MRIs are described by signal intensity, as compared to CTs where lesions are described by density.

A dark lesion on MRI is “hypointense”

A bright lesion on MRI is “hyperintense"

The most commonly used MRI sequences are T1-weighted, T2-weighted, FLAIR, and Diffusion-weighted.

• T1-weighted images are good for brain parenchyma.
• Contrast enhanced T1 with gadolinium helps differentiate pathological tissue (e.g. tumors, inflammation, infection)
• T2-weighted images are good for CSF spaces and periventricular white matter.
• Edema from a tumor, subacute stroke or hemorrhage appears bright
• Periventricular white matter scarring from multiple sclerosis appears bright
• FLAIR images are T2 images where CSF is dark.
• FLAIR is very sensitive to edema and parenchymal lesions.
• Diffusion-weighted sequences are good for cellular swelling.
• Acute ischemia appears bright on Diffusion-Weighted Imaging (DWI) and dark on Apparent Diffusion Coefficient (ADC) maps
• Some neoplasms, abscesses and toxic/metabolic/demyelinating processes can also appear bright on DWI.