Can a bacterial brain abscess caused by KPC‑producing Klebsiella pneumoniae display an elevated choline peak and reduced N‑acetyl‑aspartate peak on proton magnetic‑resonance spectroscopy, thereby mimicking high‑grade glioma or metastasis?

Can Brain Abscesses Mimic Malignant Lesions on MR Spectroscopy?

Yes, brain abscesses can display elevated choline peaks and reduced N-acetyl-aspartate (NAA) on proton MR spectroscopy, thereby mimicking high-grade gliomas or metastases, making spectroscopy alone insufficient for definitive differentiation. 1

Overlapping Spectroscopic Features Between Abscesses and Malignancies

Shared Metabolic Patterns

• Both brain abscesses and malignant tumors demonstrate decreased NAA levels, reflecting neuronal damage or absence of neuronal tissue in the lesion 1
• Both entities can show elevated choline (Cho) peaks, which in tumors indicates membrane proliferation but in abscesses reflects inflammatory cell membrane turnover 1
• Creatine (Cr) levels are typically decreased in both abscesses and neoplastic lesions, making this marker non-discriminatory 1

Critical Distinguishing Features on MR Spectroscopy

• Brain abscesses characteristically display amino acid peaks (0.9 ppm), acetate (1.9 ppm), succinate (2.4 ppm), and lactate (1.3 ppm) that are typically absent in malignant tumors 2, 3
• Anaerobic or mixed anaerobic-aerobic bacterial abscesses specifically show acetate and succinate resonances, which arise from bacterial metabolism and are pathognomonic for infection 3
• Aerobic bacterial abscesses may lack acetate and succinate, showing only lactate and amino acids, making them more difficult to distinguish from tumors 3
• High-grade gliomas demonstrate significantly elevated Cho with lipid formation, while metastases show elevated Cho with even higher lipid levels than gliomas 1

The Diagnostic Challenge with KPC-Producing Klebsiella pneumoniae

• Klebsiella pneumoniae is a facultative anaerobe, meaning the abscess may produce a spectroscopic pattern lacking the pathognomonic acetate and succinate peaks seen with obligate anaerobes 3
• Facultative anaerobic abscesses show lactate and amino acid peaks but may lack acetate/succinate, creating a pattern that overlaps significantly with malignant lesions 3
• The lactate/amino acid ratio differs significantly between anaerobic and aerobic abscesses (p=0.008), but this distinction may not reliably separate aerobic abscesses from tumors 3

Algorithmic Approach to Differentiation

Step 1: Evaluate DWI/ADC Sequences (Most Critical)

• Brain abscesses show marked central hyperintensity on DWI with corresponding low ADC values (restricted diffusion), achieving 92% sensitivity and 91% specificity 4, 5
• Malignant tumors typically show restricted diffusion only in hypercellular walls, not centrally 4
• This is the single most reliable imaging feature to distinguish abscess from tumor 4

Step 2: Analyze MR Spectroscopy Pattern

• Look specifically for acetate (1.9 ppm) and succinate (2.4 ppm) peaks, which are diagnostic of anaerobic bacterial infection 2, 3
• Presence of amino acid peaks (0.9 ppm) suggests abscess but is not specific, as some tumors may show similar patterns 3
• If only elevated Cho and decreased NAA are present without acetate/succinate, spectroscopy cannot reliably distinguish aerobic abscess from tumor 1

Step 3: Assess Enhancement Pattern and Clinical Context

• Abscesses typically show smooth, thin rim enhancement with uniformly spherical T1 hypointense center, while necrotic tumors have irregular enhancing walls 4
• Clinical presentation matters: fever, elevated inflammatory markers, and immunocompromised state favor abscess 4
• Presence of known extracranial malignancy increases probability of metastasis 4

Critical Pitfalls to Avoid

• Do not rely on MR spectroscopy alone for diagnosis, as the overlap between aerobic abscesses and malignant lesions is substantial 1
• Always combine spectroscopy with DWI/ADC sequences, which provide superior discriminatory power 4
• Prolonged antibiotic therapy (several weeks) reduces MRI sensitivity for abscess detection, potentially normalizing spectroscopic findings 4, 5
• Post-neurosurgical abscesses are particularly difficult to identify on MRI, requiring high clinical suspicion 4, 5

Definitive Diagnosis

• Neurosurgical aspiration or excision remains the gold standard for definitive diagnosis, providing material for both culture and histopathology 4, 5
• Antimicrobials should ideally be withheld until aspiration/excision in stable patients to maximize microbiological yield, preferably within 24 hours of radiological diagnosis 4
• Blood cultures are positive in 28% of brain abscess cases and should be obtained in all patients 5

Bottom Line for Clinical Practice

When encountering a ring-enhancing lesion with elevated choline and reduced NAA on MR spectroscopy, immediately evaluate DWI/ADC sequences—central restricted diffusion strongly favors abscess over malignancy. 4 If DWI shows restricted diffusion but spectroscopy lacks pathognomonic acetate/succinate peaks (as may occur with facultative anaerobes like Klebsiella pneumoniae), proceed directly to neurosurgical sampling rather than attempting further imaging differentiation. 4, 5