Specific Imaging Findings

Postoperative changes in the brain vary depending upon the amount of time elapsed between surgery and imaging. In all cases, calvarial changes consistent with a craniotomy or craniectomy indicate that the underlying parenchymal abnormalities are at least in part postoperative. In the acute postoperative setting, unenhanced CT demonstrates variable amounts of extra-axial fluid, blood and gas within the resection cavity. Edema and hemorrhage in the soft tissues overlying the craniotomy are also usually present. On MRI, gas appears as foci (frequently round) of low T1 and T2 signal and hemorrhage has a variable appearance depending on the age of blood products (usually iso- to hyperintense on T1WI). Rim of reduced diffusion along the periphery of the resection site may also represent blood products, in which case there is corresponding signal loss on T2* imaging; without T2* findings reduced diffusion is consistent with postoperative infarct. Intracranial air should resolve by 3 weeks, extra-axial fluid may be evident for up to a month. In high-grade gliomas biodegradable chemotherapeutic wafers may be left within the resection cavity. These wafers appear linear on imaging and are usually radiopaque on CT and hypointense on all MR sequences. With time, progressive local encephalomalacia and gliosis develop at the margins of a resection cavity, which may become filled with CSF.

Parenchymal contrast enhancement along the resection margins may begin to appear on MR by 24 h and is present in virtually all cases by day 7. Over subsequent days to weeks, this enhancement may become increasingly thick and nodular, but it usually resolves by 2–3 months. In rare instances, enhancement may persist for up to 8 months. Dural thickening and enhancement can persist for years and may be diffuse, not limited to the surgical bed.