Abstract:
The present invention related to a system and method for performing scatter correction in x-ray imaging systems. A pixellated solid state imaging detector is used in which an electronic window or slot is scanned across the two dimensional surface of the detector to selectively record image data. In a preferred embodiment, a collimator is used to define relative movement between an x-ray beam and the x-ray detector. A scatter correction program can be used to correct for scattering in the detected image data to provide for improved imaging in medical, scientific and industrial applications.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 60/344,306, filed Dec. 21, 2001. The entire contents of the above application is incorporated herein by reference in its entirety. 
     
    
     GOVERNMENT SUPPORT  
       [0002] This invention was supported, in whole or in part, by a grant R01 CA88792 from National Institutes For Health. The Government has certain rights in the invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    The most common method of x-ray scatter reduction is the antiscatter grid. It is a device with a series of lead blades or lamellae lined up in parallel that preferentially absorb scattered radiation and passes primarily non-scattered radiation. A variety of x-ray scatter reduction approaches using linear grids, crossed grids, focused grids, parallel grids, moving grids, and air gaps have been studied. X-ray antiscatter grids are used in x-ray imaging, and they are important for imaging the adult chest, abdomen and pelvis by conventional radiography. The main problem with antiscatter grids is their inability to provide complete “clean-up” of scattered radiation. For example, eliminating 70% of the scattered radiation may be attainable but achieving a 95% reduction in scattered radiation is extremely difficult and impractical with conventional or even with special purpose antiscatter grids. Antiscatter grids with high scatter rejection capability also absorb primary radiation. Absorption of primary radiation must be compensated with higher radiation dose to the patient in order to maintain a constant signal-to-noise ratio on the image detector. Therefore the high rejection of scattered radiation with the use of a grid is associated with higher radiation dose to the patient.  
           [0004]    Another system uses an x-ray tube collimated to irradiate only the linear detector array with a fan beam thereby preventing unnecessary exposure of other areas. Both the tube and detector move in synchrony over the region to be exposed. A problem with this scanning procedure is the time required to perform the scan, typically four to ten seconds, which for some examinations such as chest imaging, is considered slow, and patient motion can affect spatial resolution to some degree. Another system utilizes an image intensifier and thresholds that are compared on a pixel by pixel basis. This system is bulky and requires extensive computation on a frame-by-frame basis. There is a continuing need, however, for improvements in image quality, reducing x-ray exposure, smaller footprint and the cost of manufacture of such instruments, particularly for medical imaging applications.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention relates to the removal of x-ray scatter in imaging applications such as mammography, bone densitometry of the spine, the hip, the hand and other peripheral joints, chest radiography and other medical, scientific, and industrial applications. In a preferred embodiment, the invention can be added to existing flat panel imaging systems for scatter reduction with minimal modifications. Scattered x-rays can reduce image contrast which can adversely affect the diagnostic quality of images in medical applications, for example. Moreover, scattered radiation can severely affect the normally linear relationship between exposure and signal on an electronic imaging detector thereby rendering the image useless for quantitative studies such as bone densitometry. The present invention uses a scanning electronic slot without the need for image intensifiers or processing signals outside of the slot. The electronic slot can be used with a scanning mechanical slot that is controlled to match the scanning movement of the electronic slot.  
           [0006]    A preferred embodiment of the invention uses a scanning collimator that is positioned between the x-ray source and the x-ray detector to define and control an x-ray beam that is scanned across an object to be imaged and the detector surface. A preferred embodiment uses an area detector that is stationary relative to the object or patient and a scanning electronic slot or window. The shape, size and movement of the electronic slot or active region of the detector are correlated with the same parameters for the slot assembly such that the beam transmitted through the mechanical slot or window without being scattered is aligned with the electronic slot or window of the detector. As the area of the detector receiving the beam at any give time is known, scattered x-rays that are received by the detector outside the reception area do not contribute to the detected image data. A programmable computer with associated software or a dedicated processor can be used to control scanning parameters and provide the needed data processing. The individual recorded windows from each frame can be assembled to form a complete image from the scanned region using a software module.  
           [0007]    The detector can be any pixellated solid state detector such as a charge coupled device (CCD), a CMOS imager or amorphous silicon sensor or an x-ray sensitive detectors such as amorphous selenium that convert x-rays directly into electrical signals. Individual pixels can be controlled by colocated thin film transistors, for example, that allows the user to select regions of pixel elements to define the reception window at any given moment. A preferred embodiment of the invention utilizes a pre-exposure scan in which the amount of scatter is measured for the region of interest. Depending on the thickness and size of the region of interest, the x-ray source parameters, the collimator area and scan parameters and electronic slot parameters are selected, and the scan is performed. The size of the collimator slot and the electronic slot can be adjusted manually or automatically depending upon the pre-exposure step. Unlike previously described systems, no thresholding is necessary, but it can be used with this system for certain applications. Pixel elements are selectively actuated to perform readout. In another preferred embodiment of the invention, the electronic slot width and other parameters can be set automatically or manually without pre-exposure.  
           [0008]    A plurality of apertures or slots in the collimator can be used to provide for lower scan times. A preferred embodiment employs a plurality of parallel fan beams that are scanned simultaneously across the detector surface. Additionally an x-ray monitor can be used to detect and record the quality of the beams. The monitoring system can provide automatic calibration or shut-off of the system if a selected deviation occurs in the detected signal. The detector can utilize binning of adjoining pixel elements to improve scan time and signal to noise ratio.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0010]    [0010]FIG. 1A illustrates a scatter reducing system in accordance with a preferred embodiment of the invention.  
         [0011]    [0011]FIG. 1B schematic illustrating embodiments employing more than one scanning slot.  
         [0012]    [0012]FIG. 2A shows a detector system for slot scan showing a single slot scanning a flat-panel detector array.  
         [0013]    [0013]FIG. 2B illustrates an image processing system using “image stitching” to generate a fully reconstructed image.  
         [0014]    [0014]FIG. 3 illustrates an interleaved scanning process where the slot (or multiple slots) scans the detector area in either a discrete or continuous fashion.  
         [0015]    [0015]FIG. 4 illustrates an x-ray quality monitoring device-used to assess x-ray beam quality exiting from the tube.  
         [0016]    [0016]FIGS. 5A and 5B are process sequences illustrating methods of performing a scanning sequence in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    A schematic of a preferred embodiment of the invention is illustrated in FIG. 1A. The system differs from the existing slot-scan technology in that an aperture of the collimator “steers” the x-ray beam that scans a flat-panel or wide area detector  26 . Only the area illuminated by the slot assembly  14  allows x-rays  18 , from source  12  to pass through while the rest of the area blocks x-rays, preventing unnecessary exposure to the patient or other object of interest  22 . The slot width, height, and scan speed of a single slot  40  can be adjusted either manually or automatically as dictated by the diagnostic task. Multiple slot assemblies  42 ,  44  can also be used depending on the scattering  20  requirements of the application (FIG. 1B).  
         [0018]    An electronic interface  25  can be used to selectively read-out the information under the slot area. Image data can be stored in memory, processed and displayed using a data processing system or personal computer  27 . The control system and actuator  15  can be used with a bi-directional scanning capability  16 . The computer  27  can be connected to and programmed to control the system  15  as well as the readout function of interface  25 . Alternatively, post-acquisition image processing techniques can be used to reconstruct the image by adding or “stitching” different image frames. An important advantage of the invention is the lack of post-patient x-ray beam collimating. Post-patient beam collimating is traditionally considered essential to protect the detector  26  from contamination with scattered radiation. However, the present electronic slot scanning system and multi-frame electronic acquisition technique described herein provides for imaging without mechanical post-patient collimating. This is important because of the use of a post-patient slit greatly complicates the device. It requires synchronization, it is prone to produce artifacts, and it adds cost and bulk to the equipment.  
         [0019]    Digital imaging systems can be easily adapted to perform quantitative studies such as bone densitometry. Moreover, with this method the user can perform bone densitometry with scanning at two distinct energy levels with hitherto unattainable spatial detail. Although high spatial detail is not critical for all bone densitometry, there are situations where physicians can use higher spatial resolution. Most importantly, conversion to the slot scan is simple as it involves minimal hardware modifications, and the rest is done by electronic control of the digital detector.  
         [0020]    The present invention can be used for other x-ray imaging applications such as mammography. Digital mammographic systems can perform in the slot scan mode (typically within a small selected area) as an option to the conventional single snapshot acquisition. The slot scan approach in mammography can be especially useful in dense breasts where x-ray scatter interferes with visualization of subtle contrast. The detector system can comprise a cassette assembly having a size and shape suitable for replacing a standard film cassette.  
         [0021]    In this embodiment  50  (FIG. 2A), a read-out circuitry  58  can be used to selectively read the data under the slot  54  as it is traversing  56  a predefined area of detector  52 . This acts as an “electronic window” that reads the data directly under the slot that can correspond to a particular column  60  while eliminating unwanted information. A substantially “scatter free”  76  image can then be displayed after completion of the scan (FIG. 2B).  
         [0022]    A complete scanned image can be obtained in almost “real time” using this procedure and the systems can be less expensive compared to traditional slot-scan units. In this embodiment the existing flat-panel detectors and electronics can be used and multiple images (frames) can be acquired as the slot is scanned. The area under the slot can be selectively extracted from each frame  72 . Filtering can be used to remove unwanted components. The extracted portions can then be added or “stitched”  70  using image-processing techniques to generate a full resolution “scatter free” image (FIG. 2B). This method is portable and requires the addition of only the software module to control the slot assembly, detector readout and processing.  
         [0023]    Different types of scanning modes can be used for task specific imaging applications. For a continuous scan, for example, the slot scans the detector area in a continuous fashion. The direction of the scan can be changed when required. X-rays are ‘ON’ during the scan. For a discrete scan, the slot  82  scans  92  the detector area in uniform discrete steps. Here, the x-rays need to be ‘ON’ only when the slot has arrived at a specific position and remain ‘OFF’ during the transition. For an interleaved scan, the slot interleaves columns  90  when scanning the detector either in discrete steps or continuously. For example, columns 0, 2, 4 . . . n (assume ‘n’ is even) are scanned in the forward scan and columns n−1, . . . , 3, 1 are scanned when the slot returns to its start position (column  0 ) (FIG. 3). Further, the scans can be performed either from left to right as a forward scan  94 , a return scan  96  or top to bottom with reference to the patient or object of interest.  
         [0024]    In certain applications, such as bone densitometry, it is useful to monitor  100  the exit beam quality from the tube. In the present invention certain standard x-ray attenuating materials, such as aluminum, bone, or other appropriate material on or adjacent to the slot  102  (FIG. 4) and recording the intensity of the signal under the material. If the signal deviates over a predefined amount, the system controller or computer  27  is triggered by feedback signal  106  to calibrate the source  12  via connection  34  or stop under extreme circumstances. Alternatively, a small section of the flat panel detector can be used for the beam monitoring function.  
         [0025]    During a typical bone density scan, the system operates, as shown in FIGS. 5A and 5B. The operator activates the system which can include a pre-exposure sequence to measure scatter and thereby assist in setting actual scan parameters. In the pre-exposure sequence of FIG. 5A, the user initiates  111  the scan by setting an initial slot size  112 . The pre-exposure  113  is performed, the data is recorded and analyzed  114 . If the scan is not acceptable the scan can be rerun or the actual scan can then be programmed,  116  and  118 . In a preferred embodiment a database can be referenced  117  to check or refine parameters. The actual scan in FIG. 5B shows the tube voltage set  124  at one energy, such as 60kV, and an appropriate filter such as aluminum is automatically inserted in the beam; the tube current is set  126  to a relatively low value, typically 5 to 20 mA; the starting position is selected and recorded  128 , scan parameters are selected  130 , including size of scan area, rate of scan and scan format (e.g., a continuous scan, a discrete scan of selected regions, or an interleaved scan). These parameters can be set automatically on the object thickness and composition. In the case of medical imaging, this can include patient data and the portion of the anatomy to be scanned. Next, electronic slot parameters (e.g. slot width or size that can be constant, variable, asymmetric or preset at a selected value) are selected  132 .  
         [0026]    The x-ray beam is activated, and it is scanned  134  across the detector while the electronic readout is synchronized with the beam scan as described in the above modes; this image is read out  136  and stored in the computer as the “low energy” image; the scan is repeated  142  or replaced or a higher x-ray beam energy (for example 100 kV) is selected with another filter, typically aluminum or copper, or a combination of each, which can be automatically inserted in the beam; this image is acquired in the same manner of the first (low energy image); and the second image is stored in the computer as the “high energy” image. Prior to storage of each image, the columns can simply be added  144 , and in the event of border defects the operator can optionally select  146  to check for border defects and select adjacent pixel values to be averaged to correct those defects. The data can then be processed to determine the bone density of the region of interest from the low and high energy images.  
         [0027]    Portability of the scanning unit and compatibility with any existing wide area digital imager (scintillator with amorphous silicon readout, amorphous selenium with amorphous silicon readout, cadmium zinc telluride, crystalline silicon, scintillator with active or passive type-CMOS readout, scintillator with charge-coupled device detector and readout, phosphor detectors and other monolithically fabricated integrated detector devices). Additional details regarding x-ray sources, detectors and methods of scanning and processing image data can be found in U.S. Pat. Nos. 5,150,394 and 6,031,892, incorporated herein by reference in their entirety.  
         [0028]    Additional embodiments employ variable slots (which may be adaptive) for task specific applications; the use of hardware data read-out; and the use of software image processing (including “image stitching”). Adaptive scanning can be performed using the feedback control system or can be programmed for specific applications or patients.  
         [0029]    Another preferred embodiment comprises a dedicated bone densitometer. A flat-panel-based bone densitometer provides more cost efficiency than the current generation of bone densitometers. Dual energy bone densitometers can be used to make quantitative measurements of the spinal to measure bone loss, for example. Moreover, it delivers much higher performance and has fewer moving parts than the current generation of such devices that are based on mechanical scanning of the entire x-ray tube and detector.  
         [0030]    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.