MRI vs CT
MRI produces high-resolution images of soft tissue without ionizing radiation, making it superior to CT for detecting early ischemic stroke, evaluating the posterior fossa and brainstem, characterizing tumors, and assessing white-matter and inflammatory disease. CT remains faster and better for acute hemorrhage and bony detail and is used first in emergencies, while MRI is often the follow-up or problem-solving study.
The Core Sequences
A few sequences do most of the work. On T1-weighted images, fat is bright and cerebrospinal fluid (CSF) is dark, giving good anatomic detail. On T2-weighted images, fluid is bright, so most pathology (which involves increased water) stands out as bright. FLAIR is a T2 image with the bright CSF signal suppressed, which makes lesions near the ventricles and cortex more conspicuous.
Diffusion and Contrast
Diffusion-weighted imaging (DWI) detects restricted movement of water molecules and is the most sensitive sequence for acute ischemic stroke, where the infarct core is bright on DWI (with a corresponding dark ADC map). Gadolinium contrast enhances areas with blood-brain-barrier breakdown — such as tumors, abscesses, and active demyelination — highlighting them on post-contrast T1 images.
A Basic Approach
Read systematically: confirm the patient and sequences, then review the brain in an organized way — symmetry, the ventricles and CSF spaces, gray-white differentiation, and each region — comparing sequences to characterize any abnormality (is it bright on T2/FLAIR? does it restrict on DWI? does it enhance?). Correlate the imaging pattern with the clinical question, since the same signal change can have different meanings in different contexts.