Can a computed tomography (CT) scan detect all types of brain lesions?

CT Limitations in Detecting Brain Lesions

No, CT will not show all brain lesions—MRI is significantly more sensitive and detects substantially more lesions, particularly small ischemic injuries, microbleeds, subtle contusions, and lesions near the skull base or calvarium. 1

Key Sensitivity Differences

MRI detects approximately 83% more brain lesions than CT in direct comparison studies. In patients with infective endocarditis, MRI identified 137 lesions that CT missed entirely, while CT only detected 4 lesions not seen on MRI (which were false positives). Even among the 25 lesions detected by both modalities, there was only 64% concordance on lesion type. 2

Specific Lesion Types CT Commonly Misses

• Small ischemic lesions: MRI is superior for detecting punctate and small infarcts, particularly in posterior circulation 2
• Microbleeds and microhemorrhages: CT has poor sensitivity for these findings compared to MRI with susceptibility-weighted imaging (SWI) or T2* sequences 1, 2
• Diffuse axonal injury: MRI with specialized sequences detects these injuries that are typically invisible on CT 3, 4
• Microabscesses: Contrast-enhanced MRI identifies these small infectious lesions that CT cannot resolve 2
• Subtle findings adjacent to calvarium or skull base: CT has inherent limitations from bone artifact in these regions 1

Clinical Context Where CT Remains Appropriate

When CT is First-Line

• Acute trauma requiring rapid triage: CT remains the gold standard for emergency assessment of hemorrhage, skull fractures, and mass effect requiring immediate neurosurgical intervention 3, 1, 5
• Suspected acute hemorrhage: CT rapidly excludes large hemorrhages in emergency settings 2
• Calcification detection: CT is superior for identifying calcifications in conditions like tuberous sclerosis and oligodendrogliomas 3

CT Performance in Specific Scenarios

For brain metastases in lung cancer: CT is acceptable for initial staging, though MRI detects more and smaller lesions. The false-negative rate of CT is approximately 3%, meaning patients can return with brain metastases within 12 months of a negative CT scan. 3 However, MRI is the preferred modality and recommended for stage II-IV non-small cell lung cancer even without neurologic symptoms. 3

Critical Clinical Algorithm

When to Proceed Directly to MRI

1. Subacute or chronic presentations with unexplained neurological findings 1
2. Suspected inflammatory conditions (multiple sclerosis, encephalitis) 1
3. Staging advanced malignancies where brain metastases would alter management 3
4. Persistent symptoms after negative CT in traumatic brain injury 3

When CT After Negative MRI Adds No Value

The American College of Radiology explicitly does not support CT after negative MRI in the absence of new or changing symptoms, as it provides no additional diagnostic value while exposing patients to unnecessary radiation. 1

Common Pitfalls to Avoid

• Assuming normal CT excludes significant pathology: Up to 27% of patients with normal CT have abnormalities on subsequent MRI 1
• Missing small metastases: In asymptomatic patients with tumors >3 cm, 22% had brain metastases detected on MRI 3
• Overlooking posterior fossa lesions: CT has particularly poor sensitivity in this region due to beam-hardening artifact 1
• False positives on CT: Brain abscesses, gliomas, and other lesions can mimic metastases in up to 11% of cases, requiring biopsy for definitive diagnosis 3

When Contrast Enhancement Matters

Contrast-enhanced MRI identified 68 enhancing lesions (mainly abscesses and microabscesses) and improved characterization in 52% of lesions compared to only 14% improvement with contrast-enhanced CT. 2 Contrast is essential when evaluating for infection, tumor, inflammatory lesions, or vascular pathologies. 1