·
Filtration is the process of
shaping the X-ray beam to increase the ratio of photons useful for imaging to
those photons that increase patient dose or decrease image contrast.
·
Overall effect is an increase
in the mean energy of the XR beam
·
Increase contrast and decrease
patient dose
·
In a radiological examination
the x-ray beam is filtered at three different levels:
1.
The x-ray tube and its housing
(inherent filtration)
·
A Beryllium window may be used
to decrease filtration
2.
Sheets of metal placed in the
path of the beam (added filtration)
·
Ideally, absorb low-energy
photons, transmit high-energy photons
·
Utilize the photoelectric
effect (attenuation) to do this – by selecting a material with appropriate Z#.
·
Single filter or compound
(consisting of 2 or more layers of different materials with higher Z# material
faces XR tube)
·
Filtration expressed using the
equivalent of aluminum of a certain thickness
·
Filtration varies with
thickness (increase thickness – increase filtration, but over a certain
thickness for certain kVp increases in thickness may not have any significant
advantage over lower thickness)
o
<50 kVp: 0.5 mm Al
o
50 - 70 kVp: 1.5 mm Al
o
>70 kVp: 2.5 mm Al
·
See fig 6-1 C: pg 89
·
Increase filtration (increase
mean energy): Decrease exposure dose
·
Major disadvantage of
filtration is reduction in the intensity of the XR beam (see fig 6-1 C: pg 89)
·
Compensation filters (e.g Wedge
filters; Trough filters; Bow-tie filters in CT) obtain films of more uniform
density when the object part thickness changes from one side to the other
·
K-edge filters (heavy metal
filters) make use of the K-absorption edge of elements with Z# greater than 60
which offers advantages when imaging barium and iodine
o
Barium and iodine offer
greatest contrast when they absorb the XR efficiently
o
K-edge filters produce an XR
beam which has a high number of photons in the specific energy range of the
barium or iodine k-absorption edges.
o
Fig 6-3 and 6-4 C: 91
o
Molybdenum filters are use
specifically in mammography
3.
The patient
·
The polychromatic x-ray beam from the tube contains a high number of
low energy photons
·
Low-energy photons have a negligible chance of getting through the
patient, thereby contributing to patient dose, but adding nothing to the image
·
Some of the very low energy photons are stopped as they exit the
tube by the glass window (inherent filtration)
·
Beryllium provides very little filtration and is used as a window in
mammography (where low-energy photons are needed)
·
Filters may also be added to increase the filtration effect – these
are designed to preferentially absorb the low energy photons
·
Added filtration does not affect the maximum energy of the x-ray
beam spectrum
·
Added filtration will always reduce the x-ray tube output
