Factors affecting xray production

1. FACTORS AFFECTING X RAY
PRODUCTION AND INTERACTION OF
X-RAYS WITH MATTER

2. • X ray beam maybe modified by altering the beam exposure duration
(timer)
• Exposure rate (mA)
• Energy (kVp and filtration)
• Shape (collimation)
• Intensity (target-patient distance).

3. EXPOSURE TIME
• Typically measured in fractions of a second (s)
• Modifies the duration of the exposure-the number of photons generated.
• When the exposure time is doubled, the number of photons generated at all
energies in the x-ray emission spectrum is doubled.

5. TUBE CURRENT
• The quantity of radiation produced by an x-ray tube (i.e., the
number of photons that reach the patient and film) is directly
proportional to the tube current (mA) and the time the tube is
operated.
• mA setting is increased, more power is applied to the filament,
which heats up and releases more electrons that collide with the
target to produce radiation.

6. • Quantity of radiation produced is proportional to the product of
time and tube current.
• The quantity of radiation remains constant regardless of variations
in mA and time as long as their product remains constant.
• The term beam quantity or beam intensity refers to the number of
photons in an x-ray beam.

8. TUBE VOLTAGE PEAK
• Increasing the kVp increases the potential difference between the cathode
and the anode, increasing the energy of each electron when it strikes the
target.

9. Increasing the kVp of an x-ray machine increases:
• The number of photons generated.
• The mean energy of the photons.
• The maximal energy of the photons.
• The term beam quality refers to the mean energy of an x-ray
beam

11. • Exposure time, tube current (mA), and tube voltage are the three controls
found on many x-ray machines.
• It is recommended that if the tube current is variable, the operator select the
highest mA value available and always operate the machine at this setting;
this allows the shortest exposure time and minimizes the chance of patient
movement.
• If tube voltage can be adjusted, it is recommended that the operator select a
desired voltage, perhaps 70 kVp

12. FILTRATION
• It is desirable to remove low-energy photons from the beam.
• This removal can be accomplished in part by placing a metallic disk (filter) in the
beam path.
• A filter preferentially removes low-energy photons from the beam, while allowing
high-energy photons that are able to contribute to making an image to pass through.

13. • Inherent filtration consists of the materials that x-ray photons
encounter as they travel from the focal spot on the target to
form the usable beam outside the tube enclosure.

14. • The glass wall of the x-ray tube
• The insulating oil that surrounds many dental tubes
• The barrier material that prevents the oil from escaping through the x-
ray port

15. • The inherent filtration of most x-ray machines ranges from the
equivalent of 0.5 to 2 mm of aluminum.

16. • Added filtration may be supplied in the form of aluminum
disks placed over the port in the head of the x-ray machine.
• Total filtration:Added filtration + inherent filtration.
• 1.5 mm of aluminum for a machine operating at up to 70 kVp
and 2.5 mm of aluminum for machines operating at higher
voltages

17. COLLIMATION
• A collimator is a metallic barrier with an aperture in the middle used
to restrict the size of the x-ray beam and the volume of tissue
irradiated.
• ROUND AND RECTANGULAR COLIMATOR.
• Dental x-ray beams are usually collimated to a circle 2¾ inches (7
cm) in diameter at the patient’s face.
• A round collimator a thick plate of radiopaque material (usually
lead) with a circular opening centered over the port in the x-ray head
through which the x-ray beam emerges.

18. • Rectangular collimators further limit the size of the beam to
just larger than the x-ray film, further reducing patient
exposure.
• Collimators also improve image quality.

20. INVERSE SQUARE LAW
• The intensity of an x-ray beam (the number of photons per cross sectional
area per unit of exposure time) depends on the distance of the measuring
device from the focal spot.
• For a given beam, the intensity is inversely proportional to the square of the
distance from the source.
• The reason for this decrease in intensity is that an x-ray beam spreads out
as it moves from its source.
• where I is intensity and D is distance

22. INTERACTION OF X-RAYS WITH MATTER

23. • X-ray beam enters the body of a patient – interacts with hard and soft tissue
and then strikes the film
• Three interactions
• -Coherent scattering
• -Photoelectric absorption
• -Compton scattering

25. COHERENT SCATTERING
• Coherent scattering (also known as classical, elastic, or Thompson scattering)
may occur when a low-energy incident photon (less than 10 keV) passes near
an outer electron of an atom (which has a low binding energy).

26. PHOTOELECTRIC ABSORPTION
• Photoelectric absorption is critical in diagnostic imaging.
• This process occurs when an incident photon collides with a bound electron in
an atom of the absorbing medium.
• At this point the incident photon ceases to exist.
• The electron is ejected from its shell and becomes a recoil electron
(photoelectron).

27. • The extent that x-rays are absorbed by photoelectric absorption depends on
factors.
❑The wavelength or photon energy of the x rays (Kvp)
❑Thickness of the material
❑Density(mass/unit volume)of the material
❑Atomic number of the material.

28. COMPTON SCATTERING
Compton scattering occurs when a photon interacts with an outer orbital
electron.
About 62% of the photons that are absorbed from a dental x-ray beam are
absorbed by this process.

29. Approximately 30% of the scattered photons formed during a dental x-ray
exposure (primarily from Compton scattering) exit through the patient's head.
This is advantageous to the patient because some of the energy of the
incident x-ray beam escapes the tissue, but it is disadvantageous because it
causes nonspecific film darkening.
Scattered photons darken the film while carrying no useful information
because their paths are altered.