10.1 Brain & Cranium CT — Routine and Perfusion Imaging

Why This Topic Matters on the ARRT CT Exam

Head is one of three subcategories inside the Procedures domain's "Head, Spine, and Musculoskeletal" group, which alone carries 25 of the exam's 71 Procedures questions — roughly 15% of every scored item on the exam. Within Head, ARRT names eight leaf topics: brain/cranium, brain perfusion, temporal bones/internal auditory canal (IAC), orbits, sinuses, maxillofacial bones, dedicated mandible, and temporomandibular joints (TMJs). This section covers the first two — brain/cranium and brain perfusion — because routine head CT is the single most frequently performed CT study in any hospital and the acute stroke workflow is the most heavily tested clinical scenario in the entire Procedures domain. Every case vignette that opens with "sudden-onset weakness" or "head trauma" draws on the content below, and ARRT's cross-cutting "Focus of Questions" themes (anatomy/pathology recognition, protocol factors, contrast media, and additional procedures like trauma) are all exercised through brain CT scenarios.

Core Terms and Technique

A routine (noncontrast) head CT is acquired with the patient supine, gantry angled roughly parallel to the orbitomeatal line (OML) (or a reduced-dose canthomeatal angulation that spares the lens of the eye), using either sequential (step-and-shoot) axial sections — typically thinner through the posterior fossa (~2.5–3 mm) to limit beam-hardening artifact from dense skull base bone, and thicker (~5 mm) through the supratentorial brain — or a helical volume acquisition reconstructed at variable thickness. Every routine head CT is reviewed on two windows: a narrow brain window (window level [W/L] ≈ 35–45 HU, window width [W/W] ≈ 80–100 HU) to separate gray from white matter, and a wide bone window (W/L ≈ 500–700 HU, W/W ≈ 2,000–4,000 HU) to inspect the calvarium and skull base for fracture.

Typical CT numbers (Hounsfield units) on brain windows:

Traumatic intracranial hemorrhage is one of the highest-yield pattern-recognition topics on the exam:

Contrast-enhanced brain CT is reserved for suspected tumor, abscess, or infection, where breakdown of the blood-brain barrier causes abnormal enhancement; CT venography (CTV) is a dedicated contrast-timed study used to evaluate dural venous sinus thrombosis when a venous, rather than arterial, process is suspected.

Brain perfusion CT is a dynamic, time-resolved acquisition: the scanner repeatedly images a fixed brain volume at low dose while a rapid intravenous contrast bolus passes through the brain, then software deconvolves the time-density curves into four parametric color maps:

• Cerebral Blood Flow (CBF) — mL/100 g/min
• Cerebral Blood Volume (CBV) — mL/100 g
• Mean Transit Time (MTT) — seconds
• Time to Maximum (Tmax) — seconds

The clinical logic: the infarct core (already-dead tissue) shows reduced CBF and reduced CBV together. The ischemic penumbra ("tissue at risk," potentially salvageable) shows abnormal CBF/MTT and a prolonged Tmax but preserved CBV. This core-versus-penumbra mismatch is exactly what determines whether a stroke patient presenting outside the standard treatment window is still a candidate for mechanical thrombectomy.

The Alberta Stroke Program Early CT Score (ASPECTS) is a 10-point score applied to the initial noncontrast head CT in anterior-circulation (MCA-territory) stroke. Scoring starts at 10, and 1 point is subtracted for each of 10 defined regions (caudate, lentiform nucleus, internal capsule, insular ribbon, and cortical regions M1–M6) that shows early ischemic change — loss of gray-white differentiation or subtle hypoattenuation. An ASPECTS score of 7 or lower predicts a worse functional outcome and a higher risk of symptomatic hemorrhage with thrombolytic therapy.

Exam Scenario Walkthrough

A 70-year-old presents to the emergency department with sudden right-sided weakness and aphasia; last known well was 100 minutes ago. Step 1: a STAT noncontrast head CT is performed. Its purpose is not to find the infarct — an early ischemic stroke may show no visible abnormality within the first few hours — but to exclude hemorrhage, because intracranial bleeding is an absolute contraindication to thrombolytic (tPA) therapy. The technologist (or radiologist) may score ASPECTS on this same image set. Step 2: if no hemorrhage is present, CT angiography of the head and neck follows to look for a large-vessel occlusion. Step 3: if the patient is outside the standard tPA/thrombectomy window, CT perfusion defines the core-penumbra mismatch to determine candidacy for late-window mechanical thrombectomy.

A separate common vignette: an unhelmeted motorcyclist with a temporal bone fracture and a brief lucid interval before deteriorating. Noncontrast head CT shows a biconvex, lens-shaped hyperdensity — an epidural hematoma from a torn middle meningeal artery — a true neurosurgical emergency requiring immediate physician notification.

Takeaways

• Noncontrast head CT is read on both a narrow brain window (~35–45 HU level) and a wide bone window; acute blood measures roughly 50–90 HU, well above normal brain tissue.
• Epidural hematomas are biconvex and never cross suture lines but can cross midline; subdural hematomas are crescentic, can cross sutures, and never cross midline.
• The initial noncontrast head CT in acute stroke exists to rule out hemorrhage before tPA, not to diagnose the infarct — the infarct itself may be occult in the first hours.
• CT perfusion distinguishes dead core tissue (reduced CBF and CBV) from salvageable penumbra (prolonged Tmax, preserved CBV); this mismatch drives late-window thrombectomy decisions.
• ASPECTS is a 10-point score applied to the baseline noncontrast head CT, where a score ≤7 signals worse prognosis and greater thrombolysis risk.