Stroke, Hemorrhage and Targeted Skull-Base CT

Acute Ischemic Stroke: Early Non-Contrast CT Signs & ASPECTS

Cellular Pathophysiology of Early Ischemic Changes

When an intracranial artery (most commonly the middle cerebral artery, MCA) becomes occluded by an embolic thrombus, local cerebral perfusion pressure plummets. Deprived of oxygen and glucose, neuronal ATP production halts within seconds. The energy-dependent $Na^+/K^+$ ATPase pumps in cell membranes fail, allowing sodium and water to rush into the intracellular compartment. This pathological process—cytotoxic edema—causes cellular swelling without an immediate change in total tissue blood volume. Because water has a CT number of $0\text{ HU}$ while normal brain parenchymal tissue attenuates at $+30\text{ to }+40\text{ HU}$, increased tissue water can reduce attenuation, with visibility depending on timing, noise and reconstruction.

Early Signs of Ischemia on NCHCT (0 to 6 Hours Post-Onset)

In the hyperacute window, NCHCT may appear entirely normal. However, meticulous inspection on narrow stroke windows frequently uncovers subtle, early ischemic markers:

1. The Hyperdense MCA Sign: Represents the direct visualization of the intraluminal thromboembolus within the horizontal (M1) or insular (M2) segment of the middle cerebral artery. Because acute blood clots consist of tightly packed red blood cells, cellular fibrin, and concentrated globin protein with reduced serum water, the thrombus exhibits high physical attenuation ($+70\text{ to }+90\text{ HU}$), appearing strikingly bright relative to surrounding CSF and unenhanced brain parenchyma.
2. Loss of Gray-White Matter Differentiation: Under normal conditions, cerebral cortex (gray matter, $+35\text{ to }+40\text{ HU}$) appears slightly brighter than subcortical white matter ($+25\text{ to }+30\text{ HU}$) due to higher neuronal cell density and capillary blood volume. As cytotoxic edema accumulates in the cortex, gray matter density decreases by $2\text{ to }5\text{ HU}$, blurring the sharp gray-white junction.
3. The Insular Ribbon Sign: The insular cortex is supplied by distal terminal branches of the MCA with minimal collateral vascular anastomoses, making it exceptionally vulnerable to acute ischemia. Hypoattenuation and swelling of the insular gray matter ribbon produce complete loss of the normal sharp demarcation between the insular cortex and the extreme capsule.
4. Obscuration of the Lentiform Nucleus: The putamen and globus pallidus (lentiform nucleus) receive end-arterial perfusion from the medial and lateral lenticulostriate arteries arising directly from the M1 segment. M1 occlusion triggers rapid cytotoxic edema in the putamen, blurring its normally crisp lateral margin against the external capsule.
5. Sulcal Effacement: Swelling of ischemic cerebral gyri displaces extracellular fluid, compressing and obliterating the adjacent CSF-filled cortical sulci over the hemispheric convexity.

ASPECTS describes the extent of early ischemic change

The Alberta Stroke Program Early CT Score (ASPECTS) starts at 10 and subtracts one for ischemic change in each of ten MCA-territory regions: caudate, lentiform nucleus, internal capsule, insula and cortical regions M1–M6. A lower score represents more extensive visible change. A normal score does not exclude very early ischemia, and the score does not quantify every vascular territory or replace the full interpretation.

Do not use an old ASPECTS cutoff as an automatic rule to deny reperfusion treatment. The 2026 AHA/ASA stroke guideline includes treatment pathways for selected patients with large ischemic cores and lower scores. Time, symptoms, vascular occlusion, imaging and patient factors inform the stroke team's decision. The technologist should deliver the ordered imaging promptly and communicate technical limitations rather than independently determine eligibility.

CT perfusion: measured curves and estimated tissue states

CT perfusion repeatedly samples enhancement after an IV bolus over the prescribed region. Cerebral blood flow (CBF) measures flow per tissue mass and time; cerebral blood volume (CBV) measures vascular blood volume per tissue mass. Mean transit time (MTT) relates to CBV/CBF. If CBV is 4 mL/100 g and CBF is 50 mL/100 g/min, MTT is 0.08 minute, or 4.8 seconds. Carry the time conversion rather than treating 0.08 as seconds.

Tmax describes delay in a deconvolved tissue response. Common automated stroke methods estimate core using relative CBF below 30% of a reference region and hypoperfused tissue using Tmax greater than 6 seconds. The 30% criterion is relative CBF, not relative CBV. These are algorithmic estimates, not universal proof of irreversible death or guaranteed viability. Motion, incomplete bolus sampling, poor arterial input, delayed circulation and the software method can alter the maps.

A mismatch between estimated hypoperfused volume and core volume can help a qualified stroke team select patients in the appropriate treatment pathway. Do not combine different trials' time windows and volume criteria into one universal rule. Check acquisition duration, coverage and motion, and identify perfusion maps and source data clearly for interpretation. Repeated perfusion imaging adds dose and should follow an explicit clinical decision.

Reference: 2026 AHA/ASA acute ischemic stroke guideline.

Intracranial Hemorrhage (ICH): Attenuation Evolution & Classifications

Blood attenuation changes with composition and time

Acute clotted blood commonly appears hyperattenuating relative to brain. Hematocrit, clotting, anemia, dilution and timing influence its appearance. A mixture of lower- and higher-attenuation material within a hematoma can raise concern for evolving bleeding but is not the same finding as contrast extravasation on CTA. The noncontrast swirl sign and the enhanced CTA spot sign must be distinguished.

Over time, blood often becomes less attenuating and can become difficult to distinguish from brain or CSF. Do not date a hemorrhage from a universal decline of 1.5 HU per day. Evaluate morphology, location, mass effect, prior images and the clinical history. Wider subdural windows can improve visualization of extra-axial blood near bone without changing the underlying measurements.

Anatomical Classifications of Intracranial Hemorrhage

Intracranial hemorrhages are categorized into four distinct anatomical compartments, each governed by unique anatomical boundaries and physical constraints:

1. Epidural Hematoma (EDH)

• Vascular origin: EDH lies between the skull and dura. A middle meningeal arterial injury is a classic cause; venous sources also occur. The appearance must not be used to infer one certain injured vessel.

• Characteristic Morphological Shape: Forms a biconvex (lenticular / lens-shaped) collection bulging into the brain parenchyma.

• Anatomical Boundary Constraints:

  • Usually limited by cranial sutures: The outer periosteal dura is tenaciously fused to the inner skull table at the cranial sutures (coronal, sagittal, lambdoid). Dural attachment generally limits spread, forcing the blood to expand inward into a focal lenticular mass.
  • CAN cross dural reflections: EDH can cross the anatomical midline attachments of the falx cerebri or the tentorium cerebelli, extending from one hemisphere to the other along the calvarial vault.

• Clinical Presentation: Classically associated with a temporary "lucid interval"—an initial concussive loss of consciousness followed by transient clearing of mental status, followed by rapid, precipitous deterioration, ipsilateral pupillary dilation (blown pupil from uncal herniation), contralateral hemiparesis, and death if not emergently evacuated via surgical craniotomy.

2. Subdural Hematoma (SDH)

• Vascular Origin: Accumulates within the potential space between the inner meningeal dura mater and the arachnoid mater. Arises from the mechanical shearing and tearing of fragile cortical bridging veins as they traverse the subdural space to drain into the superior sagittal sinus or other dural venous sinuses. Frequently provoked by acceleration-deceleration forces in motor vehicle collisions, falls in elderly individuals with cerebral atrophy, or non-accidental trauma in infants (shaken baby syndrome).
• Characteristic Morphological Shape: Forms a crescentic (concave along brain surface, convex along calvarium) collection that blankets the cerebral hemispheric convexity.
• Anatomical Boundary Constraints:
  • CROSSES cranial suture lines: Because the subdural space is continuous beneath the calvarium and has no attachment to bony suture margins, subdural blood spreads freely across coronal, sagittal, and lambdoid suture planes from the frontal to the occipital pole.
  • DOES NOT cross dural reflections: Subdural blood cannot cross the midline attachment of the falx cerebri, nor can it cross the tentorium cerebelli without entering the interhemispheric or infratentorial spaces independently.
• Subtypes: Acute SDH ($+60\text{ to }+80\text{ HU}$, hyperdense), Subacute SDH ($+30\text{ to }+40\text{ HU}$, isodense), Chronic SDH ($0\text{ to }+20\text{ HU}$, hypodense). In elderly patients experiencing recurrent minor falls, an acute-on-chronic SDH presents with a high-density dependent layer of clotted blood settling beneath a low-density chronic fluid layer—creating a distinct fluid-fluid hematocrit level.

3. Subarachnoid Hemorrhage (SAH)

• Etiology and Distribution: Blood enters the cerebrospinal fluid-filled subarachnoid space between the arachnoid and pia mater. The most common cause overall is closed head trauma (cortical surface contusions). The most common non-traumatic (spontaneous) cause is the rupture of an intracranial saccular (berry) aneurysm (frequency varies by population), most frequently located at branching points of the Circle of Willis (anterior communicating artery, posterior communicating artery, or MCA bifurcation).
• Imaging Characteristics: Hyperdense blood fills the basal cisterns (suprasellar, prepontine, ambient, and quadrigeminal cisterns), the sylvian fissures, and tracks along the cerebral sulci over the hemispheric convexities, casting a high-density "spider" or branching outline against CSF.
• Clinical Presentation & Complications: Sudden, excruciating "thunderclap headache" described by patients as "the worst headache of my life," accompanied by nuchal rigidity and photophobia. Early high-quality CT can be highly sensitive for SAH in the appropriate clinical population, but a negative study does not universally exclude it; timing, anemia, image quality and the clinical pathway matter. Major delayed complications include hydrocephalus, from impaired CSF absorption and/or obstruction and delayed cerebral vasospasm occurring 4 to 14 days post-bleed, leading to secondary ischemic infarction.

4. Intracerebral Hemorrhage (ICH / Intraparenchymal Hematoma)

• Etiology: Rupture of arterioles directly within the brain parenchyma. The leading cause is chronic systemic hypertension, resulting in lipohyalinosis and rupture of microscopic Charcot-Bouchard aneurysms within deep perforating branches (lenticulostriate and thalamoperforating vessels). Typical hypertensive locations include the putamen / basal ganglia (frequency varies by population), thalamus (frequency varies by population), pons (frequency varies by population), and deep cerebellar hemispheres (frequency varies by population). Non-hypertensive causes include cerebral amyloid angiopathy (CAA) (characteristically producing peripheral, lobar hemorrhages in elderly, non-hypertensive patients), arteriovenous malformations (AVMs), hemorrhagic neoplasms (glioblastoma, choriocarcinoma, melanoma, renal cell metastases), and systemic coagulopathy.
• Imaging Characteristics: A well-circumscribed, focal hyperdense intraparenchymal mass surrounded by a rim of hypoattenuating vasogenic edema. Hematomas frequently produce mass effect with compression of adjacent ventricles, midline shift, and progressive risk of life-threatening subfalcine, uncal, or tonsillar herniation.

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Dedicated pituitary and internal auditory canal questions

A routine brain CT does not provide the same targeted detail as a dedicated skull-base examination. The pituitary lies in the sella turcica beneath the optic chiasm. A targeted CT, when ordered, uses appropriately thin data, a focused reconstruction field and multiplanar views to relate a sellar lesion to the sphenoid sinus, cavernous sinuses and bony sella. Contrast and phase selection follow the clinical question. MRI generally provides superior pituitary soft-tissue characterization, so CT should not be presented as the automatic first examination for every pituitary disorder.

The internal auditory canals (IACs) carry the facial and vestibulocochlear nerves between the posterior fossa and inner ear. Thin temporal-bone CT depicts canal bone, the labyrinth and adjacent petrous structures. MRI is more suitable for many nonosseous retrocochlear lesions. Match the ordered technique to an osseous canal question versus a nerve or soft-tissue question; a routine thick brain reconstruction can obscure a small bony abnormality even when the acquisition contains adequate thin data.