Abstract:
The present technique provides a novel method and apparatus for determining the motion of an internal tissue or organ undergoing dynamic movement such as a respiratory cycle. Particularly, the technique provides for a method and system for processing projection data to determine the motion rather than relying on additional data acquired from an external sensor. The motion is extracted from projection data by tracking the change over time in projections from the same projection angle and/or conjugate projection angle pairs. The extraction is possible because multiple gantry rotations occur in the time period of a single respiratory period. The motion information may in turn be used to facilitate data acquisition or image reconstruction, such as by gating techniques, to reduce or eliminate motion related artifacts.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to imaging systems and methods, and more particularly to systems and methods for extracting the motion of an object being imaged. 
     BACKGROUND OF THE INVENTION 
     Many medical imaging applications require knowledge of patient respiratory activity. Applications such as radiation therapy planning and pediatric imaging without breath hold can benefit from extraction of respiratory motion during the image acquisition process. 
     Current methodology utilizes an external sensor to obtain this respiratory activity or respiratory waveform since the breathing cycle exhibits different phases. An example of such a system uses the Varian Real-Time Position Management (RPM) Respiratory Gating System. See, Mageras G. et al.,  Initial clinical evaluation of a respiratory gating radiotherapy system , Engineering in Medicine and Biology Society, 2000, Proceedings of the 22nd Annual International Conference of the IEEE (2000), Volume 3, Pages 2124-2127. The RPM tool places a small reflective marker on the chest wall of the patient, and a video monitor tracks its position. Video analysis and signal processing steps produce the respiratory waveform based on the movement of the sensor throughout the respiratory cycle. In addition to complicating the patient setup and radiotherapy planning workflow, these respiratory monitoring devices can be expensive. 
     In four dimensional Computed Tomography (4DCT) respiratory gating utilizes a small reflective marker placed on the patient&#39;s chest or abdomen that can be detected by an infrared camera placed at the foot of the patient table. Respiratory gating allows therapists to track the patient&#39;s respiratory cycle both at the time of the CT scan for imaging and at the time of treatment. As the patient breathes, the marker is vertically displaced according to the patient&#39;s respiratory cycle and the infrared camera records the position of the marker. This information is then used to determine the patient respiratory phase at the time of Computed Tomography (CT) image acquisition. CT images are generated at multiple locations along the patient&#39;s chest and abdomen. The CT images are then retrospectively sorted into phases according to the information recorded by the infrared camera. The result of reconciling the images to the infrared camera respiratory waveform is a four dimensional (4D) image set. This 4D image set consists of a 3D volumetric image data set generated for each of multiple respiratory phases, image set of the lungs and surrounding area throughout the respiratory cycle. This is particularly useful for radiation therapy, as it is clinically relevant to understand the motion of lung and abdominal tumors throughout the respiratory cycle in order to properly plan radiation treatment. As radiation therapy technology emerges, high quality 4D reconstructions and easily implemented respiratory gating capability become increasingly important for precise tumor tracking and minimization of patient preparation and planning time. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for extracting motion from projection data that is directly related to actual respiratory activity. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The above-mentioned shortcomings, disadvantages and problems are addressed herein, which will be understood by reading and studying the following specification. 
     The present technique provides a novel method and apparatus for determining the motion of an internal tissue or organ undergoing dynamic movement such as a respiratory cycle. Particularly, the technique provides for a method and system for processing acquired projection data to determine the motion. The motion information may in turn be used to facilitate data acquisition or image reconstruction, such as by gating techniques, to reduce or eliminate motion related artifacts. 
     In one aspect, a method is provided for extracting motion from an acquired set of projection data. The acquired set of projection data is analyzed to extract timing information by tracking the changes over time of the center of mass of the tissue being monitored. Phase calculations are then performed on the timing information from one or more projection angle. The extracted motion waveform is used to synchronize CT images produced by a CT detector system with the motion of the tissue. 
     In another aspect, a method is provided for extracting motion from an acquired set of projection data. The acquired set of projection data is analyzed to extract timing information by tracking the changes over time of the center of mass of the tissue being monitored. Peak, phase, rate of change calculations are then performed on the timing information from one or more projection angle. 
     In yet another aspect, an apparatus is provided for extracting motion from projection data acquired a CT detector system. A processor, storage device, and software means are combined to acquire projection data to track the changes over time of the center of mass of a tissue being monitored. The processor as directed by the software means then samples the data at one or more projection angle to produce a motion waveform expressing the respiratory cycle. 
     In still another aspect, a computer-accessible medium having executable instructions for extracting motion from a rotating gantry in a CT detector system, the executable instructions capable of directing a processor to acquire projection data and to process the acquired projection data to produce a motion waveform expressing the respiratory cycle. 
     Systems, clients, servers, methods, and computer-readable media of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the drawings and by reading the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a system-level overview of an embodiment for acquiring projection data; 
         FIG. 2  is a block diagram of a hardware and operating environment in which different embodiments can be practiced for extracting motion from projection data; 
         FIG. 3  is a system-level overview illustrating the relationship between center of mass and respiratory phase. 
         FIG. 4  is a representation of projection intensity, removing the center portion of the projection data, and projection data represented as a sinogram. 
         FIG. 5  is a flowchart of a method for extracting motion from projection data according to an embodiment; 
         FIG. 6  is a flowchart of a method for extracting motion from projection data and for synchronizing captured images to the extracted motion according to an embodiment; and 
         FIG. 7  is a diagram of the projection data, the projection data sampled at a first projection angle, the projection data sampled at a second projection angle and conjugate sampling of the projection data in accordance to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken in a limiting sense. 
       FIG. 1  is a block diagram of an overview of a system for acquiring medical images. CT imaging system  100  solves the need in the art for managing advanced medical diagnostic imaging applications executing on a network or a computer. CT imaging system  100  includes a gantry  103 , table  106 , controllers  108 , master controller, and image reconstruction device  118 . It should be noted that other data acquisition systems are envisioned including a magnetic resonance (MRI) imaging system, a positron emission tomography (PET) system, a single photon emission computed tomography (SPECT) system, an ultrasound system, or an X-ray system. The data acquisition system obtains data including, but not limited to image data, functional image data, and temporal image data. Further examples of data include voxel data including volume information for a three dimensional region of interest (ROI), pixel data including area information for a two dimensional region of interest, and spatio-temporal data. Spatio-temporal data includes area or volume information over a selected, predetermined time period. 
     CT imaging system  100  includes a gantry  103  having an X-ray source  102 , a radiation detector array  104 , a patient support structure and a patient cavity, wherein the x-ray source  102  and the radiation detector array  104  are diametrically disposed so as to be separated by the patient cavity. In an exemplary embodiment, a patient (not shown) is disposed upon the patient support structure, which is then disposed within the patient cavity. The x-ray source  102  projects an X-ray beam toward the radiation detector array  104  so as to pass through the patient. In an exemplary embodiment, the X-ray beam is collimated by a collimate (not shown) so as to lie within an X-Y plane of a Cartesian coordinate system known to those in the art as the imaging plane. After becoming attenuated by the patient passing through, the attenuated X-ray beam is received by the radiation detector array  104 . In the preferred embodiment, the radiation detector array  104  includes a plurality of detector elements wherein each of said detector elements receives an attenuated X-ray beam and produces an electrical signal responsive to the intensity of the attenuated x-ray beam. 
     In addition, the X-ray source  102  and the radiation detector array  104  can rotate relative to the gantry  103  and the patient support structure, so as to allow the X-ray source  102  and the radiation detector array  104  to rotate around the patient support structure when the patient support structure is disposed within the patient cavity. X-ray projection data is obtained by rotating the X-ray source  102  and the radiation detector array  104  around the patient during a scan. The X-ray source  102  and the radiation detector array  104  communicate with a control mechanism  108  associated with the CT imaging system  100 . The control mechanism  108  controls the rotation and operation of the x-ray source  102  and the radiation detector array  104 . 
     The table controller  110 , X-ray controller, gantry motor controller, DAS  116 , image reconstruction  118 , and master controller  120  have the same hardware and capabilities limited only by the programming in each respective device. For the purpose of the description, all controllers are presumed to have the same hardware so a discussion to one applies to all. The master controller  120  provides computer hardware and a suitable computing environment in conjunction with which some embodiments can be implemented. Embodiments are described in terms of a computer executing computer-executable instructions. However, some embodiments can be implemented entirely in computer hardware in which the computer-executable instructions are implemented in read-only memory. Some embodiments can also be implemented in client/server computing environments where remote devices that perform tasks are linked through a communications network. Program modules can be located in both local and remote memory storage devices in a distributed computing environment. 
     The master controller  120  includes a processor, commercially available from Intel, Motorola, Cyrix and others. Master controller  120  also includes random-access memory (RAM), read-only memory (ROM), and one or more mass storage devices  124 , and a system bus that operatively couples various system components to the processing unit of master controller  120 . The memory and mass storage devices are types of computer-accessible media. Mass storage devices are more specifically types of nonvolatile computer-accessible media and can include one or more hard disk drives, floppy disk drives, optical disk drives, and tape cartridge drives. The computer readable medium can be an electronic, a magnetic, an optical, an electromagnetic, or an infrared system, apparatus, or device. An illustrative, but non-exhaustive list of computer-readable mediums can include an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (magnetic), a read-only memory (ROM) (magnetic), an erasable programmable read-only memory (EPROM or Flash memory) (magnetic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). Note that the computer readable medium may comprise paper or another suitable medium upon which the instructions are printed. For instance, the instructions can be electronically captured via optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. The processor in the master controller executes computer programs stored on the computer-accessible media. 
     Master controller  120  can be communicatively connected to the Internet  126  via a communication device. Internet  126  connectivity is well known in the art. In one embodiment, a communication device is a modem that responds to communication drivers to connect to the Internet via what is known in the art as a “dial-up connection.” In another embodiment, a communication device is an Ethernet® or similar hardware network card connected to a local-area network (LAN) that itself is connected to the Internet via what is known in the art as a “direct connection” (e.g., T1 line, etc.). 
     A user enters commands and information into the master controller  120  through input device  122  such as a keyboard or a pointing device. The keyboard permits entry of textual information into master controller  120 , as known within the art, and embodiments are not limited to any particular type of keyboard. Pointing device permits the control of the screen pointer provided by a graphical user interface (GUI) of operating systems such as versions of Microsoft Windows®. Embodiments are not limited to any particular pointing device. Such pointing devices include mice, touch pads, trackballs, remote controls and point sticks. Other input devices (not shown) can include a microphone, joystick, game pad, satellite dish, scanner, or the like. For the purpose of this description, a keyboard and a pointing device are referred to as a user interface (UI). The UI allows the user to interact with any of components in the imaging system, algorithm within any of before mentioned devices, or structure or firmware in any of the devices. The output device is a display device. Display device is connected to a system bus. Display device permits the display of information, including images, text, video and other information, for viewing by a user of the computer. Embodiments are not limited to any particular display device. Such display devices include cathode ray tube (CRT) displays (monitors), as well as flat panel displays such as liquid crystal displays (LCD&#39;s). In addition to a monitor, computers typically include other peripheral input/output devices such as printers (not shown). The controllers also include an operating system (not shown) that is stored on the computer-accessible media RAM, ROM, and mass storage device  124 , and executed by the processor in the controller. Examples of operating systems include Microsoft Windows®, Apple MacOS®, Linux®, UNIX®. Examples are not limited to any particular operating system, however, and the construction and use of such operating systems are well known within the art. 
     Master controller  120  can be operated using at least one operating system to provide a graphical user interface (GUI) including a user-controllable pointer. Master controller can have at least one web browser application program executing within at least one operating system, to permit users of the controller to access intranet or Internet world-wide-web pages as addressed by Universal Resource Locator (URL) addresses. Examples of browser application programs include Netscape Navigator® and Microsoft Internet Explorer 
     In an exemplary embodiment, the control mechanism  108  includes an X-ray controller  112  communicating with an X-ray source  102 , a gantry motor controller  114 , and a data acquisition system (DAS)  116  communicating with a radiation detector array  104 . The X-ray controller  112  provides power and timing signals to the X-ray source  102 , the gantry motor controller  114  controls the rotational speed and angular position of the x-ray source  102 , and the radiation detector array  104  and the DAS  116  receive the electrical signal data produced by detector elements  104  and convert this data into digital signals for subsequent processing. In an exemplary embodiment, the CT imaging system  100  also includes an image reconstruction device  118 , a data storage device  124  and a master controller  120 , wherein the processing device  120  communicates with the image reconstruction device  118 , the gantry motor controller  114 , the x-ray controller  112 , the data storage device  124 , an input and an output device  122 . The CT imaging system  100  can also include a table controller  110  in communication with the master controller  120  and the patient support structure, so as to control the position of the patient support structure relative to the patient cavity. 
     In accordance with the preferred embodiment, the patient is disposed on the patient support structure, which is then positioned by an operator via the master controller  120  so as to be disposed within the patient cavity. The gantry motor controller  114  is operated via master controller  120  so as to cause the X-ray source  4  and the radiation detector array  6  to rotate relative to the patient. The X-ray controller  112  is operated via the master controller  120  so as to cause the X-ray source  102  to emit and project a collimated X-ray beam toward the radiation detector array  104  and hence toward the patient. The X-ray beam passes through the patient so as to create an attenuated X-ray beam, which is received by the radiation detector array  104 . 
     The detector elements  104  receive the attenuated X-ray beam, produce electrical signal data responsive to the intensity of the attenuated x-ray beam and communicate this electrical signal data to the DAS  116 . The DAS  116  then converts this electrical signal data to digital signals and communicates both the digital signals and the electrical signal data to the image reconstruction device  118 , which performs high-speed image reconstruction. This information is then communicated to the master controller  120 , which stores the image in the data storage device  124  and displays the digital signal as an image via output device  122 . The information communicated to the master controller  120  is referred to as ROI image data. In accordance with an exemplary embodiment, the output device  122  includes a display screen having a plurality of discrete pixel elements. 
       FIG. 2  illustrates a system overview of a device  200  for extracting motion from projection data acquired with a CT detector system  100  in  FIG. 1 . The system comprises a CT detector system ( 202 ), processor  204 , storage device  206 , and software device  208  having objects  210  for performing analysis on projection data from the CT detector system  202 . The software objects  210  perform sampling of projection data at a desired set of projection angles. Further, an object such as a mask operator can crop or truncate the projection data to remove unwanted regions in an image. Further, the software modules can track the change over time in projections from the same projection angle and/or conjugate projection angle pair to derive peak, phase, and rate for the respiratory cycle (respiratory waveform). The respiratory cycle can be used to synchronize image data with the quiescent period of the respiratory cycle, avoiding motion artifacts. This allows patients with difficulty controlling their breath to have data collected during free breathing and to free scan room from the need to accommodate sensing devices for tracking the respiratory cycle. 
       FIG. 3  illustrates the overall concept of extracting motion from projection data. As shown projection data is acquired, as depicted by action  502  of  FIG. 5 , such as by the helical operation of a multi-slice detector CT system or by operation of a volumetric CT system. Item  302  is an axial slice derived from the acquired projection data. The projection data is analyzed/processed  304  over the available respiratory cycles. In the case of a helical acquisition, the projection data may include both the measured projection data as well as interpolated projection data generated by a helical interpolation technique, as is well known to those skilled in the art. All or part of the projection data may be analyzed by applying a mask operator. In particular, subsequent analysis may be limited to a subset of the projection data if the subset corresponding to the region of interest can be identified and extracted. Analysis of the subset of projection data may provide a superior signal-to-noise ratio in the region of interest compared to analysis of the full set of projection data. The center of mass correlates with the respiratory cycle as shown in item  306 . Since respiratory motion is relatively slow (10-20 breaths per minute) and the gantry mechanism rotates relatively quickly (less than or equal to 1 s per rotation) the respiratory cycle can be accurately captured with a CT detector system  100 . 
       FIG. 4  shows projection data represented in Cartesian coordinate  402  and projection data represented as a sinogram  410 . Rotation of the gantry assembly  114  around a subject results in X-ray data being acquired by detector elements (not shown) for a range of view angles, θ. A typical detector array  104  may comprise several hundred individual detector elements, such as 888 individual elements. The array  104  is positioned on the gantry at a predetermined distance (m) from the x-ray source  102 . The circular path of source  102  has a radius that is proportional to the predetermined distance. Particular values of these parameters are not critical to the present invention and may be varied according to well-known principles of CT system design. One complete gantry rotation for the gantry  114  may comprise, for example, 984 view angles. Source  102  is thereby positioned to illuminate the subject successively from  984  different directions (θ) and the detector array  104  generates X-ray data at each view angle (θ), from which projection data for separate projection views are acquired. In the Cartesian projection data  402  the x-axis is CT detector channel  404  and y-axis is attenuation value  404 . Each detector element (channel  404 ) produces a signal affected by the attenuation of the X-ray beams, and the data are processed to produce signals that represent the line integrals of the attenuation coefficients of the object along the X-ray paths. These signals are typically called projection data or just projections. The channel index α(=1, 2, . . . , α) is a sequential number given to the channels in each detector row. 
     There are occasions when the dynamics of certain organs can effect the determination of the respiratory cycle.  FIG. 3  shows an example axial slice at peak inspiration. Due to the liver motion  308  in the z-direction, the slight change in the chest-wall height is difficult to detect by center-of-mass based approaches alone. The x-dimension and y-dimension center-of-mass of the slice is dominated by the liver motion  308  in the z-dimension. An approach is presented in  FIG. 4  that focuses the center-of-mass analysis on the outside portion of the region of interest where the respiratory signal is most evident. The projections with the center portion removed. The interference from the liver ( FIG. 3 ) is eliminated. This is accomplished by masking the inner portion of each projection. This mask is calculated for each fixed projection angle as shown in  FIG. 4 . A threshold is set above which the projection is masked. In this case, roughly thirty five percent (35%) of the maximum projection value is used. The resulting threshold projection data is shown in  FIG. 4 . This allows center-of-mass calculations to focus on the motion of the chest wall and the inner portion  408  has been masked out. The projection data with the liver motion removed is represented as a sinogram  412 . 
     In the previous section, a system level overview of the operation of an embodiment was described. In this section, the particular methods of such an embodiment are described by reference to a series of flowcharts. Describing the methods by reference to a flowchart enables one skilled in the art to develop such programs, firmware, or hardware, including such instructions to carry out the methods on suitable computers, executing the instructions from computer-readable media. Similarly, the methods performed by the server computer programs, firmware, or hardware are also composed of computer-executable instructions. Methods  500 - 600  are performed by a program executing on, or performed by firmware or hardware that is a part of, a computer, such as computer or master controller  120  in  FIG. 1   
       FIG. 5  is a flowchart of a method  500  for extracting motion from projection data according to an embodiment. Method  500  solves the need in the art for extracting motion from projection data that is directly related to actual respiratory activity. 
     Method  500  begins with the acquisition of projection data in action  502 . The acquire projection data  502  is generally selected such that sufficient information is provided to calculate locations of features causing X-ray attenuation. For example, consider CT projection data in which 1000 projections are acquired per gantry rotation. In this case, each of the 1000 projection angles can be tracked over time if multiple gantry rotations occur during the acquisition. Thus, in one single rotation of the gantry a single set of projection data (1000 projections) is acquired. Since the goal is to capture the respiratory cycle we need to acquire multiple sets of projection data. To acquire multiple sets of projection data we need an equal number of gantry rotations. In the case where the gantry mechanism rotates relatively quickly (less than or equal to 1 s per rotation) a seven rotation cycle is ample for capturing the respiratory cycle. To summarize, a single rotation produces 1000 projections, a seven cycle gantry rotation will produce 7000 projections (7 rotation×1000 projections/rotation). Once the set of projection data has been acquired in action  502  control passes to action  504  for further processing. 
     In action  504 , timing information indicative of motion is extracted from the projection data. The extraction of timing information flows from the center of mass (CM) of the projections from different angles. The unit of the CM output is a detector column index. For example, prepped scan data has about 900 detector channels, so the CM for each projection would generally be in the range of 400-500. See items  402  and  406  in  FIG. 4 . It should be noted that this is only a one-dimensional CM value, not a set of coordinates. This is the CM calculation in the orthogonal direction to the projection angle. One can refer to this CM calculation as a function of time as ξ(t). The one-dimensional discrete CM formula used is: 
                 ξ   ⁡     (   t   )       =         ∑   x             ⁢     x   ·     p   ⁡     (     x   ,   t     )               ∑   x             ⁢     p   ⁡     (     x   ,   t     )             ,         
Where t is the time into the current acquisition. Note that typically projection space is referred to as p(x,φ), but in our case with multiple gantry rotations and object motion we will use p(x,t). The projection angle φ also varies periodically with t. The resulting CM function ξ(t) is shown at item  702  of  FIG. 7 . Note that ξ(t) spans all projection angles, and the calculation of ξ(t) is dominated by the periodic gantry rotation as demonstrated in  702  of  FIG. 7 . Once the timing information has been extracted control passes to action  506  for further processing.
 
     In action  506 , the timing information is processed to produce a motion waveform that represents the respiratory cycle. In order to extract a CM waveform for each fixed conjugate projection angle pair, ξ(t) is sampled accordingly for each desired projection angle pair. Due to the conjugate nature of sampling, two sample points are available per gantry rotation for the signal produced from each projection angle pair. This provides the CM as a function of time for the axis perpendicular to the projection angle φ, which we will refer to as ξ(t). Qualitatively, it can be understood that a basic respiratory waveform can be seen from this plot when φ represents a horizontal projection angle. This sampling will produce a large number of ξ φ (t) waveforms over a large range of projection angles. These many waveforms can be processed and combined using many methods. If projection angles are collected at uniform increments, then they may simply be resample to the CM sampling points and summed together. This will produce a very reliable output respiratory waveform. 
       FIG. 6  is a flowchart of a method  600  for extracting motion from projection data according to an embodiment and applying the extracted motion to synchronize CT images. Method  600  solves the need in the art for extracting motion from projection data that is directly related to actual respiratory activity. 
     Method  600  begins with the acquisition of projection data. The projection data as shown above with action  502  are acquired by rotating the gantry  114  in the CT system  100 . The acquired projection data set  602  contains the projection information that characterizes respiratory period at one point in time. By continuously rotating the detector  104  the periodicity information (rotation of the gantry) may be used to extract the periodicity of the center of mass (CM) at the region of interest, as depicted at action  504  of  FIG. 5 . Once the projection data has been acquired in action  602  control passes to action  604  for further processing. 
     In action  604 , timing information is extracted from the acquired projection data. Since the timing information is a time-varying signal, various characteristics of the signal (magnitude, phase, rate of change) are useful for synchronizing the associated imaging data. As noted earlier the time-varying signal may be a signal representative of the respiratory flows into and out of the slice of tissue being monitored by the CT detector system. This timing information can be determined by sampling the projection data  602  at one or more projection angle. The method takes advantage of multiple projections from the same or conjugate projection angles (rotating gantry) to track motion. After the timing information has been extracted control passes to action to  606  for further processing. 
     In action,  606  the timing information is sampled at different projection angles. Edge detection, center-of-mass analysis, projection sum tracking, correlation-based approaches, and an array of other approaches may be implemented. The concept remains, however, that projection differences are tracked over time for a fixed projection angle and optionally its conjugate projection angle pair, meaning the projection angle differing by 180 degrees. In CT, this approach would analyze a fixed projection angle over time that is acquired once per gantry rotation, and twice if the conjugate projection is used. As a clear extension, a separate signal could be extracted for many projection angles over time, and then these can be processed together to derive a motion signal. The sampling of the projection data at around 270 degrees, and around 90 degrees, and conjugate sampling at 270 and 90 degrees should provide an adequate amount of continuous data points to reflect an accurate waveform of the respiratory cycle. Once the sample respiratory cycle has been determined control passes to action  608  for further processing. 
     In action  608 , the CT images are synchronized to the respiratory cycle. After the timing information has been processed to provide the desired temporal correspondence with the motion of the respiratory phase, the imaging data is synchronized with the motion. In addition, the extracted motion can be used in respiratory gating in order to decrease the irradiation of healthy tissue. That is, the margins that are added in order to ensure that the tumor is within the applied field can be reduced by using respiratory gating, and thereby the proportion of irradiated healthy tissue can be decreased. As described above, the processing may yield a time value such as the start of a motion, or of another phase or temporal event during the motion or it may yield a trigger signal of some kind. Synchronizing of the imaging data can be done prospectively by using a trigger signal output to initiate acquisition of imaging data or retrospectively by comparing the time value of the start of a motion to time stamps associated with the imaging data or internal gating. See P. Salla, G. Avinash, S. Sirohey, and T. Pan. “Systems and methods for retrospective internal gating.” US Patent Application 2004/0258286 A1. The disclosure in US Patent Application 2004/0258286 A1 is hereby incorporated by reference in its entirety. After synchronizing the CT images to the respiratory cycle control passes to action  610  for further processing. 
     In action  610 , the CT images are displayed or produced in a suitable medium. In particular, the user can use the user interface  122  firstly to display reconstructed images output from the means for image reconstruction  118 , and secondly to define parameters useful for the master controller  120  to extract regions of interest from the reconstructed images in order to generate images that can be used by the user. The user interface  122  includes a window and virtual buttons. The user can use the window to display reconstructed images and destination images in which the results of operations carried out by the user on the reconstructed images are stored. Each reconstructed image is associated with a corresponding destination image. 
     In some embodiments, methods  500 - 600  are implemented as a computer data signal embodied in a carrier wave, that represents a sequence of instructions which, when executed by a processor, such as processor  204  in  FIG. 2 , cause the processor to perform the respective method. In other embodiments, methods  500 - 600  are implemented as a computer-accessible medium having executable instructions capable of directing a processor, such as processor  204  in  FIG. 2 , to perform the respective method. In varying embodiments, the medium is a magnetic medium, an electronic medium, or an optical medium. 
     Referring to  FIG. 7 , a particular implementation  700  is described in conjunction with the system overview in  FIGS. 1 and 2  and the methods described in conjunction with  FIGS. 6 and 7 . Curves  702 ,  708 , and  710  demonstrate the process of obtaining a single X f, filt (t) waveform  712 . The x-axis is projection number, and the y-axis is detector channel. The projection data and the x(t) curve is plotted in curve  702 . Label  706  represents the samples of x(t) each time the gantry passes the 270 degree projection angle. Label  704  represents samples at the 90 degree projection angle. Curve  708  is a plot of the samples of the x(t) function for the 270 degree projection angles. Curve  710  is a plot of 888 minus the samples of the x(t) function for the 90 degree projection angles. The number 888 is chosen because it is the number of detector channels. Curve  712  is the result of combining the signals from curve  708  and  710  in a conjugate sampling fashion and filtering to remove the offset. This is the waveform X f, filt (t) for f at 90 degrees. 
     Apparatus  120  components of the CT detector system  100 , and the image reconstruction  118  can be embodied as computer hardware circuitry ( FIG. 2 ) or as a computer-readable program, or a combination of both. In another embodiment, system  100  is implemented in an application service provider (ASP) system. 
     More specifically, in the computer-readable program embodiment, the programs can be structured in an object-orientation using an object-oriented language such as Java, Smalltalk or C++, and the programs can be structured in a procedural-orientation using a procedural language such as COBOL or C. The software components communicate in any of a number of means that are well-known to those skilled in the art, such as application program interfaces (API) or interprocess communication techniques such as remote procedure call (RPC), common object request broker architecture (CORBA), Component Object Model (COM), Distributed Component Object Model (DCOM), Distributed System Object Model (DSOM) and Remote Method Invocation (RMI). The components execute on as few as one computer as in computer or master controller  120  in  FIG. 1 , or on at least as many computers as there are components. 
     CONCLUSION 
     A method and apparatus for extracting motion from projection data is described. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations. For example, the embodiment was described in the context of the cine acquisition method. In general, similar techniques may be applied to helical scanning methods by applying a low-pass filter at the end to remove bias effects from patient translation. The methodology relies on tracking center of mass (CM), or more specifically, changes in the scan data due to changes in chest and abdominal anatomy throughout the respiratory cycle. Alternatively, one could consider using an external marker on the patient&#39;s chest that could be tracked in the CT raw projection data to serve as a surrogate marker for respiratory phase. Additionally, while described in procedural terms, one of ordinary skill in the art will appreciate that implementations can be made in an object-oriented design environment or any other design environment that provides the required relationships. In particular, one of skill in the art will readily appreciate that the names of the methods and apparatus are not intended to limit embodiments. Furthermore, additional methods and apparatus can be added to the components, functions can be rearranged among the components, and new components to correspond to future enhancements and physical devices used in embodiments can be introduced without departing from the scope of embodiments. One of skill in the art will readily recognize that embodiments are applicable to future communication devices, different file systems, and new data types.