Chapter 11

sunglowcitrineΠολεοδομικά Έργα

15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

77 εμφανίσεις

RAD 350 Chapter 11 Control of Scatter Radiation


Scatter radiation


while it is our enemy, is also responsible for about 50% of the overall
blackening of the image

Key factors (bullets) regarding scatter, scatter production, control of, and energy
levels, etc.:

-
Scatter radiation is INCREASED as each of the following INCREASE: kVp, field size, part
thickness (area being imaged)


Thick body parts may be compressed to improve spatial
resolution and increase contrast and radiographic detail


i.e. mam
mography

-
Increased scatter = DECREASED CONTRAST

-
Increased kVp = increased COMPTON AND DECREASE P.E.

-
Increased kVp = increased energy of scatter reaching the film

-
Increased field size = increased scatter


Beam Restricting devices:

-
aperature diaphragm (
lead sheet with a whole in it)

-
cone


like a “cheerleader megaphone”

-
extension cylinder

-
positive beam limiting device (PBL) automatically adjusts the size of the collimated
size to that of the cassette/film/image receptor size (can cone in more than the

size
of the receptor size, but NOT larger


Collimation reduces patient dose and impr
oves contrast resolution


Filtration
-

as previously discussed two types of filtration exist: inherent (inherent in the tube’s
window) and added. The added filtration is
in the collimator assembly


Also of note: the variable collimator and PBL units have “first stage and second stage
(trimmer) shutters/collimators. Also note the position of the collimator light bulb and the
mirror to reflect the light.

A light beam/rad
iation beam congruency test may be done to assess whether the light beam is
actually where the radiation beam is.


The GRID

General thoughts before beginning: Increased scatter = increased fog and DECREASED
contrast!

Amount of scatter is DIRECTLY affected
by kVp, field size, thickness of irradiated field.


A grid is made of radiopaque material (lead) and a radiolucent material )aluminum or other
“spacer” material


-
high quality grids can absorb up to 90% of scatter (remember though


as scatter is
absorbed,

density is DECREASED so MORE mAs must be used to make up for the scatter
absorbed by the grid).



Grid ratio=
height of lead strips



To the space BETWEEN the lead strips



-
increased grid ratio = INCREASED “clean up” of scatter


-
Grid FREQUENCY = lines p
er inch


usually as frequency goes up, so does “clean up”


-
General grid ratios are 8:1 or 10:1 (the higher kVp’s used


like in a dedicated chest
room, the HIGHER the grid ratio that is used


mammo uses low kVps thus lower grid ratios =
2:1 or 4:1)


CON
TRAST IMPROVEMENT FACTOR = compares contrast with a grid to without a grid


BUCKY FACTOR = attempt to measure penetration of BOTH scatter and PRIMARY radiation
through the grid


-
the HIGHER the grid ratio, the HIGHER the bucky factor


-
bucky factor INCREA
SES with INCREASING kVp

GRID SELECTIVITY = ratio of TRANSMITTED primary radiation to TRANSMITTED SCATTER


-
basically said, the HEAVIER the grid, the HIGHER it’s selectiviey and the MORE efficient
in cleaning up scatter.


Types of grids:


-
Parallel


simpl
est of all the grid types; parallel grid lines separated by interspace
material; lead strips are true VERTICAL while the x
-
ray beam is a diverging (angled) beam

with
only the CENTRAL BEAM vertical. This produces GRID CUT OFF


especially at short SID’s.
Grid
cut off prohibits radiation from reaching the film. In Potter
-
Bucky mechanisms, the grid lines
MUST run with the LONG direction of the table. Only tube tilts WITH the direction of the grid
lines can be used. The moving Potter
-
Bucky diaphragm will r
emove the grid lines from
happening as the grid moves from side to side during exposures.



-
Crossed (sometimes called cross hatch) grid


made of two parallel grids superimposed
over one another with the strips of one perpendicular to the other


THE MAIN
DISADVANTAGE OF PARALLEL AND CROSED GRIDS IS GRID CUT OFF!



-
Focused grid: a focused grid is designed to be used at ONE SID only! The lead strips are
slanted to match the angle of the beam at that “said SID” ANY SID other than the “said SID”
will result

in grid cut off.


IMPROPER POSITIONING OF THE GRID with relation to the divergent beam is the MOST
common grid problem

Focused grid problems:


-
off level : grid cut off across image; underexposed LIGHT image


-
off center: grid cut off across image;
underexposed LIGHT image


-
off focus: grid cut off toward EDGE of image


-
upside down grid: (always a board question!!!) BIG cut off toward BOTH edges of the
image with GOOD image in the center.


Grid selection is based upon: kVp’s to be used (dedicated CS
R room uses all high kVp’s


thus a
14:1 or 16:1 grid ratio); degree of clean up desired, patient dose (remember when grids are
used, pt. dose goes up).


“Air
-
gap technique” certain body parts like a lateral C
-
spine will permit the image
receptor/film to b
e placed several inches from the body part (due to the shoulders, the c
-
spine
is actually several inches away from the film/receptor). The space results in scatter diffusing
in all directions away from the patient and NOT hitting the film/receptor


thus

no need for a
grid!