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

References:

Bushberg, et al. Essential Physics of Medical Imaging, 2nd ed, 2001

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