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15-02 Gray Scale and Color Images

he introduction of color images is a recurrent and last­ing topic in dia­gnos­tic imag­ing. The contribution of colors to imaging dia­gnos­tics, in particular high-resolution images, is much debated; colors do not add any provable dia­gno­stic facts. All digital images are per se gray scale images. They can be 'artifically' co­lor­ed (pseudo colors). MR images are always gray-scale ima­­ges; colored MR ima­ges are only used as show effects. In diagnostic MR imaging, colors lack the dy­na­mic range of gray scale ima­ges and image windowing is not possible.

However, pictures of a number of MRI offsprings (e.g., MR angiography, dynamic con­trast-enhanced MRI, functional MRI, MRI tractography, PET-MRI fusion images) of­ten contain over­layed colored areas representing usually the lower resolution imag­ing tech­ni­que.

Colors are subjective qualities. They might confuse, bias, and lead to a loss of in­for­ma­tion. In general, their per­cep­tion is not well understood.

spaceholder redOne example is the semiotics of color-coding in BOLD imaging: Colors are as­sign­ed to degrees of statistical significance, then turned into colored pixels, combined into blots, and overlayed on high-resolution MR brain images to reveal 'brain ac­ti­va­tion', i.e., regions of relatively more or less blood in an excited state compared to a rest state.

However, the color scale used elicits a biased reaction in many observers — ob­fus­cat­ing a neutral interpretation of the results (for details cf. Chapter 11).

spaceholder redAnother problem is the human eye. In the central part of the retina, there are ap­­pro­­xi­­ma­­te­­ly six million cone cells which are responsible for color vision. However, 8% of the male population and 0.5% of the female population in Europe and North America suffer from a color vision deficiency.

The most common one is deu­ter­ano­maly where people (among them radiologists) perceive green, red and purple as a grayish shade and are unable to identify red or green co­lor­ed areas of these images (a simulation is shown in Figure 15-03).

Figure 15-03:
Top: Regular T1-weighted gray-scale MR images of a healthy volunteer.
Center and bottom: PET-MRI fusion images of the same per­son. The upper row of the pseudo-color ima­­ges shows the normal images; the lower row shows the same ima­ges as seen by a person with red-green deficiency (deu­ter­ano­maly).