Is acute blood hyperdense on CT and hyperintense on MRI (Magnetic Resonance Imaging) DWI (Diffusion-Weighted Imaging)?

Acute Blood Appearance on CT and MRI DWI

Yes, acute blood is hyperdense (white) on CT and can appear hyperintense on MRI DWI, though the DWI appearance is complex and depends on the specific sequence used.

CT Appearance of Acute Blood

Acute hemorrhage is consistently hyperdense (bright/white) on non-contrast CT, making it the gold standard for rapid hemorrhage detection 1.

• Acute blood appears as increased density (hyperdense) on CT due to the high protein content of hemoglobin 1.
• This hyperdensity is reliable and consistent in the acute phase, which is why CT remains the primary modality for excluding hemorrhage before thrombolytic therapy 1, 2.
• The hyperdense appearance can sometimes be confused with contrast staining after intra-arterial procedures, which is a critical pitfall 3.

MRI DWI Appearance of Acute Blood

The appearance of acute blood on DWI is more nuanced and sequence-dependent than on CT.

Susceptibility Effects on DWI

• Acute hemorrhage typically appears as signal loss (hypointense/dark) on DWI due to magnetic susceptibility effects from deoxyhemoglobin 3, 4.
• DWI uses echo-planar imaging sequences that are inherently sensitive to susceptibility changes caused by blood products 1.
• Acute deoxyhemoglobin creates magnetic field inhomogeneities that cause signal dropout (hypointensity) rather than hyperintensity on DWI 3, 4.

Detection Capabilities

• MRI with gradient-echo sequences (T2/SWI) is highly sensitive for detecting acute hemorrhage*, with studies showing MRI can detect hemorrhage that CT misses 3, 4.
• In one study, MRI detected hemorrhagic transformation in 6 of 9 patients while CT detected it in only 1 patient 4.
• Gradient-echo and susceptibility-weighted imaging demonstrate low signal (blooming artifact) in regions of acute hemorrhage 1, 5.

Critical Clinical Distinction

It is essential to understand that while acute ischemia appears hyperintense (bright) on DWI, acute hemorrhage appears hypointense (dark) on DWI due to susceptibility effects 3, 4.

• The hyperintensity on DWI represents restricted diffusion from cytotoxic edema in ischemic tissue 1, 5.
• When hemorrhage occurs within an ischemic region, it appears as a heterogeneous area of signal loss within the hyperintense ischemic area 4.
• This creates a characteristic pattern: bright ischemia with dark hemorrhagic foci 3, 4.

Practical Algorithm for Hemorrhage Detection

For Acute Stroke Evaluation:

• Use CT as first-line for rapid hemorrhage exclusion - hyperdense blood is immediately visible 1, 2.
• If MRI is performed, include gradient-echo (GRE) or susceptibility-weighted imaging (SWI) - these sequences reliably detect hemorrhage as hypointense signal 1, 5.
• Do not rely on DWI alone for hemorrhage detection - the susceptibility effect causes signal loss, not hyperintensity 3, 4.

Key Pitfalls to Avoid:

• Never assume hyperintensity on DWI represents hemorrhage - this represents ischemia with restricted diffusion, not blood 1, 5.
• Beware of contrast staining mimicking hemorrhage on post-procedure CT - MRI with susceptibility sequences can differentiate true blood from contrast 3.
• Always correlate DWI with gradient-echo sequences when using MRI for acute stroke, as this combination provides both ischemia detection and hemorrhage exclusion 1, 5, 3.

Time Evolution Considerations

The MRI appearance of blood products evolves over time based on hemoglobin breakdown 1:

• Acute phase (first week): Deoxyhemoglobin causes hypointensity on T2-weighted and gradient-echo sequences 1.
• Subacute phase (second week): Methemoglobin causes hyperintensity on T1 and T2-weighted images 1.
• This evolution does not significantly affect the DWI appearance in the acute phase, where susceptibility effects dominate 3, 4.