Magnetic Resonance Imaging (MRI) uses strong magnetic fields and radio waves to generate detailed cross-sectional pictures of the body’s internal structures. For the brain, MRI provides exceptional contrast between soft tissues, allowing physicians to visualize the gray matter, white matter, and cerebrospinal fluid (CSF) with clarity unmatched by other imaging methods. While trained specialists interpret these complex images, understanding the basic principles of how an MRI works can help you make sense of your own results. This foundational knowledge covers the visual presentation of the scan, the different types of images produced, and the common language used in reports.
Understanding the Basic Views and Anatomy
The brain is a three-dimensional structure, and an MRI scan captures this anatomy by creating a series of two-dimensional “slices” or views. These slices are taken along three standard anatomical planes, which provide different perspectives of the same structures. Visualizing these planes is the first step toward understanding the spatial context of any finding on the scan.
The axial plane, also known as the transverse plane, divides the brain into superior (upper) and inferior (lower) portions. The sagittal plane runs vertically, dividing the brain into left and right halves, which is useful for viewing structures along the midline. The coronal plane is another vertical cut, separating the anterior (front) part of the brain from the posterior (back) part.
These three planes allow for the visualization of major structures like the large, folded cerebrum, the smaller cerebellum tucked underneath, and the lower brainstem connecting the brain to the spinal cord. The ability to image in all three planes gives MRI an advantage over older technologies like Computed Tomography (CT), which traditionally collected data only in the axial plane.
Decoding the Imaging Sequences
The appearance of tissues depends entirely on the specific imaging sequence used, which is determined by varying the timing of radiofrequency pulses. These sequences highlight different tissue properties, resulting in different shades of gray, a process called “weighting.” The terms hyperintense (brighter or white) and hypointense (darker or black) describe a structure’s relative brightness compared to its surroundings.
The T1-weighted sequence is often described as “anatomy-weighted” because it is best for visualizing normal anatomy. On T1 images, fat appears bright (hyperintense), which makes the white matter appear lighter than the gray matter. Fluid, such as cerebrospinal fluid (CSF) in the ventricles, appears dark (hypointense).
The T2-weighted sequence is commonly referred to as “pathology-weighted” because fluid and areas of inflammation appear bright. In T2 images, water and CSF are hyperintense, appearing bright white. White matter appears darker than gray matter, which is the opposite of the T1 image. Since most pathological processes, such as edema or infection, involve increased fluid, they stand out brightly on T2 scans.
A third sequence, Fluid-Attenuated Inversion Recovery (FLAIR), is a modified T2 sequence sensitive to subtle abnormalities. FLAIR suppresses the signal from normal CSF, making it appear dark, even though the image is T2-weighted. This technique is helpful for identifying abnormalities located near the ventricles or the brain’s surface. By making the background CSF dark, small bright lesions are more easily seen without being obscured by the naturally bright fluid.
Common Terminology for Brain Findings
MRI reports use standardized terminology to describe deviations from a healthy brain appearance. A lesion is a general term for any abnormal area of tissue visualized on the scan. Lesions often show up as white matter hyperintensities on T2 and FLAIR images and can be associated with conditions including infections, tumors, or vascular issues.
Edema refers to swelling caused by excess fluid accumulation in the tissue, and it is a common indicator of injury or disease. Since edema involves increased fluid, it appears as a bright, hyperintense area on T2 and FLAIR sequences. Atrophy describes the shrinkage of brain tissue, often seen with aging or neurodegenerative diseases like Alzheimer’s. Atrophy is identified by widened sulci (the grooves on the brain’s surface) or enlarged ventricles.
When blood flow to a region of the brain is reduced, it results in ischemia. If the tissue dies due to prolonged lack of oxygen, it is called an infarct, which is the tissue damage related to a stroke. These stroke-related findings are identified by abnormal signal intensity in a specific vascular territory. A mass effect describes the displacement or distortion of normal brain structures caused by an adjacent abnormal growth or swelling, such as a tumor or large area of edema. Midline shift, where the center line of the brain is pushed to one side, is a measure of severe mass effect often seen after a large stroke or hemorrhage.
The Importance of Professional Interpretation
While learning the basics of MRI interpretation is informative, the final diagnosis must be left to trained professionals. Neuroradiologists specialize in interpreting medical images of the brain and nervous system. They possess the training necessary to differentiate subtle variations in signal intensity and structure that may indicate significant pathology.
Accurate diagnosis requires synthesizing imaging data with the patient’s medical history and clinical symptoms. For example, a bright spot on a FLAIR sequence could be an old scar or an active tumor; only a physician can correlate this finding with the patient’s context. Discussing the findings with your treating physician ensures the results are placed into the broader context of your health and inform the correct treatment plan. Self-evaluation should serve only as an educational exercise and never replace the guidance of qualified healthcare providers.