1. Field of the Invention
The present invention relates to a spot film device suitable for use with the radiographic system and having a grid for absorbing X-rays scattered from an object who is under radiographic process, and a grid carrier device for carrying or moving the grid between radiographic and parking positions.
2. Description of the Related Art
In the case of the spot film device for the radiographic system, object 2 to be examined is radiated with X-rays emitted from X-ray tube 1 and those X-rays penetrated through object 2 are transferred onto film 3 as an X-ray image of object 2, as shown in FIG. 1.
When object 2 is radiated with X-rays, X-ray scatter is caused from inside object 2 in addition to those effective X-ray components which include information on the X-ray image of object 2. When this X-ray scatter enters into film 3, film 3 is exposed to blur the X-ray image of object 2 on film 3. Grid 4 for absorbing this X-ray scatter is thus arranged above film 3. Grid 4 includes a plurality of lead foils 5 arranged side by side (or 40-60 lead foils per centimeter) and a plurality of interspacers 6 between these lead foils 5 through which X-rays can pass. Effective X-ray components are emitted in a certain direction, but X-rays scattered are emitted in various directions. When grid 4 is used, therefore, X-ray scattered can be absorbed by lead foils 5 and thus prevented from entering into film 3. This can prevent the X-ray image of object 2 from being blurred. On the other hand, effective X-ray components are allowed to pass through interspacers 6 to enter into film 3.
When the object is a pregnant lady or an infant, the amount of X-ray used is comparatively small. When the grid is used, however, effective X-ray components are more or less absorbed by the grid and the amount of effective X-ray components entering into the film becomes smaller not to display the X-ray image of the object on the film. Therefore, the grid is not used in this case. When the object is a grownup, the amount of X-ray used is comparatively large. Therefore, the grid is used.
When the grid is used, it happens sometimes that images (or shadows) of lead foils are projected on the film to appear as a mesh pattern thereon. The grid is thus shaken to prevent the mesh pattern from being transferred onto the film. When bones of a human body is to be radiographed, however, it is said there is no trouble even if the mesh pattern is transferred onto the film. Therefore, the grid is used but not shaken in this case.
As apparent from the above, there are cases where (1) the grid in not used, (2) the grid is used and oscillated, and (3) the grid is used but not oscillated.
In order to meet all of these cases, therefore, the spot film device is provided with the grid carrier device for moving the grid between radiographic and parking positions as well as shaking the grid when it is at the radiographic position. The radiographic position represents where the grid is positioned when it is used and the parking position denotes where the grid is positioned when it is not used.
In the case of the conventional grid carrier device, the grid is supported on the grid support frame such that the grid can oscillate. In other words, the grid is supported by one ends of plate springs, and the other ends of the plate springs are connected to the grid support frame. The grid support frame is connected to an endless belt and when this belt is rotated, the grid support frame is moved or carried between the radiographic and parking positions. When the grid support frame has reached the radiographic position, an electromagnet attached to the frame is excited to draw an iron piece attached to the grid and then demagnetized to release the iron piece. The grid is thus freely oscillated by the plate spring.
However, the free oscillating of the plate spring is damped as time goes by. The oscillating of the grid is thus damped accordingly and sometimes stopped in the worst case. This means that the grid is not reliably shaken over a certain range. In short, the conventional grid carrier system has low reliability in oscillating the grid.
Further, the grid support frame is moved by one means and the grid is shaken by another means in the case of the conventional grid carrier system. Namely, one means is the belt and another means is the electromagnet different from the belt. This causes the conventional grid carrier system to be made comparatively complicated in construction.