Abstract:
A method for determining a change in a position of a support is described. The method includes determining the change in the position of the support used in an imaging system, where determining the change includes computing the position by operating a photodetector configured to detect laser energy that provides information regarding the position.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to imaging systems and more particularly to, systems and methods for determining a position of a support used within an imaging system.  
         [0002]     In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an x-y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through a subject, such as a patient, being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.  
         [0003]     In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the subject to be imaged so that the angle at which the x-ray beam intersects the subject constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the subject includes a set of views made at different gantry angles or view angles, during one revolution of the x-ray source and detector.  
         [0004]     In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the subject. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.  
         [0005]     As CT scanners continue to use smaller detector element or cell sizes, a plurality of constraints applied to table positioning accuracy and stability become more stringent. In addition, reconstruction techniques using variable table speed during the CT scan make an accurate measurement of table position more critical.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0006]     A method for determining a change in a position of a support is described. The method includes determining the change in the position of the support used in an imaging system, where determining the change includes computing the position by operating a photodetector configured to detect laser energy that provides information regarding the position.  
         [0007]     A system for determining a change in a position is described. The system includes an imaging system comprising a support, a photodetector configured to detect laser energy that provides information regarding the position of the support, and a processor configured to compute the change in the position of the support.  
         [0008]     A system for determining a change in a position of a support is provided. The system includes an x-ray source configured to generate x-rays that pass through a subject, an x-ray detector configured to detect the x-rays after the x-rays pass through the subject, a support configured to support the subject, a photodetector configured to detect laser energy that provides information regarding the position, and a processor configured to compute the change in the position of the support.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is an isometric view of an embodiment of a computed tomography (CT) system.  
         [0010]      FIG. 2  is a block diagram of an embodiment of the CT system of  FIG. 1 .  
         [0011]      FIG. 3  is a top view of an embodiment of a system for determining a position of a support.  
         [0012]      FIG. 4  is a side view of an embodiment of a system for determining a position of a support.  
         [0013]      FIG. 5  is a front view of an embodiment of a system for determining a position of a support. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     Referring to  FIGS. 1 and 2 , a computed tomography (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of gantry  12 .  
         [0015]     The beam of x-rays generated by x-ray source  14  is collimated by a collimator. The collimated x-ray beams generated by x-ray source  14  are shaped like a fan. The collimated x-ray beams then pass through a subject  22 , such as a medical patient or a phantom, located along a z-axis. In an alternative embodiment, an object, such as a bag or a box, may be scanned by CT imaging system  10 .  
         [0016]     Detector array  18  includes a plurality of detector elements  20  which together sense the projected x-rays that pass through subject  22 . Each detector element  20  produces an analog electrical signal or an analog trigger signal that represents an intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through subject  22 . During a scan to acquire x-ray projection data, gantry  12  and a plurality of components mounted thereon rotate about a center of rotation  24 . Detector array  18  may be fabricated in a single slice or multi-slice configuration. In a multi-slice configuration, detector array  18  has a plurality of rows of detector elements  20 , one of which is shown in  FIG. 2 .  
         [0017]     Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples a plurality of analog trigger signals  33  or projection data from detector elements  20  and converts the analog trigger signals  33  to a plurality of digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed image reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 .  
         [0018]     Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a support motor controller  44  which controls a motorized support  46 , such as a couch or a pad. Support  46  supports subject  22  to position subject  22  in gantry  12 . Particularly, support  46  moves portions of subject  22  through a gantry opening  48 . Support  46  is located on top of a table or base  48 , and support  46  moves with respect to table  48 . Support motor controller  44  controls movement of support  48  in at least one of an x-direction, a y-direction, and a z-direction. The x-direction is parallel to an x-axis, the y-direction is parallel to a y-axis, and the z-direction is parallel to the z-axis.  
         [0019]     Although the specific embodiment mentioned above refers to a third generation CT system  10 , a fourth generation CT system that has a stationary detector and a rotating x-ray source or a fifth generation CT system that has a stationary detector and a stationary x-ray source may be used instead of the third generation CT imaging system  10 . In another alternative embodiment, an x-ray system including an x-ray source and an x-ray detector may be used instead of the CT imaging system  10 . In an alternative embodiment, systems and methods for determining a position of a support apply to other imaging systems, such as, a positron emission tomography (PET) imaging system, a magnetic resonance imaging (MRI) system, a CT-PET system, an ultrasound imaging system, and any other imaging system that includes support  46 .  
         [0020]     Additionally, although the herein described methods are described in a medical setting, it is contemplated that the benefits of the methods accrue to non-medical imaging systems such as those systems typically employed in an industrial setting or a transportation setting, such as, for example, but not limited to, a baggage scanning system for an airport, other transportation centers, government buildings, office buildings, and the like. The benefits also accrue to micro PET and CT systems which are sized to study lab animals as opposed to humans.  
         [0021]      FIG. 3  is a block diagram of an embodiment of a system  100  for determining a position of a support. System  100  includes an interferometer  102 , a controller  104 , a reflective surface  106 , and support  46 . Interferometer  102  is attached, such as glued and/or bolted, to a support plate  108 . In an alternative embodiment, system  100  may not include support plate  108  and interferometer  102  is attached, such as glued and/or bolted, to table  48 . Support plate  108  is attached, such as glued and/or bolted, to table  48 . As used here, the term ‘controller’ is not limited to just those integrated circuits referred to in the art as a controller, but broadly refers to a computer, a processor, a microcontroller, a microcomputer, a programmable logic controller, an application specific integrated circuit, and any other programmable circuit, and these terms are used interchangeably herein. Alternatively, controller  104  can be any of support motor controller  44 , x-ray controller  28 , gantry motor controller  30 , and computer  36 . Interferometer  102  includes a laser source  110 , a beam splitter  112 , a plurality of reflective surfaces  114  and  116 , and a photodetector  118 , such as a laser beam detector. Examples of any of reflective surfaces  106 ,  114 , and  116  include a prism and a mirror. Reflective surface  106  is attached, such as bolted and/or glued, to a side surface  120  of support  46  so that reflective surface  106  faces interferometer  102  and is in a line-of-sight of beam splitter  112 . In an alternative embodiment, side surface  120  of support  46  is reflective and system  100  excludes reflective surface  106 . In another alternative embodiment, reflective surface  106  is attached to subject  22  to determine a position of subject  22 . In another alternative embodiment, reflective surface  106  is attached to the object to determine a position of the object.  
         [0022]     Laser source  110  generates a laser beam  122  that is directed towards reflective surface  114 . An example of interferometer  102  includes a Michelson interferometer that measures a position of support  46  with a variance ranging from 0.1 micron to less than 1 micron, which is a wavelength of laser beam  122 . In an alternative embodiment, interferometer  102  does not include reflective surface  114  and laser source  110  is placed so that laser beam  122  is incident directly on beam splitter  112 . Laser beam  122  is directed by reflective surface  114  towards beam splitter  112 . Beam splitter  112  splits laser beam  122  into a plurality of laser beams  124  and  126 . Beam splitter  112  directs laser beam  126  towards reflective surface  106  and laser beam  124  towards reflective surface  116 . Reflective surface  106  reflects laser beam  126  to generate a laser beam (not shown) that is directed towards beam splitter  112 . Reflective surface  116  reflects laser beam  124  to generate a laser beam (not shown) that is directed towards beam splitter  112 . Beam splitter  112  receives the laser beams reflected from reflective surfaces  106  and  116  and splits the laser beams to generate a laser beam  128  that includes an interference fringe pattern. Beam splitter  112  directs laser beam  128  towards photodetector  118 . Photodetector  118  detects the interference fringe pattern within laser beam to generate a plurality of electric pulses  130 . For example, photodetector  118  generates a single electric pulse upon detecting one fringe within the interference fringe pattern. The interference fringe pattern is generated upon movement of support  46  along the z-axis. In an alternative embodiment, instead of interferometer  102 , a commercially available device, such as a laser distance sensor available from Keyence™ Corporation, is used to generate electric pulses  130 . Another example of the commercial available device includes a system that measures a position of support  46  with a variance ranging from and including 0.01 millimeters (mm) to 0.1 mm, and that determines a position of support  46  at a frequency ranging up to 50 kilohertz (kHz).  
         [0023]     Controller  104  receives electric pulses  130  from photodetector  118 , converts electric pulses  130  from an analog to a digital form, counts a number of electric pulses  130 , and determines a z-position of support  46  based on the count. Controller  104  includes an analog-to-digital converter that converts electric pulses  130  from an analog to a digital form. Alternatively, the analog-to-digital converter is located outside controller  104 . Controller  104  counts a number of electric pulses  130  upon converting electric pulses  130  into a digital form. Controller  104  calculates a first z-position  132  by generating a result and dividing the result by two. Controller  104  calculates the result by multiplying a wavelength of laser beam  122  with a number of electric pulses  33  counted during movement of support  46  from an initial z-position to first z-position  132 . A z-position is along the z-axis.  
         [0024]     Controller  104  outputs the first z-position  132  upon receiving analog trigger signals  33  from detector array  18 . For example, controller  104  outputs the first z-position  132  upon receiving trigger signals  33  from detector array  18  acquired at a particular view or gantry angle, such as, for example, zero degrees or one degree, of gantry  12 . Controller  104  outputs a plurality of z-positions including the first z-position  132 . As an example, controller  104  outputs the plurality of z-positions at a frequency ranging from and including 100 Hertz (Hz) to 10 kHz, where the frequency is a frequency of acquisition by CT imaging system  10  of a plurality of views of gantry  12 . As another example, controller  104  outputs the first z-position  132  upon receiving trigger signals  33  acquired at a view angle of zero degrees and outputs a second z-position upon receiving analog trigger signals  33  acquired at a view angle of twenty degrees. In the example, controller  104  determines the second z-position by counting a number of electrical pulses  33  received during a time of movement of support  46  from the first z-position to the second z-position, multiplying a wavelength of laser beam  122  with the number of the electric pulses  33  counted to generate an outcome, and dividing the outcome by two.  
         [0025]     In an alternative embodiment, controller  104  outputs the plurality of z-positions at a frequency faster than a frequency of acquisition of a plurality of views to provide feedback to support motor controller  44  or to measure a vibration of support  46 . For example, controller  104  does not wait to receiving analog trigger signals  33  acquired at a particular view angle before outputting the first z-position  132 . Controller  104  transmits the plurality of z-positions to mass storage device  38 .  
         [0026]     Image reconstructor  34  retrieves at least one of the z-positions from mass storage device  38  to reconstruct a plurality of images at a plurality of views. For example, image reconstructor  34  reconstructs a first image acquired at a first view angle and at the first z-position  132 , appends the first z-position  132  to the first image, reconstructs a second image acquired at a second view angle and at the second z-position, and appends the second z-position to the second image. Computer  32  retrieves a z-position from mass storage device  38  and the z-position is used by the operator to diagnose an image reconstructed at the z-position.  
         [0027]      FIG. 4  is a block diagram of an alternative embodiment of a system  200  for determining a position of a support. System  200  includes interferometer  102 , controller  104 , reflective surface  106 , and support  46 . Support plate  108  is attached, such as glued and/or bolted, to a ceiling  202  of a scanning room in which CT imaging system  10  is placed. In an alternative embodiment, system  200  may not include support plate  108  and interferometer  102  is attached, such as glued and/or bolted, to the ceiling  202 . In an alternative embodiment, interferometer  102  is attached to an item that suspends interferometer  102  above a height of support  46  in the z-direction. Reflective surface  106  is attached, such as bolted and/or glued, to a top surface  204  of support  46  so that reflective surface  106  faces interferometer  102  and is in a line-of-sight of beam splitter  112 . In another alternative embodiment, interferometer  102  is attached directly or via support plate  108  to a floor of the scanning room and reflective surface  106  is attached to a bottom surface  206  of support  46 . In another alternative embodiment, interferometer  102  is attached to the floor of the scanning room, system excludes reflective surface  106 , and bottom surface  206  is reflective. In an alternative embodiment, top surface  204  of support  46  is reflective and system excludes reflective surface  106 .  
         [0028]     The interference fringe pattern is generated in a similar manner described above upon movement of support  46  along the y-axis. Controller  104  receives electric pulses  130  from photodetector  118 , converts electric pulses  130  from an analog to a digital form, counts a number of electric pulses  130 , and determines a y-position of support  46  based on the count. Controller  104  calculates a first y-position  208  by generating an amount and dividing the amount by two. Controller  104  calculates the amount by multiplying a wavelength of laser beam  122  with a number of electric pulses  33  counted during movement of support  46  from an initial y-position to first y-position  208 . A y-position is along the y-axis.  
         [0029]     Controller  104  outputs the first y-position  208  upon receiving analog trigger signals  33  from detector array  18 . For example, controller  104  outputs the first y-position  208  upon receiving trigger signals  33  from detector array  18  acquired at a particular view or gantry angle, such as, for example, one degree or two degrees, of gantry  12 . Controller  104  outputs a plurality of y-positions including the first y-position  208 . As an example, controller  104  outputs the plurality of y-positions at a frequency ranging from and including 100 Hz to 10 kHz, where the frequency is a frequency of acquisition by CT imaging system  10  of a plurality of views of gantry  12 . As another example, controller  104  outputs the first y-position  208  upon receiving analog trigger signals  33  acquired at a view angle of ten degrees and outputs a second y-position upon receiving analog trigger signals  33  acquired at a view angle of twenty degrees. In the example, controller  104  determines the second y-position by counting a number of electrical pulses  33  received during a time of movement of support  46  from the first y-position  208  to the second y-position, multiplying a wavelength of laser beam  122  with the number of the electric pulses  33  counted to generate a product, and dividing the product by two.  
         [0030]     In an alternative embodiment, controller  104  outputs the plurality of y-positions at a frequency faster than a frequency of acquisition of a plurality of views to provide feedback to support motor controller  44  or to measure a vibration of support  46 . For example, controller  104  does not wait to receive analog trigger signals  33  acquired at a particular view angle before outputting the first y-position  208 . Controller  104  transmits the plurality of y-positions to mass storage device  38 .  
         [0031]     Image reconstructor  34  retrieves at least one of the y-positions from mass storage device  38  to reconstruct a plurality of images at a plurality of views. For example, image reconstructor  34  reconstructs the first image acquired at the first view angle and at the first y-position  208 , appends the first y-position  208  to the first image, reconstructs the second image acquired at a second view angle and at the second y-position, and appends the second y-position to the second image. Computer  32  retrieves a y-position from mass storage device  38  and the y-position is used by the operator to diagnose an image reconstructed at the y-position.  
         [0032]      FIG. 5  is a block diagram of an alternative embodiment of a system  300  for determining a position of a support. System  300  includes interferometer  102 , controller  104 , reflective surface  106 , and support  46 . Support plate  108  is attached, such as glued and/or bolted, to a side wall  302  of the scanning room. In an alternative embodiment, system  300  may not include support plate  108  and interferometer  102  is attached, such as glued and/or bolted, to side wall  302 . In another alternative embodiment, interferometer  102  attached to an item that suspends interferometer  102  to face reflective surface  106 . Reflective surface  106  is attached, such as bolted and/or glued, to a side surface  304  of support  46  so that reflective surface  106  faces interferometer  102 . In an alternative embodiment, side surface  304  of support  46  is reflective and system excludes reflective surface  106 .  
         [0033]     The interference fringe pattern is generated in a similar manner described above upon movement of support  46  along the x-axis. Controller  104  receives electric pulses  130  from photodetector  118 , converts electric pulses  130  from an analog to a digital form, counts a number of electric pulses  130 , and determines an x-position of support  46  based on the count. Controller  104  calculates a first x-position  306  by generating a solution and dividing the solution by two. Controller  104  calculates the solution by multiplying a wavelength of laser beam  122  with a number of electric pulses  33  counted during movement of support  46  from an initial x-position to first x-position  306 . An x-position is along the x-axis.  
         [0034]     Controller  104  outputs the first x-position  306  upon receiving analog trigger signals  33  from detector array  18 . For example, controller  104  outputs the first x-position  306  upon receiving trigger signals  33  from detector array  18  acquired at a particular view or gantry angle, such as, for example, one degree or two degrees, of gantry  12 . Controller  104  outputs a plurality of x-positions including the first x-position  306 . As an example, controller  104  outputs the plurality of x-positions at a frequency ranging from and including 100 Hz to 10 kHz, where the frequency is a frequency of acquisition by CT imaging system  10  of a plurality of views of gantry  12 . As another example, controller  104  outputs the first x-position  306  upon receiving analog trigger signals  33  acquired at a view angle of ten degrees and outputs a second x-position upon receiving analog trigger signals  33  acquired at a view angle of twenty degrees. In the example, controller  104  determines the second x-position by counting a number of electrical pulses  33  received during a time of movement of support  46  from the first x-position  306  to the second x-position, multiplying a wavelength of laser beam  122  with the number of the electric pulses  33  counted to generate an output, and dividing the output by two.  
         [0035]     In an alternative embodiment, controller  104  outputs the plurality of x-positions at a frequency faster than a frequency of acquisition of a plurality of views to provide feedback to support motor controller  44  or to measure a vibration of support  46 . For example, controller  104  does not wait to receiving analog trigger signals  33  acquired at a particular view angle before outputting the first x-position  306 . Controller  104  transmits the plurality of x-positions to mass storage device  38 .  
         [0036]     Image reconstructor  34  retrieves at least one of the x-positions from mass storage device  38  to reconstruct a plurality of images at a plurality of views. For example, image reconstructor  34  reconstructs the first image acquired at the first view angle and at the first x-position  306 , appends the first x-position  306  to the first image, reconstructs the second image acquired at a second view angle and at the second x-position, and appends the second x-position to the second image. Computer  32  retrieves an x-position from mass storage device  38  and the x-position is used by the operator to diagnose an image reconstructed at the x-position.  
         [0037]     In an alternative embodiment, controller  104  outputs a combination of at least one of the x-positions, at least one of the y-positions, and at least one of the z-positions at a frequency ranging from and including 100 Hz to 10 kHz, where the frequency is a frequency of acquisition by CT imaging system  10  of a plurality of views of gantry  12 . As an example, controller  104  outputs the first x-position  306  upon receiving analog trigger signals  33  acquired at a view angle of two degrees and outputs the first y-position  208  upon receiving trigger signals  33  acquired at a view angle of twenty degrees. As another example, controller  104  outputs the first y-position  208  upon receiving trigger signals  33  acquired at a view angle of ten degrees and outputs the first x-position  132  upon receiving analog trigger signals  33  acquired at a view angle of thirty degrees.  
         [0038]     It is noted that in another alternative embodiment, a plurality of inteferometers, such as interferometer  102 , or commercial laser distance sensors can be placed in any combination of the x, y, and z directions, to determine a position of support  46  along a combination of the x, y, and z axes. For example, interferometer  102  is placed on support  108  to obtain at least one of the z-positions and another interferometer  102  is attached to ceiling  202  to obtain at least one of the y-positions. It is noted that in all the embodiments described above, reflective surface  106  faces interferometer  102  and is in a line-of-sight of beam splitter  112 .  
         [0039]     Technical effects of the herein described systems and methods for determining a position of a support include accurately determining, in real time, a position or a vibration of support used in an imaging system. A position or vibration of support  46  moving at a variable speed in at least one of the x, y, and directions is accurately determined by using interferometer  102 . Other technical effects include associating an image with a corresponding position during an image reconstruction process. Further technical effects include correcting, by image reconstructor  34 , for artifacts in an image induced by position errors or vibrations by considering a position of support  46  during a reconstruction of the image.  
         [0040]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.