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Filtration
1)
Filter: piece of metal (typically aluminum) located between
the x-ray tube and the collimator box and in the path of the primary
beam
2)
Purpose: to remove non-diagnostic, low-energy photons from
the primary beam which in turn reduces skin dose to the pt
3)
Filters will cause partial absorption/attenuation of the
x-ray beam
a)
Attenuation/absorption: reduction in the total number of
x-ray photons remaining in the beam after passing through a given
thickness of material
4)
Inherent vs added filtration
a)
Inherent: caused by glass, oil, tube housing, port or window
i)
Roughly 0.5 mm Al equivalent
b)
Added: caused by collimator – thin sheets of ~1.0 mm Al
c)
NCRP recommendations
i)
2.5 mm Al equivalence filtration for tubes operating above 70
kVp
5)
Filtration will reduce exposure rate
6)
Filtration affects beam quality/energy/penetrating ability
a)
Average beam energy/penetrability
i)
Depends on:
(1)
kVp
(2)
Amount of total filtration in the beam
ii)
kVp also determines the minimum wavelength of the beam
iii)
Filtration determines the maximum wavelength of the
beam
iv)
Increasing either kVp or filtration will increase the average
energy of the beam, allowing it to be more penetrating and of higher
quality
b)
Beam quality is measured by its half-value layer (HVL)
i)
HVL directly measures beam quality by determining actual
penetrating ability
(1)
Federal regulation states that at 80 kVp, the half-value
layer must be 2.34 mm equivalent
(2)
Mammography has different standards because of the desire to
keep the softer x-rays; regulation call for an HVL of 40 mm
equivalence at 30 kVp
ii)
HVL: that thickness of a specified material (usually a metal)
which reduces the exposure rate to one-half its initial value
iii)
The HVL principle is utilized when extending technique charts
c)
Because filtration causes the beam to be more penetrating,
increasing filtration:
i)
Decreases density
ii)
Decreases exposure rate
iii)
Decreases contrast
7)
Types of filters
a)
Thoreau’s
i)
Compound filter used in therapy
ii)
Compound materials include tin, copper, and aluminum
iii)
250 – 450 kVp
iv)
The layering order of these metals (tin is closest to the
tube, aluminum closest to the pt) is important due to characteristic
radiation; new x-rays formed in the first layer are absorbed by the
next layer; aluminum’s characteristic radiation is absorbed in the
air
(1)
Characteristic radiation
(a)
Incoming electron collides with an inner shell electron of
the target material, displacing that electron from it’s shell
(b)
An electron from a higher shell will drop down to fill the
newly created space
(c)
Energy given off is a characteristic x-ray, called
characteristic because its energy is characteristic of the target
element and its involved electron shell
b)
Compensating filter
i)
May be made of metal or a plastic compound (EX:
boomerang)
ii)
Used where there is difficulty imaging body parts due to
varying tissue thickness and composition
iii)
A wedge filter, shaped as it is named, allows for greater
attenuation of the beam at its thicker end
(1)
Usually made of aluminum
iv)
A trough filter, lower in the middle than at sides, is used
in chest radiography to allow for greater filtration over the lung
tissue and less over the mediastinum
v)
Computer radiography incorporates its own compensating
filtration
Scatter Radiation
1)
Factors affecting the amount of scatter:
a)
Patient thickness
b)
Tissue density
i)
Total volume of body tissue = length x width x height
(1)
Length is determined by the thickness of the part
(2)
Height and width are determined by collimation
(3)
Controlling tissue volume is done via tissue compression
and/or adjusting field size
c)
Field size
d)
KVp
i)
As kVp is increased, more energy is able to reach the film
and so more scatter is produced
Beam Limitation
1)
Volume of tissue determines the amount of s/s radiation
a)
Volume = thickness x area
2)
Increased collimation means:
a)
Decreased volume of tissue irradiated
b)
Decreased s/s radiation
c)
Decreased fog
d)
Decreased density
e)
Increased contrast
f)
The effects of collimation are more evident with thick body
parts and non-grid exposures
3)
Beam limitation protects the patient from unnecessary
radiation
4)
Increasing beam limitation will decrease density, with all
other factors constant
5)
Beam limitation improves visibility of detail with technique
compensation
a)
Needed only with extreme increases in collimation, such as
going from a collimation of 14x17 to collimation of 5x5
6)
Beam limitation is the most effective method for limiting
scatter
7)
Beam limiting devices include
a)
Aperture diaphragm
i)
Essentially a metal disk with a hole in its center
ii)
Major disadvantage is that the aperture diaphragm allows more
penumbra and off-focus radiation
b)
Collimator
i)
Comprised of 2 independently-acting sets of adjustable lead
shutters
ii)
A mirror angled 45° and light bulb are set up to indicate
alignment of the central ray
(1)
If the mirror’s angulation is off, the collimator light will
not be true to the actual exposure field
iii)
Adjustable shutters allow collimated shapes to match the
shapes of cassettes
iv)
Helps to limit penumbra
v)
The collimator is the most effective of beam limiting devices
c)
Cone
i)
Disadvantage includes allowing a penumbra
ii)
Cones are useful for headwork, L5-S1 spot, sunrise, and other
small parts
8)
Positive beam limitation (PBL): automatic collimation which
automatically adjusts to the cassette size
Grids
1)
The purpose of the grid is to absorb scatter and increase
image contrast
2)
Grids are located between the patient and the film
3)
Grids absorb scatter which has already been produced
4)
Construction:
a)
Thin lead strips alternate with interspacing material
b)
Interspacing
i)
Organic (carbon-based) interspacing absorbs moisture and can
potentially warp (EX: fiber, paper, cardboard, plastic)
ii)
Inorganic interspacing is much more durable and absorbs more
radiation (EX: aluminum and the less-visible lead)
5)
Types of grids
a)
Linear
i)
Comprised of one set of lead strips extending in parallel
fashion in one direction
ii)
Strips are aligned with the long axis of the grid or the long
axis of the table
b)
Crossed / cross-hatch
i)
A second set of lead strips is set perpendicular to the first
set
ii)
This grids will not allow for the use of any tube angle
c)
Parallel
i)
Lead strips are set parallel to one another
ii)
These grids allow cut-off along the edges at shorter SIDs
d)
Focused
i)
Grid strips are angled progressively as they move further
from the grid center in order to coincide with the shape of the beam
ii)
Convergence line: imaginary line in space created by
extending the edges of angled lead strips until they meet
iii)
Grid radius / focusing distance: distance from the
convergence line to the grid
(1)
A focal range will be given on the grid
iv)
Stationary and bucky grids use linear focused grids
e)
Rhombic
i)
A type of crossed grid in which grid strips are angled with
respect to one another
6)
Grid characteristics
a)
Grid ratio
i)
Grid ratio = height of lead strips / distance between strips
ii)
The grid ratio indicates how well the grid cleans up scatter
(1)
Higher ratios mean higher absorption of scatter
iii)
Higher grid ratio means greater need for precision when
centering in order to avoid grid cut-off (increased ratio means
decreased latitude)
iv)
As grid ratio increases, mAs will need to be increased to
maintain density
v)
As grid ratio increases, contrast will increase
b)
Bucky factor / grid factor (bf)
i)
The bucky factor defines the requirement for increasing
exposure factors to maintain density with the use of a grid
c)
Grid frequency (gf)
i)
Grid frequency indicates the number of lead strips in an inch
or centimeter
ii)
As frequency increases, the strips get thinner
iii)
Grid frequencies most used in diagnostic radiography are 85 –
103 lines per inch
iv)
Thinner strips are not as visible on images, but they are not
as effective in cleaning up scatter
v)
If two grids have an equal ratio, the one with the fewer, and
thus thicker, strips will be the more efficient grid, although its
gridlines will be more visible
d)
Contrast improvement factor (gk)
i)
GK = contrast with a grid / contrast without a grid
ii)
Useful GK numbers range from 1.5 to 3.5
iii)
As the grid factor increases, the contrast improvement factor
increases
e)
Grid selectivity (gΣ)
i)
Grid selectivity = % of primary beam transmitted / % of
scatter transmitted
ii)
This number describes grid efficiency
iii)
Grids absorb around 20% to 40% of the primary beam
7)
Grid selection and use
a)
Use a grid with body parts measuring 10 centimeters or more
b)
Use a grid with kVp values over 60
Air Gap
1)
Air gap is defined by a 6” to 10” OID
2)
Air gap may be used in consideration of scatter reduction
over use of a grid since the space traversed by scatter radiation
allows it to miss striking the image receptor
3)
One disadvantage of air gap technique is magnification
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