April 3, 2010

CT Slice Thickness

Single Detector Array Scanners

  • Determined by physical collimation of the x-ray beam (green structure in the picture)
  • As this gap widens, the slice thickness increases
  • Upper limit of slice thickness depends on the width of the detector
Multiple Detector Array Scanners
  • Determined by width of the detectors (between blue lines in the picture)
  • Width of the detectors can be changed by 'binning' different numbers of individual detector elements together
  • Can be used in conventional axial scanning (no table movement) and in helical scanning protocols
Tradeoffs Respect to Slice Thickness
  • Number of detected x-ray photons increases linearly with slice thickness, for scans performed at the same kV and mAs
  • As number of detected x-ray photons increases, the signal-to-noise ratio (SNR) increases.
  • Higher SNR means better contrast resolution
  • Example: going from a 1-mm to 3-mm slice thickness triples the number of detected photons, and the SNR increases by square root of 3 = 73%
  • Increased slice thickness will reduce spatial resolution and increase volume averaging in the thickness dimension
Bushberg, et al. Essential Physics of Medical Imaging, 2nd ed, 2001

Follow RiTradiology on Facebook, Twitter or Google Friend Connect
Visit RiT Illuminations to view nice pictures of your colleague

No comments:

Post a Comment