Non-Contrast CT & CT Angiography

Non-Contrast CT & CT Angiography in Neurocritical Emergencies

Non-contrast computed tomography (NCCT) and CT angiography (CTA) represent the foundational diagnostic workhorses of neurocritical care. Due to near-universal emergency department availability, rapid scan acquisition ($<1\text{ minute}$), and absence of contraindications related to implanted ferromagnetic devices, NCCT remains the undisputed first-line imaging modality for acute coma, traumatic brain injury (TBI), suspected intracranial hemorrhage, and acute ischemic stroke.

Physical Principles & Hounsfield Unit Calibration

Computed tomography maps tissue x-ray attenuation coefficients relative to water and air, quantified via the Hounsfield Unit (HU) scale:

$$\text{HU} = 1000 \times \frac{\mu_{\text{tissue}} - \mu_{\text{water}}}{\mu_{\text{water}}}$$

On this linear scale, air is calibrated to $-1000\text{ HU}$, distilled water to $0\text{ HU}$, and dense cortical bone to $+1000\text{ HU}$.

Early Ischemic Changes on Non-Contrast CT

In hyperacute ischemic stroke, energy failure secondary to hypoperfusion induces $\text{Na}^+/\text{K}^+$-ATPase shutdown, driving intracellular sodium accumulation and cytotoxic edema. Each $1\%$ increase in tissue water decreases parenchymal CT attenuation by approximately $2.5\text{ HU}$.

Early ischemic changes become detectable within $1\text{--}3\text{ hours}$ of vessel occlusion:

• Loss of the Insular Ribbon: The insular cortex has vulnerable end-arteriolar supply from terminal M2/M3 branches with minimal collateral overlap. Edema creates early blurring and hypodensity of the insular gray-white interface.
• Obscuration of the Lentiform Nucleus: The putamen and globus pallidus lose demarcation against the surrounding internal and external capsules due to ischemia in deep lenticulostriate arterial distributions.
• Cortical Sulcal Effacement: Swelling of cortical gyri compresses subarachnoid spaces and effaces sulci over the hemispheric convexity.
• Hyperdense Vessel Signs: High attenuation within an intracranial arterial lumen denotes acute, intraluminal stagnant thromboembolism ($70\text{--}80\text{ HU}$) prior to parenchymal tissue changes. The classic hyperdense MCA sign appears along the horizontal M1 segment; the MCA dot sign represents thrombus within Sylvian fissure M2/M3 branches viewed in cross-section; and the hyperdense basilar sign indicates life-threatening vertebrobasilar occlusion.

The Alberta Stroke Program Early CT Score (ASPECTS)

The Alberta Stroke Program Early CT Score is an established, validated 10-point quantitative topological scoring system evaluating ischemic changes across the middle cerebral artery (MCA) territory.

Anatomic Topography & Scoring Rules

ASPECTS divides the MCA territory across two defined axial CT planes:

1. Ganglionic Cut (at the level of the basal ganglia and internal capsule):
   • C: Caudate head
   • L: Lentiform nucleus (putamen and globus pallidus)
   • IC: Internal capsule (posterior limb)
   • I: Insular ribbon
   • M1: Anterior MCA cortex
   • M2: Lateral temporal MCA cortex
   • M3: Posterior temporal MCA cortex
2. Supraganglionic Cut ($2\text{ cm}$ rostral to the basal ganglia, at the level of the corona radiata and bodies of lateral ventricles):
   • M4: Rostral anterior MCA cortex
   • M5: Rostral lateral MCA cortex
   • M6: Rostral posterior MCA cortex

Scoring starts at 10 points. For each defined region demonstrating distinct parenchymal hypodensity and loss of gray-white differentiation, exactly 1 point is subtracted. An ASPECTS of 10 indicates a completely normal MCA territory; an ASPECTS of 0 denotes complete, pan-territorial infarction.

• Large-core evidence: SELECT2, ANGEL-ASPECT, and other trials support expanded EVT eligibility in selected large cores. TESLA did not meet its primary superiority threshold. Do not claim that every trial proved independence or reduced mortality; assess the current guideline and the actual study population.

Traumatic Brain Injury & Intracranial Hemorrhage Patterns

Rapid characterization of extra-axial and intra-axial bleeding compartments guides emergency surgical decompression:

The Modified Fisher Scale stratifies aneurysmal SAH vasospasm risk based on cisternal blood thickness and intraventricular hemorrhage (IVH):

• Grade 1: Minimal / thin subarachnoid clot; no IVH.
• Grade 2: Minimal / thin subarachnoid clot; IVH present.
• Grade 3: Thick cisternal clot (filling cistern/fissure $\ge 1\text{ mm}$ in vertical dimension); no IVH.
• Grade 4: Thick cisternal clot; IVH present (conferring the highest risk of delayed cerebral ischemia).

CT Angiography (CTA) in Acute Neurocritical Care

CTA combines rapid helical volumetric scanning with a timed intravenous iodinated contrast bolus ($60\text{--}80\text{ mL}$ at $4\text{--}5\text{ mL/s}$) from the aortic arch through the circle of Willis to the vertex.

Large Vessel Occlusion & Collateral Assessment

• LVO Identification: CTA achieves $>95\%$ sensitivity and specificity for acute occlusions of the internal carotid artery (ICA) terminus ("T" or "L" occlusions), middle cerebral artery mainstem (M1) and primary divisions (M2), anterior cerebral artery (A1), and basilar artery.
• Collateral Assessment (Single-Phase vs. Multiphase CTA):
  • Single-phase CTA captures contrast at a single peak arterial timepoint. However, retrograde pial collateral filling is hemodynamically delayed. Single-phase acquisition often underestimates true collateral capacity in vessels where blood arrives seconds later.
  • Multiphase CTA records three sequential phases: peak arterial, peak venous, and late venous. By capturing delayed wash-in and wash-out across pial collaterals, multiphase CTA differentiates true absent collaterals from delayed but robust collateral filling, correlating tightly with penumbral preservation and clinical outcomes.

Vascular Pathologies: Dissection & Aneurysms

• Cervical Arterial Dissection: CTA demonstrates eccentric arterial wall thickening, an intimal flap, irregular luminal tapering ("string sign"), pseudoaneurysm formation, or an intramural hematoma visible as a hyperdense crescent on non-contrast imaging.
• Aneurysms and AVMs: Identifies aneurysm neck anatomy, incorporation of branch vessels, and draining vein patterns of arteriovenous malformations or dural arteriovenous fistulas (dAVF).

The CTA "Spot Sign" in Spontaneous Intracerebral Hemorrhage

In spontaneous intracerebral hemorrhage (ICH), hematoma expansion occurs in up to $30\%$ of patients within the initial hours, driving severe neurological worsening and death.

• Definition: The spot sign is defined as one or more discrete, punctate foci of contrast extravasation ($\ge 1\text{--}2\text{ mm}$) within the interior of an acute parenchymal hematoma on source CTA images, without connection to surrounding intact normal blood vessels, and exhibiting attenuation $>120\text{ HU}$.
• Clinical Significance: The spot sign represents active microvascular pseudoaneurysm rupture and ongoing arterial leakage. It is the most robust independent imaging predictor of hematoma expansion ($>33\%$ or $>6\text{ mL}$), early neurological deterioration, and in-hospital mortality. Its presence guides aggressive early systolic blood pressure lowering ($<140\text{ mmHg}$) and identifies candidates for ultra-early hemostatic intervention.