Grids Part 2 (RADS 230)

Linear Grid

has lead lines that run in only one direction

-most popular

Crossed or cross-hatched grid

lead lines that run at right angles to one another removing more scatter radiation

Which grid pattern has a higher contrast improvement factor?

the crossed grid with 2 times the ratio

One disadvantage to crossed grids is?

the positionin of the grid, it is critical and the CR must coincide with the center of the grid and no angulation can be used as it will cause grid cut off in all four direction

Grid cut off

the grids absorption of remnant radiation, not scatter, that is needed to create the image

-absorbs the good stuff

Linear grid will allow...

anglulation (cephalic & caudal) along the length of the lead lines unlike the crossed grid, but angling across the lead will result in serious cut off

Two subcategoties of linear grids (Grid Focus)

Focused grid or Parallel (nonfocused grid)

Grid Focus

the orientation of the lead lines to one another

Focused Grid

lead lines are angled to match the angle of divergence of the primary beam

-used 99% of the time

Focal distance (grid radius)

distance between the grid and the convergent point; used to determine the focal range of the focused grid

- SID grid was constructed to use

Convergent line

imaginary line formed by connecting convergent points along the length of the grid (both ends of the focused grid)

Focal Range

the recommended range of SIDs that can be used with a focused grid

ex. when the focal distance is 40in., the focal range is 36-42, it must include the number of the focal distance

Parallel (non-focused) grid

a grid with lead lines that run parallel to one another, but the beam still diverges so there will be grid cut off at the periphery of the grid

-can be partial or complete

Which linear grid is more expensive, parallel or focused grid?

Focused grid, but it is better

Grid cutoff with a parallel grid on the lateral edges is worse/increased when the SID is...

shorter/closer, because the x-rays have to divert faster to fill the entire IR so the lines flatten more

The parallel linear grid is primarily used in...

fluoro and mobile imaging because we are less aware of what out SID is exactly and with focused you have to know the SID & you have to be perfectly centered

Characteristics of a Focused Grid

allows more transmitted photons to reach IR, more difficult to manufature and harder to use, has a recommended range of SIDs and the SID has to be in that range (Focal Range), Susceptible to many errors.

Charcteristics of a Parallel Grid

grid cutoff on lateral edges, easiest to manufacture and use, has a minimum SID only, few grid errors possible

Long Dimension Grid

lead stips parallel to long axis (17 of the 14x17)

Short Dimension Grid

Lead strips perpendicular to long axis/ parallel to short axis (14 of 14x17)

When angleing the tube with a grid we must know...

the orientation of the the grid lines patter whether long or short dimension and we have to angle with the lead lines not against the CR because then they will be blocked and we'll have grid cutoff

ex. doing a potable chest with a grid, usually we do those CW but if we have a long dimension grid we wouldn't be able to angle cephalic or caudal because it would block the CR so we would have to do it LW to not have that grid cut off. But if it was short dimension we would be able to do the exam CW just fine.

Short dimension grids are useful for...

examinations where it is difficult to correctly center the CR for the long dimension grid

ex. lateral projections of skull, spine, hips, portable CW chest

Types of grids

stationary and moving (reciprocating)

Grid Cassette (stationary grids)

IR that has a grid permanently mounted on its front surface

Grid Cap (stationary grid)

contains a permanently mounted grid and allows the IR to slide behind it

Lead lines will be visible on the image with what type of grid?

Stationary grid

Moving Grids

the xray machine vibrated the grid during the exposure so grid lead lines are blurred out and not seen on the image

-moves perpendicular to long axis of grid, focused or parallel

Moving grids were developed by

Hollis Potter

Two ways moving grids can move

Reciprocating grid & Oscillating Grid

Reciprocating Grid

driven back and forth by a motor in 2 directions (approx. 2 cm)

Oscillating Grid

2-3cm tolerance on all sides (all 4 directions) between frame and grid. Four springs in each corner/ electromagnet oscillates the grid in a circle.

Disadvanatges of Moving Grids

costly, subject to failure, may cause magnification because the components create distance between grid and IR, image blur

-but still superior to stationary

A focused grid is usually why type of grid?

moving grid, but can be either or

Most common error in use of grids

improper positioning

4 grid situations that should be avoided:

-Off-Level Grid

-Off-Center Grid

-Off-Focus Grid

-Upside Down Grid

Off-level grid error

Occurs when there is no perpendicular relationship between the grid and the CR b/c of an angled/incorrect grid or tube

Off-level grid error results in

total grid cutoff across the entire image, underexposure, and decreased IR exposure

Off-level grid error is seen with what linear grid focus?

focused or parallel

Off-center grid error

occurs when the CR is not centered to the grid to match the beam divergence, CR is shifted laterally (lateral decentering)

Off-center grid error results in

patial grid cutoff across the entire image,underexposure, and decreased IR exposure

Off-center grid error is seen with what linear grid focus?

focused grid only because we have to be perfectly centered to match the beam divergence, which is why it's not used for mobile imaging

off focus grid error

occurs when grid is not positioned directly to the focal range to match beam divergenge and SID is outside of the focal distance (using an SID outside of recommemded focal range)

-parallel grids have worse grid cutoff at shorter SIDs deacresing IRE

off focus grid error is seen with what linear grid focus?

focused and parallel because we know grid cutoff is worse for shorter SIDs and the SID must be withing the focal range for that grid

upside down grid error

Total/severe grid citoff on lateral borders, but exposed center

-rare, only with focused grid

Mobile errors include:

off level (thick matress, heavy pt.), off center (blind centering to grid b/c pt. covering IR), and off focus (failure to measure SID during portables)

Grid supression

the computer removes grid lines from the image

Selection of grid with proper ration depends on many important factors:

-kVp (greater than 90 select 8:1 or higher)

-Body part (greater than 10 cm use a grid)

-Patient Dose

-Positioning Latitude

air gap technique

Alternative to using a radiographic grid to reduce scattered radiation during certain examinations

-increase OID typically 10 -15cm ~ 8:1 grid, mAs is increased abt. 10% for every cm of air gap

disadvantage of air gap technique

more magnification due to increased OID, but can be decreased by increasing SID

Virtual grids

Scatter reduction by digitally reconstructing a radiograph, pt is nearly half og what it would be with a grid

-will eventually replace physical grids

Air gap technique is most effective with..

lower kVp exams