Patent Description:
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 <NUM>-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.

<CIT> discloses the use of a high average velocity of multi-focal point x-ray source along its tomographic trajectory path without image blurring occurring in the reconstructed image, in which image clarity can be obtained by acquiring a rapid succession of x-ray image projections while switching and/or scanning the focal point of source in a direction retrograde to the source's mechanical motion, and one or more x-ray emission and detector integration periods can occur during slow-velocity segments of source trajectory.

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.

Furthermore, any numerical examples in the following discussion are intended to be nonlimiting, 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> is a schematic diagram of an exemplary dual-energy x-ray system, illustrated as a dual x-ray absorptiometry (DEXA or DXA) system <NUM>, which is also referred to as dual energy bone densitometer system capable of performing bone densitometry. The system <NUM> 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 <NUM> includes a patient table <NUM> providing a horizontal surface for supporting a subject, for example, a patient <NUM> in a supine or lateral position along a longitudinal axis <NUM>. The system <NUM> also includes a support member, for example, a C-arm <NUM>. The C-arm <NUM> has a lower end <NUM> that is positioned beneath the patient table <NUM> to support an x-ray source <NUM>. The C-arm <NUM> has an upper end <NUM> that is positioned above the patient table <NUM> supporting an x-ray detector <NUM>. Optionally, the x-ray detector may be coupled to the lower end <NUM> and the x-ray source <NUM> coupled to the upper end <NUM>. The x-ray detector <NUM> may be fabricated, for example, as a multi-element cadmium-tellurium (CdTe) detector providing for energy discrimination. The x-ray source <NUM> and the x-ray detector <NUM> may be moved in a raster pattern <NUM> so as to trace a series of transverse scans <NUM> of the patient <NUM> during which dual energy x-ray data is collected by the x-ray detector <NUM>. 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 <NUM> and the detector <NUM> 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 <NUM>. During operation, the x-ray source <NUM> produces a fan beam <NUM> having a plane that is parallel to the longitudinal axis <NUM>. Optionally, the fan beam <NUM> may have a plane that is perpendicular to the longitudinal axis <NUM>. The raster pattern <NUM> is adjusted such that there is some overlap (e.g., slight overlap of <NUM> percent) between successive scan lines of the fan beam <NUM>. The x-ray source <NUM>, the x-ray detector <NUM>, and the translation controller <NUM> communicate with, and are under the control of, a computer <NUM> which may include both dedicated circuitry and one or more processors having the ability to execute a stored program.

Referring again to <FIG>, the computer <NUM> communicates with a terminal <NUM> including a display <NUM>, a keyboard <NUM>, and a cursor control device such as a mouse <NUM> allowing for operator input and the output of text and images to the operator. In some embodiments, the computer <NUM> is located remotely from the workstation <NUM>. Optionally, the computer <NUM> may form a portion of the workstation <NUM>. 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 <NUM> 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 <NUM> 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 <NUM> also manually adjusted using the keyboard <NUM>, the mouse <NUM>, or a touch screen icon on the display itself.

During operation, the system <NUM> 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 <NUM> emits x-rays at a narrow band of energies of a few keV and in the diagnostic imaging range of approximately <NUM>-<NUM> keV. In the dual-energy mode, the x-ray source <NUM> emits radiation at two or more bands of energy emitted simultaneously or in rapid succession. The x-ray source <NUM> may also be configured to emit a single broadband energy of more than a few keV over the diagnostic imaging range. The system <NUM> may be switched between the dual energy mode and the single energy mode by increasing or decreasing the x-ray source <NUM> 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 <NUM> may emit x-rays at different energies or ranges of energies.

The x-ray source <NUM> may be configured to output a fan beam of x-rays <NUM> as shown in <FIG>. In some embodiments, the computer <NUM> controls the system <NUM> 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 <NUM> 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 <NUM> shown in <FIG> 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> depicts a schematic diagram <NUM> of a detector and related circuitry used in system of <FIG>. In general, schematic diagram <NUM> shows a detector <NUM> which can be used as detector <NUM> of <FIG>. The detector <NUM> includes an array of detector pixels <NUM> having n rows and m columns. Each of these detector pixels are connected to a charge amplifier <NUM>, one or more comparators <NUM> and a counter <NUM>. When the x-ray beam <NUM>, 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 <NUM>. 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 <NUM> is passed on to the comparator <NUM>, which discriminate the charge into different energy windows (e.g. Low and High energy windows) and provides the corresponding output to the counter <NUM>. The counter <NUM> then updates (increments) the corresponding low energy or high energy count based on the output from the comparator <NUM>. The contents (or x-ray energy count) of the counter <NUM> 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 <NUM> will not get updated. After a certain time duration (e.g., a recording period time duration), the x-ray energy count of the counter <NUM> are provided to a processor such as computer <NUM>. 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>. 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> is a schematic diagram <NUM> that describes the TDS method in accordance with an embodiment of the present technique. The schematic diagram <NUM> shows a detector <NUM> having a detector pixel array of n rows and m columns. As against <FIG>, where after the recording period time duration, the x-ray energy count of a detector pixel counter is provided to a processor <NUM>, in <FIG>, 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 <NUM> msec and total number of shifts may be <NUM> for one acquisition whereas the recording period time duration in <FIG> may be <NUM> 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 <NUM> of the x-ray beam is from right to left then the x-ray energy count shift direction <NUM> is from left to right i.e., opposite direction. A pulse diagram <NUM> at the bottom of <FIG> shows TDS shift strobes or pulses <NUM> which have a TDS shift frequency f. The TDS shift frequency f translates into a TDS shift time duration Δt<NUM>=<NUM>/f 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 <NUM> is <NUM> millisecond (f=<NUM>) then the contents of the counter of detector pixel (i, j) are shifted to the right counter of detector pixel (i, j+<NUM>) after every <NUM> 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 <NUM> 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 f 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> is a schematic diagram <NUM> depicting TDS imaging sequence of a bone of a patient. Schematic diagram <NUM> shows three steps <NUM>, <NUM>, <NUM> of TDS imaging sequence corresponding to <NUM> positions of the X-ray beam <NUM> at three different time instances. Schematic diagram <NUM> shows an object e.g., a bone <NUM> in a patient <NUM> who is lying on a table <NUM>. For all three steps <NUM>, <NUM>, <NUM>, corresponding positions <NUM>, <NUM>, <NUM> of detector <NUM> are also shown in schematic diagram <NUM>. The detector <NUM> 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 <NUM> is the first time instance when the x-ray beam <NUM> passes through the bone <NUM>. In the embodiment shown, the x-ray scanner is moving from left side to the right side in reference to the bone <NUM> at a velocity v. Thus, at step <NUM>, the x-ray beam <NUM> is at an acute angle with respect to table plane <NUM> to start with. The attenuated x-ray beam <NUM> i.e., the x-ray beam <NUM> after passing through the bone <NUM> then hits the first detector pixel of the detector <NUM> as seen in detector position <NUM>. Step <NUM> refers to a second time instance when the x-ray beam <NUM> is at a right angle with respect to table plane <NUM>. The attenuated x-ray beam <NUM> then hits the middle detector pixel of the detector <NUM> as seen in detector position <NUM>. Step <NUM> corresponds to a third time instance when the x-ray beam <NUM> is at an obtuse angle with respect to table plane <NUM>. In this instance, the attenuated x-ray beam <NUM> hits the last detector pixel as seen in detector position <NUM>.

If the TDS method is not used then the charge accumulated at detector pixels corresponding to bone imaging at positions <NUM>, <NUM> and <NUM> 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 <NUM>, <NUM> and <NUM> 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 <MAT> where v is the scanning speed or x-ray beam motion speed, a is the detector pixel dimension in the shift direction, and MO 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 MO 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., <MAT> 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 (<NUM>) 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> is a schematic diagram <NUM> depicting effect of TDS imaging on objects having different heights. The schematic diagram <NUM> shows x-ray source <NUM> and a detector <NUM>. A first object <NUM> is located at a distance O from source <NUM> and a second object <NUM> is located at a distance O' from source <NUM>. Further, the detector <NUM> or the image plane is located at a distance equal to I from the source <NUM> i.e., SID=I. In this case, if the optimum TDS shift frequency is set based on object <NUM> (i.e., SOD=O in equation (<NUM>) above) then the x-ray energy count of the detector pixels corresponding to object <NUM> 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 <NUM>.

Because object <NUM> is at a different height compared to object <NUM>, the x-ray energy count corresponding to object <NUM> 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 <NUM> 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 <NUM> for every TDS shift time duration Δt = <NUM>/f. In one embodiment, the image slippage (s) of object <NUM> per TDS shift time duration may be given as: <MAT>.

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 = <NUM> cm, O = <NUM> cm, and O' = <NUM> cm, a = <NUM> mm and N = <NUM>. In this case, in the detector plane, the total TDS blurring of object <NUM> at O' would be sN = <NUM>, compared to pixelation and motion blurring of <NUM> for object <NUM> 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>, in one embodiment, the acquired images from system <NUM> 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 <NUM> 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 <CIT> is utilized. 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>. It should be noted that the while describing TDS technique in <FIG> 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> 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>) 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 <NUM>.

Referring now to <FIG>, the fan beam <NUM> of system <NUM> diverges slightly (e.g., with a divergence angle <NUM>) as it passes from a focal spot of the x-ray source <NUM> to the detector <NUM>. This creates a triangular overlap region <NUM> formed by the intersection of the volumes of the swept fan beams for successive transverse scans. Generally, the overlap region <NUM> is formed by the intersection of the area of the fan beam <NUM> when the x-ray source and detector <NUM> are in a first position <NUM> to acquire a first scan image and the area of the fan beam <NUM>' when moved to an adjacent second longitudinal position <NUM> 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 <NUM> is collimated to reduce this divergence to a value of less than <NUM>° and preferably to approximately <NUM>° so as to reduce parallax and height dependency problems. The increased numbers of overlap regions <NUM> 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>, a first scan image <NUM> may be obtained with a transverse scan such as that at first longitudinal position <NUM> as shown in <FIG> and shows an object <NUM>, in this case, a portion of a spinal column <NUM> that is also imaged in a second scan image <NUM> acquired along second longitudinal position <NUM> as shown in <FIG>. Adjacent edges of scan images <NUM> and <NUM> have overlap portions <NUM> that are known functions of the size of the raster scan (the longitudinal increment) and the known divergence of the fan beam <NUM>. Nevertheless, the exact position of objects <NUM> within the overlap portion <NUM> of images <NUM> and <NUM> will vary depending on the height of the object <NUM>. Accordingly, images <NUM> and <NUM> must be shifted longitudinally with respect to each other to overlap by an arbitrary amount to bring objects <NUM> into alignment.

<FIG> shows a flowchart <NUM> describing a method of merging adjacent scan images of an object generated due to overlap between adjacent scans. In step <NUM>, the present invention acquires successive transverse scans to produce scan images <NUM> and <NUM> having overlap portions <NUM> as shown in <FIG>. At a succeeding step <NUM>, an object image or a bone image is extracted from each of the scan images <NUM> and <NUM> so as to highlight the image of the object or bone <NUM> of relevance in the measurement. At step <NUM>, the data of the adjacent scan images <NUM> and <NUM> within the overlap portions <NUM> 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 <NUM>. 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 <NUM> has varying or multiple height within the overlap portions <NUM>, this best fit will in fact reflect an average of that height.

The height of the bone itself is calculated at step <NUM> using simple trigonometric identities relying on the known beam angle <NUM> and a longitudinal distance D of the overlap region <NUM> determined from the overlap images. For example, a triangle is formed with base D, height as H and the beam angle <NUM>. Thus, if the beam angle and base is known then height H can be calculated using trigonometric principles. This height is used at step <NUM> to scale the image <NUM> or <NUM> so as to provide a predetermined constant magnification to the object <NUM>. 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 <NUM>, images are weighted with the weighting mask <NUM> shown in <FIG> and merged to create to a single file of image data.

<FIG> shows a flowchart <NUM> describing a method of imaging a patient in accordance with an embodiment of the present technique. In step <NUM>, 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 <NUM> 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 <NUM> higher, so then the height of the femur may be selected as <NUM>. Further, if the distance from the body of the patient to the X-ray source level is another <NUM> then the height may be selected as <NUM> + <NUM> =<NUM>.

In step <NUM>, TDS frequency is determined based on the initial height of the patient bone. The TDS frequency may be determined as in equation (<NUM>) above. Thereafter, steps <NUM>-<NUM> which are similar to steps <NUM>-<NUM> of <FIG> are performed. In other words, in steps <NUM>, 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 <NUM>. At step <NUM>, the data of the adjacent scan images within the overlap portions are correlated. The height of the bone is calculated at step <NUM>. This height is used at step <NUM> to scale the scan images so as to provide a predetermined constant magnification to the bone. Finally, at step <NUM>, scan images are weighted with the weighting mask and merged to generate the bone image.

At step <NUM>, the calculated height of the bone in step <NUM> is compared with the initial height in step <NUM>. At step <NUM> 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 <NUM>. In another embodiment, such a threshold may be <NUM> and so on. If the heights are not sufficiently close, then the method moves to step <NUM> where a new TDS frequency is determined based on the calculated height of the bone in step <NUM>. The method then moves back to step <NUM> to repeat the scan and to generate the new bone image based on the new TDS frequency determined in step <NUM>. However, if in step <NUM>, it is determined that the initial height is sufficiently close to the calculated height then the scan is finished in step <NUM> which finalizes the bone image.

<FIG> shows a flowchart <NUM> 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 <NUM>. Based on the initial height, the TDS frequency is also determined in step <NUM> based on equation (<NUM>). As against <FIG> and <FIG>, in this method, a complete scan of the bone is not performed based on the TDS frequency. Instead, at step <NUM>, 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 <NUM>, a bone image is extracted from each of the scan images. At step <NUM>, the data of the adjacent scan images within the overlap portions are correlated. The height of the bone is calculated at step <NUM>.

At step <NUM>, the calculated height of the bone in step <NUM> is compared with the initial height in step <NUM>. At step <NUM> 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 <NUM> where a new TDS frequency is determined based on the calculated height of the bone in step <NUM> and the new TDS frequency is set up in the system in step <NUM>. The method then continues to steps <NUM>-<NUM> which are similar to steps <NUM>-<NUM> of <FIG> and are performed similarly to generate the final bone image of the patient.

However, if at step <NUM> it is determined that the initial height selected in step <NUM> is sufficiently close to the calculated height in step <NUM> then the method moves to step <NUM> to resume the scan. At step <NUM>, the calculated height is used to scale the scan images generated in step <NUM> so as to provide a predetermined constant magnification to the bone. Finally, at step <NUM>, scan images are weighted with the weighting mask and merged to generate the bone image and the method is ended at step <NUM>.

<FIG> shows a flowchart <NUM> describing yet another method of imaging a patient in accordance with an embodiment of the present technique. Similar to method <NUM>, at start, an initial height of the bone with respect to the x-ray source level is determined in step <NUM>. Based on the initial height, the TDS frequency is also determined in step <NUM>. However, the main difference between method <NUM> and method <NUM> is that in method <NUM>, the TDS frequency is adjusted after every sweep of the x-ray source over the bone region of interest. For example, at step <NUM>, 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 <NUM>, a bone image is extracted from each of the scan images. At step <NUM>, 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 <NUM>. Based on the height determined in step <NUM>, a new TDS frequency is determined in step <NUM>.

At step <NUM>, 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 <NUM>. 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 <NUM> as earlier. However, if at step <NUM> it is determined that the scan is complete then at step <NUM>, the calculated height in step <NUM> is used to scale the scan images generated in step <NUM> or <NUM>. Finally, at step <NUM>, scan images are weighted with the weighting mask and merged to generate the bone image and the method is ended at step <NUM>.

<FIG> shows a flowchart <NUM> describing a method of imaging a patient in accordance with another embodiment of the present technique. At step <NUM>, 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 <NUM>, 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 <NUM>. 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 <NUM>, 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 <NUM>. Further, at step <NUM>, 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 <NUM>, 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 <NUM>, an image of the object based on detected X-ray energy at the X-ray detector is generated.

In one embodiment of the method <NUM>, 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.

Claim 1:
A medical imaging system comprising:
an X-ray source operative to transmit X-rays through an object, wherein the X-ray source is collimated to produce a diverging beam of radiation;
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 programmed to:
select an initial height of the object with respect to the X-ray source plane;
determine an initial time delayed summation (TDS) shift frequency based on the initial height, wherein the TDS shift frequency is a frequency at which a content of each detector pixel counter is shifted to a neighboring detector pixel counter;
perform a first scan of the object based on the TDS shift frequency;
determine a new height of the object based on a beam angle and an overlap of adjacent images;
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.