1725 Scatter Radiation

The four x-ray paths

• X-rays interact with patient and scatter away from the receptor

• X-rays interact and are absorbed (Photoelectric Absorption) within patient

• X-rays are transmitted through patient without interaction and strike receptor

• X-rays interact with patient (Compton Scatter) and scatter toward the receptor

Compton Scatter

X-Radiation produces noise, reducing image contrast and contrast resolution. Makes image less visible

What is Scatter Radiation

• Scatter exit radiation (Compton interactions) that reaches the image receptor does not provide any diagnostic information about the anatomic area

• Scatter radiation creates unwanted exposure on the image called fog (noise) without adding any patient information

• Scatter radiation decreases radiographic contrast

Three factors that contribute to increased scatter production

• Increased kVp

• Increased x-ray field size

• Increased patient thickness

Two principal tools used to control scatter

• Beam-restricting devices (Collimator Box)

• Grids (Controls scatter after it leaves patient and before it gets to IR)

Increasing the volume of tissue irradiated results in increased scatter production

• Patient thickness

  • Increased thickness will increase the volume of tissue irradiated

• X-ray beam filed size

  • Increased field size will increase the volume of tissue irradiated

How does higher kVp affect scatter radiation

It increases the energy of scatter radiation exiting the patient

How kVp affects scatter

• As x-ray energy increases, absolute number of Compton Interaction decrease; because using a higher kVp increases x-ray transmission

• But the number of photoelectric interactions decreases much more rapidly; reducing Overall absorption

• Thus, relative number of x-ray that undergo Compton scattering increase; Therefore increased using higher kVp results in scatter reaching the IR

• The amount and energy of scatter radiation exiting the patient depends on the kVP selected

• Exams using higher kVp produce a greater proportion of higher energy scattered x-rays

Field Size

• As field size increases, scatter radiation increases

  • Large field size allow greater amounts of exposed tissue to generate scatter radiation

  • It also decreases contrast

Patient Thickness

• Imaging thick parts of the body results in more scatter radiation

  • With increasing patient thickness, more x-rays undergo multiple scattering

• Types of tissue (muscle, fat, bone) and pathology play a role in production of scatter

• Compression devices reduce patient thickness and brings the object closer to the image receptor

• Compression reduces patient radiation dose, and improves contrast resolution

  • Ex: Mammography

Why do we want to control scatter radiation

• To improve contrast

  • Loss of contrast results from presence of scattered x-rays

• Image formation consists of both transmitted and scattered x-rays

• Also for radiation protection (Not the key reason)

Control of Scatter; Beam Restriction/Field Size Limitation

• Responsibility of radiographer

• As beam restriction increases, field size and patient dose decrease

• Serves two purposes: Limiting patient exposure and reducing the amount of scatter radiation, thus preserving contrast

• The term beam restriction, field size limitation, and collimation are often used interchangeably

  • Decrease in the size of the projection radiation field

Types of Beam Restrictors

• Aperture Diaphragm

• Cones or Cylinders

• Variable Aperture Collimator

Aperture Diaphragm

• Simplest form of beam restrictor

• Flat piece of lead (Diaphragm) that contains a hole (aperture) in the center

• Easy to use; placed directly below the x-ray tube window

• No widespread use

Cones or Cylinders

• Simply an aperture diaphragm that has an extended flange attached

  • Considered modifications of the aperture diaphragm

• Slide onto the tube directly below the window

• The position and size of the distal end act as an aperture and determine field size

  • Useful beam produced is circular

• Used to be used for headwork, seen in dentist offices

Variable Aperture Collimator

• Most commonly used

• Located immediately below window

• 2 or 3 sets of lead shutters

  • 1st stage entrance shuttering device

    • Located below the tube

    • Longitudinal and lateral blades

  • 2nd stage collimator shutter

• Light localization

  • Collimator lamp and mirror

  • Field shape; rectangular or square

Positive Beam Limiting (PBL) Devices

• Also called automatic collimators

• Automatically limits the size and shape of the IR

• Mechanically adjust the primary beam size and shape to the IR

  • X-ray beam is restricted to the image receptor in use

• Previously required by US federal law

  • Served as a way to protect patients from overexposure

  • Regulation removed in 1994

• Even with PBL, technologists should manually collimate more tightly to reduce patient radiation dose, and improve image quality

• Under no circumstances should the x-ray beam exceed the size of the IR

Increased Collimation Results in

• Field size decreases

• Patient dose decreases

• Scatter radiation decreases

• Radiographic contrast increases

Increased Field Size Results In

• Patient dose increases

• Scatter radiation increases

• Radiographic contrast decreases

Control of Scatter; Grids

• Invented in 1913 by Gustave Bucky and continues to be the most effective means for limiting the amount of scatter radiation that reaches the IR

• A device that has very thin lead strips with radiolucent interspaces; it is intended to absorb scatter radiation emitted from the patient

• Places between the patient and the IR

• Not a radiation protection device because it’s after that patient

Disadvantages of grids

• They work well to improve radiographic contrast but are not without drawbacks

• Using a grid requires additional mAs, resulting in a higher patient dose

  • Therefore grids are typically used only when the anatomic part is 10cm (4 inches) or greater in thickness, and more than 60 kVp is needed for the examination

Grid Construction

• Radiolucent interspace material separates the lead lines

  • Interspace material typically is made of aluminum or plastic fibers

• The lead lines and interspace material of the grid are covered by an aluminum front and back panel

• Grid construction is described by grid frequency and grid ratio

Grids contain thin lead strips or lines that have a precise

• Height

• Thickness

• Space between them

Grid Ratio

• The ratio of the height of the lead strips to the distance between them

• Grid ratio = h/D

Range of grid ratios

5:1 to 16:1

High-Ratio Grids

• More effective in reducing scatter

• Increase radiographic contrast

• Increase patient radiation dose

As Grid Ratio Increases

• Scatter cleanup is more effective and radiographic contrast increases

• Patient Dose increases

• Likelihood of grid cutoff increases

Grid ratio decreases

Scatter cleanup is less effective and radiographic contrast decreases

Grid Frequency

• The number of grid strips per centimeter or inch

  • Directly related to grid ratio

  • Higher frequency grids have thinner lead strips

• Can range in value from 25 - 80 lines/cm (60-200 lines/inch)

• A typical value for grid frequency might be 40 lines/cm or 100 lines/inch

Grid Performance

• The purpose of using grids in radiography is to increase radiographic contrast

• In addition to improving contrast by cleaning up scatter, grids reduce the total amount of x-rays reaching the IR

• The better the grid is at absorbing scattered photons, such as with a higher ratio grid, the fewer the photons reach the IR

• To compensate for fewer photons reaching the IR, additional mAs must be used to produce diagnostic images

Grid Conversion Factos (GCF) or Bucky Factor

Can be used to determine the adjustment in mAs needed when changing from using a grid to non-grid (or vice-versa) or for changing to grids with different grid ratios

Mathematical expression of GCF

• GCF = mAs with the grid/mAs without the grid

• When a grid is added to the IR, mAs must be increased by the factor indicated to maintain the same number of x-ray photons reaching the IR

• This requires multiplication by the GCF for the particular grid ratio

mAs multiplication factor Non-Grid

1

mAs multiplication factor 5:1 grid ratio

2

mAs multiplication factor 6:1 grid ratio

3

mAs multiplication factor 8:1 grid ratio

4

mAs multiplication factor 10:1/12:1 grid ratio

5

mAs multiplication factor 16:1 grid ratio

6

Grid Conversion Formula

mAs1/mAs2 = conversion factor1/conversion factor2

Information about a grid’s construction is contained on a label places on the tube side of the grid. The label usually states:

• The type of interspace material used

• Grid frequency

• Grid ratio

• Grid size

• Information about the range of SIDs that can be used with the grid

Grid Pattern

• Refers to the linear pattern of the lead lines of a grid

• Two types of grid pattern exist:

  • Linear

  • Crossed or cross-hatched

Linear Grid Pattern

• A linear grid has lead lines that run in only one direction

• Most popular because they allow angulation of the x-ray tube along the length of the lead lines

Cross or Cross-Hatched Grid Pattern

• Lead lines that run at right angles to one another

• Remove more scattered photons than linear grids because they contain more lead strips that are oriented in two directions

• Disadvantage

  • Applications are limited with a crossed grid because the x-ray tube cannot be angled in any direction without producing grid cutoff (absorption of the transmitted x-rays)

  • Have limited applications in radiography

Grid Types

• The orientation of the lead lines to ones another

• Two types of grid focus exist:

  • Parallel (nonfocused)

  • Focused (Matches the divergence of the x-ray beam)

Parallel Grid or nonfocused grid

• Has lead and interspace strips that run parallel to one another

  • Never intersect

• Less commonly used because the strips do not coincide with beam divergence

Focused Grid

• Has lead lines that are angled, or canted, to approximately match the angle of divergence of the primary beam

• The advantage of focused grids compared with parallel grids is that focused grids allow more transmitted photons to reach the IR

• If the lead strips in a focused grid were extended, the strips would intersect at a convergence line at the convergent point

• Convergence line is an imaginary line

Focused Grid Focal Distance

• Sometimes referred to as grid radius

• The distance between the grid and the convergent line or point

  • The focal distance is important because it is used to determine the focal range of a focused grid

• The focal range is the recommended range of SIDs that can be used with a focused grid

• Ex: a common focal range is 36-42 inches, with a focal distance of 40 inches. Another common focal range is 66-74 inches, with a focal distance of 72 inches

Stationary Grids

When grids are stationary, it is possible to closely examine and see the grid lines on the radiographic image

Reciprocating Grids

• Slightly moving the grid during the x-ray exposure blurs the grid lines

• Reciprocating grids are part of the Bucky, more accurately called the Potter-Bucky diaphragm

• The grid is located directly below the radiographic tabletop, and just above the tray that holds the IR

• Grid motion is controlled electrically by the x-ray exposure switch

• The grid moves slightly back and forth in a lateral direction over the IR during the entire exposure

Grid Cutoff

• A decrease in the number of transmitted photons that reach the IR because of some misalignment of the grid

  • Undesirable absorption of primary x-rays by the grid

• The primary radiographic effect of grid cutoff is a further reduction in the number of photons reaching the IR, resulting in a decrease in radiographic density for a film image or increase in noise caused by a decrease in x-ray photons reaching the digital IR

• May require that the radiographer repeat the image, thereby increasing patient dose yet again

• Grid ratio has a significant effect on grid cutoff, with higher grid ratios resulting in more potential cutoff

Types of Error in Grid Use - Upside Down Grid

• Occurs when a focused grid is placed upside down on the IR, resulting in the grid lines going opposite the angle of divergence of the x-ray beam

• Causes the lateral edges of the IR to be highly under exposed

  • Is easily avoided because every focused grid should have a label indicating “tube side”

  • This side of the grid should always face the tube, away from the IR

Grid Problems Off - Level Grid

Grid cutoff across image; underexposed, light image

Grid Problems Off Center Grid

Grid cutoff across image, underexposed, light image. Dark on one side of the image and light on the other

Grid Problems Off Focus Grid

Grid cutoff more severe at edges. Symmetrical grid cutoff, loss of exposure toward lateral edges of image

Moire Effect

• Also known as zebra pattern, is an artifact that can occur when a stationary grid is used during CR imaging.

• If the grid frequency is similar to the laser scanning frequency during CR image processing, then a zebra pattern can result on the digital image

• Use of a higher grid frequency or a moving grid with CR digital imaging eliminates this type of grid error

Air Gap Technique

• Alternative technique to the use of radiographic grids

• Reduces scatter radiation, enhancing image contrast

• Simple concept: Most scatter will miss the IR if there is increased OID

• The greater the gap, the greater the reduction in scatter reaching the IR

• The mAs is increased approximately 10% for every centimeter of air gap