Abstract:
An x-ray system ( 10 ) include a digital detector ( 400 ) that defines two regions: a first region ( 404 ) suitable for generating data useful for creating a patient x-ray image and a second region ( 406 ) less suitable for generating such data than the first region. A source ( 20 ) transmits x-rays through a phantom ( 420 ) located between the source and the second region ( 406 ) so that the detector ( 400 ) generates test data in the second region. A processor ( 302 ) measures at least one parameter in response to the test data and stores a value of the parameter at one point of time. The processor compares the first value with a second value of the one parameter generated at a later second point in time. The processor also generates a result signal representing the results of the comparison.

Description:
BACKGROUND OF INVENTION 
     This invention relates to x-ray detectors and more specifically relates to techniques for testing such detectors. 
     Almost all image quality evaluation methods rely on placing off-the-shelf or custom-made x-ray phantoms in the field of view. Some methods use image processing and analysis tools to automatically detect regions of interest in the acquired image of the phantom. These methods have a significant advantage over “manual” methods that rely heavily on human operators to perform these measurements. These methods also provide more consistent and objective measurements. 
     However, automating the analysis of the image of the phantom does not result in full automation of the image quality evaluation, because, like the “manual” methods, they still require intervention by a human operator to place the x-ray phantom(s) in the field of view. Experience has shown that human operators are not inclined to take the time to place the x-ray phantom in the field of view. As a result, detector problems may go undetected for some time. X-ray images generated while the detector problems go undetected can result in degraded image quality. 
     This invention addresses these problems and provides a solution. 
     SUMMARY OF INVENTION 
     The preferred embodiment is useful in an x-ray system comprising a digital detector defining a first region suitable for generating data useful for creating a patient x-ray image and a second region less suitable for generating such data than the first region. In such an environment, the detector can be tested by providing a source of x-rays and a phantom located between the source and at least a portion of the second region so that the detector generates detector test data in at least a portion of the second region in response to the x-rays. At least one parameter is measured in response to at least a portion the test data. A first value of the one parameter is stored at one point of time. A comparison is made of the first value with a second value of the one parameter generated at a second point in time later than the first point of time. A result signal representing the results of the comparison is generated. 
     By using the foregoing techniques, the detector can be tested without human intervention, thereby insuring more reliable and timely testing than has been possible in the past. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic block diagram of an exemplary form of x-ray system employing a preferred embodiment of the invention. 
     FIG. 2 is a schematic top plan view of the detector shown in FIG. 1 illustrating different regions of the detector and also schematically illustrating a preferred form of phantom made in accordance with the invention. 
     FIG. 3 is a schematic, fragmentary, side elevational view of the phantom shown in FIG.  2 . 
     FIG. 4 is an enlarged fragmentary top plan view of the phantom shown in FIG. 3 together with adjacent portions of the detector shown in FIGS. 1 and 2. 
     FIG. 5 is graph illustrating an exemplary plot of modulation transfer function versus spatial frequency of phantom grids. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a preferred form of x-ray imaging system  10  made in accordance with the invention comprises an x-ray tube  20  that generates x-rays from a focal spot  22  and directs the x-rays in relationship to a central axis CA. A digital image detector  400  detects the x-rays in a well-known manner. A collimator  320  includes collimator blades shown schematically in FIG.  1 . 
     A calibration processor  302  includes communication interface or module  304 , a keyboard  305 , a central processing unit (CPU)  306 , a memory  308  and a display unit  309 , such as a computer monitor, all coupled by a bus  307  as shown. The processor may include, for example, a microprocessor, digital signal processor, microcontroller or various other devices designed to carry out logical and arithmetic operations. Signals corresponding to an x-ray image are read from detector  400  by readout electronics  312 . The design and operation of most of the components with numbers greater than 300 are described in more detail in application Ser. No. 09/342,686, filed Jun. 29, 1999, in the names of Kenneth S. Kump et al., entitled “Apparatus And Method For x-ray Collimator Sizing And Alignment,” assigned to General Electric Company and incorporated by reference in its entirety into this specification. 
     Communication interface  304  is coupled through a modem  340  and a network  342 , such as the Internet, to a computer system  344  at a remote location  346 . Maintenance personnel at location  346  monitor computer system  344  to determine if detector  400  requires repair or maintenance. 
     FIG. 2 is a top plan view of detector  400  that defines an outer periphery  402  and an inner region  404  that is suitable for generating data useful for creating a patient x-ray image. Between region  404  and periphery  402  is a margin region  406  less suitable for generating data useful for creating a patient x-ray image than region  404 . Region  406  is typically about 2-3 millimeters (mm) wide. Within region  406  is a generally rectangular saw tooth strip phantom  420 . 
     A fragment of phantom  420  is shown in FIG.  3 . Phantom  420  comprises a frame  422  substantially transparent to x-rays and identical regions of interest (ROIs) or coupons  424  that absorb x-rays. The ROIs are separated by identical distances of about 10 mm and have dimensions of about 10 by 2 mm. 
     Referring to FIG. 4, phantom  420  may be located in one of at least two different positions. For example, phantom  420 A is located directly under cover  430  of detector  400 . The location of phantom  420 A has the advantage of being accessible for replacement and service. However, phantom  420 B may be positioned more accurately than phantom  420 A by being located inside a sealed metal box or cabinet  440 . As shown in FIG. 4, phantom  420 B is located below an aluminum graphite cover  442  and above a scintillator  444 . An amorphous silicon array  446  is located below scintillator  444  and is carried by a glass substrate  448 . A seal  450  is provided between cover  442  and array  446  to protect scintillator  444 . 
     FIG. 5 illustrates an x-ray image of a tungsten coupon sub-phantom and a modulation transfer function (MTF) curve computed based on the upper edge profile of the tungsten coupon. FIG. 5 also shows how the edge profile of a rectangular tungsten coupon can be used to compute MTF. The coupon illustrated in FIG. 5 is about 30 mm by 30 mm. The vertical axis in FIG. 5 indicates modulation strength and the horizontal axis indicates spatial frequency of the tungsten coupons. The profiles of vertical and horizontal edges of the coupon can be used to compute MTF in horizontal and vertical directions, respectively. The coupon is deliberately positioned at a slightly rotated angle with respect to the top surface of detector  400  to avoid the edge points from lining up along a row or column. 
     In general, phantom  420  is used to conduct a self-test of certain image quality (IQ) parameters of solid state digital x-ray detector  400 . Phantom  420  is located in margin region  406  of detector  400 . Image data from pixels in margin  406  of the detector is created when x-rays are transmitted through phantom  420  to detector  400 . A certain number of rows and columns of data in the margin  406  are read out but are not displayed. This is because the process used to make the detector panels does not always result in uniform deposition of the cesium iodide (x-ray scintillator) on the edges (e.g., region  406 ), compared to the rest of the panel (e.g., region  404 ). 
     By planting small x-ray phantoms, such as phantom  420 , in these unused margins (e.g., margin  406 ), it is possible to compute certain image quality parameters. For example, a narrow “edge” phantom as shown in FIGS. 3 and 4 can be used to compute the modulation transfer function (MTF), at every exposure as illustrated in FIG.  5 . Alternatively, the noise power spectrum or contrast to noise ratio, can be calculated in this margin region. 
     Specifically, for MTF, an edge-based method of computation can be utilized, based on edge profiles along the diagonal side of each saw tooth of the type shown in FIG.  3 . 
     Usually, measuring IQ parameters of a x-ray detector involves placing a known x-ray phantom in the field of view, acquiring an image and then processing it to compute the IQ parameters. The use of implanted sub-phantoms, such as phantom  420 , inside the detector  400  eliminates the need for an external phantom, and more importantly the need for an operator to place the phantom. 
     In addition to providing the necessary sub-phantoms, such as phantom  420 , and an image, a “qualifying” algorithm is used. This algorithm is executed by CPU  306  and ensures that the image data being received in margin  406  are of good enough quality. That is, the x-ray field must be uniform (or be correctable) and the detector quality must be adequate. This is important since patients will be imaged simultaneously while the self-test of the detector is being conducted. This is a feature which limits the amount of x-ray radiation received by the patient. Technologists using good practice will collimate to the interesting patient anatomy. We are relying on the scattered radiation and occasionally “raw” (un-attenuated) radiation to expose phantom  420  in margin region  406 . The qualifying algorithm computes simple statistics in the region of phantom  420  or the parts of region  406 . For example, the mean, minimum, maximum, and standard deviation of gray levels (counts) can be determined. These values are compared to predefined limits to determine if the image data is valid for subsequent calculation. Additional details about the qualifying algorithm are as follows: Step1) There is first a need to define which ROIs are acceptable for computation. An initial “Pre-calibraion” to select ROIs with acceptably low number of bad pixels, minimum conversion factor (CF), and define a response correlation to the known good area in the region of the detector suitable for creating a patient x-ray image is required. This is conducted once per detector calibration which may occur, for example, roughly yearly. 
     Step 2) Of the ROIs deemed acceptable in step 1, for each exposure, there are additional acceptance criteria such as: minimum contrast between x-ray absorbing and x-ray transparent areas, and minimum signal count. Only the ROIs passing both step 1 and step 2 criteria will be used in the calculation. 
     After the image data is qualified, CPU  306  executes another algorithm to analyze the data and produce summary data, such as MTF data. Additional details about the MTF algorithm are as follows:Calculate MTF by a) Starting with the 12 th  row or column in from the edge of the panel, for an ROI, record the signal response vs the location of the edge; b) Increment until all rows or columns crossing the edge of the imbedded phantom have been sampled; c) Fourier transform the data set; d) Extract the frequency coefficients; e) Normalize the data for each frequency and adjust per the correlation defined in step 1; f) Repeat steps a-e for each acceptable ROI; and g) average all the ROI results. 
     This summary data is then placed into log files in memory  308  that can be actively “swept” using remote diagnostic equipment embodying computer system  344 . Alternatively, the process may proactively call-out to a remote host  344  (at on-line-center) to report its data. This may be done on a scheduled timeline, or when particular events occur (e.g.: values fall below certain pre-defined levels indicating failure or imminent failure). However, as a self-test, what is important is detection of any variations in the MTF on the edges, not the absolute MTF. The creation of summary reports includes the appending of new qualifying data to the “log” files. The data includes a parameter, such as MTF. A process may be included which compares the new data or parameter (or results from trending of current plus previous data) to predefined or calculated parameter thresholds that were previously stored. CPU  306  generates a result signal indicating the results of the comparison. When these thresholds are exceeded, a result signal or a message is sent to remote computer system  344  via modem  340  and the Internet to indicate a problem or status. For example, a message indicating a problem may be sent if the MTF summary data describing an MTF curve like the one shown in FIG. 5 from a previous year is more than 10 percent different from current summary data describing a current MTF curve. All of the foregoing data and parameters may be displayed on display  309 . 
     Using implanted sub-phantoms, such as phantom  420 , in the unused margins of the detector (e.g., region  406 ) allows testing and evaluation of certain parameters of the detector during a normal patient image acquisition. This self-test capability can be used to collect IQ data during every “scan”. Analyzing the data over time can be used to identify possible change or degradation of IQ of the detector in a pro-active fashion. This design results in further automation of image quality evaluation of solid state x-ray detectors. It eliminates or minimizes the reliance on human operators to perform the IQ evaluation on a regular basis, making it possible to be truly pro-active in servicing it. 
     Those skilled in the art will recognize that the preferred embodiments may be alteredand modified without departing from the true spirit and scope of the invention as defined in the accompanying claims.