Aluminium Filter Techniques in Radiography
Introduction
Filter ApplicationsOne of the limitations of "plain film" radiography is that the X-ray beam is, for practical purposes, uniform in its intensity. A notable exception is the anode heel effect, but this is limited in its utility and effectiveness. Using aluminium filters enables the utilisation of an X-ray beam which is graduated in its intensity. The shaped aluminium filter causes variable absorption of the X-ray beam across its change in thickness. The uses are only limited by the imagination of the radiographer .
Lateral Lumbar Spine
Anatomy Filter Type Foot- DP, obl
HandAxial Calcaneum Hip, Pelvis Lateral horizontal-ray hip Lateral decubitus abdomen and decubitus barium enema abdomen Lateral Lumbar Spine Lateral thoraco-lumbar spine Lateral Sacrum/coccyx AP Thoracic spine Lateral Thoracic spine Lateral Cervical Spine PA Chest (breast compensation) AP shoulder banana filter (see this filter in the photograph at the bottom of the page) Lateral facial skull, facial bones, nasal bones OR
Oblique mandible AP Mandible
 | This lateral lumbar spine radiograph was taken with an aluminium filter attached to the Light Beam Diaphragm. The filter was positioned with its thickest end at the thoracic level. The effect of the filter is to cause a relative increase in the exposure to the L5S1 anatomy, thereby producing a more even optical density across the image. The use of an aluminium filter for lateral lumbar spine radiography is advantageous in terms of efficiency and reduction in radiation dose to the patient. The radiographer in this case has potentially saved the patient from being exposed to a large radiation dose associated with a coned lateral L5S1 view. Note that this is a radiograph not a digital image... a darkroom was involved! |
Lateral Thoracic Spine
 | This lateral thoracic spine image was taken with a "humpback" aluminium filter attached to the Light Beam Diaphragm. The filter is centred to the mid-thoracic vertebra. Note that this is a modified lateral thoracic spine position. This positioning technique is discussed on another page in this wiki. |
AP Thoracic Spine
 | This AP thoracic spine image was taken with an aluminium filter attached to the Light Beam Diaphragm (LBD). The filter is centred to the mid-thoracic vertebra with the thickest end at the top of the image. A breathing exposure technique is used to blur out the soft tissue structures. Note- this is a radiograph not a digital image It is also noteworthy that this affect is now achieved using digital radiography without aluminium filters. . |
Oblique Foot
 with aluminium filter |  without aluminium filter | These images compare an oblique (DPO) foot image with an aluminium filter to a comparable image without a filter The aluminium filter was positioned with its thickest end at the toes. The effect of the filter is to increase the exposure to the carpus, calcaneum and talus. This technique overcomes the problem of over-exposure of the phalanges, or, conversely, underexposure of the hind foot. The edge of the aluminium filter is faintly visible (arrowed) These are radiographs not digital images |
PA Thumb
 | This thumb radiograph was taken using an aluminium filter over the middle and distal phalanges. The thin edge of the filter is faintly visible (arrowed) This is a radiograph. Is this a Bennett's fracture, a Rolando fracture or neither? Search this wiki for more information. |
AP Mandible
 | This is an AP mandible non-digital radiograph taken using two identical aluminium filters. The filters are arranged with their thinnest edges touching at the midline in a butterfly wing configuration. This is a radiograph not a digital image Butterfly filter configuration shown below  |
Lateral Horizontal Ray Hip
 | This lateral hip image was taken with an aluminium filter attached to the Light Beam Diaphragm. The filter was positioned with its thickest end distally. The effect of the filter is to reduce the radiation exposure to the femur The combination of round cone and aluminium filter are powerful tools in improving the image quality of high exposure projections. Note that this is a radiograph taken with a stationary grid and is not a digital X-ray image Note the patient's neck of femur is not well demonstrated because the patient has an impacted neck of femur fracture |
Barium Enema Decubitus Abdomen
 | This image was taken using an aluminium filter with its greatest thickness positioned to correspond with right side of the abdomen. This is a radiograph not a digital image Further Reading
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Advantages of Filters
Aluminium filters are one of the most useful aids available to the plain film radiographer. A selection of filters in different thicknesses , shapes and tapers allows the radiographer to select a filter whenever there is anatomy of significant varying thickness to be imaged with a single exposure. Aluminium filters are quick and easy to use. They are also long-lasting and they have the potential to save on unnecessary expenditure on film (one exposure instead of two).
The thicker the aluminium filter, the greater the beam hardening. This will reduce the radiation dose to the patient by absorbing some of the lower energy photons from the X-ray beam
Disadvantages of Filters
The aluminium filter may produce an artifact if the edge of the filter is projected onto the film/receptor. In practice, this is of minimal concern as the image of the filter's edge is usually barely perceptible. If the radiographer is concerned that the artifact from the edge of the filter will be confusing to the radiologist, it should be labelled on the film/image.
The aluminium filter will remove lower energy photons from the X-ray beam. Whilst this will reduce the radiation dose to the patient, it will also cause a reduction in image contrast. This is not usually a problem, but is worthy of consideration when using thick filtration, and will be most noticeable when two filters are "piggy backed" together. Of course, patient anatomy is better displayed at reduced contrast rather than not at all.
A full set of aluminium filters is not cheap, but they do not get worn out with use.
The filters used on this page are all available from Gray-Tech filters in Australia. To give you some idea of the range, see the image below
My understanding is that Gray-Tech filters will send filters anywhere in the world. Check their website at <a class="external" href="http://www.gray-tech.com.au/filter_F600.html" rel="nofollow" target="_blank">http://www.gray-tech.com.au/filter_F600.html.</a>
(see thread below for further information)
I have no commercial arrangement or association with Gray-Tech filters.
Discussion
The utility of aluminium filters has waned with the introduction of digital radiography. The ability of CR and DR to display a larger range of tissue densities has largely obviated the need for aluminium filters. Despite this fact, aluminium filters still have a role in the digital X-ray suite, albeit in a diminished capacity. It is also noteworthy that many countries are still using non-digital imaging techniques and will probably continue to do so for many years to come.
The use of aluminium filters was a significant part of the craft of radiography prior to digital imaging and its loss of utility is lamented by many an old school radiographer. For those craftsman involved in developer and fixer radiography, be careful of what you wish for!
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