Chapter 16 Beam-Restricting Devices

1. Chapter 16 Beam-Restricting Devices • Three factors contribute to an increase in scatter radiation: • Increased kVp • Increased Field Size • Increased Patient or Body Part Size.

2. X-ray Interactions • a – some interact with the patient and are scattered away from the patient. • b – some are absorbed • c - some pass through without interaction • d – some are scattered in the patient • c & d are image forming x-rays.

3. Relative Contributions of Scatter to the Radiographic Image

4. Percent Interaction of Scatter and Percent Transmission through 10 cm of Tissue

5. Beam-Restricting Devices • There are two principal means to reduce scatter radiation: • Beam Restricting Devices limit the field size to reduce scatter and primary radiation. • Grids to absorb scatter before it reached the image receptor.

6. Beam-Restricting Devices • There are three principal types of beam restricting devices: • Aperture Diaphragm • Cones & Cylinders • Collimators

7. Production of Scatter Radiation • Two kinds of x-rays are responsible for the optical density, or degree of blackening on a radiograph. • Those that pass through the patient without interacting called remnant ray. • Those that are scattered through Compton interaction.

8. Kilovolt Peak • As x-ray energy increases, the relativenumber of x-rays that undergo Compton Scattering increases. • The absolute number of the Compton interactions decrease with increasing energies but the number of photoelectric interactions decreases more rapidly.

9. Field size • The size of the field or area being irradiated has a significant impact on scatter radiation. • Field size is computed in square inches or square cm

10. Field size • Scatter radiation increases as the field size increases. • The relative intensity of the scatter varies more when the field size is small than when the field is large.

11. Field size • When the field size is reduced, the resulting reduction in scatter will reduce the density on the image. • The mAs must be increased to maintain density. • The reduced scatter will improve contrast resolution resulting in improved image quality.

12. Field size • To change from a 14” x 17” to a 10” x 12” increase mAs 25%. • To change from a 14” x 17” to a 8” x 10” increase mAs 40%.

13. Patient or Part Thickness • More scatter results from imaging thick body parts compared to thin body parts. • There will be more scatter for a lumbar spine film compared to a cervical spine film. • As tissue thickness increases, more of the rays go through multiple scattering.

14. Tissue Thickness • The relative intensity of scatter radiation increases with increasing thickness of the anatomy. • The amount of primary radiation also increases to compound the scatter.

15. Patient thickness • Normally body thickness is out of our control but we can change the method of imaging to improve image quality. • With obese patients, tissue thickness is reduced when taking the film recumbent due to compression. • Be sure and measure the patient recumbent.

16. Types of Beam Restricting Devices • There are three types of beam restricting devices. • Diaphragms • Cones • Collimators

17. Types of Beam Restricting Devices • Large field sizes result in more scatter radiation that reduces image contrast.

18. Aperture Diaphragm • Aperture diaphragms are basically lead or lead lines metal devices placed in the beam to restrict the x-rays emitted from the tube.

19. Aperture Diaphragm • Apertures are the simplest form of collimation. • In this case, the aperture is used to reduce exposure to the breast tissue.

20. Aperture Diaphragm • The width or size of the aperture is fixed and can not be adjusted. • The operator must be careful when placing the aperture in the beam.

21. Cones and Cylinders • Cones and cylinders are modifications to the aperture. • Cones are typically used in dental radiography.

22. Cones and Cylinders • Most cone produce a round image on a rectangular film. • Cones are very effective at reducing scatter. • Hard to center.

23. Variable Aperture Collimator • Proper collimation of the x-ray beam has the primary effect of reducing patient dose by restricting the volume of tissue irradiated.

24. Variable Aperture Collimator • Proper collimation also reduces scatter radiation that improves contrast.

25. Light Localizing Collimator • The light localizing variable aperture collimator is the most common beam restricting device in diagnostic radiography.

26. Collimator • Not all of the x-rays are emitted precisely from the focal spot. • These rays are called off-focus radiation and they increase image blur.

27. Collimator • First stage shutters protrude into the tube housing to control the off-focus radiation. • Adjustable second stage shutter pairs are used to restrict the beam.

28. Collimator • Light localization is accomplished by a small projector lamp and mirror to project the setting of the shutters on the patient.

29. Collimator • The light field and x-ray beam should match to avoid collimator cut-off. • A scale on the collimator is used to match the beam to the film size at fixed SID’s.

30. Collimator • Many newer collimators a bright slit of light is provided to properly center the beam and the film. • Units manufactured between 1974 and 1994 has motorized shutters.

31. Collimator • A sensor in the Bucky and the motor were used to automatically collimate the image to film size. This was called a positive-beam limiting (PBL) device. • Required by the FDA.

32. Collimator • Requirement was repealed in 1994. • If the beam is not centered to the film, collimator cut-off will occur on the top or bottom of the image.

33. Collimator • If the tube is not centered to the Bucky or the film is not pushed into the Bucky, side to side collimator cut-off will occur.

34. Collimation Rules • California required three borders of collimation to be seen on the film. • Collimation must be slightly less than film size or to the area of clinical interest, whichever is smaller. • ANY exposure beyond the film is unnecessary patient exposure.

35. End of Lecture