MR Imaging (Basics)

General: 
- fat is always bright (except modified sequences like STIR) 

- bone is always dark

- marrow is somewhere in between

T1: 

- anatomy scans (resolution of normal anatomy good)
- things that are bright on T1: fat, subacute blood (=metHb), melanin, contrast. 

- fluid is dark on T1
- protip: T1 noncon is a good way to look for recent bleed (days to ~2-3 weeks) if you don't have GRE imaging.

T2

- Fluid is bright 

- FLAIR = T2 with CSF subtracted out.
- In cases of bleeds, intact RBC membrane acts as a local magnet, and will appear dark on T2.

Blood on MR: 

Time course	Substrate	T1	T2
Hyperacute	Intracellular oxyhemoglobin	Iso	Iso-br
Acute (hrs-days)	Intracellular deoxyhemoglobin	Iso	Dark
Early subacute (days-wks)	Intraceullar methemoglobin	Bright	Dark
Late subacute	Extracellular methemoglobin	Bright	Bright
Remote	Ferritin/hemosiderin	Dark	Dark

DWI 

- restriction in mobility of water molecules (brownian motion) 

- restricted diffusion shows up bright = cytotoxic edema 

- ischemia leads to failure of membrane ion pumps; cells swell with fluid; water that used to be in the extracellular space is now in the cell, which limits its motion 

- most common cause of cytotoxic edema is ischemia, but there are other pathologies: necrosis (abscess, tumor center, radiation damage), epidermoid tumor, acute demyelination, acute encephalitis, CJD, tumors with high nuclear:cytoplasmic ratio, elevated protein or subacute blood
- T2 shine through: DWI is a T2-derived study, lesions that are bright on T2 (ie tumors) will be bright on DWI as well; thus we use the the apparent diffusion coefficient (calculated from T2) to showcase intrinsically T2-bright things
- theoretically, DWI changes appear as early as 6 mins after ischemic insult and as late as 2 weeks after; imaging changes will be apparent on ADC (dark) before they are apparent on DWI
- ADC-dark lesions will eventually become bright over time as the cells complete necrosis

Gradient recalled echo/susceptibility weighted imaging 

- takes advantage of paramagnetic qualities of deoxyhemoglobin and hemosiderin to detect blood products
- most sensitive imaging modality for blood 

Contrast

- When to use: refine diagnosis, see small images. 

- Things with no BBB that enhance: Pituitatry, hypothalamus, pineal gland, choroid plexus. 

MR patterns: 

- Dural-based extraaxial mass: meningioma, dural met, granulomatous disease (sarcoid) 

- Periventricular ischemia: small vessel disease, watershed infarct