What Is Tomography?

Tomography is a family of imaging techniques that reconstruct the internal structure of an object from measurements taken along many different paths or projections through that object. The word derives from the Greek tomos (slice) and reflects the technique's goal of producing cross-sectional images without physically sectioning the object. By combining measurements from many angles and applying mathematical inversion algorithms, tomographic systems generate two-dimensional slice images or three-dimensional volumetric reconstructions that reveal interior features otherwise hidden from direct observation.

Tomography has transformed medicine, industrial inspection, geophysics, and scientific research since its clinical introduction in the 1970s. The common mathematical foundation across modalities is the Radon transform, which relates measured projections to the spatial distribution of a target property within the object.

Computed Tomography

X-ray computed tomography (CT) rotates an X-ray source and a curved detector array around a patient or industrial object, acquiring attenuation measurements at hundreds of angular positions. Filtered back-projection and iterative reconstruction algorithms convert these sinogram data into volumetric maps of X-ray attenuation, which correlate with tissue density or material composition. Clinical CT scanners achieve sub-millimeter resolution and can scan the entire chest in a single breath hold, enabling diagnosis of pulmonary embolism, cardiac disease, and oncologic conditions. Industrial CT systems examine castings, welds, and composite structures for internal porosity and cracks without destructive sectioning. NIST's biomedical imaging resources and phantom standards support performance verification of clinical CT systems.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) uses a strong static magnetic field to align nuclear spin moments in tissue hydrogen atoms, then perturbs that alignment with radiofrequency pulses and detects the relaxation signal. Spatial encoding through gradient magnetic fields localizes the signal, and Fourier reconstruction produces images with high soft-tissue contrast. MRI does not use ionizing radiation, making it preferable to CT for pediatric patients and for repeated examinations. Functional MRI (fMRI) detects blood oxygenation level-dependent (BOLD) contrast changes to map neural activity during cognitive tasks. Research published through IEEE Transactions on Medical Imaging covers reconstruction algorithms, pulse sequence design, and hardware advances for both clinical and preclinical MRI systems.

Optical Coherence Tomography

Optical coherence tomography (OCT) uses low-coherence near-infrared light and interferometric detection to generate high-resolution cross-sectional images of biological tissues. Axial resolution of 1 to 15 micrometers, far exceeding clinical CT or MRI, makes OCT the standard imaging modality for retinal assessment in ophthalmology. Swept-source and spectral-domain OCT systems acquire volumetric data at rates exceeding one million A-scans per second, enabling real-time retinal angiography without exogenous contrast agents. OCT also finds application in intravascular imaging, dermatology, and non-destructive evaluation of optical coatings and semiconductor wafers. Publications from the journal Biomedical Optics Express document the expanding clinical and industrial applications of OCT.

Electrical Tomography

Electrical capacitance tomography (ECT) and electrical impedance tomography (EIT) reconstruct the internal distribution of permittivity or conductivity from measurements made at electrodes placed around the object's boundary. ECT is applied to industrial process monitoring, visualizing the distribution of gas and solid phases inside pipelines, reactors, and hoppers. EIT has clinical applications in lung ventilation monitoring and breast cancer detection, where it offers a radiation-free, low-cost alternative to X-ray methods. The inverse problem in electrical tomography is ill-posed and requires regularization techniques to produce stable reconstructions, an active area of research in signal processing and numerical mathematics.

Applications

• Clinical diagnosis of cancer, cardiovascular disease, and neurological conditions using CT and MRI
• Non-destructive inspection of aerospace castings and composite panels using industrial CT
• Retinal disease monitoring and intravascular plaque imaging with optical coherence tomography
• Industrial process flow visualization using electrical capacitance tomography
• Lung ventilation monitoring at the bedside using electrical impedance tomography
• Geophysical subsurface mapping using seismic and electrical resistivity tomography