Computed Tomography Notes

Computed Tomography

Basic Principle of CT Scan

  • Conventional radiographs project a 3D object onto a 2D image.
  • This leads to superimposition of overlying tissues, a major limitation of conventional radiography.

Background

  • Tomography originates from the Greek word "tomos," meaning to cut, section, or layer.
  • CT uses computerized methods to acquire and transform data.
  • The components for CT systems were available 20 years before Hounsfield's demonstration in 1970.
  • Godfrey Hounsfield (EMI, Ltd.) and Alan Cormack (Tufts University) were pivotal in CT development.
  • Cormack developed the mathematics for CT image reconstruction; they shared the 1979 Nobel Prize in Physics.

Terminology

  • Spatial resolution: Ability to distinctly define small objects.
  • Low-contrast resolution: Ability to differentiate objects with similar densities.
  • Temporal resolution: Speed of data acquisition, crucial for reducing motion artifacts (e.g., in cardiac imaging).

Computed Tomography: Definition

  • CT uses a computer to process X-ray beam information after it passes through an area of anatomy.
  • Creates cross-sectional images.
  • Loaf of bread analogy: Each CT slice represents a plane in the patient's body.
  • Z-axis: Represents the thickness of the CT slice.

CT Data Elements

  • CT slice data are sectioned into elements.
  • Pixel (picture element): A 2D square representing width (X) and height (Y).
  • Thousands of pixels form a CT image on the monitor.
  • Voxel (volume element): A cube that results when the Z-axis (slice thickness) is considered with a pixel.

Beam Attenuation

  • Beam Attenuation: Reduction of X-ray beam intensity by an object.
  • Factors influencing attenuation:
    • Density
    • Thickness
    • Atomic number
  • Linear attenuation coefficient: Amount of X-ray beam scattered or absorbed per unit thickness of the absorber.

Linear Attenuation Coefficient

  • Example: For a 125-kVp X-ray beam, the linear attenuation coefficient for water is approximately 0.18 cm⁻¹.
  • This indicates that approximately 18% of photons are absorbed or scattered when the X-ray beam passes through 1 cm of water.

Hounsfield Unit (HU)

  • Hounsfield units quantify the degree to which a structure attenuates an X-ray beam.
  • Typical values:
    • Bone: 1,000
    • Distilled water: 0
    • Air: -1,000
  • Relationship: 1 HU equals a 0.1% difference between the linear attenuation coefficient of the tissue compared to water.
  • HU = 1000 * (\mu{tissue} - \mu{water}) / \mu_{water}

Factors Affecting Hounsfield Unit Measurement

  • Poor equipment calibration
  • Image artifacts
  • Volume averaging

Polychromatic X-Ray Beams

  • CT and conventional radiography use polychromatic X-ray beams.
  • These beams comprise photons with varying energies.
  • The spectrum includes photons ranging from weak to strong energies.

Beam Hardening

  • Beam hardening: Artifacts appear as dark streaks or areas of decreased density.
  • Sometimes referred to as cupping artifacts.

Volume Averaging

  • Volume averaging: Averaging of different tissue attenuation values to produce one less accurate pixel reading.
  • Thicker CT slices increase the likelihood of missing small objects.
  • Example: A 10-mm slice with a 2-mm pathologic tissue area results in averaging with 8 mm of normal tissue, potentially making the pathologic tissue less apparent.
  • Thinner slices may be useful if an area is suspicious but not definitive.

Scanning Protocols & Volume Averaging

  • Thicker cuts may be used to reduce radiation dose.
  • If scanning a large area, numerous slices are produced.
  • Scanning procedures balance image quality and acceptable radiation dose.

Pixel Size & Volume Averaging

  • Larger pixel sizes increase the chance of the pixel containing tissues of different densities.
  • Example: A pixel with equal parts calcium (600 HU) and lung tissue (-600 HU) averages to 0 HU.
  • In this case, the image pixel does not accurately reflect either the calcium or the lung tissue.

Raw Data vs. Image Data

  • Raw data: Data before pixel sectioning and Hounsfield unit assignment.
  • Image reconstruction: The process of creating an image from raw data.
  • Image data: Data included in the image after each pixel is assigned a Hounsfield unit value.
  • Prospective reconstruction: Reconstruction automatically produced during scanning.
  • Retrospective reconstruction: Generating new images from the same raw data at a later time.

Scan Modes

Step-and-Shoot Scanning

  • Used in scanning systems of the 1980s.
  • X-ray tube rotated 360° around the patient to acquire data for a single slice.
  • The motion of the X-ray tube was halted while the patient was advanced on the CT table.
  • Repeated until the desired area was covered.
  • Also called axial scanning, conventional scanning, or serial scanning.

Helical (Spiral) Scanning

  • Developed in the 1990s due to technical advancements.
  • Continuous acquisition scanning mode.
  • Key advancement: elimination of cables, enabling continuous gantry rotation.
  • Allowed for uninterrupted data acquisition, tracing a helical path around the patient.

Multidetector Row CT Scanning

  • First helical scanners used a single row of detectors, yielding one slice per gantry rotation.
  • In 1992, scanners with two rows of detectors were introduced, capturing data for two slices per gantry rotation.
  • Further improvements equipped scanners with multiple rows of detectors, allowing data for many slices to be acquired with each gantry rotation.

Imaging Planes

  • Axial
  • Coronal
  • Sagittal

Overview of CT System Operation

  • CT scan process comprises three main segments:
    • Data acquisition (GET DATA)
    • Image reconstruction (USE DATA)
    • Image display (DISPLAY DATA)