Abstract:
An imaging condition determining method for an x-ray CT apparatus for performing imaging by a helical scan. The method includes making a table of various imaging conditions in advance, extracting at least one imaging condition from the table in accordance with age, imaging region, imaging length of the patient as well as the highest-priority objective specified by a user, and displaying the exposure dose of the patient under the at least one imaging condition, enabling the user to adjust the exposure dose, and determining the imaging condition adjusted in response to the adjustment of the exposure dose to the at least one imaging condition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Japanese Application No. 2003-313774 filed Sep. 5, 2003. 
   BACKGROUND OF THE INVENTION 
   The present invention is related to an imaging condition determining method and an X-ray CT (computed tomography) apparatus, more specifically to an imaging condition determining method for an X-ray helical scan imaging CT, and an X-ray helical scan CT apparatus. 
   An X-ray CT apparatus for performing helical scan imaging has imaging condition setting performed by the user prior to the imaging. The imaging condition may be set in accordance with the subject and the objective of imaging. A variety of parameters such as tube voltage, tube current, X-ray beam thickness, image slice thickness, helical pitch and so on may be set in accordance with the imaging condition. For the imaging condition selected, the exposure dose of the patient is displayed such that the operator can decide the suitability of the imaging condition (for example, see the patent document 1 below). 
   Patent Document 1 JP-A-2003-79611 (pp 8 to 9, and  FIGS. 6 to 9 ) 
   In an X-ray CT apparatus as have been described above, if the exposure dose is not appropriate, the imaging condition is required to be revised. However it is not easy to reconfigure an alternative imaging condition consisted of a number of parameters so as to achieve the predetermined goal. In particular, the difficulty becomes increased with the cone beam characteristics elicited of the X-ray beam, along with the increase of the number of X-ray detector arrays. 
   In particular, when scanning with a thick beam, scan will terminate early, however the automatic control of the tube current in the direction of body axis will be unsubtle to increase the exposure dose. It is required to carefully select the thickness in accordance with the age of patient and the examination purpose. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to realize an imaging condition determining method and an X-ray CT apparatus, which facilitate the configuration of helical scan imaging condition for the imaging conformed to the predetermined purpose. 
   (1) The present invention in one aspect for solving the problem includes an imaging condition determining method for an x-ray CT apparatus for performing imaging by a helical scan, comprising the steps of: making a table of various imaging condition in advance; extracting a candidate of imaging condition from the table in accordance with age, imaging region, imaging length of the patient as well as the highest-priority objective specified by a user, and displaying the exposure dose of the patient under the candidate of imaging condition; enabling the user to adjust the exposure dose; and determining the imaging condition adjusted in response to the adjustment of the exposure dose to a candidate of the imaging condition. 
   (2) The present invention in another aspect for solving the above cited problem includes an X-ray CT apparatus, comprising: a memory means for storing a variety of imaging condition previously made as a table; a display means for extracting a candidate of imaging condition from the table in accordance with the age, imaging region, imaging length of the patient as well as the highest-priority objective specified by the user for displaying the exposure dose of the patient under the candidate of imaging condition; an adjusting means for enabling the user to adjust the exposure dose; and a determining means for determining an imaging condition by performing correction in response to the adjustment of the exposure dose to the candidate of imaging condition. 
   Preferably the highest-priority objective is the exposure dose for the appropriateness of the exposure dose. Preferably the highest-priority objective is any one of exposure dose, image quality and imaging speed, for the appropriateness of the exposure dose, image quality or imaging speed. Preferably the exposure dose is indicated as DLP for the appropriateness of the exposure dose. 
   When there is a plurality of candidates of imaging condition, it is preferable to determine an imaging condition based on the second priority objective specified by the user for obtaining the most appropriate imaging condition. Also the second priority objective is preferably one, which is different from the highest-priority objective, of the dose, image quality and imaging speed for obtaining the most appropriate imaging condition. 
   The imaging condition made as a table is preferably the imaging condition based on the performance for ensuring the performance. The imaging condition made as a table is preferably the imaging condition based on a simulation for diversify the imaging condition regardless of the past record. 
   In the above aspects the present invention may achieve an imaging condition determining method and an X-ray CT apparatus, which facilitate the configuration of imaging condition for helical scan imaging conformed to the predetermined object, by making a table from a variety of imaging condition, extracting a candidate of imaging condition from the table in accordance with the age, imaging region and imaging length of the patient and the highest-priority objective specified by the user and display the exposure dose of patient under the selected candidate of imaging condition, enabling the user to adjust the exposure dose, and determining the final imaging condition by performing correction in response to the adjustment of said exposure dose. 
   Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic block diagram of an X-ray CT apparatus. 
       FIG. 2  is a schematic structure of an X-ray detector. 
       FIG. 3  is a schematic diagram of an X-ray radiation and detection system. 
       FIG. 4  is a schematic diagram illustrating the relationship between the X-ray radiation and detection system and the subject. 
       FIG. 5  is a flow chart illustrating the operation of an X-ray CT apparatus. 
       FIG. 6  is a flow chart illustrating the operation of an X-ray CT apparatus. 
       FIG. 7  is an exemplary display of imaging condition candidate. 
       FIG. 8  is an exemplary display of most appropriate imaging condition. 
       FIG. 9  is an exemplary display of imaging condition candidate. 
       FIG. 10  is an exemplary display of imaging condition candidate. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The best mode for carrying out the invention will be described in greater details below with reference to the drawings.  FIG. 1  shows a schematic block diagram of an X-ray CT apparatus. This apparatus is an exemplary best mode carrying out the invention. The operation of this apparatus illustrates the exemplary best mode carrying out the invention with respect to the imaging condition determining method. 
   As shown in  FIG. 1 , the apparatus includes a scanning gantry  2 , an imaging table  4 , and an operating console  6 . The scanning gantry  2  includes an X-ray tube  20 . The X-ray beam emitted from the X-ray tube  20  but not shown in the figure will be collimated to an X-ray beam of the shape of cone, i.e., a cone beam X-ray by a collimator  22 , to irradiate an X-ray detector  24 . The X-ray detector  24  includes a plurality of detector elements arranged in an array corresponding to the sector of the X-ray beam. The structure of the X-ray detector  24  will be described later. 
   In the space between the X-ray tube  20  and the X-ray detector  24  a subject to be imaged is mounted on the imaging table  4  and carried. The X-ray tube  20 , the collimator  22 , and the X-ray detector  24  will constitute the X-ray radiation and detection system. The X-ray radiation and detection system will be described later. 
   The X-ray detector  24  is connected to a data collector unit  26 . The data collector unit  26  will gather the detected signals from each detector element in the X-ray detector  24  as digital data. The detection signal from the detector elements will be the signal illustrative of the projection of the subject by the X-ray beam. The signal will be referred to as projection data herein below or abbreviated simply to data. 
   The emission of X-ray from the X-ray tube  20  is controlled by an X-ray controller  28 . The connection between the X-ray tube  20  and the X-ray controller  28  is omitted in the figure. The collimator  22  is controlled by the collimator controller  30 . The connection between the collimator  22  and the collimator controller  30  is omitted in the figure. 
   Those parts from the X-ray tube  20  to the collimator controller  30  are mounted on the revolving unit  34  of the scanning gantry  2 . The rotation of the revolving unit  34  is controlled by the revolving controller  36 . The connection between the revolving unit  34  and the revolving controller  36  is omitted in the figure. 
   The operating console  6  includes a data processing unit  60 . The data processing unit  60  is constituted for example by a computer. The data processing unit  60  is connected to a control interface  62 . The control interface  62  is connected to the scanning gantry  2  and the imaging table  4 . The data processing unit  60  controls the scanning gantry  2  and the imaging table  4  through the control interface  62 . 
   The data collector unit  26 , the X-ray controller  28 , the collimator controller  30  and the revolving controller  36  in the scanning gantry  2  are controlled through the control interface  62 . The respective connection between those members and the control interface  62  is omitted in the figure. 
   The data processing unit  60  is connected to a data collector buffer  64 . The data collector buffer  64  is connected to the data collector unit  26  of the scanning gantry  2 . The data collected from the data collector unit  26  is sent through the data collector buffer  64  to the data processing unit  60 . 
   The data processing unit  60  is connected to a storage unit  66 . The storage unit  66  stores projection data input to the data processing unit  60  via the data collector buffer  64  and the control interface  62 . The storage unit  66  also stores a program for the data processing unit  60 . The operation of the apparatus is performed by executing the program by the data processing unit  60 . 
   The data processing unit  60  performs the image reconstruction using the projection data collected to the storage unit  66  through the data collector buffer  64 . The data processing unit  60  may be an example of the reconstruction means in accordance with the present invention. The image reconstruction may use such method as the filtered back projection. 
   The data processing unit  60  is connected to a display unit  68  and an operating device  70 . The display unit  68  is constituted of a graphic display device or the like. The operating device  70  is constituted of a keyboard equipped with a pointing device. 
   The display unit  68  displays a reconstructed image output from the data processing unit  60  and any other information thereon. The operating device  70  is manipulated by the operator to input a range of instructions and information to the data processing unit  60 . The operator uses the display unit  68  and the operating device  70  to operate interactively the apparatus. 
   A schematic structure of an X-ray detector  24  is shown in  FIG. 2 . As shown in the figure, the X-ray detector  24  is a multichannel X-ray beam detector having a plurality of X-ray detector elements  24  (ik) arranged in a two dimensional array. The plurality of X-ray detector elements  24  (ik) forms an X-ray receiving facet curved in a cylindrical concave form. 
   “i” indicates the channel number, for example i=1, 2, . . . , 1,000. “k” indicates the raw number, form example k=1, 2, . . . , 32. In the X-ray detector  24  (ik) elements having the same raw number k forms a detector element array. The number of detector element array of the X-ray detector  24  may not be considered to be limited to 32, but may be an appropriate plural number. 
   An X-ray detector  24  (ik) is formed by the combination of for example a scintillator and a photo diode. The detector unit may not be limited thereto, but may be any other forms such as a semiconductor X-ray detector element using Cadmium-Tellur (CdTe), or an ion chamber type X-ray detector unit using Xenon gas. 
     FIG. 3  shows the relationship between the X-ray tube  20 , the collimator  22  and the X-ray detector  24  in the X-ray radiation and detection system.  FIG. 3(   a ) is a schematic diagram illustrating a front view of the scanning gantry  2 ,  FIG. 3(   b ) is a side view. As shown in the figure, the X-ray beam emitted from the X-ray tube  20  is formed to a cone shaped X-ray beam  400  by the collimator  22  to irradiate on the X-ray detector  24 . 
     FIG. 3(   a ) illustrate the diversion of the cone shaped X-ray beam  400  in a direction. This direction will be referred to as the width direction herein below. The width of the cone shaped X-ray beam  400  matches the array direction of channels in the X-ray detector  24 . In  FIG. 3(   b ) there is shown an extension of the cone shaped X-ray beam  400  in another direction. This direction will be referred to as the thickness direction of X-ray beam  400  herein below. The thickness of the cone shaped X-ray beam  400  matches to the direction of apposition of a plurality of detector element arrays in the X-ray detector  24 . Two directions of extension of the X-ray beam  400  are orthogonal to each other. 
   As shown in  FIG. 4 , the subject  8  mounted on the imaging table  4  is carried in to the X-ray radiation space so as to cross the body axis over the cone shaped X-ray beam  400 . The scanning gantry  2  has a cylindrical structure containing the X-ray radiation and detection system therein. 
   The X-ray radiation space is formed in an inner space of the cylindrical structure of the scanning gantry  2 . The sliced image of the subject  8  using the X-ray beam  400  is projected to the X-ray detector  24 . The X-ray detector  24  detects X-ray transmitted through the subject  8  for each detector array. The thickness of the X-ray beam  400  radiated to the subject  8  is controlled by the opening of an aperture of the collimator  22 . 
   In parallel to the revolution of the X-ray radiation and detection system, continuous movement of the imaging table  4  in the body axis of the subject  8  as shown by the arrow  42  allows the X-ray radiation and detection system to spin on the helical trajectory around the subject  8  in relation to the subject  8 . This results in a so-called “helical scan”. 
   For one turn of scan, a plurality of (for example, about approximately 1,000) views of projection data are gathered. The collection of the projection data is performed by the system including the X-ray detector  24 , the data collector unit  26 , and the data collector buffer  64 . The projection data will be referred to as scan data herein below. Also the projection data in a view will be referred to as view data. 
   The operation of the apparatus will be described below.  FIG. 5  shows the flow diagram of the operation of the apparatus. As shown in the figure, in stage  501  the scan imaging condition is configured. The details of how to determine the imaging condition will be described later. 
   Next, in stage  503 , a helical scan is performed. The helical scan is performed in accordance with the imaging condition configured in stage  501 , by the scanning gantry  2  and the imaging table  4 . Then, in state  505 , an image is reconstructed. The image reconstruction is performed by the data processing unit  60 . The data processing unit  60  reconstructs the image using such method as the filtered back projection, based on the projection data collected by the helical scan. The reconstructed image is displayed and stored in stage  507 . The image display is performed by the display device  60 . The image storage is performed by the storage unit  66 . 
     FIG. 6  shows a flow diagram of the imaging condition configuration. As shown in figure, in stage  601 , inputs including adult/infant, age, and anatomy are set. The input of these data is performed by the operator through the operating device  70 . Input in the following stages will be performed in the same manner. The adult input or infant input is selected according to whether the patient is adult or infant; the age of the patient is also input. The anatomy input will determine the desired imaging region such as the skull, eyeball, neck, chest, abdomen, legs and so on. 
   In stage  603 , a positioning scan and start/end point input is performed. The positioning scan is performed by emitting the X-ray beam to the patient with the revolving scanning gantry  2  being stopped while the imaging table  4  is translated into the body axis of the patient. This results in a radioscopic image of the patient, or the scout image. In addition the mean X-ray amount for each position in the body axis is determined for the tube current auto control (auto mA). The start/end point input is performed by the operator using the radioscopic image to input the start point and end point of the helical scan. The imaging length of the helical scan is given by the start/end point input. 
   Next in stage  605 , the highest-priority objective input is performed. The highest-priority objective is the most important purpose in the imaging of this patient, for example for a patient having very narrow tolerance to the exposure dose such as infants the exposure dose as low as possible is the top priority. For a patient having relatively large tolerance to the exposure dose such as adults, the image quality or the imaging speed may precede the dose. In the following description, the exposure dose may also be referred to as dose, image quality to IQ, imaging speed to speed. 
   The highest-priority objective may accept either dose, IQ or speed. The dose input is performed by numerical input of DLP (dose length product). Alternatively, the dose priority input is selected, indicating the dose as low as possible. For IQ and speed, IQ priority may be input indicative of the IQ as good as possible, or speed priority indicative of the speed as fast as possible. 
   Next, in stage  607 , a candidate of imaging condition will be extracted and DLP will be displayed. The extraction of the candidate of imaging condition and DLP display are performed by the data processing unit  60 . The data processing unit  60  extracts a candidate of imaging condition in accordance with the adult/infant, anatomy, imaging length, and the highest-priority from within the imaging condition table previously stored in the storage unit  66 , and displays the DLP when imaging under the condition. 
   This DLP can be the default DLP in other words. When there are plural candidates of imaging condition, all of them are extracted, and a default DLP for each of them is displayed. The display of imaging condition candidates and default DLPs is performed on the display unit  68 . 
   The imaging condition table is a table of various imaging condition sets, including various combinations of parameters such as tube voltage, tube current, beam thickness, helical pitch, slice thickness of the reconstructed image, filter used for image reconstruction and the like. The tube current is stored as the current profile in the body axis, obtained by the positioning scan in correspondence to the X-ray transmission in the body axis of the patient. The current profile is finer when the beam thickness is thinner, and broader when the beam thickness is thicker. The fine profile is more effective for decreasing the dose than the broad profile. 
   The imaging condition table as described above can be formed from the summary of the performance in the past. The table can also be formed through the simulation of various imaging condition. 
   Next, in stage  609 , DLP is adjusted. The DLP adjustment is done by the user. The default DLP can be larger than the desired DLP, and in such a case the user adjust the displayed DLP to the desired value, using the operating device  70 . Along with the DLP adjustment, the data processing unit  60  corrects the extracted image condition such that the DLP matches to the adjusted DLP. The correction of imaging condition is such that for example the amplitude of the tube current profile is decreased entirely. When the default DLP is less than the desired one, DLP is not be adjusted hence the imaging condition is not be corrected. 
   Next, in stage  611 , second priority objective is input. The input of second priority objective is performed by the user. The second priority objective is the objective next to the highest-priority objective, and any one objective is input, which is not set to the highest-priority among the dose, IQ and speed. For example when the dose is the highest-priority, the second-priority objective can be IQ or speed. These are input as IQ priority or speed priority. The input of second priority objective may be omitted in some cases. 
   Next, in stage  613 , the most appropriate imaging condition is extracted. The extraction of the most appropriate imaging condition is performed by the data processing unit  60 . The data processing unit  60  extracts the imaging condition that matches to the second priority objective among candidates of imaging condition. The most suitable imaging condition among candidates of imaging condition is thereby extracted. 
   As can be appreciated from the foregoing description, after input of adult/infant, anatomy and start/end, it is sufficient to input the highest-priority and (if needed) second priority for automatic configuration of the most suitable imaging condition, without needs to input every parameters constituting the imaging condition, allowing the imaging condition to be determined very simply. In such an imaging condition the helical scan in the following stage  503  will be performed. By doing this the imaging result can achieve the highest-priority and second priority objectives. 
     FIG. 7  shows an exemplary display of imaging condition candidate and default DLP, both extracted along with the input of the highest-priority objective. It is assumed that DLP=100 mGycm is input for the highest-priority. With respect to this highest-priority the default DLP or an imaging condition candidate nearest thereto will be automatically extracted and displayed together with the default DLP. 
   At this point the default DLP value is 120 mGycm. For the scanning parameters constituting an imaging condition, for example, scanning time, beam thickness, helical pitch, data acquisition mode, image thickness, image interval, auto mA, tube voltage in kV, tilt angle, field of view (FOV), total scanning time are displayed. 
   For such an imaging condition candidate, by decreasing the default DLP to 100 mGycm to match to the highest-priority, the imaging condition candidate is corrected automatically in accordance thereto to obtain the most suitable imaging condition as shown in  FIG. 8 . 
     FIG. 9  shows an example case in which two imaging condition candidates are extracted. The highest-priority is assumed that DLP=100 mGycm is input. For this highest-priority, the default DLP and two nearest candidates are automatically extracted and the candidates are displayed together with the default DLP. These candidates are the first order. 
   The default DLP in both candidates is 120 mGycm. For those imaging condition candidates, when the default DLP is decreased to 100 mGycm to match to the highest-priority objective, the imaging condition candidates are automatically adjusted in accordance thereto, and the imaging condition as shown in  FIG. 10  is displayed. These candidates are the second order. 
   From within those second order candidates, any one is automatically selected which conforms to the second priority to constitute the most suitable imaging condition. If speed is selected for the second priority the second order candidate A will be selected; if IQ is selected for the second priority the second order candidate B will be selected. 
   Many widely different embodiments of the invention may be constructed without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.