14.1 CT of Brain and Spine
Key Takeaways
- Noncontrast CT is the first neuro-ICU study for hemorrhage, hydrocephalus, herniation, and fracture because it is fast and shows acute clotted blood as hyperdensity.
- Acute hematoma typically measures about 50–90 Hounsfield units and fades toward brain density over 1–3 weeks, so a subacute subdural collection can be isodense on a narrow brain window.
- ASPECTS starts at 10 and subtracts one point for early ischemic hypoattenuation in each of 10 MCA-territory regions: caudate, lentiform, internal capsule, insula, and M1–M6.
- Noncontrast CT often misses hyperacute ischemia; diffusion-weighted MRI is the study that becomes positive within minutes.
- Iodinated contrast is not required to find blood; when eGFR is below about 30 mL/min/1.73 m², prefer noncontrast CT or noncontrast MRI/time-of-flight MRA over reflexive CTA.
CT of Brain and Spine
Quick Answer: Noncontrast CT (NCCT) is the first neuro-ICU study for hemorrhage, hydrocephalus, herniation, and fracture. Acute clotted blood is hyperdense (~50–90 HU). Density falls toward brain over 1–3 weeks, so a subacute subdural can vanish on a narrow window. ASPECTS is a 10-point MCA early-ischemia score, not a perfusion map. Hyperacute infarct is often invisible on NCCT; diffusion-weighted MRI (DWI) is the sequence that turns positive in minutes. CT angiography (CTA) and CT perfusion (CTP) are named here only as pointers to the next two sections.
On the ABIM Neurocritical Care examination, a stem that starts with an unstable, unequal-pupil, ventilated patient is almost always an NCCT problem first, not an MRI problem. Independent OpenExamPrep teaching in this section covers CT of brain and spine listed under Diagnostic studies and procedural skills in the ABPN Content Specifications (02.A.1). This guide is not an ABIM or ABPN product.
Why NCCT is the first study
NCCT is available in minutes, does not require a ferromagnetic-safety checklist, tolerates pumps and metal hardware, and shows acute blood as bright. That combination is why you send the crashing patient to CT, not to a 30-minute magnet. You are looking for four immediately actionable patterns:
- Hemorrhage — intraparenchymal, extra-axial, intraventricular, or subarachnoid
- Hydrocephalus — temporal horns, ballooned third or fourth ventricle, transependymal hypodensity
- Herniation — subfalcine shift, uncal encroachment, tonsillar descent, external herniation through a craniectomy
- Fracture — calvarium, skull base, and, when the neck is uncleared, cervical and often thoracolumbar spine CT
Contrast is not required for any of those four. Iodinated CTA and CTP answer vessel-occlusion and penumbra questions; those techniques are taught in the MRI/perfusion and angiography sections that follow. Do not delay a noncontrast scan to add a CTA protocol if the patient is herniating from a hematoma you have not even seen.
Hounsfield units and windows
Hounsfield units (HU) are the CT density scale, with water near 0 HU and air near −1000 HU.
| Material | Typical HU | How you use it |
|---|---|---|
| Air | ~−1000 | Pneumocephalus, sinus air, open skull |
| Fat | ~−50 to −100 | Orbits, lipoma |
| CSF | ~0 to 15 | Ventricles, cisterns, chronic hygroma |
| White matter | ~20 to 30 | Hypodense relative to cortex |
| Gray matter | ~30 to 45 | Loss of gray–white = early ischemia or edema |
| Acute clotted blood | ~50 to 90 | ICH, SDH, EDH, SAH, hyperdense vessel |
| Calcium | >100–150 | Choroid, pineal, old cavernoma |
| Iodinated contrast / bone | hundreds to 1000+ | Vessels on CTA, fractures on bone window |
Read every head CT on at least three windows: brain (about WL 40, WW 80), subdural/blood (a wider window, for example WW 150–200, so thin extra-axial blood along the inner table is not lost), and bone. A normal-looking brain window with a missed 4 mm convexity subdural hematoma is a classic trap.
Hyperdensity timeline of blood
Fresh unclotted blood can be only mildly dense. As clot retracts over minutes to hours, attenuation rises (the hematoma looks brightest in the first 1–3 days). Hemoglobin breakdown and serum resorption then lower HU toward cortex and finally toward CSF.
| Stage | Clock | Typical CT look |
|---|---|---|
| Hyperacute | Minutes to a few hours | 40–60 HU possible; mixed density if still bleeding (swirl sign = unclotted hypoattenuating blood inside a hyperdense clot) |
| Acute | Hours to ~3 days | Peak hyperdensity ~60–90 HU; sharp margins |
| Subacute | ~3–21 days | Progressive fade; isodense to cortex around 1–3 weeks, especially extra-axial |
| Chronic | Weeks to months | Hypodense, CSF-like; chronic SDH or encephalomalacia |
Worked example: a day-0 basal ganglia hematoma measures 72 HU. On day 12 a contralateral crescent along the inner table measures 38 HU—the same as adjacent cortex—and is easy to miss unless you open a subdural window or look for sulcal buckling. Anticoagulated or actively expanding hematomas stay heterogeneous because unclotted blood sits closer to circulating blood (~30–45 HU) than to retracted clot.
Hyperdense vessel sign (MCA or basilar) is thrombus in a large artery, not parenchyma. It is a clue to large-vessel occlusion, not an ASPECTS region. Polycythemia and unenhanced vessels in a dehydrated patient can mimic it; compare with the contralateral artery.
Subarachnoid blood is hyperdense in cisterns and sulci. Sensitivity of NCCT for SAH is highest in the first 24 hours (often taught near 90–95% or higher) and falls over subsequent days. A delayed thunderclap with a normal CT still needs CSF testing or MRI (FLAIR/SWI), not reassurance from a single late scan.
Hemorrhage geometry, hydrocephalus, and herniation
| Pattern | CT geometry | Immediate worry |
|---|---|---|
| Epidural | Lentiform; usually stops at sutures | Arterial (middle meningeal) expansion, lucid interval |
| Subdural | Crescent; crosses sutures, not the midline falx freely | Mass effect; isodense subacute miss |
| SAH | Cisterns, sylvian fissures, sulci | Aneurysm until the vascular workup says otherwise |
| ICH | Parenchymal hyperdensity | Volume, mixed-density expansion, IVH, herniation |
| IVH | Cast in ventricles | Obstructive hydrocephalus |
Hydrocephalus looks like temporal horns out of proportion, a ballooned third ventricle, and periventricular hypodensity (transependymal flow). A full fourth ventricle with small laterals is a posterior-fossa outlet problem until proven otherwise.
Herniation on CT: subfalcine (septum pellucidum shift, cingulate under falx, ACA crowding), uncal (ambient cistern effacement, temporal uncus over the tentorial notch, PCA crowding), tonsillar (foramen magnum packing), upward (cerebellum crowding the tentorial notch, often after a tight posterior fossa or over-drainage), and external (brain through a craniectomy defect). Millimeters of midline shift plus a newly dilated pupil is an operative conversation, not a repeat-MRI-in-the-morning conversation.
ASPECTS, conceptually
The Alberta Stroke Program Early CT Score (ASPECTS) estimates early ischemic change in the middle cerebral artery (MCA) territory on NCCT. Start at 10. Subtract 1 for hypoattenuation or gray–white loss in each involved region:
| Level | Regions (1 point each) |
|---|---|
| Basal ganglia / insula cut | Caudate (C), lentiform (L), internal capsule (IC), insular ribbon (I), M1, M2, M3 |
| Immediately above the ganglia | M4, M5, M6 (anterior, lateral, posterior MCA cortex) |
Worked example: loss of the right insular ribbon and lentiform nucleus only means two regions, so ASPECTS 8. Add M2 hypodensity and the score is ASPECTS 7. A hyperdense MCA does not cost an ASPECTS point by itself.
ASPECTS is a burden score, not a perfusion map and not a treatment algorithm. Lower scores mean more established MCA infarct on that scan. Posterior-circulation infarcts use a separate pc-ASPECTS construct; do not force a brainstem stroke onto the MCA 10-point grid. Decisions about thrombolysis or thrombectomy belong in the acute ischemic stroke chapter. Here you only need to read the scan and report the early-change burden honestly.
When NCCT misses early ischemia
Cytotoxic edema takes time to drop HU enough to see. In the first hours, NCCT sensitivity for acute infarct is modest; a normal scan does not exclude stroke. What NCCT is excellent at in that same window is excluding hemorrhage so reperfusion discussions can start.
Missed-infarct patterns:
- Hyperacute hemispheric ischemia (minutes to about 3–6 hours) with only subtle loss of the insular ribbon, or with nothing at all
- Brainstem and cerebellum, where beam-hardening from petrous bone hides hypodensity
- Small lacunar and cortical ribbon infarcts below CT resolution
- Hyperacute posterior-circulation syndromes with a normal-looking cut through the pons
The study that detects ischemia in minutes is DWI, taught in the next section. If the stem is hemiparesis at 90 minutes, NCCT without blood and without hypodensity, the imaging diagnosis of infarct is not wait for a 24-hour CT. It is DWI, or a perfusion protocol if MRI is unavailable and you are selecting for late-window therapy—again, later sections.
Spine CT for unstable fracture screening
An uncleared, obtunded, or high-risk trauma patient gets multidetector CT of the spine, not a single lateral radiograph. CT is the bone screen: you are hunting unstable fractures that will injure the cord during a turn, a family visit, or an MRI transfer.
Denis three-column teaching: anterior column (anterior two-thirds of the body and anterior longitudinal ligament), middle column (posterior third of the body and posterior longitudinal ligament), and posterior column (posterior elements and ligaments). Disruption of two columns is treated as unstable until a spine surgeon says otherwise.
| Pattern | Why it is unstable on a test |
|---|---|
| Atlanto-occipital dissociation | Craniocervical junction shear; immediately life-threatening |
| Jefferson (C1 burst) | Stability hinges on the transverse atlantal ligament |
| Type II odontoid | High nonunion; dens separated from the body |
| Hangman (C2 pars) with displacement | Traumatic spondylolisthesis; types II/III more unstable |
| Flexion teardrop | Posterior-ligament failure plus a retropulsed body |
| Bilateral facet dislocation | Locked facets, often cord injury |
| Chance (flexion–distraction) | Horizontal shear through bone or ligament |
| Burst with retropulsion | Middle-column failure and canal compromise |
CT misses isolated ligamentous disruption, cord contusion, traumatic disc, and spinal epidural hematoma. Those are MRI questions when there is a neurologic level, unexplained hypotension with bradycardia (neurogenic shock), or a CT-negative cord. Do not claim CT cleared the cord; it cleared bone.
Contrast, kidneys, and when MRI is the better next picture
Iodinated contrast is for CTA, CTP, and contrast-enhanced CT, not for finding acute blood. Contrast-associated acute kidney injury is a real but over-called problem. Contemporary radiology–nephrology consensus (ACR/NKF 2020) places the clinically important risk mainly at eGFR <30 mL/min/1.73 m², plus AKI already in progress. Isotonic volume expansion is the prophylaxis that still makes sense in high-risk patients who truly need iodine.
When GFR is low:
- Need to exclude ICH, SAH, hydrocephalus, or herniation → NCCT, no iodine
- Need early infarct confirmation and the patient is MRI-safe → noncontrast MRI with DWI (no gadolinium required for that question)
- Need vessels and iodine is unattractive → time-of-flight MRA (angiography section) rather than reflexive CTA
- Need gadolinium MRI in advanced CKD → prefer group II macrocyclic agents; nephrogenic systemic fibrosis risk is concentrated in older group I agents at eGFR <30
An unstable, ventilated patient with a blown pupil does not go to MRI to spare the kidneys. MRI contraindications and logistics are the next section. The point here is that NCCT has no contrast nephropathy and answers the hemorrhage question.
Exam-style traps
- Calling a day-12 isodense SDH no extra-axial collection because you never opened a wide window
- Treating a normal NCCT at 2 hours as proof there is no infarct
- Scoring a hyperdense MCA as an ASPECTS point
- Giving iodinated contrast to better see the hematoma
- Clearing an obtunded neck with a single radiograph
- Sending a herniating patient to MRI as the first test
Independent practice items at /practice/abim-neurocritical-care are a study bank for this reasoning, not the computer-based examination administered by ABPN.
A ventilated trauma patient has a newly dilated pupil and a systolic blood pressure of 88 mm Hg. Which imaging study should be obtained first to look for hemorrhage, hydrocephalus, herniation, or fracture?
A 68-year-old man had a left convexity subdural hematoma on day 0 that measured 70 Hounsfield units. A follow-up noncontrast CT on day 12, read only on a narrow brain window, is called normal. What is the most likely explanation?
Noncontrast CT 3 hours after left MCA symptoms shows hypoattenuation of the insular ribbon, lentiform nucleus, and M2 cortex, with a hyperdense left M1 segment. What is the ASPECTS value?
A 71-year-old woman has aphasia and right hemiparesis that began 90 minutes ago. Noncontrast head CT shows no hemorrhage and no hypodensity (ASPECTS 10). eGFR is 22 mL/min/1.73 m². Which statement is the most accurate about confirming early ischemia?