Neuroimaging and Diagnostic Evaluation
Core concepts
Know the modality-selection logic: non-contrast CT for acute hemorrhage and trauma, CT angiography for vascular pathology, MRI with diffusion-weighted imaging for acute ischemia, and contrast-enhanced MRI for tumors and infection. Understand the physics that drive safety decisions: MRI contraindications (pacemakers, ferromagnetic foreign bodies), gadolinium and nephrogenic systemic fibrosis, iodinated contrast and renal function, and radiation dose from CT. Learn the classic imaging patterns: ring-enhancing lesions (abscess vs. glioblastoma vs. metastasis), midline shift measurement, herniation signs, and the appearance of hydrocephalus and its causes.
Traps
- A normal non-contrast CT does not exclude early ischemic stroke; DWI is the sensitive test.
- A ring-enhancing lesion is not automatically an abscess; restricted diffusion centrally favors pus, but tumor necrosis can mimic it.
- CT angiography detects large-vessel occlusion but not small-vessel disease.
- Assuming a shunt series or plain film answers a question about ventricular size; it does not.
Worked example / rule of thumb
When asked which study is "best" for a clinical scenario, ask two questions: what is the suspected pathology, and what is the patient's stability? Unstable trauma or suspected acute hemorrhage: non-contrast CT first. Suspected acute stroke within the window: non-contrast CT to exclude hemorrhage, then CTA. Suspected tumor, demyelination, or infection: contrast MRI. A useful heuristic: CT answers "blood or bone," MRI answers "tissue."