What Is X-ray Tomography?

X-ray tomography is an imaging technique that produces three-dimensional representations of an object's internal structure by recording X-ray transmission data from multiple angular orientations and applying mathematical reconstruction algorithms to synthesize cross-sectional images. The name derives from the Greek "tomos," meaning slice, and reflects the method's original goal of isolating a single plane of interest from overlapping anatomy. In computed tomography (CT), a motorized X-ray source rotates around the subject while an opposing detector array records the attenuated beam intensity, generating a set of angular projections from which a reconstruction algorithm, most commonly filtered back-projection or iterative methods, computes the three-dimensional density map.

X-ray tomography draws on physics established through Röntgen's discovery of X-rays in 1895 and on mathematics formalized by the Radon transform in 1917, but did not become practical in medicine until Allan Cormack and Godfrey Hounsfield built the first clinical CT scanners in the early 1970s, work that earned them the 1979 Nobel Prize in Physiology or Medicine.

Image Acquisition and Reconstruction

A clinical CT system houses an X-ray tube and a multi-row detector array within a rotating gantry. As the gantry completes one or more revolutions, the detector collects a large number of angular projections, each representing the integrated attenuation of the beam along every ray path through the patient. The reconstruction step inverts this process, computing the local attenuation coefficient at each volume element (voxel). Output values are expressed in Hounsfield units (HU), a normalized scale where air registers at -1000 HU and water at 0 HU, providing a quantitative basis for tissue differentiation. NIST's program on X-ray computed tomography for medical applications develops measurement standards and phantom geometries that allow scanners from different manufacturers to produce comparable quantitative images.

Industrial and Micro-CT

Industrial X-ray CT extends the same reconstruction principles to the inspection of manufactured parts, electronic assemblies, and geological samples. Micro-CT systems use finely focused X-ray spots and high-resolution flat-panel detectors to achieve voxel dimensions below one micrometer, enabling visualization of internal porosity, crack networks, fiber orientations, and assembly defects without sectioning the component. In contrast to medical CT, where dose constraints limit the number of projections, industrial systems routinely acquire thousands of views to maximize resolution. The development of X-ray CT is reviewed historically in a paper published in Medical Physics, which traces the contributions of medical physics and the AAPM from the 1970s to the present.

Synchrotron and Phase-Contrast Tomography

Laboratory CT systems rely on bremsstrahlung sources that produce a broad spectrum of X-ray energies. Synchrotron sources provide monochromatic, highly coherent X-ray beams that enable phase-contrast tomography, where differences in the X-ray phase shift, rather than only the absorbed intensity, contribute to image contrast. This approach reveals features in low-density biological tissues and polymer composites that are nearly invisible in conventional absorption CT. The National Institutes of Biomedical Imaging and Bioengineering describes the physical basis of CT, including the role of beam energy and detector geometry, in its technical overview of computed tomography.

Applications

X-ray tomography has applications across a broad range of fields, including:

• Medical diagnosis and treatment planning for tumors, vascular disease, trauma, and pulmonary conditions
• Non-destructive testing of welds, castings, and composite structures in aerospace and automotive manufacturing
• Geological core analysis and reservoir characterization in petroleum and mining engineering
• Archaeological and heritage science examination of artifacts without physical sampling
• Preclinical small-animal imaging in pharmaceutical research and developmental biology