X-ray diagnostic apparatus with a filter device

An X-ray diagnostic apparatus has a device for controlling an X-ray source with regard to the emission of an X-ray beam and a filter positioner for the introduction or removal of at least one radiation filter into or out of the beam of radiation. A monitoring device checks whether there exists a positive difference signal between an output of a dosage power regulator and a kV controller. Given the presence of a positive difference, the monitoring device controls the filter positioner with regard to the removal of the radiation filter from the beam of radiation.

BACKGROUND 
1. Field of the Invention 
The present invention is directed to an X-ray diagnostic apparatus of the 
type having at least one filter which can be placed in the path of the 
X-ray beam emitted by the X-ray source. 
2. Description of the Prior Art 
Filter devices are known which have a number of filters with respectively 
different radiation absorption characteristics, which are selectively 
positionable in the path of a beam of radiation transmitted by an X-ray 
source. 
From German OS 33 39 775, an X-ray diagnostic apparatus is known having 
such radiation filters, with means for the formation of an electrical 
density signal dependent on the density of a subject under examination. In 
addition, positioning means for the introduction of the filter are 
provided, having an electromotor that can be controlled by a control 
circuit. The density signal is supplied to the control circuit, which 
controls the electromotor in correspondence to the respective density 
signal, for the selection of an appropriate filter. 
Filters can be placed (such as by rotating a filter holder) in the path of 
the radiation in order to reduce the patient's skin exposure to radiation. 
When the filter is rotated into the beam path, the loss of image quality 
must be compensated by reduction of the tube voltage or by an increase in 
the image intensifier input dosage. Both result in an increased load on 
the X-ray source. If the limit of performance is achieved (as happens even 
for small subject thicknesses, due to the additional filtering), the tube 
voltage must be increased with a simultaneous reduction of the tube 
current, if larger subject thicknesses are also to be properly exposed. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an X-ray diagnostic apparatus 
with at least one filter selectively placable in or out of the X-ray beam 
path wherein a loss of image quality caused by the additional filter is 
avoided. 
The above object is achieved in accordance with the principles of the 
present invention in an X-ray diagnostic apparatus having a device for 
controlling the emission of an X-ray beam from an X-ray source, and a 
filter positioner for introducing or removing at least one radiation 
filter into or out of the path of the radiation beam, a monitoring device 
which determines whether a difference exists between an output of a kV 
controller and a dosage power regulator, and the monitoring device, given 
the presence of a difference, controlling the filter positioner with 
regard to removal of the radiation filter. 
An advantage achieved by the invention is that the filter remains in the 
path of the radiation as long as this is possible without loss of image 
quality. The filter is not taken out of the path of radiation until there 
is a danger of an overloading of the X-ray source, or when the voltage at 
the X-ray source exceeds a predetermined difference value in relation to 
the target value. The dose can then be reduced by the portion that was 
necessary for the additional radiation absorption and the compensation of 
the image quality loss due to the filter, while maintaining the image 
quality.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, a kV controller has an input 2 to which a kV target (reference) 
signal can be fed. The output of the kV controller 1 is connected to a 
first input 3 of a voltage supply device 4 for an X-ray source 5, with a 
feedback branch 6 leading back to the kV controller 1. A target signal is 
fed to a dosage regulator 7 at a first input 8. A first output 9 of the 
dosage regulator 7 is connected to a second input 10 of the voltage supply 
4 and to a first input 11 of a monitoring device 12. The output signal of 
the kV controller 1 is fed to a second input 13 of the monitoring device 
12. A filter positioner 14 is connected after the monitoring device 12. A 
signal identifying the desired filter is fed to the filter positioner 14 
at an additional input 15. The filter positioner 14 either places a 
desired filter 14a in the path of the beam from the X-ray source 5, or 
removes it therefrom. The dosage regulator 7 can generate, if warranted, a 
signal for the regulation of the beam from the X-ray source at a second 
output 16, and a signal for time control at a third output. These signals 
are, however, not essential for the invention, and are thus not further 
discussed. 
If, for example, a desired voltage level is chosen at the kV controller 1 
of e.g. 63 kV, and at the second input 15 of the filter positioner 14 a 
filter 14a of 0.2 mm copper (for the reduction of skin radiation exposure) 
is chosen, the filter 14a is left in the path of radiation until the limit 
of performance of the X-ray source 5 is reached. If the limit of 
performance, as shown in FIG. 2, is reached at the line identified with 
reference numeral 18, the dosage regulator 7 controls the voltage for the 
X-ray source 5 at a high level and decreases the X-ray source current. The 
monitoring device 12 detects this increase in voltage as a difference from 
the target value (63 kV), and generates an output signal for controlling 
the filter positioner 14 with regard to the removal of the 0.2 mm copper 
filter 14a, and replaces it with the next thinner filter, or takes the 
filter 14a completely out of the path of radiation without inserting a 
replacement filter (FIG. 3). Due to the use of the filter 14a, it has been 
necessary to raise the radiation dose, e.g., to 119%, in order to 
compensate for the loss of contrast caused by the filter while maintaining 
image quality. Since, however, the filter 14a has subsequently been taken 
out of the path of radiation, the dose for an equal image quality can be 
reduced to 100%, which also reduces the load on the X-ray source 5 (FIG. 
4). FIG. 5 shows that the skin radiation exposure then increases, however. 
In FIGS. 2 to 5, the removal of the filter, the reduction of the dose and 
the increase of the skin radiation exposure is shown in unbroken lines, 
with the assumption of a voltage level of 70 kV. The load limit of the 
X-ray source 5 is reached at the line identified with reference numeral 
19. The filter 14a is also taken out of the path of radiation only when 
the monitoring device 12 detects a positive voltage difference. 
By means of the invention it is thus achieved that the image quality is 
maintained by the automatic removal of the filter 14a. 
Within the scope of the invention, the X-ray diagnostic apparatus can 
include a display for indicating the radiation filter currently located in 
the path of radiation. The dosage regulator 7 can be a dose controller, as 
previously described. In this case, the actual value is measured by a 
radiation dosimeter 20 before the X-ray image intensifier 22. The dosage 
regulator 7 can, however, alternatively be a brightness controller, which 
uses the brightness at the output of the X-ray image intensifier 22 as an 
actual value. This brightness can also be obtained, for example, from an 
image signal (video signal) from an image receiver 21, such as a video 
camera. 
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.