JP2004347606A - Detection of ct target using normal curve - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To apply CT scanning of baggages on the order of 700 bags per hour, by excluding data that are not related to image reconstruction. <P>SOLUTION: By using a threshold, scan calibration with no target can be performed, even if there are objects, such as a system conveyor in the field of view. A scanner has a shape for properly detecting sheet explosives. The system runs compensation, using dark currents and temperature-dependent current offsets. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates generally to computer tomography (CT) scanners, and more particularly to a luggage scanning system using CT technology.

Various luggage scanning systems using X-rays are well known for detecting the presence of explosives and prohibited articles in baggage, luggage, etc., before loading luggage on commercial passenger aircraft. As a technique for measuring the density of a material, it is general to irradiate the material with X-rays and measure the total absorption. It is generally possible to detect explosives using x-ray equipment because the density of materials used in many explosives has characteristics that are in a range that differs from the density of materials commonly used in luggage.
Most X-ray luggage scanning systems in use today are of the "line scanner" type, which pass luggage between a stationary X-ray source (hereinafter simply referred to as the source), a stationary linear detection array and a scanner. It is common to have a stationary conveyor belt for carrying luggage between the X-ray source and the detection array. The x-ray source generates an x-ray beam that is transmitted through the package or partially attenuated by the package and received by the detection array. At each detection interval, the detection array generates data, expressed as the integral of the two-dimensionally divided density of the luggage that transmits the X-ray beam, which data is used to form one or more two-dimensional raster lines. Used. As the goods pass by the conveyor belt through the stationary source and the detection array, the scanner generates a two-dimensional image representing the density of the goods as viewed by the stationary detection array. This density image is usually displayed so that a human operator can make a decision.

Techniques utilizing dual energy X-ray sources are known to provide more than just density measurements, but also additional chemical properties of the material. Techniques utilizing dual energy X-ray sources consist of measuring the absorption characteristics in each material for two different levels of X-ray sources. These measurements show the number of atoms in addition to the density of the material. Dual energy X-ray source techniques for selective energy reconstruction of X-ray CT images are described in, for example, "Selective reconstruction of energy in X-ray computed tomography" by Alvarez Robin et al. , Vol. Nos. 5,733 to 744 and U.S. Pat. No. 5,132,998.
One purpose of such dual energy X-ray source technology is to connect to a luggage sensor to detect if there is an explosive in the luggage. Explosive materials are characterized by a known range of atomic numbers and are therefore detectable by this dual energy X-ray source. Such dual energy sources are described in co-pending U.S. patent application Ser. No. 08 / 671,202 entitled "Improved Dual Energy Source (Case Reference Number: ANA-094)." It is assigned to the same assignee as the present invention, and the description by reference is incorporated in its entirety.
U.S. Pat. No. 5,132,998 U.S. Pat. No. 4,759,047 U.S. Pat. No. 4,884,289 U.S. Pat. No. 5,132,988 U.S. Pat. No. 5,183,764 U.S. Pat. No. 5,247,561 U.S. Pat. No. 5,319,547 U.S. Pat. No. 5,367,552 U.S. Pat. No. 5,490,218 DE-A 31 50 306 A1 U.S. Pat. No. 5,473,657 “Selective reconstruction of energy in X-ray computed tomography”, Physical Society of Life Sciences, vol. 5,733 to 744

With luggage scanning systems for plastic bombs, plastic bombs are particularly difficult because they are confined within difficult-to-detect geometries. Explosives that cause serious damage to an aircraft have a weight of at least one pound or more and are long enough to be read by an X-ray scanner system unless the orientation of the explosive in the luggage is considered. , Width, and height. However, a plastic bomb that can cause enough damage to an aircraft can be large enough in two dimensions but extremely small in one dimension. This is the case, for example, when formed in a thin sheet. Detect plastic explosives, especially when the material is positioned so that the thin sheet is parallel to the X-ray beam so that it can pass through the system, because plastic bombs are not known to be explosives in the image. Is extremely difficult.
Therefore, in particular, operators are required to pay particular attention to suspicious loads. On the other hand, such extra attention causes the operator to suffer particular fatigue, and, let alone carelessness, this fatigue may also result in the system overlooking suspicious luggage.

Therefore, great value can be found in designing better performing luggage scanners. For example, such a design is described in the following U.S. Patents: For example, Don et al. (4,759,047), Glockman et al. (4,884,289), Tsutsui et al. (5,132,988), Peshman et al. (5,183,764), Kotowski, etc. 5,247,561), Krug et al. (5,319,547), Peshman et al. (5.367.552), Krug et al. (5,490,218), Hayman GmbH, etc. (German DE 31 503 06 A1) ).
At least one of these designs, described in the aforementioned Peshman et al. 5,183,764 and 5.367.552 (hereinafter referred to as 764 and 552, respectively), is commercially available at a developmental stage. Therefore, here, it is called an invision machine.
The Invision machine has a third generation CT scanner. Third generation CT scanners generally include an X-ray source and an X-ray detection system, each fixed on a radially opposite side of the annular platform or disk. The disk is rotatably mounted in the gantry and rotates continuously about an axis of rotation while X-rays pass through an object located in the object opening relative to the detection system.

The detection system includes detectors in a linear array (hereinafter also referred to as an array). The linear array is arranged in a single row in the form of a chord with the center of the curved surface at the focal point of the X-ray source. The focal point of the X-ray source is, for example, a point in the X-ray source from which X-rays are emitted. The x-ray source emits a fan-shaped beam from its focal point, through a planar image field, or as received by a detector. CT scanners have a coordinate system defined by the X, Y, and Z axes. The axes intersect each other and are perpendicular to each other so as to be rotatable about the axis of rotation at the respective centers of rotation of the disks. The fan beam is defined as the volume of space defined between one source (eg, focus) and the demand surface of the detector of the detector array exposed to the x-ray beam. Since the surface received by the linear array detector is relatively small in the Z-axis direction, the fan beam is thin in that direction. Each detector produces an output signal representative of the intensity of the x-rays emitted by those detectors. Since the x-rays are partially attenuated by the whole object in the process, the output signal emitted by each detector will represent the density of the whole object in the image field between the x-ray source and the detector.
As the disk rotates, the detection array samples periodically. Each detector between each detection array will then generate an output signal representing the density of the position of the object under scanning during each measurement. A projection image is obtained by collecting all output signals emitted by all detectors in a single row of detectors for the entire detection interval. Thus, at the projection angle, the process of X-rays from the focal point to each detector is called a "line" and increases the cross-sectional area from one point to the plane received by the detector. Act on. It is also taught as expanding the density measurement because the detector receives more surface duty than any cross section of the object through which the ray passes.

As the disk rotates around the object being scanned, the scanner produces a plurality of projection images at a corresponding plurality of projection angles. A well-known algorithm generates a CT image of the object from the projection data collected at each projection angle. The CT image shows the two-dimensional density as a cut plane in the object that the fan beam passed while the disk was rotating through various projection angles. Part of the resolution of the CT image is determined by the width of the receiving area of each detector in the plane of the fan beam. Here, the width is defined as the dimension in the direction measured in the same direction as the width of the fan beam, while the length of the detector is measured perpendicular to the fan beam parallel to the scanner's Z-axis or rotation axis. Defined as dimension in direction.
One of the important design criteria of a luggage scanner is how fast it scans the luggage. For practical use at major airports, luggage scanning must be able to scan large amounts of luggage at a very fast rate. For example, it must be at or above a level that allows 700 or more scannings per hour. In order to do this, one package must be able to scan at an average speed of 5 seconds or less. For this reason, one problem with the invision machine is that it takes a relatively long time, for example, a single turn of the disk on which the CT image produces data for one cutting plane is about 0.6 to 2.. It is mentioned in the above-mentioned Nos. 764 and 552 that it takes 0 seconds. In addition, the thinner the beam cut through the luggage for each image, the better the resolution of the image, so the CT scanner has sufficient resolution to detect a plastic bomb that is only a few millimeters thick. Must have. Assuming the required processing power if it takes 0.6 to 2.0 seconds to generate data for each cut CT image and the average load of about 70 cm in size is 700 per hour. In this case, since the luggage repeatedly moves and stops for each scan, the conventional CT luggage scanner has a margin to generate an average of two to three CT images. Obviously, the entire package cannot be scanned within the time allotted for throughput for a reasonable speed. The fact that only a few CT images are generated for a package means that many items that cannot be scanned are placed, and therefore an adequate scan cannot be provided and scanning can be completed. Means you can't.

The present invention is a scanning system that substantially compensates for the disadvantages of the prior art. In the present invention, the luggage scanning system allows scanning on the order of 700 pieces per hour without operator intervention while the luggage is moved by the scanner.
One object of the present invention is to provide a CT scanner apparatus and method that has improved the scanner by applying the data reconstruction window of the scanner to the size of each object (eg, a piece of luggage to be scanned). It is in.
From this point of view, the invention fits into the reconstruction window. The reconstruction window refers to the number of pixels to be reconstructed from the scan data to generate an image for one or the size of the load in the field of view of the scanner.
A CT imaging machine generates scan data of an object passing through a scanner by scanning the field of view. By using the size and position of the object, two positions of the pixel in the visual field can be specified. Of the two positions of this pixel, the first position is the one that is reconstructed to generate an image group of the object, while the second position is the one that does not reconstruct the specified position of the pixel group belongs to.

  Thus, for this purpose of the present invention, the pixels that provide information about the package to be scanned (eg, the first position of the pixels) are processed during image reconstruction. Pixels that are not relevant to the load (eg, the second position of the pixel) are not reconstructed. Irrelevant pixels do not include those relating to the area below the load conveyor system or those relating to the area above or adjacent to the load. By effectively discarding pixels that do not provide information about the package, the load on the reconstruction process is substantially reduced, resulting in faster reconstruction and higher processing power.

In one embodiment of the present invention, the size and position of the load in the field of view are determined by detecting the boundary of the load. This can be realized by analyzing the scan data in which the boundaries of the object are arranged in the data by using the boundary position processing method known in the prior art. In one embodiment, parallel projection data is analyzed to locate these boundaries. In an application embodiment, the scanner comprises another sensor for detecting the boundaries of the load. The sensor can be an acoustic sensor, such as a high frequency supersonic range instrument, or one or more optical devices, such as a laser, light emitting diode, infrared detector, and the like. Each of these methods is a method of return data representing an area of a package. The baggage area describes the center of the baggage and its position within the field of view of the scanner.
In another embodiment, the set of pixels identified as unrelated to the package is not reconstructed, while all the pixels related to the package are reconstructed to generate an image of the package. It is. This approach provides a complete image of each package, regardless of the size of each package, and improves the throughput of the scanner. However, because of the wide variety of luggage sizes, it is difficult to monitor and control it in terms of processing load and thus the luggage throughput. Therefore, there are scenarios that attempt to reduce the processing capacity of packages. For example, the scanner continuously processes a large amount of luggage that is of irrelevant size. Therefore, an application of the embodiment is to set the upper limit to the entire size of the reconstruction window. This is achieved by setting the maximum number of pixels to be reconstructed. This maximum pixel window is to determine the size, location and dimensions of the load as to provide the best load image within the preset pixel reconstruction limits. This method is to create a usable luggage image while maintaining a controllable level.

The adaptive reconstruction window of the present invention provides significant benefits due to the method of removing non-package related pixels analyzed from the image reconstruction process. For example, by not performing unnecessary data processing, it is possible to shorten the reconstruction time and increase the baggage processing capacity based on the baggage processing capacity required at a busy commercial airport.
In another aspect, the invention is directed to an apparatus or method for performing a calibration or an empty scan. An empty scan is the calibration of a CT system in response to individual detector responses. For example, in a conveyor system, it is difficult to remove an obstruction from the field of view of a luggage scanner, so performing an empty scan in the same manner as performing an empty scan on a conventional CT machine is not easy. The scanner of the present invention performs sky scan calibration while compensating for obstacles in the field of view.
One embodiment of the calibration is performed by first performing a scan of the field of view and obtaining complete data, including any obstacles present in the field of view. A calibration threshold is set, and for each detector, view data above the threshold is selected to calculate a calibration offset value for that detector. Values below the threshold are ignored. In one embodiment, if any data exceeds the threshold, the threshold is set so that the radiation is high enough so that it does not pass through obstacles in the field of view. Therefore, empty calibration can be performed properly. For each detector, the selected data value from the unobstructed direction is used to calculate an empty calibration value for the detector. In one embodiment, the selected values are averaged to calculate an empty calibration value. The empty calibration values are used for dimensionality during subsequent scans on the actual object to compensate for the diversity of detector-to-detector responses.

By applying the empty scan calibration data to the threshold, the data related to radiation that did not hit the obstacle is considered identical. This means that sky calibration can be performed without removing obstacles in the field of view. This is very important in a continuous scanning luggage scanning system, and removing an obstacle such as a conveyor from the field of view to perform an empty scan between luggage results in extreme inconvenience. Because. However, this technique is also useful in the field of medical CT. Removing the patient table from medical CT scanning is more inconvenient than removing the conveyor from the luggage scanner and performing a medical CT empty scan without removing the patient table. Is a major improvement in the field of medical CT. However, the null calibration of the present application is also useful in the field of medical CT.
From another perspective, the present invention also provides a method and apparatus for finding a target, such as a sheet, from three-dimensional CT image data of an object. It is well known that plastic bombs can be formed into sheet shapes. Such a sheet bomb was difficult to detect using conventional CT technology. This is because the sheet thickness of the explosive is thinner than the resolution of the conventional CT scanner. The present invention determines image data of an object if the object is a sheet-like object that is likely to be a plastic bomb.
In the apparatus and method of the present invention, an object is analyzed to determine whether it is a sheet in analyzing image data near the object surface. The object to be analyzed is defined in three-dimensional space by its boundaries or surfaces. At each of a number of locations along the surface of the object, the direction of the normal to the plane is calculated and projected toward the back of the object. At a plurality of points along the line projected from the normal line, the density of the object is obtained from the CT data for the object. Interpolation is used to calculate the data for each point. A maximum distance inside the object is set, and a density is generated up to the maximum distance. This maximum distance is selected to be greater than the expected maximum sheet thickness. After all density data has been generated for each vertical line, a distance within the object is generated such that the density decreases and drops. In general, if the density decreases at a distance less than the maximum distance, the density geodesic indicates the presence of a thin object, which is likely to be a sheet. If there is no dip above the maximum distance, an object thicker than the sheet is indicated.
The calculated reduced descent distance fits a histogram-like distribution. This histogram can be used to determine the shape of the object. The peak data appearing in the histogram at the reduced descent distance shorter than the maximum distance indicates the substantial position of the object having some thickness at the reduced descent distance. This can be used to show the sheet. The high peak data appearing in the histogram at the maximum distance indicates the substantial location of objects thicker than the expected sheet thickness. This can be used to indicate non-sheet objects. The use of surface normals and histograms to identify sheet-shaped objects is advantageous over any other method of finding a sheet. For example, one prior art approach analyzes the shape of an object by calculating it from the ratio of surface area to volume. If the ratio is high, even a thin object such as a sheet can be used for display. However, in this method, only one number with high sensitivity is calculated for the entire object, and high accuracy cannot be required. For an object having a relatively small volume with respect to a large surface, an error is likely to occur when the object has a sheet shape. On the other hand, according to the present invention, analysis can be performed at a location around the entire object. By analyzing the static distribution of thickness throughout the object, more accurate results can be obtained on the shape of the object.
In another aspect, the present invention provides an apparatus or method for compensating for dark current (eg, current generated by a detector due to X-ray absorption) in a CT system, and particularly for compensating for variations in dark current with temperature. That is. In accordance with the present invention, a calibration procedure is performed to characterize dark current temperature variations. This variation is used to create a set of detector offsets. The dark or offset current is defined at a certain temperature by each offset. In a position embodiment, a set of offsets is generated for each detector. In another embodiment, a set of offsets is utilized for all detectors. During the subsequent actual scanning of the object or region, this offset corrects the data signal emitted by the detector. The detector temperature is sensed while the area is being scanned. At each detector, the detector generates a signal such that the offset associated with the currently sensed temperature moderates the density of the object detected by the detector. Thereby, temperature fluctuation of the dark current can be compensated.
In one embodiment, the temperature variation is characterized by applying a parametric equation to the temperature-offset curve. In one embodiment, this variation is described by a Taylor series polynomial with a constant coefficient. This coefficient is obtained by the least square error method. By applying a temperature dependent offset to the detector data, the luggage scanning system of the present invention provides more accurate dark current compensation than conventional systems. In a luggage scanning environment, since the luggage is continuously scanned, the influence of temperature is more important than for medical devices. Therefore, the temperature dependence of the offset is important for maintaining the generated image quality and consequently the ability to detect the target. Therefore, the temperature dependent offset of the present invention results in a more accurate CT luggage scanner.

1, 2, and 3 show an external view, an end cross-sectional view, and a cross-sectional view as viewed from a radial direction of the luggage scanning system 100, respectively. The luggage scanning system 100 is made according to the present invention with improved ability to detect the presence of a target, such as a sheet explosive, regardless of its coordinates, and completes the scanning quickly and with CT. Bring to you. As a result, the target material can be detected at a relatively high speed with a high probability. The system 100 includes a conveyor system 110 for continuously transporting baggage or luggage 112 through the central opening of the CT scanning system 120 in the direction indicated by arrow 114. Although the conveyor system 110 is depicted as having a plurality of individual conveyor regions 122, other forms of conveyor systems may be utilized.
The CT scanning system 120 includes an annular rotating platform or disk 124, which is located in a gantry support 125 that rotates about a rotation axis 127 shown in FIG. Preferably, it is parallel to the transport direction 114 of the baggage 112. The disk 124 is driven about a rotation axis 127 by the same device, such as, for example, a belt 116 and a motor drive system 118. Or another drive, for example, assigned to the assignee of the present application and forming part of the present application, entitled "X-Ray Tomographic Scanning System (case number ANA-30CON)" by Gilbert Mackena, 19995 A drive as described in US Pat. No. 5,473,657 issued Dec. 5, may also be used. The rotating platform 124 includes a central opening 126 through which the conveyor 110 transports the baggage 112.
The system 120 includes an X-ray tube 128 and a detection array 130 disposed on a radially opposite side of the platform 124. This detection array 130 is a U.S. patent application which is filed on the same date and forms part of the present application by the same assignee as the present application under the title "Area Detection Array in a Computed Tomography Scanning System (Case No. ANA-137)". A two-dimensional array, such as a detection array as described in US Pat. The system 120 further includes a data acquisition device (DAS) 134 for processing and receiving signals emitted by the detection array 130, and an X-ray tube control system 136 for powering the X-ray tube 136 and performing other controls. And System 120 preferably includes a computer system (not shown) for processing the output of data acquisition device 134 and generating the signals necessary to operate and control system 120. The computer system may include a monitor for displaying information including the generated image. The X-ray tube control system 136 may be a dual energy X-ray tube control system as described in the aforementioned U.S. patent application Ser. No. 08 / 671,202. Although the present invention is not limited to application to this type of system, the dual energy X-ray technique for selective reconstruction of the energy of an X-ray CT image is not only indicative of the density of the material but also of the material. It is actually useful to indicate the number of atoms. System 120 includes a shield 138 that protects against radiation spreading over gantry 125 and is assembled from leads.

In one embodiment, x-ray tube 128 generates a pyramidal shaped x-ray beam. This is often referred to as a cone beam 132 and passes through a three-dimensional image field through which the baggage 112 travels while being transported through the conveyor system 110. After the luggage placed in the image field has passed, a detection array 130, which then generates a signal representative of the density of the exposed locations of the luggage 112, receives the cone beam 132. Thus, similarly, the beam defines the volume of the scanned space. Platform 124 rotates about its axis of rotation 127. The axis of rotation is such that an annular trajectory around the baggage 112 when the baggage is continuously transported by the conveyor system 110 through the central opening 126 so as to generate multiple projection images at corresponding multiple projection angles. The transport X-ray source 128 and the detection array 130 are moved to the position shown in FIG.
In a well-known manner, the signal from the detection array 130 is first acquired by a data acquisition system 134 and subsequently processed by a computer system (not shown) using CT scanning signal processing techniques. This processed data is displayed on a monitor and further analyzed by a computer system to determine the presence or absence of suspicious material. For example, this data is used to determine if there is a material having the density of the sheet bomb (including the molecular weight when a dual energy system is utilized). If this data is present, appropriate measures are taken to indicate the detection of such material to the system operator or monitor. Suitable methods include, for example, displaying on the screen of monitor 140, audio or visual, or an automatic removal device to remove packages from the conveyor for further inspection, or stopping the conveyor. As a result, suspicious packages are inspected and / or removed.
As described above, the detection array 130 may be a two-dimensional detection array capable of detecting scan data in both the X-axis and the Y-axis, let alone the Z-axis. At each detection interval, the rows of the detection array emit data from a corresponding plurality of projection images, thereby scanning the volume area of the baggage 112 simultaneously. The direction and number of detector rows is determined as a function of the desired resolution and processing power of the scanner. And it is also a function of the rotation speed of the rotation speed 124 of the rotating platform and the speed of the conveyor system 110. These parameters are preferably selected to satisfy the time required in one complete rotation of the platform 124. The conveyor system 110 is such that the volume region scanned by the detector during one platform revolution and the volume region scanned by the detection array 130 during the next platform revolution are continuous and non-overlapping. Move baggage 112 forward.

Conveyor system 110 continuously carries baggage 112, preferably at a constant speed, through CT scanning system 120, while platform 124 rotates around the baggage at a continuously constant rotational speed as the baggage passes. . Thus, the system 120 performs a spiral space CT scan on the entire bag. The baggage scanning assembly 100 uses at least some data provided by the array 130 and a helical reconstruction algorithm for volumetric CT display of the entire baggage as the baggage passes through the system. In one embodiment, the system 100, on the data described in the "Nutrated Cut CT Image Configuration System and Method (Case No. ANA-118)," which is part of the present application and is the same assignee as the present application. Is executed as nutation section reconfiguration (NSR). Therefore, the system 100 not only provides CT scanning at a given location of the baggage, but also provides a complete CT scan for each baggage without the need for pre-screening equipment. Also, the system 100 provides fast scanning. This is because the two-dimensional detection array 130 allows the system 100 to simultaneously scan a relatively large portion of the baggage that is missing in each revolution of the platform.
FIG. 4 shows a mechanical / electrical block diagram of one embodiment of the present invention. The mechanical gantry of the scanner 100 has two main parts, one of which is a disk 124 and the other is a frame (not shown). The disk 124 is a rotating member for transferring the X-ray assembly, the detector assembly 130, the data acquisition system 134, the high voltage power supply, the monitor / control assembly, the power supply assembly, and the data link assembly. The disk 124 is mechanically connected to the frame via a duplex annular contact ball bearing cartridge. The disk 124 can be rotated at a constant speed by a belt driven by the DC servo motor 505. The gantry also has an X-ray shield on the disk or frame assembly.
In one embodiment, baggage conveyor system 110 includes a single belt drive that drives at a constant speed consistent with the required throughput. An example assumed as the processing capacity is a processing capacity of processing 675 bags per hour. The belt is driven by a high torque, low speed assembly that can maintain a constant speed despite load variations. Low attenuation carbon graphite epoxy material is used as the material of the X-ray conveyor belt. The overall length of the conveyor is designed to match the average of the three bag lengths. Tunnels are arranged around the conveyor to adapt to the safety of the shelf X-ray system.
As another embodiment, a 208 V, 30 A three-phase input power supply is prepared as a main power supply for supplying power to the entire system. This input is provided by the airport where the system is located. Power is supplied from the frame through a frame brush that is continuously connected to a metal ring located on the disk 124. The low voltage power supply 501 on the disk 124 powers the computer and electronics of the DAS 134, X-ray cooling system and various other monitoring and control systems. Power is supplied from the main power supply to the conveyor monitor 503, the gantry 505, the high-voltage power supply and the X-ray cooling pump.

The high voltage power supply supplies power to the X-ray tube. This power supply provides two voltages between cathode / anode modulated at 540 Hz. The drive wave is in the form of a sine wave. This power supply also provides filament power. The power supply current is kept almost constant at both voltages.
The X-ray assembly includes a bipolar fixed anode X-ray tube 128, a heat exchange system 507, a collimator 509, shielding, an X-ray sensor, and an alignment / mounting plate. The collimator supplies an X-ray cone beam having a fan shape with a central angle of 61 ° and a width of 6 °. The heat exchange system 507 includes a pump, a radiator, a fan, and a pump. The alignment plate is used to quickly and easily replace the field when attaching the tube 128 to the disk 124. An X-ray sensor is provided for feeding back the X-ray intensity.
Dual energy X-rays hit the load, while some of the X-rays pass through and strike the detector. The detector assembly 130 includes a scintillator, a photodiode, a mounting plate, an anti-scattering plate, and a mounting main column, and also includes a main column heater having a temperature sensor 521. The detection assembly 130 performs an analog conversion from X-rays to visible photons and even currents. The anti-scattering plate is made of a material having a high atomic number in order to reduce radiation scattered on the scintillator, and is arranged at an angle in the X-ray source. A scintillator made of thick cadmium tungsten crystals is sufficient to absorb almost all of the x-rays. The X-rays are converted to visible photons. The crystal is surrounded on all sides except the bottom by an optically reflective material. Therefore, it is possible for visible photons to pass through the bottom of the crystal. The photodiode is connected to the bottom of the crystal by an optically transmissive adhesive. The photodiode emits current such that it diminishes logarithmically with the x-ray absorption of the load. The exercise is mounted on a ceramic mounting plate sized to fit some detectors. The electronic mounting plate is mounted on a base provided with a connector by soldering or epoxy bonding. Each detector mounting plate is mechanically mounted on a main mounting column having a fan beam radius and a projected image in the Z-axis direction. This main pillar is fixed to the disk 124.

DAS 134 takes the detector current, multiplexes the amplified voltage to a set of 16-bit analog-to-digital converters, and multiplexes the digital output to contactless serial data link 511. The DAS 134 generates a trigger signal according to the position of the annular portion of the disk 125. The contactless links 511 and 513 transmit high speed digital DAS data to the image reconstruction processor 515, while passing low speed monitor / control signals between the disk and the frame control computer. Data link 511 can be based on an RF transmitter and receiver. The transmission protocol may be TAXI ^™ having a transmission capability of 350 Mbit / s or more. The control link 513 can be based on wireless LAN technology. The wireless LAN technology has a similar PCMCIA card that can be attached to both the frame and the disk computer. The card can have both a transmitter and a receiver, and can be a standard Ethernet card. Therefore, the point-to-point network is in control communication with the low speed monitor.
The image reconstructor converts the digital line integrals assembled from DAS 134 at both high and low energies into a set of two-dimensional images of the cut bag. CT reconstruction is performed via a spiral cone beam solution. The reconstruction can include embedded software, a high-speed DAS port, an array processor, a DSP-based convolver, an ASIC-based backprojector, an image storage device, a UART control port, and a SCSI image output port. The array processor can perform data correction and interpolation. The reconstructor is self-contained and can sign images based on package information received on the UART interface from the frame computer.
The monitor and control system can be based on a PC embedded in the control system. All subsystems can monitor key device health and status information. This system can scan both monitor systems to sense luggage information, perform environmental control such as temperature and humidity, sense the annular position of disk 124, and trigger DAS and HVPS. . The system may also have a video and keyboard interface for performing technical diagnostics and controls. In addition, a control panel is provided for field service. The CT luggage scanner of the present invention has the ability to adapt the image reconstruction window to the luggage to be scanned to improve the luggage throughput of the system. Before reconstructing the image of the load, the system of the present invention can distinguish between pixels that are to be reconstructed and those that are not reconstructed to generate an image of the object. The reconstructed pixels should relate to the package to be scanned. Pixels unrelated to the load are not reconstructed and are therefore effectively eliminated. Rejected pixels include pixels for areas below and below the conveyor as well as areas next to and above the load. The processing time is reduced by deleting a considerable number of pixels during the reconstruction process, and consequently the throughput of the load is increased.

FIG. 5 is a schematic diagram of a field of view 350 of a scanner used to represent the adaptive reconstruction window of the present invention. The field of view 350 is shown as if a load 112 having a height h and a width w has been placed on the conveyor 110. The field of view includes a lower area 351 of the conveyor 110, an upper area 352 of the load 112, and an area 353 opposite the load 112. These areas 351, 352 and 353 are scanned by the scanner system of the present invention, while scan data is obtained from them. However, since the load 112 is not placed in these areas, the image pixels in these areas provide no information to the load and are therefore excluded from the load image reconstruction process.
FIG. 6 is a schematic diagram showing a visual field 350 displayed superimposed on the x, y Cartesian coordinate system. The scanned image of the luggage 112 is considered to have originated from a rectangular array of pixels 357. This package is considered to have N pixels in the width direction and M pixels in the height direction, and each pixel is generally considered to be of dimension p, measured in millimeters, and of equal height and width. . However, the width w of the package 112 is represented by w = Np, and the height h is represented by h = Mp. By determining the actual height and width of the package to be scanned, a number of pixels 357 given by N × M are calculated. The positions of these pixels to be reconstructed can also be determined by arranging the center of the package at the coordinates x ₀ , y ₀ .
The height h, width w, and centers x ₀ , y ₀ can be determined by determining the boundaries of the load. As shown in FIG. 6, bottom and top of the package is given respectively by the coordinate y _1, y _2, whereas left and right ends of the package are given respectively by the coordinates x _1, x _2. The center 354 is specified by x ₀ , y ₀ , where the relationship x ₀ = (x ₂ −x ₁ ) / 2, y ₀ = (y ₂ −y ₁ ) / 2 holds. The height h is given respectively by y ₂ -y _1, the width w is given respectively by the x ₂ -x _1. The column of N pixels and the column of M pixels are determined from the width w and the height h using a known dimension p to reconstruct the package image.

With the determined height, width, and position of the package, the entire pixel N × M is obtained as described above. In one embodiment, this total number of pixels is reconstructed to create an image of the package. In another embodiment, to compensate for acceptable and controllable luggage throughput, luggage reconstruction is limited to the largest pixel to be reconstructed. The desired luggage system throughput is that this maximum number of pixels be reconstructed for each luggage. In one embodiment, the maximum number of pixels is set to 25,000. The total number of pixels N × M required for reconstructing the baggage image corresponds to the preset number of pixels. If N × M is less than this limit, N × M pixels are reconstructed. However, if N × M exceeds this limit, the reconstruction window used for a particular package will fit the window of the largest possible pixel that meets that limit. Reconstruction is performed with this limited number of pixels.
As mentioned above, the height, width, center position and pixel dimensions N and M are taken from the boundary position of the load placed in the field of view of the scanner. There are several possible ways to place the boundaries. According to one embodiment, the scan data is analyzed to determine position boundary positions x _1, x _2, y ₁ and y ₂ itself. In another embodiment, a split sensor on the scanning machine is used to detect load boundaries.
FIG. 7 is a simplified schematic block diagram of one embodiment of a luggage scanning system 100 of the present invention that utilizes a split sensor to determine a luggage boundary.
The system 100 illustrated in FIG. 7 includes a CT scanner 120 and a conveyor system 110 that transports a load 110 through the scanner 120.
One or more sensors 360 include a scanner 120 that is used to detect the boundaries of package 112 as it enters scanner 120. Sensor 360 includes one or more lasers and a photodetector to sense boundaries. Alternatively, the sensor 360 comprises either an infrared sensor, a combination of a light emitting diode and a photodetector, or both. Alternatively, the sensor 360 includes a high-frequency ultrasonic semiconductor as an area finder for detecting the boundary of the package 112.
This sensor output is wired to a sensor output processing circuit 370 that performs a process of determining the boundary of the package. The detection signals generated by the detector are sent to a data acquisition device (DAS) 134 for processing the output of the detector, generating response signals, and sending them to a processing system 364. Processing system 364 also receives output from sensor processing circuitry 370 that identifies the boundaries of the package. The processing system 364 generates image data from the detected data and generates an image of the package 112.

The CT luggage scanning system of the present invention also proposes to configure the system such that it compensates for changes in detector response from detector to detector. This configuration consists in performing a sky scan in the field of view of the system. In conventional medical CT systems, all obstacles, such as the patient's table, are removed from view when a blank scan is performed. Then a complete scan of the field of view is performed and the data obtained at the detector is analyzed. In luggage scanners such as the present invention, obstacles in the field of view, such as conveyor systems, are not prepared to be removed to perform an empty scan. The present invention also provides a sky scan that can be performed without removing these obstacles from the field of view.
Figures 8a and 8B show a conventional blank scan. FIG. 8a schematically shows a conventional scanner. The scanner includes a source 204 and a detection array 202 that are counterclockwise as indicated by arrows 206 and are rotatable about a center of rotation 203 simultaneously. The source 204 and the detection array 202 rotate in a series of fields of view v about a center of rotation. In each field of view v, a series of samples s corresponding to the detectors in array 202 are acquired. Typically, the field of view is translucent to x-rays from source 204 and includes an annular window 200. FIG. 8b is a schematic diagram plotting the data signal obtained for one detector or sample s over the entire field of view v. As shown, the obstacles have been removed from the figure and the window 200 is annular and represents a constant density throughout the field of view, so that the data signal received by each detector on all fields of view is generally constant. . In a conventional sky scan, each detector in array 202 produces data as plotted in FIG. 8b. When the calibration factors are used at the detectors, a calibration factor is calculated at each detector such that the response of each detector is all equal. In this ideal conventional situation, the calibration factor calculated at each detector is independent of the field of view, since the data signal at each detector does not change with respect to the field of view. That is, the calibration factors collected by the detector are the same as the data collected by the detector for all fields of view. As a result, each detector will only be relevant to one calibration factor. However, in practice it is not accurate to assume that the response of the detector is independent of the field of view. Due to gravity and other effects, the line plotted in FIG. 8 is not exactly flat. Therefore, the calibration factor depends on the field of view. Therefore, the calibration factors for each detector for each field of view are calculated from a large calibration look-up table that occupies considerable memory space.
FIG. 9a schematically illustrates the scanning configuration of the luggage scanner system 100 of the present invention. As shown in FIG. 9a, the configuration is different from the conventional one shown in FIG. 8a. The luggage scanner 100 has the conveyor system 110 in view, leaving an obstacle in view during the calibration scan. Machine opening 126 is not as annular as a conventional machine opening. The mechanical opening of the present invention also includes a non-annular window 220 and is intended to be an annular window 200 of a conventional scanning system. As shown in FIG. 9b, these factors combine to generate a detector response during a field-dependent sky scan. FIG. 9b now plots the data signal generated by a single detector or sample s over the entire field of view v during a scan of the field scanner's field of view sky calibration. The shape of the curve represented by FIG. 9b is merely representative of the response of the field-of-view detector and should not be considered to be intended to accurately represent the response of any actual detector. is there.

To effectively perform a sky calibration scan, it is preferable to utilize only the field of view associated with rays that do not pass through obstacles. In the present invention, the data represented by FIG. 9b is analyzed for each detector, and the data associated with undisturbed rays is used to calculate the calibration of the detector. In one embodiment, this is achieved by setting a signal threshold T for the detector and processing the value of the data depending on whether it exceeds T. The threshold T is set so that data that exceeds the threshold T is generated when unobstructed radiation is generated through the field of view. A calibration factor is then determined utilizing these data values above the threshold T.
For example, as shown in FIG. 9b, two regions of view 223 and 225 generate data that exceeds a threshold T. These fields of view are assumed to have been determined from undisturbed ray paths through the field of view of a given sample s. Therefore, the data values generated in these two fields of view are used to determine a calibration value for this detector. In one embodiment, data values that exceed a threshold T are averaged, and the average is used to determine a calibration value for the detector or sample s. This results in a single calibration factor determined for the detector, which is used for all data collected by the detector in all fields of view. That is, the calibration is independent of the field of view.
However, because of the presence of an obstacle in the field of view, the response of the detector is independent of the field of view, but the present invention distinguishes between a field of view with obstacles and a field of unobstructed field. The result is a calibration independent of the detector image. This will substantially reduce memory and computation in both the process of saving calibration factors and optimizing data while actually scanning the object.
The scan data is dimensionlessized during the actual scan to correspond with the calibration factors calculated at each detector during the empty calibration scan. For each field of view and each projection on the sample, it is calculated according to the dimensionless value P _vs = ln (A _vs / D _vs ). Here, P _vs is the projection data in a particular field of view v and detected sample s, A _vs is a calibration factor obtained during calibration in the field of view v and sample s, and D _vs is the detector sample s Is the actual data collected in the field of view v. Also, as described above, in one embodiment, A _vs is the same for all v.
The threshold T can be determined in several different ways. For one approach, the threshold T is determined by multiplying the largest data value, indicated by 227 in FIG. 9b, by a constant. Preferably, this constant is slightly less than 1, for example 0.95. Choosing a relatively high threshold leads to a high level of confidence that only the values associated with the obstacles will be used in calculating the calibration factor.
The present invention also includes an apparatus and a method for detecting an object having a shape of, for example, 40 in the three-dimensional CT image data of the object. It is assumed that the object is defined by the area outside the boundary or surface, and that each pixel of the image represents the density of each object at that pixel. FIG. 10 schematically shows a three-dimensional CT image of the object 300. For simplicity, the object 300 is shown in two dimensions, but it is assumed that the present invention is applied to a three-dimensional object.

In accordance with the present invention, a series of points 302 along the surface of object 302 is identified and analyzed. By providing pixels to define the surface in three-dimensional space, a vector N perpendicular to the surface at a given location can be determined, such as by calculating the purchase of the buying surface. At each point 302, a surface vertical vector N is identified. For example, for each normal N, a normal 304 is projected back onto the object 300. A series of points 306 along the normal 304 are identified and density values are assigned to each data point 306 from the CT data of the object. When the correction between the pixel values is performed, the density at each point 306 along the normal line 304 is determined. The maximum thickness T _MAX is defined as the distance along the normal 304 where the data point 306 is calculated. The maximum thickness T _MAX is chosen to be greater than the expected thickness of the sheet.
The density distribution of point 306 along normal 304 is determined and analyzed. FIG. 11 shows two cases of data point sample distribution along the vertical direction 304 of the plane. In the distribution denoted by reference numeral 310, the density ρ is constant with respect to the object 300 up to a range exceeding the measured maximum thickness T _MAX . The distribution of the code 312, starts to decrease drop function of the density at a distance T _R. Reduced drop distance T _R denotes the thickness of the object at normal 304 of the associated surface. However, at the normal N of the special surface, the object 300 is relatively thin. Where there is no dip, the object becomes relatively thin at the relevant point 302, as represented by curve 310. In fact, the object is at least as thick as the preset maximum thickness _TMAX .
This process is repeated along the surface of the object and is repeated for a plurality of locations 302. As shown in FIG. 11, the value of T _R can be a point along the curve density decreases drops. Or, the value of T _R is calculated as the average of the curve 312 with respect to the normal N of the associated surface. As represented by reference numeral 310, T _R is set to the maximum thickness T _MAX when the distribution is not reduced drop.
As shown in FIGS. 12a and B, one of the histogram is represented for all values of T _R. FIG. 12a shows a histogram for concluding that the object 300 is a sheet. In this example, T _MAX is the peak of the histogram is set to 10mm is about 4 mm. This indicates that the T _R decreases drop for 302 points along the surface is the position of 4 mm. The thickness portions, such as below the maximum T _MAX object 300 can assert that a sheet.
The threshold value T can be set by analyzing the histogram. If the peak of the curve exceeds the threshold as shown in FIG. 12a, it can be said that the peak is a sheet having a thickness at the position of the peak along the horizontal axis. An increase in the curve at a short distance at T _MAX (10 mm) indicates that a number of measurements indicate a thickness that exceeds the maximum thickness T _MAX . This is due to measurements taken along the thin edge of the sheet showing high density that extends deep into the object. However, the peak at 4 mm is higher than the rise at 10 mm when the sheet is shown.

FIG. 12b is also made when the object 300 is not a sheet. As shown in FIG. 12b, there is a histogram peak at the maximum thickness T _MAX (10 mm). At T _MAX (10 mm), it indicates that the thickness exceeds the maximum value which is larger than the expected sheet pressure in most of the measured values. However, it indicates that the object 300 is not a sheet. How to take the top of the distribution in the T _MAX reduces the processing time, the issues related calculations with the surface normals along the sheet edge may also be solved.
In one embodiment, the statistical analysis is performed automatically in the processor. Peaks are automatically located in the histogram. Based on the position and shape of the peak, it is determined whether the object is a sheet by comparing it with data or a database in which the object exists.
The CT scanning system of the present invention also has a function of compensating for a dark current having a temperature dependency and an offset current of a detector (hereinafter simply referred to as an offset). The dark current refers to a current generated by the detector itself when the X-ray source is off, that is, while the detector does not receive X-rays. This causes scan data obtained during scanning of a standard object to be inaccurate due to residual or zero input current. Considering the inaccuracy due to dark current, the process from detector to detector depends on temperature and changes. Adjustments are calculated to offset and compensate for dark currents that wipe out these inaccuracies.
In the present invention, a temperature dependent offset is calculated for each detector. In one embodiment, a calibration procedure is performed prior to actual scanning to obtain a temperature dependent characteristic of the offset. During this calibration procedure, the current from the detector is measured for a series of detector temperatures. Multiple data points, eg, offset versus temperature points, are calculated by multiple detectors. In another embodiment, data points are acquired by each detector. In another embodiment, all lower detectors are used. The average value of the offset is used in each detector.
When the temperature-dependent characteristics are obtained, the actual scanning is performed. A temperature sensor mounted near the detector senses the temperature of the detector when scanning is performed. By comparing the current temperature with stored temperature data provided as a function, an offset value to be applied to the data selected by the scanning detector is determined. In one embodiment, a periodic scan is performed using an x-ray source that is turned off. Scanning these periodic dark current is performed in known temperature T _1. Later, when the actual scan is performed, temperature T ₂ is sensed. The difference between the dark current scan temperature T ₁ and the current temperature T ₂ is applied to the temperature dependent function obtained during the calibration procedure to identify the appropriate offset to use for detector data.

ここで、Ｔはチャンネルでの温度を示し、α_i（i＝０，１，２）は定数である。
α_iの値は較正手順を利用して決定される。オフセットに依った温度の較正値は度々必要とはならない。おそらく、年に１回または故障した検出器の交換の際に必要となる程度であろう。較正の間に検出器が使用される温度は、オフセットがスキャナーの代表的な動作温度範囲で代表値として表されるような範囲である。今、ここで、ｘ_iは温度Ｔ_i（i＝０，１，２，…，Ｎ−１，Ｎ）で計測されたオフセットである。Ｎは代表的には５である。α_iは最小二乗誤差法を使用して見積もると以下のようになる。
Ｃはα_iのマトリックス形式で以下のように表される。

マトリックスＣは以下のように求められる。

ここで、Ａは以下のように表される。

ここで、Ｂは以下のように表される。

ここで、各要素は以下のように表される。

スキャナーのオペレーションにおいて、検出器のオフセットが周期的に計測される。代表的には、動作中に、1時間に1回計測される。ここで、オフセットが計測される温度をＴ₁とする。また、ｘ（Ｔ₁）をこの第一温度におけるオフセットとしよう。スキャニング中のしばらく後に、検出器が温度Ｔ₂になったとしよう。第二温度ｘ（Ｔ₂）をにおけるオフセットの値は、以下のように表される。

実際には、Ｄ個の検出器と検出器温度が、全体として下位にあたるＤ個の検出器においてのみ計測される。Ｎは温度の読み値であって、代表的には５であろう。温度センサーによる読み値は、たとえば図４に示された5つの温度センサ５２１によって取得される。ここで、計測される温度における検出器をｄ₁（i＝０，１，２，…，Ｎ−１，Ｎ）としよう。繰り返すが、Ｎは温度の読み値の個数である。ここで検出器ｄ₁における計測温度をＴ_djとしよう。検出器ｊにおける温度Ｔ_jではＮ個の読み値を通して2次の多項式にフィッティングすることよって算出することができる。温度依存度をパラメータ形式で表すと以下の通りとなる。

ここで、β_iは最小二乗法を用いて以下のように決定することができる。

マトリックスＦは以下のように求められる。

ここで、Ｄは以下のように求められる。

ここで、Ｅは以下のように求められる。

Let x be the offset temperature dependence for a single channel to indicate the temperature dependent on the offset function of the present invention. Then, the temperature dependence of the offset is expressed as follows using the Taylor series expansion.

Here, T indicates the temperature in the channel, and α _i (i = 0, 1, 2) is a constant.
The value of α _i is determined using a calibration procedure. Calibration of the temperature due to the offset is often not needed. Probably the degree required once a year or when replacing a failed detector. The temperature at which the detector is used during calibration is such that the offset is typically represented in a typical operating temperature range of the scanner. Here, x _i is an offset measured at a temperature T _i (i = 0, 1, 2,..., N−1, N). N is typically 5. α _i is estimated as follows using the least squares error method.
C is represented in the matrix form of α _i as follows:

The matrix C is obtained as follows.

Here, A is represented as follows.

Here, B is represented as follows.

Here, each element is represented as follows.

In scanner operation, the detector offset is measured periodically. Typically, it is measured once an hour during operation. Here, the temperature at which the offset is measured as T _1. Let x (T ₁ ) be the offset at this first temperature. After some time during scanning, it tries and the detector has become to the temperature T _2. The value of the offset at the second temperature x (T ₂ ) is expressed as follows.

In practice, the D detectors and the detector temperature are measured only in the lower D detectors as a whole. N is the temperature reading and will typically be 5. The readings from the temperature sensors are obtained by, for example, the five temperature sensors 521 shown in FIG. Here, let the detector at the measured temperature be d ₁ (i = 0, 1, 2,..., N−1, N). Again, N is the number of temperature readings. Here trying to measure temperature in the detector d ₁ and T _dj. The temperature T _j at the detector j can be calculated by fitting a second-order polynomial through N readings. The temperature dependence is expressed in a parameter format as follows.

Here, β _i can be determined as follows using the least squares method.

The matrix F is obtained as follows.

Here, D is obtained as follows.

Here, E is obtained as follows.

Temperature in the detector d _i is measured by a thermocouple or resistive temperature detector (RTD).
While the present invention has been shown by way of illustration or described with reference to preferred embodiments, it has been changed in various forms or made in detail by persons having ordinary skill in the art of the present invention. The present invention is included in the present invention as long as it is within the scope of the present invention without departing from the technical concept of the present invention in the claims of the present application.

BRIEF DESCRIPTION OF THE DRAWINGS It is an external view of the luggage scanning system of this invention. FIG. 2 is a sectional end view of the system shown in FIG. 1. FIG. 2 is a cross-sectional view of the system illustrated in FIG. 1 as viewed from a radial direction. FIG. 2 is a schematic diagram showing electric and mechanical functional blocks in one embodiment of the luggage scanner of the present invention. 1 is a schematic view of a package scanner of the present invention showing a package placed on a conveyor system in the field of view. It is the figure which superimposedly displayed the visual field of the luggage scanner of this invention on a Cartesian coordinate system. 1 is a simplified block diagram of one embodiment of a luggage scanning system of the present invention when using a sensor to identify a luggage boundary. FIG. BRIEF DESCRIPTION OF THE DRAWINGS It is an external view of the luggage scanning system of this invention. FIG. 3 is a geometrical diagram showing a source, a detector array, and a field of view of a conventional CT scanner. FIG. 8B is a diagram schematically plotting data signals obtained by one detector during one scan of the visual field shown in FIG. 8A. FIG. 3 is a schematic view showing a geometric shape of one embodiment of a luggage scanner according to the present invention. FIG. 9B schematically plots data signals obtained by one detector during one scan of the field of view shown in FIG. 9A. FIG. 4 is a schematic view of a three-dimensional image of a three-dimensional object in a CT scanner. FIG. 6 is a diagram schematically plotting a density distribution of a thin object and a thick object along a direction perpendicular to a CT image plane. FIG. 4 is a schematic diagram of a histogram of a density reduction descent distance of a thin object plotted. FIG. 4 is a schematic diagram of a plot of a histogram of the density reduction descent distance of a thick object.

Explanation of reference numerals

Reference Signs List 100 Luggage scanning system 110 Conveyor system 112 Luggage 120 CT scanning system 125 Gantre support 126 Central opening 130 Detection array 134 Data acquisition device (DAS)
136 X-ray tube control system 515 Image reconstruction processor 521 Temperature sensor

Claims (10)

A method of processing scan data of an object in a field of view in a computed tomography scanning apparatus, wherein the field of view is defined from a plurality of pixels that can be reconstructed to generate an image of the field of view from the scan data. Thing,
Scanning the field of view to generate the scan data for the field of view;
A size detection step of detecting the size of the object;
A position detection step of detecting the position of the object in the field of view,
Utilizing the size and the position of the detected object, a first position in a field of view to be reconstructed when generating an image of the object, and a first position in the field of view to be reconstructed when generating the image of the object. A position specifying step of specifying a second position in the field of view which should not be performed.   The method of claim 1, further comprising reconstructing a first position in the field of view to generate an image of the object.   The scan data processing method according to claim 1, wherein the size detecting step includes a step of detecting a boundary of the object from the scan data for the visual field.   2. The scan data processing method according to claim 1, wherein the position detecting step includes a step of detecting a boundary of the object from the scan data for the field of view.   2. The scan data processing method according to claim 1, wherein the position specifying step includes a step of setting a limit on a large number of pixels to be reconstructed. An apparatus for processing computed tomography scan data of an object in a field of view of a computed tomography scanning device, the field of view comprising a plurality of pixels that can be reconstructed to generate an image of the field of view from the scan data. Defined
Means for scanning the field of view to generate the scan data for the field of view;
Size detection means for detecting the size of the object;
Position detection means for detecting the position of the object in the field of view,
Utilizing the size and the position of the detected object, a first position in a field of view to be reconstructed when generating an image of the object, and a first position in the field of view to be reconstructed when generating the image of the object. A position specifying unit that specifies a second position in the field of view that should not be detected.   7. The scan data processing device according to claim 6, further comprising means for reconstructing a first position in the field of view to generate an image of the object.   7. The scan data processing apparatus according to claim 6, wherein said size detecting means includes means for detecting a boundary of said object from said scan data for said field of view.   7. The scan data processing apparatus according to claim 6, wherein said position detecting means includes a sensor for detecting a boundary of said object. 7. The scan data processing apparatus according to claim 6, wherein said position specifying means includes means for setting a limit to a large number of pixels to be reconstructed.

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