Collimation And The Beam

Question 1

Beam restrictions

Answer 1

• device that attaches to the opening in the x-ray tube housing to regulate the size and the shape of an x-ray beam
• designed to control and minimize scatter by limiting the x-ray field size to only the anatomical structures of interest

Question 2

Purpose of beam restriction

Answer 2

• limiting patient exposure
• reducing scatter produced within patient
  
  • increasing contrast on image

Question 3

True or false: beam restriction and collimation are synonymous

Answer 3

TRUE

Question 4

Off-focus radiation

Answer 4

• ideally all filament electrons are striking the anode at the focal spot
• but some stray electrons can stray, and interact on other areas of the anode
  
  • this creates x-rays called ‘off focus radiation’
  • this type of radiation if not ideal, increases image blur, which is not ideal for image quality
  • ‘fuzzy edge’ of image: penumbra
    
    • “zone of unsharpness”
    • like blurry edge of a shadow
  • the variable aperture collimator is designed to reduce off focus radiation

Question 5

Penumbra

Answer 5

• penumbra and off-focus radiation are reduced as the beam restrictor distance is increased from the tube port
• as the distance of the beam restrictor increases from the port the more the penumbra is reduced
  
  • cylinder is more effective at reducing penumbra because it restricts the beam for a greater distance

Question 6

Beam restrictors

Answer 6

Three types

• aperture diaphragm
• cones/cylinders
• variable aperture collimator

Question 7

Aperture diaphragm

Answer 7

• simplest
• flat piece of lead or lead lined material that fits onto the x-ray tube head
• opening is usually just less than the size of the IR used
• field size controlled by: size of the opening, shape of the opening (circular, square, or rectangular) and SID
• advantages: inexpensive, easy to use
• disadvantages: need multiple diaphragms to achieve different field sizes, because it is so close to the focal spot, larger area of unsharpness around image (penumbra increased)

Question 8

Cones and cylinders

Answer 8

• modified aperture diaphragm
• extension cone or cylinder attached to a aperture diaphragm
• field size determined by the size and position of the distal end
• usually circular
• used only in special situations
• cone has a flared extension: upper diameter smaller than lower diameter
• cylinders have no flaring, same diameter from top to bottom
• advantages: inexpensive, easy to use, helps decrease penumbra around image (cylinder slightly more effective)
• disadvantages: fixed field size, shape of field is round, alignment is critical between x-ray source, cone, and IR, cones: if angle of flange is greater than angle of divergence of primary beam, not effective at restricting primary beam

Question 9

Variable aperture collimator

Answer 9

• best and most commonly used beam restrictor
• all modern general x-ray units utilize this system
• allows the most variation and flexibility in field size adjustments
• two sets of shutters, entrance shutters (limits the x-ray beam like an aperture diaphragm), purpose: to control off focus radiation
• equipped with a light source and a mirror
• light is intended to be an accurate representation of where the primary x-ray beam will be projected during procedure
• misalignment of mirror can cause anatomical structures to be cut off of the image

Question 10

What is an alignment test is a tech is concerned about the alignment of the light field and radiation field?

Answer 10

9 penny test

Question 11

Positive beam limitation (PBL)

Answer 11

• ‘automatic collimation’
• when an IR is placed in the bucky tray, automatic collimators mechanically adjust the size and shape of the radiation field to match that of the IR
• a way of protecting the patients from overexposure, but does not replace manual collimation by tech
• purpose is to ensure the field size does not exceed that of the IR
• can be built into APR (automatic programmed radiography), so when an exam is selected, the field size will automatically change

Question 12

Compton scatter interaction

Answer 12

• scatter is produced during a compton interaction
• scattered photons are not part of the useful beam
• scatter decreases the quality of the image by adding density over the entire image that isn’t related to the anatomy (losing contrast and appearing grey and dull)
  
  • image fog
  • no useful information

Question 13

Methods of reduction

Answer 13

Three primary factors that can influence the intensity of the scatter reaching the image

• kVp
• field size
• patient thickness

Question 14

Beam restriction

Answer 14

• as the x-ray beam is restricted through collimation, the amount of tissue being irradiated is decreased, results in: improved image quality, reduced patient dose
• as the field size is decreased, less scatter radiation is produced within the patient
• less patient anatomy is being irradiated, so fewer comptons interactions are occurring
  
  • less scatter reaches the IR
• “more matter, more scatter”

Question 15

Field size and density

Answer 15

• when the field size is decreased, what is happening to the density on the image? Decreasing because:
• fewer photons reaching IR
• scatter, although not useful, contributes to the density of the image
• when collimating, some of the photons have been removed due to the field size: remember we don’t ‘squeeze’ the beam when it is collimated

Question 16

Compensating for collimation

Answer 16

• increase in collimation affects the number of x-rays reaching the IR to produce the latent image
• smaller field size means fewer photons hitting patient, which results in less scatter being produced
• therefore, exposure factors should be increased when collimation is increased

Question 17

Which exposure factor?

Answer 17

• kVp is typically not increased (will increase the proportion of scatter interactions in the patient, therefore decreasing the image contrast
• mAs SHOULD BE ALTERED to maintain the exposure (will only affect density on the image, not contrast!)