RAD 254 Chapter 14Control of Scatter

Break down into: Those that reduce patient dose and those that are

geometrical in nature and those not

3 factors affecting scatter (primary)

• Increased kVp

• Increased field size

• Increased patient thickness

Spatial Resolution & Contrast Resolution

• Spatial resolution may be thought of as geometric in nature (focal spot size, emission spectrum, etc. dealing with geometric image formation

• Contrast resolution – driven by scatter and other sources of “noise”

Scatter

• Increased field sizes = more scatter – collimation is the most readily available and easiest thing to lower the amount of scatter

• Patient thickness also increases scatter – compression may be used to help avoid this (IVP’s and mammo are examples)

Beam restricting deviceslimit the radiation to the patient

• Aperature diaphram (size and resultant field size are a direct proportion – draw the damn picture and figure the problems)

• Cones and cylinders – great for absorbing scatter, but are circular shaped = great for improving contrast and removing scatter BUT requires much more mAs as a result

Variable aperture diaphram

• Mandated in 1974 by the US Dept of Food and Drug Administration (mandate removed later)– Positive Beam Limitation Devices (PBL’s)

• Automatically collimate to the size of cassette/receptor in the bucky and CANNOT be a bigger size than the film/receptor

Filtration

• Filtration also will decrease the low energy rays and limit patient dose and some scatter

The GRID

Only “FORWARD” scatter is of any benefit to the radiographic image – all

other scatter degrades the image!

Scatter = LOWER contrast

• Using a grid (alternating strips of fine leaded strips with alternating radiolucent interspace material) can effectively reduce the amount of ANGELED scatter from reaching the film/recepter

Grid terms

• Grid ratio = height of the lead lines divided by the interspace WIDTH

• Grid frequency/lines per inch = the more lines per inch, the more clean up

• Grid clean up = scatter w/o a grid vs scatter reaching film with a grid AKA “Contrast Improvement Factor”

• Grid function = improve image contrast

Bucky Factor

• Refers to the AMOUNT of radiation to the patient with a grid vs W/O a grid.– Higher the grid ratio the higher the “bucky

factor”– The higher the kVp, the higher the “bucky

factor”

• Grid weight refers to how heavy it is – duh – the more lead, the heavier it is

Grid Types

• Parallel

• Crossed (cross-hatch)

• Focused– Focused - crossed

Grid Problems

• Grid cut-off = short SID’s result in the vertical, parallel strips absorbing the “diverging” beam at the outer margins of the grid/film/rescepter MOST pronounced at short SID’s

• Most grid problems are “positioning” related– Uneven grid/off level grid– Off centered (lateral decentering)– Off focus grid– Upside down focused grid

Focused Grid Misalignment

• Off level = grid cutoff across image; underexposed image (light)

• Off center = ditto

• Off focus = CR centered to one side or the other of a focused grid

• Upside down grid = severe grid cut-off (no density) at both sides of the image

Grid Ratio Selection

• 8:1 grid is the most widely used

• Grid ratio is kVp driven– Higher kVp’s warrant higher grid ratios– Higher grid ratios = higher patient dose (more

radiation needed to produce an image)– As kVp increases past MAXIMUM OPTIMUM

kVp, patient dose INCREASES

mAs – Grid considerationsAs grid ratio increases, so much

mAs

• 5:1 grid = 2 X mAs

• 8:1 grid = 4 X mAs

• 12:1 grid = 5 X mAs

• 16:1 grid = 6 X mAs

Air gap technique

• By allowing the scatter radiation to “diffuse” in the atmosphere after the patient but BEFORE the film results in a higher contrast image as the scatter diffuses and does NOT reach the film

– C-spine lateral is a good example of this