CT scan

Computed tomography is a medical imaging technique that generates detailed cross-sectional views of internal structures using X-rays and computer-based reconstruction methods. It has become one of the most important diagnostic tools across modern healthcare, offering rapid, accurate visualisation of soft tissues, bones, organs, and blood vessels. Operated by radiographers or radiology technologists, computed tomography is widely used in emergency medicine, oncology, neurology, cardiology, and numerous screening applications. Since its development in the 1970s, the method has advanced significantly, evolving from early axial scanners to modern multislice helical, dual-energy, and hybrid systems.

Background and Development

Computed tomography emerged in the early 1970s as a major technological innovation in radiological science. Its creation is credited to physicist Allan Cormack and engineer Godfrey Hounsfield, who jointly received the 1979 Nobel Prize in Physiology or Medicine for their contributions. CT scanning was initially referred to as computed axial tomography due to its acquisition of axial slices; however, as techniques expanded to allow multiplanar and volumetric imaging, the simpler term computed tomography became standard.
Early scanners were limited to head imaging and used a sequential acquisition process that required several minutes to obtain a single slice. Developments in detector technology, gantry design, and computing power transformed the field, enabling dynamic imaging, improved spatial resolution, three-dimensional reconstructions, and significantly reduced scan times. These improvements also expanded the range of indications and made CT suitable for time-sensitive clinical scenarios such as trauma, stroke assessment, and cardiac evaluation.

Principles of Operation

Computed tomography relies on differential X-ray attenuation by tissues of varying densities. An X-ray tube rotates around the patient while detectors measure the intensity of transmitted radiation. Data collected from multiple angles are processed using tomographic reconstruction algorithms to produce cross-sectional images or “slices”. These images may be displayed in axial, coronal, and sagittal planes or reconstructed into three-dimensional volumes.
Modern CT scanners typically employ a rotating gantry containing an X-ray tube and detector array. As the gantry rotates, the patient table moves through the scanner, enabling continuous volumetric acquisition. The use of high-speed rotation and multislice detectors allows for rapid imaging of large anatomical regions. CT imaging is particularly valuable when MRI is contraindicated, such as in patients with certain metallic implants or pacemakers.

Types of CT Scanners

Various CT scanner designs exist, each suited to different applications and performance requirements.
Sequential CTSequential, or step-and-shoot, CT acquires images slice by slice. The patient table moves incrementally, halting during each rotation of the X-ray tube for data acquisition. This method provides high-quality images but requires more time and may produce artefacts if patient movement occurs between steps. While largely replaced by helical scanners, sequential CT remains useful in specific high-precision examinations.
Spiral (Helical) CTSpiral or helical CT rotates the X-ray tube continuously while the patient table moves at a constant speed, creating a helical path of acquisition. This method reduces scan time, improves continuity of data, and supports advanced three-dimensional reconstructions. Helical CT scanners dominate the modern market due to lower production costs and high clinical utility. Some designs incorporate two X-ray sources positioned at offset angles to improve temporal resolution, particularly for cardiac imaging.
Electron Beam TomographyElectron beam tomography uses a fixed anode ring and electronically steered electron beam to generate X-rays without physically rotating a tube. Because the beam steering is extremely rapid, EBT offers excellent temporal resolution, making it particularly useful for imaging fast-moving structures such as the heart. However, limited anatomical coverage and the cost of large X-ray tube assemblies have restricted widespread adoption.
Dual-Energy CTDual-energy CT employs two different X-ray energy levels to generate two datasets, enabling material differentiation based on energy-dependent attenuation properties. Techniques include dual-source systems, dual-layer detectors, and rapid kilovoltage switching in single-source scanners. Dual-source CT uses two X-ray tubes and detector arrays mounted at 90 degrees within the same gantry. This configuration increases temporal resolution and reduces motion artefacts, particularly in cardiac imaging. Dual-energy imaging supports applications such as kidney stone characterisation, bone marrow oedema evaluation, and improved contrast in vascular studies.
CT Perfusion ImagingCT perfusion imaging evaluates blood flow dynamics within organs by acquiring repeated images during and after the administration of a contrast agent. Key parameters include blood flow, blood volume, and mean transit time. CT perfusion is valuable in assessing cerebral ischaemia, identifying the penumbra in acute stroke, and evaluating myocardial and tumour perfusion. Sensitivity for cardiac use is lower than other forms of CT, but it is well established in neuro-imaging.
PET-CTPositron emission tomography–computed tomography combines functional and anatomical imaging in a single system. PET assesses metabolic activity through radiotracer uptake, while CT provides structural detail. Image fusion enhances localisation and interpretation of functional abnormalities. PET-CT is a major tool in oncology for tumour detection, staging, and treatment monitoring. It is also used in cardiology and neurology for evaluating perfusion, inflammation, and degenerative processes.

Clinical Applications

Computed tomography holds a central role in diagnostic radiology, offering rapid and comprehensive assessment of a broad range of conditions.
Head ImagingCT of the head is a primary investigation in acute neurological emergencies. It rapidly detects ischaemic infarction, intracranial haemorrhage, skull fractures, calcifications, and neoplastic lesions. Hypodense regions often represent oedema or infarction, whereas hyperdense regions may indicate haemorrhage or calcified structures. Head CT is also integral to image-guided neurosurgical procedures such as stereotactic surgery and radiosurgery.
Neck ImagingContrast-enhanced CT is frequently used to evaluate neck masses, upper airway pathology, and lymph node enlargement. It provides accurate delineation of soft tissues, vessels, and thyroid lesions. CT also commonly identifies incidental thyroid abnormalities and assists in staging thyroid malignancies.
Thoracic ImagingCT of the lungs assesses a variety of acute and chronic pulmonary conditions. High-resolution CT (HRCT) is widely used to characterise interstitial lung diseases such as pulmonary fibrosis, pneumatosis, and inflammatory processes. Inspiratory and expiratory scans may be performed to detect air trapping or small airway disease. Incidental pulmonary nodules are common findings, and established guidelines recommend conservative surveillance when nodules remain stable for two years in individuals without prior malignancy.
Vascular and Cardiovascular ImagingCT angiography produces detailed images of blood vessels using intravenous contrast. It is essential for assessing aneurysms, stenosis, embolism, and vascular malformations. In cardiac imaging, rapid gantry rotation and high temporal resolution minimise motion blurring, enabling coronary artery evaluation and analysis of cardiac morphology. Dual-source and high-pitch helical acquisitions improve image quality in patients who cannot tolerate breath-holding or beta-blocker administration.
Abdominal and Pelvic ImagingCT is crucial for evaluating abdominal pain, trauma, malignancy, infection, and gastrointestinal diseases. It assists in diagnosing appendicitis, bowel obstruction, hepatic lesions, renal calculi, and inflammatory conditions. Multiphase contrast studies allow assessment of organ perfusion, vascular supply, and tumour enhancement patterns.

Significance and Considerations

Computed tomography has greatly expanded diagnostic capabilities across medicine. Its speed, accuracy, and versatility make it indispensable in acute care, oncology, and complex surgical planning. However, CT involves exposure to ionising radiation, and inappropriate or excessive use can pose risks. Professional guidelines advise cautious use of whole-body scans in asymptomatic individuals and recommend adherence to evidence-based surveillance intervals for incidental findings.