How CT Scans Work: X-Ray Technology, Image Reconstruction, and Clinical Applications

Computed Tomography (CT), historically called CAT (Computerized Axial Tomography) scanning, uses ionizing radiation—not electromagnets—to generate detailed cross-sectional images of the body.

Core Physics and Acquisition

X-Ray Generation and Detection

An X-ray tube rotates around the patient, emitting a fan-shaped or cone-shaped beam. Opposite detectors (solid-state scintillators) measure transmitted photons. Modern multi-detector CT (MDCT) systems have 64–320+ detector rows, enabling sub-millimeter isotropic resolution.

Data Acquisition and Reconstruction

• Raw data (sinograms): Attenuation measurements at each projection angle.
• Filtered Back Projection (FBP) or Iterative Reconstruction (IR): Mathematical algorithms convert projections into 3D voxel matrices. IR reduces noise and radiation dose by 30–60%.
• Hounsfield Units (HU): Standardized scale: air = -1000, water = 0, cortical bone = +1000 to +3000.

Contrast Enhancement

• Iodinated IV contrast: Opacifies vessels, enhances organ parenchyma, characterizes lesions.
• Oral/rectal contrast: Distinguishes bowel lumen from adjacent pathology.
• Timing phases: Arterial, portal venous, delayed (equilibrium), excretory—tailored to clinical question.

Clinical Indications

• Trauma: Pan-scan (head, cervical spine, chest, abdomen, pelvis) in <5 minutes.
• Oncology: Staging, restaging, treatment response (RECIST criteria).
• Vascular: CTA for PE, aortic dissection, coronary calcium scoring, peripheral runoff.
• Neurology: Hemorrhage, stroke (CT perfusion), hydrocephalus.
• Musculoskeletal: Complex fractures, surgical planning.

Radiation Dose and Safety

• Effective dose: 1–25 mSv per exam (chest CT ~7 mSv; abdomen/pelvis ~15 mSv).
• ALARA principle: Justification, optimization (kVp modulation, automatic exposure control, IR), dose monitoring.
• Pregnancy: Shielding, alternative imaging (US, MRI) when feasible.

Advances

• Photon-counting CT: Energy-resolved imaging, improved contrast-to-noise, K-edge imaging.
• AI-assisted reconstruction: Deep learning for ultra-low-dose diagnostic quality.
• Dual-energy CT: Material decomposition (uric acid, iodine, calcium), virtual non-contrast images.

CT remains a cornerstone of diagnostic imaging, balancing speed, spatial resolution, and widespread availability with ongoing dose-reduction innovations.