JP5017350B2 - X-ray computed tomography system - Google Patents

Description

  The present invention relates to an X-ray computed tomography apparatus.

  As is well known, the X-ray computed tomography apparatus calculates the X-ray absorption rate of tissues such as organs as an index of CT value based on that of water based on the amount of X-rays received in the subject ( An image (tomographic image) is obtained by performing reconstruction. Image noise is inevitable in the reconstructed image. Image noise typically defines a variation in CT values of a homogeneous phantom image as a standard deviation, and is generally abbreviated as an image SD.

  The image SD is determined according to a plurality of condition items such as imaging slice thickness, tube voltage, tube current and the like in the scan conditions and the subject. In order to make a diagnosis by observing the reconstructed image, for example, in order to distinguish whether a minute shadow on the image is noise or a tumor, it is necessary to consider the image SD that the image has. In other words, it is necessary to set scan condition items such as imaging slice thickness, tube voltage, and tube current so that the image SD has an appropriate value that can identify the tumor to be examined as noise.

  However, as in Japanese Patent Application Laid-Open No. 11-235334, it is not easy to grasp the value of the image SD only by inputting many condition items such as imaging slice thickness, tube voltage, tube current and the like. Furthermore, even if the image SD can be grasped, recognizing how much noise appears with respect to the value of the image SD requires a lot of experience and knowledge and is not easy. For this reason, it has been difficult to set scan conditions appropriately.

JP-A-11-235334

  An object of the present invention is to provide an X-ray computed tomography apparatus capable of more suitably setting scan conditions.

According to a first aspect of the present invention, X is performed to perform an X-ray scan on an imaging target region of a subject and image data reconstruction based on projection data collected by the scan according to a scan condition including a plurality of condition items. In a line computed tomography apparatus, a means for inputting a desired value relating to an image noise index, a storage means for storing data relating to a sample image having a reference value relating to an image noise index, and a desired value relating to an image noise index The input image noise from the data related to the sample image based on the means for associating the recommended value of the scan condition, the desired value related to the input image noise index, and the reference value related to the image noise index Simulated image data generating means for generating simulated image data corresponding to a desired value of the index of Serial generated simulated image data, and and means for displaying the recommended value of the associated scanning conditions.
According to a second aspect of the present invention, X is performed to perform an X-ray scan on an imaging target region of a subject and image data reconstruction based on projection data collected by the scan according to a scan condition including a plurality of condition items. In the line computed tomography apparatus, based on a storage means for storing sample image data having a reference value for an image noise index, a desired value for an image noise index, and a reference value for the image noise index, the sample Simulated image data generating means for generating simulated image data corresponding to a desired value of the image noise index from image data, means for associating a recommended value of a scanning condition with a desired value for the image noise index, Means for displaying the generated simulated image data and the recommended value of the associated scan condition Comprising a.

  According to the present invention, the scan condition can be set more suitably.

The figure which shows the structure of the X-ray computed tomography apparatus by embodiment of this invention. The flowchart which shows an example of the operation | movement procedure of this embodiment. The figure which shows an example of the scan plan setting screen displayed in the step of S2 of FIG. The figure which shows an example of the image SD confirmation screen displayed in the step of S5 of FIG. FIG. 3 is a supplementary explanatory diagram of image SD calculation processing in the step of S9 in FIG. The figure which shows an example of the determination method of the weighting coefficient with respect to two sample images in the step of S10 of FIG. The figure which shows an example of the simulation image display screen displayed in the step of S11 of FIG. The figure which shows an example of the display screen by the image noise simulation apparatus by embodiment of this invention.

  Embodiments of an X-ray computed tomography apparatus according to the present invention will be described below with reference to the drawings. The X-ray computed tomography apparatus includes a rotation / rotation (ROTATE / ROTATE) type in which an X-ray tube and an X-ray detector are rotated as one body, and a large number of detection elements in a ring shape. There are various types such as a fixed / rotated (STATIONARY / ROTATE) type in which only the X-ray tube is rotated around the subject, and the present invention can be applied to any type. Here, the rotation / rotation type that currently occupies the mainstream will be described. In addition, to reconstruct one slice of tomographic image data, projection data for about 360 ° around the periphery of the subject is required, and projection data for 180 ° + view angle is also required in the half scan method. . The present invention can be applied to any reconstruction method. Here, the former will be described as an example. In addition, the mechanism for converting incident X-rays into electric charges is based on an indirect conversion type in which X-rays are converted into light by a phosphor such as a scintillator and the light is further converted into electric charges by a photoelectric conversion element such as a photodiode. The generation of electron-hole pairs in semiconductors and their transfer to the electrode, that is, the direct conversion type utilizing a photoconductive phenomenon, is the mainstream. Any of these methods may be employed as the X-ray detection element, but here, the former indirect conversion type will be described. In recent years, the so-called multi-tube X-ray computed tomography apparatus in which a plurality of pairs of X-ray tubes and X-ray detectors are mounted on a rotating frame has been commercialized, and the development of peripheral technologies has progressed. Yes. The present invention can be applied to both a conventional single-tube X-ray computed tomography apparatus and a multi-tube X-ray computed tomography apparatus. Here, a single tube type will be described.

  FIG. 1 shows the configuration of an X-ray computed tomography apparatus according to this embodiment. The gantry 1 includes an X-ray tube 10 and an X-ray detector 23. The X-ray tube 10 and the X-ray detector 23 are mounted on the ring-shaped rotating frame 12 so as to face each other. The rotating frame 12 is rotated around the Z axis by the gantry driving device 25. The central portion of the rotating frame 12 is opened, and the subject P placed on the top 2a of the bed 2 is inserted into the opening. Between the X-ray tube 10 and the opening, a slit 22 for changing the X-ray irradiation width in accordance with the beam pitch (also referred to as imaging slice thickness) is disposed. The couch 2 is equipped with a couchtop drive unit 2b for moving the couchtop 2a with respect to the direction of its long axis (parallel to the rotation axis).

  A tube voltage is applied between the cathode and anode of the X-ray tube 10 from the high voltage generator 21, and a filament current is supplied to the filament of the X-ray tube 10 from the high voltage generator 21. X-rays are generated by applying a tube voltage and supplying a filament current. In order to achieve high-speed continuous rotation, the X-ray tube 10 and the high voltage generator 21 are electrically connected via a slip ring.

  The X-ray detector 23 is a single slice type or multi-slice type detector. If the X-ray detector 23 is a single slice type, for example, 916 element rows in which a plurality of X-ray detector elements having a square light receiving surface of 0.5 mm × 0.5 mm are arranged in a row in the channel direction Y, for example. Have If the X-ray detector 23 is a multi-slice type, for example, 40 element rows are arranged in parallel in the slice direction Z.

  A data acquisition device 24 generally called a DAS (data acquisition system) converts a signal output from the detector 23 for each channel into a voltage signal, amplifies it, and further converts it into a digital signal. This data (raw data) is taken into the computer unit 3 outside the gantry. The preprocessing unit 34 of the computer unit 3 performs correction processing such as sensitivity correction on the raw data output from the data collection device 26 and outputs projection data. This projection data is sent to and stored in the data storage device 35 of the computer system 3.

The computer system 3 includes a pre-processing unit 34 and a data storage device 35, a system controller 32, an input device 39 including a keyboard and a mouse, a display 38, a display processor 37, a scan controller 33, a reconstruction unit 36, and a scan plan setting. The support system 40 includes an image SD calculation unit 41, a simulated image generation unit 42, and an image SD simulator 43. The reconstruction unit 36 reconstructs image (tomographic image) data based on the projection data stored in the data storage device 35. The data storage device 35 stores the projection data generated by the preprocessing unit 34 and the tomographic image data reconstructed by the reconstruction unit 36 and is used for generating simulated image data in the simulated image generation unit 42. Sample image data is stored in advance.

  The sample image data is image data obtained by actually scanning a phantom that faithfully reproduces the internal composition of the human body or a subject that has been accepted for use as a sample image, and is predetermined data corresponding to the scanning conditions at the time of the scan. Image SD (the image SD of this sample image is referred to as a reference image SD). The simulated image generation unit 42 generates an image based on the image SD calculated by the image SD calculation unit 41 based on the scanning conditions set by the scan plan setting support system 40 and the reference image SD included in the sample image data. Image data (simulated image data) having noise corresponding to the image SD calculated by the SD calculation unit 41 is generated from the sample image data. The processing time required to generate the simulated image data from the sample image data can be much shorter than the processing time required to process the sample projection data based on the image SD and reconstruct the simulated image data. it can. Thereby, when setting the scanning conditions, the work of checking the image SD on the actual image and the correction work of the scanning conditions can be made realistic. Hereinafter, a process of generating simulated image data from sample image data will be described together with a scan condition setting process.

  FIG. 2 shows a typical flow when applying a process of generating simulated image data from sample image data in the scan condition setting process in the present embodiment. First, in S1, a scanogram relating to the whole body or a part of the subject is taken. As is well known, for example, the X-ray tube 10 fixes the rotating frame 12 at an angle of 0 ° where the X-ray tube 10 directly faces the subject on the top 2a, and the top 2a is continuously with the subject at a constant speed. This is performed by continuously irradiating the subject with X-rays while being moved, and repeatedly detecting the X-rays transmitted through the subject with a predetermined period by the X-ray detector 23. Thereby, image data similar to the X-ray plane image, that is, scanogram data can be generated.

  Next, scan conditions are set. The scan conditions are set with the support of the scan plan setting support system 40. The scan plan setting support system 40 has functions necessary for facilitating the setting of scan conditions by an operator in an interactive format. For example, when items such as patient information, examination purpose, examination site, etc. are input, the scan plan setting support system 40 creates and presents at least one scan condition candidate corresponding thereto.

  FIG. 3 shows a scan condition setting support screen constructed by the scan plan setting support system 40. On the scan condition setting support screen, along with patient information, gantry information, and scanogram, a candidate list of scan conditions is displayed below them. The main condition items included in the scan condition represent the scan mode (single slice scan, multi slice scan, helical scan), imaging range, tube voltage, tube current, and time required for one rotation of the X-ray tube 10. The scanning speed, the beam pitch indicating the ratio of the top plate movement distance per rotation of the X-ray tube 10 to the X-ray beam width during helical scanning, and the top plate movement distance per rotation of the X-ray tube 10 during helical scanning. The bed speed, imaging slice thickness, reconstruction function, image slice thickness, S-FOV representing the diameter of the imaging area, D-FOV representing the diameter of the reconstruction area, and the like are included. The radiologist can select the desired candidate from the presented scanning condition candidates and correct the candidate value of the desired condition item as necessary to set the scanning condition with less effort. it can.

  The scan condition setting support screen includes a button labeled “Confirm image SD”. In S3, when the “image SD confirmation” button is clicked, the image SD simulator 43 is activated. The image SD simulator 43 captures the set scan plan data from the scan plan setting support system 40 and also captures scanogram data from the data storage device 35 (S4). The image SD simulator 43 constructs and displays the image SD confirmation screen shown in FIG. 4 from the captured scan plan data and scanogram data (S5).

  As shown in FIG. 4, the image SD confirmation screen includes an upper left scanogram display area, an upper right simulated image display area, an imaging region display field, a body thickness display field, a water equivalent thickness display field, an imaging range display field, an imaging time Display field, scan mode display field, exposure reduction display field, imaging slice thickness display field, image slice thickness display field, FOV display field, beam pitch (Pitch) display field, bed speed display field, reconstruction function (function) display field Tube voltage (kV) display column, tube current (mA) display column, scan speed display column, image SD display column, dose (CTDI, DLP) display column, and window level / window width display column. Of these display fields, in the display fields other than the body thickness display field, the water equivalent thickness display field, and the image SD display field, the part names or numerical values of the corresponding items included in the scan plan data are initially inserted.

  The body thickness is numerically input via the input device 39 on the image SD confirmation screen. Alternatively, the body thickness is calculated from the scanogram data by the image SD simulator 43. The water equivalent thickness is calculated by the image SD simulator 43 from the inputted or calculated body thickness. Alternatively, the water equivalent thickness is calculated directly from the scanogram data by the image SD simulator 43. There are various methods for calculating the water equivalent thickness from the body thickness, and any method may be adopted. An example of a method for calculating the water equivalent thickness from the body thickness is the diameter of the water phantom based on the ratio of the pixel value in the scanogram for the subject to the pre-collected scanogram pixel value for a cylindrical water phantom of known diameter. There is a method for estimating the water equivalent thickness of the subject. In order to reduce the error, in practice, the square root of the ratio of the pixel value integration of the local area of the same size in the scanogram for the subject to the pixel value integration of the local area in the scanogram for the cylindrical water phantom of known diameter is calculated. The water equivalent thickness of the subject is estimated by multiplying the diameter of the water phantom.

  The image SD confirmation screen includes a button labeled “Display simulated image”. When the “simulated image display” button is clicked in S7, sample image data is also taken into the simulated image generation unit 42 from the data storage device 35 under the control of the image SD simulator 43 (S8). Actually, in the data storage device 35, a plurality of data, here two pieces of sample image data are associated with each of a plurality of parts. Two sample images associated with the same part have different images SD. One sample image (first sample image) has an image SD (first reference image SD) of 2.0, for example, and the other sample image (second sample image) has an image of 50.0, for example. It has an image SD (second reference image SD). As is well known, the image SD has a property that the smaller the value, the higher the image quality, and the higher the value, the lower the image quality.

  Next, in S <b> 9, the image SD calculation unit 41 calculates an image SD as an index related to image noise based on the set value of a specific condition item in the scan plan and the calculated water equivalent thickness. . There are various methods for calculating the image SD, and any method may be adopted. For example, the image SD is calculated by the following method.

  In this method, the image SD is obtained based on the imaging slice thickness, tube voltage, tube current, reconstruction function, beam pitch, and water equivalent thickness. The relationship of the image SD with respect to the water equivalent thickness is determined by scanning a plurality of types (for example, five types) of water phantoms with different diameters in order with all the condition items of the scanning conditions fixed at the reference value. The image SD is obtained, and as shown in FIG. 5A, the relationship between the water phantom thickness and the image SD is obtained by an exponential approximation formula. The approximate expression data is stored in advance in the data storage device 35. The relationship of the image SD to the imaging slice thickness is that the other condition items of the scanning condition are fixed to the reference value, and scanning is repeated while changing the imaging slice thickness for a specific water phantom, and the image SD of each image is obtained. Obtained by an approximate expression as shown in FIG. The approximate expression data is stored in advance in the data storage device 35. The relationship between the image voltage SD and the tube voltage is determined by repeating scanning while changing the tube voltage for a specific water phantom with the other condition items of the scan condition fixed to the reference value, and obtaining the image SD of each image. As shown in FIG. 5C, it is obtained by an approximate expression. The approximate expression data is stored in advance in the data storage device 35. The relationship of the image SD to the tube current is determined by repeating scanning while changing the tube current for a specific water phantom with other condition items of the scan condition fixed at the reference value, and obtaining the image SD of each image. As shown in FIG. 5D, it is obtained by an approximate expression. The approximate expression data is stored in advance in the data storage device 35. The relationship between the image SD and the beam pitch is determined by repeating helical scanning while changing the beam pitch for a specific water phantom with the other condition items of the scan condition fixed to the reference value, and obtaining the image SD of each image. Thus, the approximate expression is used. The approximate expression data is stored in advance in the data storage device 35. The relationship between the image SD and the reconstruction function is that, with all condition items of the scan condition fixed at the reference value, the scan is performed once for a specific water phantom, and the image reconstruction is performed while changing the reconstruction function. Repeatedly, the image SD of each image is obtained and obtained by an approximate expression. The approximate expression data is stored in advance in the data storage device 35.

  The image SD calculation unit 41 uses these five types (six types at the time of helical scan) of approximate expression data to correspond to the condition items (imaging slice thickness, tube voltage, tube voltage) corresponding to the scan conditions set on the scan plan setting support system 40. Current, beam pitch, reconstruction function) set values, and image SD values corresponding to the body thickness of the imaging target region of the subject were specified, and the values of the specified images SD were normalized by the respective normalization coefficients By multiplying the value, the imaging target region of the subject is scanned with the set value of the scan condition item, and when the reconstruction is performed using the selected reconstruction function from the obtained projection data, the image is The value of the image SD is calculated and actually estimated.

  Next, in S10, based on the image SD calculated by the image SD calculation unit 41, an image (simulated image) having a noise corresponding to the image SD is generated from the sample image data by the simulated image generation unit 42. . The simulated image generation unit 42 calculates two types of sample image data in which the images SD associated with the imaging target parts captured from the data storage device 35 are different for the section between the first and second reference images SD. Simulated image data is generated by interpolating two types of sample image data according to the position of the image SD. As is well known, the interpolation process is a process of multiplying the pixel values at the corresponding positions of the first and second sample images by the first and second weighting coefficients and adding them. The values of the first and second weighting factors are determined so that the sum is 1.0. As the determination method, the first sample image is multiplied by the ratio of the distance between the first reference image SD and the calculated image SD to the distance between the first and second reference images SD. A second coefficient that is determined as a weighting factor and that multiplies the second sample image by a ratio of the distance between the second reference image SD and the calculated image SD to the distance between the first and second reference images SD. It may be determined according to a so-called simple distance interpolation process that is determined as a weighting factor, or may be determined according to a predetermined multi-order function as shown in FIG.

  By generating simulated image data from two types of sample image data having different reference images SD as described above, the processing time can be shortened. In the above description, the example in which the simulated image data is generated from two types of sample image data has been described. However, for one type of sample image data having a reference image SD corresponding to a high image quality such as 2.0, the reference Simulated image data may be generated by adding a noise corresponding to the ratio between the image SD and the calculated image SD, and conversely, a reference image SD corresponding to a low image quality such as 50.0 is provided. Simulated image data may be generated by applying noise reduction to one type of sample image data to a degree corresponding to the ratio between the reference image SD and the calculated image SD, or three or more types. By interpolating from three or more types of sample image data having the reference image SD with a weighting factor corresponding to the ratio between the reference image SD and the calculated image SD. It may generate the image data.

  Next, the image SD simulator 43 constructs and displays the image SD confirmation screen illustrated in FIG. 7 including the image SD calculated by the image SD calculation unit 41 and the simulated image generated by the simulated image generation unit 42. (S11).

  The radiologist corrects the set values of the imaging slice thickness, tube voltage, tube current, reconstruction function, and beam pitch as needed via the input device 39 (S12). After the correction, when the “simulated image display” button is clicked in S13, the process returns to S9, and the image SD calculation unit 41 corrects the correction value of the corrected scan condition item and the uncorrected scan condition in the scan plan. The image SD is recalculated based on the set value of the item and the calculated water equivalent thickness. In S10 and S11, the simulated image data corresponding to the recalculated image SD is obtained from the same sample image data by the simulated image generating unit 42 based on the image SD recalculated by the image SD calculating unit 41 in a short time. Generated and displayed at home.

  Viewing the result, the radiologist re-corrects the values of the imaging slice thickness, tube voltage, tube current, reconstruction function, and beam pitch as necessary (S12). The process of S8 to S13 is repeated several times, and the radiologist views the image SD and the degree of noise corresponding to the image SD on the simulated image, while photographing slice thickness, tube voltage, tube current, reconstruction function, The beam pitch can be approximated to an optimum value. The optimum value is defined as a value corresponding to the fact that the tumor or the like to be examined can be clearly confirmed without being buried in noise and the exposure is minimized.

  When a button labeled “Reflect” in the image SD confirmation screen is clicked (S14), the image slice thickness, tube voltage, tube current, reconstruction function, and beam pitch are controlled under the control of the image simulator 43. The correction value data is supplied to the scan plan setting support system 40 (S15). The scan plan setting support system 40 replaces the values of the corresponding items of the scan plan with the correction values related to the supplied imaging slice thickness, tube voltage, tube current, reconstruction function, and beam pitch. By clicking the “Confirm” button in FIG. 3, the corrected values related to the imaging slice thickness, the tube voltage, the tube current, the reconstruction function, and the beam pitch corrected on the image SD confirmation screen are determined as the setting values of the scan conditions. By clicking the “Close” button in FIG. 7, the image SD confirmation screen is closed (S16).

  An X-ray computed tomography apparatus as an image noise simulation apparatus comprising the data storage device 35 for storing the sample image data, the image SD calculation unit 41, the simulated image generation unit 42, the display processor 37, the display 38, and the input device 39. You may make it provide independently. As shown in FIG. 8, an image SD simulation screen is displayed. On this simulation screen, body thickness, water equivalent thickness, scan mode, exposure reduction on / off, imaging slice thickness, image slice thickness, FOV, beam pitch, bed speed, reconstruction function, tube voltage, tube current, scan By setting the speed to an arbitrary value and clicking the “simulated image display” button, an image SD corresponding to the set value and a simulated image having a noise level corresponding to the image SD are displayed almost immediately. . By selecting the head, chest, abdomen and lower limbs on the body mark of the scanogram displayed in the upper left area of the screen, the simulated image of the selected part is switched almost immediately. As an application of the image noise simulation device, it can be placed close to the console of an X-ray computed tomography device that is not equipped with an image noise simulation function, which is equivalent to the state-of-the-art machine equipped with it. With the assistance, the scan condition can be approximated to the optimum value. It is also possible to use the image noise simulation device as an inexperienced instructor or as a training device for a radiographer. In addition, since it is physically separated from the X-ray computed tomography apparatus, the image noise simulation apparatus can be used in various scenes.

  In the above description, when a scan condition or an imaging region is input, an image SD corresponding to the input is calculated, and a simulated image corresponding to the calculated image SD is generated and displayed. However, if the value of the image SD desired by the doctor, engineer, or the like is directly input together with the imaging region, as described above, based on the image SD reference value of the sample image data and the input desired value of the image SD. Thus, a simulated image corresponding to the input image SD may be generated and displayed from the sample image data corresponding to the input imaging region. Further, a plurality of recommended values related to scan conditions are associated with a plurality of imaging regions and images SD and stored in the data storage device 35, and scan conditions corresponding to the input imaging regions and the input images SD are stored. The recommended value may be displayed together with the simulated image.

(Modification)
The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention at the stage of implementation. Furthermore, the above embodiment includes various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. For example, some constituent requirements may be deleted from all the constituent requirements shown in the embodiment.

  The present invention may be used in the field of X-ray computed tomography apparatuses that can set scan conditions more suitably.

  DESCRIPTION OF SYMBOLS 1 ... Stand, 2 ... Bed, 3 ... Computer unit, 10 ... X-ray tube, 12 ... Rotating frame, 21 ... High voltage generator, 22 ... Slit, 23 ... X-ray detector, 24 ... Data acquisition device (DAS) , 25 ... gantry driving device, 32 ... system controller, 33 ... scan controller, 34 ... preprocessing unit, 35 ... data storage device, 36 ... reconstruction unit, 37 ... display processor, 38 ... display, 39 ... input device, 40 ... Scan plan setting support system, 41 ... Image SD calculation unit, 42 ... Sample image generation unit, 43 ... Image SD simulator.

Claims (2)

In an X-ray computed tomography apparatus that performs X-ray scanning on an imaging target region of a subject and reconstruction of image data based on projection data collected by the scan according to a scanning condition including a plurality of condition items,
Means for inputting a desired value for an index of image noise;
Storage means for storing data relating to a sample image in which a reference value relating to an index of image noise and a recommended value for scanning conditions are associated ;
Simulated image data for generating simulated image data corresponding to a desired value of the input image noise index based on a desired value related to the input image noise index and a reference value associated with the sample image Generating means;
An X-ray computed tomography apparatus comprising: means for displaying the generated simulated image data and a recommended value of the associated scan condition.   The X-ray computed tomography apparatus according to claim 1, wherein the scan condition includes an image slice thickness, a tube voltage, a tube current, a reconstruction function, and a beam pitch.

Images