Beam-Restricting Devices

• Three factors contribute to an increase in scatter radiation:

• Increased kVp

• Increased Field Size

• Increased Patient or Body Part Size.

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.

Relative Contributions of Scatter to the Radiographic Image

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

kVp Photoelectric Compton Total %

Transmission

60 70 % 30% >99% <1%

70 60 % 40% > 99% < 1%

80 46% 52% 98% 2%

90 38% 59% 97% 3%

100 31% 63% 94% 6%

110 18% 83% 93% 7%

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.

Beam-Restricting Devices

• There are three principal types of beam restricting devices:

• Aperture Diaphragm

• Cones & Cylinders

• Collimators

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.

Kilovolt Peak

• As x-ray energy increases, the relative numberof 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.

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

Field size

• Scatter radiation increases as the field size increases.

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.

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%.

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.

Tissue Thickness

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

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.

Types of Beam Restricting Devices

• There are three types of beam restricting devices.

• Diaphragms

• Cones

• Collimators

Types of Beam Restricting Devices

• Large field sizes result in more scatter radiation that reduces image contrast.

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.

Aperture Diaphragm

• Apertures are the simplest form of collimation.

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

Cones and Cylinders

• Cones and cylinders are modifications to the aperture.

• Cones are typically used in dental radiography.

Cones and Cylinders

• Most cone produce a round image on a rectangular film.

• Cones are very effective at reducing scatter.

• Hard to center.

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.

Variable Aperture Collimator

• Proper collimation also reduces scatter radiation that improves contrast.

Light Localizing Collimator

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

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.

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.

Collimator

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

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.

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.

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.

Collimator

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

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.

Collimation Rules

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