Detection of hemorrhagic transformation of cerebral infarcts with Ultra-low field MRI

Purpose: Hemorrhagic transformation (HT) within ischemic infarcts is a critical post-stroke complication that significantly influences secondary prevention strategies, particularly the timing and safety of anticoagulation. Ultra-low field (ULF) MRI (0.064 T, Swoop, Hyperfine) offers bedside neuroimaging, but its ability to detect hemorrhagic transformation compared to high field MRI (1.5 T or 3T) and CT remain uncertain. This study aimed to evaluate the diagnostic accuracy of ULF-pMRI for detecting hemorrhagic transformation in ischemic stroke patients. Methods: We retrospectively analyzed 106 patients with ischemic infarcts who underwent both ULF-pMRI and standard neuroimaging (high-field MRI or CT). The presence of hemorrhagic transformation was assessed by two independent readers (trained neuroradiologists) and by consensus reading in univariant analysis. Diagnostic accuracy metrics sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated with the standard method (high-field MRI or CT within two days of ULF-pMRI) as the reference. Agreement was quantified using Cohen’s Kappa, and ROC analysis was performed to assess discriminatory performance. Results: Of 97 stroke patients, n = 20 (20.61 %) showed hemorrhagic transformation. Mostly scattered small petechia and subarachnoid hemorrhage were found as hemorrhagic transformation in standard imaging. Reader 1 detected hemorrhagic transformation in ULF-pMRI with a sensitivity of 0.17, specificity 1, PPV 1.00, and NPV 0.80 and Reader 2 detected hemorrhagic transformation in ULF-pMRI with a sensitivity of 0.11, specificity 1.00, PPV 1.00, and NPV 0.00, while consensus reading showed sensitivity of 0.10, specificity 1.00, PPV 1.00, NPV 0.81. The McNemar test demonstrated a significant difference between ULF-pMRI and the standard method (p < 0.001), reflecting systematic underdetection of hemorrhagic changes by ULF-pMRI. Conclusion: Ultra-low field MRI showed very low sensitivity for detecting mainly petechial hemorrhagic transformation within infarct areas in this study. That restricts its reliability for therapy-relevant decision-making such as early anticoagulation after acute ischemic stroke.