Chapter 5

Filter

 

 

Filtration is the process of increasing the mean energy of the x-ray beam by passing it through an absorber. The primary x-ray beam is polychromatic, that is, the beam contains a spectrum of photons of different energies and the average energy is one-half to one-third of the peak energy. Many of the photons produced are low energy and, if they escape through the glass window of the tube, they are absorbed by the first few centimeters of tissue and contribute nothing to the exposure of the film. Only the higher energy photons can penetrate the patient and reach the film to assist in making the radiograph (Table 5-1) . The dose of radiation received by the patient is highest in the first few centimeters of tissue because of absorption of this low energy portion of the x-ray beam. The amount of scattered radiation is higher with an unfiltered beam because of the number of low energy photons. So, it is advantageous to both the patient and to the technician to use a filtered x-ray beam.

The x-ray beam is filtered by absorbers at 3 levels;

The technician has no control over the inherent filtration or over the patient. He can ensure that added filtration is present to absorb the low energy portion of the primary x-ray beam. Ideally, the filtration would absorb all of the low energy photons from the beam and transmit all high energy photons. This ideal can only be partially reached. 

5.1 Inherent Filtration

This is the filtration that results from the absorption of the low energy portion of the x-ray beam as it passes through the x-ray tube and housing. The amount of inherent filtration by an x-ray tube is strongly dependent on the wave form of the tube current and the tube voltage. The materials responsible for inherent filtration are:

Inherent filtration is dependent also on tube voltage and wave form (Trout, Kelley, and Furno, 1956; Reinsma, 1960). 

Table 5-1

X-RAY ATTENUATION*

 Attenuator

% of Beam Remaining

Target

100

Glass (.78 mm Al equiv.)

6.6

Oil (.07 mm Al equiv.)

5.9

Bakelite (.05 mm Al equiv.)

5.6

Added Filter (0.5 mm Al equiv.)

4.2

Collimator mirror and face plate (1.0 mm Al equiv.)

2.4

Air (negligible loss)

2.4

30 cm patient

0.01

Grid (varies with grid type)

0.01

Film

<0.01

*(Trout, 1963)

 

Inherent filtration is usually measured in aluminum equivalents. The filtering material is compared with the amount of aluminum that is necessary to cause the same amount of attenuation of the primary x-ray beam as the material in question. 

5.2 Added Filtration

The filtration resulting from other material placed in the primary x-ray beam in addition to the inherent filtration is called added filtration. This can be altered by the technician. Materials used are dependent on the energy of the x-ray beam and represent a compromise. Aluminum (atomic number 13) is used for low-energy radiation producing units such as would

 

Table 5-2.

MATERIALS RESPONSIBLE FOR INHERENT FILTRATION.*

Absorber

Thickness (mm)

AluminumEquivalents(mm)

glass envelope

1.4

0.78

insulating oil

2.36

0.07

bakelite window

1.02

0.05

Total

-

0.90

  * (Trout, 1963)

be found in most diagnostic x-ray facilities. Copper (atomic number 29) is used for high-energy radiation producing units in conjunction with aluminum. The aluminum absorbs the characteristic radiation that originates from the interaction of the x-ray beam with the copper. The characteristic radiation from copper is 8 keV, which is energetic enough to reach the skin and increase skin dosage. The characteristic radiation that originates from the interaction of the primary x-ray beam with aluminum is 1.5 keV and is absorbed by the air gap between the filter and the patient. 

5.3 Effect of filtration

Recommendations from the National Council on Radiation Protection and Measurements for total filtration are (NCRP 1136) 

kVp Level

Total Inherent and Added Filtration

< 50

0.5 mm aluminum

50-70

1.5 mm aluminum

> 70

2.5 mm aluminum

In most veterinary radiology facilities, it is necessary to have added filtration of 1.5 -2.0 mm of aluminum to ensure that the total filtration meets the recommendations listed above.

 Effect of Filtration on the X-Ray Beam Filtration of the primary x-ray beam reduces the total number of x-ray photons in the beam but, more importantly, it selectively removes a greater portion of the low-energy photons that do not contribute to the production of a radiograph (Fig. 5-1, Table 5-3) . The intensity of the x-ray beam on the high energy side of the spectrum is reduced somewhat but not to as great an amount as noted on the low energy side of the spectrum. 

Effect of Filtration on Patient Exposure (Table 5-4)Exposure dose to the skin for comparable density radiographs of a phantom (18 cm thickness) with various thickness of aluminum filtration are listed. The 

 EFFECT OF FILTRATION ON PHOTON ENERGY.

Filtration of the x-ray beam results in the absorption of the low energy portion of the beam. Note that the effect of the filter is less in the higher energy portion of the beam.

 

Table 5-3

AVERAGE PHOTON ENERGY FOR A 85 kVp BEAM

(0.5 MM AL INHERENT FILTER).*

Added filter

Average photon energy

none

29.3 kev

0.5 mm A

32.5 kev

1.0 mm Al

33.8 kev

1.5 mm Al

35.6 kev

2.5 mm Al

38.5 kev

 *(Trout, 1963)

 

The exposures were made at 60 kVp without any filtration. Then the exposures were made with 0.5, 1.0, and 3.0 mm of aluminum added. The exposure dose to the skin is cut up to 80 percent by the addition of 3.0 mm of aluminum. The importance of this to the patient is easily appreciated (Trout, et al, 1952).

Another study evaluated the radiation intensity associated with mAs with added filters of different thickness. Average settings required to radiograph a dog's abdomen are 5 mAs and 70 kVp. With no added filtration, the exposure in air at the table top would be 66.5 mr while the exposure with 2.0 mm of aluminum added filtration would be 16.0 mr. The difference at lower kVp settings is even more impressive. An exposure at 50 kVp and 5 mAs results in 38.0 mr if no added filtration is used and only 7.5 mr if 2.0 mm aluminum is added (Trout, 1952). 

Effect of Filtration on Exposure Factors Filtration results in reduction in the intensity of the x-ray beam. Even though most of the reduction is in the lower energy photons, there still is the necessity of increasing the exposure time or mA setting to compensate for the small loss of the higher energy photons. Even with this increase in radiation from increased machine settings, the patient receives less radiation during production of the radiograph than from the unfiltered beam. The effect on exposure factors is given so that you may more fully understand the value and the effect of filtration on the primary x-ray beam. You should not be concerned about having to remember the increase in exposure time needed to compensate for the filtration, since a filter of given thickness is placed in the primary beam and is not changed. Your

 

 

TABLE 5-4.

EFFECT OF FILTRATION ON PATIENT EXPOSURE.

 

Aluminum Exposure Dose Filtration to SkLn (mr)

60 kVp

% Decreasein Exposure Dose

None

2,380

--

0.5 mm

1,850

22

1.0 mm

1,270

47

3.0 mm

465

80 

*(Trout et al, 1952)

 

 

Table 5-5.

EFFECT OF FILTRATION ON EXPOSURE TIMES.*

60 kVp- lOO mA 130 kVp-100 mA

Aluminum Filtration

Exposure Time(sec)

% Increase in Exposure time

Exposure Time

%increase in Exposure time

None

1.41

--

0.12

--

0.5 mm

1.61

14

0.12

--

1.0 mm

1.64

17

0.12

--

3.0 mm

2.14

52

0.12

--

 *(Trout, 1952)

 

technique chart is developed with the filter in place, and you do not have cause to remove the filter during normal radiographic procedures.

Exposure times for comparable density radiographs of a phantom (18 cm thickness) for low and high energy radiation with various thicknesses of aluminum filtration are listed (Table 5.5). Even with the heaviest filtration (3 mm of aluminum), the exposure time need only be increased by approximately 50 percent. The filtration has much less effect on the 130 kVp exposures and no change in exposure time is needed (Trout, 1952). 

5.4 Wedge and Trough Filters

Because of the great unevenness of tissue thickness encountered in veterinary radiography, it is desirable to consider the use of wedge or trough filters. These filters are made of aluminum or lead acrylic that is 30% lead by weight and fully transparent, and have the same effect on the primary x-ray beam as the filters discussed before . A difference exists in the uneven thickness of the filter that causes unequal absorption of the primary x-ray beam and creates an x-ray beam of uneven energies. The value in the use of this type of filter comes in radiography of parts of unequal thickness or mass. Less radiation is required to make the exposure of the thin or less dense part and more radiation is require to make the exposure thicker of more dense parts. Adaptor plates that fit into existing cone tracks or magnetic mounting are used to hold the filters.

The wedge filter creates an x-ray beam that is more energetic and contains more photons on one side of the x-ray field. This is especially of value in small animal radiography if there is a requirement to radiograph the abdomen on a deep-chested dog in a dorsoventral direction. The

region of the stomach and liver is much thicker than the region of the urinary bladder. By using the wedge, it is possible to direct the higher energy portion of the beam toward the cranial aspect of the abdomen, while the less energetic portion of the beam is used to make the exposure of the thinner caudal abdomen. The caudo-cranial radiograph of the stifle of the horse is an example of a similar problem in large animal radiography, where a highly penetrating beam is needed proximally but not as strong a beam is needed more distally near the tibia. The wedge can be placed to supplement or cancel the heel effect.

The trough filter uses the same principle of unequal absorption of the primary beam but creates a different pattern of radiation. The edges of the field are less energetic than the center. This is of primary value in making the dorsoventral study of the thorax. The exposure required on the edges of the radiograph are not as great because of the radiolucent lung tissue. However, the heart, mediastinum, and vertebral column create greater tissue density in the center of the field and require greater exposure for a diagnostic study. The unequal field of radiation partially compensates for this marked difference in tissue density. 

5.5 Beryllium Window Tubes

In some research studies, it is desirable to make exposures with a very soft (low kVp) x-ray beam. This would be useful in making exposures of thin tissue specimens of several cm thickness. A beryllium window tube can be profitably used in this type of work because it produces an x-ray beam with low energy and the tube has a small inherent filtration. This is because the glass window normally found in the x-ray tube is replaced by a window made of beryllium (atomic number 4). In addition, there is no tube housing or insulating oil to act as an inherent filter. Because of its low atomic number, beryllium absorbs much less of the low energy portion of the x-ray beam than does glass found in a tube envelope of conventional x-ray tubes. Recently, mammography has become an important diagnostic tool in examination for breast cancer in women. Because of the requirement of visualizing only soft tissue shadows, beryllium window tubes have been used to excellent advantage.

References

  • Trout, E.D.: The Life History of an X-ray Beam. Radiol. Tech., 35:161-170, 1963
  • Trout, E.D., Kelley, J.P. and Cathey, G.A.: The Use of Filters to Control Radiation Exposure to the Patient in Diagnostic Roentgenoloty. Amer. J. Roentgenol., 67:942-952, 1952.