Read the head CT in a fixed order — extra-axial blood, brain, ventricles and cisterns, bone — and measure the three numbers the neurosurgeon acts on: thickness, midline shift and cistern status.
Orient first
- Acute blood is hyperdense (roughly 50–70 HU) only for the first days; an anaemic patient, a coagulopathic patient or a hyperacute bleed can be isodense. Swirling mixed density inside a haematoma means active bleeding into it.
- Extradural blood is biconvex and stops at sutures (dura is bound there) but can cross the midline and the tentorium; subdural blood is crescentic, crosses sutures, and stops at the falx and tentorium.
- Mass effect kills before the blood does. The basal cisterns (ambient, quadrigeminal, suprasellar) are the pressure gauge: open, compressed or effaced.
Acquire the study
- Non-contrast CT head: thin axial reconstructions with SAGITTAL and CORONAL reformats — a thin subdural along the tentorium or the vertex is seen on coronals and missed on axials.
- Three windows every time: brain (~W80/L40), a narrow SUBDURAL window (~W150–200/L50–80), and BONE on the thin-slice kernel.
- Include the craniocervical junction; add CT cervical spine per the trauma criteria in use, and CTA when the skull-base fracture pattern or mechanism meets BCVI screening criteria.
- MRI is not the acute study; it is for unexplained deficit with a near-normal CT (diffuse axonal injury: SWI and DWI), or for the brainstem.
The manoeuvre
- Extra-axial first, on the SUBDURAL window: convexities, along the falx and tentorium, the middle cranial fossa floor and the posterior fossa. Measure the maximum THICKNESS of any collection perpendicular to the inner table, in mm.
- Measure MIDLINE SHIFT in mm at the level of the septum pellucidum (foramen of Monro), from the ideal midline drawn between the anterior and posterior falx attachments.
- Grade the BASAL CISTERNS: open, compressed, or absent — and check the fourth ventricle and the ambient cisterns for posterior fossa mass effect.
- Parenchyma: contusions at the classic coup and contrecoup sites (inferior frontal lobes, anterior temporal poles), and small haemorrhages at the grey–white junction, corpus callosum and dorsolateral brainstem (diffuse axonal injury).
- Subarachnoid blood in sulci and the sylvian fissures, and intraventricular blood in the occipital horns (dependent layering).
- Herniation: subfalcine (cingulate under the falx), uncal (temporal horn dilatation on the opposite side, effaced suprasellar cistern), tonsillar, and upward.
- Bone window: the fracture line, its depression in mm relative to the inner table, whether it crosses a VENOUS SINUS or the middle meningeal groove, and pneumocephalus.
What confirms it
- Extradural haematoma: biconvex, bounded by sutures, usually under a fracture across the middle meningeal groove or a venous sinus.
- Subdural haematoma: crescentic, crosses sutures, limited by the dural reflections; acute is hyperdense, subacute can be isodense and is found by the displaced grey–white junction and effaced sulci.
- Traumatic SAH is convexity-predominant beside contusions; blood filling the basal cisterns with little else should raise an aneurysmal bleed that caused the fall.
What licenses you to exclude it
- A normal CT does not exclude diffuse axonal injury — if the deficit is out of proportion to the CT, say so and recommend MRI with SWI and DWI.
- A normal early CT does not exclude a delayed bleed or blossoming contusion in an anticoagulated patient; state the time from injury and the anticoagulation status if known.
The classic misread
- Reading only the brain window: a thin acute subdural against the inner table is invisible without the subdural window.
- Calling a hyperdense tentorium or falx normal on an axial slice without checking the coronal reformat.
- Missing an isodense subacute subdural — look for sulci that do not reach the inner table and a buckled grey–white junction.
- Measuring midline shift at the pineal or the third ventricle and quoting it as the septum pellucidum shift.