general decrease in blood supply, e.g., in shock)
Cerebral venous sinus thrombosis.A stroke without an obvious explanation is termed cryptogenic (of unknown origin); this constitutes 30–40% of all ischemic strokes.There are various classification systems for acute ischemic stroke. The Oxford Community Stroke Project classification (OCSP, also known as the Bamford or Oxford classification) relies primarily on the initial symptoms; based on the extent of the symptoms, the stroke episode is classified as total anterior circulation infarct (TACI), partial anterior circulation infarct (PACI), lacunar infarct (LACI) or posterior circulation infarct (POCI). These four entities predict the extent of the stroke, the area of the brain that is affected, the underlying cause, and the prognosis. The TOAST (Trial of Org 10172 in Acute Stroke Treatment) classification is based on clinical symptoms as well as results of further investigations; on this basis, a stroke is classified as being due to (1) thrombosis or embolism due to atherosclerosis of a large artery, (2) an embolism originating in the heart, (3) complete blockage of a small blood vessel, (4) other determined cause, (5) undetermined cause (two possible causes, no cause identified, or incomplete investigation).
SPECT documents cerebral blood flow, whereas PET with an FDG isotope shows cerebral glucose metabolism. CT scans may not detect an ischemic stroke, especially if it is small, of recent onset, or in the brainstem or cerebellum areas (posterior circulation infarct). MRI is better at detecting a posterior circulation infarct with diffusion-weighted imaging. A CT scan is used more to rule out certain stroke mimics and detect bleeding. The presence of leptomeningeal collateral circulation in the brain is associated with better clinical outcomes after recanalization treatment. Cerebrovascular reserve capacity is another factor that affects stroke outcome – it is the amount of increase in cerebral blood flow after a purposeful stimulation of blood flow by the physician, such as by giving inhaled carbon dioxide or intravenous acetazolamide. The increase in blood flow can be measured by PET scan or transcranial doppler sonography. However, in people with obstruction of the internal carotid artery of one side, the presence of leptomeningeal collateral circulation is associated with reduced cerebral reserve capacity. Underlying cause
When a stroke has been diagnosed, various other studies may be performed to determine the underlying cause. With the current treatment and diagnosis options available, it is of particular importance to determine whether there is a peripheral source of emboli. Test selection may vary since the cause of stroke varies with age, comorbidity and the clinical presentation.