Beam-Restricting Devices

Transcription

1 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 kvp Photoelectric Compton Total % Transmission % 30% >99% <1% % 40% > 99% < 1% 80 46% 52% 98% 2% 90 38% 59% 97% 3% % 63% 94% 6% % 83% 93% 7%

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 relative number 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 Scatter radiation increases as the field size increases. Field size

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 The relative intensity of scatter radiation increases with increasing thickness of the anatomy. Tissue Thickness

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 with compression.

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 Apertures are the simplest form of collimation. Aperture Diaphragm

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. It is always designed for a particular film size and SID Chest room has a fixed aperture diaphragm

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

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 If the beam is not centered to the film, collimator cut-off will occur on the top or bottom of the image. Collimator

33 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. Collimator

34 Collimation Rules Collimation must be slightly less than film size or to the area of clinical interest, whichever is smaller.