X-Ray Production & Exposure Factors

The x-ray tube

Cathode: heated tungsten filament releases electrons (thermionic emission); a molybdenum focusing cup aims them at the target.
Anode: tungsten target set in copper (which conducts heat away). The focal spot is the area struck; a smaller effective focal spot gives a sharper image (line focus principle).
Over 99% of the electrons' energy becomes heat; under 1% becomes x-rays.
Bremsstrahlung (braking) radiation makes most of the beam, with a continuous energy spectrum; characteristic radiation (electron ejected from an inner shell of tungsten) is a small share.
Aluminum filtration removes low-energy photons that would only add skin dose: at least 1.5 mm Al at or below 70 kVp and 2.5 mm above 70 kVp. Collimation limits beam size: round beam no more than 2.75 in (7 cm) at the skin; rectangular collimation cuts patient dose substantially.

Control	Changes	Image effect
mA (tube current)	Number of electrons, so number of photons (quantity)	Higher mA → darker image (more density); no change in contrast
Exposure time	Number of photons	Longer → darker; mA × time = mAs
kVp (tube voltage)	Photon energy and penetration (quality), also some quantity	Higher kVp → darker and LOWER contrast (long gray scale); lower kVp → higher contrast (short scale)
Source-to-receptor distance	Intensity follows the inverse square law	Longer distance → lighter image unless exposure is increased; less magnification and sharper image

Inverse square law: intensity ∝ 1/distance². Moving from 8 in to 16 in cuts intensity to ¼, so exposure must be multiplied by 4; from 8 in to 12 in needs (12 ÷ 8)² = 2.25 times the exposure.
Sharpness improves with a small focal spot, a long source-object distance, a short object-receptor distance and no motion.

Interaction	Share (approximate)	Meaning
Coherent (Thomson) scatter	About 7–8%	Photon changes direction without ionization; minor fog
Photoelectric absorption	About 30%	Photon fully absorbed; creates image contrast and most patient absorbed dose; more likely in dense, high atomic number tissue
Compton scatter	About 60%	Photon loses part of its energy and scatters; main source of image fog and of scatter exposure to the operator
No interaction	Remainder of the beam	Passes through to form the image

Saying kVp only changes darkness: it also changes contrast.
Saying mA changes beam energy: mA changes quantity, kVp changes quality.
Doubling exposure (instead of quadrupling) when the distance is doubled.
Assigning patient dose mainly to Compton scatter: photoelectric absorption contributes most of the patient's absorbed dose.

Pick an answer to see right away whether it is correct and why.

Which change produces a radiograph with LOWER contrast (longer gray scale)?
The source-to-receptor distance is increased from 8 in to 16 in. To keep the same image density, exposure time must be:
Most x-ray photons produced in a dental tube come from:
What is the purpose of aluminum filtration?
Which interaction is the main cause of scatter exposure to the operator?

0 of 5 answered

Answer key: 1 B · 2 D · 3 B · 4 A · 5 D

Dental School Map · Original study material, written and checked by our team; always confirm clinical decisions against current guidelines.