Beam Restricting Devices

Sergeo Guilbaud

Abstract

Beam limiting devices are used to control the amount of scatter radiation that is created as a result of Compton’s interaction with the body’s tissues. Scatter hinders the visualization of detail by adding additional density to the radiographic image. Scatter is also responsible for increasing the patient’s radiation dosage, decreasing image contrast and impairing the visibility of detail. By controlling the amount of scatter radiation that is produced, there is improvement in detail, enhancement of contrast and a reduction in radiation dosage to the patient. Beam limiting devices are types of radiographic tools used to reduce the production of scatter radiation.

Objectives

At the conclusion of this lecture, each participant should be able to:

1. Identify the principle factors that are responsible for the creation of scatter radiation.

2. Identify the three types of Beam limiting devices.

3. Identify the advantages and disadvantages of each beam limiting device.

4. Identify the rationale for using Positive Beam Limitation (PBL).

5. Describe the ancillary devices that may be used to further reduce the amount of scatter radiation reaching the image receptor system.

Principle factors creating scatter

Kilovoltage (kV)

Irradiated Tissues

Principle factors creating scatter

Kilovoltage (kV) Affects beam penetrability

As kVp increases, the percentage of x-ray photons that undergo Compton’s interaction increases.

B/C Compton interactions are responsible for creating scatter, as the kVp increases, the percentage of primary photons that will scatter also increases.

Principle factors creating scatter

As kVp increases, the percentage of photons that undergo photoelectric absorption decreases. Thus, the patient receives

significantly less radiation.

Principle factors creating scatter

Scattered photons from Compton’s interaction are of no value to the radiographic image b/c they do not assist in demonstrating the anatomic structures of interest. (add additional density to film)

The majority of the overall radiographic density on a film is created by scattered photons.

Thus, a decrease in Compton’s scatter will improve the image quality by removing the scatter that reaches the film.

Principle factors creating scatter

Irradiated Tissue Tissue volume and atomic number

affect the amount of scatter produced during a radiographic exposure.

Tissue volume irradiated is controlled by the radiographer.

When the amount of tissue volume increases, so does the amount of scatter radiation.

The larger the field size, the more photons to interact with tissue, this results in greater amount of scatter production.

Principle factors creating scatter

Remember: More scatter is created in bone than in soft

tissue. When the beam is restricted, less scatter

radiation is produced thus, less scatter reaches the film.

The technical factors may also have to be manipulated.

There may need to be an increase in the technical factors to compensate for the reduction in the overall density on the film.

Beam Restrictors Three types: 1. Aperture diaphragms. 2. Cones/cylinders. 3. Collimators.

Aperture Diaphragm

A flat sheet of metal with a hole cut in the middle attached to the tube head. The simplest of beam limiting devices

both in design and ease of use. This was the original type of beam

limiting device. Use is now limited to dedicated chest

units, where field size and SID do not vary.

Aperture Diaphragm

Aperture Diaphragm

Advantages: 1. Simple design 2. Low cost 3. Ease of use

Disadvantages 1. Increase in

penumbra. This is a result of the

diaphragm’s close proximity to the target.

An increase in off-focus (stem radiation) radiation reaching the film. This can result in images, similar to shadows beyond the field.

Aperture DiaphragmFormula for determining diaphragm field size.

Projected image size =SID X diameter of diaphragmdx from focal spot to diaphragm

e.g. A diaphragm is placed 5” from the FS. The diameter of the opening in the aperture is a 2” circle. What would be the projected image size if the SID is 40”?

40 x 2 = 80 -------- --- = 16 inches 5 5

Cones/Cylinders

Circular aperture diaphragms with metal extensions.Cone – has a flaring extension w/ upper diaphragm smaller than bottom (flared end).Cylinder – does not flare but, may be equipped w/ an extension sleeve which can expand or collapse to cavy the beam restriction.

Cones/Cylinders

Cones/Cylinders

Advantages:1. Inexpensive.2. Simple to use.3. Reduce penumbra and off-focus

radiation.4. Provide better beam restriction at a

greater distance from the focal spot.

Cones/Cylinders

Disadvantages: Fixed field sizes unless equipped with

extension sleeve. Flared cones are no better at reducing

penumbra than aperture diaphragms.

Cones/Cylinders

Formula for determining Cone field size.

Projected image size = SID x lower diameter of coneDx from focal spot to bottom of cone

e.g. The diameter of the lower rim of a cone is 3”. The bottom of the cone is 16” from the focal spot. What will be the size of the projected image at 40” SID?

40 x 3 120Projected image = -------- = ----- = 7.5 “ circle

16 16

Collimators

Collimators Consists of two sets of lead shutters at right

angles to one another which move in opposing pairs.

Each set moves symmetrically from the center of the field.

They can be adjusted to correspond to an infinite number of squares or rectangular field sizes.

Bottom shutters serve to reduce penumbra along the periphery of the beam.

Upper shutters help reduce amount of off-focus radiation.

CollimatorsConstruction:Two sets of shutters moving

in opposite pairs.Light field provided by light

source.Mirror mounted at 45

degrees within path of beam to reflect light source. Provides additional beam filtration.

Collimators

Advantages:1. Permit infinite number of field

sizes.2. Provides light source as an aid to

proper tube, part and film alignment.

3. Only one device providing all said functions.

Collimators

Disadvantages:1. Expensive to manufacture.2. Light requires replacement with heavy

usage.3. Whenever light source is changed,

proper alignment must be checked (light-field congruency test.

4. Collimator accuracy testing must be performed at regular intervals.

Positive Beam Limitation (PBL) Devices

Automatic Collimators Automatically reduce the collimated area to

the size of the film placed in the Bucky tray. When operating properly, they leave a small

unexposed area on all four borders of the film.

State regulations allows the collimation to fall within plus or minus 2% of the SID.

Can be overridden by the radiographer. Assure that area exposed does not exceed

image receptor size.

Ancillary devices used to reduce the scatter reaching the film

Normally designed to fill a special need. Lead blockers Lead Masks

Ancillary Devices

Lead Blockers A sheet of lead-impregnated rubber

that can be cut to a size or shape. Especially useful in preventing scatter

radiation from reaching the film especially when imaging the Thoracic & Lumbar spines in the projection and the shoulder when done on the table.

Lead Masks A sheet of lead-impregnated rubber

that can be cut to a size or shape to correspond to a particular field size then, secured to the end of a collimator.

Most common: lead mask used during cerebral angiography.

References

Bushberg et al, The Essentials of Physics and Medical Imaging, Williams & Wilkins Publisher.

Bushong, S., Radiologic Science for Technologists, Physics, Biology and Protection, 8th Edition, C.V. Mosby Company.

Carlton et al, Principles of Radiographic Imaging, An Art and Science, Delmar Publishing.

Curry et al, Christensen’s Introduction to the Physics of Diagnostic Radiology. 4th Edition, Philadelphia, PA, Lea and Febiger, 1990.

Selman, J., The Fundamentals of X-Ray and Radium Physics, 8th Edition, Charles C. Thomas Publisher.