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DWI: Bright Isn't Always Restricted

Fellowship-level neuroradiology teaching by E. Brooke Schrickel, MD. Open this topic in the interactive reading room →

Watch · concise explainerVisual summary

Core clinical idea

Bright on DWI is not the same as stroke. Diffusion is sensitive but not specific, so read the whole family (DWI + ADC + parent EP-T2) together, separate true restriction from T2 shine-through and T2* from blood, then assign the brightness to a mechanism.

Bottom line

Bright on DWI isn't stroke until ADC and the parent agree — read the family, time it, then name the mechanism of brightness.

Core workstation questions

  • Is it dark on ADC (true restriction) or bright on ADC (T2 shine-through)?
  • Do the shiny yin-yang edges mean blood (T2*) — do I need an FSE-based diffusion to be sure?
  • How old is it (DWI bright up to ~a month; ADC pseudonormalizes earlier) — could this be a pseudonormalized lesion needing post-gadolinium?
  • If not a vascular-territory infarct, which mechanism is this (cellular tumor, abscess, encephalitis, toxic/metabolic, venous)?
  • Would high b-value or FSE-based diffusion change what I can see (subtle brainstem/CJD; skull base/metal/blood)?

What changes reporting / management

  • Use ADC + post-gadolinium to avoid missing a pseudonormalized infarct in the subacute/post-reperfusion window.
  • When shiny edges suggest hemorrhage, re-image with FSE-based diffusion (PROPELLER/BLADE) to strip T2* before calling restriction.
  • DWI-positive lesion after TIA flags high short-term infarct risk — supports more urgent triage/stroke workup.
  • Don't assume acute DWI restriction is irreversibly dead tissue in a treated patient — rare reversal occurs with effective recanalization.

Practical traps

  • Calling T2 shine-through (bright DWI, bright ADC) a stroke.
  • Mistaking hemorrhage T2*/susceptibility (yin-yang edges, falsely restricted-looking ADC) for infarct.
  • Missing a pseudonormalized lesion around the ~1-week-plus window.
  • Mistaking a restricting abscess or highly cellular tumor (lymphoma, PNET/medulloblastoma, epidermoid, cellular mets) for infarct.
  • Trying to interrogate skull base / near-metal regions with echo-planar diffusion despite distortion.

Teaching pearls

  • True restriction is bright on DWI and dark on ADC — everything else is shine-through until proven otherwise.
  • Diffusion never forgets its mother: shiny yin-yang edges mean blood, not infarct.
  • DWI bright lasts ~a month; ADC pseudonormalizes earlier — use both to date the stroke.
  • When EP distortion or blood spoils the read, go FSE-based (PROPELLER/BLADE).
  • High b-value pulls out subtle brainstem ischemia and CJD ribboning.
  • Most abscesses and cellular tumors restrict — bright is a differential, not a diagnosis.

Teaching visuals

“Bright on DWI” — decision treeDWI is a high-b image; bright signal can be true restriction or T2 shine-through. Always pair with ADC.DWI brightstarting observationADC low?(truly restricted)NOT2 shine-throughADC not low — not restrictionor artifact / T2 effectYESVascular territory+ clinical stroke?YESAcute infarctfits a vascular territorytime it with ADCNORestricted but not a stroke — work the mechanismlow ADC confirmed, no vascular territory: reach for these causesHemorrhagecheck SWIAbscess / puscentral restrictionHypercellular tumorlymphoma, high gradeEncephalitis / CJDcortical ribbonActive demyelinationleading edge of plaqueToxic-metaboliccytotoxic edemaRule: bright DWI + low ADC = restricted diffusion. Then ask “is it vascular?” before reaching for the rarer causes.
"Bright on DWI" decision tree
DWI & ADC over time after infarctConceptual curves — timing is approximate. The key is that ADC pseudonormalizes before DWI fades.lowhighsignal intensityminutes–hrs~1 week~1 monthmonthstime →DWI signal (bright)ADC valueADC pseudonormalizesACUTEDWI bright + ADC lowSUBACUTEDWI still bright, ADC pseudonormalizedPitfallA bright DWI with a normal/high ADC is subacute, not hyperacute — don’t over-call timing from DWI alone.
DWI & ADC over time after infarct
Bright DWI that isn’t a typical infarctAll show low ADC. Group by mechanism, then use the “tell” to land the diagnosis.Ischemic, but not a routine arterial strokeVenous infarctnon-arterial territory; dural sinus / cortical vein clotAir / fat / DIC embolipunctate foci at gray–white junction (“starfield”)MELAScrosses vascular territories; +lactate on MRSBloodHemorrhageblooms “yin-yang” on SWI;DWI signal is paramagnetic, not cellularPusAbscess / empyemacentral cavity restricts;smooth “onion-skin” rim, ring-enhancingHypercellular tumorLymphoma, high-grade, medullodense nuclei pack cells; solid mass, low ADC throughoutEncephalitis / prionCJDcortical ribboning + basal ganglia onhigh-b DWI; HSV → limbicActive demyelinationAcute MS / ADEM plaquerestriction at the advancing leading edge of the lesionToxic-metabolicsymmetric, often deep GM / WM;hypoglycemia, CO, drug toxicitycytotoxic edema in a non-vascular, bilateral pattern
Bright DWI that isn't a typical infarct

Source lectures

  • Diffusion Weighted Imaging (DWI): How to be Right When It's Bright

Educational material for radiology residents and neuroradiology fellows. Nothing here drives individual patient care, and it contains no patient data.