Stroke is a public health issue and a priority for our institution. MRI plays an essential role in the management of stroke, its contribution being diagnostic, etiological and prognostic.

Among the MRI sequences used in stroke emergencies, the diffusion sequence plays a key role in highlighting ischemic lesions as early as the hyperacute phase, even though the other sequences in the protocol do not reveal any anomaly. This sequence alone conditions the management of patients, particularly in the context of "thrombolysis emergencies".

It has been shown that the sensitivity of the diffusion sequence for the detection of ischemic lesions can directly depend on acquisition parameters such as b value, slice thickness or spatial resolution.

Recent advances in MRI now allow us to perform diffusion sequences with higher spatial resolution. The matrix is an important acquisition parameter of MRI sequences defining the ability of the sequence to distinguish 2 pixels in the acquisition plane. The higher the matrix, the higher the spatial resolution of the sequence in the acquisition plane.

At the Saint-Joseph Hospital, we have a new 3T MRI since September 2020 allowing the acquisition in clinical routine of a more resolved diffusion sequence: 160x200 matrix ("high resolution" diffusion, HR), against 128x140 ("standard" diffusion usually). These two sequences are acquired in particularly short acquisition times (1 minute 37 seconds). This HR diffusion sequence is performed as part of routine care since September 2020 for specific situations: absence of lesion highlighted on the standard diffusion sequence while the patient has a suggestive symptomatology (especially for lesions of the brainstem), search for lesion in other vascular territories (thus in favor of an embolic origin) in a patient who presents an isolated ischemic lesion or ischemic lesions in a single territory.

It has been reported in the literature that increasing the spatial resolution can reveal small lesions that were not visible on more conventional sequences. There is a clear rationale for seeking to improve the detection of small lesions (<5 mm) because their detection may have important therapeutic implications for many patients (particularly in the context of thrombolysis emergencies, transient ischemic attacks, or amnesic strokes).

Inclusion Criteria:

• Patients with age ≥ 18 years
• Patients who underwent a 3T brain MRI for suspected stroke/TIA (MRI performed within 7 days of clinical symptomatology) and for whom the "HR" diffusion sequence was considered useful for diagnosis
• French-speaking patient

Exclusion Criteria:

• Patient under guardianship or curatorship
• Patient deprived of liberty
• Patient under court protection
• Patient objecting to the use of his data for this research

These patients were referred for suspected stroke between September 1, 2020, and July 31, 2021, to the Paris Saint-Joseph Hospital.