DIRECT AND INDIRECT METHODS FOR OBTAINING COLOR X-RAY IMAGES

V. A. BERDONOSOV, V. I. GORBUNOV, A. A. POPOV

Radiography using black-and-white film does not make it possible to detect small changes in the density of the darkening of the emulsion on an X-ray image. It has been established that the human eye, in the range of optical densities from 0 to 3, can distinguish only several hundred gradations of gray shade. This is a substantial shortcoming of black-and-white radiography, since small changes in the density of darkening are of very great importance for determining defects in a specimen.

An ordinary black-and-white X-ray photograph contains only one evaluation parameter—brightness of the gray shade (optical density). A color image, unlike a black-and-white one, makes it possible to obtain two additional parameters when assessing the quality of the inspected object: color and saturation, owing to which a significant increase occurs in the informational capacity of the radiograph (a person is capable of subjectively distinguishing several thousand colors and shades of color).

Research in the field of color radiography has been underway for a comparatively long time.

As early as 1951, the first color radiographs of the human body in the world were obtained [1]. From that time the successful development of color radiography began.

In the initial period of developing methods for obtaining color X-ray images, work was conducted mainly in the USA and England. Recently, considerable efforts in this direction have been undertaken by such countries as Japan, the GDR, France, Canada, and the USSR [2].

It should not be thought that attempts to replace black-and-white radiography with color radiography are due to economic or technical factors: the development of methods for obtaining color X-ray photographs promises great prospects because of their high informational capacity.

Methods of Color Radiography

At present, the development of methods of color radiography is being carried out along two main lines (indirect and direct methods) (Fig. 1).

Indirect methods involve the use of a monochromatic black-and-white radiograph, whose gray gradation is then, by one method or another, converted into colors and shades of color.

In direct methods, color photographic materials are used

or special color radiographic films, which are exposed to X-ray or γ-radiation, with or without the use of intensifying screens.

Indirect Methods

In [2] a method is described for converting a black-and-white X-ray image into a color one; its essence consists in repeated chlorination and bromination of a black-and-white X-ray film in combination with repeated development, successively, in special coloring solutions. When superimposed in layers, the dyes form a color image whose information content, in comparison with a black-and-white one, is substantially increased.

Of considerable interest is the monochromatic method in color radiography. Its essence is as follows. The human eye is most sensitive to the light energy of the green region of the visible spectrum. From color theory it is known that fuchsin (purple) objects are the most effective absorbers of the green hue. This phenomenon can be successfully used to improve the information content of an X-ray image. Thus, for example, the authors of [3], in order to obtain fuchsin-tinted radiographs, chose the method of metallic toning. Photometry in green light showed a significant increase in the contrast of the purple image in comparison with an ordinary black-and-white one. After processing, the image of the defect began to differ from the background by 0.09 units of optical density, whereas before toning the difference in optical densities was only 0.03 units of optical density.

A color image can be obtained not only on the black-and-white film itself. For example, one can make a copy of a black-and-white negative on special flexichrome film, as described in [2], and then color this film with dyes of various colors.

A copying method for obtaining a color X-ray image is fairly widespread; in this method, negative copies of the X-ray image are made on two positive films. Then the positives (each under its own color filter) are projected one after another onto color photographic paper or onto a projection screen [2].

The spectrum of X-radiation consists of a mixture of waves of different energy, which, in accordance with their physical properties, produce different photochemical effects. However, in an ordinary black-and-white image this is not noticeable, since in the emulsion layer the photochemical effects of waves of different energy are superimposed and mutually cancel one another.

To obtain radiographs whose color depends not only on intensity but also on the radiation energy, the object to be inspected is radiographed repeatedly with X-rays of different hardness, duration, and intensity. In this case exposure may be carried out successively for each film (multiple exposure) or simultaneously (single exposure), using filters to select the X-rays. Each film is then colored in its own color [2].

This method can be successfully applied in cases where, in the inspected part, it is necessary to reveal materials of different density and atomic weight. An experiment was performed in which a wedge, consisting on one side of lead and on the other of tin, was irradiated with X-radiation with maximum energies of 35 kV and 125 kV

Fig. 1. Schematic overview of methods for obtaining a color image

• Decoding
• Negative
• Television
• Without the use of auxiliary means
• Projection screen
• Negative
• Projection screen
• Negative
• Application by printing on paper
• Without the use of auxiliary means

• Negative

• Color indication

• Multilayer chromogen development
• Coloring by absorption of dyes
• Conversion to a color television image
• Printing on color paper
• Simultaneous projection through filters
• Monochromator
• Coloring with a vanillin solution
• Application by printing on paper
• Single-layer chromogen development
• Simultaneous projection through a color filter
• Ordinary color development
• Development with intermediate exposure
• Ordinary color development
• Application by printing on paper
• Ordinary color development
• Ordinary color development

• Special development

• Shooting technique

• Color separation
• Color separation
• One-shot method
• Multiexposure method
• Monochromatic protection
• Monochromatic protection
• One-shot method
• One-shot method
• Monochromatic protection
• One-shot method
• Monochromatic protection
• Monochromatic protection
• One-shot method
• Simultaneous or without the use of color screens
• Simultaneous or without the use of color screens
• Color rulers
• Color ruler
• Color photography
• Color photography
• Ordinary method

• Projection method

• Recording material

• Black-and-white X-ray film
• Reversal method
• Color radiography
• Color photographic paper
• Color X-ray film
• Ordinary methods
• Direct methods
• Color photographic paper
• Color X-ray film
• Color radiography
• Color photographic paper
• Color X-ray film
• Ordinary method
• Color photographic paper
• Color X-ray film
• Color photographic paper
• Color X-ray film
• One-shot method
• Color radiography

Fig. 2. Method for material recognition.

(Fig. 2). The image obtained with an energy of 35 kV was colored red, the other green.

In this case, because of the different spectral course of the process of absorption of X-ray radiation, the heights of the Pb and Sn steps, which at an energy of 35 kV have identical absorptive properties, at 125 kV give different values of absorption; as a result, when the images are superimposed it is possible to distinguish lead from tin.

It should be noted, however, that the difference in the absorption process of \(\gamma\)-rays of different energies becomes noticeable only at relatively large wavelengths. In connection with this, this effect can hardly be used in production inspection of materials of great thickness or density, where increased voltage most often has to be used.

Direct methods

Direct methods using photographic materials

The possibility of using photographic materials in color radiography is based on the different sensitivity of emulsion layers to X-ray or \(\gamma\)-radiation.

Usually a balanced color film or photographic paper has the characteristics shown in Fig. 3. If, however, the film is exposed to X-rays or \(\gamma\)-rays, then, owing to the different sensitivity of the emulsion layers to this type of radiation, the film proves to be unbalanced both in contrast and in sensitivity, as a result of which different color shades appear in accordance with the intensity of the incident radiation (Fig. 4).

As in ordinary black-and-white radiography, exposure of color film may be carried out with or without the use of intensifying screens [4]. The principal drawback of exposure without the use of intensifying screens is an inadmissibly long exposure time, since the photographic material used is of low sensitivity.

Fig. 3. Characteristic curves of a color material balanced for light sensitivity and contrast. \(D\) — color optical density; \(H\) — illumination intensity.

Fig. 4. Characteristic curves of ORWOcolor PC7 positive film when exposed to X-rays \((U = 80\ \text{kV})\). \(D\) — color optical density; \(Q\) — X-ray intensity.

is sensitive to the action of ionizing radiation. The use of metallic intensifying screens reduces the exposure time severalfold while simultaneously improving the sharpness and color contrast of the image.

Since ordinary photographic film is sensitized to visible light, the use of fluorescent intensifying screens entails an even more substantial reduction in exposure time. In practice, a combination of fluorescent and metallic intensifying screens is usually used.

The authors of [4] carried out a number of experiments to determine the best combination of intensifying screens. Transillumination of aluminum using films of the ORWOcolor NT17 and ORWOcolor PC7 types with various combinations of intensifying screens \((\mathrm{Pb}\ 0.15/\mathrm{Pb}\ 0.15,\ \mathrm{M100}\ (M=\mathrm{CaWO}_4)/\mathrm{Pb}\ 0.15;\ \mathrm{M200}/\mathrm{Pb}\ 0.15;\ \text{standard}/\mathrm{Pb}\ 0.15;\ \mathrm{ZnS}/\mathrm{Pb}\ 0.15,\ \mathrm{ZnCdS}/\mathrm{Pb}\ 0.15,\ \mathrm{M100}/\mathrm{M100},\ \mathrm{ZnCdS}/\mathrm{M100},\ \mathrm{M100}(\mathrm{ZnS}))\) showed that the combination \(\mathrm{ZnCdS}/\mathrm{Pb}\ 0.15\) gives the minimum exposure, approximately three times smaller than with screens of the \(\mathrm{M100}/\mathrm{Pb}\ 0.15\) type.

It should be noted that not all types of photographic materials are suitable for color radiography.

Studies have shown that negative types of film are not advisable to use in this case, which can be explained by the practically identical behavior of the color-density curves of the emulsion layers of the film (Fig. 6) [5].

Positive film gives an undoubtedly larger number of colors and shades, as is evidenced by a comparison of Fig. 5 with Fig. 6.

A significant place in color radiography using photographic materials is occupied by the method of increasing color contrast [6]. It consists in dividing the development procedure into two stages (the stage of initial development and the stage of subsequent development), between which the film is “fogged” with visible light. In doing so, it is necessary that illumination of the film be carried out with green, red, or yellow light, to which the lower emulsion layers are sensitive. Areas exposed to X-radiation are not affected by the “fogging,” since the silver image present in the upper layer, which appears

![Figure 5 graphs]

Fig. 5. Partial color densities of ORWO color PC7 film with various combinations of screens when transilluminating Al. \(D_B\) is the color density behind the blue filter; \(D_G\) is the color density behind the green filter; \(D_R\) is the color density behind the red filter.

Fig. 6. Partial color densities of ORWO color NT 17 film when transilluminating Al. \(D_B\) is the color density behind the blue filter; \(D_G\) is the color density behind the green filter; \(D_R\) is the color density behind the red filter.

![Figure 6 graph]

after the initial-development stage, acts on the lower layers as a black filter.

Thus, fogging the film during development increases the image density and the color contrast in those cases where, because of excessive thickness or density of the material, the film has been underexposed to a considerable extent by the X-rays.

Differentiation of the optical densities of an X-ray image arises as a result of changes in the density or atomic weight of the object being transilluminated. Moreover, if changes in optical density that appear as a result of nonuniform density of the specimen are independent, over a wide wavelength range, of the voltage on the tube, then differences in optical density caused by changes in atomic weight depend to a large degree on the radiation energy, especially in the energy range of 40–60 keV.

To determine the cause of the change in the optical density of an X-ray image, it was proposed to expose simultaneously two films at once, with a filter placed between them that absorbs the soft components.

After photochemical processing, two radiographs are obtained simultaneously: the first in the region of soft radiation, and the second in the region of harder waves. In this case, changes in density and in the atomic number of the material are conveyed in different tones and shades [2].

In this method, it is recommended to use a red fluorescent screen for the upper film and a blue fluorescent screen for the lower film.

Direct methods using color X-ray films. The simplest color X-ray film consists of two emulsion layers, each of which has its own contrast coefficient, thereby achieving a certain change in the color and brightness of the image when the thickness or density of the specimen changes [2].

A similar film was created in the USSR by the MNIRRI institutes.

and the Research Institute Khimfotoproekt. This color X-ray film is a two-layer photographic material. Each of the film layers contains a color, non-diffusing component which, during color development, forms a purple image in one layer and a green image in the other. When exposing the film, it is necessary to use different X-ray intensifying screens of the “Standart” type, SB, US, and others.

Another variety of color X-ray materials is the film proposed in [7]. This film consists of two emulsion layers containing sensitizers and color components, between which an absorbing screen is located (Fig. 7).

Fig. 7. Structure of color radiographic film.

Diagram labels: X-rays; emulsion layer (blue-green color); filter; emulsion layer (red color); base.

After exposure and photographic processing, a color image appears on the film; its coloration depends not only on the intensity but also on the wavelength of the X-ray radiation.

With an appropriate choice of the thicknesses and compositions of the emulsion and filter layers, it is possible to achieve a certain sensitivity of both layers, depending on the wavelength of the X-ray or γ-radiation.

From the diagram of the color density of the emulsion layers as a function of the wavelength of the incident radiation, it is evident that at a wavelength of 0.9 Å the densities of both layers are the same, but when the energy of the radiation is decreased or increased, the equality is disrupted, respectively, in the direction of an increase in the density of the blue-green or red color (Fig. 8).

Fig. 8. Dependence of color density on the wavelength of the incident radiation.

Diagram labels: \(D_1/D_2\); blue-green; red; \(\lambda, \text{Å}\). Axis marks: 3, 2, 1, 0.8, 0.5; 0.3, 0.5, 1.0, 2.0.

The number of shades in an X-ray image can be increased if a larger number of emulsion layers is used. Thus, for example, a good multicolor image is produced by a film consisting of three emulsion layers, each of which contains dyes of a definite color of subtractive color synthesis.

Conclusions

1. Differentiation of a material by thickness and density is revealed in the image through changes in brightness, as well as in color and saturation, which increases the informativeness of a color X-ray image in comparison with a black-and-white one.

2. A color X-ray image shows the tendency—an increase or decrease—in the change of the thickness (or density) of the material being transilluminated.

3. A change in color permits a qualitative determination of the dimensions of a defect in the direction of transillumination.

4. When the “overexposure” method is used, the range of thicknesses being evaluated is greater than in black-and-white radiography.

Prospects

The necessity and expediency of color radiography are beyond doubt. The question is to select, from among all possible methods of producing a color radiographic image, the most promising one.

Indirect methods, evidently, because of their laboriousness and complexity, will not become widely used. Although the development of these methods is continuing even now.

The use of photographic materials was developed in the initial period of the development of color radiography methods, when it was necessary to obtain data on the effect of ionizing radiation on multilayer materials, and on the principle of obtaining an image on multilayer materials in radiography.

Because of the low sensitivity of photographic material to ionizing radiation, and the long and laborious processing time, color photographic film can also hardly be called a promising material suitable for radiography.

The accumulated data on the effect of ionizing radiation on multilayer materials make it possible to create a specialized color film.

The use of a specialized color film offers a number of advantages over other methods. These are the short exposure and processing time and good sensitivity.

Taking into account that this method is at the initial stage of development (for example, the domestic film RTs-2 is currently being refined in order to improve its characteristics) and that at MNIRRI a film is being developed whose image-assessment parameters (color, saturation, brightness) will depend not only on intensity but also on the wavelength of the incident radiation, which will sharply increase the sensitivity of the film, it must be assumed that the method using specialized color film is the most promising.

LITERATURE

1. G. E. Donovan. Radiography in Colour, The Lancet, 1951, April, 14.

2. H.-V. Richter, D. Linke. Grundlagen und Verfahren der Color-Radiografie, Die Technik, 21 (1966).

3. N. S. Beger, I. E. Staroba. Color Radiography in Monochrame, D3, Sixth International Conference of Nondestructive Testing, June 1970, Hannover (Germany).

4. H.-V. Richter, D. Linke. Color-Radiografie, ein neues Verfahren der industriellen Werkstoffprüfung, Die Technik, 21, 1966.

5. H.-V. Richter, D. Linke. Color-Radiografie, Die Technik, 21, 1966.

6. N. S. Beyer, and I. E. Staroba. Besearch and Development in Color Radiography, Materials evaluation, 1968, vol XXVI, 8.

7. I. Blum. Nouvelles emulsions sensibles an x rayonnements invisibles, Science et industries photographigues, 1958, 29.