Headaches are among the most common complaints, experienced by nearly everyone. While most headaches are harmless, the worry that a headache might signal a serious underlying problem often leads people to consider getting a brain scan. Deciding when a diagnostic imaging test, such as an MRI scan, is necessary depends on differentiating between a benign headache and one that suggests a serious neurological issue. This distinction is important for patient safety and appropriate resource use.
Identifying Headaches That Require Immediate Imaging
The decision to order an MRI is guided by specific “red flag” symptoms that suggest a secondary headache, meaning the pain is a symptom of a structural problem. A sudden, extremely severe headache, often described as the “worst headache of life,” requires immediate evaluation, as this presentation is characteristic of a subarachnoid hemorrhage (bleeding around the brain). This acute onset, known as a thunderclap headache, reaches maximum intensity within sixty seconds and suggests a vascular emergency.
Any headache accompanied by a new, abnormal finding on a neurological examination mandates urgent imaging. This includes symptoms like weakness or paralysis on one side of the body, sudden vision changes, double vision, or difficulty with balance and coordination. These focal neurological deficits suggest that a structural lesion is interfering with specific brain functions.
Headaches that change significantly in pattern, frequency, or severity, or those that are new and progressive, raise suspicion for a space-occupying lesion. A new headache appearing after the age of 50 also increases concern for conditions like giant cell arteritis, tumors, or stroke. Other concerning features include a headache worsened by the Valsalva maneuver (coughing, straining), a headache accompanied by fever or a stiff neck, or new headaches in a patient with a known history of cancer or a compromised immune system. These symptoms suggest an underlying infection, inflammation, or malignancy that warrants prompt investigation.
Understanding What Imaging Reveals
When a patient presents with red flag symptoms, an MRI scan is often the preferred imaging modality. The MRI uses powerful magnetic fields and radio waves to create highly detailed images of the brain’s soft tissues, making it superior to a Computed Tomography (CT) scan for many conditions.
The primary utility of the MRI is identifying structural pathologies that cause secondary headaches. This includes detecting brain tumors, which may cause pain by increasing intracranial pressure or irritating pain-sensitive structures like the meninges. The scan can also reveal signs of cerebral aneurysms (balloon-like bulges in blood vessel walls) and arteriovenous malformations (AVMs), which are abnormal tangles of blood vessels.
MRI is effective for visualizing signs of past or acute bleeds, inflammation from infections like meningitis or encephalitis, and structural issues such as hydrocephalus (the buildup of fluid in the brain). The detailed images allow clinicians to evaluate for pressure on the brainstem, which can be caused by conditions like Chiari malformations. While CT scans are faster and commonly used in acute emergency settings, the MRI provides greater tissue contrast necessary for a definitive diagnosis.
Why Routine Headaches Do Not Require an MRI
The vast majority of headaches are classified as primary headache disorders, meaning the headache itself is the primary condition and not a symptom of an underlying structural disease. These include common types such as tension-type headaches and migraines, which are caused by complex neurobiological processes involving nerve signaling and brain chemistry, rather than a physical lesion.
For patients with a stable, established pattern of a primary headache disorder and a normal neurological examination, the likelihood of finding a significant abnormality on a brain scan is extremely low. Studies show that the prevalence of a clinically significant intracranial abnormality in patients with typical migraines and a normal exam is less than one percent, similar to that in the general population. Routine imaging in these cases is not medically necessary and offers minimal diagnostic value.
Ordering an MRI unnecessarily carries distinct drawbacks, including significant financial costs to the healthcare system and the patient. A common issue is the detection of incidental findings, which are minor abnormalities unrelated to the headache discovered by chance. These findings, such as small white matter spots or tiny cysts, can cause considerable anxiety for the patient and often lead to a cascade of further, unnecessary testing and follow-up scans. Therefore, for a typical, stable headache pattern without any red flags, clinical practice guidelines recommend against routine brain imaging.