ARNE ENGSTRÖM, M.D. Department for Physical Cell Research. Karolinska Institutet, Stockholm, Sweden
The absorption of x-rays in matter follows simple and clear-cut laws. Therefore, it is to be expected that histo- and cytochemical methods based on the absorption of x-rays can be developed into valuable tools for the cytochemist. The great problem, however, is to make an image of a biological sample by x-rays with sufficiently high resolving power so that a cell of ordinary size (about 10 μ in diameter) or a part thereof (structures 1 to 5 μ in diameter) can be resolved. There are several possible ways of doing this. One is to make an absorption image of the sample in the scale 1 : 1 with adequate resolving power so that the image can be considerably magnified optically, e.g., in a microscope. The procedure is called microradiography, and the small x-ray picture is a microradiogram. If the sample being radiographed consists of a biological tissue, the procedure is sometimes called historadiography. Another principle has recently been described, according to which considerable progress in the construction of a real x-ray microscope based on total reflection of x-rays has been achieved ( Kirkpatrick and coworkers (32, 33, 34)]. Theoretically, the x-ray microscope has a high resolving power. For details and a survey of the literature, one should consult Kirkpatrick and Pattee (34) and Dyson (10).
The different principles of x-ray image formation will be dealt with in this chapter, and the errors, quantitative aspects, and possibilities of the historadiographic procedures will be discussed. Heretofore, most of the historadiographic methods used in biological research have been based on direct microradiography, and these methods will therefore be treated in detail.
The technique of microradiography is not new. A few years after Roentgen's discovery of x-rays, Haycock and Neville in England used a radiographic procedure in an experiment which was a direct forerunner of the present-day microradiographic technique. They used the radiographic technique for distinguishing between gold and sodium in an alloy. The investigation was performed at unit magnification.