This clinical trial aims to compare two methods of administering polymyxin B for treating severe brain infections caused by carbapenem-resistant Gram-negative bacteria (CRGNB). The two methods are: 1) delivering polymyxin B directly into the brain (intracranial administration) and 2) combining this with an intravenous (IV) infusion. The goal is to determine which approach is more effective in clearing the infection, improving patient outcomes, and influencing the concentration of polymyxin B in the cerebrospinal fluid (CSF). Participants in this study will be monitored for both effectiveness and safety, with a focus on CSF polymyxin B levels, to find the best treatment strategy for these challenging infections.

Inclusion Criteria:

1. Age ≥ 18 years old, regardless of gender.
2. Patients with cerebrospinal fluid (CSF) cultures showing carbapenem-resistant Gram-negative bacteria (such as Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa) and polymyxin MIC ≤ 0.5 µg/ml.
3. Diagnosed with intracranial infection and have been receiving polymyxin B treatment for at least 5 days, with an external ventricular drain (EVD) in place for continuous drainage.

Exclusion Criteria:

1. Patients with known allergy to polymyxin B or other severe allergic histories that may affect the study.
2. Pregnant women or other conditions that the investigator deems unsuitable for participation in the study.

Withdrawal Criteria:

1. Patient requests to withdraw from the study.
2. Patient's cerebrospinal fluid drug concentration results do not pass quality control center approval.