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ORIGINAL ARTICLE Table of Contents   
Year : 2002  |  Volume : 12  |  Issue : 2  |  Page : 169-178
Retrieval of true color of the internal organ of CT images and attempt to tissue characterization by refractive index : Initial experience

National Centre for Experiental Mineralogy and Petrology, Allahabad-211002, U.P, India

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To retrieve the true color of the internal organs and various tissues of the CT images obtained in gray scale, by determining the value of color components, density, CT (attenuation) value, refractive index (RI) and gray value of various tissues in vivo thereby revealing physics of true color sensation of tissue in various path-physiological states and to digitize the tissue and parenchyma by R.I. Materials and Methods: Physical density, CT value (No.) and refractive index of the tissues of goat and biopsy materials of various patients are determined in the laboratory and integrating such data with gray value of CT images. A computer generated color image of the organs and tissue are thus retrieved. Value of the red (R), green, blue (B) components (out of 256 shades) of true color of the tissues are obtained by a special technique of optics. Results: From all these experimental data a mathematical equation has been derived to show the relationship of CT No., refractive index and density of tissue which explains the underlying physics of true color sensation (wave length) of tissue in different patho-physiologic states. The refractive index is found to be unique for each tissue. Conclusion: All these determined values are assigned for gray value to produce a computer generated true color image of various tissue of CT images obtained in gray scale. Color of the tissue in turn determines R. I. a tool for tissue characterization.

Keywords: Tissue characterization, refractive index(R.I.), laboratory tests, true color

How to cite this article:
Biwas T K, Gupta A K. Retrieval of true color of the internal organ of CT images and attempt to tissue characterization by refractive index : Initial experience. Indian J Radiol Imaging 2002;12:169-78

How to cite this URL:
Biwas T K, Gupta A K. Retrieval of true color of the internal organ of CT images and attempt to tissue characterization by refractive index : Initial experience. Indian J Radiol Imaging [serial online] 2002 [cited 2020 Dec 4];12:169-78. Available from:

The images of the various organs and tissues derived from CT, MRI and Ultrasound are displayed in gray scale, not in their true color. The color is an important physiological parameter. The collimated X'Ray photon of CT, the radio frequencies (the radiant energy) of MRI and ultra sound do not possess the property to encode and transport the color of the tissue. The purpose of the article is to understand the underlying science of the color of the tissue in different patho-physiologic state and to reveal the relationship of CT number, gray, red, green, blue value (of the true color) of the tissues and refractive index. Retrieval of the true color of the organs and tissue is possible by using their CT No, gray, red, green, blue color value and characterization of tissue from their refractive indices. Actually the dominant wave length of the color of the substance is reflected and others are absorbed and color of the substance is seen [1]. It is impossible to appreciate the color of the closed internal organs by reflection of light. Retrieval of the true color of the organ and tissue, by ordinary white light, is also not possible because it can not penetrate the whole thickness of the body through and through. Even the strongest LASER fails to do so because red LASER is attenuated by the hemoglobin [2].


The two major criteria of the color image formation are : i) to obtain the value of red, green, blue components of various organs and tissues and (ii) a photon which can penetrate through the body, upon which the color informations can be integrated with its important property. If an imaginary radiomagnetic wave is available which penetrates through the body like X'Ray and decodes color signal of the tissue like light of visible spectrum as well as gets attenuated by the tissue then the answer can be obtained. When the electromagnetic wave beams through matter, both dispersion and absorption (attenuation) of the wave by the matter occur. Refractive index (R.I.) is based on this dispersion property of the matter. [3] A relationship of the attenuation of matter with its refractive index has been determined after analyzing the results obtained in the laboratory [4].

The mathematical relationship has been established, which leads to the correlation of the attenuation number, color sensation (wave length), R. I. and density of tissue [4] :

Color Sensation of tissue (wave length λ)

= CT N0 x R. I. x 100 x d


CT NO = CT number of the tissue

= (us - uw) X1000


us = attenuation coefficient of the material ,uw=attenuation coefficient of water [5],[6],[7]

R. I. = refractive index [8]

p = density of tissue d =density of water

If the value is expressed in Angstrom () ie. in terms of wave length, any value obtained by this equation between 3800 to 7800 corresponds exactly with the color of the visual spectrum. [4] [Figure1] Any value below 3800 appears colorless and value above 7800 looks white. In the CT images the colorless substances like C.S.F., air, gas, water have been assigned black

A brief review of the underlying physics of R. I. and color, trichromatic theory and basis of additive color mixture of red, green and blue has been discussed below [1].

A brief review of the underlying physics of R. I. and color, trichromatic theory and basis of additive color mixture of red, green and blue has been discussed below [1].

Physics of Color Sensation

Appreciation of color is a function of the light adapted eye and is entirely the property of the cones (color receptors). Eye can distinguish seven colors of the spectrum and an infinite number of intermediate shades of 160 different colors in the visible spectrum. The spectral colors are expressed in terms of wave lengths(l) rather than frequency units. [9].[10] According to the trichromatic theory the sensation of many colors are produced by the combined stimulation of Red (6270-7800) Green (5000-5780) and Blue (4640-5000) color at different proportion. The three different photoreceptors of retina after being stimulated transmit different types of signal to visual cortex. The basis of additive color mixture are (1) Brightness (2) Hue - dominant wave like RED (R), GREEN (G) and BLUE (B) (3) Saturation (purity). These are all psychological concepts while radiant quantities are physical [9].

Refractive Index

In the theory of the refraction of electromagnetic waves on passing from a vacuum into a material, it is considered that the electric vector in the incident beam will bring about forced vibration of the electrons which already have free vibration of their own. The electromagnetic wave thus originating from the forced vibration of the electron are the waves of the refracted beam in the material. Sellmeire's law [8].[3] analyses the mathematical relationship of the refractive index for the material. In the case of visible light, this equation gives result for the refractive index and for the variation of the refractive index with wavelength (ie. the dispersion) of materials which can be related to practically determined values.

Materials and Methods

Four important parameters to produce color images are : (1) determination of R G B color components of the tissue (2)tissue attenuation - to determine CT number (3) physical density of the tissue (4) Refractive Index (R.I) or tissue permeability. The methods of determining these important parameters have been described below. Tissue of the goat from the slaughter house and biopsy materials after surgery were collected and utilized.

Determination of the Color Components

The following experiment of optics helped to get the primary component of the original color of the tissue.

n a dark room 100% saturated (made by gelatine filter) narrow beam of Red (R), Green (G) and Blue (B) light beam (light source - 100 watt white lamp) are allowed to pass one after another through thin slice (5 to 8mm) of various tissue and organs like liver, kidney, lung, muscle, brain heart clotted blood etc and are photographed serially [Figure - 2]a,b,c. These R, G and B predominant photographs of the tissue carry the red green and blue components in different proportion. If the three R G B colored images of the organ or tissue are allowed to be super imposed (2d) one by one in the same frame of the color photographic film (during taking of the picture) then they will overlap one upon another in appropriate proportion producing almost original color of the organ.

The RGB color photographs of the tissue are separately analysed by a computer to know about the following informations: :

1) Color shades - out of 256 shades - a) Red b) Green, C) Blue [14]

2) Amount of Hue - that means how much color is there [1],[14],[16]


CT number of various tissues, blood, C.S.F., bile are determined by a 3rd generation CT scanner (WiproGE, CT 1800i). CT or attenuation number represents the linear coefficient of attenuation of various organic tissue in relation to water and expressed in Hounsfield unit (HU) [10],[11],[12],[13] CT No. of the non contrast image is mainly taken. Small amount (10 to 20 cc) of non-ionic contrast is used in Contrast enhanced CT [6]

3) Density : ( p )

Density of the various issue and organs are determined first by determining volume by dipping the tissue in a water filled measured cylinder and then weighing the tissue by a digital balance to note the mass.

Density ( p ) =: mass gm/cm3


Results have been discussed.

Refractive Index

Refractive index of tissues of various organs were determined by a) Abbey's Refractometer and b) by oil index method.[14]

a) Refractometer- With the help of a Sodium (yellow predominant) lamp, refractive index (R.I) was measured specially of the tissue of R.I. below 1.462

b) Oil index method was used to determine R.I. of bone, cartilage and fatty tissue by utilizing special Cargille fluid and looking for the inward outward movement of "Becke line". Temperature correction was also made by the following method :

(-) dn0/dt = 3.72x10-4/C

Determination of R. I. of various tissues in different diseases and neoplasm is yet to be complete. The values of R.I. is tabulated in [Table - 1].


The correlation of CT No. Gray value [16] and RGB color components of various tissue, organs and parts of the central nervous system are tabulated. Increase in CT No causes increase in gray value [7] and a tissue or a lesion looks red within certain range of CT No and gray value as for example in acute hematoma (CT No-61-86 and gray value 165-226). Similar value is obtained in the periphery of a solid tumor, metastasis and granuloma. In chronic hematoma the CT No 28 HU and gray value (49-66) are relatively low and the color is blackish brown (derived from the R-8-14 G 3-5 and B - 0 value). Degenerated central portion of a tumor or a granuloma also shows blackish appearance as derived from the low gray value (50-80) and low CT No (30-50 HU). The gray value and CT No is found moderately high of flowing blood and the color value is obtained as red. The CT No and gray value of colorless substance and tissue are markedly low. The bones and calcium containing tissue show high gray value andCT No and begin to look white when R-237 G-237 B-237. (Range of white is 237 to 255 of gray shades)

Density of air/gas is 003295 gm/cc.; CSF, water have density of 1. Fatty tissue, white and gray matter have the density of 0.9. Most of the parenchyma have density of 1. Flowing (uncoagulated) blood has density of 1.1 ; clotted blood (hematoma) 1.4 to 1.67, calcification has density of 1.6 and bone 1.8

The Refractive indices of the tissue and organs are unique (table - 1) Various organ and tissue like serum, lung, and gall bladder and gray matter have R. I ranging from 1.33 to 1.35. Other organs and tissue like liver, kidney, spleen clotted blood have refractive index around 1.43 to 1.44 Fat predominant organs have R I more than 1.46. However cartilage and bones have high R. I. (> 1.49) A definite influence of R.I on the color of the tissue and organ is obvious from various results. To express the value of R. I. in a magnified scale 1 and decimel sign of the value have been dropped out eg. 1.443 becomes 443 in the magnified scale.

Discussion (Table 4 through table 9)

To recap the relationship of CT No. Refractive index and physical density with the color sensation (expressed in Angstrom)

Color sensation (wave length l)

= (CT X R.I. x 100) x d


CT = CT number of the tissue, R. I. = refractive index,

p = density of tissue, d = density of water.

In general, the CT number of any material will shift as a function of the difference between the atomic number of that material and the atomic number of water. CT number determines directly the effective atomic number or electron density of the tissue instead of the attenuation coeeficient [7]. So importance of the density of tissue (p) and density of water (d) are important in the above expression of color sensation (wave length). From the Sellmeire's law it is obvious that electron density of the tissue influences the R. I. and emergent wave length (wave length responsible for color sensation). The incident wave length (l) of the electromagnetic wave remains unchanged [8],[3]

It has been observed in this study that CT number has a distinct relationship with the color though tissue containing elements like iron of hemoglobin / myoglobin, calcium and fatty tissue have further influence. Wave length of value less than 3800 looks colorless and more than 7800 appears white.

CT number of the substance between - 1000 to +20 are usually colorless like gas, air, CSF water. In these substance wave length is less than 3800. (water generates a wave length of maximum 2600). Here it needless to mention that the -ve sign of the value of CT No. is ignored. The exception here is the fatty tissue (CT number -60 to -160HU). The high refractive index (1.472) and low density

(0.9) of fatty tissue have definite role producing whitish appearance which is quite obvious from the above equation (wave length = 1390 A to 2616 A). Any tissue with density (CT number) 90 HU, R.I of 1.465 and density of 1.4 looks white (wave length 9417 A) and does not look red. On the other hand a hematoma of attenuation value of 86 HU, R.I of 1.465 and physical density of 1.62 color sensation or wave length would be 7778 A [Figure - 3]a,b. A chronic hematoma with markedly low attenuation and R. I appears to be brownish black [Figure - 4]a,b. Similarly bone or calcification with CT No. of 100, R.I of 1.556 and physical density [1].[8] derives a wave length of 8644 A which is beyond the last limit of the visible color spectrum (7800A) C.S.F. edema etc have attenuation number (CT number 5-23 HU) and are colorless. Similarly hair, gases have CT number approximately - 300 to 900 HU and are colorless. In the colorized CT they have been assigned black. Calcium containing tissues have CT number more than 100, like calcifications, bones, teeth (CT No - 150 - 1800 HU) etc look white.

Blood Supply

Blood supply of the organ has a direct relationship with the color. Highly vascular organs are appreciated red or reddish and the attenuation is high ranging from 58 to 86 HU. This is due to the high iron (Z=atomic No. = 26) [5],[10],[11],[12] content of the Hemoglobin. In an inflammatory lesion or if the blood brain barrier disruption occurs as in the neoplasm (so there will be increased CT number) red colored margin or peripheral rim of the pathological lesion is seen [Figure - 5]a, b, c and [Figure - 6]a, b, c eg. abscess, granuloma, metastasis and high grade astrocytoma or glioblastoma. A low grade astrocytoma naturally will not show this.

In the brain blood vessels and highly vascular organs (red colors) show attenuation number 50 to 54 HU like straight sinus, vein of Galen and choroid plexus. Acute hematoma measures about 60 to 86HU and are bright red. Chronic hematoma looks blackish to dark brown (Density - 28 HU). Mid brain, pons, basal ganglia and internal capsule are also light orange. Gray matter looks reddish brown (41 to 44 HU). Highly vascular Meningioma and pituitary adenoma have increased density (CT No) and look red.

In the thorax Great vessels, Cardiac chambers show increased CT numbers and are red colored [Figure - 7] Mediastinal tissue shows yellowish orange appearance. Bronchogenic Carcinoma gives a variable appearance of red, orange and yellow color [Figure - 8]

The normal Gall bladder looks bluish. Its attenuation value is 32 to 36 HU and when its relation with the R.I. of 1.35 and density [1] is taken into account the color sensation falls into the bluish spectrum (wave length 4320 to 4867 A - (the blue predominate wave length is 4300) [Figure - 9]. It is indeed difficult to explain the color of the bile probably due to its change in the R.I. in different state. Noninfected bile gives a dark green color (5040A). An infected bile may be whitish due to gross change of R.I. and attenuation value (CT No.) R.I. of bile changes appreciably if it is preserved for a long time. Probably a delicate balance of the bile pigment, bile salt and cholesterol in the physiological state influences the R. I. which is completely different when measured in vitro.

It is an interesting point to note that though liver, gall bladder, pancreas and kidney are closely situated and there is not much of difference in the CT No, their color is different. It is mainly due to their respective refractive indices which play a major role for generating their color.

Renal tumors, specially renal cell carcinoma and adrenal tumors appears reddish in color. The area of tumor degeneration looks blackish. A fibroid uterus looks reddish yellow.

Fatty tissues are yellowish white or white (CT number - 60 to - 160). White matter in the brain looks off white to yellowish white (34 to 36 HU). Here low density (0.9) and increased refractive index may have certain role for generating this color, because the deducted wave length is 9813 A.

Conversely, from the wave length (color) of the unknown tissue / parenchyma or a lesion of a colored image, along with the CT No and density, R. I. of the tissue etc can be determined. It can be corroborated with the value already determined, and thus the character of the tissue can be determined.

R. I. And Tissue Characterization

The initial experience of various laboratory data leads to the hypothesis that each tissue or parenchyma has unique R. I. This R. I. is influenced by the density of the tissue and cell type. Density depends upon : a) Number of cellularity and b) Inflammatory state.

a) Different parenchyma has unique R. I. For example R. I. of Liver, spleen, kidney are different. Increased cellularity will increase the R.I eg. Neoplasm and in malignancy R.I. should have specific value. Similarly diminished cellularity will cause low value of R. I. Different disease will have specific pattern of changes of R. I. For example the R. I of liver in fatty infiltration and storage disease will have specific values. Determination of the R. I. of the pathological tissue is yet to be completed.

b) Inflammation - increased vascularity, exudation, pus cells, scavengers and damaged cell membrane are the factors which will modify the density of the tissue and thus will change the R. I of a tissue/ organ

Computer Generated Colored Images

With the help of a special software the color images are produced in a computer. According to the relation of gray value and attenuation number the R G B value of the organs and tissues are assigned. As raw data of the pixel is not available in the work station so emphasis has to be given on the gray value. Different settings are used for brain, abdomen and thorax.

At first the image from the scanner is captured in the work station. To get uniform colorization and to eliminate noise, tonal correction of the gray value of the images is the required. The image is converted to 256 indexed format (24 bit). The program has been created in such a way that it interprets the gray value of each pixel; then it colorises pixel by pixel according to its assigned value.


We sincerely thank Prof. P. Wooman and Dr Mrs. Elizabeth Sengupta, Department of Neuro Pathology and Surgical Pathology and Electron Microscopy of the University of Chicago, U.S.A. for providing us various slides and photographs of the pathological specimen.

We are grateful to Mr Rajendra Prasad Shaw, the photographer who has taken all the photographs used for optical study for last ten years and also for the article.

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Correspondence Address:
T K Biwas
Biswas X-Ray and Scan Centre, G T Road west, Asansol-4, 713304, W.B
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Source of Support: None, Conflict of Interest: None

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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10]


[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5], [Table - 6]

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