Patent Description:
The invention generally relates to computed tomography systems ("CT systems") and, more particularly, the invention relates to calibrating and/or verification of x-ray computed tomography devices/CT machines used to measure objects.

Coordinate measuring machines (CMMs) are the gold standard for accurately measuring a wide variety of different types of work pieces. For example, CMMs can measure critical dimensions of aircraft engine components, surgical tools, and gun barrels. Precise and accurate measurements help ensure that their underlying systems, such as an aircraft in the case of aircraft components, operate as specified.

Inaccurate measurements can have catastrophic effects. Accordingly, to ensure that CMMs deliver accurate measurements, the CMM industry has developed well-defined accuracy verification standards, procedures, and gauging tools to calibrate and verify the underlying machines taking these measurements. To those ends, a CMM verification procedure typically requires hard gauges that are traceable for uncertainty calculations, and designed in such a way to ensure that they (i.e., the gauges) are dimensionally stable.

<CIT> shows an apparatus for calibrating an x-ray computed tomography imaging device (e.g., a CT device) with a plurality of objects formed from a material that is visible to x-rays, and a base at least in part fixedly supporting the plurality of objects so that each of the plurality of objects contacts at least one of the other objects.

<NPL> describes the basic principles of CT metrology, putting emphasis on issues as accuracy, traceability to the unit of length (the meter) and measurement uncertainty. The paper shows the aptitude of CT metrology to: (i) check internal dimensions that cannot be measured using traditional coordinate measuring machines and (ii) combine dimensional quality control with material quality control in one single quality inspection run.

<NPL> describes a geometrical standard for testing X-ray-micro-computertomography measuring systems (µCT) and methods for manufacturing and calibration. This standard enables the testing of µCT measuring systems to use test procedures of classical coordinate measurement technique according to ISO <NUM> resp. VDI/VDE <NUM> as well as the nominal-actual comparison of µCT data against reference data.

In accordance with one embodiment of the invention, a method of calibrating or verifying the dimensional accuracy of an x-ray computed tomography machine controls the x-ray computed tomography machine to produce a gauge reconstruction (a 3D reconstruction of the gauge). The gauge has a first base supporting two or more objects, and a second base supporting two or more objects. The first base and the second base form a perpendicular configuration, and each of the plurality of objects is secured on at least one of the first base and the second base. Each of the objects has a center, and the distance between the centers of each object is known ("known center distance values"). The method then measures, in the reconstructed three-dimensional volume or three-dimensional derived surface of the gauge ("gauge reconstruction"), the distance between at least two objects to produce measured center distance values, compares the measured center distance values against the known center distance values, and uses the comparison to determine if there is a distance error (i.e., the gauge is incorrectly measuring distances-e.g., out of specification) in the gauge reconstruction.

Among other things, the gauge reconstruction may include a point cloud. To reduce interference with the x-rays, at least one of the first base and the second base may have at least one hole through solid base material. Moreover, the second base may contact the first base along an edge of the first base. To ensure uniformity, each of the objects may substantially spherically shaped and identically sized and include a similar material (e.g., ruby).

In some embodiments, at least one of the objects contacts both the first base and the second base to at least in part form a straight line with other objects. For example, where first, second, and third objects of the plurality of objects form a substantially straight line, the method may measure between the first and second objects, and measuring between the first and third objects. A fourth object of the plurality of objects also may be part of the substantially straight line. In that case, the method may measure between the fourth object and at least one of the first, second and third objects. One of the noted first, second, third, or fourth objects may contact the two bases.

The method also may produce a plurality of 3D gauge reconstructions of the gauge from a plurality of x-ray projection images in different orientations. Then, the method may measure center distance values of the objects in each of the 3D gauge reconstructions against the respective known center distance values for calibrating the computed tomography machine. In this or other cases, if the method also determines that there is no distance error, then it may responsively maintain the calibration settings in the x-ray computed tomography machine. Conversely, if the method determines there is a distance error, it responsively may modify the calibration settings of the x-ray computed tomography machine. Those calibration settings are modified as a function of at least one of the differences.

The objects may include a given type of object, and the first base and the second base each may support at least two of the given type of object. Moreover, the first and second bases are formed from a base material, and the plurality of objects are formed from an object material. To enhance contrast under x-rays, the object material may have a higher attenuation to x-rays than that of the base material.

The x-ray computed topography machine may scan around the gauge in a number of manners. For example, it may scan around the gauge about an axis of rotation that diverges with either a) the first plane, b) the second plane, or c) both the first plane and the second plane.

Illustrative embodiments of the invention are implemented as a computer program product having a computer usable medium with computer readable program code thereon. The computer readable code may be read and utilized by a computer system in accordance with conventional processes.

In accordance with another embodiment, a gauge for calibrating or verifying the dimensional accuracy of an x-ray computed tomography machine has a plurality of objects formed from a material that is visible to x-rays. The plurality of objects are configured to receive x-rays without changing shape, and each one has both substantially the same shape and an object attenuation value to x-rays. The gauge also has a substantially planar first base fixedly supporting a first set of the plurality of objects, and a substantially planar second base fixedly supporting a second set of the plurality objects. The first base is connected to the second base to form a substantially right angle with the second base. The first and second sets of objects have a common object-i.e., at least one object is fixedly supported by both bases. The first base and second base have respective first and second base attenuation values. For appropriate contrast to x-rays, the object attenuation value is greater than the first and second base attenuation values.

Those skilled in the art should more fully appreciate advantages of various embodiments of the invention from the following "Description of Illustrative Embodiments," discussed with reference to the drawings summarized immediately below.

In illustrative embodiments, an apparatus for calibrating (or verifying) an x-ray computed tomography machine enables more accurate measurements of a work piece-particularly, more accurate measurements of smaller dimensions of a work piece (e.g., in the sub-millimeter level, such as to the micrometer or nanometer level).

To that end, the apparatus has a first base that holds a plurality of objects in place. The apparatus also has a second base that holds a plurality of objects in place. The first base and the second base are positioned substantially perpendicular/orthogonal to one another. The perpendicular configuration of the first base and the second base positions objects on orthogonal planes, which allows measurement calibration and verification at steep and orthogonal angles. This arrangement further enables independent characterization of vertical and horizontal errors. Details are discussed below.

<FIG> and <FIG> schematically show an x-ray machine/computed tomography device <NUM> that may use a calibration gauge configured in accordance with illustrative embodiments. It should be noted that although this discussion primarily relates to calibration, principles of various embodiments apply to verification of the accuracy of x-ray computed tomography machines <NUM>. Accordingly, discussion of calibration is not intended to limit all embodiments of the invention.

Among other things, the x-ray CT machine <NUM> may be a computed tomography system (a/k/a a "CT system" or a "CT machine") that produces a three dimensional model of a work piece positioned within its interior (referred to as a "work piece reconstruction"). To those ends, the x-ray CT machine <NUM> has a housing <NUM> forming an interior chamber for containing, among other things (see <FIG>), <NUM>) a work piece <NUM> to be measured, <NUM>) an x-ray source <NUM> (also referred to as an "x-ray gun <NUM>") for generating x-rays, <NUM>) a rotary stage <NUM> for rotating the work piece <NUM>, and <NUM>) a detector <NUM> for detecting x-ray attenuation caused by the work piece after it is radiated by the x-ray gun <NUM>. As known by those in the art, the detector <NUM> subsequently produces an x-ray image of the work piece. Returning to <FIG>, an access door <NUM>, which may be formed from a transparent material, provides access to the interior for adding and removing work pieces <NUM>. For example, the work piece <NUM> may be a cardiovascular stent commonly used in coronary angioplasty procedures. A control panel <NUM> on the side of the machine <NUM> acts as the control interface for an operator.

To produce the 3D model of the work piece <NUM> (the "reconstruction"), the CT system moves the work piece <NUM> relative to the x-ray guns <NUM>. For example, the CT system may rotate the work piece <NUM> a full <NUM> degrees on the rotary stage <NUM>, relative to the x-ray gun <NUM>, to form the 3D reconstruction, and take multiple x-ray images (known in the art as "projections" or "projection angles" and noted above) of the work piece <NUM> during rotation. During and/or after rotating the work piece <NUM>, a model building module (e.g., post-processing software executing on a local microprocessor or microcontroller) converts the data of all the projections into the noted 3D model of the work piece <NUM> -the noted reconstruction. Stated another way, a 3D reconstruction typically is a 3D volume of a plurality of x-ray projections of the work piece <NUM>. This data can be stored in memory, used to generate a point cloud, and/or used to measure the workpiece <NUM> (discussed below). Measuring the reconstruction generally produces more accurate measurements than other prior art techniques, such as measuring an x-ray projection on a display (e.g., measuring based on pixels). This is particularly apparent in the coordinate measurement machine (CMM) industry, where very small differences in accuracy (e.g., <NUM> millimeter or less) are significant. This is in contrast to many other applications, such as medical applications, that typically do not require such fine accuracy.

It is this 3D model-which may be a software model-that may be measured to confirm the dimensional accuracy of the work piece <NUM>. Thus, even if the work piece <NUM> is a small medical device, such as a cardiovascular stent, then the measurement software may precisely measure selected features of the stent, such as its radius, wall thickness, etc..

If the CT system <NUM> is not properly calibrated, however, then these work piece measurements may be inaccurate. This is particularly problematic in metrology applications, which often require precise measurements. Accordingly, the operator or some other person should calibrate the CT system <NUM> prior to use. Such CT systems <NUM> used for precise measurement of workpieces <NUM> have verification standards, which define specifications and procedures for testing coordinate measuring machines with sensors relying on the principle of X-ray computed tomography. One such verification standard is provided by The Association of German Engineers ("VDI") and The Association for Electrical, Electronic & Information Technologies ("VDE") in VDI/VDE Directive <NUM> Part <NUM>, and titled "Computed tomography in dimensional measurement. " Undesirably, however, the inventors know of no highly reliable and fine pitch technology, mechanism, or technique to conveniently verify the accuracy of conventional CT systems <NUM> to VDI/VDE verification standards at relatively steep angles. Recognizing that technical problem, the inventors have developed a highly accurate, fine pitch calibration/verification gauge that fills this deficiency in the art. Use of this gauge solves the problem that the prior technology known by the inventors lacks-the inability of such prior art technology of more efficiently calibrating and verifying such angles.

Specifically, <FIG> schematically show an illustrative gauge <NUM> for calibrating and/or verifying the CT machine <NUM>. The gauge <NUM> has a first base <NUM> supporting a plurality of discrete objects <NUM> that act as guideposts in the calibration and/or verification process. More specifically, in illustrative embodiments, the objects <NUM> include three, four, or more spheres (also identified by reference number "<NUM>") that are ground or lapped to have very precise qualities (for example, precise symmetry, shape, size, volumes, centers, geometry, etc.. In illustrative embodiments, the spheres <NUM> are certified by some reliable and well-known third party to have certain measurement qualities. In some embodiments, the spheres <NUM> are free-standing, immovable, and independent structures-they are not integral to or even connected to each other. Instead, spheres <NUM> merely contact the first base <NUM>. In alternative embodiments, the spheres also may contact each other. As discussed in greater details below, some embodiments position the spheres <NUM> so that their centers form a measurement axis/straight line.

As noted above and discussed in greater detail below, the first base <NUM> couples the spheres <NUM> into place along pre-defined positions on the first base <NUM>. The coupling may be accomplished by ultrasonic welding, adhesive and/or other techniques known to those skilled in the art. In particular, the spheres <NUM> are constrained so that they cannot move in any other manner, e.g., they cannot rotate or translate relative to the first base <NUM>. Other embodiments, however, may permit non-translational motion, such as rotation relative to the first base <NUM>.

The gauge <NUM> also has a second base <NUM> that supports a plurality of other, similar or identical discrete objects <NUM>. In fact, the spheres <NUM> are coupled one or more surfaces of the second base <NUM>. For example, spheres <NUM> may be coupled with both of the largest surfaces of the second base <NUM>, and/or on one of the side surfaces. As shown in the figures, the first base <NUM> preferably is mounted to be substantially perpendicular to the second base <NUM>. As shown, the first base <NUM> and the second base <NUM> form an open region free of objects <NUM>-e.g., there are no other spheres <NUM> between the orthogonally mounted bases <NUM> and <NUM> (and no other bases <NUM> or <NUM>). The first base <NUM> and second base <NUM> preferably are secured together in a "T" configuration (i.e., their combination generally resembles an upper case "T"). It should be understood that the term "substantially perpendicular" is intended to encompass various embodiments that are virtually perfectly perpendicular/orthogonal, as well as embodiments that are within an acceptably small tolerance of virtually perfectly perpendicular.

In a manner similar to the first base, <NUM>,the second base <NUM> supports its spheres <NUM> in one or more parallel (or non-parallel) planes that are different than the one or more parallel (or non-parallel) planes of spheres <NUM> the first base <NUM> supports. While illustrative embodiments show the second base <NUM> as substantially perpendicular to the first base <NUM>, this is not intended to limit all embodiments. Indeed, an acute, perpendicular or obtuse angle may be formed by the intersection of a longitudinal axis of the first base <NUM> with a longitudinal axis of the second base <NUM>. Illustrative embodiments showing the second base <NUM> as planar and substantially perpendicular to the first base <NUM> are just exemplary and not intended to limit all embodiments. The second base <NUM> may be non-planar (not shown), and thus may support spheres <NUM> that lie on different tangential planes.

In illustrative embodiments, the second base <NUM> has x-ray attenuation features <NUM> (<FIG>). The features <NUM> are intended to minimize the amount of x-ray attenuation and photon deflection caused by the gauge <NUM> during scanning by the CT machine <NUM>, while allowing the gauge <NUM> to maintain its structural integrity. More specifically, in illustrative embodiments, the features <NUM> comprise one or more holes (also identified by reference number "<NUM>") through the solid material forming the second base <NUM>. These holes <NUM> may be precisely shaped and positioned to minimize interference with x-ray intensity. Optimization of the hole parameters refers to the maximum reduction of x-ray attenuation without significant effect on the structural integrity of the second base <NUM>. To that end, structural analysis may be performed to optimize the hole characteristics (e.g., shape, size, etc.). Although shown as circular in the figures, the holes <NUM> can take on different shapes and sizes. The size and shape of the holes <NUM> depends on the material used to make the second base <NUM>, as well as the number of spheres it will support.

Because a platform (e.g., the rotary table <NUM> of <FIG>) supports the first base <NUM>, it can be formed from different materials, such as a less-rigid, less-x-ray attenuating material than that of the second base <NUM>. To simplify production, however, illustrative embodiments of the gauge <NUM> form first base <NUM> and second base <NUM> from the same material. For example, the first and second bases <NUM> and <NUM> may be formed from boron nitride, and/or other materials having low coefficients of thermal expansion. A low coefficient of thermal expansion allows the geometric dimensions to remain relatively stable when temperature fluctuates. Some embodiments may have a structure similar to a truss that connects the spheres <NUM> along its joints.

The features <NUM> in the second base <NUM> are not limited to holes. A number of methods may be employed to reduce the amount of material that attenuates or deflects x-ray intensity. For example, portions of the second base <NUM> may be thinned, thereby reducing the amount of x-ray attenuation. Some embodiments may have recessed portions or grooves along the surface of the second base <NUM>. In some embodiments, the spheres <NUM> may sit in the grooves. The second base <NUM> may have grooves with substantially straight and flat surfaces. For example, the surface of the groove may form a V-shape with an angle of between about <NUM> and <NUM> degrees. Additionally, or alternatively, the gauge <NUM> may also have protrusions on which the spheres <NUM> may be locked.

Furthermore, although <FIG> show the second base <NUM> positioned along the edge of the first base <NUM>, other embodiments may position the bases <NUM> and <NUM> in a different manner relative to each other. Specifically, the second base <NUM> may attach to the first base <NUM> along any point, such as the middle of the top face of first base <NUM>. Additionally, in some embodiments, the first base <NUM> does not necessarily attach/couple/fasten to the second base <NUM> at its bottom end. The first base <NUM> may attach to the second base <NUM> anywhere, including in the middle of the second base, again to form a "T" configuration (as noted above), or in a cross (e.g., "+") configuration. It should also be noted that not all embodiments require the first base <NUM> to be attached to the second base <NUM>. In some embodiments, the bases may not be attached, and/or may not be in contact with one another.

To calibrate the CT system <NUM>, a calibration module measures the distance between some identifiable regions of the various objects <NUM>. For example, in the sphere <NUM> embodiment discussed above, the calibration module may measure between the centers of the one or more of the spheres <NUM>. If the object <NUM> was not in the form of a sphere <NUM> (e.g., in the form of a protrusion, cube, cylinder, irregular shape, etc.. ), then the identifiable region could be the center or some other area, such as an end, a discontinuity, a corner, the intersection of two portions, etc. Even if the objects <NUM> are spheres <NUM>, the identifiable portion could be an outside region.

Accordingly, it is important for the spheres <NUM> to be visible on the x-ray images. To that end, the spheres <NUM> preferably are formed from a material having a higher attenuation to x-rays than that of the base <NUM>. For example, the spheres <NUM> may be formed from aluminum oxide, such as ruby or sapphire, or other material with a low thermal expansion and x-ray attenuation near the middle of the range of intensity values of the CT system <NUM>. As previously noted, the base(s) <NUM> and <NUM> may be formed from a ceramic material with a high stiffness and a low thermal expansion, but with an x-ray attenuation that is relatively low when compared to the material of the spheres <NUM>. This differential in attenuations should be selected to provide good contrast and a clear separation between the surfaces of interest (i.e., the spheres <NUM>) and the base(s) <NUM> and <NUM>. For example, as noted above, the base(s) <NUM> and <NUM> preferably may be formed from boron nitride while the spheres preferably may be formed from ruby. Those in the art should understand that the base(s) <NUM> and <NUM> and spheres <NUM> may be formed from other materials having similar relative properties.

The coefficient of thermal expansions of the spheres <NUM>, the first base <NUM>, and the second base <NUM> preferably are as low as possible, such as no greater than that for steel. The first bases <NUM> and <NUM> also preferably are in a specified form to accurately support two or more spheres <NUM> in a precisely straight line. As noted herein, this line should be straight within a predefined error, such as <NUM> micron. This precision applies to spheres <NUM> that are on a single base <NUM> or <NUM>, and also lines formed by spheres <NUM> on different bases <NUM> and <NUM> (e.g., one sphere on the first base <NUM> and another sphere on the second base <NUM>).

To form a substantially straight line, the surfaces of the spheres <NUM> and the base <NUM> should be precisely configured. Specifically, as shown in <FIG> and <FIG> (discussed in greater detail with regard to <FIG>), the base <NUM> surface of this embodiment preferably is very planar, smooth, and straight. Ideally, each sphere <NUM> contacts the first base <NUM> at one infinitesimally small, discrete point. Of course, in illustrative embodiments, this contact is not infinitesimally small. The point of contact effectively forms a single force vector in a direction that is normal to the longitudinal axis of the first base <NUM>.

Those skilled in the art should drive toward that end by using the more finely and accurately produced spheres <NUM>. The spheres <NUM> thus may be formed to have a very fine precision. For example, the spheres <NUM> have a diameter with a precision to at least <NUM> millimeters. Specifically, as used herein, a precision of at least <NUM> millimeters may have an even finer precision, such as <NUM> millimeters, <NUM> millimeters, <NUM> millimeter, <NUM> millimeters, etc. As another example, the spheres <NUM> may have a diameter of <NUM> millimeters, within some known tolerance, such as <NUM> millimeters. All spheres <NUM> of the same gauge <NUM> may be the same size, or different. In either case, the diameters of the spheres <NUM> are known to the precision noted. Accordingly, illustrative embodiments can detect a variance of the reading by the CT machine <NUM> by an amount on the order of the precision of the sphere <NUM>-down to the micrometer or nanometer level.

It should be noted that the above description regarding the first base <NUM> may apply to the second base <NUM>, and vice-versa. For example, the first base <NUM> may have x-ray attenuation features, or be shaped to form a non-planar surface in a manner similar to the second base <NUM>.

<FIG> shows a verification and/or calibration process that uses the gauge <NUM> in accordance with one embodiment of the invention. As noted above, this process provides a technical solution to a technical problem associated with prior art calibration and verification technology known by the inventor. Among other things, execution of this process ensures that the CT machine <NUM> is operating properly-a critical function for such a device. Catastrophic results can subsequently occur during use of a measured part if the CT machine is not operating properly-in this case, mis-measuring critical elements, such as stents or airplane propellers, could cause significant injuries.

It should be noted that this process is simplified from a longer verification and/or calibration process that may use the gauge <NUM>. Accordingly, the process may have other steps that those skilled in the art may use. In addition, some of the steps may be performed in a different order than that shown, or at the same time. Those skilled in the art therefore can modify the process as appropriate.

The process begins at step <NUM> by selecting a prescribed orientation for the gauge <NUM> within the x-ray CT machine <NUM>. For example, the first prescribed orientation may be relative to the X-axis of the CT machine <NUM>. Next, the process physically positions the gauge <NUM> on the rotary stage <NUM> within the interior chamber of the CT machine <NUM> in the prescribed orientation (step <NUM>). After the gauge <NUM> is positioned in the prescribed orientation within the interior chamber, the x-rays gun <NUM> and detector <NUM> cooperate to image the gauge <NUM> (step <NUM>). To that end, the rotary table preferably rotates the gauge <NUM> a full <NUM> degrees. The axis of rotation in this example diverges (i.e., not parallel with) from at least one of the bases <NUM> and <NUM>. In this example, the axis of rotation may be shown by line <NUM> of <FIG>, which is generally perpendicular to the first base <NUM>. During this time, the x-ray gun <NUM> and detector <NUM> cooperate to generate a plurality of sequenced images/projections of the gauge <NUM> for subsequent processing. Each image may be stored in memory for subsequent reconstruction and processing.

After the CT machine <NUM> finishes imaging the gauge <NUM>, the process constructs a three-dimensional model ("3D model") of the gauge <NUM> (step <NUM>). A model engine (or model building module) thus uses the data from the successive images to construct the 3D model-a gauge <NUM> reconstruction, which also can be stored in memory. Although not necessary, rendering software may render the 3D model, and then rotate or otherwise move the ultimate 3D model in a viewer, thus showing the details of the gauge <NUM>.

Step <NUM> then measures the 3D model elements (e.g., between reconstructed spheres <NUM>) to determine if the CMM measurements are accurate-verifying accuracy. To that end, the process measures between preselected points within the gauge <NUM> reconstruction. For example, the process may measure from the center of each sphere <NUM> to the center of one or more of the other spheres <NUM>. This step thus produces a plurality of values for verification in subsequent steps.

Some embodiments comply with VDI-VDE verification standards. Among other things, VDI-VDE verification standards specify that length measurement error tests compare known measurements with five measurements in each of seven different spatial directions. <FIG> schematically show an embodiment of the gauge <NUM> made to comply with such verification standards. Accordingly, the gauge <NUM> has a plurality of precisely positioned spheres <NUM> that permit five measurements in each of seven different spatial directions. As with other gauge embodiments, the first base <NUM> and the second base <NUM> are configured substantially perpendicularly to each other. This substantially perpendicular configuration provides a wider variety of potential measurements than that of a 2D standard gauge. Specifically, measurements may be taken in three dimensions, at steep angles, and at a fine pitch.

<FIG> shows examples of six different spatial directions <NUM>-<NUM> for measuring 3D model elements (e.g., spheres <NUM>). The top view of <FIG> shows a seventh example spatial direction <NUM>. Each of those directions effectively forms a straight line or vector. Each straight line in this embodiment has four or more spheres <NUM>, although other embodiments may have fewer spheres <NUM>. The spheres in a given line may be positioned on opposite sides of the second base <NUM>, across the first base <NUM>.

These spatial directions/lines are merely exemplary and not intended to limit embodiments of the invention. As shown, various embodiments are not limited to directions along each of the orthogonal planes. Measurements may be taken in spatial directions that traverse both planes formed, for example, by a top surface <NUM> of the first base <NUM> and a front surface <NUM> of the second base <NUM>. For example, placing two spheres <NUM> on the top surface <NUM> of the first base <NUM> and two spheres <NUM> on the front surface <NUM> and rear surface <NUM> of the second base <NUM> may create a spatial direction if the four spheres <NUM> are aligned. One of those spheres <NUM> may be coupled/supported/contacting both the first and second bases <NUM> and <NUM>. Such a sphere <NUM> thus can be shared by different lines, minimizing the required number of spheres <NUM> in the gauge. For example, lines <NUM> and <NUM> share a sphere <NUM> that contacts both bases <NUM> and <NUM>. A person having skill in the art should know how to position spheres <NUM> on the gauge <NUM> to create multiple spatial directions. Some embodiments may place spheres <NUM> within the holes <NUM> to provide another measurement point.

It should be noted that some of the lines <NUM>-<NUM> may appear in the figures to contact spheres <NUM> that would make the lines not straight. For example, line <NUM> does not pass through the sphere spaced away from the top surface <NUM> of the first base <NUM>. Such a suggestion is merely a limitation of a <NUM>-D drawing.

The substantially perpendicular configuration of the first base <NUM> and the second base <NUM> enables fine pitch measurements in three dimensions. For example, a line (also referred to as a "measurement axis") may be formed by a sphere <NUM> positioned near the top of the second base <NUM>, and a first base sphere <NUM> slightly offset from the intersection of the bases <NUM> and <NUM>. Additionally, more spheres <NUM> may lie within that axis. Some embodiments of the first base <NUM> and/or the second base <NUM> have protrusions on which some of the spheres <NUM> rest to facilitate fine pitch measurements.

While a spatial direction may be formed between two spheres <NUM>, preferred embodiments use at least four axially aligned spheres <NUM> to form a single spatial direction/measurement axis. <FIG> shows an example of such a measurement axis-line <NUM>. Four first base spheres <NUM> aligned in a single spatial direction <NUM> enable six different measurements (M1-M6) in the spatial direction <NUM>. Specifically, measurements may be taken between the center of each adjacent sphere (M1-M3), between the centers of the first sphere <NUM> and the fourth sphere <NUM> (M4), between the centers of the first sphere <NUM> and the third sphere <NUM> (M5), and between the centers of the second sphere <NUM> and the fourth sphere <NUM> (M6). The method may use any five of these measurements, or all six of the measurements.

The actual distance between the prespecified points is known; in preferred embodiments, those distances are certified. For example, the known distance between the centers of two spheres <NUM> can be <NUM> millimeters. The known distance between the centers of two other spheres <NUM> could be <NUM> millimeters. Other embodiments can vary the distance between the different spheres <NUM>.

Step <NUM> compares those different measured distances against the known distances and determines if there are discrepancies, which indicates errors (step <NUM>). For example, the process simply may use logic to determine the difference between the various measurements and the known distances. This difference is the calibration error of the machine <NUM>. Using the example above, if the measured distance between the first two spheres <NUM> (known distance <NUM> millimeters) is <NUM> millimeters, then the CT machine <NUM> has an error of <NUM> millimeters and thus, should be appropriated adjusted/re-calibrated. Illustrative embodiments complying with the VDI-VDE standard compare the calibration error in five measured distances in each of seven spatial directions (<NUM> total measurements).

Accordingly, if the process detects errors beyond some preset limits or tolerances (e.g., detecting this exemplary <NUM> millimeter error) and the process is a calibration process, then step <NUM> corrects the error by refining the initial calibration settings of the CT system <NUM>. Of course, if errors are not beyond the noted preset limits, then the process does not adjust the calibration settings. After correcting the errors by step <NUM>, or if there are no errors from step <NUM>, the process continues to step <NUM> to determine if calibration or verification is complete. If it is complete, then the process ends. If not complete, then the process may change the prescribed orientation of the gauge <NUM>. For example, the prescribed orientation can be moved to be orthogonal to the initial prescribed orientation. By doing this, the operator can test various different axes within the machine.

Some embodiments may skip <NUM>. Instead, such embodiments may execute step <NUM> after the CT machine <NUM> images all orientations of the gauge <NUM>.

Potential applications of various embodiments may include (in addition to those noted above or reiterating those above):.

Accordingly, using the reconstruction to measure prescribed distances provides precise measurements to verify or calibrate the CT system <NUM>. This is especially important in metrology applications, which, as noted above, often require highly precise and fine measurements. The inventor expects such a method to be superior to simply measuring x-ray projections on a pixelated display device, which can have an error that is unacceptable in many metrology applications. Moreover, use of three or four objects <NUM> in a single line, and sharing objects <NUM> between the two bases <NUM> and <NUM> similarly enhances gauge efficiency (e.g., gauge manufacture, and reduced set-up for comprehensive characterization of system accuracy).

Various embodiments of the invention may be implemented at least in part in any conventional computer programming language. For example, some embodiments may be implemented in a procedural programming language (e.g., "C"), or in an object oriented programming language (e.g., "C++"). Other embodiments of the invention may be implemented as a pre-configured, stand-along hardware element and/or as preprogrammed hardware elements (e.g., application specific integrated circuits, FPGAs, and digital signal processors), or other related components.

In an alternative embodiment, the disclosed apparatus and methods (e.g., see the flow chart described above) may be implemented as a computer program product for use with a computer system. Such implementation may include a series of computer instructions fixed either on a tangible, non-transitory medium, such as a computer readable medium (e.g., a diskette, CD-ROM, ROM, or fixed disk). The series of computer instructions can embody all or part of the functionality previously described herein with respect to the system.

Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, optical or other memory devices, and may be transmitted using any communications technology, such as optical, infrared, microwave, or other transmission technologies.

Among other ways, such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over the network (e.g., the Internet or World Wide Web). In fact, some embodiments may be implemented in a software-as-a-service model ("SAAS") or cloud computing model. Of course, some embodiments of the invention may be implemented as a combination of both software (e.g., a computer program product) and hardware. Still other embodiments of the invention are implemented as entirely hardware, or entirely software.

Claim 1:
A method of calibrating or verifying the dimensional accuracy of an x-ray computed tomography machine (<NUM>), the method comprising:
controlling the x-ray computed tomography machine to image a gauge (<NUM>) to produce a 3D gauge reconstruction with reconstructed features (<NUM>) of the gauge, the gauge comprising a plurality of objects (<NUM>) formed from an object material, a first base (<NUM>) supporting two or more of the objects, and a second base (<NUM>) supporting another two or more of the objects, the first and second bases formed from a base material, the object material having a higher x-ray attenuation value than the base material,
the first base forming a first plane, the second base forming a second plane, the first plane and second plane being substantially perpendicular to each other,
each of the objects being secured on at least one of the first base and the second base, whereby the perpendicular configuration of the first base and the second base positions the objects (<NUM>) on the orthogonal planes in multiple spatial directions,
each of the objects having a center, the distance between the centers of each object being known center distance values;
measuring, in the 3D gauge reconstruction, the distance between at least two objects to produce measured center distance values;
comparing the measured center distance values against the known center distance values; and
using the comparison to determine if there is a distance error in the x-ray computed tomography machine.