Neuroimaging in Neuropsychiatry Qurat Khan

Basics of reading MRIs

Planes

Viewed in three planes Axial, coronal, and sagittal

Shades of Gray Matter

Routine MRI is presented as black and white images Shades of gray are described in terms of “signal intensity” “Low signal intensity” means “dark,” and “high signal intensity” means “bright” 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 to remember this? “den – SITY” rhymes with “C-T”

Weights

The most commonly used techniques are T1-weighted, T2-weighted, Flair, and Diffusion-weighted

T1 weighted images Useful for brain parenchyma Brain appears dark gray, CSF is black Tumor- hypointense Fat- hyperintense Abscess- hypointense MS- hypointense foci Gadolinium contrast on 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 T1 weighted images

T2 weighted images CSF appears bright white Brain appears light gray Used to check the size/symmetry of ventricles, edema, sub acute stroke or hemorrhage, MS T2 weighted images

T2 Changes Periventricular ischemic foci Acute/subacute ischemic stroke- hyperintense- detect 80% of infarctions before 24 hours. May be negative up to 2-4 hours post-ictus; hyperintensity reaches its maximum between 7 and 30 days. After this it starts to fade Hemorrhagic stroke, acute and sub acute- hyperintense on T2 Edema (Inflammation, vasculitis, infection)- poorly differentiated hyperintense foci Abscess- hyperintense foci MS: hyperintense foci, periventricular and subcortical white matter, infratentorial structures, corpus callosum Tumor- hyperintense foci PML- asymmetrical, hyperintense foci with predilection for parietal and occipital lobes Central pontine myelinolysis- hyperintense lesion in the central part of the pons Migraine- hyperintense foci on T2 Virchow-Robin space-hyperintense on T2 Fat- hypo intense T2 Changes

Multiple Sclerosis

FLAIR Is “a T2 with dark CSF,” Very sensitive to edema and to parenchymal abnormalities like a low grade glioma FLAIR

Purpose: to visualize acute ischemia- positive in the acute phase and then becomes more bright with a maximum at 7 days. Will be positive for approximately for 3 weeks after onset A person has new onset of stroke symptoms. CT shows no blood, proving that stroke is not hemorrhagic. Stroke will appear as bright signal on DWI images, and dark on diffusion ADC sequence ADC will be of low signal intensity with a max at 24 hours, then will increase in signal intensity and finally becomes bright in the chronic stage DWI

L MCA stroke

CT

Bone Air- Jet black CSF Brain

Hyperdense: acute intraparenchymal hemorrhage, calcification, tumor, implant Hypodensity: ischemic stroke BUT changes may not appear upto 24 hours after the stroke Brain

For tumor or infection (abscess) Contrast

MCA infarction