Purpose—To develop a new MRI method to detect and characterize brain abscesses using the CEST contrast inherently carried by bacterial cells, namely bacCEST.
Methods—Bacteria S. aureus (ATCC #49775) and F98 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=4) were acquired using two B1 values (i.e., 1 and 3 μT) and compared. The bacCEST signal of the brain abscesses in the rats (n=3) receiving ampicillin (i.p. 40 mg/kg twice daily) was acquired before, 4 and 10 days after the treatment.
Results—The bacCEST signal of S. aureus was characterized in vitro as a strong and broad signal in the range of 1 to 4 ppm, with the maximum contrast occurring at 2.6 ppm. The CEST signal in S. aureus-induced brain abscesses was significantly higher than that of contralateral parenchyma (P=0.003). Moreover, thanks to their different B1-independece, brain abscesses and tumors could be effectively differentiated (P=0.005) using ΔCEST(2.6ppm,3μT-1μT), defined by the difference between the CEST signal (offset=2.6ppm) acquired using B1 = 3 μT and that of 1 μT. In treated rats, bacCEST MRI could detect the response of bacteria as early as 4 days after the antibiotic treatment (P=0.035).
Conclusion—BacCEST 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.