Radiation Physics and X-ray Equipment
Radiation Physics and X-ray Equipment
This domain tests how the X-ray tube generates a beam and how generator settings alter it. Questions target tube components, rectification, beam quality and quantity, and attenuation with distance.
Core concepts
- Thermionic emission from the tungsten filament releases electrons; the focusing cup directs them to the anode focal spot. A smaller focal spot sharpens the image but limits mA.
- Electrons convert to heat (over 99%) and X-rays (under 1%). Bremsstrahlung produces a continuous spectrum; characteristic radiation produces discrete K-shell peaks.
- kVp controls penetration and also output; mAs controls quantity. Exposure rises with mAs and approximately with kVp squared to kVp to the fifth power.
- Full-wave rectification uses both half-cycles; high-frequency generators give a more constant potential and higher effective kVp.
- Air kerma obeys the inverse-square law.
Worked example
A tube produces 400 µGy at 100 cm. At 200 cm: I2 = 400 × (100/200)² = 400 × 0.25 = 100 µGy. Doubling distance spreads the beam over four times the area, so intensity falls fourfold.
Traps
- Saying kVp changes only quality; it changes output too.
- Confusing mA (current) with mAs (current × time).
- Treating the heel effect as focusing; it reduces intensity on the anode side.