What is the clinical significance of blooming on Susceptibility Weighted Imaging (SWI) MRI images, particularly in adults with a history of stroke or trauma?

Blooming on Susceptibility-Weighted Imaging (SWI) MRI

Clinical Significance

Blooming artifact on SWI MRI represents areas of magnetic susceptibility difference—most critically indicating blood products (hemosiderin, deoxyhemoglobin), intravascular thrombus, calcium deposits, or iron accumulation—and serves as a highly sensitive marker for microhemorrhages, hemorrhagic transformation, and vascular pathology that directly impacts prognosis and treatment decisions in stroke and trauma patients. 1, 2

Key Diagnostic Applications

In Acute Stroke

• SWI detects hemorrhage 3-6 times more sensitively than conventional gradient echo sequences and far exceeds CT sensitivity, identifying hemorrhagic transformation in acute infarcts that would be missed on standard imaging 1, 3, 4
• Blooming within a vessel lumen on SWI indicates intravascular thrombus, which helps localize arterial occlusion and may guide endovascular intervention decisions 1, 2
• Asymmetrically prominent cortical vein blooming suggests impaired venous drainage and tissue hypoxia, providing hemodynamic information about stroke severity 5, 4
• SWI can detect acute intracerebral and subarachnoid hemorrhage as early as 6 hours after onset, when CT may still be negative 4

In Traumatic Brain Injury

• SWI is 3-6 times more sensitive than conventional T2 gradient echo for detecting traumatic microhemorrhages associated with diffuse axonal injury (DAI)*, which is the primary application in moderate-to-severe TBI 1
• The American College of Radiology recommends MRI with SWI when initial CT is normal but unexplained neurologic findings persist after head trauma (Class I recommendation) 1, 6
• SWI detects 30% more TBI-related lesions compared to conventional CT and standard MRI sequences 6
• Microhemorrhage burden on SWI correlates with presenting Glasgow Coma Scale scores, though the relationship between microhemorrhage number and long-term outcomes remains controversial 1

Prognostic Implications

Mortality and Morbidity Impact

• In intracerebral hemorrhage, hematoma expansion detected on repeat imaging is associated with a nearly 5-fold increase in clinical deterioration, poor outcome, and death 1
• The presence of multiple microbleeds on SWI may indicate bleeding-prone angiopathy (cerebral amyloid angiopathy, hypertensive vasculopathy), though data on whether this increases hemorrhagic transformation risk after thrombolysis remains mixed 1
• In TBI, identification of microhemorrhages may predict injury severity, though this remains controversial and does not typically alter acute management 1

Quality of Life Considerations

• Approximately 15% of mild TBI patients with normal CT will have persistent neurocognitive sequelae at 1 year, and early MRI with SWI showing microhemorrhages may have prognostic value for predicting these outcomes 1, 6
• Detection of chronic microbleeds helps identify patients at risk for recurrent stroke—lobar microbleeds suggest amyloid angiopathy while deep microbleeds suggest hypertensive vasculopathy 1

Differential Diagnosis of Blooming

Paramagnetic vs. Diamagnetic Substances

• Blood products (hemosiderin, deoxyhemoglobin) and iron deposits are paramagnetic and cause blooming on SWI 2, 5, 7
• Calcium is diamagnetic and also causes blooming, but can be differentiated using phase images or specialized paramagnetic/diamagnetic-specific SWI processing 5, 7
• Filtered phase images show opposite signal characteristics: paramagnetic substances (blood) appear dark while diamagnetic substances (calcium) appear bright 7

Specific Pathologies

• Cavernous malformations show characteristic "popcorn" appearance with mixed signal and prominent blooming from hemosiderin deposition 2, 4
• Cerebral microbleeds in vasculitis, CADASIL, or amyloid angiopathy appear as small punctate foci of blooming 4
• Superficial siderosis shows linear blooming along brain surfaces in Alzheimer disease and amyloid angiopathy 5

Critical Clinical Pitfalls

Avoid These Common Errors

• Do not assume all blooming represents hemorrhage—correlation with CT or phase images is essential to exclude calcification, particularly in tumors or vascular malformations 5, 7
• Do not rely solely on SWI for acute hemorrhage detection in the hyperacute phase (<6 hours)—CT remains more reliable in the first few hours after symptom onset 4
• Do not use the presence of a few microbleeds as an absolute contraindication to thrombolysis—large numbers of microbleeds may increase risk, but small numbers have not consistently shown increased hemorrhagic transformation risk 1
• Do not order SWI as initial imaging in acute trauma or stroke—noncontrast CT remains first-line for rapid triage and treatment decisions 1

Practical Imaging Algorithm

When to Order SWI

1. After acute stroke when standard MRI shows infarct but clinical severity seems disproportionate—look for additional microhemorrhages or venous congestion 1, 4
2. In TBI when CT is negative but neurologic deficits persist—SWI may reveal DAI not visible on CT 1, 6
3. For stroke etiology workup—identify microbleeds suggesting specific vasculopathies (amyloid angiopathy, CADASIL, vasculitis) 1, 4
4. When vascular malformation is suspected—SWI enhances detection of cavernomas and venous anomalies 2, 4

Technical Considerations

• SWI requires high-resolution 3D gradient echo sequences with both magnitude and phase data 2, 5
• Imaging appearance varies significantly between vendors and acquisition parameters 5
• No gadolinium contrast is needed for SWI in trauma or acute stroke settings 1