Optimisation in general radiography
CJ Martin, PhD, FIPEM, FioP
Health Physics, Gartnavel Royal Hospital, Glasgow, Scotland
Radiography using film has been an established method for
imaging the internal organs of the body for over 100 years. Surveys carried out
during the 1980s identified a wide range in patient doses showing that there
was scope for dosage reduction in many hospitals. This paper discusses factors
that need to be considered in optimising the performance of radiographic
equipment. The most important factor is choice of the screen/film combination,
and the preparation of automatic exposure control devices to suit its
characteristics. Tube potential determines the photon energies in the X-ray
beam, with the selection involving a compromise between image contrast and the
dose to the patient. Allied to this is the choice of anti-scatter grid, as a
high grid ratio effectively removes the larger component of scatter when using
higher tube potentials. However, a high grid ratio attenuates the X-ray beam
more heavily. Decisions about grids and use of low attenuation components are
particularly important for paediatric radiography, which uses lower energy
X-ray beams. Another factor which can reduce patient dose is the use of copper
filtration to remove more low-energy X-rays. Regular surveys of patient dose
and comparisons with diagnostic reference levels that provide a guide
representing good practice enable units for which doses are higher to be
identified. Causes can then be investigated and changes implemented to address
any shortfalls. Application of these methods has led to a gradual reduction in
doses in many countries. � 2007 Biomedical Imaging and Intervention Journal.
All rights reserved.
Keywords: Radiography, dental radiography, X-ray film, automatic exposure control, anti-scatter grid
Radiography using film has been the primary tool in
radiology for over a century. The radiation dose to the patient was given only
minor consideration during the early days. As the number of examinations
performed has increased and data on the long term risks of cancer arising from
ionising radiation exposure has emerged, more attention has been focussed on
keeping the doses received to a minimum.. National programmes were set up to
assess doses from radiological examinations in developed countries. A survey
carried out in the UK in the early 1980s showed that mean doses from similar
radiographic examinations varied by a factor of seven between different
hospitals  and a factor of a hundred was present between doses for
individual patients. The National Evaluation of X-ray Trends (NEXT) program has
painted a similar picture in the United States . It was apparent that in
many hospitals the dose levels were much higher than required to provide a
sufficiently high-quality image for the radiologist to make a diagnosis. Since
that time more emphasis has been placed on the need to optimise imaging
conditions to minimise the risk to patients from radiation exposure .
The quality of an image and the anatomical detail seen
within it depend on the properties of the imaging system and the radiation
used. In general, use of more radiation will improve the quality of the image
within certain limits, but will give the patient a higher radiation dose,
although other factors also need to be considered. The important aspects of
optimisation are to first recognise the level of radiographic image quality
that is required to make a diagnosis. Next to determine the technique that
provides that level of image quality with the minimum dose to the patient. The image
quality should be sufficient to ensure that any clinical diagnostic information
that could be obtained is imaged. However, the radiation dose to the patient
should not be significantly higher than necessary. Finally the procedures
should be reviewed from time to time to ensure that any dose reduction that has
been achieved does not jeopardise the clinical diagnosis.
Assessment of radiation dose and image quality
Before discussing optimisation in radiography in more depth,
it is worth considering briefly the ways in which dose and image quality can be
measured. There are several different quantities that are used for evaluating
doses to patients. The dose quantities that can be measured for radiographic
exposures are the entrance surface dose (ESD) and the dose-area product (DAP).
The ESD is the dose to the skin at the point where an X-ray beam enters the
body and includes both the incident air kerma and radiation backscattered from
the tissue. It can be measured with small dosimeters placed on the skin, or
calculated from radiographic exposure factors coupled with measurements of
X-ray tube output [4, 5, 6]. The DAP is the product of the dose in air (air
kerma) within the X-ray beam and the beam area, and is therefore a measure of
all the radiation that enters a patient. It can be measured using an ionisation
chamber fitted to the X-ray tube. DAP and ESD can be used to monitor, audit and
compare radiation doses from a wide variety of radiological examinations. To
provide a comparator that could be used to achieve more uniformity in patient
doses for similar examinations in different hospitals, diagnostic reference
levels (DRLs) or guidance levels for particular examinations have been
established in terms of the ESD or DAP. National DRLs have been set up or proposed
by various organisations based on surveys of doses in a large number of
hospitals [7, 8, 9]. Conventionally, the third quartile of the distribution of
mean doses from each of the hospitals in the survey for the particular
examination is used as a guide in setting the DRLs, so that mean doses for
three quarters of the hospitals are below the DRL and one quarter of them are
above [7, 10]. DRLs proposed for a selection of radiographic examinations are
given in Table 1 for adults and Table 2 for children [11, 12]. The mean dose in
a hospital for a selection of patients of average weight should be less than
the relevant DRL. If the DRL is exceeded, this should trigger an investigation
into whether further optimisation is needed. This paper reviews the various
factors that could contribute to higher doses, which may need to be considered
if the dose for an examination is found to be too high. A requirement for
countries in the European Union to establish and use DRLs has been included in
a European Directive . Adoption of an optimisation strategy with national
and local DRLs in the UK has lowered patient doses, as demonstrated by the
gradual reduction in third quartile values derived from UK-wide surveys of mean
doses for large numbers of hospitals by the National Radiological Protection
Board (NRPB) (Table 3) [9, 11].
Effective dose attempts to provide a quantity, related to
the risk of health detriment for a reference patient in terms of stochastic
effects in the long term . It equates the uniform dose to the whole body
that would have a similar level of risk and takes account of doses to
radio-sensitive organs in different parts of the body. However, the effective
dose can only be derived from calculations. These are based on computer
simulations of the interactions of X-rays as they pass through the various
organs within the body. The organ doses must be estimated from measurable dose
quantities resulting in large uncertainties in the values. Effective dose is
useful for comparing doses from different types of examination in general terms
for a reference patient, and assessing changes in the dose for a reference
patient during the process of optimisation. Another quantity that is simple to
derive and can be equally useful, but whose application has declined since the
introduction of effective dose, is the energy imparted to the body by an X-ray
exposure [15, 16]. This includes all the energy absorbed from an X-ray beam,
and so gives a more complete picture of the relative harm than a measurement of
ESD, but does not include the complications and approximations involved in the
calculation of effective dose.
A radiographic image provides a representation of the
spatial distribution of tissue components as variations in the optical density
of film. Image quality can be quantified in terms of the characteristics;
contrast, sharpness (or resolution), and noise. Contrast is a result of the
different attenuations of X-radiation in tissue; sharpness is the capability to
display small details; and noise refers to the random fluctuations across the
image that tend to obscure the detail. Evaluation and diagnosis from the image
requires structures of interest to be distinguished against the background. The
difference between the film optical density of a structure of interest and that
of the background can be thought of as the signal. Random fluctuations across
the film can occur, which are superimposed on the image. These are referred to
as noise, and result from a number of causes; quantum mottle due to statistical
variations because of the finite number of photons; the granularity or finite
grain size of the film; and anatomic variations in structure density through
the tissue. The fluctuations affect the detection of low contrast structures.
An optimised radiograph should be limited by quantum mottle. If quantum mottle
is less than noise due to one of the other factors, then it is likely that the
film / screen combination chosen is slower than necessary and the dose to the
patient is greater than it needs to be. Objective methods of evaluating image
quality measure the imaging performance in terms of the signal reproduction for
details of different sizes, using quantities such as the modulation transfer
function (MTF), and their visibility within the noise generated by the imaging
system, using the detective quantum efficiency (DQE) [17, 18]. The DQE
characterises the performance of the radiographic system in terms of the
efficiency with which the image information is reproduced. These variables are
used in standards laboratories and film company research laboratories for
evaluating the performance of screen / film systems.
Medical image quality is related to the subjective
interpretation of visual data. It represents the clinical information contained
in the image. It is more important that the observer interprets the image
appropriately than whether the appearance of the image is pleasing to the eye.
The ideal set of parameters to describe image quality should measure the
effectiveness with which an image can be used for its intended purpose.
However, since the interpretation and diagnosis made from an X-ray involve
subjective opinions from the radiologist, results are likely to vary at
different centres. Guidelines have been set up by the European Commission (EC) for
assessing the basic aspects of quality for clinical radiographic images
dependent on technique and imaging performance [8, 19]. This sets out
diagnostic requirements against which the observer can judge an image. These
requirements include aspects related to physical technique and production of
anatomical structure for a normal individual. Visualisation of anatomical
structures which should be clearly observed in particular types of radiograph
are assessed as well as image detail which should be reproduced. This ensures
that techniques employed within a department provide clinical images of
acceptable quality and any changes made to reduce doses do not have a
detrimental effect on the clinical image. Examples of criteria that may be
affected by choice of technique are given in Table 4. The EC guidelines 
also contain examples of parameters that are considered to represent good
radiographic technique (Table 5).
Results from both practical measurements and theoretical
simulations are included in this paper to illustrate how the different factors
involved in radiographic imaging affect the radiation dose to the patient.
Exposures have been made on anthropomorphic phantoms to mimic radiographs
performed with a range of tube potentials for a selection of projections. ESDs
have been assessed and effective doses have been calculated. Spreadsheet
calculations have been performed using data sets for X-ray spectra, and these
have been used to predict the responses of film / screen systems with different
tube potentials, and various filter options .
Photon fluence or radiation intensity
Screen / film combinations
The most important factor in the optimisation of
conventional radiography is the choice of screen / film combination. The X-ray
film is sandwiched between two screens inside a light-tight cassette. Each
screen has a layer of a fluorescent phosphor, such as calcium tungstate or
gadolinium oxysulphide, which converts X-ray photons into visible light
photons. The spectral emission of the phosphor must be matched to the
sensitivity of the film. Calcium tungstate, the traditional phosphor used in
radiographic screens, emits blue light, terbium activated gadolinium
oxysulphide, the phosphor used in rare earth screens manufactured by Kodak,
Agfa and Fuji emits green light, and yttrium tantalite used by Du Pont in the
Ultravision system emits ultraviolet light. Using a film in the wrong type of
cassette would require an X-ray exposure of higher magnitude. A definite
relationship exists between film optical density and radiation exposure for
every screen / film combination, and this can be described by a characteristic
curve. Examples are given in Figure 1a. Exposures must be constrained to within
the range that will produce perceptible differences in film blackening for the
human visual system. Thus the range of exposures to be used for any radiograph
is pre-determined through the choice of fluorescent screen and film. While the
dynamic range of film is very limited, digital imaging systems have wide
dynamic ranges, enabling images with acceptable contrast to be obtained for a
broad range of exposure levels.
The sensitivities of different systems depend on the
absorption properties of the phosphors. Relative sensitivities for a selection
of phosphors to X-ray beams corresponding to different tube potentials are
shown in Figure 2. Gadolinium and other rare earth atoms have greater
absorption at photon energies above 50 keV than calcium tungstate, and as a
result, the rare earth screens have better sensitivities for X-ray beams with
tube potentials above 70 kVp. Yttrium tantalate has an X-ray photon energy
dependence that is similar to calcium tungstate, but a higher absorption and
sensitivity. The thickness chosen for the phosphor layer is a compromise
between radiation dose and image quality. A thick screen will have a high
efficiency for conversion of X-rays to light, but the image will be more
blurred as some of the X-ray photon interactions will occur further away from
the film and therefore the light photons produced will spread out further
before reaching the film. Thin screens result in better resolution but require
a higher radiation exposure. Sensitivities of gadolinium oxysulphide screens of
different thickness are compared in Figure 2. The sensitivity of screen / film
combinations is quantified in terms of a speed index, which relates to the
reciprocal of the dose to the cassette (in mGy) required to produce an optical
density of 1.0 above the base plus fog level. It is analogous to the film speed
employed in conventional photography. A higher speed index corresponds to a
faster film and less radiation will be required to produce an image, although
the radiograph will be noisier (more grainy). A speed index of 400 has been the
standard for general radiography in Europe since the late 1980s [8, 21].
However, before that time, speed index combinations of 200 were widely used and
may still be the combinations employed in many countries. In the UK, 200 speed
index film cassettes will be used for imaging fine detail, for example to
visualise fractures in the extremities. 600 or 800 speed indices are very high
speed systems, but may be satisfactory for some applications such as lumbar
spine and lumbar sacral joint imaging [22, 23].
Knowledge of the speed index of a film/screen combination
plays an important role in optimisation, and a combination used with a low
speed index is the most probable reason for exposures being high. Speed indices
may be measured by deriving characteristic curves from films exposed to a range
of dose levels. Various phantoms may be used to simulate the spectrum
transmitted through the body and methods have been described in the literature
. Although 20 cm thick water or Perspex provide the closest approximation
to the spectrum, a 20 mm thick aluminium phantom may provide a more practical
alternative with a transmitted spectrum not too dissimilar from that of tissue
(Figure 3). The transmission of copper, which is sometimes used for such
measurements does not resemble tissue transmission as closely and will
therefore give slightly different results. Sections of the film to be tested
should be exposed to a range of air kerma levels, covering the full range of
optical density from 0.2 to over 2.0. This is normally achieved by using a single
film, and covering parts of the cassette with lead. Higher exposure levels may
be achieved by leaving parts of the film uncovered for several exposures.
Measurements of optical density can then be plotted against the air kerma that
is incident on the cassette in the form shown in Figure 1a. An assessment of
the speed index can be calculated from the reciprocal of the air kerma in mGy
to give an optical density of 1.0 above the film base plus fog level.
The image contrast and the range of exposure levels to be
reproduced are also important� factors in the choice of screen / film system.
The contrast is defined in terms of the slope of the characteristic curve
(Figure 1a), which can also be quantified from the measurements described. A
high contrast screen / film combination equates to a steep slope for which the
film optical density varies rapidly with dose and tissue attenuation (Figure
1b). The contrast is linked to the relative amount of film blackening produced
by different exposures and also to the differences in tissue attenuation that
can be imaged. A high contrast film will produce better visualisation of subtle
variations in tissue structure. However, a high contrast film will be
unsuitable for imaging the chest, which contains tissues such as the lung,
heart and spine that have very different attenuations. For this, a combination
that will give an acceptable level of contrast over a wide range of exposure,
referred to as a wide latitude film, is required (Figure 1). For intra-oral
dental radiography, film sensitive to X-rays and backed by lead foil is placed
in the mouth. E-speed film is recommended and this should not require the ESD
to be greater than 2.5 mGy.
To produce an image on film with an acceptable level of
contrast, the exposure must be within a relatively narrow range of doses. The
exposure factors used will be optimised through the experience of the
radiographers, and exposure charts employed for each X-ray unit. The charts
provide a guide to the best factors for different examinations for a patient of
standard build. However, adjustments will need to be made for patients of
To achieve a consistent exposure level, an automatic
exposure control (AEC) device is usually employed in fixed radiographic imaging
facilities. This comprises a set of X-ray detectors behind the patient that
measure the radiation incident on the cassette. The detectors are usually thin
ionisation chambers. Exposures are terminated when a pre-determined dose level
is reached, thereby ensuring that similar exposures are given to the image
receptor for imaging patients of different sizes. The important parameter
involved in radiographic image formation is optical density, so film is used in
setting up the AEC to give a constant optical density. The variation in
relative exposure with tube potential is determined by the phosphor
sensitivities (Figure 2). Plotting this data relative to the response at 80 kVp
gives an indication of the variation in relative dose level with tube potential
that is required when setting up AECs for different screen / film combinations
(Figure 4). A spectrum similar to that transmitted through tissue should be
used to set up an AEC system. Two hundred millimetre thick phantoms of water or
Perspex are suitable for this, but if these are not available, then 20 mm of
aluminium provides an acceptable alternative, giving a similar transmitted
spectrum (Figure 3). A metal filter will be thinner and can be attached to the
X-ray tube light beam diaphragm and so may be more convenient to use. However,
filters of heavier metals such as copper are less suitable. This is because the
filter must be very thin (e.g. 0.5 mm copper), to obtain an energy spectrum
similar to that transmitted through 200 mm of water and the exposure times
required to give usable film densities will be too short for standard X-ray
generators. If thicker filters are used (e.g. 2 mm copper), the spectra differ
from those transmitted through tissue (Figure 3). Therefore the dependence of
film density on kVp is different from that for tissue.
Parts of the cassette used for the assessment can be
shielded to avoid use of new films for each X-ray exposure. A 1.5 mm to 2 mm
thick lead disc, about 150 mm in diameter, from which a 10o � 20o
segment has been cut, provides a useful tool for this (Figure 5). A positioning
device consisting of a Perspex sheet, with a hole to place the disc within and
a lip to hold it in place can be fixed on the surface of the cassette. The disc
should be rotated through the segment angle between exposures so that a new
segment of film is left unshielded each time. Different kVs and different
phantom thicknesses should be used to cover the range of exposures required in
clinical practice, with different AEC chambers selected to terminate the
exposure, to test the performance under a variety of conditions simulating
clinical practice. A careful record of the disc orientation and sequence of
exposures must be made to allow interpretation. Coins or other metal objects
placed in suitable positions provide useful markers on the film. Most
radiographic units have standard relationships between exposure and tube
potential relating to different screen / film systems, and the purpose of the
measurements is to ensure that the most appropriate AEC relationship for the
combination is selected.
Radiography is performed using mobile equipment, but the
quality of the image is likely to be lower, because radiographic cassettes
cannot be aligned as accurately as with a fixed unit, and the distance of the cassette
from the X-ray tube will be variable . An AEC cannot be used to terminate
the exposure, so an exposure chart is essential. The output of mobile X-ray
units is lower than for fixed ones, so the range of exposures that can be
obtained is limited and longer exposure times may be required. Therefore,
mobile radiography should only be used in situations when an examination on a
fixed installation is not feasible.
Exposure levels for most radiographic techniques will be
determined by air kerma measurements in the X-ray beam. Evaluating a dose for a
panoramic dental X-ray unit is more difficult, as it is necessary to integrate
the dose from the exposure over the period during which the X-ray tube is moved
around the head. This is measured in terms of the dose in the X-ray beam
multiplied by the beam width or �dose-width product�. The dose-width product
can be determined from the beam characteristics at the receiving slit measured
over one rotation, either by a small detector that can be placed at the centre
of the X-ray beam, multiplied by the beam width, measured using film, or using
a CT chamber attached perpendicular to the slit . A reasonable value for
the dose-width product is 75 mGy mm, or it can be multiplied by the height of
the X-ray beam at the receiving slit to derive the DAP, for which the DRL will
be of the order of 100 mGy cm2.
X-ray beam quality
Radiation quality refers to the proportions of photons with
different energies within an X-ray beam. The contrast between different
structures in an X-ray image results from removal of photons from the primary
beam. The radiation quality influences the image quality and radiation dose
through the mechanisms by which the X-ray photons of different energy interact
with the tissue [20, 27]. Few photons with energies below 30 keV will be
transmitted through 20 cm of tissue or water (Figure 3), so metal filters are
placed in the X-ray beam which remove more of the low energy photons. X-ray
beams which contain more photons with energies between 30 keV and 50 keV give
better image contrast, but a greater proportion of the photons are absorbed in
the body, so a larger radiation intensity must be used to obtain sufficient
photons to form an image. The radiation quality of the X-ray beam chosen for
each radiological examination should be selected to achieve the best compromise
for the clinical task. The factors that determine the radiation quality are the
tube potential and the beam filtration. Factors recommended by the EC for
radiographs of a patient of standard size are given in Table 5.
The potential applied to the X-ray tube determines both the
maximum photon energy and the proportion of high energy photons. The optimum
potential will depend on the part of the body being imaged, the size of the
patient, the type of information required and the response of the image
receptor. Figure 6 shows the reduction in incident air kerma that is the result
of using higher tube potential to gain the same level of film blackening, for
different phosphors used in radiographic screens. The ESD will be reduced by
about 50% if the tube potential is increased by 10 kV. Figure 6 also shows that
the exposure required for a calcium tungstate screen would be typically 50%
higher than for a gadolinium oxysulphide screen. Tube potentials used for
radiographic examinations have been established through experience. 80 kV to 85
kV are typical values used for radiographs of the abdomen, pelvis and lumbar
spine antero-posterior (AP) views for an average patient. X-ray beams with tube
potentials of 50 kV to 60 kV will give better contrast, but fewer photons will
be transmitted. These are used for thinner regions of the body, such as the
arms, hands and feet. 85 kV to 90 kV X-rays will provide better beam
penetration and a lower radiation dose, but poorer contrast. They are employed
for thicker, more attenuating parts of the body, such as the lumbar spine
lateral projection. Standard kV ranges have been recommended for a selection of
common radiographic examinations based on practices in different countries
(Table 5) . Patient doses will be significantly greater if lower tube
potentials than those recommended are used [5, 28]. As the thickness of the
part of the body to be imaged or of the patient increases, the exposure will
need to be increased (Figure 7). If the tube potential remains the same, the
ESD is about doubled for each additional 50 mm of tissue in the range 80 kVp to
100 kVp, and will increase by 2.5 to 3 times at 60 kVp. Therefore the tube
potential will normally be increased for larger patients to keep the dose at a
reasonable level. Using a higher tube potential results in poorer contrast and
tends to produce more scatter, further reducing the image quality.
The reduction in effective dose when tube potential is
increased is less than that in ESD or DAP, because the surface dose is
proportionately higher with lower tube potentials. Plots showing the reduction
in effective dose for a reference patient with tube potential were derived from
practical experiments using anthropomorphic phantoms and are shown in Figure 8.
The ESD was measured and then multiplied by conversion coefficients to estimate
the effective dose . Anthropomorphic phantoms are useful because they
provide a normal anatomy reference patient which can be X-rayed multiple times
using different exposure factors. Artificial lesions can be manufactured using
Blu-Tack or similar materials to allow assessment of details of varying size.
This type of investigation may be undertaken for assessment and evaluation of
possible alternative techniques and therefore contribute to optimisation.
Thin sheets of metal such as aluminium or copper are
incorporated into diagnostic X-ray tubes to reduce the proportion of low energy
photons, as few are transmitted through the patient and contribute to the
image. A filter equivalent to at least 2.5 mm of aluminium is incorporated as
standard into medical X-ray tubes and is required by national guidance [30,
31]. Copper will absorb a higher proportion of the lower energy photons than
aluminium, which contribute significantly to patient ESD. The disadvantage of
using copper filters is that an increased tube output is required to compensate
for the additional attenuation. With tube potentials of 70-80 kV, reductions of
over 50% in ESD and 40% in effective dose can be achieved by using a 0.2 mm
thick copper filter, but the tube output would need to be increased by about
50% to provide the necessary air kerma level . Rare earth filters such as
erbium have been investigated as possible alternatives to copper for imaging
thinner tissue structures in paediatric and dental radiography. The advantage
was their perceived ability to attenuate higher energy photons (>60 keV),
and lower energy ones, therefore providing a narrower energy spectrum. However,
apart from dental radiography, they have not provided significant advantages
over copper filters.
Other factors in optimsation
Scattered radiation and use of low attenuation components
Once an X-ray beam has been transmitted through a patient,
no new information can be obtained, but different components can be selectively
emphasised or suppressed. Radiation scattered from tissues within the body,
increases the level of random background noise on the film and this degrades the
visibility of low contrast details. The amount of scattered radiation can be
reduced by means of an anti-scatter grid. The grid consists of a plate
containing thin strips of lead lying perpendicular to the plate surface, which
are sandwiched between a low attenuation inter-space material such as fibre or
paper. X-ray photons that do not change direction as they are transmitted
through the patient pass between the lead strips with little attenuation,
whereas scattered photons are more likely to be attenuated by the lead strips.
The lead strips may be parallel, but can be angled towards the focal spot of
the X-ray tube to improve transmission. The grid attenuates the transmitted
primary beam and removes scattered radiation, which requires a higher intensity
X-ray beam resulting in a higher radiation dose to the patient. The inter-space
material may be carbon fibre, low attenuation plastic, or aluminium, and the
grid cover is often made of aluminium . Using a grid with aluminium
interspaces is likely to double the dose required without a grid, whereas a
grid with fibre inter-space only increases the dose by about 50%. However, the
amount by which the radiation level will need to be increased depends on the
grid characteristics and the tube potential. In paediatric radiography, carbon
fibre or other low attenuation material should be used for all components
between the patient and the film, because the attenuation of the low kVp X-rays
is greater. For example, patient exposure can be increased by about 5% at low
tube potentials by an aluminium grid cover. Use of a low attenuation X-ray
couch is also particularly important for paediatric radiography, as this can
attenuate the beam by 20% to 30%.
The decisions to use a grid or not, and the choice of the
technique employed for scatter reduction are important and involve image
quality and dose, and depend on the application. An anti-scatter grid should
only be used if more diagnostic information will be obtained as a result. A
radiographic examination of an adult abdomen performed without a grid is
unlikely to show the detailed tissue structure required for diagnosis, whereas
a similar examination for a young child is likely to be satisfactory, because
much less scattered radiation is generated. In Figure 9 data on the DAPs for
paediatric examinations of the pelvis are plotted as a function of the
equivalent diameter of the patient, calculated from the weight and height .
The figure displays discontinuity in the dose data that corresponds to the size
of patient for which the technique was changed. A grid was not used for smaller
children who were placed directly on the cassette on the X-ray couch. As a
result there was no attenuation by the grid or X-ray couch and the focus to
film distance was reduced. Older children were placed on the standard X-ray
table and a grid was employed.
For examinations where a grid is used, the choice of grid
characteristics is important for optimising imaging performance [32, 34]. The
anti-scatter grid types are categorised by the strip density N, the grid ratio
r, and the material used for the interspace. The strip density, i.e., the
number of strips per cm, determines whether the grid can be used in a
stationary mode or must be moved during the exposure to prevent the appearance
of lines on the image. Strip densities over 60 strips per cm do not require
mechanical movement. The grid ratio between the depth of the lead strips in the
direction of the X-ray beam and the width of the fibre interspace perpendicular
to the beam direction determines the effectiveness of the grid in removing
scattered radiation, but also affects the transmission of the primary beam.
Typical values for the grid ratio are between 18:1 and 8:1 with higher ratios
being more effective in removing scatter. For thicker parts of the body, such
as the lumbar spine, for which there are relatively large amounts of scatter,
the use of high grid ratios (e.g. 16:1 � 18:1) with a high tube potential will
give better image quality. When there is less scatter, a lower grid ratio (8:1)
can be used, with a lower tube potential to give the desired contrast level. A
12:1 grid may provide an acceptable compromise allowing a range of general
radiography with a single grid.
Much debate has taken place about the best technique for
chest radiography over the years. Less scatter is produced by the lung that has
a lower tissue density, but the heart and spine scatter radiation. A lower tube
potential (65 kV � 70 kV) without a grid was the favoured technique in the UK,
to give good detail in the lung, but a higher tube potential (110 kV � 130 kV)
with a grid is now favoured, as this improves detail visibility in the higher
attenuation mediastinum and produces better image quality over the whole image.
When tube potentials of 100 kVp or above are employed for chest radiography, a
high scatter fraction is produced and a high selectivity grid (12:1 to 18:1)
should be used. An alternative method for reducing the level of scattered
radiation in chest radiography is to use an air-gap of 200 mm or 300 mm between
the patient and the radiographic cassette . Scatter spreads out in all
directions from the patient, whereas the primary beam spreads out from the
X-ray tube focus. Thus when a gap exists between the patient and the film, a
smaller proportion of the scattered radiation reaches the cassette. One effect
of the air gap is to magnify the image, because the magnification is related to
the ratio of the distances of the film and the patient from the X-ray tube. As
a result, a larger focus to film distance of 3 m to 4 m is required when an
air-gap is used to reduce the magnification to fit the whole image on the film.
The output from the X-ray tube should also be increased to compensate for the
greater distance. An air gap is not as effective as a grid in removing scatter
and a slightly lower tube potential is required to achieve the same contrast
level as a grid.
For paediatric examinations that have lower scatter levels,
a lower grid ratio together with a lower tube potential that produce a better contrast
level may provide a more satisfactory result. However, Aichinger et al 
report that a grid with a high grid ratio (e.g., 15:1), if properly designed,
can be better suited to paediatric radiography than a grid with a low ratio
(e.g. 8:1), as recommended by the CEC , in which the thickness of the lead
strips may be greater. Performance depends crucially on grid design.
Beam collimation and X-ray projection
Collimation of the X-ray beam is an important factor in
optimisation. Good collimation will both minimise the dose to the patient and
improve image quality, because the amount of scattered radiation will increase
if a larger volume of tissue is irradiated. Collimation is particularly
important in paediatric radiography since the patient�s organs are closer
together and larger fields are more likely to include additional radiosensitive
organs. Collimation in most cases depends on the technique of the radiographer,
but regular quality assurance by checking that the X-ray beam and the field from
the light beam diaphragm are accurately aligned is important, particularly for
�Beam collimation in dental radiography is achieved through
use of a fixed cone and the traditional aperture size is 60 mm diameter [30,
37]. In older units which used a focus to film distance of 100 mm, a
substantial proportion of the face was exposed. Optimisation has involved two
stages, an increase in the focus to skin distance to 200 mm, and incorporation
of a smaller rectangular aperture similar in size to the film. Both these have
contributed to a reduction in the volume of tissue irradiated (Figure 10).
However, the use of a smaller beam size means that alignment of the film is
crucial. Therefore film holders placed in the mouth, with which the X-ray tube
collimator can be aligned, should be used. Optimisation in dental radiography
through the use of 65-70 kVp instead of 50 kVp, use of faster E speed film, and
more accurate beam collimation can reduce the effective dose for a dental
radiograph by a factor of ten.
Another aspect that influences the effective dose, is the
projection chosen for a radiograph. The organs and tissues lying closer to the
surface on which radiation is incident will receive higher radiation doses. If
organs that are more sensitive to radiation are further from the surface on
which the X-rays are incident, the X-ray beam will be attenuated by overlying
tissues, and the doses to the organs will be lower. Therefore for some
examinations the projection taken can influence doses to particular organs and
the effective dose. Chest examinations will normally be taken using a
postero-anterior (PA) projection, to minimise the dose to the breast tissue and
oesophagus. Many of the abdominal organs are closer to the anterior surface, so
a PA radiograph of the abdomen is also likely to have a lower effective dose.
Effective doses for the antero-posterior (AP) view can be 50% higher for chest
and abdomen radiographs, and even higher for low tube potentials, as shown by
ratios of AP/PA effective doses for a few examinations plotted against tube
potential (Figure 11). The discontinuity in the curve for chest examinations is
a result of the method used for calculation of effective dose  and does not
have any practical significance. Doses for AP and PA projections of the pelvis
are similar, as the sensitive organs are located more centrally within the
lower abdomen. Right and left lateral views of the pelvic area with similar
exposure factors may also give different effective doses, since the exposure of
the descending saecum will increase the dose to the colon for the right lateral
The risks from exposure to an embryo or foetus are greater
than those to children or adults , so decisions involving investigations of
pregnant women should be made carefully. The examination should only be
performed if the risk of not making a diagnosis at that stage is greater than
that of irradiating the foetus. Where the examination can be delayed without
undue risk to the patient, this may be the better option, or if an acceptable
technique using non-ionising radiation is available, this may be employed. If
it is necessary to carry out a radiograph of the abdomen for a woman who is
pregnant, the PA projection would reduce the dose to the foetus as much as possible.
The final stage in the production of a radiograph is
processing the film. If processing conditions are not optimal, the film will
require a higher radiation dose in order to provide an acceptable film density.
Chemicals should be changed regularly, and the processing conditions, such as
temperature and development time should be carefully optimised. A system of
quality control that involves checking temperatures of processing chemicals and
carrying out sensitometry, involving development of a test strip of film
exposed to a range of light levels ensures optimal performance. These checks
should be carried out daily to monitor performance in terms of film density,
contrast and background fog level. The performance levels of processors that
have a relatively low workload need to be monitored carefully. Since film
processing affects the film density, it influences the speed index. Thus the
measurements of the characteristic curve for a film, discussed earlier, will
also reveal problems with processing.
Processing can be a particular issue with dental
radiography. A simple step wedge can be constructed using a spatula of low
attenuation material with layers of lead foil taken from dental film. The step
wedge with 0, 1, 2, 3, 4, and 5 thicknesses of lead foil can be placed on top
of an intra-oral dental film and an exposure made with standard settings under
standard conditions (Figure 12). If a film is taken with optimised processing,
it can be considered the reference standard. Checks can then be made by
comparing future results with the reference standard to identify any
Discussion and Conclusions��������
The formation of images of the body involves interplay
between many different factors. To achieve the correct balance between patient
dose and image quality it is necessary to understand the way in which the
images are formed, and to know the factors that influence the image quality and
the radiation dose received by the patient, so that the appropriate options can
be selected. The most important choice in radiography is the speed of the
screen / film combination used. Rare earth systems with speed indices of about
400 are recommended for general radiography (Table 5)  and 600 may be
appropriate for certain lumbar spine projections [22, 23]. Tests on the system
speed should be carried out from time to time to check the performance, and if
it is suspected that this may be a factor contributing to higher patient doses.
Consistent exposures can be achieved using an AEC device. The AEC should be set
up whenever a new type of screen / film system is introduced into a department,
as sensitivities of different phosphors vary with tube potential in different
ways. A simple shield device which allows each part of the film to be used for
a single exposure enables film densities with different tube potentials and
phantom thicknesses to be compared using a single film when setting up an AEC.
Education in techniques for reduction of patient dose,
coupled with periodic review of doses to feed back data to individual
departments, provides the best way of achieving optimisation [6, 23]. Surveys
of patient dose in terms of ESD or DAP and comparisons with DRLs should be
carried out every few years . When working in isolation, it is difficult to
judge whether the dose to the patient for a particular examination is higher
than it ought to be. The establishment of DRLs is a crucial step in
optimisation as it enables hospitals to compare doses with established values
to represent good practice. But if patient doses are found to be high, it is
important that reasons are thoroughly investigated and shortfalls in equipment
or technique addressed. There are many factors that can be involved, but the
pattern of higher doses can give a clue to the possible reason. If doses for
all examinations within a department are high, then the most likely cause is
the screen / film combination used, or associated factors such as the
processing. If certain X-ray units within a department have higher doses, this
could be due to factors such as grid characteristics, incorrect alignment of
the grid with the X-ray beam, or a relatively high table attenuation. If only
certain examinations have higher doses, this may involve factors in
technique, such as the choice of tube potential. In many cases if doses are
above the DRL, there may be several factors involved.
The tube potential selected should be appropriate for the
degree of contrast required and the thickness of the part of the body being
imaged. The importance of grid characteristics and the interplay with exposure
factors should not be forgotten. A higher tube potential gives rise to more
scatter and requires use of a higher grid ratio. However, using a lower tube
potential that produces less scatter with a lower grid ratio may result in
better overall contrast for some examinations. Decisions about grid
characteristics and whether a grid should be used in the first place should be
based on the range of examinations performed. The use of low attenuation
materials in couches and grids is particularly important for paediatric
examinations, since the lower tube potential X-ray beams employed are more
highly attenuated and the tissues of paediatric patients are more sensitive to
radiation damage. Incorporation of an additional 0.1 mm or 0.2 mm of copper can
give a significant reduction in ESD and should be considered, particularly for
units used for paediatric and other examinations.
Dose reduction without regard for image quality could
produce images that are inadequate for diagnosis and must be avoided.
Alternative options for optimisation can be investigated using anthropomorphic
phantoms or other phantoms designed to simulate imaging of detail within water,
Perspex or other attenuating material designed to mimic clinical imaging
situations. If any change in technique is introduced to optimise performance,
an evaluation of a selection of clinical radiographs should be carried out,
using criteria such as those in Table 4 to ensure that the image quality
obtained is appropriate for the clinical task.
Despite all the effort to optimise radiography in recent
years, the doses for similar examinations in different hospitals still vary
substantially. However, the dose levels have gradually fallen as demonstrated
by the gradual decline in the third quartile of survey data for the UK (Table
3) . This has been achieved by carrying out optimisation through the
performance of regular equipment quality assurance [24, 37] and periodic
patient dose surveys to ensure that lower dose levels are maintained.
Figure 1 Curves showing the properties of three films, two 400 speed index screen / film combinations with differing contrasts and film latitudes and one 200 speed index; a) characteristic curve showing the variation in optical density with air kerma incident on the cassette, and b) variation in contrast with incident air kerma.
Figure 10 Arrangements for taking dental intra-oral radiographs with a short cone, a long cone and a long cone with a rectangular aperture, comparing the volume of tissue irradiated, which is shaded.
Figure 11 Ratios of effective doses for AP and PA projections for a reference patient as a function of tube potential for a selection of common radiographs .
Figure 12 Simple dental step wedge test object with five different attenuation steps created using a dental spatula and different thicknesses of lead foil, taken from used dental film. A reproduction of the type of image that might be produced is also shown. The phantom is placed on top of the dental film at the end of the cone and an exposure taken with a standard setting. Subsequent exposures should be repeated under the same conditions, with the film placed on the same surface, as different scatter conditions may affect the image.
Figure 2 Relative sensitivity for different tube potential X-ray beams for three phosphors used in conventional radiography; gadolinium oxysulphide, yttrium tantalite and calcium tungstate, each 200 �m thick. Relative sensitivities are also shown for different thicknesses of gadolinium oxysulphide phosphor. The X-ray spectra applied are those transmitted through 2.5 mm aluminium and 200 mm of water.
Figure 3 Spectra transmitted through filters comprising 200 mm water, 200 mm Perspex, 20 mm aluminium, and 2 mm copper, which are often used to harden X-ray beams to simulate the X-ray spectra transmitted through patients.
Figure 4 Relative dose setting for an AEC device against tube potential for different phosphors each 200 �m thick, for X-ray spectra transmitted through 2.5 mm aluminium and 200 mm of water. Data are also shown for a 200 �m thick barium fluorohalide phosphor used in computed radiography.
Figure 5 Tool that can be used to record a number of X-ray exposures on one film for setting up radiographic unit AEC devices, showing (Left) the tool itself and (Right) a simulation of the image that might be obtained.
Figure 6 Graphs of relative entrance surface dose against tube potential to obtain an image for a 200 mm thickness of soft tissue with three phosphors used in radiographic cassettes; gadolinium oxysulphide, calcium tungstate and yttrium tantanate. All calculations are performed for 200 �m thick phosphor layers and an X-ray beam filtered by 2.5 mm of aluminium.
Figure 7 Graph of relative entrance surface dose against thickness of tissue to obtain an image with a rare earth screen / film combination using different tube potentials.
Figure 8 Variation in effective dose for a reference patient with tube potential for several different projections. The results were obtained through imaging of anthropomorphic phantoms with a rare earth screen / film combination, and calculation of values for the effective dose based on measurements of the entrance surface dose using conversion factors .
Figure 9 Plot of DAP against equivalent patient diameter for pelvis radiographs of children . Smaller children were placed directly on the radiographic cassette on top of the X-ray couch, while for older children the cassette was placed in the bucky tray behind an anti-scatter grid.
Table 1 Suggested values for diagnostic reference levels for radiographs of adult patients [11 and local data]
Table 2 Suggested DRLs for individual radiographs on paediatric patients in terms of ESD [8, 12]
Table 3 Third quartile values for ESDs (mGy) from NRPB reviews of UK national patient dose data 
Table 4 Image quality criteria relating to X-ray equipment and exposure factors
Table 5 Examples of good radiographic technique 
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|Received 20 October 2006; accepted 25 October 2006
Correspondence: Health Physics, Gartnavel Royal Hospital, Glasgow, G12 OXH, Scotland, United Kingdom. Tel.: +44 0141 211 3387; Fax: +44 0141 211 6761; E-mail: firstname.lastname@example.org (Colin Martin).
Please cite as: Martin CJ,
Optimisation in general radiography, Biomed Imaging Interv J 2007; 3(2):e18
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