The question of the correct exposure conditions is one of the most troublesome factors in successful radiography and calls for a great deal of experience, for it is not easy to reduce it to any general or simple rule which will fit all cases. The quantity of X-rays obtained from a tube is measured by the number of milliamperes of current passing through the tube, and exposure is usually expressed in terms of the product of milliamperes and time, as milliampere seconds. Both the intensity and the quality of the X-rays govern exposure, and the latter also depends, there fore, upon the voltage impressed on the X-ray tube. If we wish to obtain a radiograph of a substance easily penetrable by X-rays, such as an arm, we should use a voltage of say 70,000 volts; but, on the other hand, if we wished to radiograph a thick mass of steel we should have to employ a voltage from 200,000 volts upwards. The diagram shows a typical exposure chart which has been worked out under practical conditions for mild steel.
Something has been said about the difficulty of overcoming the bad effect of scattered X-rays on a radiograph when such scattering occurs at the surfaces and edges of the specimen, and also in the surrounding medium, but the scattering which occurs inside the specimen has also a most deleterious effect. Much of this scattering is lost by absorption in the specimen but a great deal of it will emerge and affect the photographic emulsion, and as by its very nature it has no definite direction it will cast no definite shadow but it will merely produce a general fog. The manner in which technique seeks to overcome this difficulty is by the use of what is known as a grid. This piece of apparatus has many different forms but the principle involved is always the same. By the interposition of suitably disposed absorbing sur faces between the specimen and the film, the radiation which has not a suitably defined direction is absorbed and so prevented reaching the photographic emulsion. The diagram illustrates the general method of arrangement and shows the effect of the absorb ing strips. It is obvious that the use of such a grid will result in an image of the strips appearing in the photographs as a series of lines. Although in some cases this may not detract from the
value of the radiograph, yet in others it may be very undesira ble. In the majority of such grids provision is made to over come this difficulty by arranging for the system of absorbing surfaces or grids to travel across the film during the exposure at a uniform speed and in a direction at right angles to the length of the grids. Thus each part of the film is covered in turn for the same time by each grid so that no shadow results. The use of these grid diaphragms for all radiography, both medical and industrial, has become universal practice and the consequent improvement in general results is very striking.