Use of diffusion-weighted magnetic resonance imaging in empyema after cranioplasty.

External decompression can be an effective treatment for acute intracranial hypertension, but the cranial defect must be repaired. The most serious complication of cranioplasty is late infection. Confusing an empyema that occurs after cranioplasty with a fluid collection (haematoma or liquor) can have catastrophic consequences, such as the development of cerebritis. The goal of this study was to assess the ability of diffusion-weighted (DW) magnetic resonance imaging (MRI) to diagnose empyema after cranioplasty. DW MRI and apparent diffusion coefficient (ADC) maps were studied in six patients with surgically verified empyema after cranioplasty. The findings were compared with those in five patients who had surgically verified haematoma or liquorrhoea. In the patients with empyema, the lesion was hyperintense, whereas the fluid collections (haematoma and liquorrhoea) were visualized as hypointense lesions. The ADC maps showed that empyema had a significantly lower intensity than the fluid collections (haematoma or liquorrhoea). DW MRI can be used to identify empyema after cranioplasty and can help to differentiate it from other fluid collections. Hence, this is a useful additional imaging modality for the diagnosis of empyema after cranioplasty.