In MRI, a susceptibility-weighted imaging (SWI) sequence is performed on a patient with suspected microhaemorrhages. SWI uses a fully-flow-compensated long TE gradient echo sequence combined with filtered phase information. Microhaemorrhages appear as dark foci (blooming artefact) on SWI because:

• A Haemosiderin and deoxyhaemoglobin are paramagnetic, causing local magnetic field inhomogeneity and accelerated T2* decay ✓
• B Haemorrhage contains calcium which is diamagnetic and reduces the local field
• C Microhaemorrhages enhance on gadolinium due to BBB breakdown
• D The long TR used in SWI suppresses the signal from haemoglobin

Explanation

Haemosiderin (and early deoxyhaemoglobin/intracellular methaemoglobin) are paramagnetic substances that create local magnetic field inhomogeneities. In gradient echo sequences (including SWI), there is no 180° refocusing pulse, so T2* (T2-star) dephasing is not corrected — paramagnetic foci cause accelerated spin dephasing and signal loss, appearing as dark 'blooming' foci larger than the actual lesion. SWI phase imaging amplifies this susceptibility contrast, making microhaemorrhages, calcifications, and venous blood visible.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.