MOD 6 - Grid and Scatter Reduction

Scatters’ influence on IQ

degrades contrast resolution by creating a “fog“ as they do not accurately represent the internal structure of the object, since they come from different paths directly from the source

scatter vs. blur

• blur: sharpness/detail, magnification, distortion

• scatter: exposure, subject contrast, level of noise

Three factors that contribute to scatter production

• kV selection

• collimated field size

• density of anatomy

Effect of increased kV

increased photon energy causes

• decreased photoelectric absorption AND Compton interactions BUT Compton scatter decreases less → produced compton scatter has more energy to reach the detector

• increased energy causes produced scatter to have a higher chance of reaching the receptor

effects of tissue density

as tissue density increases → the chance of photon and matter interaction increased → increased scatter production

effects of field size

as field size increases → more tissue area is exposed to radiation → more tissue interaction → more scatter generation

how to reduce the effect of field size

using collimation

Grid design

• wrapped in Al casing

• consisted of fine lead strips that absorb scatter xrays; placed // and spaced evenly

• interspace between lead strips made of radiolucent materials (Al or plastic fiber material)

Function of Grid

• absorb scattered photons while allowing transmitted and mildly off focus scatter photons before the receptor → improves image contrast

Consideration of Grid Usage and PT Safety

grids will also absorb useful beams, therefore we need to increase our exposure factor to compensate for the lost useful beams → which increases PT dose

Types of Grids

• parallel

• focused

• crossed

• oscillating

Parallel / Linear Grid Construction

spaced out, vertically orientated lead strips

Parallel / Linear Grid Usage

for smaller body parts (eg. hip joint)

grid cut off

when some useful photons are absorbed by the grid because of the diverging beam, more prominent on the sides

how to reduce grid cut off

reduce beam divergence

• increase SID

• tight collimation

Focused Grid Design / Function

lead strips are aligned to the path of the divergent beam reducing grid cut off

Considerations when using Focused Grids

• use recommended SID = focal range/distance

• CR has to be centered to the centre of the grid

Crossed Grid Design

two parallel grids stacked on top of each other

Crossed Grid Function

effective at reducing scatter therefore improved IQ, but still exhibit grid cutoff at the periphery (like parallel grids)

Crossed Grid Disadvantage

substantial technique increase, therefore not commonly used in radiography

Oscillating Grids Design / Function

either parallel or focused design within a motorized case that jiggles the grid during the exposure to improve scatter absorption while simultaneously blurring the appearance of grid lines

Grid Ratio

shows us how much scatter radiation absorbing ability a grid has (greater h / higher ratio = more lead strips = more absorption)

• h (height) / D (distance)

Higher Grid Ratio increases

• scatter absorption

• PT exposure

• potential for grid cut off

Grid Frequency

considers the number of lead strips per inch or cm

Higher grid frequency means and will cause

decreased interspace width → lead strips becomes closer → increased useful beam absorption → increased exposure factors → increased PT dose

Grid Conversion Factor Formulas

GCF Chart with Grid Ratios

Types of Grid Errors

• grid cut off

• grid lines

6 Ways Grid Errors Occurs

• inherent

• off-level

• off-center

• off-focus

• backwards grid

• grid lines

Inherent Grid cut off and how to reduce effect

occurs at the periphery of parallel grids when collimation is opened to the maximum field width

reduce effect by

• reducing beam divergence

• replace parallel grid with focused grid

Off-level and how to reduce effect

• occurs when the grid is tilted or off-level resulting in majority of the useful beam being absorbed

• hence, portable grids MUST be perpendicular to the CR

Off-center and how to reduce effect

• occurs when the grid is off-center causing grid cut off

• ensure that the CR is ALIGNED and CENTERED to the grid

Off-focus and how to reduce effect

• occurs when the incorrect SID (focal range) is applied to a focused grid causing major grid cut-off

• ensure correct focal range is applied

Backwards Grid and how to reduce effect

• occurs when portable focused grid is applied backwards → near complete image cut off due to opposing angles of the divergent beam and the focused grid lines

• ensure grid is not on backwards

Grid Lines and how to reduce this effect

• inherent grid artifacts when using a portable grid as there is no oscillating or reciprocating mechanism that serves to blur the distinct lines

• not much can be done to eliminate these lines

Field of View (FOV) on CR/DR

FOV on CR/DR is the component of the whole imaging plate

Significance of FOV

allows us to determine the Pixel size

Pixel Size Formula / Calculations

• Pixel size = Fixed FOV/Matrix Size

  • Fixed FOV = CR plates whole size

  • Matrix Size = pixel area arrangement

  • Pixel size = micron size pixel that can be displayed

Visualized field of view

also known as the collimated field; the changing field of view when collimating

Pixel size and image distortions

define pixels in our receptor image are being redistributed across the corresponding pixels in the monitor when displayed, causing our original image to visually magnify if a smaller image size (finger) is being displayed onto a large monitor, and vice versa

Collimators / X-ray Beam Restrictions Function

• to decrease the field that a patient is being exposed to

• acts as an added filtration for the x-ray beam

Old xray beam restriction designs

cylindrical or cone shaped metallic structures added to the base of the x-ray tube

Modern xray beam restriction designs

collimator box added to the x-ray housing's port hole

Collimator Composition

• several sets of lead shutters (controllable with external knobs)

• light bulb

• mirror

Effects of Multiple layers of collimators

• helps with the image sharpness

• gives multiple possibilities to reduce the amounts of off-focus radiation from reaching the patient

Effects of Collimation

• lowers exposed field on PT and receptor

• increases visual quality of images as it reduces field size

• reduces noise

Buffer region

2.5cm region around the edge of the interested region that should be included to compensate for the misalignment of the collimator lightbox

Positive Beam Limitation (PBL)

sensor systems that determine the size of a cassette (if CR) and limit the collimator from expanding beyond the border sizes of the imaging plate

PBL and Collimation

regardless of PBL, collimation should still ALWAYS be done manually by the tech as per our SC35 safety code manual from Health Canada