Patent Number: 048037168
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS A high voltage supply 1 is shown in the drawing which feeds an x-ray tube 2, and which is connected to the mains via a line 3. The x-ray tube 2 emits an x-ray beam 4 having a central ray indicated at 10. The x-ray beam 4 during a radiograph exposure penetrates an examination subject 5, passes through a radiation measuring chamber 6, passes through a secondary radiation grid 7, and is incident on an x-ray film 8 in an x-ray film cassette 9. During an exposure, the secondary radiation grid 7 is moved perpendicularly to the central ray 10 by a motor 11, so that the lamellae of the grid 7 which are directed at the focus of the x-ray tube 2, do not form an image on the film 8. An electrical output of the radiation measuring chamber 6 is connected to an input of an integrator 13 through an amplifier 12. The output of the integrator 13 is an electrical signal corresponding to the actual value of the radiation dose, and is supplied to one input of a comparator 14. Another input of the comparator 14 is supplied with an electrical signal on a line 15 corresponding to the rated value of the radiation dose which is required for an optimum film blackening. When the actual value and the rated value of the radiation dose are the same, the comparator 14 supplies a signal to the high voltage supply 1 which de-energizes the supply and thus shuts off the x-ray tube 2. The input signal of the integrator 13 (the current i) is proportional to the dose rate D. The exposure time is fixed by this signal. This signal is supplied through an amplifier 17 to a motor control unit 18 which controls the speed of the motor 11 and thus the speed of movement of the secondary grid 7. The signal on line 15 is also supplied to the amplifier 17 and controls the amplification factor thereof. The adjustment speed of the secondary radiation grid 7 is shown in the drawing as X.sub.R. This adjustment speed X.sub.R will be higher as the anticipated exposure times becomes shorter. The rated value and the actual value of the dose are calculated according to the following equation: ##EQU1## wherein T is the exposure time, D.sub.actual is the actual value of the dose rate and t is real time. The following relationship derives for the path X.sub.R traversed by the secondary radiation grid: ##EQU2## wherein X.sub.R, as above, is the speed of the secondary radiation grid. This speed can be determined according to the following relationship: ##EQU3## The dose rate can be maintained constant during the exposure time, or may be a function of the time. According, X.sub.R is then constant or a function of time t. For adequate blurring, a path distiance X.sub.R which the secondary radiation grid 7 must cover during an x-ray exposure is fixed. In order to avoid differences in optical density due to the secondary radiation grid 7 being off-centered, one half of the path distance X.sub.R is present on each side of the central ray 10. Instead of controlling the speed of the secondary radiation grid as discussed above, a control circuit for this speed wherein the rated value is fixed dependent on the anticipated exposure tme may also be provided. Although modifications and changes may be suggested by those skilled in the art it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.