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1. (WO2018094121) SYSTEM AND METHOD FOR MOTION ESTIMATION AND COMPENSATION IN HELICAL COMPUTED TOMOGRAPHY
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SYSTEM AND METHOD FOR MOTION ESTIMATION AND COMPENSATION IN HELICAL COMPUTED

TOMOGRAPHY

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 62/423,166 entitled, "System and Method for Motion Estimation and Compensation in Helical Computed Tomography", filed on November 16, 2016, which is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to the field of image processing, and in particular to image processing in medical applications. In particular, the present invention is directed to a method for motion estimation and motion compensation of a helical computed tomography (CT) scan of an object of interest.

Motion is one of the most critical sources of artifacts in helical cone-beam computed tomography (CT). Motion artifacts result from discrepancies between the requirement that the object remain unchanged during the scan and reality, in which the object changes ("deforms'' or "moves") during the scan. Motion of the patient, whether voluntary or involuntary, during image acquisition may result in motion artifacts in the reconstructed image. Involuntary motion, such as respiration or cardiac motion, may result in motion artifacts. While there are techniques known in the art for the reconstruction of motion compensated images from circular CT scan data, the known techniques do not address the unique conditions of a helical CT scan, wherein the field of view (FOV) is continuously changing.

Accordingly, what is needed in the art is an improved system and method for motion estimation and compensating for motion by reducing motion artifacts produced during image reconstruction from helical computed tomography (CT) scan data.

SUMMARY OF THE INVENTION

The present invention provides an improved system and method for estimating motion and compensating for motion by reducing motion artifacts produced during image reconstruction from helical computed tomography scan data. In various embodiments, the system and method can also be applied to more general helical-like trajectories, including variable pitch helices and helices in which the axis of rotation changes.

In one embodiment, the present invention provides a method for estimating and compensating for motion by reducing motion artifacts in an image reconstruction from helical computed tomography (CT) scan data of an object of interest, which includes, collecting helical computer tomography (CT) scan data of an object of interest, wherein the scan data is acquired using a radiation source to generate a cone beam and a radiation detector to detect the cone beam. The method further includes, selecting a plurality of center-points along a trajectory of the radiation source and identifying a plurality of pairs of sections along the trajectory of the radiation source, wherein each of the plurality of pairs of sections is associated with one of the plurality of center-points and wherein a first section of each of the pairs of sections and a second section of each of the pairs of sections are positioned on opposite sides of the center-point. In a particular embodiment, the sections are separated from each other by an angular distance equal to π. The method additionally includes, selecting a subset of the plurality of pairs of sections and reconstructing, for each pair of the subset, a first partial image from the scan data of the first section and a second partial image from the scan data of the second section and performing image registration of the first partial image and the second partial image for each pair of the subset to estimate a deformation that transforms the first partial image into the second partial image, wherein the deformation is representative of motion of the object of interest during the scan. Following image registration of the partial images, the method further includes, generating a motion compensated image by reconstructing the object of interest using the scan data and the estimated deformation.

In an additional embodiment, the invention provides a system for estimating and compensating for motion by reducing motion artifacts in an image reconstruction from helical computed tomography (CT) scan data of an object of interest. The system includes, a memory for storing a helical computer tomography (CT) scan data of an object of interest and a data processor. The data processor is configured for estimating and compensating for motion by reducing motion artifacts in an image reconstruction from the helical computed tomography (CT) scan data of an object of interest. As such, the data processor is adapted for loading the helical CT scan data from the memory, selecting a plurality of center-points along a trajectory of the radiation source and identifying a plurality of pairs of sections along the trajectory of the radiation source, wherein each of the plurality of pairs of sections is associated with one of the plurality of center-points and wherein a first section of each of the pairs of sections and a second section of each of the pairs of sections are positioned on opposite sides of the center-point. In a particular embodiment, the sections are separated from each other by an angular distance equal to π. The data processor is further configured for selecting a subset of the plurality of pairs of sections and for reconstructing, for each pair of the subset, a first partial image from the scan data of the first section and a second partial image from the scan data of the second section, for performing image registration of the first partial image and the second partial image for each pair of the subset to estimate a deformation that transforms the first partial image into the second partial image, wherein the deformation is representative of motion of the object of interest during the scan, for generating a motion artifact compensated image by reconstructing the object of interest using scan data and the estimated deformation.

The present invention additionally provides an embodiment including one or more non-transitory computer-readable media having computer-executable instructions for performing a method of estimating and compensating for motion by reducing motion artifacts in an image reconstruction from helical computed tomography (CT) scan data of an object of interest. The method includes, collecting helical computertomography (CT) scan data of an object of interest, wherein the scan data is acquired using a radiation source to generate a cone beam and a radiation detector to detect the cone beam. The method further includes, selecting a plurality of center-points along a trajectory of the radiation source and identifying a plurality of pairs of sections along the trajectory of the radiation source, wherein each of the plurality of pairs of sections is associated with one of the plurality of center-points and wherein a first section of each of the pair of sections and a second section of each of the pair of sections are positioned on opposite sides of the center-point. In a particular embodiment, the sections are separated from each other by an angular distance equal to π. The method additionally includes, selecting a subset of the plurality of pairs of sections and reconstructing, for each pair of the subset, a first partial image from the scan data of the first section and a second partial image from the scan data of the second section and performing image registration of the first partial image and the second partial image for each pair of the subset to estimate a deformation that transforms the first partial image into the second partial image, wherein the deformation is representative of motion of the object of interest during the scan. Following image registration of the partial images, the method further includes, generating a motion artifact compensated image by reconstructing the object of interest using scan data and the estimated deformation.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:

FIG. 1 is an illustration of an exemplary helical computer tomography (CT) scanner, in accordance with an embodiment of the present invention.

FIG. 2 is an illustration of a portion of a helical trajectory and associated section pairs for estimating motion artifacts in a helical CT scan, in accordance with an embodiment of the present invention. The source trajectory is shown projected onto the plane perpendicular to the rotation axis.

FIG. 3 is an illustration of an auxiliary reconstruction volume for compensating for motion by reducing motion artifacts, in accordance with an embodiment of the present invention. The illustrated volume is visible from one source position.

FIG. 4 is an illustration of the overlap of volumes visible from the section pairs for estimating motion artifacts in a helical CT scan, in accordance with an embodiment of the present invention.

FIG. 5 is an illustration of the PI lines within the overlap volume, in accordance with an embodiment of the present invention.

FIG. 6 is a flow diagram illustrating a method for motion estimation and compensation in a helical CT scan, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates an exemplary embodiment of a helical CT scanner system 100 according to the present invention. This embodiment is intended to be exemplary and the present invention will be described as relating to medical imaging. However, this is not intended to be limited and various embodiments of the present invention may also be used for other purposes, such as baggage scanning for security and material analysis.

As shown in FIG. 1 , a helical CT scanner system 100 includes a gantry 102 which is rotatable around a rotational axis 105. The gantry 102 carries a radiation source 110 that forms a cone shaped radiation beam 115. The radiation beam 115 emitted from the radiation source 110 is focused on an object of interest 120 positioned in the center of the gantry 102. A radiation detector 125 is positioned on the gantry 102 opposite from the radiation source 110. The radiation detector 125 comprises a plurality of detector elements for measuring the intensity of the cone shaped radiation beam 115 after it passes through the object of interest 120.

During a scan of the object of interest 120, the radiation source 110 and the radiation detector 125 are rotated with the gantry 102 in a direction indicated 145. The object of interest 120 is additionally positioned on a movable table 130 which moves the object of interest 120 along a direction parallel to the rotational axis 105 of the gantry 102. As such, helical scan trajectory is created around the object of interest 120 and helical CT scan data of the object of interest is obtained.

Following the helical scanning of the object of interest 120, the radiation detector 125 provides the collected helical CT scan data to a data processor 135. The data processor 135 is adapted for reconstructing an image from the measurements provided by the radiation detector 125. The image generated by the data processor 135 may then be provided to a display 140 for subsequent viewing.

The data processor 135 is additionally adapted to perform motion estimation and motion compensation to correct for motion in the scan data provided by the radiation detector 125. Motion estimation and compensation may be performed by the data processor 135 as detailed below.

In the case of a relatively long helical scan trajectory C, each voxel is

reconstructed using its own section of C, denoted as
. Suppose that the parametric description of be the parametric interval corresponding to In a helical

CT scan, usually, the source of radiation moves along the scan trajectory C with a constant speed, so that the parameter s may be identified with time. Let f denote the dynamic

object, wherein s represents time. It is therefore implied that for every voxel
the value of / is reconstructed not at some reference time s0, which is the same for all
but at some time , wherein ff is the midpoint of the interval Accordingly, in the case of helical


scanning it is desired to reconstruct only the function


In a circular scan, the source "sees" the same FOV (field of view) at all times, and this allows one to reconstruct
In contrast, in a helical scan, the FOV constantly changes. This explains the difference between a circular scan and a helical scan and why a different motion artifact estimation and compensation method is required.

With reference to FIG. 2, it is necessary to find a volume which can be reconstructed using two sections of the helix 200 located at a distance π apart. As shown in FIG. 2, referring to these two sections of the helix by their corresponding parametric intervals as L and /+, interval /_ 205 is centered at the point s0 - π/2, 215 and the interval /+ 210 is centered at the point s„ + TT/2 220. S 230 identifies the direction of rotation of the scan. As a result of symmetry, if motion estimation (ME) is performed based on the partial angle constructions (PARs) computed from the two intervals, then the estimated motion model will correspond to the time s0 225.

Assume that Δα 235 represents the range of directions (in the parallel beam geometry) that is used for computing PARs, as shown in FIG. 2. Let be half the fan angle of the

ROI (region of interest), wherein r is center radius 240 and R 245 is the source-to-center of rotation distance. As such,

(1)


Thus, the width of each s-interval is


Once the mid-point s0 is fixed, the desired volume is the set of points x that are "visible" from all source positions
305 denote the set of points visible when the source is located at s 300, as shown in FIG. 3, then:

(2)

Then, the desired volume is
p A vertical cross-section 400 through Voverlap(s0), 405 is illustrated in FIG. 4. Since it is desirable to run the scan with a high pitch value, it is clear that the volume
405 is located rather close to the PI lines 410, and additionally PI lines 500 in FIG. 5., with one endpoint located inside /_(s0), and the other endpoint located inside I+(s0), as illustrated by one such PI line in FIG. 4.

Letting L(a, b) be the PI line passing through the points the following surface S(s0)

500 (sometimes referred to as a "potato chip") can be defined, as shown in FIG. 5:

The PI lines in (Eq. 3) have the following properties: their bottom endpoints are in L(s0) 205, their top endpoints are in /+(s0) 210, and they are symmetric about s0 225. When the helical pitch is high, Voverlap (s0) 405 is a fairly thin" set containing S(s0) 500. As such, the potato chips, 5(s0) 500 for different s0 225 do not intersect.

Based on the previous discussion, the present invention provides a method for motion estimation, which includes, first, choose a set of equispaced points,
covering the entire scan length and denote the reconstruction grid as


Second, perform motion estimation, which includes:

1) Loop over all st. For each st repeat the following steps.

2) For each
such that the midpoint of the PI interval of the voxel coincides with st. Thus,



with reference to Eq. 3. Also, find the top and bottom boundaries of
p }

3) Reconstruct two PARs /_ and /+ using the intervals L.(st) and L(s{), respectively. The PARs will be reconstructed in the coordinates (x.y.h). These coordinates are converted to physical coordinates by the formula

The value of ft is constrained to the interval


is sufficiently small so that the resulting volume is strictly

4) Perform image registration by finding the shifts

(4)


In discussing the geometric meaning of the minimization problem shown in Eq. 4, recall that the points all belong to the surface 5(st)- The term then

corresponds to the values of the symmetrically distorted surface inside the

volume / . By solving Eq. 4, it is guaranteed that the values of /+ and /_ on the two distorted

surfaces coincide. Once a solution to Eq. 4 is found, it is assumed that the points on the surface S(st) move according the following formula:

Since the sets
(s{) are fairly thin, one option is to register two surfaces rather than two volumes.

For additional noise stability, it may be required that motion vectors
be two-dimensional, i.e. have zero h -component. This way, Eq. 4 is equivalent to registering two 2D images. The downside of this approach is that motion away from the surface S(st) is ignored. If motion vectors are 3D, then the accuracy of the motion estimation is increased, but noise stability is decreased.

Since the problems of Eq. 4 are independent from each other for all 1 = 1, 2, to insure that the estimated motion vectors change smoothly from one I to the next, the problems of Eq. 4 can be combined for all I and a regularizer can be added that enforces smoothness of


along I.

The third step involves motion-compensated reconstruction. It is proposed that the most straight forward motion compensation reconstruction can be used, wherein motion is accounted for at the backprojection step. Thus, when reconstructing, all that is necessary is to find where any given voxel is located at the current time. This can be done by using the following method steps:

1) Given a voxel
, find its PI line.

2) Compute the mid-point smld of the PI parametric interval of the voxel. This implies that In other words, the potato chip containing

the given voxel is found.

3) Find the interval


4) Find the motion vector of the voxel by interpolating between



and


5) For a given source position/time s, the new position of the voxel is calculated by using the following formula, with reference to Eq. 5:

(6)


With reference to FIG. 6, a method for estimating and compensating for motion by reducing motion artifacts in an image reconstruction from helical computed tomography (CT) scan data of an object of interest is provided 600. The method 600 may be implemented using the data processor 135, the radiation source 110 and the radiation detector 125 of FIG 1.

At a first step 605, the method 600 includes collecting helical computer tomography (CT) scan data of an object of interest, wherein the scan data is acquired using a radiation source to generate a cone beam and a radiation detector to detect the cone beam.

Following the collection of scan data, the method continues at step 610 by selecting a plurality of center-points along a trajectory of the radiation source, followed by step 615 for identifying, for each of the plurality of center-points, a pair of sections along the trajectory of the scan data, wherein a first section of the pair of sections and a second section of the pair of sections are positioned on opposite sides of the center-point.

A typical center-point may be denoted s0, a first section of a pair may be denoted as /_(s0) and a second section of a pair may be denoted as /+(s0). The sections should not be too long, so that the amount of object motion during the time the source moves along the section is negligible. At the same time, the length of the section should not be too short, so that they would allow incomplete reconstruction of a part of the object of interest such that some of its features are clearly recognizable.

In one embodiment, the center-points are equidistant from each other along the trajectory scan and the length of the first and second sections in each pair are equal. In an additional embodiment, the positions of the center-points do not necessarily have to be uniform and the lengths of the sections do not have to be equal. For example, the center-point positions and lengths of pairs of sections along the trajectory of the radiation source could be based upon an external signal that includes motion information of the object of interest. In an exemplary embodiment, an ECG (electrocardiogram) signal could be collected concurrently with the helical scan data and, during times when the cardiac motion of a patient is slow, the center-points could be spaced farther apart, and when the cardiac motion of a patient is fast, the center-points could be spaced closer together. Accordingly, the motion information signal can be used to determine the spacing between each of the plurality of center-points and a location of each of the plurality of center-points. Similarly, when the cardiac motion of a patient is slow, the lengths of the sections in a pair of sections along the trajectory can be increased and when the cardiac motion of a patient is fast, the lengths of the sections in a pair of sections along the trajectory could be decreased. As such, the motion information signal from an external source, such as an ECG, could be used to improve the motion estimation and compensation method. At a next step 620, the method continues by reconstructing, for each pair of sections, a first partial image from the scan data of the first section and a second partial image from the scan data of the second section. The image reconstructed using scan data corresponding to I+(s0) is denoted as
and the image reconstructed using scan data corresponding to 7_(s0) is denoted as
. The set of points where the images are reconstructed should be the same for both sections. This set of points is denoted as V(s0). Due to the limited extent of the

detector, the set of points is not too large and is generally located in a small neighborhood of lines (chords) connecting points on I+(s0) with points on /_(s0). In particular, these reconstructions pertain to a subset of the object of interest. Since the sections of the trajectory /+(s0) and /_(s0) generally cover a limited angular range, reconstructions of


are referred to as Partial Angle Reconstructions (PARs). The reconstructions of and

do not nave to attempt to recover the attenuation coefficient exactly, they can instead reconstruct a modified image of the object, e.g. with enhanced edges. The main requirement for the reconstruction is that be very close to each other if the object does not

have any motion during the scan.

Image registration of the first partial image and the second partial image is then performed at step 625 to estimate a deformation that transforms the first partial image into the second partial image, wherein the deformation is representative of motion of the object of interest during the scan. In performing the image registration, the deformation that transforms one image into another is taken to be an estimate of the deformation that the object is undergoing in a neighborhood of the region V(s0) at the time close to s0. By interpolating motions estimated in regions V(s0) for all center-points s0, a global deformation function is obtained for a region of interest in the object of interest. This deformation function has the property that deformation at different points x in the object is estimated not for all times, but at fixed times, depending on the time interval when that point was irradiated by the radiation source.

At step 630, the method concludes by generating a motion compensated image by reconstructing the object of interest using the scan data and the estimated deformation.

As such, in various embodiments, the present invention provides an improved system and method for estimating motion and reducing motion artifacts produced during image reconstruction from helical computed tomography scan data

The above exemplary embodiment is not meant to be limiting and variations of the exemplary embodiment are within the scope of the present invention.

In general, the method is applicable for more general helical-like trajectories, e.g. variable pitch helices, helices in which the axis of rotation changes somewhat (e.g., as in thermal drift), etc.

Other methods may also be used for computing PARs. Such methods can be based on Local Tomography (LT), or exact or quasi-exact reconstruction.

Instead of estimating the motion (or, equivalents, deformation) of the object, the method can also be used for calibration of the scan (i.e. determination of relevant scan parameters, e.g. position of the central ray, or source-to-center of rotation distance, etc.) In an additional embodiment, the calibration can be performed on the fly during the scan or subsequent to the

scan. The calibration can also be performed by finding one set of parameters for the entire scan or by computing these parameters as functions along the scan trajectory.

In various embodiments, the source trajectory may consist of helical-like turns in one direction followed by helical-like turns in the opposite direction.

The proposed method of the present invention can be used in conjunction with other motion estimation algorithms. For example, if a preliminary motion model is obtained using fiducial markers, then the proposed algorithm can be used as a second step for finding an improved (i.e. more accurate) motion model.

The present invention may be embodied on various computing platforms that perform actions responsive to software-based methods. The following provides an antecedent basis for the information technology that may be utilized to enable the invention.

The computer readable medium described in the claims below may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any non-transitory, tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device.

A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device. However, as indicated above, due to circuit statutory subject matter restrictions, claims to this invention as a software product are those embodied in a non-transitory software medium such as a computer hard drive, flash-RAM, optical disk or the like.

Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wire-line, optical fiber cable, radio frequency, etc., or any suitable combination of the foregoing. Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, C#, C++, Visual Basic or the like and conventional procedural programming languages, such as the "C" programming language or similar programming languages.

Aspects of the present invention are described with reference to illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general-purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

It should be noted that when referenced, an "end-user" is an operator of the software as opposed to a developer or author who modifies the underlying source code of the software. For security purposes, authentication means identifying the particular user while authorization defines what procedures and functions that user is permitted to execute.

It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.