X-ray computed tomography

X-ray computed tomography
Name	X-ray computed tomography
Caption	A modern CT scanner.
MeshID	D014057

X-ray computed tomography. It is a medical imaging technique that uses computer-processed combinations of many X-ray measurements taken from different angles to produce cross-sectional (tomographic) images of specific areas of a scanned object. This allows the user to see inside the object without cutting. The method is used in radiology for diagnostic purposes and in various industries for non-destructive testing.

Principles of operation

The fundamental principle relies on the differential attenuation of X-ray beams as they pass through materials of varying density. A rotating X-ray tube and a ring of stationary detectors, or a rotating detector array, are positioned around the patient. As the beam passes through the body, structures like bone and soft tissue absorb X-rays to different degrees, a phenomenon first studied by Wilhelm Röntgen. The resulting data, a set of projections, represents line integrals of the linear attenuation coefficient, forming the mathematical basis for the Radon transform.

Data acquisition

Data acquisition involves the systematic collection of transmission measurements. The patient lies on a motorized table that moves through the gantry, which houses the X-ray source and detectors. Modern systems, such as those from Siemens Healthineers or GE Healthcare, use a slip-ring design to allow continuous rotation. The two primary scan modes are axial (step-and-shoot) and helical (spiral), the latter pioneered by Willi Kalender. Key parameters controlled by the radiographer include tube current, kilovoltage peak, and slice thickness.

Image reconstruction

Image reconstruction is the computational process of converting raw projection data into a cross-sectional image. The most common algorithm is filtered back projection, which applies a convolution filter to the projections before back-projecting them onto the image matrix. Iterative reconstruction techniques, which model the physics of the scan and statistical noise, are increasingly used by companies like Philips to improve image quality. The final image is a digital matrix of voxels, each assigned a value on the Hounsfield scale, named after Godfrey Hounsfield.

Applications

Its primary application is in diagnostic medicine, providing critical information for oncology, traumatology, and neurology. It is indispensable for assessing stroke, pulmonary embolism, and appendicitis. Outside medicine, it is used in materials science for analyzing composite materials, in archaeology for examining artifacts like the Antikythera mechanism, and in security for baggage screening at airports like Heathrow Airport. Industrial CT scanning is vital for inspecting aerospace components and electronic assemblies.

Technical specifications and variants

Technical specifications define system performance, including spatial resolution, contrast resolution, and scan time. Major variants include cone beam computed tomography, widely used in dentistry and image-guided radiation therapy, and positron emission tomography–computed tomography, which combines anatomical and functional data. High-resolution micro-CT systems are used in preclinical imaging. Innovations like dual-energy CT, available on systems from Canon Medical Systems, allow material decomposition, improving tissue characterization.

History and development

The theoretical foundation was laid by Johann Radon and his transform in 1917. The first commercially viable scanner was invented independently by Godfrey Hounsfield of EMI Laboratories and Allan McLeod Cormack of Tufts University; they shared the 1979 Nobel Prize in Physiology or Medicine. The first clinical scan on a patient was performed at Atkinson Morley Hospital in 1971. Rapid evolution followed, from first-generation translate-rotate scanners to the development of slip-ring technology enabling helical CT by Toshiba in the late 1980s, revolutionizing scan speed and volume coverage.

Category:Medical imaging Category:Radiology Category:Tomography