The Relationship Between Arachnoid Cysts and the Subarachnoid Space by Hounsfield Unit Value in Computed Tomography Scans: Identification of Isolated and Communicating Arachnoid Cysts in a Cohort Study

Background: Arachnoid cysts are formations containing cerebrospinal fluid (CSF). They may be associated with the subarachnoid space. This feature is a factor that can affect the change in the size of arachnoid cysts over time. Hounsfield Unit (HU) value measurements provide rational numeric representation of imaging of tissues in computed tomography (CT) examinations. In this study, we aimed to define isolated or communicated arachnoid cysts, and whether arachnoid cysts communicate with the subarachnoid distance, with the HU values obtained in CT examinations. Methods: Patients with radiological diagnosis of arachnoid cyst were identified retrospectively by examining the CT database. HU value measurements were recorded over the Picture Archiving and Communication System (PACS) in the area where the arachnoid cyst was detected. HU value measurements are a method that rationally shows the amount of radiograph absorption of tissues in CT examinations. HU value measurements were performed in cerebrospinal fluid (CSF) and arachnoid cysts, and whether arachnoid cysts had equal HU values with CSF. They were defined as homogeneous or heterogeneous by measuring HU values in different regions of arachnoid cysts. Results: Data on 478 cases were obtained. An average of 2.82 CT scans were detected per case. A change in arachnoid cyst size was detected in 47 (9.8%) cases. In cases of arachnoid cysts of varying size, the mean HU value of arachnoid cysts was 9.32±1.93 (6.43±1.67 in the lateral ventricle and 7.04±1.71 in the fourth ventricle). HU value of arachnoid cysts with a change in size significantly differed from CSF. Conclusion: Arachnoid cysts with HU values equal to CSF are not real arachnoid cysts, and significant volume changes are not expected. Arachnoid cysts, which have a different HU value than CSF, may secrete a different fluid from CSF. This type of arachnoid cyst is a real cyst that can be detected in various sizes on CT examinations at different times since they are not in communication with the subarachnoid space.