Stroke Imaging (Week 4--Hathout)

Question 1

3 findings on CT that aid in diagnosis of stroke

Answer 1

1) Sulcal effacement: cytotoxic edema causes enough mass effect to efface sulci in infarct zone; latest of early signs to appear but sometimes seen within 3 hours
2) Loss of gray-white matter differentiation: due to cytotoxic edema in gray matter (neurons) which causes hypodensity of gray matter, particularly in insular ribbon (between insular cortex and external capsule)
3) Dense vessels indicating intravascular clot: earliest sign seen before any edema but only in 10-15% of strokes; in MCA or carotid artery terminus

Question 2

Sensitivity of non-contrast CT in diagnosing early infarct

Answer 2

31% in first 3 hours

40-60% in first 6 hours

Question 3

Compare CT, MRI and DWI for diagnosing stroke

Answer 3

Time frame 6 hours, one study showed:

CT sensitivity 45%

MRI sensitivity 18%

DWI sensitivity 100%

Question 4

What is DWI imaging based on?

Answer 4

Diffusion-weighted imaging (DWI) reflects diffusion coefficient D of water molecules

In stroke, there is decrease in water diffusion in the infarct zone

Intracellular water is more restricted than interstitial water and ventricle/free water

Pure water in ventricle loses a lot more signal than brain parenchyma when using DWI

Infarct zone has LESS MR signal loss = brighter

Question 5

Stroke on DWI and ADC

Answer 5

Stroke on DWI is bright

Stroke on ADC is dark

Question 6

Difference between T2 and DWI for seeing edema

Answer 6

All edema is brighter on T2

DWI allows us to distinguish between cytotoxic edema (infarct) and vasogenic edema (tumor and infection) because they have different D values

Infarct is bright on DWI and tumor/infection is dark on DWI

Question 7

1/3 of the MCA Rule

Answer 7

If patient already has signs of large stroke on CT then high likelihood that brain has already infarcted (brain is already dead) so IV tPA won’t help and could increase intracranial bleed

So if CT shows that infarct involves more than 1/3 of MCA territory, don’t give tPA

Question 8

Dense vessel sign

Answer 8

“Hyperdense MCA sign” or HDMCAS

If HMCAS on CT within 90 minutes of stroke, don’t give tPA because already major neurological deficit and bad outcome if you give tPA; also probably means big clot that won’t be dissolved by tPA

Give intra-arterial thrombolysis or clot retreival (with MERCI)

Question 9

Ischemic penumbra model of stroke therapy

Answer 9

Use DWI and PWI to find penumbra zone: PWI positive but not DWI positive

Penumbra is hypoperfused but viable, is “at risk” but can benefit from thrombolytic therapy

If there is a large penumbra zone (large PWI-DWI mismatch), should give tPA

Question 10

Why do we do CT?

Answer 10

To determine who to WITHOLD IV tPA from

If patient has abnormal CT, don’t want to give tPA because brain is already dead and IV tPA won’t help, will only hurt

Question 11

PWI

Answer 11

Perfusion weighted imaging, PWI, requires IV administration of gadolinium

Gadolinium reduces T2 time, makes tissue darker

Generate perfusion map (mathematically generated)

Brightness is less blood flow

Question 12

Tumor on DWI and ADC

Answer 12

Opposite of infarct!!

Tumor is dark on DWI

Tumor is bright on ADC

Note: old infarct behaves like a tumor too!

Question 13

Old infarction on CT

Answer 13

Very low density in affected vascular territory, sometimes even cystic

Area becomes atrophic, with ipsilateral ventricle enlargement