Abstract:
A method for obtaining data includes scanning an organ with an imaging system emitting X-rays and modulating the emitted X-rays with an organ specific bowtie addition.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to medical imaging and, more particularly, to medical imaging using bowtie filters. 
     At least some known bowties used in current Computed Tomographic (CT) scanners are designed for general uses. For example, a General Electric LightSpeed scanner commercially available from General Electric Medical Systems of Waukesha Wis. has a head bowtie for the head and pediatric applications and a body bowtie for adult body scans. The body bowtie was designed to provide a fairly uniform X-ray flux on the detector surface after the X-rays pass through the body, therefore providing relatively equivalent image quality (noise) for the whole imaging area. This, however, may not be necessary if one is only interested in specific organs, such as a heart, and may introduce extra surface dose to the patient that may not improve the image quality of the specific organs that one is interested in. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a method for obtaining data includes scanning an organ with an imaging system emitting X-rays and modulating the emitted X-rays with an organ specific bowtie addition. 
     In another aspect, a method for scanning an object with an imaging system having a bowtie filter is provided. The method includes positioning a bowtie addition in the imaging system, and scanning an object. 
     In yet another aspect, a collimator assembly for an imaging system is provided. The collimator assembly includes a bowtie filter, and a bowtie addition positioned proximate the bowtie filter. 
     In still another aspect, an imaging system is provided. The imaging system includes a radiation source, a radiation detector positioned to receive X-rays from the source, a bowtie filter positioned between the radiation source and the radiation detector, a bowtie addition positioned between the radiation source and the radiation detector, and a computer operationally coupled to the radiation source and the radiation detector, the computer is configured to scan objects. 
     In another aspect, a Computed Tomography (CT) imaging system includes a radiation source, a radiation detector positioned to receive X-rays from the source, a bowtie filter positioned between the radiation source and the radiation detector, a bowtie addition positioned between the radiation source and the radiation detector, and a computer operationally coupled to the radiation source and the radiation detector, the computer is configured to perform CT scans. 
     In one aspect, a Computed Tomography (CT) imaging system includes a radiation source, a radiation detector positioned to receive X-rays from the source, a bowtie filter positioned between the radiation source and the radiation detector, a bowtie addition comprising a plurality of thick sections interspersed with a plurality of thin sections positioned between the bowtie filter and the radiation detector, and a computer operationally coupled to the radiation source and the radiation detector, the computer is configured to perform CT scans of hearts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial view of a CT imaging system. 
         FIG. 2  is a block schematic diagram of the system illustrated in FIG.  2 . 
         FIG. 3  is a more detailed view of the collimator assembly shown in FIG.  2 . 
         FIG. 4  illustrates an X-ray modulation  24  corresponding to the bowtie addition shown in  FIGS. 2 and 3 . 
         FIG. 5  illustrates an image comparison from three different sets of data. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In some known CT imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of an x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at the detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile. 
     In third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. 
     In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units” (HU), which are used to control the brightness of a corresponding pixel on a cathode ray tube display. 
     To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. 
     Reconstruction algorithms for helical scanning typically use helical weighing algorithms that weight the collected data as a function of view angle and detector channel index. Specifically, prior to a filtered backprojection process, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The weighted data is then processed to generate CT numbers and to construct an image that corresponds to a two-dimensional slice taken through the object. 
     To further reduce the total acquisition time, multi-slice CT has been introduced. In multi-slice CT, multiple rows of projection data are acquired simultaneously at any time instant. When combined with helical scan mode, the system generates a single helix of cone beam projection data. Similar to the single slice helical, weighting scheme, a method can be derived to multiply the weight with the projection data prior to the filtered backprojection algorithm. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     Also as used herein, the phrase “reconstructing an image” is not intended to exclude embodiments of the present invention in which data representing an image is generated but a viewable image is not. However, many embodiments generate (or are configured to generate) at least one viewable image. 
     Referring to  FIGS. 1 and 2 , a multi-slice scanning imaging system, for example, a Computed Tomography (CT) imaging system  10 , is shown as including a gantry  12  representative of a “third generation” CT imaging system. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of gantry  12 . Detector array  18  is formed by a plurality of detector rows (not shown) including a plurality of detector elements  20  which together sense the projected x-rays that pass through an object, such as a medical patient  22  between array  18  and source  14 . A collimator assembly  19  is positioned between array  18  and source  14 . Collimator assembly  19  includes a known bowtie filter  21  and a bowtie addition  23 . Bowtie addition  23  is fabricated from any material suitable for fabricating known bowtie filters. In one embodiment, bowtie addition  23  is positioned between bowtie filter  21  and array  18 . Alternatively, addition  23  is positioned between bowtie filter  21  and source  14 . Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence can be used to estimate the attenuation of the beam as it passes through object or patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted therein rotate about a center of rotation  24 .  FIG. 2  shows only a single row of detector elements  20  (i.e., a detector row). However, multi-slice detector array  18  includes a plurality of parallel detector rows of detector elements  20  such that projection data corresponding to a plurality of quasi-parallel or parallel slices can be acquired simultaneously during a scan. 
     Rotation of components on gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of components on gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detector elements  20  and converts the data to digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high-speed image reconstruction. The reconstructed image is applied as an input to a computer  36 , which stores the image in a storage device  38 . Image reconstructor  34  can be specialized hardware or computer programs executing on computer  36 . 
     Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28 , and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44 , which controls a motorized table  46  to position patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 . 
     In one embodiment, computer  36  includes a device  50 , for example, a floppy disk drive, CD-ROM drive, DVD drive, magnetic optical disk (MOD) device, or any other digital device including a network connecting device such as an Ethernet device for reading instructions and/or data from a computer-readable medium  52 , such as a floppy disk, a CD-ROM, a DVD or an other digital source such as a network or the Internet, as well as yet to be developed digital means. In another embodiment, computer  36  executes instructions stored in firmware (not shown). Computer  36  is programmed to perform functions described herein, and as used herein, the term computer is not limited to just those integrated circuits referred to in the art as computers, but broadly refers to computers, processors, microcontrollers, microcomputers, programmable logic controllers, application specific integrated circuits, and other programmable circuits, and these terms are used interchangeably herein. Although the specific embodiment mentioned above refers to a third generation CT system, the methods described herein equally apply to fourth generation CT systems (stationary detector-rotating x-ray source) and fifth generation CT systems (stationary detector and x-ray source). Additionally, it is contemplated that the benefits of the invention accrue to imaging modalities other than CT. Additionally, although the herein described methods and apparatus are described in a medical setting, it is contemplated that the benefits of the invention accrue to non-medical imaging systems such as those systems typically employed in an industrial setting or a transportation setting, such as, for example, but not limited to, a baggage scanning system for an airport or other transportation center, and that addition  23  is used for objects other than organs. 
     Herein are described novel methods and apparatus for use in imaging systems. In one aspect, the use of an organ-specific bowtie addition (e.g., bowtie addition  23 ) together with bowtie  21  for imaging specific organs results in the use of less dose but equal noise as with the use of bowtie  21  singularly. A set of organ-specific bowtie additions are made to modulate the X-ray flux coming out of the tube-bowtie assembly (e.g., collimator assembly  19 ) based on the specific organs that a physician is interested in. In one embodiment, the addition has a smooth surface and can be moved in and out of collimator assembly  19  easily by an operator. In an exemplary embodiment, bowtie addition  23  is mounted within collimator assembly  19  such that the operator can remove bowtie addition  23  without tools. Additionally, the operator can replace bowtie addition  23  with another bowtie addition without the use of tools. The additions maintain a majority of the x-ray flux coming out of bowtie  21  for the interested organ area while reducing X-ray flux for other areas, therefore reducing the whole body dose. Special bowtie additions may be made for, but certainly not limited to, the CT applications of cardiac, lung, and liver. 
     As illustrated in  FIG. 3 , bowtie addition  23  is a specific example of a bowtie addition for cardiac imaging. Bowtie addition  23  has a smooth surface and is able to move in and out of collimator assembly  19  easily. For cardiac imaging, one can use a known head bowtie or a modified head bowtie that provides more attenuation of the X-ray towards the edge of the body than the current head bowtie. This modified head bowtie provides more X-ray flux than the current body bowtie near the center of the imaging field of view where the heart is located, while reducing the dose to the whole body by at least 20%. This modified head bowtie can also be used for the general head and pediatric scans. Combined with this modified head bowtie, a bowtie addition for cardiac imaging is also used such as bowtie addition  23 . Cardiac bowtie addition  23  is designed to account for the fact that the heart is not located exactly at the center of the imaging field of view, and that X-ray flux requirement for the lung area is substantially less. In the exemplary example of a cardiac bowtie addition as shown in  FIG. 3  includes a plurality of thick sections  60  interspersed with a plurality of thin sections  62 . A middle thick section  64  is less thick than the other thick sections  60 . Although thick section  64  can be between one-third and two-thirds the thickness of other thick sections  60 , thick section  64  is always about one-half the thickness of the other thick sections  60 . In another example, bowtie addition  23  has more than 5 sections. In yet another example, bowtie addition  23  has 9 sections. In still another embodiment, bowtie addition  23  has 12 sections. In a further embodiment, bowtie addition  23  has at least one but less than 5 sections. In an additional embodiment, bowtie addition  23  has 4 sections. An X-ray modulation  66  is shown in FIG.  4 . X-ray modulation  66  corresponds to bowtie addition  23  having 5 sections. 
     Cardiac bowtie addition  23  has been evaluated using patient scans. Two sets of cardiac scans were obtained at both 320 mA and 200 mA for clinical evaluation. Lower dose scans (210 mA average) with the X-ray modulated according to the cardiac bowtie addition was simulated using a noise addition tool based on the original 320 mA scans. These three sets of scan data were reconstructed using the standard reconstruction algorithm. Image noises were measured at three different locations on three sets of images.  FIG. 5  shows the image comparison from the three different sets of data. Also shown on the images are the noise measurements. The comparison indicates that (1) using the current body bowtie for cardiac imaging, the noise increases as the mA decreases. The discrepancies of the edge noise numbers were caused by the current fan beam reconstruction algorithm. After the two numbers were averaged, they still follow the inverse square root of the mA rule. And (2), with the use of bowtie addition  23 , the noise measurements in the heart area of the simulated lower dose scans were about the same as the original scans, even through the average mA (dose) has decreased by 30%. 
     Exemplary embodiments of methods, systems, and assemblies for facilitating a reduction in patient dose are described above in detail. The methods, systems, and assemblies are not limited to the specific embodiments described herein, but rather, components of each methods, systems, and assemblies may be utilized independently and separately from other components described herein. In addition, each methods, systems, and assemblies component can also be used in combination with other components described herein. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.