Infographics / CT Physics

Single- vs multi-slice dose (overbeaming)

Two beam-and-detector cross-sections compared. In single-slice CT the x-ray beam is collimated to roughly match the single detector, so little penumbra is wasted. In multi-slice CT the beam is wider than the active detector rows, so penumbra over-runs both edges (overbeaming) and is wasted dose. A helical z-axis band shows extra rotations added beyond the planned imaged length (over-ranging), making helical dose exceed axial. Key points explain that wider detector arrays are more efficient and that reconstructed slice thickness sets noise, not single-slice dose.

Single-slice versus multi-slice CT dose efficiency Two beam-and-detector cross-sections compared. In single-slice CT the x-ray beam is collimated to roughly match the single detector, so little penumbra is wasted. In multi-slice CT the beam is wider than the active detector rows, so penumbra over-runs both edges (overbeaming) and is wasted dose. A helical z-axis band shows extra rotations added beyond the planned imaged length (over-ranging), making helical dose exceed axial. Key points explain that wider detector arrays are more efficient and that reconstructed slice thickness sets noise, not single-slice dose. Single-slice vs multi-slice CT: beam geometry and dose efficiency imaged / used wasted dose Single-slice Multi-slice (MDCT) focal spot beam ~ detector wider z-coverage per rotation one detected slice beam matched to detector many detector rows beam over-runs edges: overbeaming Helical over-ranging extra rotations at each helical end, so helical dose > axial planned imaged length over-range over-range Why multi-slice dose can exceed single-slice Multi-slice covers more z per rotation (faster) but is less dose-efficient per imaged length. Overbeaming: the total beam is wider than the active rows, so edge penumbra is wasted dose. Waste is a larger fraction for narrow total collimation; wider detector arrays are more efficient. Over-ranging: helical adds rotations beyond the planned length, so helical dose exceeds axial. Reconstructed slice thickness alone sets noise, not single-slice dose.
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