How Does a CT Scanner Create Images?

In computed tomography (CT), an X-ray tube and detector array rotate around the patient inside a doughnut-shaped gantry. As the tube emits a narrow X-ray fan beam, different tissues absorb different amounts of radiation — bone absorbs more than soft tissue, which absorbs more than air. The detectors record the transmitted X-ray intensity at thousands of angles. A computer then uses mathematical reconstruction algorithms (originally filtered back-projection, now often iterative methods) to generate a cross-sectional image (slice). Modern multi-detector CT scanners have up to 320 detector rows and can image the entire heart in a single rotation lasting about 0.3 seconds, producing sub-millimetre resolution.