5.1 X-ray and Computed Tomography (CT)

X-ray imaging uses electromagnetic radiation to create pictures of internal body structures. It relies on different tissues absorbing X-rays at varying rates, allowing doctors to see bones, lungs, and other organs.

CT scanning takes X-ray technology further, using rotating X-ray beams and detectors to create detailed cross-sectional images. This advanced technique offers high-resolution views of organs, bones, and blood vessels, revolutionizing medical diagnostics and treatment planning.

X-ray Imaging Principles and Generation

Principles of X-ray imaging

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• Based on differential absorption of X-rays by tissues in the body
  • X-rays are electromagnetic radiation with wavelengths shorter than visible light
  • As X-rays pass through the body, they are attenuated (absorbed or scattered) to varying degrees depending on tissue density and composition
    • Bone attenuates X-rays more than soft tissues due to higher density and calcium content
    • Air attenuates X-rays the least, allowing them to pass through easily (lungs)
• X-ray generation occurs in an X-ray tube
  • Consists of a cathode (negative electrode) and an anode (positive electrode) in a vacuum
  • Electrons emitted from heated cathode filament and accelerated towards anode by high voltage (typically 50-150 kV)
  • When accelerated electrons strike anode target (usually tungsten), X-rays produced through two main processes:
    • Bremsstrahlung radiation: Electrons decelerated by strong electric fields of anode atoms, converting kinetic energy into X-ray photons
    • Characteristic radiation: Electrons collide with and eject inner shell electrons of anode atoms, causing outer shell electrons to fill vacancies and emit X-rays with specific energies (K-shell, L-shell)

Computed Tomography (CT) Imaging

Components of CT scanners

• X-ray tube: Generates fan-shaped beam of X-rays that rotates around patient
• Detector array: Measures intensity of X-rays that pass through patient at various angles
  • Detectors typically solid-state or gas-filled devices that convert X-rays into electrical signals (scintillators, photodiodes)
• Gantry: Houses X-ray tube and detector array, rotates them around patient
• During CT scan:
  1. Patient lies on motorized table that moves through gantry opening
  2. X-ray tube and detector array rotate around patient, acquiring X-ray projections from multiple angles
  3. X-ray projections processed by computer using complex algorithms to reconstruct cross-sectional images (slices) of body
  • Reconstruction involves solving linear equations using techniques such as filtered back projection or iterative reconstruction

Advantages vs limitations of CT

• Advantages:
  • High spatial resolution: Produces detailed cross-sectional images with sub-millimeter resolution
  • Excellent contrast resolution: Differentiates between tissues with small differences in density
  • Fast acquisition times: Modern scanners acquire images in seconds
  • Versatile: Can image bone, soft tissue, and blood vessels (brain, lungs, abdomen)
• Limitations:
  • Ionizing radiation exposure: Delivers higher radiation doses than conventional X-rays, may increase risk of radiation-induced cancers
  • Limited soft tissue contrast: Not as effective as MRI in differentiating between soft tissues with similar densities
  • Artifacts: Images can be affected by beam hardening, motion, and metal artifacts, degrading quality
  • Iodinated contrast agents: Some scans require iodinated contrast, which can cause allergic reactions or contrast-induced nephropathy in some patients

Applications of X-ray and CT

• X-ray imaging applications:
  • Skeletal imaging: Evaluation of fractures, dislocations, bone abnormalities
  • Chest radiography: Assessment of lung diseases, heart size, placement of medical devices
  • Dental radiography: Detection of tooth decay, gum disease, jaw abnormalities
  • Mammography: Screening and diagnostic imaging of breast for early cancer detection
• CT imaging applications:
  • Neuroimaging: Evaluation of brain tumors, hemorrhage, stroke, intracranial pathologies
  • Oncology: Staging and follow-up of cancers (lung, liver, pancreas)
  • Cardiovascular imaging: Assessment of coronary artery disease, aortic aneurysms, pulmonary embolism
  • Musculoskeletal imaging: Evaluation of complex fractures, spinal disorders, joint abnormalities
  • Abdominal imaging: Detection of appendicitis, kidney stones, abdominal aortic aneurysms
  • Virtual colonoscopy: Non-invasive screening for colorectal cancer using CT images of colon