focal spot
The effective area of the anode where X-rays are produced. Smaller focal spots produce sharper images but dissipate heat less efficiently — at high mA, only a larger focal spot can sustain the heat load. Modern X-ray tubes typically offer dual focal spots to balance these competing requirements.
Why it matters to buyers: Small focal spot (~0.3–0.5 mm) for detail imaging — magnification mammography, fine vascular work, mammographic spot views. Large focal spot (~0.6–1.2 mm) for high-output exposures — cardiac CT at high mA, interventional cine, large-patient body imaging. Dual focal spot tubes support both modes from one tube.
Why it matters to engineers: Focal-spot size degrades with anode wear — visible as image unsharpness creep over time. Annual physicist focal-spot measurement (slit / pinhole / star pattern) tracks the drift; out-of-spec focal-spot is a tube end-of-life signal.