Patients within this arm (only arm of the study) undergo burr hole sonography

All ultrasound examinations were conducted alongside the routine postoperative CT scan between 3-7 days postoperatively. To minimize potential influences of the patient's head position on the distribution and thickness of the subdural fluid, we first carried out the CT scan with the patient and their head in a supine position. Immediately after the CT scan, the patient remained on the same examination bed, and burr hole ultrasound was performed in the identical supine position. The burr hole was centered in the image, with the outer table surrounding the burr hole displayed as a horizontal line. The content of the burr hole could be differentiated by its higher echogenicity from the subdural residual hematoma or residual fluid. The brain's surface and overlying leptomeninges, however, displayed higher echogenicity compared to the subdural fluid. The subdural hematoma's thickness was measured at its maximum extent.

maximum extent of the hematoma (chronic subdural hematoma, CSDH) thickness under the burr hole in axial and coronal plane