What is the appearance of acute blood on CT, MRI (Magnetic Resonance Imaging) T1, T2, DWI (Diffusion-Weighted Imaging), and SWAN (Susceptibility-Weighted Imaging)?

Acute Blood Appearance on Neuroimaging

Acute blood appears hyperdense (bright white) on CT, isointense to hypointense on T1-weighted MRI, markedly hypointense (dark) on T2-weighted MRI, hypointense (dark) on DWI, and markedly hypointense with blooming artifact on SWAN/gradient-echo sequences. 1

CT Imaging of Acute Blood

• Acute hemorrhage appears hyperdense (high density/bright) on non-contrast CT due to the high protein content of hemoglobin, with Hounsfield values consistent with acute blood 2, 1
• CT should ideally be performed within 1 week of symptom onset to reliably demonstrate the high density characteristic of recent hemorrhage 2
• The American Heart Association recommends CT as the primary modality for excluding hemorrhage before thrombolytic therapy due to its reliable and consistent hyperdensity in the acute phase 1
• CT has been the assumed gold standard for detecting acute intracranial hemorrhage, though this is based primarily on practical experience and level C studies 2

MRI T1-Weighted Imaging of Acute Blood

• In the first week, acute blood appears isointense to brain tissue on T1-weighted images due to increased deoxyhemoglobin 2
• The central portion of acute hemorrhage may show isointense signal corresponding to the acute clot 3
• Acute thrombus may be of low intensity on T1-weighted sequences in the very early phase 2

MRI T2-Weighted Imaging of Acute Blood

• Acute hemorrhage demonstrates markedly diminished signal intensity (hypointensity) centrally on T2-weighted images relative to surrounding white matter 4
• In the first week, venous thrombus and acute blood appear hypointense on T2-weighted images owing to increased deoxyhemoglobin 2
• Surrounding edema may produce moderately increased signal intensity (hyperintensity) of adjacent white matter on T2-weighted images 4

DWI (Diffusion-Weighted Imaging) of Acute Blood

• Acute hemorrhage appears hypointense (dark) on DWI due to susceptibility effects from blood products, which is the critical distinction from acute ischemia that appears hyperintense (bright) 1
• DWI uses echo-planar imaging sequences that are inherently sensitive to susceptibility changes caused by intraparenchymal blood products 2, 1
• Acute hemorrhage may show hyperintense signal on DWI in some cases, but this is typically accompanied by areas of hypointensity 3

Critical Pitfall to Avoid

• Do not confuse acute ischemia with acute hemorrhage on DWI: acute ischemia is hyperintense (bright), while acute hemorrhage is hypointense (dark) due to susceptibility effects 1
• This distinction is essential for treatment decisions, as thrombolytic therapy is contraindicated in hemorrhage 2

SWAN/Gradient-Echo/Susceptibility-Weighted Imaging of Acute Blood

• Acute blood demonstrates markedly hypointense signal (blooming artifact/signal dropout) on gradient-echo and susceptibility-weighted sequences 2, 1
• Magnetic susceptibility imaging (T2*-weighted sequences including SWAN, GRE, SWI) detects very small amounts of deoxyhemoglobin and other paramagnetic compounds 2, 5
• These sequences are particularly sensitive for detecting acute hemorrhage, with gradient-echo MRI demonstrating 96% concordance with CT for acute hemorrhage detection 2
• The American Heart Association states that gradient-echo sequences reliably detect intracranial hemorrhage, allowing MRI to serve as the sole initial imaging modality for acute stroke evaluation 2, 5
• A thrombosed dural sinus or acute hemorrhage demonstrates low signal on gradient-echo and susceptibility-weighted images due to paramagnetic products of deoxyhemoglobin 2

Practical Clinical Algorithm

For rapid hemorrhage exclusion:

• Use non-contrast CT as first-line imaging, where hyperdense blood is immediately visible 1
• CT can be performed and interpreted within minutes, making it ideal for emergency triage 2

If MRI is the initial modality:

• Include gradient-echo (GRE) or susceptibility-weighted imaging (SWI) sequences, as these reliably detect hemorrhage as hypointense signal 2, 1
• A streamlined MRI protocol including DWI, FLAIR, gradient-echo can be performed in approximately 10 minutes 5
• MRI with gradient-echo sequences has been shown to be at least as accurate as CT for detecting acute intracranial hemorrhage 2

Time-Dependent Evolution Caveat

• The MRI appearance of blood products evolves over time based on hemoglobin breakdown, with deoxyhemoglobin causing the characteristic hypointensity on T2-weighted and gradient-echo sequences specifically in the acute phase (first week) 2, 1, 4
• By the second week, thrombus contains methemoglobin, which results in hyperintensity on both T1- and T2-weighted images, representing the subacute phase 2