Infographics / CT Physics

Helical acquisition & pitch

Top: an X-ray tube coil wraps around a horizontal patient, showing that continuous gantry rotation (slip-ring) combined with continuous table motion traces a helix; the helical data are interpolated to axial slices. Bottom: pitch is defined as table feed per rotation divided by collimated beam width, compared as a tight low-pitch coil (overlapping turns) versus a stretched high-pitch coil (spaced turns with z-gaps), with increasing pitch giving shorter beam-on time, lower dose, and coarser z-resolution.

Helical CT scanning: geometry and the effect of pitch Top: an X-ray tube coil wraps around a horizontal patient, showing that continuous gantry rotation (slip-ring) combined with continuous table motion traces a helix; the helical data are interpolated to axial slices. Bottom: pitch is defined as table feed per rotation divided by collimated beam width, compared as a tight low-pitch coil (overlapping turns) versus a stretched high-pitch coil (spaced turns with z-gaps), with increasing pitch giving shorter beam-on time, lower dose, and coarser z-resolution. Helical CT scanning: geometry and the effect of pitch Continuous rotation (slip-ring) + table motion → the focal spot traces a helix around the patient. spin patient (z-axis) X-ray tube helical path table translation (+z) Helical projection data are interpolated to reconstruct axial slices at any chosen z-position → slice position/spacing chosen retrospectively (including overlapping slices). Pitch = table feed per rotation (d) ÷ total collimated beam width (W) Same beam width W in both; only the helical turn spacing d changes. d low pitch: overlapping turns (pitch < 1) d high pitch: spaced turns, z-gaps (pitch > 1) increasing pitch • fewer rotations per scan length → shorter beam-on time (time ∝ 1/pitch) • same tube output spread over more z → lower patient dose (CTDIvol ∝ 1/pitch) • wider z-sampling, more interpolation → coarser longitudinal (z) resolution Reconstructed slice thickness ≥ collimated beam width; minimum slice = one detector row.
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