INTRODUCTION

The usefulness of diffusion-weighted imaging (DWI) for the diagnosis of acute ischemic stroke is widely known, and early diagnosis and early initiation of therapy lead to improved prognosis. It is important to diagnose correctly by DWI because that is the image firstly taken in the stroke MRI. However, both infarction and hemorrhage show hyperintensity on DWI, and their image findings are similar to those of brain abscess. Therefore, it is necessary to check other image sequence like T2*WI and MR angiography. However, because the low-intensity findings on DWI differ from each other in these diseases, attention to these findings may lead to a more accurate diagnosis using DWI alone. At this time, we tried to prove the usefulness of the low signal intensity on DWI for differentiating these diseases in this image interpretation study.

METHOD

This retrospective study included 15 consecutive cases of untreated brain abscess diagnosed pathologically or clinically between 2012 and 2019. One patient was excluded because of a history of head surgery near the abscess. 20 consecutive cases of acute ischemic stroke and 26 consecutive cases of acute hemorrhagic stroke were collected. One representative DWI was selected from each case and 60 DW images were randomly classified into 2 groups. Six radiologists (7 to 20 years of experience) reviewed one group without any guidance and examined the accuracy of classification of brain abscess, cerebral infarction, and hemorrhage. The interpretation of another group was carried out after the following explanation; In brain abscesses, a thin, smooth, low-signal intensity rim surrounds the strong high-signal intensity. Cerebral hemorrhage is an irregular, heterogeneous mixture of high and low intensity on DWI, and the rim is nonuniform in thickness. The strong high signal can be also often recognized around the low intensity rim. In cerebral infarction, low signal rim is not observed, but in the basal ganglia, iron deposition may be seen as rim. The diagnostic accuracy before and after explanation was statistically analyzed and p-value less than 0.05 was considered significant.

RESULT

Each diagnostic radiologist spent 14.8 minuites for the first reading (average, range: 10.5 -19.5 min.). The accuracy was 70.3% (average, range: 50 -80%). The reading time after the explanation about the low intensity rim sign was 7.3 min. (5.0 -8.5 min). The accuracy was increased to 85.8 % (70 -96.7 %, p = 0.025, paired t-test). The accuracy was correlated with years of experience of each radiologist (Pearson's correlation coefficient of 0.841, p = 0.036).

DISCUSSION

Brain abscess, cerebral infarction, and cerebral hemorrhage show focal hyperintense lesion on DWI and their imaging findings are similar, but more accurate diagnosis was possible by focusing on low intensity rim surrounding the lesion. The mechanism of formation of low signal rim in brain abscess is considered to be the T2 shortening effect of free radical secreted from aggregated macrophages. On the other hand, the low signal intensity in the cerebral hemorrhage is mainly caused by the T2 shortening effect and the heterogeneity of the local magnetic field from the extracellular hemoglobin and deoxyhemoglobin. Because the low intensity is thought to originate from the displaced erythrocyte, the low signal rim at the periphery may also become serpentine and nonuniform in thickness, reflecting the heterogeneous formation of the hematoma. Although the high-intensity of DWI has attracted attention, the low-intensity is also considered to be useful for diagnosis.

CONCLUSION

In the diagnosis of stroke MRI, not only the high signal intensity but also the low signal area on DWI can be used for more accurate diagnosis.

Acknowledgements

No acknowledgement found.