Abstract:
An apparatus and method for use in correcting for table sag within a dual imaging system wherein the dual system includes two separate imaging configurations that define first and second imaging areas arranged sequentially along an imaging axis and that generate first and second imaging data sets, the apparatus including at least one sensor for identifying table sag and a compensator for modifying at least one data set to correct for table sag prior to combining the data sets to form a single image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   The field of the invention is medical imaging systems and more specifically includes systems for compensating for misalignment of two or more image data sets due to support table deflection. 
   The medical imaging industry has developed many different types of imaging systems that are useful for diagnostic purposes. Each different system typically has particular uses for which it is advantageous. For example, computerized tomography (CT) systems that employ X-rays are useful for generating static images of bone and the like while positron emission tomography (PET) systems are useful for generating dynamic or functional images of dynamic occurrences such as blood flow and the like. 
   For various reasons it is advantageous to generate images that include both static and functional characteristics. To this end one solution has been to sequentially use separate imaging systems to gather both functional and static imaging data sets and then combine those sets or corresponding images to generate unified functional/static images. For example, first a CT system may be used to generate a CT image and second a PET system may be used to generate a PET image, the two images being combined thereafter to generate the unified image. 
   Unfortunately, systems having two separate imaging configurations have several shortcomings. First, there has to be some way to align the functional and dynamic images so that the unified image reflects relative anatomical positions precisely. To this end fiducial markers have been employed. For example, a metallic button with a positron emitter can be placed on the surface of a patient&#39;s skin which is detectable by both the CT and PET systems. By aligning the marker in the resulting images the images can be aligned. 
   Second, where two separate imaging configurations are employed a patient has to be moved from one configuration to the next between acquisition sessions. Movement increases the likelihood that the patient&#39;s positions during the two imaging sessions will change thus tending to reduce the possibility of accurate alignment (i.e., relative positions of organs or the like could change during movement). The possibility of misalignment is exacerbated by the fact that often imaging session schedules will not allow both CT and PET imaging processes to be performed during the same day. Thus, overall diagnostic value of the resulting unified image can be reduced appreciably through movement between acquisition periods. 
   One solution to eliminate the need to move patient&#39;s between acquisition periods is to provide a dual CT-PET imaging system like the one illustrated in FIG.  1 . In these types of systems both a CT imaging configuration  14  and a PET imaging configuration  16  are arranged sequentially along a single translation axis  19  with their relative positions fixed. A support  12  for a support table  18  is positioned adjacent the system with the table  18  moveable along the translation axis  19 . Here the CT and PET systems can be used simultaneously or sequentially to acquire both CT and PET sets of imaging data in a relatively short time and without moving the patient from one configuration to another. The end result is less patient movement, less time to gather required data and better alignment of resulting images to provide a more accurate unified image. 
   One problem with dual imaging systems is that each of the CT and PET configurations typically include a gantry to support a detector or series of detectors laterally displaced from the translation axis  19 . For this reason the translation axis  19  is relatively long and the support table  18  needs to extend a relatively long distance in order to accommodate the system configurations. 
   While every effort is made to provide stiff supports and tables so that vertical alignment within CT and PET imaging areas can be maintained, when a patient is positioned on a table and the table is extended to accommodate the axial length of dual imaging systems it has been found that the tables often sag such that the CT and PET data sets collected are not aligned along the translation axis  18 . Exacerbating matters is the fact that over time stiffness of some supports and tables has been known to deteriorate. While stiffer tables and supports is an option, increased stiffness is a relatively expensive proposition as exotic configurations and materials have to be used to achieve greater stiffness. 
   BRIEF SUMMARY OF THE INVENTION 
   An exemplary embodiment of the invention includes an apparatus wherein at least one sensor senses, during data acquisition with a support table extended, the position of the table, a determiner uses the table position signals to determine the relative positions of the first and second detectors (corresponding to first and second imaging configurations such as a CT configuration and a PET configuration, respectively) with respect to the table and a compensator uses the relative positions to modify at least one of the data sets prior to the sets being combined to form a unified image. 
   In one embodiment the apparatus includes two sensors so that two table segment positions can be tracked during data acquisition and both table position signals can be used to modify the data set. In another embodiment both the first and second data sets can be modified as a function of the table position signals. 
   These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefor, to the claims herein for interpreting the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a schematic view of a prior art combined CT-PET imaging system; 
       FIG. 2  is a schematic view of the present invention including two separate table position sensors; and 
       FIG. 3  is a flow chart illustrating one method according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings wherein like reference numbers indicate similar components throughout the several views and, more specifically, referring to  FIG. 2 , therein is illustrated an exemplary embodiment of the present invention in the context of a combined CT-PET imaging configuration  10 . Configuration  10  generally includes a support  12 , a table  18 , an imaging system  21 , first and second sensors  50 ,  52 , respectively, a determiner  42 , a compensator  40  and an interface  30 . Support  12  is similar to the various types of table supports known in the art and therefore will not be described here in detail. Similarly, table  18  is similar to known tables and therefore will not be described here in detail. Suffice it to say that table  18  is mounted on top of support  12  for motion in at least the direction along the length of table  18 . When used in conjunction with imaging system  21 , table  18  can be stepped along a translation axis  19  defined by system  21  as described in more detail below. 
   System  21  includes both a CT imaging configuration  14  and a PET imaging configuration  16 . CT configuration  14  includes a CT detector array  15  and an opposing source  17  mounted for rotation about a gantry  23 . The space between source  15  and detector  17  defines a CT imaging area  60 . Similarly, PET system  16  includes oppositely facing PET detectors collectively referred to by numeral  62  that, when rotated about a PET gantry  21  define an annual PET imaging area  64 . The CT and PET configurations  14 ,  16 , respectively, are in a fixed position with respect to each other and such that the imaging areas  60  and  64  are adjacent and spaced along translation axis  19 . As illustrated, when table  18  is moved into and through imaging areas  60  and  64  in a direction parallel to axis  19 , a first segment or end  22  of table  18  first passes through imaging area  60  and then through area  64  while end  20  of table  18  remains secured to support  12 . 
   It should be appreciated from  FIG. 2  that, when table  18  is extended as illustrated, table  18  tends to sag or deflect downward at extended end  22 . This is particularly true in the case of relatively large patients that may have to be supported by table  18 . Thus, because table  18  deflects downward, when CT data is acquired via configuration  14 , the relative vertical position of the portion of the patient being imaged will be at a first height, whereas, when the table is extended further so that the same portion of the patient is imaged via PET configuration  16 , the portion imaged will be at a relatively lower height. In addition, assuming that a patient&#39;s head is positioned at end  22  of table  18 , when the table is extended such that the portion of the patient being imaged is moved from within CT imaging area  60  to imaging area  64 , table  18  will likely deflect even more as additional weight is located further from end  20 . Thus, in addition to the portion of the patient being imaged being at a lower vertical position, that portion will also be skewed downwardly. In this case, clearly the CT data collected for the portion of the patient being imaged will not be aligned with the PET data collected. 
   To compensate for the misalignment of the CT and PET data, referring still to  FIG. 2 , in one embodiment of the invention first and second table position sensors  50  and  52  are provided adjacent the translation axis  19  and outside the imaging areas  60  and  64 . As illustrated, in one embodiment, sensor  52  is positioned adjacent system  21  and on a side opposite support  12 . In addition, as illustrated, first sensor  50  is positioned between CT imaging configuration  14  and PET imaging configuration  16 . Thus, sensors  50  and  52  are used to determine the heights of different table segments thereabove, the relative heights being identified by distances  80  and  82 , respectively. Sensors  50  and  52  can take any of various forms including ultrasonic sensors, laser sensors, acoustical sensors, optical sensors, light sensors, magnetic sensors, and any other type of distance determining sensor known in the art. 
   Referring still to  FIG. 2 , data acquired by imaging configurations  14  and  16  is provided to compensator  40 . Table position signals from sensors  50  and  52  are provided to determiner  42 . 
   Determiner  42  uses the position signals to determine the relative positions of at least one and preferably both the of the CT and PET detectors with respect to table  18  during acquisition. To this end, the positions of the CT and PET detectors are always known as they are either stationary or the positions are precisely controlled. In addition, the positions of each of sensors  50  and  52  are known as the those positions are also fixed in the illustrated embodiment. Thus, the relative positions between each of sensors  50  and  52  and the CT detector  14  and the PET detectors  62  is known. Combining those known relative positions with the table positions from sensors  50  and  52 , determiner  42  can easily determine the relative positions of the CT and PET detectors to table  18 . 
   The relative positions are provided to compensator  40 . Compensator  40  can be programmed to either modify the raw acquired data sets from the CT and PET detectors to compensate for the misalignment associated with distances  80  and  82 , may compensate one set of the raw data, may compensate a final CT image prior to generating a unified image or may compensate both the final CT and PET images prior to generating a unified image. After compensator  40  modifies data to eliminate the affects of the misalignment, compensator  40  combines PET and CT images to generate a unified image which is then provided to interface  30  for review by a system user. 
   Referring now to  FIG. 3 , a method  100  according to the present invention is illustrated. Referring also to  FIG. 2 , at process block  102  position sensors  50  and  52  are provided adjacent imaging areas  60  and  64  along translation axis  19 . At block  104 , a patient is positioned on table  18  and the table and patient are positioned with respect to CT detector  14 . At block  106  the vertical positions of table  18  identified by numerals  80  and  82  are determined by sensors  50  and  52 , respectively and those position signals are provided to determiner  42 . At block  108  CT configuration  14  is used to collect CT data which is provided to compensator  40 . 
   Next after CT data has been collected for the portion of the patient to be imaged, table  18  is extended further along axis  19  until the portion of the patient to be imaged is located within PET imaging area  64 . This positioning of the patient with respect to the PET detector  62  is identified by block  110 . Continuing, at block  112 , the positions  80  and  82  of table  18  are determined again using sensors  50  and  52  and those position signals are provided to determiner  42 . 
   At block  114  PET detectors  62  are used to collect PET data which is again provided to compensator  40 . At block  116  the position signals received by determiner  42  are used to determine the relative positions of detectors  14  and  62  with respect to table  18  during each of the CT and PET acquisitions. Next, at block  118  the relative positions of table  18  are used to modify at least one of the CT or PET data sets, and, perhaps both of the data sets. Finally, at process block  120  compensator  40  uses the compensated or modified data to generate a unified image including both the CT and PET data which is then displayed on interface  30 . 
   It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, as indicated above, the compensator  40  illustrated in  FIG. 2  can be used to either modify one or both sets of raw acquired data. In the alternative, the compensator  40  can modify final CT and PET images prior to generating a unified image although such compensation may be less accurate than compensation involving raw data. In addition, it should be appreciated that the present invention could be carried out using one position sensor if the relative height of table end  20  (see  FIG. 2 ) were known or if CT configuration  14  were relatively thin (e.g., a few millimeters). 
   To apprise the public of the scope of this invention, the following claims are made: