“Does a stroke always show up on a brain scan?”

Not a mini stroke, which is one that lasts less than 24 hours.

It is still a possibility. MRI should be done.

Stroke without hemorrhage is often detected on CT but may required a time interval from the onset of stroke symptoms. It is very useful. A negative exam does not rule out a stroke, which is a clinical diagnosis. If negative, then we go to MRI, which has properties that make it exquisitely sensitive to stroke and its etiologies.

MRI scans are very reliable for diagnosing stroke, but a small stroke, or a stroke that resolves quickly (transient ischemic attack, or TIA) may not show up on an MRI. CT scans are often done in the ER on patients with stroke symptoms to evaluate for possible bleeding in the brain. A CT scan does not diagnose a stroke per se, but if a stroke is diagnosed clinically, and an MRI scan is not immediately available, a CT scan can help determine treatment, such as whether or not to give clot busting drugs.

CT scan doesn't show in first 24 hours ischemic stroke.

Unfortunately, no answer is simple in medicine. The first issue is what type of stroke, and the second issue is what type of scan. Stroke is a general term and can mean any acute or sudden onset brain event. The 2 major types of events are ischemic and hemorrhagic. Ischemic stroke is the blockage of blood flow to a portion of the brain so that some amount of brain tissue dies. The second is a hemorrhagic stroke which means there is a bleed in the brain. Just for confusion an ischemic stroke can become hemorrhagic if the dead brain tissue bleeds. Brain scan can refer to many diagnostic tests. The most common is a CAT scan which uses x-rays to take a picture of the brain. In the earliest phases of an ischemic stroke the CAT scan is going to be normal as it takes some time for the rain tissues to die after it loses its blood supply. This is the period of time you hope to reach the hospital so that an doctors can save that portion of the brain. By a few hours it should start to become subtly apparent on a CAT scan and by 24 hours If blood flow is not restored will be apparent. The amount of tissues that dies can vary quite a locked so that a very small area maybe difficult to see on the CAT scan even after 24 hours. A brain bleed is usually immediately apparent on a CAT scan. MRI is a remarkable test using magnetic fields to see the brain. Much more information can be gained including an ischemic stroke in its most early phases and even if only a very small amount of brain tissue is affected. Most types of brain bleeding can also be readily apparent on MRI though the earliest phases can be the most tricky.

Sorry for the complexity of the answer, but I hope you now see it is not a simple question.