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CT principles: HU values, window, pitch, noise and artefacts
In CT every voxel receives a value in Hounsfield units (water 0, air -1000), the window decides what is seen, pitch decides speed and dose, noise falls with mAs and thicker slices — and artefacts come from beam hardening, metal, motion or a faulty detector.
Why it matters in the exam
CT questions in the exam are short and technical. HU values: water 0, air -1000, fat about -100 to -50, soft tissue 20–60, fresh blood about 50–80, and bone above 400. Window width sets the HU range displayed in grey levels: a narrow window increases contrast (brain, liver), a wide window lowers contrast and shows a large range (lung, bone). The window level should be near the HU of the tissue examined. Pitch is table travel per rotation divided by total beam width; a pitch above 1 shortens the scan and lowers dose, at the cost of resolution along the axis.
Quantum noise depends on the number of photons reaching the detector: it falls when mAs or kV is raised, with thicker slices and larger voxels, and with a soft reconstruction kernel; it rises in a large patient. Fine detail is seen with thin slices, a sharp (bone) kernel, a small FOV and a small focal spot. Iterative reconstruction reduces noise compared with filtered back projection, and so allows a lower dose. Dual-energy CT separates materials: iodine maps, "virtual non-contrast" images, distinguishing uric acid from calcium stones, virtual monoenergetic images and reduction of metal artefact.
Artefacts: beam hardening — dark streaks between two dense structures (for example between the petrous bones at the skull base); metal artefact — bright and dark streaks around implants; motion — doubling and blurring; partial volume — an averaged value between two tissues in a thick slice; ring artefact — a miscalibrated or faulty detector. The "bow-tie" filter lowers dose at the edges of the body and balances the signal to the detectors. The anode–cathode axis lies along the Z axis so the heel effect does not alter the beam across the patient. Dose metrics: CTDIvol in mGy and DLP in mGy·cm. Injection phases: arterial about 25–35 seconds, portal venous about 60–70 seconds, delayed 3–5 minutes.
Key points
- Water 0, air -1000, fat negative, bone above 400.
- Narrow window — high contrast.
- High pitch — fast and low dose.
- Noise falls with mAs, thick slices and iterative reconstruction.
- Ring — detector; dark streaks between bones — beam hardening.
Summary table
| Topic | What it determines | Change that improves it | Cost |
|---|---|---|---|
| Window width | Displayed contrast | Narrow window → high contrast | A small HU range is seen |
| Pitch | Speed and dose | High pitch → fast, low dose | Resolution along the axis falls |
| Noise | Number of photons | High mAs, thick slice, iterative reconstruction | Dose or sharpness |
| Spatial resolution | Fine detail | Thin slice, sharp kernel, small focal spot | More noise |
| Artefacts | Image quality | Recognise: hardening, metal, motion, ring | Ring = faulty detector |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
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Sources
- Bushberg et al., The Essential Physics of Medical Imaging, 4th edition (2020) — Computed tomography
- AAPM Report 204: Size-Specific Dose Estimates in Pediatric and Adult Body CT Examinations, 2011 — CTDIvol and patient size
- ACR–SPR Practice Parameter for the Performance of Computed Tomography, 2022 — Technical factors and dose optimisation
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.