Abstract:
Systems and methods for non-intrusive analysis and display of internal features of wooden objects are provided. In embodiments of the system, a log is passed through a CT scanner in one continuous motion. One or more x-ray sources revolve around the log generating x-ray beams that traverse contiguous cross-sections of the log. An array of x-ray detectors detects x-rays that traverse the log for variations in the attenuation of rays. The detected attenuation is converted into spiral scan data that corresponds to projections in different contiguous cross-sections traversed by the x-rays. An image processor reconstructs spiral scan data into two dimensional cross-sectional images of the log by processing and formatting scan data using a planar reconstruction technique. The system renders three-dimensional views based on two-dimensional images.

Description:
BACKGROUND 
     1. Field of Invention 
     The present invention is directed to non-intrusive analysis and visual display of internal features of logs and, in particular, to systems and methods for scanning, detecting, and displaying internal features of a log, using computerized tomography. 
     COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material, which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     2. Related Art 
     The value of a board generally depends on the size of the board and number of defects in the board. Accordingly, sawmills, particularly when sawing valuable logs, attempt to maximize the yield of boards having large areas that are relatively free of defects. Naturally, a log with fewer defects provides better quality boards or lumber. Therefore, the commercial value of a log directly depends on the type and number of defects in the log. 
     Defects in a log commonly correspond to variations in a log&#39;s composition or density. Such variations often arise from the natural growth process of the tree and correspond to knots, voids, or decay in the body of the tree. Some defects such as cracks or decay on the exterior of a log are clearly visible, but many internal defects or the extent thereof (e.g., interior decay and internal knots) are not fully visible to the naked eye. A buyer, typically, evaluates a log by considering the log&#39;s shape, external indicators of internal defects, and knowledge of lumber grades. While experts such as log graders and sawyers are highly skilled in the evaluation of logs, they cannot possibly detect all internal defects of a log. Therefore, it is difficult to accurately distinguish a high quality log over a lower quality log that has a similar external appearance. 
     A person could more accurately estimate the value of a log if he could view and inspect the internal defects and undesirable features of the log. Accordingly, methods and systems that can efficiently detect and reveal the defects in the interior composition of a log with reasonable accuracy would be very helpful in evaluating a log&#39;s worth. 
     SUMMARY 
     One or more embodiments of the invention are directed to a system and method for non-intrusive analysis and visual display of the internal features of logs, using CT scanning technology. The system and method described herein may be also applicable to scanning objects other than a log. In one embodiment of the system, a log passes through a CT scanner in one continuous motion. The scanner includes an aperture to receive the log and one or more x-ray sources that revolve around the log, as it passes through the system, generating x-ray beams that traverse multiple cross-sections of the log. 
     An array of x-ray detectors detects attenuation of the x-ray beams that traverse the log. The attenuation of the x-ray beams is measured based on changes in the intensity of the x-ray beams as they pass through the log. These measurements are converted into scan data. The scan data contains information about multiple cross-sections of the log. A computer system reconstructs the scan data into two-dimensional or cross-sectional images of the log, by using a standard planar CT reconstruction technique. 
     The computer system also uses the scan data collected from multiple cross sections to construct three-dimensional images of the log. A three-dimensional image data set, in accordance with one aspect of the system, can thus be produced from data from two-dimensional images. In certain embodiments, interpolation, surface rendering, and ray tracing techniques may be used to produce three-dimensional image data sets, as well. In certain embodiments of the invention, three-dimensional image data sets are used to produce two-dimensional images with three-dimensional features using perspective and shading techniques. 
     According to one or more aspects of the invention, the system includes a retractable log driver (Push Dog) that drives the log on a transport bed through the scanner. In certain embodiments, the transport bed is preferably constructed of at least two parallel metal rails. The rails have a gap in the area where the x-rays pass through to avoid image distortion and interference with the scanning process. A loader or loading arm coupled to a mechanical actuator loads a log from a staying mechanism onto the transport bed. The loading arm, in a certain embodiment, includes one or more rails that form a portion of the transport bed when the loading arm is engaged in a loaded position. The loading arm can be activated in multiple positions to receive, load, or eject a log. 
     Embodiments of the system include an unloading arm for unloading logs from the transport bed onto a receiving mechanism. The receiving mechanism transitions the logs to a storage area, for example. The staying and receiving mechanisms can be represented by a variety of devices (e.g., “transport”, “log haul”, “hourglass rollers”, “log kicker, etc.). The unloading arm may also include one or more rails that form a portion of the transport bed. 
     Before the log driver engages the log, a sensor mechanism determines whether a log loaded on the system can go through the aperture of the CT scanner. If the sensor detects an oversized log, the log is ejected from the transport bed. In certain embodiments, the log is ejected when the loading arm moves to an eject position. If the sensor determines that the log can safely clear the aperture of the scanner, then the log driver engages at least one end of the log and drives the log through the scanner. The driver is configured to displace the log at a speed that allows the scanning system to capture sufficient data from multiple cross-sections of the log for display and analysis purposes. 
     A processing system converts captured data, also referred to as scan data, into two or three-dimensional density distribution images that display the defects within the body of the log so that the defects are recognizable by a human operator or a computing system. In one or more embodiments, an end unit (e.g., hold back dog) engages the log at the end opposite to the point of engagement of the log driver to add stability to the positioning and movement of the log on the transport bed. Once the complete length of the log has been driven through the scanner, the log driver is retracted to its original position so that another log can be loaded. The end unit can further include marking systems that mark the log for sawing or identification. 
     Other embodiments of the invention may include a revolving belt mechanism with multiple log drivers installed at successive intervals around the belt so that a second log driver is in position to engage a second log when the scanning process for the first log is complete. This avoids the requirement for having a retracting mechanism. In either implementation, the loading arm is configured in such a way to allow for loading a second log immediately after the first log has gone through the scanning process. 
     The scan data and the images produced as the result of CT scanning and reconstruction are useful to a log purchaser or sawyer to evaluate the worth and usefulness of the log for producing certain quality boards. For example, the system can reconstruct scan data to simulate a longitudinal cut through the log producing the image of a virtual board. The system can also reconstruct other images such as three-dimensional views of the log that display the interior defective regions of the log, the type of defect (e.g., decay, cracks, knots, etc.), and the exact location of each undesirable feature. 
     Certain images, according to an embodiment of the invention, are produced so that the exterior surface of the log appears transparent and defects appear opaque and/or color-coded so that the defects are easily identified. In accordance with one aspect of the invention, the system may determine an optimal cutting solution for the log based on information provided about the log&#39;s intended use, the type of each defect, and respective distance between the defects. Other embodiments may provide a purchaser with a log&#39;s grade, estimated value, or most suitable or profitable cutting solution. 
     In accordance with another aspect of the invention, a log is marked with a reference mark that indicates the orientation of the log during the scanning process. This information can be used during sawing or image processing to accurately identify the location of the defective or undesirable regions in the log. Using a reference mark, a sawyer or a computerized sawing machine can appropriately position the log&#39;s orientation for a specific cut. The reference mark can be simple stripes or markings painted on one or both ends of the log. The markings can include more sophisticated coding such as bar codes or magnetic strips that contain additional information about the log (e.g., grade, sample cross-sectional views, sawing solutions, etc.). The reference marks can be placed on the logs by a marking mechanism. In one embodiment of the system, the marking mechanism is integrated into the log driver, end unit, or both. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating components of a log scanning system in accordance with an embodiment of the invention. 
     FIG. 2 is a perspective view of the system of FIG. 1, in accordance with one or more embodiments. 
     FIG. 3 illustrates a perspective view of a log CT scanner, according to an embodiment of the invention. 
     FIGS. 4A and 4B illustrate the log CT scanner of FIG. 3, according to an embodiment of the invention, in mount and dismount positions. 
     FIG. 5 is a block diagram illustrating the components of the scanner in conjunction with a corresponding computing system that processes the data produced by the scanner, in accordance with an embodiment of the invention. 
     FIG. 6 illustrates a staying or receiving mechanism for transporting logs to and from a scanning system. 
     FIG. 7 is a perspective view of the system of FIG. 1 illustrating the loading arm in a receiving position, in accordance with an embodiment of the invention. 
     FIG. 8 is a flow diagram illustrating a method of processing logs for scanning by the system, according to an embodiment of the invention. 
     FIG. 9A is a cross-sectional view illustrating the loading arm in a receiving position in accordance with an embodiment of the invention. 
     FIG. 9B is a cross-sectional view of the system illustrating the loading arm in a loading position. 
     FIG. 9C is a cross-sectional view of the system illustrating the loading arm in an ejecting position. 
     FIG. 9D is a perspective view of an embodiment of the system including a spring-loaded plate. 
     FIG. 10 is a perspective view of the log driver, according to one of the embodiments of the invention. 
     FIG. 11 is perspective view illustrating the log driver engaging a log and driving it through the scanner of the system, according to an embodiment of the invention. 
     FIG. 12 is a perspective view illustrating the loading arm receiving a second log, as the log driver drives the first log through the scanner of the system, according to an embodiment of the invention. 
     FIG. 13 is a perspective view illustrating the log driver as it retracts after scanning is completed. 
    
    
     DETAILED DESCRIPTION 
     The invention relates to a system and method for scanning, detecting, and displaying internal features in a log by using computerized tomography (CT) techniques. One skilled in the art will appreciate that the teachings of the present invention can be equally applied to the detection and display of inner features of any object using any scanning technology. In the following description, numerous specific details are set forth to provide a more thorough description of embodiments of the invention. The invention, however, may be practiced without some or all of these specific details. In some instances, certain well-known features have not been described in detail so as not to obscure the more relevant aspects of the invention. 
     System Architecture Overview 
     In accordance with one or more aspects of the system, a CT scanner in combination with a computing system and other corresponding equipment is used to scan and graphically display the interior defects in a log in a manner that is recognizable by a human operator. FIG. 1 is a block diagram illustrating the primary components of the system. As shown, system  300  generally includes a first staying mechanism  310 , an infeed structure  320 , a CT scanner  330 , an outfeed structure  340 , and a receiving mechanism  350 . An operator cabin  360  may be also included in embodiments of the system between transport mechanisms  310  and  350 , for example, to house the computing system and control mechanisms that operate the above components of the system. 
     Infeed structure  320 , scanner  330 , and outfeed structure  340  include rails, in certain embodiments of the system, that are longitudinally aligned on a substantially horizontal plane to collectively compose a transport bed  420  (FIG.  2 ). The rails, in certain embodiments, are straight and parallel but aligned along a curved surface to form transport bed  420  such that the rails can support and cradle the outer surface of a log. Transport bed  420  in certain embodiments is a continuous medium that supports and guides a log through scanner  330 . 
     Staying mechanism  310  is utilized to transport one or more logs from a storage area to infeed structure  320 . The logs, before being placed on staying mechanism  310 , can be debarked and readied for scanning. Infeed structure  320 , as described in further detail below, includes mechanical components for receiving a log from staying mechanism  310  and driving the log through scanner  330 . Scanner  330  is an electromagnetic or x-ray scanner capable of scanning a log to detect defects. Outfeed structure  340  unloads logs from system  300  onto receiving mechanism  350 . Receiving mechanism  350  is a transport mechanism, similar in structure to staying mechanism  310 . Receiving mechanism  350 , however, operates in a reverse direction to transfer a log back to the storage area, for example. 
     FIG. 2 is a perspective view of an embodiment of system  300  with operator cabin  360  located at a closer proximity to infeed structure  320  and further away from outfeed structure  340 . As shown, in accordance with one or more aspects of the invention, transport mechanisms  310  and  350  include conveyor or chain driven mechanisms for transporting multiple logs to and from the scanning area. Infeed structure  320  and outfeed structure  340  include rails that extend through scanner  330  to form transport bed  420 . Transport bed  420  is preferably constructed out of a plurality (e.g., four) of parallel steel rails that extend along the log&#39;s path through scanner  330  to support and guide the log&#39;s movement. The rails may be made of any suitable material (e.g., steel) that can smoothly support movement of the log on transport bed  420 . In accordance with one aspect of the invention, system  300  includes a tunnel structure for shielding a human operator from harmful radiation generated by scanner  330 . 
     Scanning System and Method 
     Scanner  330 , illustrated in further detail in FIG. 3, is a CT scanner (e.g., CTX-5000, CTX 9000, CTX 9300 scanning unit manufactured by Invision Technologies, Newark, Calif.). Other scanning mechanisms that utilize electromagnetic radiation to calculate density distribution within selected cross-sections or regions of an object may also be utilized. Scanner  330  is between infeed  320  and outfeed  340 . Scanner  330  includes a gantry  337  with an opening in form of aperture  335  to receive a log  405 . As shown in FIG. 3, scanner  330  can have a stand-alone structure that can be functionally integrated with infeed and outfeed structures  320  and  340  to provide a continuous transport bed  420  for a log to travel on. In embodiments of the invention, scanner  330  includes base frame  332  for mounting gantry  337  at a height suitable for receiving a log  405  from infeed  320 . 
     Aperture  335  is preferably circular in shape and preferably has a diameter of 85 to 110 centimeters to receive logs of approximately 25 to 100 centimeters in diameter, for example. As described in further detail below, log driver  410  (FIG. 2) is a driving mechanism utilized for driving log  405  through aperture  335 . Log driver  410  engages and pushes log  405  so that log  405  moves in a controlled continuous manner along transfer bed  420  in an axial direction. In embodiments of the invention, oversized logs that cannot fit through aperture  335  are detected and unloaded before log driver  410  engages the oversized log. 
     FIGS. 4A and 4B illustrate a system according to an embodiment of the invention in which a scanner  330  can move between a mounted position and a dismounted position. This movable scanner allows the transport of oversized logs from infeed  320  to outfeed  340  without having to drive the log through scanner  330 . 
     FIG. 4A illustrates scanner  330  in the mounted position. In certain embodiments, scanner  330  includes guide rails  510  that extend in a substantially horizontal plane through scanner  330 . Guide rails  510  optionally include a gap to prevent guide rails  510  from interfering with X-rays projected through the log during scanning. As shown, infeed  320  and outfeed  340  include matching horizontal rails  520 . In this embodiment, each of guard rails  510  lines up with a matching one of rails  520  when scanner  330  is in the mounted position. Rails  510  and  520  are positioned along a curved surface to cradle the log and resist movement of the log perpendicular to the lengths of the rails. In the mounted position, guide rails  510  engage rails  520  forming one continuous transport bed  420  (possibly including a small gap in a scanning region inside scanner  330 ). Guide rails  510  help to guide and support the controlled movement of log  405  in an axial direction from infeed  320  through scanner  330  and over to outfeed  340 . 
     In certain embodiments, to maximize the signal to noise ratio of the scan data and avoid any gap in transport bed  420 , all or a portion of guide rails  510  and/or rails  520  are made of low attenuation material (e.g., plastic) that is not highly detectable by scanner  330 . Alternatively, some embodiments of the system have a gap in guide rails  510  inside scanner  330 . The gap is sufficiently wide (e.g., about 1 cm) to allow an unobstructed scanning region within scanner  330  such that an x-rays generated by the scanner  330  do not traverse guide rails  510  during the scanning process. 
     FIG. 4B illustrates the system when scanner  330  is in the dismounted position. In accordance with one or more aspects of the invention, a base frame  332  extends out in a direction substantially perpendicular to the direction of movement of log  405 . Scanner  330  can move across the extension to clear transport bed  420 . Certain embodiments of the invention include an external set of guide rails  530 . In the dismounted position, guide rails  530  engage rails  520  forming one continuous transport bed  420 . As such, an oversized log can be transported from infeed  320  to outfeed  340  without going through aperture  335 . 
     This aspect of the invention is useful, for example, when a log loaded onto infeed  320  is too large to pass through aperture  335 . Other embodiments that allow the removal of scanner  330  from path of travel of log  405  on transport bed  420  are possible. For example, an embodiment may include a lifting mechanism to lift scanner  330  above transport bed  420  at a height that would clear the log. Certain embodiments may include wheels under scanner  330  to allow removal of scanner  330  from in between infeed  320  and outfeed  340 , respectively, so that an oversized log can be transported from one side to the other. 
     FIG. 5 illustrates a block diagram of the various physical components of scanner  330  and a corresponding computing system  626 . Scanner  330  scans logs for defects and produces scan data. Computing system  626  processes the scan data e.g., converts scan data to image data. Scanner  330  includes gantry  337 , an x-ray source  646 , a detector array  650 , and a data acquisition system (DAS)  652 . Gantry  337  houses a high voltage power supply (HVPS)  644  coupled to x-ray source  646 . X-ray source  646  creates an x-ray fan beam  648  that traverses log  405  during scanning. Detector array  650  is opposite to x-ray source  646 . Detector array  650  measures the intensity of x-rays that passed through log  405 . Detector array  650  can include multiple detector elements, multiple segmented detector elements, an array of single detectors, or continuous media responsive to x-rays. 
     Detector array  650  intercepts x-rays attenuated by log  405  as the log passes through x-ray fan beam  648 . The x-ray detectors in the detector array convert x-ray intensities into electrical signals. These electrical signals are digitized by the data acquisition system  652  to form scan data. Software in workstation  664  or back projector  680  manipulating the corresponding scan data produces cross-sectional images of the log which show the log&#39;s internal features. In certain embodiments of the invention, x-ray source  646  and detector array  650  revolve around log  405  during the scanning process. 
     Alternatively, x-ray source  646  and detector array  650  may remain stationary while log  405  axially rotates in the field of scan. In either implementation, log  405  moves at a constant linear speed relative to scanner  330  during the scanning process. In some embodiments, log  405  moves as described across a transport medium through scanner  330 . In other embodiments, log  405  remains stationary and scanner  330  moves along the length of log  405  during the scanning process. 
     In the exemplary embodiments of the invention, log  405  continuously travels through fan beam  648 , while x-ray source  646  revolves around log  405 . As the result of the continuous movement of log  405  during the scanning process, a volume of attenuation measurements is taken that includes information about the cross-sections of log  405 . In particular, scanning log  405  produces scan data that corresponds to a spiral slicing of log  405 . 
     A portion of the spiral corresponding to all or part of one or more rotation around log  405  provides raw scan data for CT processing that determines a density distribution for a cross-section of log  405 . The raw scan data produced, as the result of this continuous scanning, is sometimes referred to herein as spiral scan data. This is in contrast to conventional scanning methods that produce scan data that includes information about multiple distinct cross-sections in multiple parallel planes through the log. 
     Detector array  650  determines the spiral scan data by measuring the intensity of x-rays after the rays pass through the log, and computing system  626  converts the spiral scan data into the density distributions for cross-sections of the log. Because knots, bark, decay, sapwood, heartwood, voids, and other log features usually have different densities, these features can be distinguished based on detected variations in density. For example, a knot in the body of a log may have a higher density in comparison to a void or a crack. In one or more embodiments, the collection of the density distributions for evaluated set of cross-sections of the log forms a three-dimensional data structure sometimes referred to herein as the image data for the log. Images representing the inner features of the scanned log can be rendered from the image data. 
     In accordance with an aspect of the system, the spiral scan data is communicated to data acquisition system  652  and computing system  626 . Computing system  626  processes the spiral scan data to generate the density distributions and images that identify and display the internal features of the log. The relationship between attenuation and density is linear in woody materials. Thus, higher density for a certain region may indicate a knot, for example, while lower density may indicate a crack in log&#39;s body. Detector array  650  is connected to data acquisition system  652 . Data acquisition system  652  converts all detector measurements into spiral scan data in a digital format, for example. In embodiments of the system, data acquisition system  652  exchanges control signals with a rotating control module  654  that provides the means for x-ray source  646  to rotate about the scan object (e.g., log  405 ). 
     In embodiments of the invention, gantry  337  includes rotating control module  654  that controls the communication and transfer of data and power between the rotating and non-rotating portions of scanner  330 . Rotating control module  654  is electrically coupled to the non-rotating portion of scanner  330  (e.g., stationary control module  660 ) via a slip ring  658  having multiple contacts. Through slip ring  658 , electrical input power is transferred from HVPS  644  to other electrical component within gantry  337 . Digital data signals and control signals are also transferred to and from gantry  337  through slip ring  658 . In one or more embodiments of the invention, a wireless transmitting and receiving system (e.g., an RF Ring) transmits data and control signals between the rotating and non-rotating portions of the scanner and slip ring  658 . By using slip ring  658 , gantry  337  rotates continuously without the need to use winding/unwinding mechanisms to connect rotating components of the system to the stationary ones, via cables, for example. Slip ring  658  is coupled through a slip ring bus  659  to stationary control module  660 . Stationary control module  660  provides control signals to scanner  330  and motion controller  622 . 
     Image Reconstruction System and Method 
     In one or more embodiments of the invention, spiral scan data acquired by scanner  330  is transmitted to computing system  626  via stationary control module  660  for reconstruction into two or three-dimensional images or data structures. Images reconstructed from the spiral scan data correspond to cross-sections of the scanned portions of log  405 . This cross-sectional data is analyzed by components of computing system  626 , illustrated in FIG. 5, and is converted to image data. Computing system  626  includes a workstation  664 . Workstation  664  is coupled to a monitor  666  for controlling the operation of various components of the system. A system operator interacts with the workstation  664  via a keyboard  668 , a mouse  670 , and/or other user interface devices. Memory device  672  and tape drives  674  are also provided, in accordance with one or more embodiments of the invention, for data storage. 
     In certain embodiments, a CT image reconstruction system processes the spiral scan data using a standard planar CT image reconstruction technique and ignore the longitudinal motion occurring during one rotation around log  405 . Alternatively, special spiral scan reconstruction techniques can produce an accurate detailed image of a particular cross-section of an object. However, certain embodiments of the invention do not utilize a spiral reconstruction technique to determine density distributions because the overhead associated with using such reconstruction techniques is relatively high and not cost effective. As such, certain embodiments use standard planar CT reconstruction techniques to construct cross-sectional images of log  405 . These well known techniques include Direct Fourier Reconstruction and Filtered Back Projection. 
     Generally, using a standard planar CT reconstruction technique to determine scan data from spiral scanning of an object can result in a distorted image that does not clearly or accurately distinguish between the internal features of the object. However, in accordance with one aspect of the invention, scanner  330  rotates around a log at about 60 rpm, while the movement of the log during a single rotation is about 5 cm. Other rotation and movement rates may be used as desired. Because wood composition is relatively consistent within successive cross-sections in a longitudinal direction, the movement during a scanning revolution does not greatly change the raw data. Additionally, features smaller than one cm are often of little interest. 
     Accordingly, a standard planar reconstruction method can produce images and density distributions that are sufficiently accurate for determining certain defects in the log. A planar reconstruction method, in contrast to more sophisticated and resource intensive methods, allows the system to operate with maximum throughput and efficiency while slightly compromising image quality. One embodiment of the invention uses a Direct Fourier Reconstruction method to reconstruct data obtained from preferably about 250° of rotation of x-ray source  646  around the log. This is equal to about 180° plus the x-ray fan angle. During the 250° rotation, the log moves approximately 3.5 cm. 
     In accordance with certain aspects of the system, the image of a cross-section of log  405  is constructed from the data acquired during 250° of rotation. Since the log is moving during data acquisition, half of the data is collected during the first 125° of rotation on either side of the selected cross section, so the image produced contains contributions from features on either side of the cross section. However, the reconstruction technique emphasizes the contributions of features scanned during the central 110° of rotation (i.e., 180°−fan angle). Although a single acquisition needs 250° of rotation, images can be reconstructed using overlapping data. For example, an image can be produced from scan data centered at about 90°, 180°, 270°, and 360° (plus the initial 70° corresponding to the fan angle) to produce 4 images per rotation. 
     Log  405  moves approximately 6 centimeters, for example, for every rotation of the x-ray source  646  to reconstruct images of cross-sections spaced 1.5 centimeters apart. Preferably, the cross-section determined using the above reconstruction method consists of an array of 512×512 density values. Images of other cross-sections of the log are generated by linear interpolation of the density values for the points in between the reconstructed points. The above-referenced angles and distances are approximations and are provided by way of example. Of course, other rotation angles and distances may be used within the course and scope of this disclosure. 
     In certain embodiments, workstation  664  is preferably coupled to a real time VME computer  676  that provides additional mathematical computing power. The VME computer  676  preferably includes memory device  678 . A back projector  680  is provided in conjunction with VME computer  676 . Back projector  680  processes the spiral scan data to generate planar image data, in accordance with one aspect of the system. The image data may be displayed on monitor  666 . As described above as an alternative to CT reconstruction using back projector hardware  680 , the Direct Fourier Reconstruction method may be used. As is known in the art, such a method does not require a back projector  680 , instead, standard computers or array processors are used, in one or more embodiments. 
     In accordance with one aspect of the invention, based on the density of certain defects, computing system  626  can be programmed to detect and display specific defects so that different defects can be easily distinguished from one another. For example, the system can be programmed to construct an image of the log where undesirable features of the log such as voids or decay are highlighted. Embodiments of the system are configured to display the heartwood or other desirable wood as translucent or transparent, for example. The internal undesirable features may be presented instead in various colors or opaque shades to indicate different types of defects. For example, voids and decay can be displayed as white, while knots can be displayed as black. Additionally, the physical appearance of the grain at certain longitudinal cross-sections can be simulated by generating a longitudinal cut through CT cross-sectional images in shades of brown, for example. 
     Exemplary Mechanical Embodiment of the System 
     Aside from the novel aspects of the system directed to scanning and imaging, one or more aspects of the system are directed to a system and method for handling logs before, during, and after the scanning process. FIG. 6 illustrates a transport mechanism for transporting logs to and from transport bed  420 , according to one or more embodiments of the system. Transport mechanisms  310  and  350 , mentioned earlier, include a frame  710 , a motor drive  720 , and one or more conveying belts  730 . Motor drive  720  is installed on frame  710  and includes an engine rotatably engaged to one or more rollers for rotating the rollers in a clockwise or counterclockwise direction. The rollers are preferably tapered inwardly (e.g., in the shape of an hourglass) to snugly receive belts  730 . Belts  730  are tightly fit around the rollers to provide a conveying bed for carrying logs to and from system  300 . In embodiments of the invention, other driving or power transferring mechanisms may be used to provide a non-binding conveying bed. For example, belts  730  maybe replaced by chains or other equivalents engaged to sprockets instead of rollers. 
     FIG. 7 illustrates transport mechanisms  310  and  350  in operational relationship with other system components, according to one or more embodiments. Staying mechanism  310  delivers logs to transport bed  420  for scanning. Receiving mechanism  350  receives scanned logs from transport bed  420  and moves the scanned logs away from transport bed  420 . In certain embodiments of the invention, transport mechanisms  310  and  350  have the same construction but operate to move logs in different directions. Once staying mechanism  310  delivers log  405  system  300 , system  300  performs a series of operations to load log  405  on transport bed  420 . FIG. 7 illustrates the relative positions of components  310 ,  330 ,  350 , and  420  of system  300  with respect to one another before log  405  is loaded onto transport bed  420 . As shown, log driver  410  is in a retracted position ready to engage log  405  after log  405  is loaded. 
     FIG. 8 is a flow diagram illustrating a method of loading and unloading log  405  on transport bed  420 , in accordance with one or more aspects of the invention. Transport bed  420 , as described earlier, includes infeed and outfeed structures  320  and  340 , with scanner  330  mounted there between. In certain embodiments, each of infeed  320  and outfeed  340  includes movable and non-movable parts. As shown in FIG. 9C, the movable part of infeed  320  includes a loading arm  920  that includes one or more rails  930 . The non-movable part includes one or more rails  940  supported by a frame  950 . FIGS. 9A through 9C include cross-sectional views of system  300  with loading arm  920  in three different positions. FIGS. 9A,  9 B, and  9 C illustrate loading arm  920  in a receiving position, a loading position, and in an ejecting position, in accordance with one or more aspects of the invention. 
     To load the logs transported from staying mechanism  310  onto transport bed  420 , loading arm  920  is moved to a receiving position at step  810 . Loading arm  920  is rotationally attached to an axle  960  supported by frame  950 . Loading arm  920  and axle  960  are coaxial, sharing a common pivot point. By the virtue of this relationship, loading arm  920  rotates about axle  960 , for example, through 180° by means of an actuator  910 , such that loading arm  920  can be lowered into a receiving position to receive log  405  from staying mechanism  310 . With loading arm  920  lowered, at step  820 , log  405  moves or rolls onto loading arm  920  from staying mechanism  310 , as the result of log  405  reaching the terminal end of the conveying belt of staying mechanism  310  and loading arm  920  being lower than the terminal end of staying mechanism  310 . 
     Referring to FIGS. 9A through 9C, actuator  910  is, for example, a hydraulic actuator that controls the movement of loading arm  920  in multiple positions. Actuator  910  preferably is suited to lift 7,000 pounds in weight, for example. Longitudinally, loading arm  920  extends along the receiving side of infeed  320  and is sufficiently wide and long to support a log on rails  930 . As shown, the outer side surface of loading arm  920  that faces staying mechanism  310  is curved, for example, like the outer surface of a section of a cylinder. Consequently, loading arm  920  can extend close to the edge of staying mechanism  310 , as loading arm  920  moves in a curved path from the receiving position to the loading or ejecting position. In addition, as shown in FIG. 9B or  9 C, loading arm  920  when in the loading or ejecting position is sufficiently tall to serve as a barrier that prevents logs on staying mechanism  310  from falling. Rails  930  of loading arm  920  substantially extend the full length of infeed  320  in accordance with an aspect of the invention. Similarly, rails  940  on frame  950  substantially extend the full length of the non-movable part of infeed  320 . 
     In certain embodiments, the non-movable part of infeed  320  also includes a spring-loaded plate  970  attached to frame  950  and extending along the side of loading arm  920 . FIG. 9D is a perspective view of an embodiment of the system including a spring-loaded plate. As shown, the spring-loaded plate is positioned next to loading arm  920  to absorb the impact that loading of log  405  can cause to frame  950 . Thus, when log  405  is loaded on the system as shown in FIG. 9A (e.g., when loading arm  920  is in a receiving position) log  405  rests on rails  930  and spring-loaded plate  970 . 
     As shown in FIG. 9B, when loading arm  920  moves to the loading position, at step  830 , rails  930  and  940  are aligned to form a portion of transport bed  420  that supports and guides log  405  through scanner  330 . As log  405  is driven further through scanner  330 , the weight of log  405  is gradually transferred from rails  930  and  940  to guide rails  510  and thereafter to rails of outfeed  340 . A preferred embodiment of the system includes at least two pairs of coplanar rails for supporting the curved outer surface of log  405 . In embodiments of the invention where scanner  330  includes guide rails  510 , rails  930  and  940  are aligned to meet guide rails  510  such that a smooth continuous transport bed is provided for log  405  as it is being driven through scanner  330 . 
     FIG. 9B illustrates loading arm  920 , at step  830 , in the loading position. Loading arm  920  raises log  405  onto a loading position so that log  405  rests on transport bed  420  made up of rails  930  and  940 . As shown, log  405  is supported by transport bed  420  and is appropriately aligned so that log driver  410  can drive log  405  through scanner  330 &#39;s aperture  335 . In accordance with certain aspects of the invention, transport bed  420  is preferably coupled to an input sensor. The input sensor detects the presence of log  405  on transport bed  420 . Referring to FIG. 5, the sensor is coupled through control system  626  to motion controller  622 . Motion controller  622  controls the movement of log driver  410  along transport bed  420 . When a log is initially loaded on the system, the log&#39;s exact position on transport bed  420  is determined through data derived from the input sensor, the transport bed  420 , and motion controller  622 . 
     In certain embodiments of the invention, a sensor included on transport bed  420  detects oversized logs (i.e., logs with a cylindrical volume or diameters larger than that of aperture  335 ). The sensor may be alternatively installed on other components of the system (e.g., log driver  410 ). Thus, step  840  determines whether the log loaded on transport bed  420  can go through aperture  335 . If an oversized log is detected then, at step  870 , the log is ejected from transport bed  420  as loading arm  920  is moved to an ejecting position, as shown in FIG.  9 C. Moving loading arm  920  to ejecting position causes rails  930  and  940  to move out of alignment and shift transport bed  420  in an angle so that the oversized log is ejected from system  300 . The input sensor in embodiments of the invention can be mounted on system  300 &#39;s frame or any other suitable position. If the input sensor does not detect an oversized log then, at step  850 , log driver  410  engages log  405  at one end. At step  860 , log driver  410  drives log  405  through scanner  330  in a linear direction along transport bed  420 . 
     FIGS. 9D and 10 show perspective views of a log driver  410  suited for driving log  405 , according to one or more embodiments of the invention. As shown in FIG. 10, the portion of log driver  410  that engages log  405  includes a toothed member to allow for a better grip on and stabilization of log  405 . A motor drive engaged through a cable or chain driven mechanism posers log driver  410 , for example, so that log driver  410  can retractably move forward and backward in a linear direction along transport bed  420 . In embodiments of the system, log driver  420  is configured to generate sufficient force (e.g., 5,000 pounds) to move a log and to travel at the approximate speed of 20 centimeters per second. Log driver  420  is also configured to retract, in accordance with one aspect of the invention, at a proximate speed of 335 centimeters per second, for example. Other embodiments of the invention may include driving mechanisms with multiple log drivers installed in successive intervals such that a second log driver will be in position to engage a second log by the time the scanning process for the first log has been completed. This avoids the need for a retracting mechanism. 
     FIG. 11 illustrates an embodiment of the system, where log driver  410  is driving log  405  through scanner  330  during the scanning process. A radiation tunnel covering the scan area is not shown in FIG. 11 for clarity, but can be included in certain embodiments. FIG. 12 illustrates system  300 , after log  405  has been driven through and scanned by scanner  330 . FIG. 13 illustrates log driver  410  retracting to its initial position as log  405  is being unloaded from outfeed  340 . In certain embodiments of the system, outfeed  340  has the same configuration as that of infeed  320 , but operates in reverse order. Thus, outfeed  340  is made up of movable and non-movable parts. The movable part, for example, includes an unloading arm operated by a hydraulic actuator that controls the movement of the arm in various positions. 
     The non-movable part and unloading arm of outfeed  340  each include rails that extend in a longitudinal direction. In combination, the rails on each part compose the portion of transport bed  420  that receives a log after the log is driven out of scanner  330 . While a log moves on to outfeed  340 , the unloading arm remains in a position with rails are parallel to and in a substantially horizontal planes with the corresponding rails of the non-movable part. In this position, the rails are aligned to form a stable transport bed  420  for receiving a log emerging from end of scanner  330 . To unload the log, the unloading arm is lowered causing the transport bed tilt towards receiving mechanism  350 . The created slope causes the log to roll down towards receiving mechanism  350 . 
     In accordance with certain embodiments, a second log can be loaded on transport bed  420  via infeed  320  before or while the first log is unloaded from transport bed  420  via outfeed  340 . The moving portions of infeed  320  and outfeed  340  are controlled and moved independently. Thus, for example, while the unloading arm of outfeed  340  moves to an unloading position, the loading arm of infeed  320  can move to a loading position. In an alternative embodiment, the movable portions of infeed  320  and outfeed  340  can be incorporated into an integrated hydraulic arm such that while the arm of outfeed  340  moves down to unload a log, the arm of infeed  320  moves down to receive another log. In addition to log driver  410 , certain embodiments of the system also include an end unit (e.g., hold back dog). Structurally, the end unit is comparable with log driver  410 . It engages a log at the end opposite to location of engagement of log driver  410  and provides sufficient resistance against the driving force of log driver  410  to further stabilize and secure a log&#39;s movement on transport bed  420 . 
     In certain embodiments of the system, a marking device is included in the end unit and/or log driver  410  for placing one or more reference marks on the log. The reference marks indicate the orientation and positioning of a log on the system and can guide an automated sawing machine or a sawyer with cutting the log. In one embodiment, the marking device is a self-contained module that includes one or more spray heads for painting stripes on the ends of the log. The marking system can further include internal paint pots and one or more sensors for limiting over spray. In accordance with one or more embodiments, the marking system may include bar code applicators and readers. A bar code can be coded to include useful information about the log (e.g., length, size, weight, grade, etc.). Alternatively, memory storage mediums such as magnetic strips can be used to store more detailed information, such as optimal cutting solutions or image data for a scanned log. 
     Embodiments of this invention can be used in conjunction with an on-line sales model for presenting scanned logs for sale on a worldwide network, and a software system that efficiently presents internal features of a log in a two-dimensional data structure. One or more of these embodiments are described in U.S. patent application filed on Feb. 23, 2001, Ser. No. 09/792,650, the entire content of which is incorporated by reference, herein. 
     Thus, a system and method for analyzing and displaying the internal features of a log is described in conjunction with one or more embodiments. It should be understood, however, those system embodiments disclosed here are provided by way of example. Other methods or system architectures and implementations for transporting or scanning a log may be utilized. These and various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. The invention is defined by the following claims and their full scope of equivalents.