Cone Beam CT
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Direct answer
A cone-beam CT scanner acquires a volumetric dataset in one short arc: a cone-shaped X-ray beam from a small focal spot passes through the patient onto a large flat-panel detector while the source-detector pair rotates 180-360 degrees in roughly 5-40 seconds, and a Feldkamp-type reconstruction converts the projection series into isotropic voxels as fine as 0.08-0.4 mm. Because the entire volume is captured in a single rotation rather than slice by slice, the geometry suits small and medium body regions — dentoalveolar imaging, maxillofacial surgery, temporal bone, and kilovoltage image-guided radiotherapy on the linac. Effective doses for dental applications run from tens to a few hundred microsieverts depending on field of view, and the installation requires AERB registration like any medical X-ray unit.
What you must remember
- Acquisition geometry: single 180-360 degree rotation, continuous or pulsed exposure, flat-panel detector (typically 15 x 15 cm to 30 x 40 cm), one rotation = one volume; voxel size 0.08-0.4 mm gives true isotropic resolution superior to fan-beam CT for small bony detail.
- Field-of-view doctrine: small (under 5 cm, single jaw quadrant), medium (5-10 cm, both jaws) and large (over 10 cm, skull base to hyoid) — FOV must match the question, since dose scales with imaged volume.
- Reconstruction: Feldkamp (FDK) cone-beam back-projection; circular trajectories leave artefacts at the scan extremes, one reason long Z-axis coverage degrades accuracy.
- Hounsfield reliability: CBCT numbers are approximate, not water-calibrated — never report nodule density or enhancement from CBCT.
- Artefact profile: metal streak from amalgam implants, ring artefacts from faulty detector rows, beam-hardening "cupping" in dense bone, scatter-induced streaks (no anti-scatter grid on most small-FOV units), and motion blur from a 20-40 second exposure.
- IGRT role: kilovoltage CBCT on the linac verifies patient position against the planning CT immediately before treatment.
- Indian regulation: dental and medical CBCT units fall under AERB radiation safety requirements for X-ray equipment — registration through the AERB e-LORA system, quality assurance at acceptance and periodic intervals, per current AERB directives.
Choosing CBCT over MDCT or panoramic radiography
A 26-year-old needs implant planning in the mandibular molar region; the panoramic image cannot display buccolingual width or the canal's 3D course, so a 5 x 5 cm small-FOV CBCT is chosen. Set the smallest FOV that covers the implant site plus a margin, select the finest voxel the unit allows (0.08-0.125 mm for dentoalveolar work), orient the arc to keep the region of interest at the rotation centre where resolution is best, and instruct the patient to bite gently and stay motionless for the 15-20 second exposure. The reconstructed volume yields cross-sectional, panoramic and 3D reformats; the inferior alveolar canal is traced and its buccolingual relation to the planned implant measured directly.
Contrast that with the linac bunker at the same hospital. Before each fraction the kilovoltage CBCT rotates about the patient in the treatment position, registers to the planning scan, and the couch shift appears on the console. Here CBCT is deliberately low-dose and soft-tissue-poor — a positioning tool, with the radiographer judging setup error against organ motion and prosthesis artefact.
How the exam frames it
Examiners love the "why is CBCT cheap on dose but poor on Hounsfield numbers" question. The answer: a single rotation with a cone beam delivers far lower mAs than a stacked fan-beam helical acquisition, but the wide beam suffers heavy scatter that flat panels correct only approximately, and without a water-calibrated reference the CT numbers drift. The second trap is calling CBCT "3D radiography with no dose concern": a large-FOV scan's effective dose approaches that of a low-dose MDCT, so justification still applies. A viva favourite asks which artefact a faulty detector row produces — the ring, because the same pixel orbits the reconstruction centre; saying "streak" suggests metal and loses the mark.
Frequently asked questions
How does cone-beam CT differ from multidetector CT in acquisition?
CBCT captures the whole volume in one cone-beam rotation on a flat panel, while MDCT irradiates thin fan-beam slices as the table feeds — CBCT wins on resolution and dose for small fields, MDCT on soft-tissue contrast and Hounsfield accuracy.
Why are Hounsfield units from CBCT unreliable?
Wide cone beams generate heavy scatter without a grid and lack water-based calibration, so CBCT numbers are approximate and should not be used for density quantification.
Which artefact indicates a defective detector row in CBCT?
A ring artefact centred on the rotation axis, since the faulty pixel traces a circle through every projection; streaks instead point to metal or photon starvation.
What field of view should be selected for single-implant planning?
A small FOV of about 5 cm or less confined to the region of interest, because scan dose scales with the irradiated volume and the smallest FOV that answers the clinical question is the correct one.
What is the role of CBCT in image-guided radiotherapy?
Kilovoltage CBCT on the linac reproduces a low-dose 3D image of the patient in the treatment position, which is matched to the planning CT to compute and apply daily couch corrections.