Postoperative versus spontaneous intracranial abscess: Diagnostic value of the apparent diffusion coefficient for accurate assessment

Purpose: To assess the diagnostic value of apparent diffusion coefficients (ADCs) in spontaneous intracranial abscesses compared with those occurring after primary neurosurgery. Materials and Methods: This retrospective comparative study was approved by the local institutional review board. The requirement to obtain written informed consent was waived. A total of 73 consecutive patients who underwent preoperative diffusion-weighted magnetic resonance (MR) imaging from February 2005 to May 2015 were divided into two cohorts depending on whether the abscess was identified after a primary neurosurgical procedure (n = 43) or was a spontaneous abscess (n = 30). Abscesses were assessed for ADC by two readers independently. ADCs were described as medians with interquartile range. The Mann-Whitney and x² tests were used to compare ADCs between the two cohorts, and x² automatic interaction detection analysis was used to classify the ADCs into categories according to the cohorts. Results: The median ADC of postoperative abscesses was 1.34 3 1023 mm2/sec (interquartile range, 1.00-1.62 3 1023 mm2/sec), which was significantly higher than the median ADC of spontaneous abscesses, 0.68 3 1023 mm2/sec (interquartile range, 0.58-0.79 3 1023 mm2/sec; P, .001). Twenty four (83%) of the abscesses with ADCs less than 0.79 3 1023 mm2/sec were found to be spontaneous, while only six (27%) abscesses with ADCs between 0.79 and 1.33 3 1023 mm2/sec and no abscesses with ADCs greater than 1.33 3 1023 mm2/sec were spontaneous (P, .001). Conclusion: Significantly higher ADCs were found in postoperative abscesses compared with those in spontaneous abscesses. Diffusion-weighted imaging with ADC mapping is not sufficient for excluding postoperative intracranial abscess and should not be used as the key diagnostic modality in this situation.