Abstract:
A system and method of medical imaging is designed to reduce a patient&#39;s X-ray exposure during scanning based upon patient size and task dependency. The system includes receiving a task and patient size dependency input and determining threshold levels based on the received inputs to separate imaging data into a number of projection sets for further image processing and reconstruction of an image. Each projection set can then be independently processed based on the type of task and/or the patient size to allow reduced and modified X-ray doses dependent on the task and/or specific patient to be scanned.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to medical imaging and, more particularly, to a system and method of imaging a region of interest (ROI) based upon patient size and/or task selection, preferably in computed tomography systems.  
           [0002]    Typically, in computed tomography (CT) imaging systems, an X-ray source emits a fan-shaped beam toward an object, such as a patient. The beam, after being attenuated by the patient, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the X-ray beam by the patient. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing unit for analysis which ultimately results in the formation of an image.  
           [0003]    Generally, the X-ray source and the detector array are rotated with a gantry within an imaging plane and around the patient. X-ray sources typically include X-ray tubes, which conduct a tube current and emit the X-ray beam at a focal point. X-ray detectors typically include a collimator for collimating X-ray beams received at the detector, a scintillator for converting X-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator.  
           [0004]    In one known CT imaging system used to image an ROI, imaging of a patient is conducted by moving the patient through a gantry. Preferably, it is desirable to minimize the patient&#39;s exposure to X-rays. To do so, improved signal processing has allowed the use of lower dose CT scans, such as the commercially available 0.5 second CT scanner. However, for larger and heavier patients, low signal streaking problems are known to occur due to low tube current values for certain angular views. One proposed solution to the low signal streaking problem is to determine a threshold based upon clinical evaluation of large or heavy patient scans. The determined threshold is then fixed, and corrections during image processing are performed based upon a signal strength that corresponds to X-rays being attenuated by large or dense objects. Problems arise, however, when reducing the dose in CT scans further, and in particular, for smaller patients and task dependent scans.  
           [0005]    There is a need for a system that can apply the lowest possible patient doses based on patient size, especially for pediatric patients, and/or based on a task to be performed. Setting fixed patient thresholds to correct for low signal streaking problems in medium and smaller size patients does not improve reconstructed images of the patients, but may expose such patients to unnecessary X-ray radiation. Furthermore, certain sub-regions of the ROI may require a lower image resolution, or alternatively, a particular task such as Cardiac Artery Calcification Scoring may require a lower image resolution as compared to Cardiac Artery imaging thereby permitting application of a lower patient dose of radiation.  
           [0006]    Since lower radiation exposure is an on-going goal in X-ray and CT development, it would be desirable to have an imaging system capable of processing imaging data according to an automated selection of a patient size and/or task dependency to reduce a patient&#39;s X-ray exposure during scanning of the patient.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0007]    The present invention provides a system capable of processing imaging data according to selection of a patient size and/or task dependency to reduce a patient&#39;s X-ray exposure during scanning of the patient, and a method of processing imaging data that solves the aforementioned drawbacks.  
           [0008]    A system and method of computer tomography imaging to reduce a patient&#39;s X-ray exposure based upon patient size and/or task selection prior to scanning of the patient are provided. The system includes a high frequency electromagnetic energy projection source to project X-rays towards an object, such as a patient. A detector receives the high frequency energy attenuated by the patient, and a plurality of electrical interconnects is configured to transmit detector outputs to a data processing system. The system also includes a computer capable of receiving a task and patient size dependency selection input and determining a threshold level based on the received inputs to separate the detector outputs into a number of projection sets for further image processing to reconstruct an image.  
           [0009]    In accordance with one aspect of the present invention, a method of processing imaging data for a radiation emitting medical device includes the steps of providing a task and patient size dependency selection and setting a first threshold level based on the task and patient size dependency selection. The method also includes the steps of acquiring imaging data and separating the imaging data into a plurality of projection sets based on the first threshold level. The method further includes the step of uniquely processing the imaging data of each projection set to reconstruct an image.  
           [0010]    In accordance with another aspect of the invention, a computed tomography system is provided. This system includes a high frequency electromagnetic energy projection source to project high frequency energy towards an object and a detector to receive high frequency electromagnetic energy attenuated by the object. The detector produces outputs that are transmitted to a data processing system by a plurality of electrical interconnects. The system further includes a computer programmed to receive the detector outputs and a task and patient size selection input, and determine threshold levels based on the received task and patient size selection input. The computer is further programmed to separate the detector outputs into a plurality of projection sets based on the threshold levels, and reconstruct the separated plurality of projection sets to produce a visual image.  
           [0011]    In accordance with yet another aspect of the invention, a computer-readable medium having stored thereon a computer program having a set of instructions that, when executed by a computer, will cause the computer to receive a selection signal of a task and patient size input, and determine at least one threshold based upon the received selection signal. The computer program also has instructions to receive imaging data signals acquired with low-dose radiation, and synthesize the imaging data signals into a plurality of projection sets. The computer further includes instructions to process each projection set based on the selection signal and the threshold, and to reconstruct a visual image with improved artifact reduction.  
           [0012]    Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.  
         [0014]    In the drawings:  
         [0015]    [0015]FIG. 1 is a perspective view of a CT imaging system incorporating the present invention.  
         [0016]    [0016]FIG. 2 is a perspective block schematic diagram of the system illustrated in FIG. 1.  
         [0017]    [0017]FIG. 3 is a flow chart showing a process of the present invention and implemented in the system of FIGS. 1 and 2. 
     
    
     DETAILED DESCRIPTION  
       [0018]    A system and method is described for a computed tomography (CT) system capable of imaging an ROI. It will be appreciated by those of ordinary skill in the art that the present invention is equally applicable for use with different CT system configurations. Moreover, the present invention will be described with respect to the detection and conversion of X-rays. However, one of ordinary skill in the art will further appreciate, that the present invention is equally applicable in other imaging modalities.  
         [0019]    Referring to FIGS. 1 and 2, an exemplary computed tomography (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an X-ray source  14  that projects a beam of X-rays  16  toward a detector array  18  on the opposite side of the gantry  12 . Detector array  18  is formed by a plurality of detectors  20  which together sense the projected X-rays that pass through a medical patient  22 . Each detector  20  produces an electrical signal that represents the intensity of an impinging X-ray beam and hence the attenuated beam as it passes through the patient  22 . During a scan to acquire X-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . Detector array  1   8  and detectors  20  can be any number of high frequency electromagnetic energy detectors, such as gas-filled, scintillation cell-photodiode, and semiconductor detectors as is know to those skilled in the art of detector design.  
         [0020]    Rotation of gantry  12  and the operation of X-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an X-ray controller  28  that provides power and timing signals to an X-ray source  1   4  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detectors  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 reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 .  
         [0021]    Computer  36  also receives commands and scanning parameters, such as patient size and task dependency, from an operator via console  40  that has a keyboard for entering commands and scanning parameters. 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 speed controller  44  which controls a variable speed table  46  during imaging of a patient  22  within gantry  12 . Particularly, table  46  is configured to move a patient  22  through a gantry opening  48  along an axis  50 , and may include a single or multiple speed settings.  
         [0022]    In operation, a patient  22  or object is positioned within the CT scanner or imaging device  10  on the variable speed table  46  with a selected region of the patient chosen for scanning adjacent to the gantry  12 . A technician or health-care operator enters input into the operator console  40 , thereby defining a ROI or scanning region such as a thorax of the patient  22 , which includes a cardiac region  52  and a pair of non-cardiac regions  54 . The computer  36  then instructs the table speed controller  44  to move the table  46  towards the gantry opening  48  causing the patient  22  to enter the gantry opening  48 . Control mechanism  26  causes X-ray controller  28  to provide power and timing signals to X-ray source  14  while the gantry motor controller  30  causes rotation of gantry  12  to conduct an imaging scan of the patient  22  passing through the gantry  12 .  
         [0023]    After scanning the ROI, detectors  20  send the X-ray data acquired to DAS  32  and image reconstructor  34  for digitalization and image reconstruction. Computer  36  then processes the digitized X-ray data to provide a reconstructed image of the ROI on display  42 .  
         [0024]    Referring to FIG. 3, a flowchart illustrating the steps of a method and acts associated with a computer program in accordance with the present invention implemented in the system shown in FIGS. 1 and 2 are shown. The method and/or computer program is initiated at  100  by a technician or CT scanner operator who provides input into the computer at  102  to select a task and/or patient size dependency for a particular ROI. Generally, such operator-entered input can further include a starting position and an ending position along a common axis, such as axis  50  of FIG. 1 for conducting a scan. A patient size dependency query is then determined at  104 , and if patient size dependency is selected  106 , a scout scan is acquired  108 . After acquiring the scout scan  108 , the method and/or computer program proceeds to automatically determine a threshold at  110 , and receive a task (if any) and set the thresholds accordingly  112 . If patient size dependency is not selected  114 , the method and/or computer program receives also receives a task (if any) at  112  and sets the thresholds accordingly. After thresholds are set  112 , the method and/or computer program allows interactive threshold adjustment at  116  to change the threshold.  
         [0025]    After allowing interactive threshold adjustment  112 , initial projections are acquired  118  using projection techniques known to those skilled in the art. For example, in one embodiment using parallel projection CT scanners, a patient in a two dimensional plane (x, y) is irradiated by an X-ray source. Alternatively, other sources such as ultrasound and MRI may be used. The radiation emitted by the source penetrates the patient along straight lines in the two-dimensional plane and is attenuated as it passes through the patient. A detector measures such attenuated signals and calculates the projection measurement data as line integrals using the following equation:  
           P   n ( j )=∫∫ f ( x,y )∂( x  cos  n   i   +y  sin  n   i   −r   j ) dxdy ,   (Eqn. 1)  
         [0026]    wherein P n (j) are the calculated projections.  
         [0027]    The acquired projections  118  that are lower than a defined threshold, T low  are truncated to modify the projections  120 . Preferably, the truncated projections are modified based on their initial values. The modified projections  120  are then smoothed  122 . In one embodiment, the modified projections are grouped into first, second, and third projection sets having projection data above a first threshold, between the first and a second threshold, or below a third threshold respectively. Preferably, the first set of projections are smoothed using a lower order, 3-point smoothing technique, the second set of projections are smoothed using a medium order, 5-point smoothing technique, and the third set of projections are smoothed using a higher order, 7-point smoothing technique.  
         [0028]    After smoothing the projections  122 , error projections, E n (j) are formed  124  and modified based on each error projection&#39;s strength  126 . In a preferred embodiment, the error projections are modified according to the following equations:  
           E   n ( j )=P n ( j )− P   n ( j ) smoothed    (Eqn. 2)  
           E   n ( j ) modified   =E   n ( j )* M _factor n ( j ), and   (Eqn. 3)  
           M _factor n ( j )=exp(−1.0* P   n ( j )/ C _factor),   (Eqn. 4)  
         [0029]    wherein C_factor is a constant that depends on the threshold selections, P n (j) are the initial projections, P n (j) smoothed  are the smoothed projections, and M_factor n (j) is the modification factor that modifies the error projections, E n (j) to form the modified error projections, E n (j) modified . After modification  126 , the error projections are formed into a final set of projections  128 .  
         [0030]    The method next decides at  130  whether the initial projections are greater than a first threshold, and if so  132 , performs Fourier deconvolution on the final set of projections  134 . If the initial projections are not greater than the first threshold  136 , an image is reconstructed  138 . Similarly, the Fourier deconvoluted projections  134  are used to reconstruct an image at  138 . The method then ends at  140 .  
         [0031]    As previously discussed and in accordance with one aspect of the present invention, a method of processing imaging data for a radiation emitting medical imaging device, such as a CT scanner, includes the steps of providing a task and patient size dependency selection and setting a first threshold level based on the task and patient size dependency selection. The method also includes the step of acquiring imaging data for image reconstruction, and separating the imaging data into a plurality of projections sets based on the first threshold level. The method further includes the step of uniquely processing the imaging data of each projection set prior to reconstruction of the image.  
         [0032]    In accordance with another aspect of the invention, a computed tomography system is provided. This system includes a high frequency electromagnetic energy projection source to supply a patient dose or project high frequency energy towards a patient or object, and a detector to receive high frequency electromagnetic energy attenuated by the patient or object. The detector generates outputs that are transmitted to a data processing system by a plurality of electrical interconnects. The system also includes a computer programmed to receive the detector outputs, and a task and patient size selection input. The computer determines threshold levels based on the received task and patient size selection input, and is further programmed to separate the detector outputs into a plurality of projection sets based on the threshold levels. The computer is also programmed to reconstruct the separated plurality of projection sets, preferably after further image processing, and produce a visual image.  
         [0033]    In accordance with yet another aspect of the invention, a computer-readable medium having stored thereon a computer program having a set of instructions that, when executed by a computer, will cause the computer to receive a selection signal of a task and patient size input and determine at least one threshold based upon the received selection signal. The computer program also includes instructions to receive imaging data signals acquired with low-dose radiation and synthesize the imaging data signals into a plurality of projection sets. The computer program further includes instructions to process each projection set based on the selection signal and the threshold to reconstruct a visual image.  
         [0034]    The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the append the steps ofing claims.