Computed Tomography – Imaging Using Multiple X-ray Projections

Computed Tomography (CT) is a transformative imaging technology that reconstructs three-dimensional (3D) cross-sectional images from multiple X-ray projections acquired around an object or patient. By leveraging sophisticated mathematical algorithms, CT provides unparalleled insight into internal structures, supporting medical diagnostics, scientific research, and industrial quality control. Below is a comprehensive glossary of key CT concepts and technologies.

A

Absorption Contrast

Absorption contrast refers to the fundamental mechanism in X-ray imaging that differentiates internal structures based on the variable absorption of X-rays by different materials. Denser or higher atomic number materials (like bone or metal) absorb more X-rays than soft tissue, creating visible contrast in images. This principle is crucial for distinguishing anatomical features in CT scans and is influenced by X-ray energy, material composition, and imaging parameters.

Acquisition Time

Acquisition time in CT is the total duration required to collect all X-ray projection data necessary for image reconstruction. Factors influencing acquisition time include the number of projections, detector speed, gantry rotation, and scan protocol (e.g., helical vs. step-and-shoot). Modern CT scanners can achieve full-body scans in seconds, while high-resolution or micro-CT systems may require longer durations. Minimizing acquisition time reduces motion artifacts and improves patient comfort.

Algorithm (Reconstruction Algorithm)

A reconstruction algorithm in CT converts a series of two-dimensional (2D) X-ray projections into a 3D volumetric image. The most common is filtered back projection (FBP), offering speed and simplicity, but iterative reconstruction algorithms (such as ART, ML-EM, or MBIR) provide superior image quality, especially in low-dose or sparse-data scenarios. Recent advances include machine learning-based reconstruction for faster and more accurate imaging.

Anode (in X-ray Source)

The anode is the positively charged electrode inside an X-ray tube. High-speed electrons from the cathode strike the anode (commonly tungsten), producing X-rays via bremsstrahlung and characteristic emission. Rotating anodes, used in medical and high-performance CT, dissipate heat more efficiently, allowing higher tube currents and shorter exposures. Anode design impacts X-ray intensity, spectrum, and focal spot size, influencing image resolution and scanner longevity.

B

Beam Hardening

Beam hardening occurs when lower-energy X-ray photons are preferentially absorbed as the beam passes through matter, increasing the average energy of the beam. This can cause artifacts such as cupping and streaks in CT images, particularly near dense structures like bone or metal implants. Correction techniques include pre-filtration, calibration algorithms, and dual-energy CT to minimize diagnostic errors and improve quantitative accuracy.

Biomedical Engineering

Biomedical engineering merges engineering principles with medical and biological sciences to advance CT technology. Biomedical engineers design scanner hardware, optimize reconstruction algorithms, develop safer and more effective protocols, and innovate new applications, such as molecular imaging and automated diagnostics. Their work ensures CT systems meet international safety and performance standards and continue to evolve for clinical, industrial, and research use.

C

Collimator

A collimator shapes and narrows the X-ray beam, ensuring only rays traveling along desired paths reach the detector. Pre-patient collimators define slice thickness and reduce scatter, while post-patient collimators minimize detection of scattered photons. In specialized systems like multi-pinhole FXCT, collimators enable simultaneous multi-angle data collection for molecular imaging. Proper design and alignment are critical for image quality and quantitative accuracy.

Computed Tomography (CT)

Computed Tomography (CT) is a 3D imaging technique that reconstructs internal structures from multiple X-ray projections taken at different angles. It surpasses conventional radiography by providing volumetric data, enabling visualization of anatomy, materials, or defects in any plane. CT is vital in medicine (for diagnosis and treatment planning), industry (for non-destructive testing), and research. Key performance indicators include spatial, contrast, and temporal resolution, all governed by international safety and quality standards.

Contrast Agent

A contrast agent is a material administered to enhance the visibility of specific tissues or structures in CT imaging. Most clinical agents are iodine-based, increasing X-ray attenuation in blood vessels and organs. Other agents (e.g., barium, gold nanoparticles) are used for specialized or research applications. The choice and administration route are tailored to the diagnostic task, with careful management to minimize allergic reactions and toxicity.

D

Detector (X-ray Detector)

The X-ray detector is the sensor array that captures transmitted X-rays after they pass through the object. Modern CT detectors use either scintillator materials (converting X-rays to light, then to electrical signals) or direct-conversion semiconductors (converting X-rays directly to charge). Advanced detectors may use photon-counting technology for enhanced spectral resolution. Detector design impacts spatial resolution, noise, and scan speed, and must comply with strict calibration and safety standards.

Dose (Radiation Dose)

Radiation dose in CT is the amount of ionizing radiation absorbed during a scan. It is measured as absorbed dose (gray, Gy), equivalent dose (sievert, Sv), CT dose index (CTDI), and dose length product (DLP). Managing dose is critical to minimize health risks, especially in repeated or pediatric scans. Techniques include automatic exposure control, iterative reconstruction, and protocol optimization, guided by international safety standards.

E

Energy Resolution

Energy resolution describes a detector’s ability to distinguish between X-ray photons of different energies. High energy resolution is essential in spectral, dual-energy, and fluorescent X-ray CT for material discrimination and accurate quantitative imaging. Semiconductor detectors (CdTe, HPGe) provide superior energy resolution compared to scintillator-based systems, and their use is expanding in advanced clinical and research CT applications.

Ex-vivo / In-vivo Imaging

Ex-vivo imaging is performed on samples or tissues outside a living organism, allowing for higher resolution and longer scans. In-vivo imaging occurs within living organisms, enabling real-time study of biological processes. In-vivo CT requires careful dose and motion management, while ex-vivo imaging permits more aggressive imaging parameters. Both approaches are important in research, preclinical studies, and translational medicine.

F

Field of View (FOV)

The field of view (FOV) is the maximum area that a CT scanner can image in a single scan. Determined by detector array size, X-ray source position, and mechanical limits, FOV ranges from 25 cm (head) to over 50 cm (body) in medical CT, and can be as small as a few millimeters in micro-CT or nano-CT. Selecting the proper FOV balances coverage, spatial resolution, and scan time for the specific application.

This glossary is a living resource for professionals and students seeking to understand the principles and components of computed tomography. For more detailed information or to discuss specific applications, please contact our imaging experts.