Thread by @daniel_gewolb

Tips & tricks of DWI to help narrow the differential

Ddx:
Stroke
Abscess
Hypercellular tumor
Hematoma
Epidermoid cyst
Encephalitis
Seizure
Demyelination
Toxic/metabolic disorders
CJD
Other stuff I’m forgetting
#Neurology #neurosurgery #radres #MedTwitter #MedEd @TheASNR

Anything that traps fluid can restrict diffusion! Here are some tricks I use to narrow the ddx

1️⃣STROKE
Cytotoxic edema due to trapped intracellular fluid leads to restriction

Look for wedge shaped restriction in a vascular territory

2️⃣ABSCESS
Trapped purulent material leads to LIGHT BULB BRIGHT restriction

DWI is excellent for differentiating tumor from pyogenic abscess as the abscess will have CENTRAL restriction

Abscess should also have vasogenic EDEMA, ENHANCEMENT, and possible dual rim sign (T2 & SWI)

3️⃣HYPERCELLULAR TUMOR (lymphoma, medulloblastoma, embryonal tumor, germinoma, glioblastoma, etc)

Densely packed tumor cells trap fluid in between

Hypercellular tumor continued

Primary CNS Lymphoma

▶️Central diffusion restriction

▶️Homogenous enhancement

▶️Low T2 signal (less cytoplasm and more nucleus so less water in cells and lower T2 signal)

▶️Hyperdensity on CT

▶️Periventricular location

Hypercellular tumor continued

▶️Glioblastoma or high grade glioma

Variable but may have more eccentric or nodular restriction around areas of necrosis and heterogeneous enhancement

4️⃣HEMATOMA
RBCs trapped in serum and fibrin can restrict on DWI (though blood can also be dark on DWI from susceptibility)

Hyperdensity on CT is a giveaway but this may fade overtime or you may not have a CT

Look for a rim of HYPERINTENSITY ON T1 and HYPOINTENSITY on SWI

5️⃣DEMYELINATION

High signal on DWI is predominantly due to T2 SHINE THROUGH

True restriction may be seen at the LEADING EDGE (along the margin) in acute demyelination possibly from cytotoxic edema, reduced fiber tract organization, or myelin fragments

(This example is PML)

6️⃣EPIDERMOID CYST

Tightly organized epithelial layers cause a light bulb bright restriction

ADC tends to be ISOINTENSE TO BRAIN PARENCHYMA (not super dark), possibly from movement of fluid between layers (at least that’s how I think of it)

Epidermoid cyst continued

▶️CSF intensity on T1 & T2

▶️Dirty on FLAIR

▶️DO NOT ENHANCE! (May have a tiny rim of enhancement along edge but NO CENTRAL)

Bonus cases

7️⃣SEIZURE
Shows gyriform or cortical restricted diffusion (often in the mesial temporal lobe)

Examples in 2 different patients

8️⃣ENCEPHALITIS
Diffusion restriction in the insula and temporal lobes favors herpes encephalitis, though any encephalitis can cause restriction

Herpes is usually bilateral but asymmetric and may have patchy enhancement and hemorrhage

Case of herpes

9️⃣CJD
Diffusion restriction is seen in the basal ganglia, thalami, and cortex. This can be asymmetric

🔟Many Toxic/metabolic disorders

Hepatic encephalopathy
Acute toxic leukoencephalopathy
Hypoxia
Methotrexate toxicity
Drug abuse
CO poisoning
Many more