CT Head — Unenhanced and Post-Contrast
Foramen magnum to vertex for both series.
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
- Suspected intracranial infection (abscess, empyema, meningeal disease) where MRI is not available or not tolerated.
- Suspected intracranial metastases or a mass lesion in a patient who cannot undergo MRI.
- Assessment of a known lesion for interval change when prior imaging was also contrast-enhanced CT.
Technique
- A post-contrast delay of roughly 3-5 minutes is conventional for parenchymal and meningeal enhancement; some centres extend this for suspected metastatic disease.
- This protocol is a fallback. Contrast-enhanced MRI remains substantially more sensitive for small enhancing lesions and for meningeal disease.
Where it goes wrong
- Scanning too early after injection produces a vascular study rather than an enhancement study and under-calls small lesions.
- Omitting the unenhanced series makes calcification, blood products and enhancement indistinguishable.
- A negative contrast-enhanced head CT should not be reported as excluding metastases when MRI is achievable.
Clinical questions that reach this study
Contrast
Typically 60-100 mL of non-ionic iodinated contrast at a modest rate; the acquisition is an equilibrium one, so injection rate matters far less than for angiography.
- The unenhanced series is retained: without it, enhancement cannot be distinguished from intrinsic density.
Acquisition
- Reconstruction
- Matched slice thickness and position for the two series so they can be compared directly.
- 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.
- Delayed / equilibrium (washout) phaseQuestion-dependent: ~3–5 min for hepatic equilibrium/washout, ~15 min for adrenal absolute-washout calculations. Confirm locally.
Intravascular and interstitial compartments have equilibrated and contrast is being cleared, so most normal tissue is falling in attenuation. The transferable principle is that the diagnostic information is now in the RATE OF CHANGE rather than in the absolute density: tissues with rapid capillary exchange and a small interstitium wash out quickly, whereas fibrous, myxoid or otherwise expanded interstitial spaces retain contrast and become relatively dense. That single mechanism underlies delayed enhancement of scar and fibrosis, retained enhancement in cholangiocarcinoma and haemangioma fill-in, and the arithmetic of adrenal washout — all of which require a matched earlier acquisition to be interpretable at all.
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.
- Prior contrast reaction and elective premedication· nurse pre scan
- Intravenous access adequate for the planned injection· radiographer at scan
- Metformin and iodinated contrast· radiographer at scan
- Child-sized technique and contrast dose· radiographer at scan
- Kidney function and intravenous iodinated contrast· radiographer at scan