Brain MRI: A Systematic Reading
Weights and Planes
MRI images are commonly viewed in three planes: axial, coronal, and sagittal.
Shades of Gray Matter
The routine MRI is presented as black and white images, not color. The various shades of gray are described in terms of their “signal intensity.” “Low signal intensity” means “dark,” and “high signal intensity” means “bright.”
Remember that this is different from CT, where lesions are described in terms of “density.” A dark lesion on CT is called “hypodense,” and a bright lesion is “hyperdense.” How can you remember this? The word “den – SITY” rhymes with “C – T.”
MRI Weighting: T1, T2, others
The most commonly used techniques for MRI imaging are T1-weighted, T2-weighted, Flair and Diffusion-weighted.
T1 weighted images are useful for brain parenchyma. Brain appears medium gray and CSF is dark gray, and air is nearly black. Most tumors appear dark, with low signal intensity. Fat has high signal intensity on T1 but “drops out” on T2 weighted images where it becomes dark. Gadolinium contrast added to the T1 may “light up” a tumor or abscess. If a lesion has dark signal on T1, and bright signal with gadolinium, it is a “contrast enhancing” lesion. The enhancement may be the same throughout the lesion, making it “homogeneously enhancing.”
Or it may enhance only around the edges, making it “ring enhancing” as in a brain abscess, or in this lung met.
T2 weighted images are great to evaluate CSF spaces, which appear bright white. Brain appears light gray. Use the T2 sequence to check the size and symmetry of the ventricles, and look for bright signal of edema surrounding an aggressive tumor or a subacute stroke or hemorrhage. Also check the T2 images for Multiple Sclerosis, with its characteristic periventricular white matter changes–bright splotches around the lateral ventricles. The bright signal is the scarring of the myelin sheath in the white matter–the “sclerosis” shows up on T2 images.
The flair sequence is “a T2 with dark CSF,” very sensitive to edema and to parenchymal abnormalities like a low grade glioma. In fact a grade 1 astrocytoma will be virtually invisible on T1, but will be unmistakable on Flair. The flair sequence is useful for surveillance of a low grade glioma, as you watch for signs of enlargement like fattening of the gyri.
Diffusion weighted images have one purpose: to visualize an area of acute ischemia. Say a person has new onset of stroke symptoms. A CT shows no blood, proving that the stroke is not hemorrhagic. An MRI will show the stroke as bright signal on the Diffusion-weighted images, and dark on the diffusion ADC sequence.
An MRI is the study of choice for tumor, multiple sclerosis, and ischemic stroke. Add gadolinium contrast to evaluate tumor and abscess. For brain hemorrhage, however, CT is the go-to study.