CT Head — Unenhanced
Foramen magnum to vertex, including the skull base and calvarium.
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
When to use it
- Acute head injury meeting a validated decision rule.
- Suspected acute stroke, as the first acquisition before any vascular or perfusion imaging.
- Sudden severe headache where subarachnoid haemorrhage is being considered.
- New focal neurological deficit, reduced consciousness or seizure of unclear cause.
- Suspected shunt malfunction or hydrocephalus, and post-operative or post-procedural checks.
- Any situation where MRI is the better test but the patient cannot tolerate it or it cannot be obtained in time.
Technique
- Helical or sequential axial acquisition angled to avoid the orbits where the technique permits.
- Two reconstruction kernels are the norm: a smooth kernel for grey-white differentiation and a sharp kernel for bone.
- Narrow "stroke" or "subdural" window settings are review settings, not extra acquisitions.
Where it goes wrong
- Adding intravenous contrast to a query-haemorrhage study defeats it: enhancing vessels and acute blood are both dense, and the unenhanced comparison no longer exists.
- Beam-hardening streak between the petrous bones mimics and conceals posterior fossa lesions; a repeat with different angulation or a thin-section review is more useful than a second full scan.
- Motion is the dominant cause of a non-diagnostic study in the agitated patient — physical settling before the scan beats repeating it.
- Sensitivity for subarachnoid haemorrhage falls with time from ictus; a negative scan late after onset does not have the same meaning as a negative scan in the first few hours.
Clinical questions that reach this study
Contrast
Acquisition
- Breathing
- Not applicable; the patient is asked to keep still and avoid swallowing.
- Reconstruction
- Soft-tissue brain kernel at around 5 mm axial with thin-section source data retained for reformats, plus a bone kernel. Thin axial reconstructions improve detection of small extra-axial collections adjacent to the inner table.
- Preparation
- No preparation for the unenhanced study. Remove hairclips, earrings and dentures where practical; metallic hair accessories cause streak artefact across the posterior fossa.
Phases
Each phase is authored once and shared across every protocol that uses it, so the physiology below is the same wherever you meet it.
- Non-contrast (unenhanced)No injection. Acquired before any contrast is given.
Shows intrinsic tissue attenuation, and nothing else. The transferable principle is that contrast is anti-signal for anything that is already dense: calcification, acute haemorrhage, urinary and biliary calculi, iodine-containing or haemorrhagic fluid, and intrinsic fat all lose conspicuity, or become uninterpretable, once surrounding tissue enhances. It is also the only baseline against which enhancement can be measured, so any protocol that quantifies enhancement or washout (a lesion "enhances by X HU", adrenal absolute washout, renal mass characterisation) is arithmetically impossible without it. Conversely, an unenhanced series adds dose and no information whenever the question is purely about vascularity or perfusion.
Safety checks this protocol carries
Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.
- Child-sized technique and contrast dose· radiographer at scan