Acute Blood Appearance on SWI
No, acute blood is NOT white on SWI—it appears dark (hypointense/black) due to the paramagnetic properties of deoxyhemoglobin, though the appearance varies depending on the exact timing and blood product stage. 1, 2
Understanding Blood Evolution on SWI
SWI is exquisitely sensitive to paramagnetic substances and detects hemorrhage as early as 6 hours after onset, appearing as low signal (dark/black). 3 The key principle is that SWI exploits magnetic susceptibility differences of blood products including deoxyhemoglobin, intracellular methemoglobin, and hemosiderin. 4, 5
Stage-Dependent Appearance
Hyperacute to acute blood (first 24-48 hours): Contains deoxyhemoglobin, which appears hypointense (dark/black) on SWI due to its paramagnetic properties 1, 3
Early subacute phase (days 2-7): As blood evolves and contains intracellular methemoglobin, it remains hypointense (dark) on SWI 2, 4
Late subacute to chronic phase: With extracellular methemoglobin and eventual hemosiderin deposition, blood products continue to appear hypointense (dark) on SWI, with hemosiderin being particularly prominent 1
Critical Clinical Caveat
It is a common misconception that blood is always black on SWI—hemorrhage has variable appearance in different stages, and not all dark lesions on SWI represent blood. 2 The appearance depends on:
- Timing of imaging relative to hemorrhage onset 1
- Specific blood product present (deoxyhemoglobin vs. methemoglobin vs. hemosiderin) 1, 4
- Location and concentration of blood products 5
Practical Implications
SWI is 3-6 times more sensitive than conventional T2 gradient-echo sequences* for detecting hemorrhagic axonal injuries and microbleeds 1
SWI can detect acute intracerebral parenchymal and subarachnoid hemorrhage reliably within the first 6 hours 3
Other substances also appear dark on SWI including iron, certain forms of calcium, and air—so low signal does not automatically equal blood 2, 4, 5
In cerebral venous thrombosis, thrombosed vessels demonstrate low signal on SWI due to paramagnetic blood products 1