Color in Medical Imaging

The field of radiology dates back to 1896 with the first direct x-ray exposure of film by Roentgen. From the next 80 years the exposure of black-and-white film by x-rays, and later, from calculated images from CAT scans, ultrasound scans, and MRI scans would dominate the hardcopy world of medical imaging. The uses of color were largely experimental and limited until the development of real-time color Doppler imaging in the early 1980s. Since that time, the uses of color have grown rapidly as 3D visualizations and multi-modality or multi-spectral images become widely utilized. The rapid growth of imaging techniques that combine anatomical information with additional functional or molecular information is driving color to the forefront, since additional information needs to be fused in the renderings. Thus, 100 years after Roentgen’s experiment, a century of monochrome imaging is giving way to an emerging need for color displays of medical images. The Monochrome Era 1900-1980 The direct exposure of film by x-rays dominated radiological imaging for at least the first half-century after Roentgen’s initial exposure of the human hand (Fig. 1). Due to the increasing understanding of the need to limit exposure to x-rays, a major early focus of development was on increasing the sensitivity of film, and developing contrast agents for enhancing the vasculature and uptake of contrast by organs and lesions. Many new techniques for obtaining tomographic and 3D image data sets were developed in the second half of the 20th century; however, the great majority of CAT scans, ultrasound scans, and MRI scans up to the early 1980s were rendered and interpreted as monochrome images (Fig. 2, 3, 4). One early use of color involved multiple x-ray exposures of the same object using different x-ray energy (Fig. 5) [1]. It is an empirical fact that the absorption of x-rays within bone and soft tissues is dependent on the wavelength (energy) of the x-ray source. Thus, multiple exposures of different x-ray wavelengths, when rendered as color channel information, were studied for the additional differential information that they might contain. However, this technique had a number of disadvantages, including the need for higher total x-ray exposure, and it did not achieve widespread use. Fig. 1. Wilhelm Roentgen with X-ray radiograph of his wife's hand. (http://www.learnxrf.com) Fig. 2. CT image of the human liver. (Permission from Dr.