General Screen Film Radiography and Its Limitations · the radiographic image. A polyenergetic x-ray beam produced within the x-ray tube anode has a low number of high energy photons

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Chapter 2

General Screen Film Radiography and Its Limitations

X-rays have been in use for over a century since the report of their discovery by

Röentgen (1896). In the health and medical areas, x-rays have been used for both

diagnosis and treatment of patients’ conditions. In diagnostic medical imaging

(radiography), x-rays are used in a wide variety of examinations. Examples of

medical imaging examinations can be found in Table 2.1.

Table 2.1 Types of medical imaging examinations using x-rays as the

energy source (Ballinger, 1991; Fauber, 2000; Gunn, 2002)

general radiography – single projection images

fluoroscopy – single projection imaging to display function or anatomical

motion

angiography – a rapid series of single projection images to capture

motion of blood flow

computed tomography (CT) – multiple projection imaging to display a

cross-section of the anatomy

bone mineral densitometry (BMD) – analysis of the bone strength.

X-rays are electromagnetic (EM) radiation with characteristics of short wavelengths,

very high frequencies and very high energy. High energy EM radiation exhibits

characteristics of both waves and discrete bundles of energy called photons. X-ray

photon energy is usually measured in kilo-electron volts (keV) and diagnostic x-ray

energies are typically 10 to 150 keV. Such high energy EM radiation is ionising.

Ionising radiation has potential harmful effects when it irradiates human tissue

(Bushberg et al, 2002; Curry et al, 1990; Graham & Cloke, 2003).

In diagnostic medical imaging, x-ray production and its attenuation in matter is well

described by Bushberg et al, (2002), Bushong (2001), Curry et al (1990), Dowsett,

Kenny & Johnston (1998), Graham & Cloke, (2003), Thompson et al (1994) and

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Webb (1988). An important consideration for this project is that a heterogeneous

intensity x-ray beam results from differential attenuation within the patient’s

anatomy.

The exit intensities of the x-ray photons will depend on the properties of the body

within the irradiated area and the characteristics of the x-ray beam. The mechanisms

that result in reduction of x-ray intensities are the types of interaction with matter,

such as the photoelectric effect and Compton scatter. Exit intensity of the x-ray beam

can be considered as a function of the probability that an x-ray photon will have an

attenuating event. Properties of the body that affect attenuation are effective atomic

number of the anatomical material, electron density of the anatomical material, and

thickness of the anatomical area or the distance the x-ray photons travel through

anatomy. As the effective atomic number of the tissue increases, the probability of an

interaction of the x-ray photon with the anatomy increases; as electron density of the

anatomy increases, the probability of an interaction of the x-ray photon with the

anatomy increases; and as the x-ray photon’s path distance through the body

increases, the probability of an interaction of the x-ray photon with the anatomy

increases. The degree of attenuation is also affected by the energy of the photons of

the entrance x-ray beam. As x-ray photon energy increases, the probability of an

interaction of the x-ray photons with the anatomy decreases (Bushberg et al, 2002;

Curry et al, 1990; Graham & Cloke, 2003).

X-ray beams are comprised of photons of many different energies, and as such are

polyenergetic. A characteristic of an x-ray beam that affects the rate of attenuation,

and hence the exit intensity of the beam, is its effective energy. Effective energy of

the x-ray beam can be considered as the weighted mean of the x-ray photon energies

within the beam (Bushberg et al, 2002; Carlsson & Carlsson, 1984). A common

means of measurement of the effective energy of the x-ray beam is the measurement

of the half value thickness (HVT) of the material that is irradiated. The HVT is the

thickness of the material irradiated that reduces the intensity of the exit x-ray beam to

half of its entrance intensity. Common materials used for the measurement of

effective energy of an x-ray beam are aluminium (Al) and copper (Cu). HVT, at

diagnostic x-ray energies, is typically stated in millimetres of Al or Cu (Bushberg et

al, 2002; Carlsson & Carlsson, 1984).

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Attenuation of polyenergetic x-ray beams can then be considered as a function of

their effective energy or HVT. As the effective energy of x-ray beams increases, the

probability of interactions of the x-ray photons decreases. A higher effective energy

x-ray beam will result in a higher exit intensity than when the effective energy of the

entrance beam is lower.

Subject contrast is the differences in exit intensities of the x-ray beam, within the

irradiated field, that result from different rates of attenuation within the irradiated

body. Large subject contrast results from large differences in attenuation rates. Large

subject contrast will typically result from a low effective energy beam as well as

large differences in attenuation (Bushong, 2001; Fauber, 2000; Gunn, 2002).

Subject contrast produces differences in optical densities that can be visualised

within the resulting image. Optical density differences are known as radiographic or

image contrast. The degree of radiographic contrast depends on the subject contrast

and also depends on factors within the image receptor (Fauber, 2000; Gunn, 2002;

Thompson et al, 1994).

2.1 Filtration and Shaping of the X-ray Beam

Polyenergetic x-ray beams used in medical imaging have a maximum photon energy,

which corresponds to the peak kilovoltage (kVp) applied across the x-ray tube. The

kVp is determined by the radiographer, depending upon the desired characteristics of

the radiographic image. A polyenergetic x-ray beam produced within the x-ray tube

anode has a low number of high energy photons and there are an increasing number

of photons at each energy level as the photon energy decreases from the maximum

energy. This is depicted graphically as the dashed line in Figure 2.1 (Curry et al,

1990). In Figure 2.1, the maximum photon energy is 150 keV which results from

setting 150 kVp across the x-ray tube.

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Figure 2.1 Typical energy spectrum of a 150kVp x-ray beam (Curry et al, 1990,

p.33)

Filtration is the removal of x-ray photons from the beam by attenuation when the

beam is passed through a medium (Bushberg et al, 2002; Curry et al, 1990;

Thompson et al, 1994). There are two types of general filtration: inherent and added

filtration.

X-ray photons are produced by interaction of an accelerated electron with a target

atom. The atoms producing the x-ray photons are often at depth within the target. As

the photons travel within the target itself, attenuation can occur. As the photons

travel further through the x-ray tube, attenuation can again occur within the glass of

the x-ray tube, within the cooling oil that surrounds the glass x-ray tube and within

other parts of the x-ray tube housing and collimator. This is the process of inherent

filtration. Lower energy photons have a high probability of attenuation. Photons with

energies below 15 keV are fully attenuated by the x-ray tube’s inherent filtration

(Bushberg et al, 2002; Curry et al, 1990; Graham & Cloke, 2003; Thompson et al,

1994). The solid line in Figure 2.1 depicts a typical shape of the x-ray spectrum

following inherent filtration of the Bremsstrahlung and characteristic radiation.

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Added filtration is the addition of other material in the path of the x-ray beam prior to

the x-ray beam entering the patient’s anatomy. The prime purpose of added filtration

is the further removal of low energy photons from the x-ray beam. The effect of

added filtration on the shape of the x-ray spectrum can be seen in Figure 2.2 (Curry

et al, 1990). Low energy photons, if allowed to enter the patient’s body, would have

a high probability of being fully attenuated by the anatomy and hence increasing the

absorbed dose to the patient without contributing to the image (Bushberg et al, 2002;

Curry et al, 1990; Thompson et al, 1994).

Figure 2.2 X-ray spectra at 90kVp with added filtration (filtered) and without

added filtration (unfiltered) (Curry et al, 1990, p.89)

Added filtration will remove more lower energy photons from the beam than higher

energy photons. Added filtration assists in lowering the patient absorbed dose.

Another effect of placing added filtration in the path of the x-ray beam is that of

shifting the effective energy of the beam. In Figure 2.2 it can be seen that the

addition of filtration to the beam has increased the effective energy of the beam.

Added filtration can be any material that is placed in the path of the x-ray beam. It is

usually measured in equivalent thickness of aluminium. When the prime purpose is

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dose reduction, added filtration material is of uniform thickness across the entire

x-ray field. Minimum amounts of added filtration are usually regulated by statutory

authorities. In Australia, these standards are regulated by various state Acts and

Regulations such as the New South Wales Radiation Control Act (NSW) (1990), the

Radiation Control Regulation (NSW) (2003) and the Radiation Guidelines 6 (2004).

Aluminium and copper are the most common added filtration material (Bushberg et

al, 2002; Curry et al, 1990; Thompson et al, 1994). Typical thicknesses of added

filtration material in diagnostic radiography are 2 – 3 mm of Al or 0.1 mm of Cu

(Carlsson & Carlsson, 1996). The use of erbium filter material has been reported by

Chakera et al (1982), Shrimpton et al (1988) and Cranage et al (1992). Other authors

(Koedooder & Venema, 1986; MacDonald-Jankowski & Lawinski, 1992; Regano &

Sutton, 1992; Sanborg et al, 1993; Tapiovaara et al, 1999; Villegran et al, 1978)

have reported on the use of other filtration material in medical x-ray use such as

samarium (Sm), gadolinium (Gd), holmium (Ho), ytterbium (Yb), tungsten (W),

yttrium (Y), niobium (Nb) and other metals and materials.

Different filter materials attenuate the x-ray beam in different manners and hence

produce different dose rates to the patient. The prime purpose of many authors

(Chakera et al, 1982; Kohn et al, 1988; MacDonald-Jankowski & Lawinski, 1992;

Regano & Sutton, 1992; Sanborg et al, 1993; Shrimpton et al, 1988) has been to

document the improved dose reduction when particular filter materials are placed in

the x-ray beam.

The choice of filtration material can have effects other than dose reduction in

medical imaging. Kohn et al (1988) used five radiologists to compare image quality

when different filter materials were used in the x-ray beam. The radiologists found

no difference in image quality of skull images when Al, Cu and Y filter material

were used. The radiologists preferred images of the hands when Al was the filter

material used. Image contrast was reduced when Cu filter material was used in

paediatric barium examinations (Hansson et al, 1997). In low kVp dental

radiographic examinations reported by Shibuya et al (2000), filter materials of Al,

Nb, Gd and a composite material of Cu, Al and tin were used. In this case, the

composite material improved diagnostic performance over the other filter materials.

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Williamson et al (1994) reported on the use of K-edge filtration materials, where the

binding energy of the K-edge was within the diagnostic medical imaging range of

energies, between 17 and 68keV. One of the conclusions drawn by Williamson et al

(1994) was that reduced image contrast occurred when K-edge filters were used

compared to when Al filters were used.

2.2 Film/Screen Radiography

X-ray intensities that exit the body are recorded for viewing and storage. In general

radiography, the x-ray image may be recorded using a variety of means. The type of

general radiography is named after the recording media used. The types of general

radiography are listed in Table 2.2.

Table 2.2 Types of general radiography recording media

(Bushberg et al, 2002; Bushong, 2001; Gunn, 2002)

film/screen (F/S) radiography using film and intensifying screens

digital radiography (DR) such as:

computed radiography (CR) using photostimulable phosphors

flat panel systems such as:

direct radiography using photoconductor material with thin film

transistors

indirect radiography using intensifying screens and detectors.

Material that fluoresces under irradiation from x-rays was first noted by Röentgen

(1896). Intensifying screens for use with film as a joint recording medium were first

designed by Thomas Edison (Thompson et al, 1994). The purpose of the intensifying

screen is to convert the x-ray energy to light. The use of intensifying screens

increases the efficiency of exposure of the film over exposure directly by the x-ray

beam. The use of intensifying screens decreases the absorbed dose received by the

patient compared to x-rays directly exposing the film. Films are typically exposed by

95 – 99% light and 1 – 5% x-ray photons when intensifying screens are used. A

measure of the screen’s efficiency is its intensifying factor (IF). IF is a comparison of

exposures with and without the use of intensifying screens to achieve the same

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optical density on the film (Bushong, 2001; Curry et al, 1990; Fauber, 2000;

Thompson et al, 1994 ).

Many different fluorescent materials have been used in intensifying screens.

Different materials fluoresce at different wavelengths of light. Examples of some

fluorescent materials and their spectral emission used in intensifying screens are:-

• calcium tungstate (CaWO4) – blue light (peak response ≈ 420nm);

• gadolinium oxysulphide: terbium activated (Gd2O2S:Tb) – green light (peak

response ≈ 550nm);

• lanthanum oxybromide: thulium activated (LaOBr:Tm) – blue light (peak

response ≈ 380 to 420nm).

(Bushberg et al, 2002; Curry et al, 1990; Graham, 2003)

X-ray films are designed to be sensitive to the specific spectral emission of a given

intensifying screen. Consequently, films and screens are matched for optimised

efficiency of conversion of x-ray photons to optical densities on the film. The F/S

combinations can exhibit different characteristics of conversion efficiency (speed),

spatial resolution or detail visualised in the image. It is generally accepted that high

speed F/S combinations will result in lower spatial resolution. The use of a higher

spatial resolution and lower speed F/S combination will result in a high absorbed

dose to the patient (Bushong, 2001; Fauber, 2000; Gunn, 2002; Thompson et al,

1994).

Optical density (OD) is a logarithm of the ratio of the amount of incident light to the

transmitted light through the x-ray film when viewing the image. OD measurements

on x-ray film range between the base fog of the film, typically an OD of 0.2, to the

maximum density of the film, Dmax, at a typical OD of 3.5 to 4. When the number of

x-ray photons reaching the F/S is high, the OD is high. When there has been no x-ray

exposure to the F/S, the OD measured is that of the base fog of the film. High speed

F/S combinations require fewer x-ray photons to achieve the same OD as lower

speed F/S combinations (Fauber, 2000; Gunn, 2002).

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The OD at a specific point on the film is dependant upon several factors. These

factors are the initial number of photons produced in the x-ray tube, loosely referred

to as exposure; the amount of attenuation of the x-ray beam as it travels through the

body; the F/S speed; and the film processing (Fauber, 2000; Gunn, 2002; Thompson

et al, 1994).

Another characteristic of F/S combinations is F/S latitude. Latitude is the range of

relative exposures required to produce a range of OD. The range of OD is usually

between values of 0.25 and 2.0. Figure 2.3 (Curry et al, 1990, p.159) shows plots of

OD resulting from various exposures reaching the F/S. If a small range of exposures

produces these ODs, the latitude is said to be narrow (Figure 2.3 a.). If a larger range

of exposure produces the same ODs, the latitude is said to be wide or broad (Figure

2.3 b.).

Figure 2.3 Characteristic curves showing: a. Narrow latitude; b. Wide latitude

(Curry et al, 1990, p.159)

The dynamic range of an x-ray film is equivalent to the film’s latitude. The range of

OD on an x-ray film, typically 0.25 to 3.0, is equivalent to a dynamic range of 1:100

(Dowsett et al, 1998).

The effective energy of the x-ray beam can also affect film latitude. An increase of

effective energy of the beam, either through increasing the kVp or through the

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addition of filtration to the beam, will increase the latitude (Bushberg et al, 2002;

Curry et al, 1990; Fauber, 2000; Gunn, 2002).

2.3 Radiographic Contrast

Radiographic contrast is a measure of the difference between the OD of one region in

the image and the OD of another region. Radiographic contrast can be measured

objectively through the use of densitometers, which measure OD at a point on the

x-ray film. Measurement of two ODs at different points on a film will provide a

measurement of radiographic contrast. Radiographic contrast can also be assessed

subjectively. When an x-ray image comprises mainly areas of high OD (black) and

low OD (white), it is referred to as exhibiting high radiographic contrast. When the

image has a broad range of OD, typified by the highest OD not being black and

lowest OD not being white, it is referred to as exhibiting low radiographic contrast or

wide latitude. Radiographic contrast is directly proportional to the angle, γ, of the

straight line regions of the characteristic curve of the F/S (Bushberg et al, 2002;

Fauber, 2000; Gunn, 2002). Comparison of angles of the straight line of the F/S

characteristic curves can be seen in Figure 2.3. Figure 2.3a has a high γ and high

radiographic contrast, whereas Figure 2.3b has a low γ and low radiographic contrast.

Figure 2.4 shows a comparison of high and low radiographic contrast images. The

image in Figure 2.4a has an appearance of strong blacks and whites. It exhibits high

radiographic contrast. The bones of the feet are better visualised in this image than in

Figure 2.4b. Figure 2.4b exhibits low radiographic contrast. This image displays the

soft tissue regions such as muscles better than Figure 2.4a (Bushong, 2001; Fauber,

2000; Gunn, 2002).

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Figure 2.4 Radiographic images with:

a. High radiographic contrast

b. Low radiographic contrast

Radiographic contrast within the image results from the variable attenuation of the

x-ray beam and from the latitude. High radiographic contrast occurs when there are

large differences in attenuation within the x-ray field and/or there is a narrow F/S

latitude or dynamic range and/or there is a low effective x-ray beam energy. Low

radiographic contrast or wide latitude occurs when there are small differences in

attenuation within the x-ray field and/or there is a wide F/S latitude or dynamic range

and/or there is a high effective x-ray beam energy. A low or high effective x-ray

beam energy is set by radiographers as either a low or a high kVp setting. The

amount of added filtration is usually constant within the x-ray tube and as such is not

usually considered to have an effect on radiographic contrast (Bushong, 2001;

Fauber, 2000; Gunn, 2002).

It is a general principle in radiography to maximise radiographic contrast for most

radiographic examinations. Bones are better visualised with high radiographic

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contrast images. Soft tissue examinations and chest radiographic examinations are

exceptions to this general rule. High latitude films and/or high kVp techniques are

often used in the radiographic examination of the chest. Soft tissue such as muscle

has inherent low attenuation differences and hence has low subject contrast within

the tissue. Radiographic contrast is still maximised for soft tissue radiographic

examinations. Low radiographic contrast images will result from imaging soft tissue

anatomical regions (Bushong, 2001; Fauber, 2000; Gunn, 2002).

Radiographic contrast within the image is optimised so as to display the desired

anatomical area under examination. Optimisation is effected through the

radiographer’s selection of appropriate exposure factors such as the kVp and F/S

combination.

2.4 Limitations of Film/Screen Radiography

The dynamic range of a medical imaging system is the system’s ability to record the

signal and represent the anatomical detail. In general radiography, the signal is the

radiation that exits the body. The dynamic range will depend upon attenuation

characteristics of the body. In some instances the F/S recording devices may have a

narrower dynamic range than that of the exit radiation (Dowsett et al, 1998).

In F/S general radiography, radiographers must choose the desired contrast

appearance of the radiographic image. Radiographers may produce a radiographic

image with high contrast or a radiographic image with low contrast. Radiographers

can not produce a single image that demonstrates both high and low radiographic

contrast at the same time. Two radiographic exposures on two films must be made to

achieve an examination that has both high and low radiographic contrast or wide

latitude.

X-ray photon intensities or exposures that exit the anatomy must fall on the linear

region of the F/S characteristic curve. The linear region of the F/S characteristic

curve is shown in Figure 2.3 and is typically between OD values of 0.5 to 2.0. The

linear region of the F/S characteristic curve is effectively the dynamic range of the

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F/S system. If x-ray exit intensities are outside this latitude or dynamic range, any

change in exposure will not result in a proportional change of OD that could be

visualised on the film.

Many regions of the body have areas that have both very high and very low

attenuation within a single x-ray field. Examples of such regions of the body are the

chest, where the lungs have low attenuation and the thoracic spine has relatively high

attenuation and the shoulder where peripheral regions have lower attenuation. Such

anatomical areas have a high dynamic range of exit x-ray intensities. When a low

effective energy (low kVp) x-ray beam is used, the difference in attenuation within

these areas is accentuated, and a very high radiographic contrast image will result. If

the exit beams that emerge from two or more regions are above the exposure

required to produce the Dmax of the image these regions will appear black on the

image. In this example, the exit intensities are above the dynamic range of the F/S

system. A linear increase in exposure will not result in a corresponding linear

increase in OD. No radiographic contrast difference between these regions will be

visualised and as such these areas will be indistinguishable. Similarly, if two or more

regions attenuate the beam so that no x-ray photons exit, the regions appear white

and are indistinguishable. This occurs even if the anatomical regions have different

levels of attenuation characteristics. Here the exit intensities are below the dynamic

range of the F/S system.

The usual solution to this problem in radiography is to increase the effective energy

of the beam. Radiographers increase kVp to increase the so-called penetration of the

beam. As kVp is increased there is a corresponding increase of the x-ray beam’s

effective energy. As the effective energy of the x-ray beam is increased there is a

decrease in attenuation of the beam and an increase in intensity of the exit beam. The

dynamic range of the exit radiation is reduced. The advantage of this method is that

latitude is increased and all anatomy is visualised in one image. The γ of the F/S

system is decreased and the linear response region of the characteristic curve is

increased. The disadvantage is that radiographic contrast will decrease across all

anatomy visualised in the image. The general principle of maximising radiographic

contrast has been upheld. An alternative method could be to undertake two

radiographic exposures to optimise radiographic contrast across all anatomy within

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the field of interest. This approach is not generally undertaken as it requires two

exposures of ionising radiation and an increase in absorbed dose by the patient.

There are many anatomical regions of the body where large attenuation differences

occur or where there is a wide dynamic range within the x-ray field. Some examples

are provided in Table 2.3.

Table 2.3 Examples of regions where wide dynamic ranges exist within the

anatomy (Ballinger, 1991)

Anatomical Region

Explanation of Wide Dynamic Range

chest large attenuation differences between the air-filled lungs and the thoracic

spine

thoracic spine large anatomical thickness differences between the superior and inferior

portions of the thoracic spine

shoulder large anatomical thickness differences between the edges of the

shoulder and the chest region within the image

facial bone lateral image required to display both soft tissue and bony anatomy

cervical spine lateral image required to display both the cervical vertebra and the

cervico-thoracic junction – large attenuating differences between the

cervical vertebra and the cervico-thoracic junction regions

cervical spine lateral image required to display both soft tissue and bony anatomy

thoraco-lumbar

spine

lateral image required to display both the thoracic and the lumbar spine

hip / neck of

femur

cross-table lateral used – large attenuation differences between the

pelvic region and the neck of femur within the image

femur large anatomical thickness differences between the superior and inferior

regions of the femur

feet anatomical thickness differences between the tarsal region and the

metatarsal/phalanges of the foot

hands lateral image required to display both the metacarpals and the

phalanges

abdomen horizontal ray required to show air/fluid differences – attenuation

differences are further increased when barium is introduced to outline

the gut

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2.5 Shaped Tissue Compensation Filters

Filtration material that is non-uniform in thickness may be placed in the x-ray beam

to compensate for the varying degrees of attenuation of the anatomy within the

irradiated area (Bushong, 2001; Curry et al, 1990; Thompson et al, 1994). Curry et al

(1990) describe the use of a wedge shaped filter material. Wedge shaped filters are

occasionally used to obtain radiographic images of more uniform optical density

when a part being examined diminishes greatly in thickness from one side of the

x-ray field to the other. (Curry et al, 1990)

According to Bushong (2001), one of the most difficult tasks facing the radiographer

is to produce an image with a uniform optical density when examining a body part

that varies greatly in thickness or tissue composition. Bushong recommends the use

of a compensating filter when this occurs. A trough shaped filter is recommended for

chest radiographic examinations and a wedge shaped filter for examinations of the

feet.

Thompson et al (1994) recommend the use of compensating filters when there are

large differences in tissue density within the anatomical region and the goal is to

visualise the entire structure without making additional exposures. They describe the

use of wedge filters and trough filters. Figure 2.5 shows sectional planes through

tissue compensation filters (TCFs) and their relationship to the x-ray beam and the

anatomy.

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Figure 2.5 Cross-sections through wedge and trough TCFs (Thompson et al,

1994)

Other authors discuss various other uses and shapes of TCFs. Feczko et al (1983)

used various shaped filters in horizontal-beam lateral decubitus radiograph

examinations of barium filled abdomens. They (1983) performed 30 examinations, of

which 28 showed marked quality improvement through the use of TCFs. The shapes

of the TCFs used are shown in Figure 2.6.

Crow, Guinto & Segura (1983) examined the use of TCFs to compensate for thinner

anatomical regions when undertaking arch aortograms. TCFs have been reported to

improve the quality of radiographs of the shoulder (Vezina, 1985). Gray, Hoffman &

Peterson (1983) and Butler et al (1986) have reported on the use of TCF in

radiographic examinations of the scoliotic spine. A single exposure was used to

obtain excellent quality radiographs of the lower leg using a TCF (Petersen & Rohr,

1987). Marugg et al (1990) designed a holder to allow the use of multiple TCFs

during one radiographic examination.

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Figure 2.6 Cross-sections through tissue compensation filters used for

horizontal-ray barium examinations (Feczko et al, 1983, p.849)

TCFs have proved to be useful devices to assist in overcoming the F/S limitation

where large subject attenuation differences exist within the irradiated field. The

insertion of a varying thickness TCF into the x-ray beam attenuates the beam by

different amounts within the field. There is an assumption that, prior to entering the

anatomy, the x-ray beam is uniform in intensity across the field. Insertion of the TCF

into the entrance beam alters the intensities across the field. The TCF modifies the

intensities within the entrance beam so that the exit beam has a reduced dynamic

range. The result is that the exit beam is more uniform or has a narrower dynamic

range whilst still containing exit intensity differences so that OD differences exist

within the image.

TCF sizes and shapes are predetermined. Radiographers select the most appropriate

size and shape for a particular radiographic examination and patient shape, from a

limited range. Modification to the TCF size and shape can not be made easily.

Goodsitt et al (1998) examined TCF use in mammography. Their object was to

develop a range of TCF shapes to suit individual breast shapes. They concluded that

three or four TCF shapes would be needed to match the contour shape of the

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compressed breast for all mammography examinations. Mammography

radiographers would then be able to select the most appropriate TCF shape for

individual mammography examinations.

The advantage of using a TCF in F/S radiography where there are anatomical regions

with large attenuation differences within the x-ray field, is that all of the anatomy can

be visualised with only one exposure of ionising radiation to the patient.

Radiographers can select an appropriate kVp to optimise radiographic contrast

without the concern of under- or over-exposing areas of anatomy within the x-ray

field. Radiographic contrast is maximised over the entire image whilst still enabling

all anatomy to be visualised.

Radiographic image optimisation is generally within a radiographer’s autonomous

duties, and the appropriate use of TCFs in the radiographic examination is part of

image optimisation. Selection of the appropriately shaped TCF and precise

placement of the TCF within the beam are judgement issues of the radiographer

performing the radiographic examination.

2.6 Computed Radiography

Computed radiography (CR), as an alternative to screen-film general radiography, is

another means of image capture, storage and display in diagnostic radiography. The

first CR system was announced by the Fuji Photo Film Company in 1981

(Schaetzing et al, 1990). Since then such systems have been manufactured and

marketed by a number of different companies. There are many advantages and

disadvantages between CR and F/S. These are discussed in detail in Chapter 4.

Computed radiography uses photostimulable phosphors (PSPs) to capture a latent

image on the imaging plate. PSPs are barium fluorohalides activated with europium

(BaFX:Eu2+ where the halide X is typically bromine or iodine). X-ray photon energy

ionises the Eu atoms and the free electrons become trapped in so called F-centres.

During reading out of the image, a red laser scans and exposes the imaging plate. The

photon energy of the red laser light is absorbed by the F-centres, allowing the trapped

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electron to become mobile and reabsorbed by the Eu atoms. During this de-

excitation, energy is released in the form of blue-green light. A picture element or

pixel is a quantised value that represents a small area at location (x, y) in a digital

image (Baxes, 1994; Jain, 1989). The intensity of the blue green light is recorded and

converted to a pixel value. The resulting CR image is comprised of multiple rows

and columns of pixels representing the x-ray photon intensities at locations (x, y)

(Bushberg et al, 2002; Bushong, 2001; Dowsett et al, 1998; Weiser, 1997).

One advantage of CR over F/S is the increased dynamic range of CR images (Balter,

1990; MacMahon & Vyborny, 1994; Schaetzing et al, 1990; Siebert, Shelton &

Moore, 1996; Weiser, 1997). Dynamic range in CR is equivalent to F/S exposure

latitude. Figure 2.7 (Bushberg et al, 2002) provides a comparison of the dynamic

ranges of CR and typical F/S exposure latitude. CR is linear to x-ray exposure over a

greater range of exposures than is F/S. CR has a resulting increased exposure latitude

or dynamic range over F/S. A typical dynamic range of the imaging plate used in CR

to capture the image is 1,000:1 (Vuylsteke & Schoeters, 1994).

Radiographic contrast in F/S is limited by the exposure latitude. High radiographic

contrast results when the exposure latitude is narrow. The greater dynamic range of

CR allows x-ray photon exit intensities from high or low attenuating anatomical

regions, which could fall outside the exposure latitude of F/S, to be recorded. In

Figure 2.7, exposures of 0.01 and 0.1 would result in the same OD (white) using an

F/S combination. Using these exposures in CR, differences in CR signal exist and

OD differences can be visualised that would not be visualised using F/S.

Viewers of CR images can take advantage of CR’s greater dynamic range through

manipulation of the displayed brightness and contrast of the image. Look-up tables

(LUTs) are means of controlling the displayed brightness and contrast of digital

images (Artz, 1997; Baxes, 1994; Freedman & Artz, 1997b). The image pixel values

are converted through the LUT to display values visualised on the computer monitor.

CR images can be displayed with a broad or narrow displayed contrast. Narrow

displayed contrast images are similar in appearance to a narrow radiographic contrast

in F/S radiographic imaging. Broad displayed contrast images are similar in

appearance to a wide radiographic contrast in F/S radiographic imaging.

24

Figure 2.7 Dynamic range of CR vs exposure latitude of F/S (Bushberg et al,

2002, p.296)

Where there are anatomical regions with large attenuation differences within the

x-ray field, the advantage of CR over F/S is that all of the anatomy can be visualised

with one exposure of ionising radiation to the patient. The displayed CR image can

then be adjusted to optimise the displayed brightness and contrast of individual

anatomical regions within the image.