Hygiene Atlas

Atlas · Plate IX · Dental Radiography · 01

X-ray production

X-rays form when electrons from a heated tungsten filament strike a tungsten target; almost all of that energy becomes heat, and most of the useful beam is braking (bremsstrahlung) radiation. kVp sets penetration and contrast, while mA and exposure time set quantity and density. Filtration, collimation and the inverse square law shape what reaches the patient.

High-yield facts · 13

  1. X-rays form when electrons boiled off the heated tungsten filament (cathode, thermionic emission) strike the tungsten target (anode).T7
  2. Over 99% of the electron energy becomes heat; under 1% becomes x-rays. A copper stem carries heat away from the target.T7
  3. Most dental x-rays are bremsstrahlung ("braking") radiation; characteristic radiation is a small share.T7
  4. The molybdenum focusing cup aims the electron stream at the focal spot; a smaller focal spot gives a sharper image.T7
  5. Dental units typically run at about 60-70 kVp (range up to about 90) and about 7-15 mA.T7
  6. kVp controls beam energy (penetration) and contrast; mA and exposure time control the quantity of x-rays (density).T7
  7. Higher kVp = longer gray scale = low contrast (better for periodontal bone); lower kVp = short scale = high contrast (better for caries).T7
  8. Aluminum filtration removes weak, long-wavelength x-rays: at least 1.5 mm Al at up to 70 kVp and 2.5 mm Al above 70 kVp.T7S19
  9. Collimation limits beam size: a round beam must be no more than 2.75 in (7 cm) at the skin.T7
  10. Rectangular collimation cuts patient dose by up to about 60-70% (some sources say up to 5 times) compared with round.S19T7
  11. Inverse square law: intensity varies inversely with the square of the distance (double the distance = one-quarter the intensity).T7
  12. If the source-to-receptor distance doubles, exposure time must be multiplied by 4 to keep the same density.T7
  13. Half-value layer is the thickness of aluminum that cuts beam intensity in half; it measures beam quality.T7

Facts are written in our own words; each tag links to its source. Values marked "sources vary" differ between references, so check the one your program uses.