Patent Number: 056489960
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a preferred embodiment of a tangential CT scanner system in accordance with the present invention. The system has a penetrating energy source 1. For example, a suitable x-ray source is in the 300 kV to 450 kV output range such as the Pantak 320 kV high frequency constant potential x-ray tube. As shown in FIG. 2, the system includes a plurality of detectors 3, generally x-ray detectors, positioned in a linear or arcuate arrangement that functions as a detector array 2. As depicted, the detector array 2 shown has about 160 solid state x-ray detectors. When activated, the penetrating energy source 1 emits radiated energy that passes through an object 6 to be scanned, depicted in FIGS. 1, 2 and 5 as a simple cylinder. The radiated energy is received by the detector array 2, forming a planar fan-shaped beam of penetrating energy or fan-beam 5. The detectors 3 measure the intensity of the fan-beam 5 as received. The fan-beam 5 is attenuated as it passes through various features of the object 6. The varying intensities of radiation are registered by the detectors 3 and define the features of the object 6 as it is scanned. The tangential CT scanner is ideally suited for analysis of circular type objects 6, such as rocket motors or barrels of toxic or nuclear waste. It should be understood that a multiplicity of objects such as pistons and turbines can also be analyzed with the tangential CT scanner. In addition, other types of noncylindrical objects can also be scanned. With respect to rocket motors, manufacturers need to analyze the rocket fuel and other sections of the interior of the motor, without destroying the motor. Analysis of the rocket motor fuel for unbond defects, voids and inclusions is critical as any defect or impurity can cause failure of the rocket motor or result in the payload arriving at the wrong destination. For example, unbond defects are a problem because they are very difficult to detect, as they are very narrow and lie along the inner surface of the rocket motor. Indeed, normal radiographic methods cannot detect them. Accordingly, it is common in the industry to use tangential film radiographic methods to detect such defects. Using this method, an object is interposed between a film and an x-ray source, which penetrates the object. When the film and x-ray are parallel to the defect, it appears in the film. Because only a limited number of tangential film radiographs can be taken, however, unbond defects can be missed. Failures initiated by such defects can be catastrophic. For a simultaneous CT scan of the entire volume of the object 6, the entire object 6 must fit within the beam 5, as shown in FIG. 3. When the entire object 6 fits within the x-ray beam 5, the tangential CT scanner system simultaneously collects CT cross-sectional data for all slices. In essence, the height or length of the object 6 must lie within the beam 5 of penetrating energy for simultaneous scanning of the entire volume of the object 6 as shown in FIG. 3. The fan-beam 5 is defined by the length of the array 2 (L) and the angle .phi.. Therefore, the size of the object 6 to be scanned dictates both the length of the array (L) 2, which can be varied by increasing or decreasing the distance between each detector 3, and the distance (X) between the array 2 and the energy source 1. These parameters, in turn, control the size of the angle .phi.. In an exemplary embodiment, 160 detectors 3 are used in the detector array 2 and can accommodate a ten inch long rocket motor. The number of detectors 3 and the space between them does not dictate, however, the level of geometrical resolution of the displayed image. Rather, resolution is a function of rotation and translation speeds. The number of detectors 3 only determines the number of cross-sectional CT slices. Several types of scanning are available. For example, the object 6 can be translated relative to the energy beam 5 without concurrent rotation. This type of scan generates a radiographic image of the object 6. Conversely, the object 6 can be rotated relative to the energy beam 5 without concurrent translation. This type of scan provides a single tangential layer of the object 6. Finally, to obtain full volume data, the object 6 is simultaneously rotated and translated relative to the energy beam 5. When the scanner is activated to collect full volume data, a drive device 70 rapidly rotates the object 6 to be scanned about an axis 8 parallel to the orientation of the detector array 2. Typically, the objects 6 to be scanned are cylindrical in nature, such as rocket motors or drums holding nuclear or toxic waste. The speed of rotation can be varied, depending on the degree of resolution required, but is generally about 10-100 rpm. In a gantry embodiment, used for scanning circular objects 6 as shown in FIGS. 1 and 2, the object 6 is rotated about its own cylindrical axis 8, or axis of symmetry, and is supported by two rollers 9 and 10; a drive roller 9 that induces rotation, and a recorder roller 10 that has an encoder 19, such as the Dynapar encoder, Model No. M2005001003107. In an exemplary embodiment, the drive roller 9 is connected to a servo-motor 60 with a timing belt 45. The encoder 19 tracks the azimuthal location of the object 6 as it rotates. As data points are recorded, the position of the object 6 is recorded and transmitted to a computer 23. The azimuthal location of the object 6 provides the exact path of the x-ray beam 5 through the object 6 at any given time. This information is used in analyzing the data. For example, it can provide the exact location of any flaw or defect found by the tangential scanning system. When the data from the tangential CT scanner is used for reconstructing cross-sectional CT images, the azimuthal location information is used, during the back projection, for proper image reconstruction. Alternatively, FIG. 6 shows an embodiment where the drive device consists of a conveyor system 18 and a turntable 20 mounted on the conveyor system 18. The object 6 is supported by the turntable 20, which rotates the object 6 within the energy fan-beam 5. The rotating object 6 is then translated through the fan-beam 5 by the conveyor system 18. With such an embodiment, the conveyor system 18 is marked with positioning markings for detection by encoders that read the markings. The encoder allows the computer to record the exact position for each reading. Similarly, the rotational device, or turntable 20, has markings which can be recorded by the computer for each data point reading. The encoder on the rotational turntable 20 marks the exact azimuthal location of the object 6. Again, these encoder readings are useful for locating the flaw in the object and for back projection of cross-sectional CT slice image reconstruction. When using the gantry embodiment as shown in FIGS. 1 and 2, the fan-beam 5 is translated past the rotating object 6 by moving the detector array 2 relative to the spinning object 6 as shown in FIGS. 1 and 4. Typically, the rate of translation is in the range of about 1 inch/minute to 10 inches/minute. The location of the object 6 is noted for each data point reading by recording the exact position of the array 2 as it rotates about the object 6. Alternatively, the object can be translated in a direction perpendicular to the plane defined by the beam of penetrating energy. Translation can be either linear or arcuate. In either case, the position of the object 6 relative to the beam 5 is recorded for each data point reading. An example of arcuate translation is shown in FIGS. 1 and 4, where the detector array 2 is slowly rotated about a focal point 13 containing the energy source 1. An arm 12, depicted as a C-shaped arm, is attached at one end to a frame 17 at the focal point 13. The detector array 2 is connected to the other end of the arm 12. An actuator 14 is attached to a midpoint of the arm 12 and to a first support bracket 50 at pivot point 15. The first support bracket 50 is mounted on the frame 17. A support arm 40 also interconnects the arm 12 and a second support bracket 52 mounted on the frame 17. As the actuator 14 extends or retracts, it rotates about the pivot point 15 and causes the arm 12 to rotate about the focal point 13. Thus, the fan-beam 5 is translated through the object 6. Translation of the fan-beam 5 is in a direction perpendicular to the plane of the fan-beam 5 as shown in FIG. 4. As it rotates, the fan-beam 5, formed between the energy source 1 and the detectors 3, forms a tangential path in the object 6. As the object 6 is translated through the beam 5, the diameter of the tangential path is varied. The detectors 3 measure the energy intensity through all possible tangents of the object 6 as the fan-beam 5 is moved across the diameter of the object 6. For example, in FIG. 4, the fan-beam 5 is shown tangentially passing through the object 6 at radii R1, R2 and R3. Because the object 6 simultaneously rotates and translates through the fan-beam 5, data defining the object 6 is collected in a spiral form. By analogy, the object 6 resembles a roll of paper towels being unrolled a single layer at a time. Data is typically collected at a rate of about 100,000 to 500,000 readings/second. For a typical 180 degree scan, the translation involves placing the fan-beam 5 at a tangent to the object's outermost diameter and passing the fan-beam 5 through the object 6 to the center of the object 6 or until the entire radius (R) of the object 6 is covered by the tangential path as shown in FIG. 4. A 360 degree scan requires scanning from the outer diameter of the object 6 to the other outer diameter, or until the full diameter (D) of the object 6 is covered by the tangential path. Alternatively, the object 6 can be scanned from a predetermined diameter to another predetermined diameter. With this invention, therefore, the collection of extraneous data is avoided with all types of scans, including the 180 degree and the 360 degree scan. In contrast, third and fourth generation scanner systems collect useless data when the diameter of the object is less than the size of the x-ray fan-beam. Because the tangential CT scanner's beam 5 is started at the object's outer diameter, and stopped when it reaches either the center, the opposite end of the diameter or any other predetermined diameter, no extraneous data is generated or collected. As the fan-beam 5 traverses the rotating object 6, each detector 3 collects data for one cross-sectional slice of the object 6. A cross-section slice is defined as that area being scanned by one detector 3 and is typically an area lying perpendicular to the axis of rotation 8 and the plane of the beam 5. Therefore, the data collected for each cross-section is collected by a single detector 3. Because only one detector 3 is utilized for each cross-sectional slice, the tangential scanner does not produce any artifacts due to detector variations. Furthermore, as the fan-beam 5 passes over the object 6, all detectors 3 in the detector array 2 simultaneously collect data for a different cross-sectional slices at their respective levels. Therefore, data is collected for the entire volume of the object 6 with only one translational pass. This data can be reconstructed later by computer software into a variety of images representing the interior of the object. To obtain better resolution of the images generated from the collected data, the rotation and translation speeds can be varied to increase the number of rays collected. Rays are defined as the number of data points collected across the diameter of the object 6. The views represent the number of directions from which the data points are collected through the object 6. The number of rays and views determine the ultimate resolution of the reconstructed cross-sectional image. In the tangential scanner system, the number of rays in the data set is equal to the number of rotations experienced during the data collection. Accordingly, as the number of rays, i.e. rotations, is increased, better spatial resolution is achieved. Conversely, the number of rotations can be limited to reduce overall scanning time. Resolution is also dictated by the number of data points or views collected during any single rotation. The more points collected, the better the spatial resolution. Again, the rotation and translation speeds can be altered to permit more or less data point readings. In essence, the tangential system can collect data with any number of rays and views simply by varying the rotation and translation speed parameters, or data collection rate. There is no limitation, therefore, to the number of rays or views in the collected data sets that are used to reconstruct the object displays or images. A system computer 23 is used to collect the data from each detector 3 in the array 2. The computer 23 responds to the resulting signals from the detectors 3 to construct a tomographic x-ray image of the object 6. The computer system 23 communicates with the detector array 3 through communication links 30. For a meaningful display of the tangential data set, the entire data set is organized in the computer system's memory. In an exemplary method for data collection, the tangential data is organized for each successive layer in the object 6. First, a two dimensional plane of data is generated. The number of detectors 3 in the detector array 2 represent one axis of this data plane and the number of data points taken through one 360 degree rotation represents the other axis of the plane, as shown in FIGS. 5 and 6. Many such data planes are generated, where each data plane represents one rotation of the object 6. These data planes are then stacked one over the other for successive rotations (layers) of the object 6 to organize a cube of data, or data cube 80, in the computer memory as shown in FIG. 5. A layer of data is made up of a tangential data sets collected at successively different depth levels in the object 6. Data collected and stored in this manner can be utilized in a variety of ways for visually displaying computer generated images for data analysis. Typically, a processor 16 equipped with software or logic, reconstructs the data into three types of images on a computer screen 65, or display unit such as a CRT. The three modes of display correspond to viewing the image of the data cube 80 through three perpendicular planes. In the first mode, the system displays the tangential planes, as they were stacked originally. In the second mode, various vertical cuts through the data cube are displayed. This mode shows the entire data collected by a single detector 3. The data in this mode is similar to the sinogram data from any conventional CT scanner system. This data can be used to reconstruct the cross-sectional image of CT slices. The data from each detector 3 represents one CT slice. In the third mode, various horizontal cuts through the data cube 80 are displayed. This mode show the entire data set collected through a single orientation of the object 6 and it is equivalent to a radiographic view of the object 6. Many such radiographic views are available through various orientations of the object. The first mode, or the tangent display, is equivalent to unrolling a roll of paper one layer at a time. Each unrolled sheet can be individually displayed. Data displayed in this mode is very sensitive to defects located parallel to the surface of the object 6 such as unbond defects in rocket motor casings. Within a single layer of the tangent data, features of the object 6, such as unbond defects, will show up once in the data when the x-ray beam 5 is exactly at the tangent of the defect. The same feature shows up twice in the display when the x-ray beam is inside the "object" circle, i.e., once when the defect is towards the x-ray tube and again when it is towards the detector side. In the second mode, the data is displayed in the form of sinograms. In this mode, the computer 23 displays the data for an individual slice of each cross-section of the object 6. Each sinogram display is created from data points gathered by a single detector 3. The individual sinogram from each detector 3 is a complete set of raw data which can be used to reconstruct individual CT slices. Therefore, the number of possible sinogram displays is directly correlated to the number of detectors 3. Because each sinogram display is organized from data gathered by one detector 3, the sinogram display does not contain any artifacts. A sinogram has an amplitude and a phase corresponding, respectively, to the radial and azimuthal locations of the feature being displayed. The intensity of the sinogram curve demonstrates the size of the feature being analyzed. The third and final mode for displaying the data is the radiographic display. Such a display shows the data collected in a plane cutting the data cube 80 along a fixed azimuthal angle, or the data corresponding to the image of the object without rotation. Successive similar cross sections show digital radiographs of the object 6 as it rotates around its axis. The radiographic display is helpful in extracting the location and intensity of the features. Together, all three display modes provide a better understanding of the feature being evaluated. Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. As such, it is intended that the foregoing detailed description be regarded as illustrative rather than limiting and that it is the appended claims, including all equivalents thereof, which are intended to define the scope of the invention.