## Microhemorrhage Detection in Neuroimaging **Key Point:** Susceptibility-weighted imaging (SWI) and gradient echo (GRE) sequences are the most sensitive MRI techniques for detecting microhemorrhages because they are exquisitely sensitive to magnetic field inhomogeneities caused by hemosiderin and deoxyhemoglobin. ### Comparison of Imaging Modalities for Microhemorrhage Detection | Modality | Sensitivity | Mechanism | Clinical Use | |----------|-------------|-----------|---------------| | **CT (non-contrast)** | Very low (<5%) | Poor detection of small bleeds; only detects macroscopic hemorrhage | Acute stroke rule-out | | **T2-weighted MRI** | Low–moderate (30–50%) | T2 hypointensity from hemosiderin, but less sensitive than SWI | Routine brain imaging | | **SWI / GRE MRI** | Very high (>95%) | Extreme sensitivity to magnetic susceptibility changes from iron-containing blood products | **Gold standard for microhemorrhages** | | **DWI** | Low | Detects acute ischemia, not hemorrhage | Acute ischemic stroke | **High-Yield:** SWI sequences show microhemorrhages as small round foci of signal loss ("blooming artifact") that appear larger than they actually are due to the magnetic field distortion. This blooming effect increases sensitivity but requires careful interpretation to avoid overestimating lesion size. **Clinical Pearl:** In cerebral amyloid angiopathy (CAA), microhemorrhages are predominantly **lobar** (cortical and subcortical white matter), whereas in hypertensive microangiopathy, they are **deep** (basal ganglia, thalamus, pons). SWI can differentiate these patterns and guide diagnosis. **Mnemonic:** **SWI-GRE = SUSCEPTIBLE** — Susceptibility-Weighted Imaging and Gradient Recall Echo sequences are most sensitive for iron-related signal changes (hemosiderin, deoxyhemoglobin). 
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