From the Guidelines
Cerebral infarction appears as a hypointense (dark) area on T1-weighted pulse sequence MRI in the acute to subacute phase, typically 16-24 hours after the stroke occurs, but diffusion-weighted imaging (DWI) is much more sensitive for detecting acute infarcts within minutes to hours of onset. This imaging finding represents tissue that has undergone ischemic necrosis due to inadequate blood supply. The hypointensity on T1 occurs because of increased water content (edema) in the infarcted tissue, which prolongs T1 relaxation time 1. As the infarction evolves over days to weeks, the signal characteristics change, with the lesion potentially becoming isointense or even hyperintense on T1 in the late subacute to chronic phases due to the presence of methemoglobin or protein-rich fluid.
Key Points to Consider
- T1-weighted imaging alone is not optimal for early stroke detection
- DWI has a high sensitivity (88% to 100%) and specificity (95% to 100%) for detecting infarcted regions, even at very early time points, within minutes of symptom onset 1
- For comprehensive stroke assessment, multiple MRI sequences including T2, FLAIR, DWI, and ADC mapping should be used together to determine the age, extent, and characteristics of the infarction, which helps guide appropriate treatment decisions 1
- The American Heart Association recommends using either NECT or MRI before intravenous tPA administration to exclude ICH and to determine whether CT hypodensity or MRI hyperintensity of ischemia is present 1
Recommendations for Imaging
- For a patient within a 3-hour time period from onset of symptoms, either NECT or MRI is recommended before intravenous tPA administration to exclude ICH and to determine whether CT hypodensity or MRI hyperintensity of ischemia is present 1
- For patients beyond 3 hours from onset of symptoms, either MR-DWI or CTA-SI should be performed along with vascular imaging and perfusion studies, particularly if mechanical thrombectomy or intra-arterial thrombolytic therapy is contemplated 1
From the Research
Cerebral Infraction in T1-Weighted Pulse Sequence MRI
- Cerebral infarction can be visualized on T1-weighted MRI, with large infarcted lesions of the cortex-subcortex often being detected earlier than small lesions of the basal ganglia and brainstem 2.
- T1-weighted images can be used to detect infarcted lesions, with some studies showing that lesions can be detected on T1-weighted image earlier than on T2-weighted image 2.
- The signal intensity of T1-weighted imaging can be decreased in areas with contrast enhancement, which can be useful in differentiating hemorrhagic infarct from non-hemorrhagic "incomplete infarct" 3.
- T1-weighted MRI can also be used to evaluate carotid plaque signal hyperintensity, which is associated with ipsilateral ischemic events 4.
- The degree of acute perfusion or diffusion abnormalities measured on MRI prior to treatment onset can help predict the evolution of brain infarction 5.
Detection of Cerebral Infarction
- MRI is useful for early diagnosis of ischemic cerebral infarction, and may elucidate some aspects of the pathophysiology of ischemic stroke 2.
- T1-weighted MRI can detect infarcted lesions, but the difference between infarction and perifocal edema may not be clear even on MRI 2.
- Susceptibility weighted imaging (SWI) can be used to differentiate hemorrhagic infarct from non-hemorrhagic "incomplete infarct" based on T1 hyperintensities 3.