Abstract:
An apparatus and method for an X-ray dosimeter support, comprises of a material-poor support-layer, held by a frame elevated by at least one foot, comprising optimization for low back-scatter, high measurement-repeatability and ease of assembly and disassembly.

Description:
RELATED APPLICATIONS 
     This application claims the benefit under 35 USC 119(e) of U.S. Provisional Application No. 60/914,070, filed on Apr. 26, 2007, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to quality assurance of radiography systems. 
     More specifically the invention relates to the measurement of the output of an X-ray source during acceptance testing and periodic image quality control testing on general radiography imaging equipment. 
     BACKGROUND OF THE INVENTION 
     The radiation detectors may be powder phosphor screens or needle image plates (needle IP), direct radiography detectors (amorphous silicon, amorphous selenium, Cmos (complimentary metal oxide), phosphor detector arranged for direct radiography etc.) or the like. 
     A radiation image is recorded on such a detector (also called ‘plate’) by exposing it to an x-ray field. The radiation image which is temporarily stored by the detector is read out in a read out system (also called ‘digitizer’), where the exposed detector, is scanned with light of an appropriate wavelength and where the image-wise modulated light emitted by the detector upon stimulation is detected and converted into a digital image signal representative of the radiation image. 
     The signal to noise ratio (SNR) or normalized noise power spectrum ((N)NPS) of the image data must be analysed in order to evaluate the diagnostic capacity of the radiographic system for different uniform dose levels to be able to study its behaviour over the dynamic range. 
     Instead of using different detectors for each dose setting, a phantom target is used that contains a number of sub-targets each with a known absorption level for x-ray exposure. 
     Exposure of the detector then gives an image that contains the raw data needed for SNR or (N)NPS calculation. Every sub-target contains a region of interest (roi) with a known and constant attenuation for a known X-ray beam quality (energy spectrum) and is exposed to a uniform radiation field. 
     It is clear then that the output of the X-ray source must be exactly known, since it determines the radiation image that is used in the quality control testing of the SNR or the (N)NPS of the digitizer. 
     When a dosimeter is used to measure the output of an X-ray source, or during beam quality evaluation, the environment of the dosimeter ideally has to be free of surrounding material that would scatter the X-rays to the dosimeter and corrupt the measurement. This problem is normally solved by balancing the dosimeter on a web of thin wires, or using an assembly with a telescopic arm, in an attempt to keep scattering material as far away as possible from the dosimeter. 
     SUMMARY OF THE INVENTION 
     Current dosimeter systems usually work, but are clumsy, take time to configure, and have a low repeatability. 
     The current invention can be used for acceptance testing and periodic image quality control on general radiography imaging equipment to position the dosimeter for the determination of the “Radiation Quality Aluminium RQA 5 Tube Voltage” according to the IEC 61267 standardized measurement geometry and for input dose measurements. 
     The invention provides an apparatus and method for an X-ray dosimeter support, including a material-poor support-layer, held by a frame elevated by at least one leg or foot. 
     The support-layer, including a membrane, a perforated membrane, a fabric, or a mesh, is equipped with a detection-chamber alignment hole, and has detection-chamber or X-ray beam centering markers arranged on it. 
     The X-ray entry surface of the frame, or the sidewall of the frame facing the dosimeter (the X-ray exit surface), or both, are equipped with an X-ray shield. 
     The at least one leg of the apparatus is hollow, and inclined towards the X-ray source, and the frame is equipped with an X-ray shield above the at least one foot. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
         FIG. 1  is a perspective view of the X-ray dosimeter support according to one embodiment of the present invention; 
         FIG. 2  is a top plan view of the X-ray dosimeter support according to one embodiment of the present invention, after disassembly; and 
         FIG. 3  is a side schematic view of the X-ray dosimeter support showing different embodiments according to the present invention; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows support device  100  with low back-scatter contribution that is easily put together and allows for a quick re-establishment of circumstances, with a high repeatability of the experiment as a result. 
     The support device  100  is stool-like with a frame  118  that is supported by at least one leg; preferably three legs  112 A,  112 B,  112 C are provided, for stability. The “seat” of the support device  100  is a “tambourine skin” support layer  114  made of very thin tear-free material with a central hole  116  in it. The support layer  114  is stretched over the frame  118   
     In the current embodiment of the device  100 , this support layer  114  is a non-perforated membrane. In other embodiments it could be a perforated membrane, a fabric, or a mesh. As a matter of fact, any material-poor, i.e., low X-ray scattering characteristics, support layer would do. 
     As shown in  FIG. 2 , in a preferred embodiment, the device  100  is capable of easy disassembly/assembly. The legs are easily removable which makes all the components of the device fit in a “pizza box” like case. In more detail, each of the legs  112 A,  112 B,  112 C is provided with a bolt end-portion that mates with corresponding threaded holes  132  in the frame  118 . 
       FIG. 3  illustrates the use of the support device  100 . The device  100  is oriented within the X-ray beam  212  of an X-ray source  210  such that the legs  112 A,  112 B,  112 C and the frame  118  that upholds the support layer  114  are outside the X-ray cone  212  produced by the source  210 . The only material interfering with the beam cone  212  is the support layer  114  on which a dosimeter  214  resides. The hole  116  in the support layer  114  has to guarantee that the direct X-rays from the source  210  that pass through the detector cell  216  of the dosimeter  214  are not scattered back to the detector cell  216  by material so close to the detector cell. Since the dosimeter  214  has to be positioned right above the hole  116  in the support layer  114  the X-rays that traverse the dosimeter will travel through free air afterwards to ensure pure direct X-ray measurement. To guide the positioning of the dosimeter the support layer or the frame of the stool can have detection-chamber or X-ray beam centering markers  120  on it (see  FIGS. 1 and 2 ). 
     Measurements have shown that with this construction the scattering is minimal, and that the results are as good as with any other solution proposed in the prior art. It must be clear from the description that ease of use and repeatability of the tests are superior to those of the solutions in the prior art. 
     In the current embodiment the device  100  has three legs  112 A- 112 C that give minimal weight and maximal stability. 
     The invention as described will only give acceptable results if the X-ray cone  212  is collimated in such a way that back-scatter can only come from the support layer  114  of the stool-like device  100 . If this condition cannot be satisfied nor guaranteed, extra precautions must be taken, which lead to another embodiment of the invention. 
     In embodiments in which the material of the stool device  100  is composed will scatter X-rays to the dosimeter  214 , the construction of the stool has to be such that this back-scatter is minimal. Therefore, the legs  112 A- 112 C of the stool preferably should be hollow and have an inclination to ensure that any X-ray from the source that directly hits the leg, must travel through the leg over its whole length. Thus, the legs are splayed outward as shown by reference number  112 ′ in this example. This minimizes the impact surface of the leg. To even improve on this, the impact surface of the leg can be shielded by a lead plaquette  252  built into the frame  118  positioned such that there is no straight line from any point in the leg to the source that does not cross the plaquette. 
     Also the frame  118  of the support layer  114  itself will cause back-scatter. Therefore the material the frame  118  is made of (and that holds the thin membrane on which the dosimeter is positioned) is also preferably shielded by an annular lead ring  250  in such a way that there is no straight line from any point in the frame material  118  to the dosimeter  214  that does not cross the lead ring  250 . 
     The lead ring  250  itself must be constructed as a section of a cone of which the top is the X-ray source to ensure minimal back-scatter from said ring too. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.