SU152053A1 - Electric Exposure Meter for Medical Radiography - Google Patents

Description

The known electrical exposure meters for medical radiography use an exposure relay that shuts off the X-ray unit after a predetermined amount of X-ray radiation passes through the sensitive element of the relay. This type of exposure meter determines the exposure time for X-ray imaging only during the operation of the X-ray unit, and the optical density of the blackening of the X-ray negative is not monitored. The choice of the exposure time (at a given optical density of the blackening of the negative), depending on the parameters of the object to be removed, is impossible before turning on the device when working with known exposure meters.

The proposed electric exposure meter eliminates these disadvantages by using an electrical simulator that solves an approximate equation to determine the optical density of the blackening of the negative.

The drawing shows a circuit diagram of the proposed electric exposure meter, solving an approximate equation:

D Klg - to (nlgv 4-lgJ + Igt - 2lgF)

to determine the optical density of the blackening of the negative, where D is the optical density of the blackening of the negative;

V-voltage on the X-ray tube;

/ - anode tube current;

t-shutter speed;

No. 152053- 2 F-focal length of the device;

An n-variable, depending on the thickness of the subject (typically 3-5);

k is the coefficient of proportionality, depending on the thickness of the subject and the sensitivity of the negative material.

The autotransformer. 1 with taps 2 supplies the primary windings of measuring transformers 3 and 4 with stabilized voltage. The secondary windings 5 of transformer 3 are used for switching the voltage on an X-ray tube (not shown) with the help of switch 6 and are brought out in such a way as to reproduce the voltage proportional to l Ig V. The secondary winding 7 of the transformer 4 has branch lines 8, which serve for switching the circuit when the x-ray tube anode current value is changed by the switch 9. Dena so as to reproduce the voltage value proportional Ig /. Otpayki 10 windings // displayed in such a way as to reproduce the voltage proportional to 21. Desoldering 12 of the secondary winding / transformer 4 is derived in such a way as to reproduce a voltage value proportional to Ig.

The voltages from the windings 5, 7, 11 and 13 are algebraically added, creating a voltage at the output proportional to the sum:

n gV + lgI + gt-2 gF.

The sum of the voltages is applied to the measuring device 14, graduated in cm thick, through a variable resistance of 15 and 16. The arrow indicates the thickness of the object corresponding to the specified blackening density of the film. For a properly exposed image, it is sufficient that the thickness values on the instrument are equal to the measured value of the object thickness.

Since the value of n in the conditions of X-ray diagnostics varies from three (pictures of the foot and hand) to five (pictures of the torso and skull), an adjustment is made to the scheme for the thickness of the object. For this purpose, a compensation transformer 17 is used in the circuit, the primary winding of which is connected in parallel with the output of the circuit and the secondary transformer is connected in series with the primary winding of transformer 3 so that it automatically increases the voltage across it when the voltage at the output of the circuit increases, i.e. with increasing thickness.

The proposed electric exposure meter makes it possible to determine the value of the shutter speed necessary to ensure a given optical density of the blackening of the negative before turning on the x-ray apparatus.

Subject invention

An electrical exposure meter for medical radiography, characterized in that, in order to determine the exposure time depending on the parameters of the object to be photographed (for a given optical density of the blackening of the negative) before turning on the x-ray machine, it uses an electrical modeling device that solves an approximate equation to determine the optical density of the blackening negative.