Magnetic susceptibility imaging is a magnetic resonance imaging (MRI) technique that uses the magnetic properties of tissues and the BOLD (blood oxygen level-dependent) effect. It allows a better visualization of venous structures and hemorrhagic lesions. These sequences are now used in clinical routine.

The extreme sensitivity of these sequences to the oxy/deoxyhemoglobin ratio makes it possible to describe a new MRI semiology, particularly in the context of cerebral ischemia. The interest of the analysis of the venous network signal, which can reflect cerebral perfusion, has been reported.

However, the influence of the hematocrit level on the signal of the venous network in magnetic susceptibility imaging has not been evaluated at present. It seems important to better define the influence of hematocrit level on the signal of the veins with this sequence to avoid potential diagnostic errors.

Inclusion Criteria:

• Patient whose age ≥ 18 years
• Patient referred to GHPSJ for suspected stroke
• Patient who had a brain MRI including the magnetic susceptibility sequence
• Patient for whom a blood count with hematocrit was performed at the same time (within 7 days between the MRI and the blood test)
• French-speaking patient

Exclusion Criteria:

• Patient under guardianship or curatorship
• Patient deprived of liberty
• Patient under court protection
• Elimination of potential biases: recent injection (48 h) of gadolinium, pathology likely to modify the venous signal in magnetic susceptibility imaging (notably arterial occlusion or stenosis), high flow oxygenation, sickle cell disease, thalassemia.
• Patient objecting to the use of his/her data for this research

These patients were referred for suspected stroke, between January 1, 2016, and May 1, 2021. who had a brain MRI including the magnetic susceptibility sequence Patients for whom a blood count with hematocrit was performed at the same time (within a 7-day interval between the MRI and the blood test).