Digital colour radiography

Digital colour radiography W. H. Tait University of East London, London E15 4LZ, UK Received 4 November 1992; revised 26 May 1993

The production of digital colour radiographs is discussed and the interpretation and possible advantages of such radiographs are examined. Two specific processes are identified. One is direct colour radiography in which radiographs obtained at different beam energies are colour coded and combined to form a true colour image. The other is colour subtraction radiography in which the primary radiographs are subtracted and the resulting difference radiographs are colour coded and combined. Both formats generate colour radiographs in which intensity and colour are related to the material composition of the object. They are found to have some advantages over single energy, monochrome pictures. They provide greater latitude and contrast, in terms of object structure, improved image interpretation, by facilitating long range visual correlation and feature interpolation, quantitative information for analysis of single and multiple radiographs, and the possibility of elemental analysis. The direct colour radiographs are additive with respect to the primary colours and, like monochrome radiographs, provide good intensity contrast. The subtraction radiographs retain the primary colours and offer better colour contrast but poorer intensity contrast. Because of the subtraction process, they are noisier images. Keywords: digital radiography, colour, energy

In X-radiography, energy selection is necessary to optimize the penetration of X-rays through the object being radiographed. Except at absorption edges, X-ray attenuation coefficients decrease smoothly with increasing photon energy, so a high energy produces greater penetration of the object and a qualitatively different radiograph from that obtained with a low energy beam. In addition, attenuation decreases with the density and, especially, the atomic number (Z) of the absorber, so, in general, high photon energies are required for dense, high Z materials, while lower energies are best for the less dense, low Z materials. The beam quality has to be tuned to match the absorptive properties of the object. This approach is satisfactory for objects of relatwely uniform composition, but it is less successful for more contrasty objects with a wide range of attenuation factors. In such cases, the beam quality can be adjusted for maximum information return from a particular region of interest, or an intermediate setting can be used to achieve a compromise image quality over the whole object. Alternatively, two or more radiographs can be obtained at different energies and viewed either separately or combined in some way. One technique involves dual energy subtraction [1'2]. Two 0963-8695/93/040171-06 © 1993Butterworth-Helnemann Ltd

radiographs are obtained at different kVp. The high energy beam shows detail of high Z components while the low energy beam is attenuated by both high and low Z components. Subtraction gives the separate effect of the low Z components. Another technique, which can be regarded as a form of energy radiography, is absorptiometry, in which attenuation coefficients are calculated by dual beam absorption. This has been developed into methods that are widely used for bone density measurements t3'41 and NDT applications. Colour radiography is often associated with the use of pseudo colour, to represent intensity levels and improve the latitude and contrast resolution of the images [5,6J. True colour radiography is a process in which image colour is related to photon energy. Early attempts at true colour radiography used photographic methods to combine three radiographs taken at different energies into a single colour radiograph tTj. The present study employed digital image capture and colour processing techniques to extend this principle. Component radiographs were obtained at different beam energies, processed, colour coded and combined. The aims were to examine ways of producing true colour radiographs and to determine their properties and possible advantages over single energy, monochrome radiographs.

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Detection system X-ray detection systems are primarily imaging or spectrometric, and the only way to measure both the position and energy of incident photons is to adapt one type or the other. The present work was concerned with imaging processes, and the choice was between fluorescent screens and film, with energy selected, rather than measured, by the simple expedient of adjusting the kVp of the X-ray machine For digital colour radiography, fluors have some advantages. They can be examined directly by a television camera whose output can be recorded electronically and stored directly in the computer. In addition, there is no need to move any part of the system during a series of exposures, so all members of a series of exposures are precisely registered with respect to each other They can then be added or subtracted with no loss of spatial accuracy and minimum processing, so the final colour radiograph is produced almost In real time. On the other hand, fluors have poor spatial resolution compared to films and, being real-time devices, cannot integrate radiation exposure, except by electronic frame integration. Image intensifiers improve the detection efficiency but they still produce relatively noisy images, which contribute large errors to the subtractlve digital processes As a preliminary to the present study, photographic integration of colour filtered, energy radiographs was examined, but the results were noisy and colour biased by the emission spectrum of the fluor. Films are less convenient in many respects, but they offer higher resolution and better contrast, so they were used in this study. Registration was achieved by using a digital subtraction procedure to locate each radiograph in the same position when it was digitized Energy selection was achieved by altering the kVp of the tube, so each exposure related to a continuous photon energy distribution tsl ranging from about 20 keV to the maximum beam energy. The energy resolution was not as good as could have been achieved by a combination of filtering and kVp adjustment, but was sufficient to demonstrate the advantages of colour radiography. The X-ray beam can be regarded as monoenergetic, with an energy value equal to the mean photon energy and a F W H M energy resolution ranging from about 20%, at the lowest kVp, to about 60% at the highest energy. The number of component radiographs that can be stored and processed depends on the specifications of the computer system and its display hardware These parameters limited the present study to combinations of four radiographs of each object, obtained at the operating potentials, 30, 70, 110 and 150 kVp.

image is to be printed, a black component is added for the same purpose In colour radiography, the same process can be employed, but some consideration must be given to the choice of component colours. The primary colours would be a suitable choice for three-component energy radiographs but, in general, there may be more than three such components to accommodate, so the theory and practice of digital colour radiography must be extendable to any number. Colours should be allocated to these components in such a way as to optimize the colour resolution and intensity balance Thus, the full range of colours must be employed, so that the saturation colour is pure white, and the relative intensities of the components should be equal In the present work, four components were used and a number of colour schemes were examined. One is based on the combination of red, green, cyan and blue, for low to high energies. This corresponds to the colour-energy relationship exhibited by the physical properties of visible light An alternative, using these colours in the reverse order, is more compatible with the psychological interpretation of colour, going from blue at low energy to the hot, red colour at high energy. No particular advantage was found for one scheme or the other. The components were combined by allocating them to different colour planes in the computer display. VGA screen modes include a high resolution mode with 640 by 480 pixels and 1 bit per colour plane, and a low resolution mode with 320 by 200 plxels offering 2 bits per colour plane. SVGA graphics adaptors provide higher resolution modes and a 640 by 480 pixel mode with 2 bits per colour plane. It is also possible to integrate screen images photographically to produce a 640 by 480 image with 4 bits per colour plane. This and the low resolution VGA mode were used in the present work

Physical interpretation When a film radiograph is viewed by transillumlnation, or digitized, the light intensity, I, transmitted through a picture element is given by an expression of the form: log(l) = log(/o) - D

(1)

where I o is the incident, unattenuated light intenstty and D is the optical density at the point of measurement. D can be expressed as a function of log(X) in the form of the characteristic curve of the film. This is an S-shaped curve with a linear working area given by. D = DO + g log(X)

(2)

where Do is a constant and g is the absorption coefficient per unit area of the film.

Colour processing and display In colour photography, blue, green and red primary colour images are combined to form a single colour picture. At each picture element, the three components are added, in proportion to their relative intensities, normalized to the characteristics of the colour filters used. If the display is on a computer screen, a further white component is added to control overall intensity. If the

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The exposure X reaching the film is determined by absorption in the object being radiographad. If an exposure of Xo enters the object, the exit exposure is given bylog(X) = log(Xo) -

k#x

(3)

where #x is the aggregate product of attenuation coefficients and relative thicknesses of all materials in the ray path, at the mean photon energy, and k is a constant.

Digital co/our radiography: W. H. Tait

component radiograph, indicating that the use of single energy curves is a reasonable approximation.

A

The component radiographs were obtained by kVp adjustment, and processed by histogram normalization, as required by Equations (5) and (6). The minimum intensities were reduced to the same zero, black values, to make the X-ray entrance exposures equal and to utilize the full contrast range. The maximum, white intensities were similarly adjusted to make use of the full range of

RelaC£ve AJeaoz'pt£v£ty,

~x

Figure 1 Energy response curves for radtographs obtained at 30 kVp (left), 70 kVp, 110 kVp and 150 kVp (nght)

From these equations, it can be seen that the light intensity transmitted through any point on the film is given by an expression of the form: log(l) = a + b # x

(4)

This curve relates viewed or digitized light intensity, I, to a physical property of the object, namely, its attenuation factor, /~x. It is the basis of X-ray absorptiometry measurements.

Figure 2 Rad=ograph of an electrm shaver test object, obtained at 30 kVp

The attenuation coefficient,/~, is a function of the object absorptivity, dZ", where d is the density and Z the atomic number of the absorber. It is also a function of the incident photon energy, E, being proportional to E -=. The value of n varies from about 5, at moderate to high energies, down to 0, at very low energies. Over the same range, the value of m changes from about 7/2 to 1. It is possible, then, to express log(I) as a function of d Z ~, or E-=, instead of #x. Each curve has the same S shape as the characteristic curve, with a linear region given by a formula similar to Equation (4), namely. log(l) = A + B dZ"

(5)

log(I) = A + C E -=

(6)

where A, B and C are constants, for the same film, exposures and viewing conditions. The values of n and m may vary for light elements where scattering can make a significant contribution to the attenuation process.

Figure 3 Radmgraph of an electnc shaver test object, obtained at 70 kVp

These equatmns define the response curves of the film as a function of absorptivity, for constant energy, and as a function of photon energy, in a constant medium. They are illustrated in Figure 1 as functions of absorptivity for different photon energies.

Colour radiographs The energy response curves shown in Figure 1 can be applied to the component radiographs of an electric shaver. These were obtained at 30, 70, 110 and 150 kVp and are shown in Figures 2-5, respectively. Attenuation factors decrease smoothly with increasing photon energy, except at strong absorption edges, so the curves are parallel, but the separation decreases with increasing energy, as predicted by Equation (6). Each curve shows the range of dZ" values displayed in the associated

' Figure 4 Radiograph of an electr=c shaver test object, obtained at 110 kVp

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Digital colour radiography W H Tait good intensity contrast along w~th the added colour contrast. If the colour scheme is reversed to blue, cyan, green and red the outcome would be an integrated set of cold colours

Figure 5 Radiographof an electnc shaver test oblect, obtained at 150 kVp

grey scales. The resulting colour balance should be independent of measurement conditions. The most charactenst~c feature of digital colour radiographs can be predicted from the response curves. Prowded the human eye can d~stingmsh colour mtensity a well as grey scale intensity, the latitude, in terms of the range of visible absorptivity, is considerably extended compared to that of a single energy exposure. The effect can be confirmed by comparing the monoenergetic images of Figures 2 to 5 with the colour radiograph of the shaver shown in Figure 9. This is a 320 by 200 pixel ~mage, with 2 bits per colour. It has poorer resolution than the grey scale radiographs but clearly demonstrates the increased range of shaver components displayed. For comparison, Figure 10 shows a colour radiograph of a computer mouse displayed in 640 by 480 pixels and 4 bits per colour. The same latitude can be reproduced m grey scales, by using a different film or by combining the separate ~mages either digitally or by visual comparison, but the gradient of a single curve spanning the same range of absorptivity ~s much less than the gradients of the separate curves in Figure 1. This does not mean that the intensity contrast is different, since both response curves increase from zero illumination to saturation white in the same range of object absorptivity, but the colour radiograph has an additional colour contrast which must mmprove overall contrast, in terms of object composition. Another characteristic property of these radiographs is an additive colour scheme. This is predicted from the response curves and illustrated in Figure 9. The colour at any point on the radiograph represents a particular value of absorptivity and includes contributions from all the radiographs whose curves pass through that value. If the colours used to &splay the 30, 70, 110 and 150 kVp radiographs are red, green, cyan and blue, the displayed colours are red, orange, yellow and white. For example, the colour corresponding to an absorptivity just less than the minimum of the blue curve is saturation red plus saturation green plus some cyan, giving a net yellow result. In the shaver radiographs, the plastic casing is red/orange, the denser metal parts are orange/yellow and the very dense, high atomic number transformer is white. The effect Is similar to the hot body scheme used in medical imaging t9,1°] and has the advantage of retaining

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Because the response curves are fixed to particular colours and absorptivity values, colour adds a quantitative property to the energy radiograph. It facilitates long range visual correlations Areas of equal absorptivity are easily identified by their colour and visual extrapolation and interpolation processes are Improved. Monochrome images allow only comparison of adjacent areas. If the response curves are fixed absolutely, even longer range comparisons become possible, from one radiograph to another. This would allow X-ray colour to be used for visual analysis in the same way that visible light ~s used The ad&twe colours are produced by the shapes of the response curves Object structures that show m one component radiograph, and in one component colour, are always present m all the lower energy components so the colour displayed at any point ms the integral of colours up to that representing the highest energy present. The model breaks down if there is any anomalous absorption in the object. This can occur where there are strong K or L absorption edges m the attenuation coefficients At these discontlnmties, absorption increases sharply over a short energy range, tending to increase the relative brightness of the component radiograph associated with that energy interval and &stort the associated response curve. For a typical mimmum beam energy of 20 kVp, this effect would only be observed for elements w~th atomic numbers greater than about 42 and they would have to be substantml fractions of the ray paths to produce vlsmble results. Nevertheless, it presents a possible means of identifying these as anomalous colours in the additive scheme.

Colour subtraction radiographs An alternative approach is to extend the pnnciple of dual subtraction radiography to multiple energies. Each component image is now the difference between the adjacent energy pairs used for the direct process. The last, and highest, energy component is left unsubtracted, to display the remainder of the object structure. All of these component radiographs are histogram normalized to the same entrance exposure and maximum absorption, and expanded to cover the full range of intensity levels. The effect is described by Figure 6, which shows a set of subtracted colour response curves, all normalized to the same maximum value, and m Figures 7 and 8, which show two of the subtracted ra&ographs of the shaver test object. The curves are no longer monotonically increasing and are much less overlapping than those of Figure 1. They indicate that subtracted colour radiographs should display relatively short ranges of absorptivity in the primary colours. Highly absorptive structures no longer overlap the lower absorptivity ones to the same extent, as evidenced by the black regions in both radiographs. Also, the intensity gradient averages at about zero, even becoming negative in some regions. Subtractmn radiographs have more colour contrast but less intensity contrast than direct colour ra&ographs.

Digital co/our radiography: W. H. Tait quality is also reduced by the increase in image noise due to the subtraction and normalization processes. Quantitative analysis is considerably improved by the increase in colour contrast, allowing better resolution of absorptivities. Visual interpretation, by long range correlation, extrapolation and interpolation, within a single radiograph and, if colour schemes remain constant, from one radiograph to another, are also improved in accuracy. However, anomalous effects may go unrecognized because they would not introduce a new colour.

|

I

O

~lt£vl

10 ~

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100 0

px

Figure 6 Subtracted energy response curves for rad,ographs obtained at 30-70 kVp (left), 70-110 kVp, 110-150 kVp and

150 kVp unsubtracted (nght)

Figure 7

30-70 kVp

Subtracuon radiograph of the electrBc shaver, at

Discussion The quality of a digitized radiograph is inferior to that of the original film radiograph. It is sampled onto a limited number of pixels and quantized into a limited number of grey scales, with added camera noise. In effect, the information capacity is reduced. The transition from monochrome to colour introduces further degradation of visual quality, since each component image is compressed into a relatively small number of bits. The advantage of digitization is that it allows the images to be processed. In particular, they can be colour processed to produce radiographs such as those displayed in Figures 9-12. The first two are direct colour radiographs, while the remaining two are subtracted colour radiographs. Figures 9 and 11 are low resolution, VGA displays of shaver radiographs, while Figures 10 and 12 are high resolution, 4 bits per colour plane, images of a computer mouse and a stopwatch, respectively. In all of these radiographs, there is an increased latitude compared to that of a monoenergetic radiograph, in that they display a greater range of object structures. This is predictable from the energy response curves and exactly as would be expected from the use of several exposures. However, the effect is now contamed in a single image, permitting direct comparisons between features that might otherwise have been impossible.

70-110 kVp

These include the long range correlations and other visual processes that facilitate image interpretation in colour pictures, compared to monochrome ones. In Figure 10, the solder points on the printed circuit board are more easily identified and counted than in monochrome, and the orange coloured restraining catches in the top right and bottom left of the stopwatch picture, in Figure 12, are clearly of equal absorptivity and material composition. Energy-related colour facilitates other visual processes, such as the extrapolation and interpolation of structures against a background of overlaying features, as illustrated in all the colour images.

These predictions are confirmed by the subtraction radmgraphs shown in Figures 11 and 12. The former is a low resolution picture of the electric shaver, with 2 bits per coiour, and the latter is a 640 by 480, 4 bits per colour, radiograph of a stopwatch. They are much more colourful than the direct radiographs but have uneven intensity contrast. They more easily depict all the object features, from the low absorptivity plastic to the very dense transformer, but lack the familiar intensity contrast that assists in the recognition of object features. Image

The colour radiograph is quantitative to a greater extent than a monochrome one. The colours allow absolute determinations as well as comparisons. It is an effect used m other imaging processes, such as digital subtracUon angiography, in gamma camera images, and in absorptiometry, where the purpose is analytical, that is, to make intensity measurements rather than imaging. In this case, both results are available. The coiour of any part of the image represents a measure of object absorptivity, while preserving the comparative effect of a recognizable picture. It allows the colour radiograph to be used for elemental analysis.

Figure8

Subtra(~Uon radiograph of the electrm shaver, at

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Figure 9 Colour radmgraph of an electr)c shaver displayed m 320 by 200 plxels w~th 2 bits per colour plane, red, green, cyan and blue for low to high energ)es

Figure 11 Subtracted radiograph of an electr)c shaver dmplayed m 320 by 200 pixels with 2 bits per colour plane, red, green, cyan and blue for low to h)gh energies

Figure 10 Colour radiograph of a computer mouse displayed )n 640 by 480 plxels with 4 bits per colour plane, red, green, cyan and blue for low to h=gh energ)es

Figure 12 Subtracted radmgraph of a stopwatch displayed m 640 by 480 p=xels with 4 bits per colour plane, red, green, cyan and blue for low to high energies

Anomalous absorption enhances the analytical nature of the colour radiograph by permitting the detection and measurement of atomic number, rather than the more ambiguous absorptwity It is possible only for relatively high atomic numbers, in excess of about 42. The process ~s not illustrated in any of the pictures shown here, but can be demonstrated for heavy metal test objects. In general, the enhancement is quite small, unless the element responsible is a substantial component of the ray path. Its effect is to produce an unexpected colour which is obvious in the additive colour schemes of the direct energy radiographs.

Digital colour radiography is a useful addition to present X-radiographic techniques and further developments may be expected to extend its advantages. These include improvements in computer display technology, imaging systems with better energy resolution and photon count rate capability, and advances m real-time radiographic systems. Both the direct and the subtractive formats should find applications in medicine as well as NDT.

The overall conclusion is that true colour radiographs can be produced by digital processes, and in at least two different forms. Both relate colour to photon energy and object composition, but differ in other respects. The direct colour radiograph is forced to use a colour scheme that lacks colour contrast but has the advantage of good intensity contrast and low noise. This is a relatively short step away from the [amiliar monochrome image and better for feature recognition. The subtractive mode provides good colour contrast but poor intensity contrast and, as illustrated in Figure 12, increased image noise. It zs better for quantitative analysis, but not elemental analysis by anomalous absorption.

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8 Birch, IL, Man/all, M. and A r d r ~ G. M. 'Catalogue of Spectral Data for D]agnoshc X-Rays' SRS-30 Hospital Phys,cists' Association (1979) 9 Milan, J. and Taylor, K. J. W. J Chn Ultrasound 3 (1975) pp 171-173 10 Houston, A. S. J Nucl Med 21 (1980) pp 512-517