3 Intro to Neurorad I Flashcards
[SOME OF THIS IS STRAIGHT FROM MIKE SUN’S NOTES. THANKS, MIKE!]
Compare and contrast the technique used to obtain a CT vs. MRI of the brain.
CT compiles xray slices obtained using ionizing radiation; only one pass you need to take / interpret
MRI:
- uses magnetic field to align protons in a tissue
- sends a radiofrequency pulse through the magnet which temp. aligns them against that field
- reads the energy given by protons as they relax back to alignment as the “ECHO” or “MR signal”
- need to take 4-6 pulse sequences each at 3-5 mins (long test-time esp. for clautrophobic pts) and then interpret them (also longer time)
Describe the differences in conspicuity of normal anatomic structures on CT and MRI (including the different types of MR pulse sequences)
CT:
- more radio dense structures (e.g. bone) attenuate/absorb more of the x-rays; appear WHITE
- less radio-dense structures (e.g. air) attenuate/absorb little-to-no x-rays; appear BLACK
- measured in Hounsfield units (ie water = 0; air = -1000)
- Gray matter is SLIGHTLY HIGHER in attenuation (more is absorbed) than white matter, which causes the GM to be LIGHTER (can see deeper cortical WM tracts).
MRI:
- Will depend on which type of aquisition, or “pulse sequence” (e.g. T1-weighted T2-weighed, FLAIR, etc.) used
- Generally: high signal intensity = white; low = dark/black
- Look at ventricles for CSF and corpus collosum for WM to figure out what kind you are looking at
- T1-weighted
- best for soft tissue
- water/CSF/subarachnoid space = dark/low signal = gray/black
- fat = high signal = white
- GM lwr sig (ie darker) than WM; GRAY MATTER is GRAYER on T1
- T2-weighted
- opposite of T1
- GRAY MATTER is WHITER on T2
- FLAIR
- shows fluid in abnormal places (e.g. w/in parenchyma)
- Free fluid is dark like T1
- Fluid in parenchyma is bright like T2
- Diffusion
- Areas w/ restricted diffusion (e.g. stroke, cytotoxic edema) are bright
Explain what IV contrast does and why it might be used.
Enchances areas that otherwise blend in on non-contrast CT. Use when you suspect tumor, e.g.
CT:
- uses iodine-based dye
- increases attenuation of tissues it enters = WHITER on CT
- normal brain parenchyma does not enchance b/c of BBB
- helps visualize brain parenchyma, pituitary gland, meninges, choroid plexus, arteries/veins
- don’t use when there’s breakdown of BBB (e.g. 2° to infection, inflammation, etc.) b/c dye will go everywhere
MRI:
- Uses gadolinium which ∆s proton relaxation times
- will appear whiter on MRI
- augments MRI scans to improve dx capabilities
- helps, e.g., show angiogenesis in tumors
- Always use T1 w/ contrast MRI
What structures does IV contrast normally enhance?
Both CT and MRI:
- vessels
- meninges
- some pathologies (e.g. meningioma) when:
- they cause BBB breakdown
- they involve creation of new blood vessels (angiogenesis/neovascularity of tumors)
What are the strengths and weaknesses of CT vs. MRI?
CT Strengths
- fast (head CT ~5 secs)
- accessible (right in the ER usually)
- cheaper than MRI
- good for looking at:
- acute bleeds
- fractures
- brain swelling and mass effect (to r/o impending brain herniation)
- hydrocephalus
- can decr. need for biopsy
CT Weaknesses
- uses ionizing radiation
- not as good for soft tissue/white matter lesions
MRI Strengths
- superior soft tissue contrast c/w CT
- Good for looking at:
- edema
- blood of variable ages
- cortex abnormalities
- white matter abnormalities
MRI Weaknesses
- Slower
- More expensive
- Not good for bone b/c no water in cortex of bone
How does acute blood appear on CT?
high attenuation
When IV contrast is used for a head CT, does it affect the density of gray and white matter structures of the brain?
No, (or “normally no”)
cannot cross BBB
You have a pt w/ remote hx of colon cancer who presented w/ suddent onset of worst headache of his life. You suspect subarachnoid hemorrhage and are ordering a head CT.
Should you order it w/ or w/o IV contrast? Why?
Order non-contrast CT of the head
Dunno why?
