Is MRI (Magnetic Resonance Imaging) DWI (Diffusion-Weighted Imaging) the most sensitive imaging modality to determine if early brain ischemic lesions are present in patients with suspected Transient Ischemic Attack (TIA)?

From the Guidelines

MRI with diffusion-weighted imaging (DWI) is the most sensitive imaging modality for detecting early brain ischemic lesions in patients with suspected transient ischemic attack (TIA). This is because DWI can identify acute ischemic changes within minutes of onset, even when conventional CT scans appear normal, with a high sensitivity (88% to 100%) and specificity (95% to 100%) for detecting infarcted regions 1. The technique works by measuring the random Brownian motion of water molecules, which becomes restricted in areas of cytotoxic edema that develop rapidly after ischemia.

Key Benefits of MRI DWI

• Can detect relatively small cortical lesions and small deep or subcortical lesions, including those in the brain stem or cerebellum, areas often poorly or not visualized with standard MRI sequences and NECT scan techniques 1
• Allows identification of the lesion size, site, and age
• Can identify subclinical satellite ischemic lesions that provide information on stroke mechanism
• Has been shown to be associated with a higher risk of recurrent ischemic events in TIA patients 1

Importance of Early Detection

For optimal results, MRI-DWI should be performed as soon as possible after symptom onset, ideally within 24-48 hours, as the sensitivity decreases over time. Identifying these lesions is crucial for accurate diagnosis, appropriate risk stratification, and implementing proper secondary prevention strategies such as antiplatelet therapy, statins, and management of vascular risk factors. Studies have shown that approximately 30-50% of clinical TIA patients have positive DWI findings, indicating that many events classified clinically as TIAs actually represent small completed infarcts 1.

Comparison with Other Imaging Modalities

Compared with CT, advantages of MRI for parenchymal imaging include the ability to distinguish acute, small cortical, small deep, and posterior fossa infarcts; the ability to distinguish acute from chronic ischemia; identification of subclinical satellite ischemic lesions that provide information on stroke mechanism; the avoidance of exposure to ionizing radiation; and greater spatial resolution 1. However, limitations of MRI in the acute setting include cost, relatively limited availability of the test, relatively long duration of the test, increased vulnerability to motion artifact, and patient contraindications such as claustrophobia, cardiac pacemakers, patient confusion, or metal implants.

From the Research

Sensitivity of MRI DWI in Detecting Early Brain Ischemic Lesions

• MRI DWI is a sensitive imaging modality for detecting early brain ischemic lesions in patients with suspected TIA, with a detection rate of approximately 30-50% 2, 3, 4, 5.
• The sensitivity of MRI DWI in detecting ischemic lesions is higher when performed within 12 hours of symptom onset, and in patients with symptoms of speech impairment, and among individuals younger than 60 years 2.
• The combination of early diffusion-weighted MRI and perfusion-weighted MRI can document the presence of a cerebral ischemic lesion in approximately half of all patients who present with a suspected hemispheric transient ischemic attack (TIA) 2.

Comparison with Other Imaging Modalities

• MRI DWI has demonstrated clear superiority over other conventional imaging modalities in the detection of hyperacute cerebral ischemia 3.
• The presence of high-signal lesions on DWI decreases nonlinearly with time since symptom onset, and increases with National Institutes of Health Stroke Score and with age 4.
• DWI provides additional information in patients with subacute TIA or minor stroke, such as clarification of clinical diagnosis or vascular territory, which can influence management in approximately 14% of patients 4.

Clinical Correlations

• Patients with positive DWI scans are more likely to have had symptom duration >1 hour, motor deficits, and aphasia than patients with negative DWI scans 3.
• The combination of symptom duration >1 hour, motor deficits, and aphasia is 100% specific for an abnormality on DWI 3.
• DWI abnormalities are closely related to intracranial vascular occlusive lesions, and the combination of DWI and MRA is useful for detecting large-artery lesions in patients displaying a classical lacunar syndrome 5.

Serial Brain MRI in TIA Patients

• Up to 40% of patients with TIA demonstrate lesions on diffusion-weighted magnetic resonance imaging (DWI), but the time course of these ischemic lesions is not well known 6.
• Serial brain MRI in TIA patients shows that some lesions can vanish soon after symptom onset, while others can be visible only after a certain delay 6.
• Changes in DWI findings are observed in approximately 14% of patients, including presence of ischemic lesions despite normal initial DWI, increase in lesion size, or partial or complete lesion reversibility 6.