A brain MRI is a non-invasive imaging procedure that uses a strong magnet and radio waves to generate detailed images of the brain and its surrounding tissues. This technique does not involve radiation and is considered the most sensitive imaging test for the head. Healthcare providers order brain MRIs to diagnose or monitor neurological conditions and investigate symptoms such as persistent headaches, seizures, or changes in vision. The images allow for a clear visualization of the brain’s internal structures, helping doctors assess and diagnose a wide range of medical issues.
Characteristics of a Normal Brain MRI
A normal brain MRI serves as a baseline, presenting a clear and predictable picture of healthy brain structures. One of the most immediate observations is the brain’s symmetry; the left and right hemispheres should appear as near-perfect mirror images of each other. Any significant deviation from this symmetry can be an initial sign that something is amiss.
The images also show a sharp distinction between the different types of brain tissue. Gray matter, which contains the brain’s neuronal cell bodies, appears on the outer surfaces of the brain, while white matter, composed of nerve fibers that connect different brain regions, is situated deeper inside. In a healthy scan, these tissues have a well-defined contrast. The cerebrospinal fluid that cushions the brain appears in specific cavities called ventricles, which should be of a normal size and shape.
On various MRI sequences, which are different ways of generating images to highlight specific tissues, a normal brain shows expected signal intensities. T1-weighted sequences are useful for examining anatomy, while T2-weighted sequences are often used to detect pathological changes. A key characteristic of a normal scan is the absence of any unexpected bright or dark spots, which could represent inflammation, bleeding, or other tissue damage. There should be no masses, swelling, or abnormal fluid collection, indicating the brain’s structure is intact.
Identifying Pathological Abnormalities
Pathological abnormalities on a brain MRI are findings that indicate a disease process or injury. Structural abnormalities, such as brain tumors, appear as masses that can displace or distort the surrounding normal brain tissue. Radiologists can often distinguish between different tumor types based on their shape, location, and how they appear after the injection of a contrast agent, making certain tissues more visible.
Vascular problems are another common finding. An ischemic stroke, caused by a blood clot, can be identified very early on an MRI using a special technique called diffusion-weighted imaging, where it appears as a bright area. Hemorrhagic strokes, which involve bleeding in the brain, are also clearly visible. Aneurysms, which are weak, bulging spots on the walls of blood vessels, can be detected before they rupture, allowing for preventative treatment.
Inflammatory and demyelinating conditions like Multiple Sclerosis (MS) have a characteristic appearance. MS causes lesions, or plaques, in the white matter where the protective coating of nerve fibers has been damaged. These lesions often appear as bright spots on T2-weighted images and can be found in specific locations, such as near the ventricles or in the brainstem. The presence of both new and old lesions helps confirm the diagnosis.
Traumatic brain injuries can result in a range of findings, including bruising known as contusions, swelling referred to as edema, and various forms of bleeding within or around the brain. Infections may also be identified, presenting as localized areas of pus called abscesses or as inflammation of the meninges, the membranes covering the brain.
Age-Related Changes and Normal Anatomical Variants
As people age, certain changes in the brain are expected and not necessarily indicative of disease. One of the most common is a gradual decrease in brain volume, known as atrophy. This natural shrinking process typically begins after the age of 40.
Another frequent finding in older adults is the appearance of white matter hyperintensities, which are bright spots seen on specific MRI sequences. These are often related to changes in small blood vessels in the brain and are seen in a majority of people over the age of 60. While extensive white matter changes can be associated with conditions like high blood pressure, mild findings are often considered a normal part of aging. These changes are distinct from the specific lesions seen in conditions like MS.
Beyond age-related changes, some individuals have unique anatomical features they were born with, known as normal anatomical variants. Examples include a cavum septum pellucidum, which is a small, fluid-filled space between the two frontal horns of the lateral ventricles, or enlarged perivascular spaces, which are fluid-filled channels that follow the path of blood vessels. These variants are like anatomical birthmarks and usually have no clinical significance.
The Significance of Incidental Findings
An incidental finding is an abnormality discovered unintentionally during a brain MRI performed for an unrelated reason. For example, a scan ordered to investigate headaches might reveal a small, unrelated abnormality. Their discovery can cause anxiety, but in most cases, they are not a cause for alarm.
Common examples of incidental findings include pineal cysts, which are small, fluid-filled sacs in the pineal gland. Another frequent incidental discovery is a developmental venous anomaly (DVA), which is a common variation in the way veins are formed in the brain. These are considered benign and very rarely cause symptoms.
Other incidental findings might include small meningiomas, which are typically slow-growing, non-cancerous tumors arising from the membranes covering the brain. They are often so small and slow-growing that the best course of action is to monitor them with follow-up scans rather than immediate intervention. The decision of whether to treat or monitor an incidental finding depends on its size, location, and whether it is causing any symptoms.
How Radiologists Interpret and Report Results
After an MRI is completed, a radiologist, a doctor specializing in medical imaging, interprets the series of images and compiles a detailed report. This report is then sent to the referring healthcare provider who ordered the scan. The report uses specific medical terminology to accurately describe what was seen.
The report typically includes a “Findings” section, where the radiologist details the state of various brain structures. If the scan appears normal, the report might use the term “unremarkable.” The report will also describe any detected abnormalities, noting their size, location, and characteristics. This is often followed by an “Impression” section, which summarizes the most important findings and the radiologist’s conclusion.
Sometimes, a report may include the phrase “clinical correlation is recommended.” This means that the significance of a particular finding is unclear from the images alone and must be considered in the context of the patient’s symptoms and overall health. The referring physician uses this report, along with the physical exam and other tests, to make a final diagnosis and determine the next steps for care.