What Is Computed Tomography?

Computed tomography (CT) is a medical and industrial imaging technique that uses a rotating X-ray source and a ring of detectors to acquire multiple two-dimensional projection measurements of an object, then reconstructs those projections computationally into cross-sectional images or full three-dimensional volumes. The term tomography derives from the Greek word for slice, reflecting the technique's ability to resolve internal structures in discrete planar sections without physical cutting.

CT was developed in the early 1970s by Godfrey Hounsfield and Allan Cormack, who shared the 1979 Nobel Prize in Physiology or Medicine for the invention. The key advance over conventional radiography was the mathematical reconstruction step: instead of superimposing all tissue layers onto a single film, CT uses the filtered back-projection algorithm or iterative reconstruction methods to assign an attenuation value to each volumetric element (voxel) independently, producing images in which structures at different depths are clearly resolved rather than blurred together.

X-Ray Computed Tomography

In clinical CT, a motorized gantry rotates an X-ray tube around the patient while detectors on the opposite side measure transmitted intensity at hundreds of angular positions. Each rotation produces a set of line-integral measurements called a sinogram; the reconstruction algorithm inverts this data to produce a two-dimensional slice of tissue. Modern multi-detector CT (MDCT) scanners acquire sixteen to over 320 simultaneous slices per rotation, enabling whole-organ imaging in a single breath-hold and true volumetric datasets with isotropic resolution below one millimeter. Attenuation values in CT images are expressed in Hounsfield units (HU), calibrated so that water is zero and air is approximately -1000 HU, giving clinicians a quantitative scale for distinguishing tissue types. The physics and instrumentation underlying this technique are documented in the NIBIB science overview of computed tomography.

PET/CT Multimodality Medical Imaging

Fusing CT with positron emission tomography (PET) produces a complementary pair of images: CT supplies high-resolution anatomical detail while PET maps metabolic or molecular activity through the distribution of a radiolabeled tracer. In combined PET/CT scanners, both acquisitions are performed in a single session on the same table, and the CT scan also provides attenuation correction data for quantitative PET reconstruction. This multimodality approach has become the standard of care in oncologic staging, where the anatomical precision of CT is needed to localize the metabolic hotspots identified by PET. A published review of emerging clinical applications of CT documents how PET/CT fusion has displaced separate sequential imaging in many centers. SPECT/CT, which pairs single-photon emission tomography with CT, follows the same integration principle for a different class of tracers.

Image Reconstruction and Computational Advances

The quality and dose of CT imaging depend heavily on the reconstruction algorithm. Filtered back-projection, the classical approach, is fast but amplifies noise when the number of projections is reduced. Iterative reconstruction methods model the acquisition physics more accurately and apply regularization to suppress noise, permitting lower radiation doses for equivalent or superior image quality. Model-based iterative reconstruction and deep learning reconstruction, in which neural networks trained on paired low-dose and reference images are used to denoise outputs, have substantially reduced patient dose in routine clinical CT. Published benchmarks for these methods appear in Nature Reviews Methods Primers on X-ray computed tomography.

Applications

Computed tomography has applications across medical and non-medical domains, including:

• Oncologic staging and tumor volume assessment
• Cardiac and vascular imaging with contrast enhancement
• Trauma evaluation for complex fractures and internal hemorrhage
• Industrial non-destructive testing of welds, castings, and composites
• Archaeological analysis of artifacts and paleontological specimens
• Security screening of baggage and cargo