How do non contrast ct scans detect strokes? i was wondering how non contrast ct scans help diagnose strokes, since the non contrast would means that the parts of the brain with and without blood would all look the same, right?

A : A ct scan generates a series of images based on the density of the tissues being scanned. In general turns, the brain is made up of two different tissue densities: gray matter and white matter. Normally, gray matter and white matter are easily distinguished in a noncontrast ct. However, following a stroke, the brain cells which lose their blood supply die and become edematous (swollen with fluid). This changes the appearance of the tissue on the ct images and the gray and white matter can no longer be differentiated. These changes generally take place approximately six hours after the stroke occurs. Mri (specifically diffusion-weighted imaging) is can detect a stroke shortly after symptoms begin.

It often doesn't: Non-contrast head ct will miss a stroke in the very early (within hours) period. An MRI is needed to catch an early stroke. The exception is if it is a "hemorrhagic" or bleeding stroke. This will be detected on ct. Early ct is performed to rule out hemorrhage. A stroke of sufficient size can be detected on a delayed (over 6-12hours) ct.

A screening test: The earliest sign of a stoke on a ct scan is edema or swelling of the part of the brain that had the stroke. It may take several hours for this to show up on a ct scan, but ct scans are widely available 24/7, while MRI may not be. Ct can will show if the stroke is hemorrhagic, which will mean that treating the stroke with tpa (alteplase) is contraindicated.