This article states following about dehydration headaches:
Our brains are 80% water. When you become dehydrated your brain tissue loses water causing your brain to shrink and pull away from the skull. This triggers the pain receptors surrounding the brain, giving you a headache.
Dehydration also causes your blood volume to drop which in turn lowers the flow of blood and oxygen to the brain. In response the blood vessels in the brain dilate leading to swelling and inflammation, worsening the headache.
Which of these symptoms should be visible on an MRI scan?