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SWI: Where It Blooms and Why

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

Watch · concise explainerThe distribution is the diagnosis

Core clinical idea

SWI is a single-purpose high-sensitivity detector for things that distort the local field (blood/hemosiderin, calcium, iron, deoxyhemoglobin). Read it as 'is there bloom, where, and in what pattern?' — the distribution of the bloom is the diagnosis, not gray-white/edema/anatomy.

Bottom line

SWI is a blooming detector — the distribution is the diagnosis, and a negative GRE doesn't count.

Core workstation questions

  • Is the microhemorrhage distribution lobar (amyloid) or deep gray (hypertensive)?
  • Do these FLAIR lesions have a central vein (MS) or sit between veins (small-vessel)?
  • Is there a blooming clot pointing to an occlusion I should chase on CTA?
  • Are prominent deoxygenated draining veins marking oligemic/at-risk tissue or venous thrombosis?
  • Could 'innumerable foci' be DAI burden or malaria/cysticercosis given the history?
  • Is the filtered phase image even available, or did the scanner send only magnitude/SWI?
  • On this vendor's phase display, are the veins dark or bright — what's my reference polarity?
  • Does the lesion match the veins (blood) or oppose them (calcium)?

What changes reporting / management

  • Name the microhemorrhage distribution (lobar vs deep gray) and put the implied differential in the impression, not just 'microhemorrhages present'.
  • Report a perivenular/central-vein distribution of FLAIR lesions as supportive of demyelination to curb unnecessary white-matter workups.
  • Use a blooming clot on SWI to prompt CTA review for a peripheral/occluded vessel; report prominent deoxygenated draining veins as oligemic territory.
  • Flag the elevated hemorrhage risk of venous infarcts when SWI shows sinus/venous thrombosis with petechial bleed.
  • When the question is calcium vs blood and there is no CT, request/use the filtered phase image; magnitude SWI alone cannot answer it.
  • Calibrate polarity to the veins first (standardize so venous blood is dark, then calcium is bright), inverting grayscale/narrowing window to a dark-CT appearance if needed.
  • State the calibration in the read: lesion matches venous signal = blood; lesion opposes venous signal = calcium.
  • Use MIPs for venous continuity/relationship, not for the calcium-vs-blood call.

Practical traps

  • Reading SWI for gray-white differentiation, edema, or anatomy instead of susceptibility.
  • Treating a negative GRE as a clean study when SWI is substantially more sensitive to microhemorrhage.
  • Overcalling radiation change or tumor neovascularity as primary hemorrhage without the treatment context.
  • Missing CNS malaria or calcified neurocysticercosis that is near-invisible on T2/GRE but florid on SWI.
  • Reading the phase map without first setting the vein reference — vendor polarity can flip the entire interpretation.
  • Memorizing 'bright = calcium' as if polarity were fixed across vendors.
  • Forcing a calcium-vs-blood determination through aliasing artifact in large or heavily mineralized/hemorrhagic lesions.
  • Trying to separate calcium from blood on magnitude/post-processed SWI instead of the filtered phase image.

Teaching pearls

  • Lobar microbleeds sparing deep gray = amyloid; deep gray microbleeds = hypertensive.
  • MS plaques have a vein through the center; small-vessel disease sits between the veins.
  • A blooming clot on SWI sends you back to the CTA to find the occlusion.
  • Venous infarcts bleed more than arterial — call the risk.
  • DVA is a don't-touch variant; SWI may be the only sequence that shows it.
  • Magnitude SWI can't separate calcium from blood; the filtered phase image can.
  • Always calibrate to the veins first — vendor polarity flips the whole interpretation.
  • Matches venous signal = blood; opposite = calcium.
  • Aliasing in big lesions defeats the phase call — don't force it.
  • MIPs are for venous continuity, not the calcium-vs-blood distinction.

Source lectures

  • Susceptibility Weighted Imaging
  • Susceptibility Weighted Imaging: Calcium vs. Blood

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