It might be thought that all X – rays are the same but this is not the case. The energy of X – rays increases with increasing frequency, and the penetrating power of X – rays into different types of body tissue varies with energy and frequency. This is best shown by the attenuation constant, %mu , a number such that the intensity of X – rays falls to a a fractionfor ever distance %mu that the X- rays penetrate tissue.

When making an X – ray image it is desirable that bone and muscle be clearly distinguished. This means that the attenuation coefficients be as different as possible to increase the contrast. From the above table those X – rays with energy equal to 50 keV are most suited for this.

In fact X – rays may be used to distinguish between different tissues and even show fine detail with soft tissue by picking X -rays with that energy for which the contrast is maximised for the type of tissues that need to be distinguished.