Infographics / Fluoroscopy

Brightness gain, minification/flux gain and conversion factor

Brightness gain equals minification gain multiplied by flux gain. Minification gain is (input diameter divided by output diameter) squared and comes from focusing a large input phosphor onto a small output phosphor. Flux gain, about 50 to 70, comes from accelerating electrons through 25 to 35 kilovolts so each releases many more light photons. A comparison shows a full 35 cm input field giving a conversion factor of 12, versus a magnified 15 cm input field where minification gain falls by (15/35) squared so the conversion factor drops to about 2.2, giving a dimmer image; automatic brightness control then raises the dose.

Image intensifier brightness gain and the magnification-mode dose penalty Brightness gain equals minification gain multiplied by flux gain. Minification gain is (input diameter divided by output diameter) squared and comes from focusing a large input phosphor onto a small output phosphor. Flux gain, about 50 to 70, comes from accelerating electrons through 25 to 35 kilovolts so each releases many more light photons. A comparison shows a full 35 cm input field giving a conversion factor of 12, versus a magnified 15 cm input field where minification gain falls by (15/35) squared so the conversion factor drops to about 2.2, giving a dimmer image; automatic brightness control then raises the dose. Image intensifier: brightness gain and the magnification trade-off brightness gain = minification gain × flux gain = output screen brightness ÷ input screen brightness · typically several thousand Minification gain = (D input ÷ D output)² Large input phosphor focused onto a small output → brighter per unit area. e.g. (25 cm ÷ 2.5 cm)² = 100 Flux gain ≈ 50–70 Electrons accelerated through ~25–35 kV; each then releases many more light photons at the output. (not extra electrons per input photon) Magnification mode: a smaller input field lowers minification gain Full field of view 35 cm input output conversion factor = 12 minification gain: reference Magnified field of view 15 cm input output conversion factor ≈ 2.2 12 × (15 ÷ 35)² ≈ 2.2 Dimmer image → automatic brightness control (ABC) raises the dose.
Practise FluoroscopyDownload SVG

More in Fluoroscopy

Practice the Physics paper

FRCRX is a Part 1 Physics question bank: true/false items with explanations, timed mocks and topic analytics.

Start question practiceSee plans