The transparency of the human body to X-rays depends on the spectral quality of the radiation which is in turn governed by the voltage applied to the anode of the X-ray tube. Ionizing radiation arises from the abrupt arrestation of electron movement, whether in an X-ray tube or in the atom nucleus of a radioactive substance. The radiation frequency depends on the force of impact. The higher the anode voltage, the greater the acceleration of electrons towards the anode, the greater the impact, and the higher the frequency of the radiation emitted. The human skeleton absorbs more radiation energy from lower frequency radiation and consequently the shadows appear heavier on radiograms. This is fine for imaging fractures, but disadvantageous for a balanced picture of the bone and soft tissue of the speech articulators. Higher frequency radiation is more suitable for pictures of speech movements in order to lighten the shadow of especially the mandible relative to the soft tissues of the tongue, lips, velum and pharynx. This is achieved by using a high anode voltage, in our case 120 kV. Copper and aluminium filters are also used to further improve the spectral quality by removing low frequency radiation. The cathode current determines the intensity of the radiation by controlling the electron flow towards the anode and is set so as to achieve an adequate exposure in the camera.
Safety requirements have become increasingly stringent, decade by decade. What follows summarizes the understanding of radiation hazards in the 1970s and 1980s when our motion films were made. Radiation intensity is measured in roentgens (a measure of the ionization of the air by radiation). A radiation dose is measured in rads (the amount of radiation energy aborbed by biological material, 1 rad is 100 ergs/gram). A rough relation is that exposure to diagnostic X-radiation of 1 roentgen gives a dose of about 1 rad (depending on the actual exposure time and the part of the body examined). A third unit, the rem, is defined as a comparable radiation dose that has the same biological effect on man as exposure to 1 roentgen of X-rays. A few examples will add some perspective. The average adult annual radiation dose in southern Sweden from cosmic, natural and environmental radiation sources is about 100 mrad. A diagnostic X-ray examination of the abdomen gave doses up to 5 rad for, say, a dozen still radiograms. The safety limit for personnel employed in X-ray units and nuclear power stations was a cumulative dose of 5 rad/annum.
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© Sidney Wood and SWPhonetics, 1994-2011
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