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IEC 60336: X-Ray Tube Assemblies Focal Spot Characteristics — How Focal Spot Size Determines CT Imaging Resolution
CT Image Sharpness Is Determined by X-Ray Tube Focal Spot Size — Smaller Spot = Sharper Image
IEC 60336:2005 specifies X-ray tube assembly focal spot characteristics measurement. The effective focal spot size (not the manufacturer nominal value) determines CT and DR image spatial resolution — smaller spot means smaller penumbra and sharper edges.
| Nominal Spot | Application | Effective Size (W×L) | Resolution |
|---|---|---|---|
| 0.3 (Micro-focus) | Industrial CT, small-animal imaging | ≤0.3×0.3 mm | >20 lp/mm |
| 0.6 (Small focus) | Mammography | ≤0.6×0.9 mm | ~10 lp/mm |
| 1.0 (Standard) | General radiography | ≤1.0×1.4 mm | ~5 lp/mm |
Effective focal spot is measured using a pinhole camera method with a 0.03 mm tungsten pinhole (magnification ≥2×). The Line Spread Function (LSF) is measured, and the Full Width at Half Maximum (FWHM) defines the effective focal spot size. Nominal and effective values can differ by 30–50%.
TNLab — CT image sharpness is not determined by pixel count, but by the effective size of the X-ray tube focal spot.
