Patent Publication Number: US-2023136752-A1

Title: System and method for imaging a subject

Description:
BACKGROUND 
     The subject matter disclosed herein relates generally to medical diagnostic imaging systems, and more particularly to a system and method for acquiring patient bone images. 
     Dual-energy imaging systems such as a bone densitometer includes an x-ray source that emits a collimated beam of dual-energy x-rays to image a patient. An x-ray detector is positioned with respect to the x-ray source to receive the x-rays passing through the patient. The x-ray detector produces electrical signals in response to the received x-rays. The electrical signals are converted to digital signals that are utilized by the imaging system to generate images of the patient. 
     Measurements of the x-ray absorption by an object at two different x-ray energies can reveal information about the composition of that object as decomposed into two selected basis materials. In the medical area, the selected basis materials are frequently bone and soft tissue. The ability to distinguish bone from surrounding soft tissue allows x-ray images to yield quantitative information about in vivo bone density for the diagnosis of osteoporosis and other bone disease. 
     Current bone densitometers use frame-based data acquisition methods to scan patients. In this mode the use of highly-pixellated 2-D detectors generates large volumes of data with typically low x-ray statistics per pixel that complicates image reconstruction. Therefore, there is a need for an improved system and method to acquire bone density information. 
     BRIEF DESCRIPTION 
     In accordance with an embodiment of the present technique, a medical imaging system having an X-ray source operative to transmit X-rays through an object is provided. The X-ray source is collimated to produce a diverging beam of radiation. The medical imaging system also includes a detector operative to receive the X-ray energy of the X-rays after having passed through the object, wherein the detector includes detector pixels arranged in at least one row and a processing system. The processing system is programmed to select an initial height of the object with respect to the X-ray source plane and determine an initial time delayed summation (TDS) shift frequency based on the initial height. The processing system is also programmed to perform a first scan of the object based on the TDS shift frequency and determine a new height of the object based on a beam angle and an overlap of adjacent images. The processing system is further programmed to determine a new TDS shift frequency based on the new height of the object if the initial height and the new height are not substantially same and perform a second scan of the object based on the new TDS shift frequency and generate an image of the object based on detected X-ray energy at the X-ray detector based on the first scan and the second scan. 
     In accordance with another embodiment of the present technique, a method for imaging an object is provided. The method includes providing an X-ray source operative to transmit X-rays through an object and providing a detector operative to receive the X-ray energy of the X-rays after having passed through the object. The X-ray source is collimated to produce a diverging beam of radiation. The method also includes selecting an initial height of the object with respect to the X-ray source plane and determining an initial time delayed summation (TDS) shift frequency based on the initial height. A first scan of the object is performed based on the TDS shift frequency and further a new height of the objected is determined based on a beam angle and adjacent images overlap. The method further includes determining a new TDS shift frequency based on the new height of the object if the initial height and the new height are not substantially same and performing a second scan of the object based on the new TDS shift frequency. Finally, the method includes generating an image of the object based on detected X-ray energy at the X-ray detector based on the first scan and the second scan. 
     In accordance with yet another embodiment of the present technique, a medical imaging system having a multi-energy X-ray source operative to transmit X-rays through a patient is provided. The X-ray source is collimated to produce a diverging beam of radiation. The medical imaging system further includes a detector operative to receive the X-ray energy of the X-rays after having passed through the patient and a processing system. The detector includes detector pixels arranged in at least one row. The processing system is programmed to perform a scan of the patient based on a time delay summation (TDS) frequency and to generate at least two images of a patient bone corresponding to the multi-energy levels of the multi-energy X-ray source. The processing system is further programmed to determine a patient bone mineral density (BMD) based on the at least two images. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG.  1    is a schematic diagram of a dual-energy x-ray imaging system illustrating a full body scan, in accordance with an embodiment of the present technique; 
         FIG.  2    is a schematic diagram of a detector and related circuitry used in  FIG.  1   , in accordance with an embodiment of the present technique; 
         FIG.  3    is a schematic diagram of the Time-Delayed Summation (TDS) technique, in accordance with an embodiment of the present technique; 
         FIG.  4    is a schematic diagram depicting a TDS imaging sequence of a bone of a patient, in accordance with an embodiment of the present technique; 
         FIG.  5    is a schematic diagram depicting effect of TDS imaging on objects having different heights, in accordance with an embodiment of the present technique; 
         FIG.  6    is a schematic diagram of the x-ray source and linear detector of  FIG.  1    showing the divergence angle of a fan beam of radiation and a region of overlap in the fan beam for adjacent scan images, in accordance with an embodiment of the present technique; 
         FIG.  7    is a schematic diagram of two scanned images before and after merging, in accordance with an embodiment of the present technique; 
         FIG.  8    is a flowchart describing a method of merging adjacent scan images of  FIG.  7   , in accordance with an embodiment of the present technique; 
         FIG.  9    is a flowchart describing a method of imaging a patient, in accordance with an embodiment of the present technique; 
         FIG.  10    is a flowchart describing another method of imaging a patient, in accordance with an embodiment of the present technique; 
         FIG.  11    is a flowchart describing yet another method of imaging a patient, in accordance with an embodiment of the present technique; and 
         FIG.  12    is a flowchart describing a method of imaging a patient in accordance with yet another embodiment of the present technique. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers’ specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Furthermore, any numerical examples in the following discussion are intended to be non-limiting, and thus additional numerical values, ranges, and percentages are within the scope of the disclosed embodiments. 
     Exemplary embodiments of dual-energy x-ray systems and methods for acquiring, for example bone and tissue information are described in detail below. In particular, a detailed description of an exemplary dual-energy x-ray system will first be provided followed by a detailed description of various embodiments of methods and systems for generating patient anatomy images that may be used to diagnose a medical condition such as osteoporosis for example. In one embodiment, the system and method may be used for acquiring and measuring bone mineral density, bone tissue information, and other bone related information from patient bone images. In another embodiment, the system and method may be used for determining body composition which distinguishes lean and fat tissue in regions which do not contain bone. 
     At least one technical effect of the various embodiments of the systems and methods described herein is to acquire accurate patient anatomy such as bone images using a dual-energy x-ray imaging system. In some embodiments, a single dual-energy x-ray scan, and more particularly, a single body scan is used to acquire image information for a number of different bones, from which bone lengths are determined. 
       FIG.  1    is a schematic diagram of an exemplary dual-energy x-ray system, illustrated as a dual x-ray absorptiometry (DEXA or DXA) system  10 , which is also referred to as dual energy bone densitometer system capable of performing bone densitometry. The system  10  constructed in accordance with various embodiments is configured to measure at least an area of a bone, a length of a bone, a bone mineral content (BMC), a bone mineral density (BMD), and a tissue thickness or density. The BMD is calculated by dividing the BMC by the area of the bone. During operation, an x-ray beam with broadband energy levels is utilized to scan a subject, for example, to scan a human subject to image the bones of the human subject. The acquired images of the bones are used to diagnose a medical condition such as osteoporosis. The images may be generated in part from determined bone density information acquired during a dual-energy x-ray scan. 
     The system  10  includes a patient table  12  providing a horizontal surface for supporting a subject, for example, a patient  14  in a supine or lateral position along a longitudinal axis  16 . The system  10  also includes a support member, for example, a C-arm  18 . The C-arm  18  has a lower end  20  that is positioned beneath the patient table  12  to support an x-ray source  22 . The C-arm  18  has an upper end  24  that is positioned above the patient table  12  supporting an x-ray detector  26 . Optionally, the x-ray detector may be coupled to the lower end  20  and the x-ray source  22  coupled to the upper end  24 . The x-ray detector  26  may be fabricated, for example, as a multi-element cadmium-tellurium (CdTe) detector providing for energy discrimination. The x-ray source  22  and the x-ray detector  26  may be moved in a raster pattern  28  so as to trace a series of transverse scans  30  of the patient  14  during which dual energy x-ray data is collected by the x-ray detector  26 . The transverse scanning procedure generates either a single image or quantitative data set, from a plurality of scan images acquired across a patient, wherein the x-ray source  22  and the detector  26  are either longitudinally aligned with the superior-inferior axis of the patient or transversely from the patient’s left to right. Scanning a patient using a transverse motion facilitates minimizing the time between acquisitions of adjacent scan images because the transverse direction across the patient is shorter than the longitudinal direction across the patient. Thus, transverse scanning can reduce the severity of patient motion artifacts between scan images allowing the images to be more accurately merged. 
     The transverse scanning motion is produced by actuators (not shown) under control of a translation controller  32 . During operation, the x-ray source  22  produces a fan beam  34  having a plane that is parallel to the longitudinal axis  16 . Optionally, the fan beam  34  may have a plane that is perpendicular to the longitudinal axis  16 . The raster pattern  28  is adjusted such that there is some overlap (e.g., slight overlap of 10 percent) between successive scan lines of the fan beam  34 . The x-ray source  22 , the x-ray detector  26 , and the translation controller  32  communicate with, and are under the control of, a computer  40  which may include both dedicated circuitry and one or more processors having the ability to execute a stored program. 
     Referring again to  FIG.  1   , the computer  40  communicates with a terminal  42  including a display  44 , a keyboard  46 , and a cursor control device such as a mouse  48  allowing for operator input and the output of text and images to the operator. In some embodiments, the computer  40  is located remotely from the workstation  42 . Optionally, the computer  40  may form a portion of the workstation  42 . The computer is adapted to perform one or more processing operations. The acquired bone and tissue information, for example, image and density information may be processed and displayed in real-time during a scanning session as the data is received. Additionally or alternatively, the data may be stored temporarily in a memory device on the computer  40  during a scanning session and then processed and displayed in an off-line operation. The information may also be stored in a long-term storage device (e.g., hard-drive or server) for later access, such as during a follow-up scan of the same patient and useful to monitor, for example, the change in bone and tissue density over a period of time. The display  44  includes one or more monitors that present patient information, including the scanned image and the bone length images to the operator for diagnosis and analysis. The displayed images may be modified and the display settings of the display  44  also manually adjusted using the keyboard  46 , the mouse  48 , or a touch screen icon on the display itself. 
     During operation, the system  10  is configured to operate in either a dual energy x-ray mode or a single energy x-ray mode. In the single energy mode, the x-ray source  22  emits x-rays at a narrow band of energies of a few keV and in the diagnostic imaging range of approximately 20-150 keV. In the dual-energy mode, the x-ray source  22  emits radiation at two or more bands of energy emitted simultaneously or in rapid succession. The x-ray source  22  may also be configured to emit a single broadband energy of more than a few keV over the diagnostic imaging range. The system  10  may be switched between the dual energy mode and the single energy mode by increasing or decreasing the x-ray source  22  voltage and/or current. The system may also be switched between the dual energy mode and the single energy mode by removing or adding a K-edge filter. It should be noted that the x-ray source  22  may emit x-rays at different energies or ranges of energies. 
     The x-ray source  22  may be configured to output a fan beam of x-rays  34  as shown in  FIG.  1   . In some embodiments, the computer  40  controls the system  10  to operate in the single energy mode or dual-energy mode to determine the bone or tissue information of at least some of the scanned body. The single energy mode generally enables higher resolution images to be generated. The acquired images may then be used to measure, for example, bone density or other bone and tissue characteristics or content. As discussed above, the dual-energy x-ray scan may be a rectilinear scan of the entire patient body, which may be performed in a transverse-type scanning sequence as described above. During the dual-energy x-ray scan an image of the entire body of the patient may be acquired, which includes image information relating to the bones and tissue in the body. The full body or total body scan of the entire body may be performed as a single scanning operation, which may be a low dose mode scan. In some embodiments, instead of a full body or total body scan, individual rectangular regions of the body may be performed, which may be single sweep scans. Once the scan of the patient, or a portion thereof, is completed, the dual energy signals provided by the detector  26  are deconstructed into images of two basis materials, such as bone and soft tissue. The high and low energy signals can also be combined to provide a single energy mode having superior signal to noise ratio for imaging purposes. 
     The detector  26  shown in  FIG.  1    may be embodied as either a linear array of detector elements, a side linear array of detector elements, which includes two transversely separated rows of detector elements, or a stacked array detector in which the detector elements are stacked along a direction of propagation of the radiation and are selectively sensitive to low and high energy spectrums, respectively. 
       FIG.  2    depicts a schematic diagram  100  of a detector and related circuitry used in system of  FIG.  1   . In general, schematic diagram  100  shows a detector  102  which can be used as detector  26  of  FIG.  1   . The detector  102  includes an array of detector pixels  104  having n rows and m columns. Each of these detector pixels are connected to a charge amplifier  106 , one or more comparators  108  and a counter  110 . When the x-ray beam  112 , after passing through a subject or the patient, hits a particular detector pixel (e.g., pixel (n, m)), the detector pixel (n, m) converts the x-ray energy into a charge which is amplified by a charge amplifier  106 . It should be noted that if the x-ray beam hits a border of the two detector pixels, two detector pixels may receive the x-ray energy which will be converted into two charges. Output of charge amplifier  106  is passed on to the comparator  108 , which discriminate the charge into different energy windows (e.g. Low and High energy windows) and provides the corresponding output to the counter  110 . The counter  110  then updates (increments) the corresponding low energy or high energy count based on the output from the comparator  108 . The contents (or x-ray energy count) of the counter  110  are generally stored in a memory register of the corresponding detector pixel. It should be noted that if there is no charge from the x-ray detector pixel i.e., when no x-ray beam falls on the detector pixel, the counter  110  will not get updated. After a certain time duration (e.g., a recording period time duration), the x-ray energy count of the counter  110  are provided to a processor such as computer  40 . Based on the number of counts from multiple counters, the processor finally determines the patient image which may be a bone image or a patient tissue image. 
     In the present technique, a Time-Delayed Summation (TDS) technique is used instead of the embodiment shown in  FIG.  2   . The TDS technique is a process of adding together multiple exposures of the same object as it passes a detector. TDS technique allows images to be taken of moving objects. In the TDS technique, the incremented sums of x-ray energy counts are shifted to a neighboring detector pixel’s memory register in synchronization with the x-ray beam motion, such that an object (e.g., a bone) in the scanned subject (e.g., a patient) remains on an x-ray line between a single point in the detector plane and source’s focal spot. In the TDS method, this shifting synchronization is dependent on the object’s magnification, detector pixel pitch and scan speed. Notably, objects at heights other than the optimum for time-delay synchronization will be blurred since signals through that object will be distributed across multiple points in the detector plane 
       FIG.  3    is a schematic diagram  200  that describes the TDS method in accordance with an embodiment of the present technique. The schematic diagram  200  shows a detector  202  having a detector pixel array of n rows and m columns. As against  FIG.  2   , where after the recording period time duration, the x-ray energy count of a detector pixel counter is provided to a processor  40 , in  FIG.  3   , the x-ray energy count of the detector pixel counter is shifted to a neighboring detector pixel counter after a TDS shift time duration. For example, in one embodiment, TDS shift time duration may be 0.56 msec and total number of shifts may be  60  for one acquisition whereas the recording period time duration in  FIG.  2    may be 6 msec i.e., the recording period time duration may be longer than the TDS shift time duration. The shifting of x-ray energy count from one counter to the next is done for all the detector pixel counters in synchronization with the x-ray beam motion. 
     For example, if the x-ray scanner is sweeping across the patient and if the scan direction  204  of the x-ray beam is from right to left then the x-ray energy count shift direction  206  is from left to right i.e., opposite direction. A pulse diagram  210  at the bottom of  FIG.  3    shows TDS shift strobes or pulses  212  which have a TDS shift frequency f. The TDS shift frequency f translates into a TDS shift time duration Δt 0 =⅟f between two pulses. Thus, whenever a TDS shift pulse is received, the counter contents of corresponding detector pixels are shifted to the neighboring counter. As an example, if the TDS shift time duration between two pulses  212  is 1 millisecond (ƒ=1 khz) then the contents of the counter of detector pixel (i, j) are shifted to the right counter of detector pixel (i, j+1) after every 1 millisecond, where i and j corresponds to row number and column number respectively. In other words, the contents of all detector pixel counters corresponding to row i are shifted to their corresponding right column. Moreover, the contents of the last column N are shifted to a main detector memory buffer  208  and then finally to a processor for generating image of the object. In general, by shifting the contents of the counter to the neighboring counter and then adding them together, multiple x-rays passing through a bone of the patient are integrated as the x-ray beam passes through that bone over a given time duration. The TDS shift frequency ƒ depends on a plurality of parameters such as a velocity v of the x-ray source motion and will be explained in more detail in subsequent paragraphs. 
       FIG.  4    is a schematic diagram  300  depicting TDS imaging sequence of a bone of a patient. Schematic diagram  300  shows three steps  302 ,  304 ,  306  of TDS imaging sequence corresponding to 3 positions of the X-ray beam  308  at three different time instances. Schematic diagram  300  shows an object e.g., a bone  310  in a patient  312  who is lying on a table  314 . For all three steps  302 ,  304 ,  306 , corresponding positions  316 ,  318 ,  320  of detector  322  are also shown in schematic diagram  300 . The detector  322  includes N number of detector pixel columns, each pixel having dimension a. Thus, the total detector width d equals d=Na. 
     When the x-ray scanner is sweeping across the patient, step  302  is the first time instance when the x-ray beam  308  passes through the bone  310 . In the embodiment shown, the x-ray scanner is moving from left side to the right side in reference to the bone  310  at a velocity v. Thus, at step  302 , the x-ray beam  308  is at an acute angle with respect to table plane  314  to start with. The attenuated x-ray beam  324  i.e., the x-ray beam  308  after passing through the bone  310  then hits the first detector pixel of the detector  322  as seen in detector position  316 . Step  304  refers to a second time instance when the x-ray beam  308  is at a right angle with respect to table plane  314 . The attenuated x-ray beam  324  then hits the middle detector pixel of the detector  322  as seen in detector position  318 . Step  306  corresponds to a third time instance when the x-ray beam  308  is at an obtuse angle with respect to table plane  314 . In this instance, the attenuated x-ray beam  324  hits the last detector pixel as seen in detector position  320 . 
     If the TDS method is not used then the charge accumulated at detector pixels corresponding to bone imaging at positions  316 ,  318  and  320  would remain at the same detector pixels i.e., first, middle and last detector pixel. This would result in bone image being distributed across the whole detector and so the final bone image would be blurry. Alternatively, the frame rate could be increased to reduce blurring, at the cost of increasing data volume and re-registration of frame in image reconstruction. However, in the TDS method, the contents of detector pixels are continuously shifted to the neighboring pixel counter at a TDS shift frequency till the bone is completely imaged and finally the charge corresponding to bone image at positions  316 ,  318  and  320  gets accumulated in a last detector pixel. The contents of the last detector pixels are then read out by the processor for generating the bone image. In other words, bone image charge at all the detector positions gets integrated in one detector pixel instead of getting distributed across the entire detector resulting in a less-blurry or clearer image of the bone. 
     In one embodiment, the TDS shift frequency is synchronized with the x-ray beam motion and is given as 
     
       
         
           
             f 
             = 
             
               
                 
                   M 
                   o 
                 
                 v 
               
               a 
             
           
         
       
     
      where v is the scanning speed or x-ray beam motion speed, a is the detector pixel dimension in the shift direction, and M O  is the magnification factor of the object i.e., bone. The magnification factor depends on the distance of the bone away from the source. For example, the closer the bone is to the source, the greater the magnification of that bone on the detector plane. In general, the magnification factor M O  of the object is given as an x-ray source to image distance (SID) divided by the x-ray source to object distance (SOD) i.e., 
     
       
         
           
             
               M 
               O 
             
             = 
             
               
                 SID 
               
               / 
               
                 SOD 
               
             
           
         
       
     
      The image is measured in the detector plane so the x-ray source to image distance can also be considered as source to detector distance. It can be seen from equation (1) above that the TDS shift frequency (i.e., the frequency at which the contents of the detector pixel counters are shifted to the neighboring detector pixel counter) depends on the distance between the object and the x-ray source i.e., SOD. So, if a wrong SOD is used while determining the TDS shift frequency then the content of the detector pixel counter won’t be shifted in perfect synchronization with the x-ray beam motion resulting in a sub-optimal image of the bone. 
       FIG.  5    is a schematic diagram  400  depicting effect of TDS imaging on objects having different heights. The schematic diagram  400  shows x-ray source  402  and a detector  408 . A first object  404  is located at a distance O from source  402  and a second object  406  is located at a distance O′ from source  402 . Further, the detector  408  or the image plane is located at a distance equal to I from the source  402  i.e., SID=I. In this case, if the optimum TDS shift frequency is set based on object  404  (i.e., SOD=O in equation (2) above) then the x-ray energy count of the detector pixels corresponding to object  404  would shift to neighboring pixel counters in the same row. Thus, the shifting would happen in synchronization with the x-ray beam motion but in an opposite direction of movement of the x-ray beam motion. This results in a less blurry image of the object  404 . 
     Because object  406  is at a different height compared to object  404 , the x-ray energy count corresponding to object  406  would not shift to neighboring pixel counters in synchronization with the x-ray beam motion. In other words, the shifting of x-ray energy count corresponding to object  406  to the neighboring pixel counter may fall behind the speed of the x-ray beam motion resulting in a slippage of the image of object  406  for every TDS shift time duration Δt = ⅟ƒ. In one embodiment, the image slippage (s) of object  406  per TDS shift time duration may be given as: 
     
       
         
           
             s 
             = 
             a 
             
               
                 o 
                 − 
                 o 
                 &#39; 
               
               
                 o&#39; 
               
             
           
         
       
     
     It should be noted that this slippage is independent of velocity v and there is no transverse bias in the position. In an example, assume that I = 73 cm, O = 20 cm, and O′ = 30 cm, a = 0.2 mm and N = 32. In this case, in the detector plane, the total TDS blurring of object  406  at O′ would be sN = 2.1 mm, compared to pixelation and motion blurring of 0.4 mm for object  404  at O which is optimally synchronized. Thus, it can be seen that knowing the height of the object is very important in determining the optimal TDS shift frequency for capturing the best image of the object. 
     Referring back to  FIG.  1   , in one embodiment, the acquired images from system  10  as described above are further used to measure, for example, bone mineral density (BMD) or other bone and tissue characteristics or content. In one embodiment, the system involves two X-ray beams with differing energy levels that are directed toward a target bone for measurement. In a preferred embodiment, a single multi-spectral x-ray beam is used. After soft tissue absorption is subtracted out of the x-ray image, BMD can be determined from the absorption of each beam by bone. In another embodiment, the two images corresponding to the two energy levels (i.e., high and low) are aligned and mathematically combined to produce the necessary bone density information according to mathematical algorithms known in the art (e.g. basis set material decomposition). The system  10  uses the TDS technique described above to acquire object images. However, the height of the object needed to determine the optimal TDS shift frequency is generally not known in advance. In one embodiment, the height of the object is an estimate based on prior data. The prior data may be historical data of the same object or data of similar objects obtained in past. 
     In another embodiment, the technique presented herein first determines the height of the object and then accordingly determines the optimal TDS frequency. For example, in one embodiment, the object height determination technique as described in patent U.S. Pat. No. 6081582 is utilized. The U.S. Pat. No. 6081582 is incorporated by reference herein for object height determination purpose and adjacent scan image merging purpose. In general, the object height is determined based on adjacent scan images which are the result of use of a fan beam in transverse scan as explained with respect to  FIG.  6   . It should be noted that the while describing TDS technique in  FIG.  4    above, only one sweep of the scanner over an object was considered and how the charge data for that sweep is integrated was explained. On the contrary,  FIG.  6    described below corresponds to a transverse scan of the object. For transverse scan, the x-ray source and the x-ray detector are moved in a raster pattern ( FIG.  1   ) so as to trace a series of scans of the patient during which dual energy x-ray data is collected by the x-ray detector  26 . 
     Referring now to  FIG.  6   , the fan beam  34  of system  10  diverges slightly (e.g., with a divergence angle  67 ) as it passes from a focal spot of the x-ray source  22  to the detector  26 . This creates a triangular overlap region  50  formed by the intersection of the volumes of the swept fan beams for successive transverse scans. Generally, the overlap region  50  is formed by the intersection of the area of the fan beam  34  when the x-ray source and detector  26  are in a first position  52  to acquire a first scan image and the area of the fan beam  34 ′ when moved to an adjacent second longitudinal position  54  to acquire a second scan image. As will be appreciated by those skilled in the art, overlap between adjacent scans distorts or blurs the imaged produced by combining the scan images near the edges of the scan images. The blurring is caused by a dependency of the projected image on the height of the imaged structure (the “height dependency problem”) that displaces the relative position of the structure in the two scan image as a result of the different angles of illumination of the structure by the adjacent beams of radiation. In the present invention, the fan beam  34  is collimated to reduce this divergence to a value of less than 10° and preferably to approximately 4° so as to reduce parallax and height dependency problems. The increased numbers of overlap regions  50  required by the small fan beam angle are rendered acceptable by the reduced parallax of the beams and additional imaging processing steps to be described. 
     Referring now to  FIG.  7   , a first scan image  64  may be obtained with a transverse scan such as that at first longitudinal position  53  as shown in  FIG.  6    and shows an object  56 , in this case, a portion of a spinal column  66  that is also imaged in a second scan image  68  acquired along second longitudinal position  54  as shown in  FIG.  6   . Adjacent edges of scan images  64  and  68  have overlap portions  70  that are known functions of the size of the raster scan (the longitudinal increment) and the known divergence of the fan beam  34 . Nevertheless, the exact position of objects  56  within the overlap portion  70  of images  68  and  66  will vary depending on the height of the object  56 . Accordingly, images  68  and  64  must be shifted longitudinally with respect to each other to overlap by an arbitrary amount to bring objects  56  into alignment. 
       FIG.  8    shows a flowchart  80  describing a method of merging adjacent scan images of an object generated due to overlap between adjacent scans. In step  81 , the present invention acquires successive transverse scans to produce scan images  64  and  68  having overlap portions  70  as shown in  FIG.  7   . At a succeeding step  82 , an object image or a bone image is extracted from each of the scan images  64  and  68  so as to highlight the image of the object or bone  56  of relevance in the measurement. At step  84 , the data of the adjacent scan images  64  and  66  within the overlap portions  70  are then correlated for a series of overlaps of progressively greater longitudinal amount to deduce an optimal or best fit overlap for the particular imaged object  56 . Optimal overlap occurs when adjacent sweeps image the same region of bone in their respective overlap regions. Therefore, optimal overlap is determined by minimizing the difference in estimated bone in each sweep’s overlap region. It will be recognized that to the extent that imaged object  56  has varying or multiple height within the overlap portions  70 , this best fit will in fact reflect an average of that height. 
     The height of the bone itself is calculated at step  86  using simple trigonometric identities relying on the known beam angle  67  and a longitudinal distance D of the overlap region  50  determined from the overlap images. For example, a triangle is formed with base D, height as H and the beam angle  67 . Thus, if the beam angle and base is known then height H can be calculated using trigonometric principles. This height is used at step  88  to scale the image  64  or  68  so as to provide a predetermined constant magnification to the object  56 . This correction for scaling provides increased accuracy to quantitative uses of the data, for example, in density measurements and provides improved merging of the image data to the extent that both images have similar magnification. Finally, at step  90 , images are weighted with the weighting mask  77  shown in  FIG.  7    and merged to create to a single file of image data. 
       FIG.  9    shows a flowchart  500  describing a method of imaging a patient in accordance with an embodiment of the present technique. In step  502 , the method includes selecting an initial height of the bone with respect to the x-ray source level. Selecting the initial height of the bone may include making an estimate based on prior data. The prior data may be historical data of the same patient or data of similar objects obtained in past. For example, the femur may be 5 cm deep inside the body based on historical averages for that body type. But for a particular patient, previous measurements may have shown that femur height is an extra 5 cm higher, so then the height of the femur may be selected as 10 cm. Further, if the distance from the body of the patient to the X-ray source level is another 10 cm then the height may be selected as 10 + 10 =20 cm. 
     In step  504 , TDS frequency is determined based on the initial height of the patient bone. The TDS frequency may be determined as in equation (1) above. Thereafter, steps  506 - 516  which are similar to steps  81 - 90  of  FIG.  8    are performed. In other words, in steps  506 , successive transverse scans of the patient are acquired based on the TDS frequency. The transverse scan generates scan images of the bone having overlap portions. A bone image is then extracted from each of the scan images in step  508 . At step  510 , the data of the adjacent scan images within the overlap portions are correlated. The height of the bone is calculated at step  512 . This height is used at step  514  to scale the scan images so as to provide a predetermined constant magnification to the bone. Finally, at step  516 , scan images are weighted with the weighting mask and merged to generate the bone image. 
     At step  524 , the calculated height of the bone in step  512  is compared with the initial height in step  502 . At step  520  it is determined if the initial height is sufficiently close with the calculated height. In one embodiment, a threshold value may be used to determine whether the initial and calculated heights are sufficiently close. In one embodiment, such a threshold may be 1 cm. In another embodiment, such a threshold may be 2 cm and so on. If the heights are not sufficiently close, then the method moves to step  518  where a new TDS frequency is determined based on the calculated height of the bone in step  512 . The method then moves back to step  506  to repeat the scan and to generate the new bone image based on the new TDS frequency determined in step  518 . However, if in step  518 , it is determined that the initial height is sufficiently close to the calculated height then the scan is finished in step  522  which finalizes the bone image. 
       FIG.  10    shows a flowchart  600  describing another method of imaging a patient in accordance with an embodiment of the present technique. As earlier, at start, an initial height of the bone with respect to the x-ray source level is determined in step  602 . Based on the initial height, the TDS frequency is also determined in step  602  based on equation (1). As against  FIGS.  8  and  9   , in this method, a complete scan of the bone is not performed based on the TDS frequency. Instead, at step  604 , based on the TDS frequency at least two or three sweeps of the x-ray source over the bone region of interest are carried out to generate scan images of the bone having overlap portions. In steps  606 , a bone image is extracted from each of the scan images. At step  608 , the data of the adjacent scan images within the overlap portions are correlated. The height of the bone is calculated at step  610 . 
     At step  612 , the calculated height of the bone in step  610  is compared with the initial height in step  602 . At step  614  it is determined if the initial height is sufficiently close with the calculated height. If the heights are not sufficiently close, then the method moves to step  624  where a new TDS frequency is determined based on the calculated height of the bone in step  610  and the new TDS frequency is set up in the system in step  626 . The method then continues to steps  628 - 638  which are similar to steps  81 - 90  of  FIG.  8    and are performed similarly to generate the final bone image of the patient. 
     However, if at step  614  it is determined that the initial height selected in step  602  is sufficiently close to the calculated height in step  610  then the method moves to step  616  to resume the scan. At step  618 , the calculated height is used to scale the scan images generated in step  604  so as to provide a predetermined constant magnification to the bone. Finally, at step  620 , scan images are weighted with the weighting mask and merged to generate the bone image and the method is ended at step  622 . 
       FIG.  11    shows a flowchart  700  describing yet another method of imaging a patient in accordance with an embodiment of the present technique. Similar to method  600 , at start, an initial height of the bone with respect to the x-ray source level is determined in step  702 . Based on the initial height, the TDS frequency is also determined in step  702 . However, the main difference between method  600  and method  700  is that in method  700 , the TDS frequency is adjusted after every sweep of the x-ray source over the bone region of interest. For example, at step  704 , based on the TDS frequency-at least two initial sweeps of the x-ray source over the bone region of interest is carried out to generate scan images of the bone having overlap portions. In steps  708 , a bone image is extracted from each of the scan images. At step  710 , the data of the adjacent scan images within the overlap portions is correlated. As described earlier, the height of the bone is calculated at step  712 . Based on the height determined in step  712 , a new TDS frequency is determined in step  714 . 
     At step  718 , it is determined whether the scan is completed or not. It should be noted that the scan is completed when last sweep has been acquired. All scans have a user-prescribed length to cover the site of interest. This scan length determines a fixed number of sweeps for the scan. Thus, if the last sweep of the fixed number of sweeps is not completed, then the new TDS frequency is set up in the system in step  716 . Based on the new TDS frequency, at step another sweep of the x-ray source over the bone region of interest is carried out. The method then continues to step  708  as earlier. However, if at step  718  it is determined that the scan is complete then at step  720 , the calculated height in step  712  is used to scale the scan images generated in step  704  or  706 . Finally, at step  722 , scan images are weighted with the weighting mask and merged to generate the bone image and the method is ended at step  724 . 
       FIG.  12    shows a flowchart  800  describing a method of imaging a patient in accordance with another embodiment of the present technique. At step  802 , an X-ray source is provided which is operative to transmit X-rays through an object. The X-ray source is collimated to produce a diverging beam of radiation. In one embodiment, the X-ray source may operate in a single energy mode or a dual energy mode. At step  804 , the method includes providing a detector which is operative to receive the X-ray energy of the X-rays after having passed through the object. In one embodiment, the detector may be embodied as either a linear array of detector elements, a side linear array of detector elements, which includes two transversely separated rows of detector elements, or a stacked array detector in which the detector elements are stacked along a direction of propagation of the radiation and are selectively sensitive to low and high energy spectrums, respectively. The method further includes, selecting an initial height of the object with respect to the X-ray source plane at step  806 . Selecting the initial height of the bone may include making an estimate based on prior data. The prior data may be historical data of the same patient or data of similar objects obtained in past. 
     At step  808 , the method includes determining an initial time delayed summation (TDS) shift frequency based on the initial height. In general, TDS shift frequencies depend on x-ray beam motion speed, a detector pixel dimension, and a magnification factor of the object on the detector, wherein the magnification factor of the object depends on the height of the object. Based on the initial TDS shift frequency a first scan of the object is performed in step  810 . Further, at step  812 , a new height of the object is determined based on beam divergence characteristics and adjacent images overlap. The adjacent images are result of intersection of a first area of an x-ray fan beam when the x-ray source and the detector are in a first position and a second area of the x-ray fan beam when the x-ray source and the detector are in a second position. 
     If the initial height and the new height are not substantially same then at step  814 , a new TDS shift frequency based on the new height is determined and a second scan based on the new TDS shift frequency is performed. Finally, at step  816 , an image of the object based on detected X-ray energy at the X-ray detector is generated. 
     In one embodiment of the method  800 , the first scan of the object includes performing only a couple of x-ray source sweeps across the object and the second scan of the object includes a full scan of the object. In this specific embodiment, remaining x-ray source sweeps of the first scan are performed if the initial height and the new height are substantially same. In another embodiment, the first scan and the second scan are both full scan of the object. In yet another embodiment, the first scan and the second scan are both single x-ray sweeps of the object and the steps are repeated till the complete scan is over. 
     In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
     The techniques presented and claimed herein are referenced and applied to material objects and concrete examples of a practical nature that demonstrably improve the present technical field and, as such, are not abstract, intangible or purely theoretical. Further, if any claims appended to the end of this specification contain one or more elements designated as “means for [perform]ing [a function]...” or “step for [perform]ing [a function]...”, it is intended that such elements are to be interpreted under 35 U.S.C. 112(f). However, for any claims containing elements designated in any other manner, it is intended that such elements are not to be interpreted under 35 U.S.C. 112(f). 
     This written description uses examples to disclose the present subject matter, including the best mode, and also to enable any person skilled in the art to practice the subject matter, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.