CT Scan Windowing and Leveling

Windowing

• Windowing is an electronic manipulation of data to represent a limited range of HU values, enabling different structures to be imaged.
• Window level represents the central HU of all numbers within the window width, controlling density.
• When the window level is decreased, the CT image will be brighter, and vice versa.
• Increasing the window level helps view lungs and other airways, while decreasing it helps view denser bones.

Window Width

• Window width converts the HU of all tissues of interest and displays them as various shades of gray, controlling contrast.
• When the window width is decreased, the CT image will be brighter, and vice versa.
• Increasing the window width increases contrast in the image, making it good for looking at differences in soft tissue.

Tissue Densities

• Hyperdense: areas with the highest density (bone/white)
• Isodense: areas between hyperdense and hypodense
• Hypodense: areas with the least density (air/black)

Image Quality Characteristics

Spatial Resolution

• Refers to the degree of blurring, ability to distinguish between objects or structures that differ in density.
• Factors affecting spatial resolution:
  • Field of view: increases in FOV decrease spatial resolution
  • Pixel size: smaller pixel size increases spatial resolution
  • Focal spot size: larger focal spot decreases resolution
  • Magnification: increasing magnification decreases resolution
  • Motion of the patient: affects spatial resolution
  • Pitch: higher pitch decreases resolution
  • Kernels: edge enhancement kernels have higher resolution than soft tissue
  • Slice thickness: larger slice thickness decreases resolution
  • Detector size: increasing detector size decreases resolution

Contrast Resolution

• Ability to distinguish materials of one composition from another without regard to size and shape.
• Refers to the ability of the scanner to demonstrate small differences in tissue contrast.
• One of the major advantages in CT scanning.

Noise

• Defined as the percent standard deviation of a large number of final images as graininess.
• Refers to the fluctuation of CT number between points in the image for a scan of uniform matter such as water.
• Characterized by a grainy appearance of the image.
• Factors influencing system noise:
  • KVP and filtration
  • Pixel size
  • Slice thickness
  • Detector deficiency
  • Patient dose

Major Types of Noise

Quantum Mottle or Quantum Noise

• Result of too few photons reaching a detector after being attenuated by the body.
• Factors that result in quantum noise:
  • Anatomical structure size
  • Reduction of slice thickness without increasing technical factors
  • Decreasing pixel size
  • Scatter radiation

Electronic Noise

• Noise contained within the image caused by vibrations of any physical components, especially rotational components or power fluctuations.

Computational Noise

• Primarily caused by statistical fluctuations that occur from reconstruction mathematics essential to produce CT images.
• Factors influencing computational noise:
  • Voxel size, slice thickness, matrix, FOV
  • Filter algorithm, mAs

Linearity

• The CT scanner must be calibrated so that water is consistently represented as zero HU, and other tissues are represented by their appropriate CT value.