MRI detection of bacterial brain abscesses and monitoring of antibiotic treatment using bacCEST

PurposeTo develop a new MRI method to detect and characterize brain abscesses using the CEST contrast inherently carried by bacterial cells, namely bacCEST. MethodsBacteria S. aureus (ATCC #49775) and F98 and 9L glioma cells were injected stereotactically in the brains of F344 rats to form abscesses and tumors. The CEST signals of brain abscesses (n=4) and tumors (n=7) were acquired using 2 B-1 values (i.e., 1 and 3 mu T) and compared. The bacCEST signal of the brain abscesses in the rats (n=3) receiving ampicillin (intraperitoneal injection 40mg/kg twice daily) was acquired before, 4 and 10 days after the treatment. ResultsThe bacCEST signal of S. aureus was characterized in vitro as a strong and broad signal in the range of 1 to 4ppm, with the maximum contrast occurring at 2.6ppm. The CEST signal in S. aureus-induced brain abscesses was significantly higher than that of contralateral parenchyma (p=.003). Moreover, thanks to their different B-1 independence, brain abscesses and tumors could be effectively differentiated (p=.005) using CEST(2.6ppm, 3 mu T-1 mu T), defined by the difference between the CEST signal (offset=2.6ppm) acquired using B-1=3 mu T and that of 1 mu T. In treated rats, bacCEST MRI could detect the response of bacteria as early as 4 days after the antibiotic treatment (p=.035). ConclusionBacCEST MRI provides a new imaging method to detect, discriminate, and monitor bacterial infection in deep-seated organs. Because no contrast agent is needed, such an approach has a great translational potential for detecting and monitoring bacterial infection in deep-seated organs.