Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Stroke ?
    ### Response:
    The most commonly used imaging procedure is the computed tomography or CT scan, also known as a CAT scan. A CT scan is comprised of a series of cross-sectional images of the head and brain. Because it is readily available at all hours at most major hospitals, produces images quickly, and is good for ruling out hemorrhage prior to starting thrombolytic therapy, CT is the most widely used diagnostic imaging technique for acute stroke.
    