Patent Description:
The CT device reconstructs a three-dimensional image from a plurality of projected images acquired while rotating a sample or a gantry. At that time, if the rotation axis of the sample deviates from the center of the detector with respect to the X-ray source (center shift), the quality of the reconstructed image is deteriorated as compared with the ideal image without center shift.

To cancel such center shift, conventionally, a method of estimating the center shift amount to correct it has been considered. For example, in the technique described in Non-Patent Document <NUM>, a synchrotron collimated beam is used to measure the center shift amount by scanning <NUM> degrees, and the total variation (TV) is used as an index for estimating the center shift amount.

However, in such a case where a part of the sample is made of metal, since the variation of the pixel value becomes large at the boundary position of the reconstructed image of the part, the total variation does not always increase or decrease in accordance with the center shift amount.

The present invention has been achieved in view of such circumstances, and an object thereof is to provide a center shift amount estimating apparatus, a method and a program capable of accurately identifying the center shift amount by using an image area avoiding a region where a pixel value is extremely different from the surrounding even if there is such a region.

According to the present invention, it is possible to accurately specify the center shift amount by using the image area avoiding a region where pixel values are extremely different from the surrounding even if there is such a region.

Next, embodiments of the present invention are described with reference to the drawings. To facilitate understanding of the description, the same reference numerals are assigned to the same components in the respective drawings, and duplicate descriptions are omitted.

A CT device irradiates a sample with a cone-shaped or parallel beam of X-rays from all angles, and acquires a distribution of the absorption coefficient of the X-rays, that is, a projected image, by a detector. To irradiate X-rays from any angles, the CT device is configured to rotate the sample stage with respect to the fixed X-ray source and the detector or the gantry integrated with the X-ray source and the detector.

Thus, the distribution of the linear absorption coefficient f of the sample can be inferred from the contrast of the projected image of the sample obtained by performing projection from various angles. Then, it is called reconstruction that a three-dimensional line absorption coefficient distribution is obtained from two-dimensional projected images. Basically, backprojection of the projected images is performed.

In the CT device as described above, adjustment is performed so that the rotation center of the sample or the gantry is positioned on a straight line connecting the center of the X-ray source and the center of the detector. Misalignment of the center of rotation (center shift) from the straight line connecting the center of the source and the detector deteriorates the reconstructed image. <FIG> is a schematic diagram showing the configuration of a CT device where a center shift occurs, with a view of the configuration in the rotation axis direction. A center shift Y1 occurs between the straight line R1 connecting the X-ray source F0 with the rotation axis P0 and the center line R2 of the detector D0.

<FIG> are exemplary schematic diagrams of reconstructed images of <NUM>° scanning measurements respectively in the absence and presence of center shift. For example, for the CT device <NUM>, nano3DX (registered trade mark) produced by Rigaku corporation can be used. In devices that use parallel-beams, such as nano3DX, measures are performed by <NUM>° scanning. An image <NUM> of a characteristic structure such as a point appears in the reconstructed image <NUM> when there is no center shift, while a semicircular artifact <NUM> appears in addition to the image <NUM> of the characteristic structure in the reconstructed image <NUM> when there is a center shift.

<FIG> are exemplary schematic diagrams of reconstructed images of <NUM>° scanning measurements respectively in the absence and presence of center shift. For example, for the CT device <NUM>, CTLabHX made by Rigaku corporation can be used. With devices using cone beams, such as CTLabHX, <NUM>° scanning measurement is performed. The image <NUM> of the characteristic structure appears in the reconstructed image <NUM> when there is no center shift, however, the image <NUM> of the characteristic structure is blurred as a whole in the reconstructed image <NUM> when there is a center shift.

In the present invention, an index for quantitatively evaluating the degree of deterioration of an image is introduced, and the center shift amount is automatically and uniformly calculated. Specifically, an index for quantitatively evaluating the degree of deterioration of the reconstructed image is calculated. The value is calculated for the center shift amount of the search range, and the center shift amount when it takes the extreme value is searched. Thus, it is possible to obtain an image in which blurring and artifacts are reduced. As the index, for example, an index using differentiation such as total variation (Total Variation, TV) can be cited. Further, sharpness may be used as an index using the standard deviation value of the image.

The calculation of the total variation is performed by summing the differential values at each pixel in the region with respect to the region of interest of the image f(x, y) of a z-section as shown in Equation (<NUM>). When a plurality of images are specified in the z-direction, the average value thereof is used.

<FIG> is a graph showing the relationship between the center shift amount and the total variation in <NUM>° scanning measurement. In the case of a <NUM>° scanning CT machine, semicircular artifacts should increase the total variation of the image. The change in the total variation when the detector is virtually shifted by numerical calculations can be obtained, and it can be inferred that the center shift Y1 giving the minimal value T1 is the center shift amount at the time of the measurement.

<FIG> is a graph showing the relationship between the center shift amount and the total variation in <NUM>° scanning measurement. In the CT device for a <NUM>° scan, the total variation should be small if the image is blurred. The change in the total variation when the detector is virtually shifted is obtained by numerical calculation, and it can be inferred that the center shift Y2 giving the maximal value T2 is the center shift amount at the time of the measurement.

<FIG> is a schematic diagram showing a configuration of a whole system <NUM> including a CT device <NUM> and a processing apparatus <NUM> connected thereto, an input device <NUM>, and a display device <NUM>. Here, the CT device <NUM> shown in <FIG> is configured to rotate the sample with respect to the X-ray source <NUM> and the detector <NUM>, however, is not limited thereto, and may be configured to rotate a gantry in which the X-ray source and the detector are integrated.

The processing apparatus <NUM> is connected to the CT device <NUM>, and controls the CT device <NUM> and processes the acquired data. The processing apparatus <NUM> may be a PC terminal or a server on a cloud. The processing apparatus <NUM> estimates the deviation between the rotation axis with respect to the X-ray source and the center of the detector in the CT device. The input device <NUM> is, for example, a keyboard or a mouse, and performs input to the processing apparatus <NUM>. Display device <NUM> is, for example, a display, and displays a projected image or a reconstructed image.

As shown in <FIG>, the CT device <NUM> includes a rotation control unit <NUM>, a sample stage <NUM>, an X-ray source <NUM>, a detector <NUM> and a driving unit <NUM>. X-ray CT is performed by rotating the sample stage <NUM> installed between the X-ray source <NUM> and the detector <NUM>. Note that the X-ray source <NUM> and the detector <NUM> may be installed on a gantry (not shown), and the gantry may be rotated with respect to a sample fixed to the sample stage <NUM>.

The CT device <NUM> drives the sample stage <NUM> at a timing instructed by the processing apparatus <NUM>, and acquires a projected image of the sample. The measurement data is transmitted to the processing apparatus <NUM>. The CT device <NUM> is suitable for use in precision industrial products such as semiconductor devices, however, can be applied to an apparatus for animals as well as an apparatus for industrial products.

The X-ray source <NUM> emits X-rays toward the detector <NUM>. The detector <NUM> has a receiving surface for receiving X-rays, and can measure the intensity distribution of X-rays transmitted through the sample by a large number of pixels. The rotation control unit <NUM> rotates the sample stage <NUM> at a speed set at the time of CT measurement by the drive unit <NUM>.

<FIG> is a block diagram showing a configuration of a processing apparatus <NUM> (center shift amount estimating apparatus). The processing apparatus <NUM> is configured by a computer formed by connecting a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and a memory to a bus. The processing apparatus <NUM> is connected to the CT device <NUM> and receives information.

The processing apparatus <NUM> comprises a measurement data storage unit <NUM>, a device information storage unit <NUM>, a reconstruction unit <NUM>, a region specifying unit <NUM>, the temporary correction unit <NUM>, a method determination unit <NUM>, an index analyzing unit <NUM>, a plot outputting unit <NUM>, a center shift amount specifying unit <NUM> and a correction unit <NUM>. Each unit can transmit and receive information via the control bus L. The input device <NUM> and the display device <NUM> are connected to the CPU via an appropriate interface.

The measurement data storage unit <NUM> stores measurement data acquired from the CT device <NUM>. The measurement data include rotation angle information and corresponding projected images. The device information storage unit <NUM> stores device information acquired from the CT device <NUM>. The device information includes device name, beam shape, geometry at the time of measurement, scanning method, etc. The reconstruction unit <NUM> reconstructs a three-dimensional image from the projected images for the reconstruction.

The region specifying unit <NUM> specifies a region of interest in the uncorrected image. The uncorrected image is an image reconstructed from the projected images. Preferably, the region specifying unit <NUM> specifies the region of interest based on the user specification. For example, a UI function that allows a region of interest to be set, such as specifying a rectangular region by mouse operation, is used.

This allows the user to optionally specify the image region as the determination material. It is preferable that the user sets the image region so as to include an area where the characteristic structure appears. For example, when an index value is calculated based on an image including an area where a point-like structure appears, noise is less and it is easy to search for extreme values. In addition, it is also preferable to set the region so as not to include an area of metal and an area where an artifact appears due to the metal. It is possible to reduce the adverse effect on the center shift correction by specifying an area where the absorption coefficient is extremely lowered to set a region so as to avoid the area.

The region of interest is preferably a two-dimensional region on a cross section perpendicular to the rotation axis. For the shape of the region of interest, an arbitrary shape may be specified and the size of the shape may be specified. Further, any number of coordinate points may be defined on the image. Thus, it is possible to easily and effectively specify the region of interest. However, it is not necessarily required to be a two-dimensional region on one cross section, and a plurality of z cross sections may be selected and a two-dimensional region may be set for each cross section.

The temporary correction unit <NUM> acquires the temporarily corrected image in the region of interest reconstructed from the projected images with correcting the assumed center shift. "Assumed" refers to changing the center shift in a gradual and trial manner to generate a plurality of corrected reconstructed images. Thus, by specifying the actual center shift amount using the region of interest specified in the uncorrected image, it is possible to accurately specify the center shift amount by using an image region avoiding the area where the pixel values are extremely different from the surrounding, for example, an area in which the metal is imaged.

The method determination unit <NUM> determines the scanning method of the CT device. The method determination unit <NUM> determines the scanning method of the CT device with the information received from the CT device. It is preferable to set whether to search the maximal value or the minimal value in relation to a beam shape or a projection angle range. Note that the scanning method may be determined in the scan angle range of the projected images for the subject. For example, a <NUM>° to <NUM>° scan may be dealt as a <NUM>° scan. Thus, it is possible to easily and reliably determine the scanning method. In this way, automatic center correction is enabled for both <NUM>° and <NUM>° scans.

The index analyzing unit <NUM> calculates an index representing variation of pixel values in the temporarily corrected image, and searches for an extreme value of the index value with respect to the assumed center shift amount. Thus, it is possible to provide an index for quantitatively evaluating the degree of deterioration of the reconstructed image.

The index analyzing unit <NUM> automatically selects an extreme value search algorithm according to the determined scanning method, and searches for a maximal value or a minimal value of the index. In the case of a <NUM>° scan using the parallel beam method, the minimal value is searched, and in the case of a <NUM>° scan using the cone beam method, the maximal value is searched. As a result, it is possible to search for the minimal value and the maximal value in accordance with each of the <NUM>° scan and the <NUM>° scan by executing a single program. As a result, the center shift amount can be quantitatively estimated.

In parallel with the search for the extreme value, the index analyzing unit <NUM> performs statistical processing and shape determination on a plot of the index with respect to the assumed center shift amount. A profile fitting is performed on the plot of the index value with the assumed center shift amount by an appropriate function.

The plot outputting unit <NUM> outputs a plot of the index with respect to the assumed center shift amount. As a result, the user can check the plot to determine whether or not the search for the extreme value is appropriate, and can take necessary measures.

The center shift amount specifying unit <NUM> specifies the actual center shift amount with respect to the extreme value. That is, the center shift amount specifying unit <NUM> specifies the center shift amount when a specific extreme value corresponding to the scanning method is obtained.

The correction unit <NUM> corrects the actual center shift amount, and acquires a corrected image reconstructed from the projection images. Thus, it is possible to obtain a reconstructed image in which the artifacts caused by the center shift are removed. In the processing apparatus <NUM>, both the functions of the estimation of the center shift amount and the setting of the region of interest according to the scanning method are not indispensable, and only one of them may be used.

A sample is installed in the CT device <NUM>, and projected images are acquired while the sample is irradiated with X-rays under predetermined conditions. The CT device <NUM> transmits the device information, such as a scanning method, and the acquired projected images as measurement data to the processing apparatus <NUM>.

<FIG> is a flowchart showing the operation of the processing apparatus <NUM> (center shift amount estimating apparatus). First, the processing apparatus <NUM> acquires measurement data and device information (step S1). Next, a projected image is output by using the obtained measurement data and device information (step S2), and a reconstructed image is generated and output (step S3).

The input of the processing conditions such as the setting of the region of interest by the user, the number of steps, the step width, and the like are received with respect to the output of the reconstructed image (step S4). Preferably, the region of interest is specified firstly on the Z component (parallel to the rotation axis) and secondly the XY component (perpendicular to the rotation axis). For the region of interest set in this manner, the index calculation loop is performed in accordance with the input processing conditions. The range of the center shift for being changed is a search range for an extreme value, and the width, step, and center position of the search range are determined in advance. For the center position, for example, by using projected images of θ = <NUM>° and <NUM>° around the sample rotation axis, a value estimated from the residual of the images may be used.

In the loop, a certain center shift amount is assumed, and the assumed center shift amount is corrected to generate a reconstructed image (step S5). An index is calculated based on the corrected reconstructed image (step S6). It is determined whether or not the number of steps reaches a predetermined numerical value and the loop condition is completed (step S7). If the loop condition is not completed, the process returns to step S5. When the loop condition is completed, the process proceeds to step S8. Thus, an index is obtained for each of a plurality of assumed center shift amounts.

Next, profile fitting is performed on the index values obtained for the plurality of assumed center shift amounts (step S8). The obtained profile function can be used to determine extreme values. Statistical processing or shape determination is performed on the plot of the index values (step S9). An analysis result is obtained from these processes, and the user can recognize whether the measurement time is short or not and information on the motion of the sample. The plot of the index value and the analysis result are output (step S10). Note that whether or not steps S9 and S10 are performed is arbitrary, and the steps can be performed in parallel with the following steps S11 to S15.

Next, it is determined whether or not the scanning method of the CT device <NUM> is <NUM>° scanning (step S11). When the scanning method is a <NUM>° scan, a maximal value of the index with respect to the assumed center shift amount is searched for (step S12). If the scanning method is not a <NUM>° scan, the scanning method is assumed to be a <NUM>° scan, and a minimal value of the index with respect to the assumed center shift amount is searched for (step S13). Note that, instead of the determination as to whether the scan is a <NUM>° scan or not, the determination as to whether the scan is a <NUM>° scan or not may be performed.

The center shift amount with respect to the extreme value searched in this manner can be specified as the actual center shift amount (step S14). A reconstructed image is generated by correcting the measurement data with the actual center shift amount obtained, and output (step S15). Thus, the quality of the reconstructed image can be improved.

The processing apparatus <NUM> acquires information of the scanning method as device information from the CT device <NUM>. When the scanning method is a <NUM>° scan, a maximal value of the index representing the variation of the pixel values is searched to specify the center shift amount. On the other hand, when the scanning method is a <NUM>° scan, a minimal value of the index representing the variation of the pixel values is searched to specify the center shift amount.

<FIG> expresses an example schematically showing a reconstructed image <NUM> generated directly from the measurement data. Reconstructed image <NUM> shown in <FIG> shows a cross section perpendicular to the z-axis (rotation axis). For this cross sectional image, the user can specify a rectangular region of interest <NUM>. Thus, the region of interest <NUM> can be specified by inputting an area for a certain z-axis cross section. At the time, the region of interest <NUM> includes an image <NUM> of the characteristic structure from which information is desired to be extracted, and the region of interest <NUM> is set so that an image <NUM> of the structure made of a metal material or the like is removed from the region of interest <NUM>.

<FIG> expresses an example showing a display screen <NUM> at the time of correction. The button <NUM> is a button for instructing acquisition of measurement data. For example, when the button <NUM> is clicked, a window listing the measurement data files can be displayed so that a specific file can be specified from the window. The image <NUM> is a projected image obtained from the measurement data. The image <NUM> is a reconstructed image obtained from the measurement data.

The user can specify the position of the line <NUM> on the image <NUM> to specify a cross section perpendicular to the z-axis of the reconstructed image. Further, the user can move the position of the boundary <NUM> on the image <NUM> to specify the region of interest as the xy region on the z-axis cross section of the reconstructed image. An index may be obtained for the two-dimensional region on the reconstructed image.

The display frames 660a and 660b respectively indicate the number of steps and the step width in changing and assuming the center shift as the loop condition of the processing. Further, the display frame <NUM> is used for inputting the center of the search range before the start of the processing, and displays the specified actual center shift amount after the extreme value search is completed. AutoCenter button <NUM> is a button for instructing the execution of the index analysis.

The processing apparatus <NUM> may display a plot of the index versus the assumed center shift amount. The plot of the index against the center shift amount can be applied to the evaluation for the appropriateness of the original measurement. The user may realize indications of modifications from the plot.

After the profile fitting is performed on the plot of index values by an appropriate function, statistical processing and shape determination can be performed. The result of statistical processing of the plot shows the variation of the index value. For example, if the variation is large, the processing apparatus <NUM> can indicate that the noise of the reconstructed image is large and the measurement time is short. The result of the shape determination of the plot shows how many peaks and valleys there are in the profile. For example, if there are two peaks or valleys, there are two points for extreme values, therefore the processing apparatus <NUM> can indicate a motion of the sample other than the center shift. If there are no peaks or valleys for extreme values, there may be no center or metal effects in the region of interest. If the analysis results show these trends, the processing apparatus <NUM> may offer modification of the cause.

The processing apparatus <NUM> may automatically indicate a modification of the measurement condition or the analysis condition from the tendency of the plot. For example, an indication that the exposure time should be extended may be displayed depending on whether or not the sum of squares of the residuals caused by fitting an appropriate function to the plot exceeds a predetermined value.

The above technique is presupposed to be applied to a case that the center shift is constant along the z-axis, however, may be applied to a case where the center shift is not constant. The tilt of the rotation axis (detector) can be calculated by changing the z value to estimate the center shift amount on multiple sections. With such an assisting function, the adjustment work performed by the installation service person for half a day at the time of delivery of the CT device is shortened.

The cross section of the bamboo skewer was observed using the system <NUM> configured as described above. A nano3DX (registered trade mark) with <NUM>° scan produced by Rigaku was used as the CT device <NUM>. <FIG> is a graph showing a plot of total variation in a <NUM>° scanning measurement.

<FIG> are reconstructed images of z=<NUM> cross sections respectively with assumed center shifts of -<NUM> (minimal value) and <NUM>. It can be seen that bamboo cells clearly appear in the image of <FIG>, however, semicircular artifacts occur in the image of <FIG>.

A CTLabHX with <NUM>° scan produced by Rigaku was used as the CT device <NUM> to observe a bread. <FIG> are reconstructed images of z=<NUM> cross sections respectively with assumed center shift amounts of -<NUM> (maximal value) and <NUM>. In the image of <FIG>, the shape of voids in the bread is clearly seen, but in the image of <FIG>, the shape of voids can be seen to be obscure. <FIG> are pixel values of z=<NUM> cross sections respectively with assumed center shift amounts of -<NUM> (maximal value) and <NUM>. Both are obtained by outputting the pixel values of the range of A-B (<NUM> pixels) in the <FIG>. It is confirmed that <FIG> shows that the change of the pixel values at the boundary between the bread body and the void is clear, whereas <FIG> shows that the change of the pixel values at the boundary between the bread body and the void is unclear.

<FIG> are graphs showing plots of the total variation respectively when the variation of the index values is large and when the variation of the index values is small. The total variation shown in the <FIG> is the result of the index analysis when measured with short exposure times, and there is not enough tendency in plotting the total variation with respect to the center shift amount. On the other hand, the total variation shown in <FIG> is the result of an index analysis when the measurement is performed with a long exposure time, and it is clearly seen that the minimal value occurs around the center shift amount -<NUM> and -<NUM> from the plot of the total variation. Thus, the index can be used for quantitative evaluation of the quality of the image. It can also be used to determine whether the exposure time is sufficient or to determine whether or not there is a motion other than the center shift.

Claim 1:
A center shift amount estimating apparatus (<NUM>) for estimating a deviation between a rotation axis of a sample and a center of a detector (<NUM>) with respect to an X-ray source (<NUM>) in a CT device (<NUM>), the apparatus (<NUM>) comprising:
a region specifying unit (<NUM>) for specifying a region avoiding an area with extremely low absorption coefficient or an area with extremely different pixel values from a surrounding area as a region of interest in a reconstructed uncorrected image,
a temporary correction unit (<NUM>) for assuming different center shift amounts and correcting each of the assumed center shift amounts to reconstruct a temporarily corrected image with respect to the region of interest,
an index analyzing unit (<NUM>) for calculating an index representing a total variation, performed by summing the differential values at each pixel in the region with respect to the region of interest of the image in each of the temporarily corrected images and searching a maximal value of the index when the scanning method is a <NUM>° scan and searching a minimal value of the index when the scanning method is a <NUM>° scan as an extreme value of the index, and
a center shift amount specifying unit (<NUM>) for specifying the center shift amount assumed in the temporarily corrected image in which the index has the extreme value as an actual center shift amount.