WO2015115309A1 - X線ct装置、および、x線ct装置用画像演算装置 - Google Patents
X線ct装置、および、x線ct装置用画像演算装置 Download PDFInfo
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- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
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Definitions
- the present invention relates to an X-ray CT apparatus, and more particularly to a technique for suppressing image quality degradation caused by system noise during low-dose imaging.
- the X-ray CT apparatus irradiates X-rays from around the subject and images the X-ray absorption coefficient distribution of the subject from projection data acquired at a plurality of projection angles. As the amount of X-ray irradiation increases, an image with less noise can be acquired and the image quality improves. On the other hand, the influence of the X-ray exposure on the human body has been regarded as a problem in recent years, and techniques for obtaining image quality necessary for a doctor's diagnosis in low-dose imaging with a reduced dose of X-ray are being actively studied.
- the noise that affects the image of the X-ray CT apparatus is broadly divided into photon noise caused by fluctuations in X-ray photons and system noise added in the data acquisition system.
- the former changes according to the amount of X-ray irradiation, while the latter shows a noise amount specific to each data collection system. Therefore, when the X-rays incident on the detector at the time of low-dose imaging are weak, the ratio of system noise to the output signal from the data acquisition system increases.
- Patent Document 1 discloses a technique for preventing an increase in noise components due to logarithmic conversion when converting measurement data having a large proportion of noise components into projection data because the X-ray dose incident on the detector is small. ing.
- the measurement data value is greater than or equal to the predetermined value, it is converted into projection data by the logarithmic function as before, but when the X-ray dose incident on the detector is low and the measurement data is less than the predetermined value, the logarithmic function
- the projection data is converted using a function instead of.
- the bias correction method described above refers to the measurement data value of the target element of the detector with reference to the measurement data value of the neighboring element of the target element.
- the correction is performed by repetitive filtering while maintaining the average value between the target element and the adjacent element.
- the measurement data after the bias correction is converted into projection data by a logarithmic function, and image reconstruction is performed from the projection data.
- the successive approximation reconstruction method from the measurement data does not convert the measurement data into projection data, but performs successive approximation reconstruction on the measurement data to reconstruct the image.
- the successive approximation reconstruction method from the measurement data does not convert the measurement data into projection data, but performs successive approximation reconstruction on the measurement data to reconstruct the image.
- photon noise and system noise included in the measurement data are modeled, and an image is calculated using a successive approximation method based on the model.
- the method of performing bias correction on measurement data and then converting to projection data to perform image reconstruction, or the method of performing successive approximation reconstruction from measurement data has weak measurement data values and some signals are 0 Even with the following values, the image quality degradation of the reconstructed image can be reduced.
- the bias correction is an iterative filtering and the iterative image reconstruction method uses an iterative solution method, both of them require a lot of calculation time.
- the image reconstruction is performed again by changing the reconstruction conditions of the data once captured.
- the projection data is generated and reconstructed by normal logarithmic transformation processing without performing the successive approximation image reconstruction method or bias correction processing.
- the measurement data can be restored from the projection data when the image was quickly generated and used to generate a high-accuracy image, the image can be imaged even when reconstructing it again rather than storing the measurement data. Making it easier to do quickly.
- the projection data obtained by logarithmically converting the measurement data requires a smaller data range than the measurement data, the data storage capacity can be reduced.
- the logarithmic function has a characteristic that the value increases to infinity when the variable approaches 0, the measurement data cannot be accurately restored from the projection data stored with a finite data size.
- the measurement data before offset cannot be restored even by inverse logarithmic conversion. Cannot be restored.
- An object of the present invention is to provide a technique capable of accurately restoring measurement data from projection data and removing system noise contained in the measurement data.
- the present invention provides an X-ray generator that irradiates a subject with X-rays, a data acquisition device that detects X-rays that have passed through the subject, a signal processing device, and a reconstruction calculation device.
- An X-ray CT apparatus provided with The signal processing device processes the output signal of the data collection device to obtain measurement data including a signal value of 0 or less, and converts the measurement data by a predetermined function including a logarithmic function to generate projection data .
- the reconstruction calculation device reconstructs the projection data and generates an image.
- the above-described predetermined function is a function in which an inverse function exists for a value greater than or equal to a predetermined negative number. By applying the inverse function to the projection data, measurement data including a signal value of 0 or less within a predetermined range is restored from the projection data.
- measurement data can be accurately restored from projection data, so that system noise included in the restored measurement data can be removed, and image quality can be improved.
- FIG. 1 is a block diagram showing a configuration of an X-ray CT apparatus according to a first embodiment.
- 1 is a block diagram showing a configuration of a signal processing device 124 of the X-ray CT apparatus of Embodiment 1.
- FIG. FIG. 3 is an explanatory diagram illustrating a display screen example of the display device 120 in the first embodiment.
- the flowchart which shows the operation
- FIG. 3 is a flowchart showing the image reconstruction operation of the signal processing device 124 and the reconstruction calculation device 125 of the first embodiment.
- Graph showing logarithmically transformed data z of Embodiment 2 A block diagram showing a configuration of a signal processing device 124 according to the second embodiment.
- movement which the signal processing apparatus 124 of Embodiment 2 converts the output signal of the data acquisition device 108 into projection data.
- FIG. 5 is a block diagram showing a configuration of a signal processing device according to a third embodiment.
- the X-ray CT apparatus of this embodiment includes an X-ray generator 102 that irradiates a subject 110 with X-rays, and a data collection device that detects X-rays that have passed through the subject 110 108, a signal processing device 124, and a reconstruction calculation device 125.
- the signal processing device 124 processes the output signal of the data collection device 108 to obtain measurement data including a signal value of 0 or less, and converts the measurement data by a predetermined function including a logarithmic function to convert the projection data. Generate.
- the reconstruction calculation device 125 reconstructs the projection data and generates an image.
- the above-mentioned predetermined function is a function in which an inverse function exists for a value greater than or equal to a predetermined negative number. Therefore, by applying the inverse function to the projection data, measurement data including a signal value of 0 or less within a predetermined range is restored.
- measurement data can be accurately restored from projection data including a predetermined range of 0 or less, so that system noise included in the restored measurement data can be removed.
- the signal processing device 124 includes a positive number converter 11 and a logarithmic converter 18 as shown in FIG.
- the positive number converter 11 converts negative data of measurement data into positive number data using a predetermined positive number conversion function.
- the logarithmic converter 18 performs logarithmic conversion on the positive number data converted by the positive number converter 11 to generate projection data.
- the positive number conversion function of the positive number converter 11 is a monotonically increasing function for negative numbers in a predetermined range. By using a monotonically increasing function as a positive number conversion function, measurement data having a signal value of 0 or less within a predetermined range can be restored from projection data by an inverse function of the monotonically increasing function.
- the X-ray CT apparatus of the first embodiment is provided with a storage device 123 that stores the generated projection data, and the signal processing device 124 reads out the projection data stored in the storage device 123, and has a predetermined range. It is desirable to arrange a restorer 24 that restores measurement data including a signal value of 0 or less. Further, the signal processing device 124 is preferably provided with a correction device 30 for correcting the measurement data restored by the restoration device 24. As a result, system noise included in the restored measurement data can be removed by the correction device 30.
- the reconstruction calculation device 125 includes a successive approximation reconstruction unit 125b that performs successive approximation image reconstruction on the measurement data restored by the restorer 24 to generate an image.
- the X-ray CT apparatus of the first embodiment further includes an input device 121 that accepts selection of normal image reconstruction or image reconstruction using measurement data restored by the decompressor 24 from the operator. Is desirable.
- the input device 121 can accept the selection by displaying a screen for accepting selection on the display screen of the display device 120 as shown in FIG. 4, for example.
- the signal processing device 125 restores the projection data stored in the storage device 123 by the restorer 24. .
- FIG. 1 is an external view of the X-ray CT apparatus of the embodiment
- FIG. 2 is a block diagram showing an internal configuration of the X-ray CT apparatus.
- the X-ray CT apparatus includes a scanner 100 used for imaging, a bed 109 for moving a subject, an input device 121, an image calculation device 132, and a display device 120.
- the image calculation device 132 includes a signal processing device 124 that processes data obtained by the data collection device 108, a reconstruction calculation device 125, and an image processing device 126.
- the input device 121 and the display device 120 constitute an input / output device 131 together with the storage device 123.
- the input / output device 131 and the image calculation device 132 constitute an operation unit 133.
- the scanner 100 includes an X-ray generation device 102, a data collection device 108, a collimator 114, and a rotating body 115 that carries these and rotates around the subject 110.
- the data acquisition device 108 includes an X-ray detector 111, a preamplifier 110, and an A / D converter 109.
- the scanner 100 includes a driving device 112 that rotationally drives the rotating body 115, a high voltage generation device 103, an X-ray control device 104, a scanner control device 113, a central control device 105, a bed control device 106, and a bed movement measuring device 107.
- a collimator control device 101 and the like are provided.
- the input device 121 of the operation unit 133 is composed of a mouse, a keyboard and the like. Imaging conditions (bed movement speed, tube current, tube voltage, slice position, etc.), reconstruction parameters (region of interest, reconstruction image size, backprojection phase width, reconstruction filter function, etc.), measurement used for image reconstruction Accepts input such as data selection from the operator.
- the display device 120 displays the reconstructed image, the input reception screen of the input device 121, and the like.
- the central control device 105 sends control signals required for imaging to the X-ray control device 104, the bed control device 106, and the scanner control device 113 based on the imaging conditions received by the input device 121. Thereafter, when the operator operates the imaging start button, the X-ray control device 104, the bed control device 106, and the scanner control device 113 receive an imaging start signal and start an operation for imaging.
- the X-ray controller 104 outputs a control signal to the high voltage generator 103.
- the high voltage generator 103 applies a high voltage to the X-ray generator 102.
- the X-ray generator 102 irradiates the subject 110 with X-rays.
- the scanner control device 113 outputs a control signal to the drive device 112.
- the driving device 112 causes the rotating body 115 on which the X-ray generator 102, the X-ray detector 111, the preamplifier 110, and the like are mounted to circulate around the subject 110.
- the bed control device 106 controls the operation of the bed 109 on which the subject is placed, and moves the bed 109 in the still or body axis direction.
- the X-rays emitted from the X-ray generator 102 are limited in the irradiation area by the collimator 114, are irradiated onto the subject 110, pass through the subject 110 while being absorbed (attenuated) by each tissue in the subject 110, It is detected by the X-ray detector 111 of the data acquisition device 108.
- the X-ray detector 111 includes a plurality of detection elements arranged in a two-dimensional direction (a channel direction and a column direction orthogonal thereto). Signal detection by the X-ray detector 111 is performed at discrete positions (views) of the rotating body 115 in the circulation direction.
- the detection signal of the X-ray detector 111 is converted into a current, amplified by the preamplifier 110, converted into a digital signal by the A / D converter 109, and output to the signal processing device 124.
- the signal processing device 124 includes a pre-processor 12, a positive number converter 11, a logarithmic converter 18, a correction processor 19, a restoring device 24, and a correction device 30.
- the preprocessor 12 performs predetermined preprocessing on the output signal of the data collection device 108 to obtain measurement data x.
- An example of the positive number conversion function ⁇ (x) is shown in Expression (1).
- the lower threshold s ( ⁇ 0), the upper threshold t (> 0), and L (> 0) are arbitrary constants.
- the upper threshold t satisfies the condition of t ⁇ 2L ⁇ s.
- a graph of the function ⁇ (x) in Equation (1) is shown by a solid line in FIG.
- the value y of the conversion function ⁇ (x) takes a value not less than L, and the range of x ⁇ t increases linearly.
- ⁇ (x) is a constant value, for example, L value in the range of s ⁇ x.
- the logarithmic converter 18 applies the conversion function ⁇ (y) to the positive number data y converted by the positive number converter 11 to generate the logarithmically converted data z.
- An example of the conversion function ⁇ (y) is shown in Equation (2).
- q and g are positive numbers and are constants determined according to the size of the storage area of the storage device 123 that stores the projection data a and the accuracy desired as the projection data a. .
- the correction processor 19 performs a predetermined correction process on the logarithmically converted data z to obtain projection data a, and outputs the projection data a to the reconstruction calculation device 125 and also outputs to the storage device 123 for storage.
- the restorer 24 reads the projection data a stored in the storage device 123, and restores measurement data x including a signal value of 0 or less within a predetermined range.
- the correction device 30 performs a predetermined correction process such as bias correction on the restored measurement data x to reduce system noise included in the measurement data x.
- the measurement data after system noise reduction is input to the logarithmic converter 18.
- the positive number converter 11 described above includes a discriminator 13, a threshold value setter 14, and first, second and third converters 15, 16, and 17.
- the threshold setter 14 holds thresholds s, t, and L used for the functions ⁇ (x) and ⁇ (y).
- the threshold s is a negative number and t is a positive number.
- the threshold values s, t, and L can be changed to arbitrary values by the operator operating the input device 121. It is also possible to employ a configuration in which the signal processing device 124 measures the system noise dispersion value ⁇ unique to the system of the X-ray CT apparatus and calculates s and t based on the dispersion value ⁇ . This calculation method will be described later.
- the discriminator 13 reads the threshold values s, t, and L from the threshold setting device 14 and distributes the measurement data x output from the preprocessor 12 to the first, second, and third converters 15, 16, and 17.
- the first converter 15 receives the measurement data x equal to or greater than the threshold value t shown in FIG. 5, and outputs the input signal as it is as the positive number data y.
- the second converter 16 receives the measurement data x that is smaller than the threshold value t and greater than or equal to s, and generates and outputs positive number data y by converting it into a positive number by the function ⁇ (x).
- the third converter 17 receives measurement data x smaller than the threshold value s, converts it into a constant data value L, and outputs it as positive number data y.
- the restorer 24 includes an inverse correction processor 29, an inverse logarithmic converter 25, a restorer discriminator 28, a first inverse converter 26, and a second inverse converter 27.
- the inverse correction processor 29 performs a process opposite to the process of the correction processor 19 on the projection data a stored in the storage device 123, and restores the logarithmically converted data z described above.
- the inverse logarithmic converter 25 transforms the restored logarithmic transformed data z with the inverse function of the transformation function ⁇ (y) to restore the positive data y.
- the restoring device discriminator 28 reads t and L from the threshold setting device 14 and distributes the restored positive number data y to the first and second inverse converters 26 and 27.
- the first inverse converter 26 receives positive number data y equal to or greater than the threshold value t, and outputs the input signal as it is as measurement data x.
- the second inverse converter 27 receives positive number data y that is larger than the threshold L and smaller than t, converts it by the inverse function of the conversion function ⁇ (x) used in the second converter 16, and outputs it as measurement data x To do. Thereby, the measurement data x corresponding to the positive number data y larger than the threshold value L can be restored. Therefore, the measurement data x is restored even in the negative range of s ⁇ x ⁇ 0.
- the correction device 30 removes system noise by performing bias correction processing or the like on the restored measurement data x.
- the measurement data x that has been restored and from which system noise has been removed is input to the logarithmic converter 18, logarithmically converted, and corrected by the correction processor 19 to obtain projection data b.
- the reconstruction calculation device 125 generates an image by successive approximation image reconstruction processing from the reconstruction unit 125a that reconstructs an image from the projection data a and b from the correction processor 19 and the measurement data x output from the decompressor 24. And a successive approximation reconstruction unit 125b. Since the restored projection data b is generated from the measurement data x from which the system noise is removed, unlike the projection data a that is quickly generated without removing the system noise, the reconstruction unit 125a performs a reconstruction process. Thus, an image from which artifacts due to system noise are removed can be generated. Further, the restored measurement data x is subjected to successive approximation image reconstruction processing by the successive approximation reconstruction unit 125b, thereby generating an image from which artifacts due to system noise are removed.
- the reconstructed image is stored in the storage device 27 in the input / output device 26 and is displayed on the display device 120 as a CT image.
- the image processing device 126 performs image processing on the reconstructed image according to the operation of the operator.
- the signal processing unit 124 causes the display device 120 to display a selection receiving screen as shown in FIG. 4 in order to receive selection of the processing mode from the operator.
- the selection reception screen includes an object information display unit 120a for displaying information such as an image of the object 110, projection data, and imaging conditions, an icon 21 for receiving an instruction to start normal reconstruction, and measurement data x 2 includes an icon 22 for receiving an instruction to perform reconstruction after performing bias correction, and an icon 23 for receiving an instruction to perform successive approximation reconstruction from the measurement data x.
- the operator selects the processing mode by operating the input device 121 and selecting one of the icons 21 to 23.
- the signal processing device 124 acquires the output signal of the data collection device 108 and processes it as shown in steps 31 to 34 in FIG.
- step 31 when the signal acquired from the data collection device 108 is compressed for transmission, the preprocessor 12 is restored to the original number of bits.
- measurement data x is obtained by applying signal value correction to subtract the output signal in a state where X-rays are not exposed from the data restored to the original number of bits.
- the positive number converter 11 converts the measurement data x into recoverable positive number data y using the above-described conversion function ⁇ (x).
- the discriminator 13 reads the threshold values s, t, and L from the threshold value setter 14, and the measurement data x output from the preprocessor 12 is first, second, and third according to the function ⁇ (x) of the equation (1).
- the second converter 16 receives the measurement data x of s ⁇ x ⁇ t, generates positive number data y by the function ⁇ (x) of the above equation (1), and outputs it.
- the third converter 17 receives the measurement data x of x ⁇ s, converts it all to L by the function ⁇ (x), and outputs it as positive number data y.
- ⁇ (x) In the case of x ⁇ t, there is no decrease in the accuracy of data due to conversion and inverse conversion. However, in order for the argument x and its value ⁇ (x) to correspond one-to-one, ⁇ (x) needs to be a monotonically increasing function in the narrow sense in the range of s ⁇ x ⁇ t.
- s is a parameter for adjusting the upper limit value for maintaining the data accuracy and the lower limit value of the data to be rounded up.
- the operator can set arbitrary s and t so as to satisfy the condition of t ⁇ 2L ⁇ s. Note that the value of L is arbitrarily set to satisfy the condition described in step 33 of FIG.
- the factor that the measurement data x takes a value of 0 or less is system noise.
- the system noise can be generally modeled by a normal distribution, and has a mean value of 0 and a variance value ⁇ depending on the data collection device.
- the dispersion value ⁇ can be calculated experimentally or by simulation. Therefore, by setting s corresponding to the value of the variance value ⁇ , it is not necessary to set s smaller than necessary, and the ratio of data to be rounded up can be reduced. For example, in the normal distribution of the system noise variance ⁇ 2 , it is known that about 68% of the entire data exists in the ⁇ range and about 95% exists in the 2 ⁇ range. Therefore, for example, by setting s according to the equation (3), the ratio of the measurement data x rounded up to L by the conversion function ⁇ (x) can be reduced. In view of the capacity of the storage area of the storage device 123 for storing the projection data a and the data accuracy, ⁇ is preferably set to 6 or less empirically.
- the coefficients are converted based on s and L in advance, so that a large amount of calculation is not generated. Is possible.
- Equation (4) the conventional conversion function ⁇ (x) is first shown in Equation (4). Further, the graph of the formula (4) is shown by a one-dot chain line in FIG.
- L (> 0) is an arbitrary constant.
- step 33 the converted signal y calculated in step 32 is logarithmically converted using the logarithmic function ⁇ (y) of equation (2) to obtain logarithmically converted data z.
- the maximum value of the projection data a is a value obtained by performing correction by the correction processor 19 on the maximum value r of the logarithmically converted data z represented by the equation (5).
- the value obtained by correcting the r in Expression (5) by the correction processor 19 in the next step 34 is the upper limit (for example, a 16-bit signed integer) of the storage area of the projection data a in the storage device 123. It is desirable to set q in Equation (2) empirically so that it falls within the upper limit. Further, g in Expression (2) is set empirically based on the range and accuracy of the CT image value reconstructed from the projection data a. Furthermore, L is set empirically as the lower limit value of the valid positive number data y from the range of the projection data a corresponding to r.
- One of the effects of the present invention is that the lower threshold value of the measurement data that can be restored after logarithmic conversion can be expanded from the positive number L of the conventional method to the negative number s while maintaining the maximum value r of the logarithmically converted data z.
- the logarithmic transformed data z generated in step 33 is shown by a solid line graph in FIG.
- a conventional conversion function ⁇ (x) is used in step 32, and in step 33, data after logarithmic conversion obtained in the same manner as in the present embodiment is shown in a dot-dash line graph in FIG.
- the log-transformed data is a function having r as a maximum value, and the projection data a has the same size.
- the logarithm-transformed data z in the comparative example takes all r values within the range of x ⁇ L, so there is no inverse function and the value of the measurement data x is restored.
- the logarithmically transformed data z of the present embodiment is a monotonically decreasing function even in the range of s ⁇ x ⁇ L, an inverse function exists and the value of the measurement data x can be restored.
- step 34 for the data z after logarithmic conversion obtained in step 33, reference correction by the value of the reference detector, air correction by data taken without the subject 110, phantom correction to suppress the beam hardening effect Etc. to obtain projection data a.
- the projection data a is stored in the storage device 123 in the input / output device 131.
- the value of the reference detector used for various conversions, the data obtained without the subject 110, and the parameters of the function used for phantom correction are also stored in the storage device 123 in the input / output device 131. .
- the reconstruction calculation device 125 acquires the projection data a from the storage device 123 (step 61), and confirms the processing mode selected by the operator from the input device 121 using the screen of FIG.
- the reconstruction calculation device 125 reconstructs the image of the subject 110 by the normal reconstruction unit 125a (step 63). ).
- the reconstructed image is displayed on the subject information display unit 120a of FIG.
- steps 31 to 34 for generating projection data a and steps 61 to 63 for generating an image by normal reconstruction the processing for removing system noise is not performed. Can be generated. Therefore, it is suitable when a quick image display is required, such as an emergency patient.
- the projection data a is stored in the storage device 123 through the above-described steps 31 to 34, it is possible to perform processing for reducing system noise using the projection data a. Thereby, it is possible to execute a processing mode for generating a highly accurate image with few artifacts. This will be described below.
- the operator instructs to perform image reconstruction using measurement data in which system noise is reduced by bias correction.
- the operator instructs to select an icon 23 that instructs to reconstruct an image in which artifacts due to system noise are reduced by performing successive approximation image reconstruction from measurement data.
- the operator confirms the image of the subject information display unit 120a, and selects the icon 22 or 23 if necessary.
- the reconstruction calculation device 125 confirms the processing mode in step 62 of FIG. 8, and when the icon 22 for instructing bias correction is selected, the reconstruction calculation device 125 sends the measurement data from the projection data a to the decompressor 24 of the signal processing unit 124. Instruct to restore x. The restorer restores the measurement data x (step 64).
- the operation of the restorer 24 will be described in detail with reference to FIG.
- the inverse correction processor 29 of the restorer 24 performs a correction process opposite to the correction process of the correction processor 19 on the projection data a stored in the storage device 123 to restore the logarithmically converted data z (step 121).
- the inverse logarithmic converter 25 transforms the restored logarithmic transformed data z by the inverse function of the transformation function ⁇ (y) to restore the positive number data y (step 122).
- the restoring device discriminator 28 reads t and L from the threshold setting device 14 and distributes the restored log-transformed data z to the first and second inverse transformers 26 and 27.
- the first inverse converter 26 receives positive number data y equal to or greater than the threshold value t, and outputs the input signal as it is as measurement data x.
- the second inverse converter 27 receives positive data y that is larger than the threshold value L and smaller than t, converts it by the inverse function of the function ⁇ (x) used in the second converter 16, and outputs it as measurement data x. .
- the measurement data x is restored not only in the positive number but also in the range of s ⁇ x ⁇ 0 (step 123).
- the correction device 30 of the signal processing device 123 removes system noise by performing bias correction processing or the like on the restored measurement data x (step 65 in FIG. 8).
- the bias correction processing refers to the value of measurement data of the target element of the detector 111 with reference to the value of measurement data of the neighboring element of the target element, and the positive value of the target element, and between the target element and the neighboring element. In this method, correction is performed by iterative filtering while maintaining the average value. Since the bias correction process is a well-known technique described in Non-Patent Document 1 and the like, detailed description thereof is omitted here.
- the measurement data x from which the system noise has been removed by bias correction is input from the correction device 30 to the logarithmic converter 18, logarithmically converted, and corrected by the correction processor 19 to obtain projection data b. (Step 66).
- the reconstruction calculation device 125 reconstructs the projection data b by the normal reconstruction unit 125a and generates an image. Since the projection data b has system noise removed, an image in which artifacts due to the system noise are reduced can be obtained. The obtained image is displayed on the subject information display unit 120a of the display screen of FIG.
- the reconstruction calculation device 125 confirms the processing mode in step 62 of FIG. 8, and when the icon 22 is selected by the operator to instruct to perform successive approximate reconstruction from the measurement data x, The restoration unit 24 of the processing unit 124 is instructed to restore the measurement data x from the projection data a. The restorer 24 restores the measurement data x (step 68).
- step 68 Since the operation in step 68 is the same as that in step 64 in the case of bias correction, description thereof is omitted here.
- the restored measurement data x is transferred to the successive approximation reconstruction unit 125b of the reconstruction calculation device 125, and an image is reconstructed by performing successive approximation reconstruction processing on the measurement data x. Specifically, for example, photon noise and system noise included in measurement data are modeled, and an image is calculated using a successive approximation method based on the model. As a result, an image with reduced influence of system noise can be obtained.
- the successive approximation image reconstruction method is a widely known method disclosed in the above-mentioned Non-Patent Document 2 and the like, and thus detailed description thereof is omitted here.
- the obtained image is displayed on the subject information display unit 120a of the display screen of FIG.
- an image with reduced system noise can be generated by restoring the measurement data x from the projection data a projected on the storage device 123.
- Both quick image display and high-accuracy image display corresponding to it can be realized. Therefore, when quick image display is required, such as in an emergency patient, an image is generated by normal reconstruction without performing system noise reduction processing (step 63), and then bias correction or It is possible to generate and display a highly accurate image that has been subjected to processing for reducing system noise by successive approximation image reconstruction or the like (steps 64 to 67 and steps 68 to 69).
- the image reconstruction device 125 acquires the projection data a corresponding to the image selected by the operator when acquiring the projection data a in step 61 of FIG. This makes it possible to reconstruct the image with reduced system noise by restoring the measurement data x not only for the projection data a of the image captured immediately before but also for the projection data a of the image captured in the past. Become.
- Embodiment 2 The X-ray CT apparatus of Embodiment 2 will be described.
- the signal processing device 124 includes the positive number converter 11 and the logarithmic converter 18 and converts the measurement data x into the positive number data y using the function ⁇ (x), and then the function ⁇ (y).
- the present invention is not limited to this configuration.
- the measurement data x is converted into logarithmically converted data z by one conversion function ⁇ (x).
- the conversion function ⁇ (x) for example, as shown in Expression (6), a function in which a conversion function for converting the measurement data x into positive data is included in the logarithmic function argument is used.
- the lower threshold s ( ⁇ 0), the upper threshold t (> 0), and L (> 0) are arbitrary constants.
- the conversion function ⁇ (x) of Equation (6) converts the measurement data x into logarithmically converted data z as shown by the solid line graph in FIG.
- FIG. 11 shows the configuration of the signal processing device 124 in the X-ray CT apparatus of the second embodiment.
- the signal processing device 124 of the second embodiment has the same configuration as the signal processing device of FIG. 3 of the first embodiment, but instead of the positive number converter 11 and the logarithmic converter 18 of the first embodiment.
- a positive number conversion / logarithmic converter 211 having these functions is provided.
- the signal processing device 124 according to the second embodiment includes a preprocessor 12, a positive number conversion / logarithmic converter 211, a correction processing unit 19, a restoration unit 224, a correction unit 30, and a logarithmic converter 218.
- the logarithmic converter 218 is arranged for logarithmically converting the measurement data x restored by the restorer 224.
- the preprocessor 12, the correction processor 19, and the correction device 30 are the same as those in the first embodiment.
- the positive number conversion / logarithmic converter 211 includes a discriminator 13, a threshold value setter 14, and first, second and third logarithmic converters 215, 216 and 217.
- the threshold setting device 14 holds the thresholds s, t, and L as in the first embodiment.
- the discriminator 13 reads out the threshold values s, t, and L from the threshold setting device 14 in the same manner as in the first embodiment, and the measurement data x output from the preprocessor 12 is converted into first, second, and third logarithmic converters. Sort to 215, 216, 217.
- the first logarithmic converter 215 receives the measurement data x with t ⁇ x shown in FIG. 10, performs logarithmic conversion with the function ⁇ (x) with t ⁇ x in the equation (6), and outputs the data z after logarithmic conversion. .
- the second logarithmic converter 216 receives the measurement data x of s ⁇ x ⁇ t, converts it to a positive number by the function ⁇ (x) of s ⁇ x ⁇ t in equation (6), and performs logarithmic conversion to logarithmic conversion Output as post-data z.
- the third logarithmic converter 217 receives the measurement data x of x ⁇ s, converts it all into r by the function ⁇ (x) of equation (6), and outputs it as logarithmically converted data z.
- the correction processor 18 corrects the logarithmically converted data z to generate projection data a.
- the restorer 224 includes an inverse correction processor 29, a restorer discriminator 28, a first inverse logarithmic converter 226, and a second inverse logarithmic converter 227.
- the inverse correction processor 29 is the same as in the first embodiment, and restores logarithmically transformed data z from the projection data a.
- the restoring device discriminator 28 reads t and L from the threshold setting device 14 and distributes the restored log-transformed data z to the first and second inverse logarithmic converters 226 and 227.
- the first inverse logarithmic converter 226 receives the data z after logarithmic conversion equal to or less than q ⁇ gln (t), converts the data z by the inverse function of the function ⁇ (x) used in the first logarithmic converter 215, and obtains measurement data. Output as x.
- the second inverse logarithmic converter 227 receives the logarithmically transformed data z that is larger than q ⁇ gln (t) and less than or equal to r, and converts it by the inverse function of the function ⁇ (x) used in the second converter 216. Output as measurement data x. As a result, the measurement data x is restored even in the negative range of s ⁇ x ⁇ 0.
- the correction device 30 removes system noise by performing bias correction processing or the like on the restored measurement data x.
- the measurement data x that has been restored and from which the system noise has been removed is input to a logarithmic converter 218, logarithmically converted, and corrected by the correction processor 19 to obtain projection data b.
- FIG. 12 is a flow of receiving the output signal of the data collection device 108 and sequentially converting it into measurement data x and projection data a, which is the same as the operation shown in the flow of FIG. 3 of the first embodiment.
- the first to third logarithmic converters 215 to 217 of the positive / logarithmic converter / logarithmic converter 211 perform the operations of steps 32 and 33 in this flow as step 232 in one step.
- FIG. 13 is a flow for restoring the measurement data x from the projection data a, which is the same as the operation of the flow of FIG. 9 of the first embodiment, but in the second embodiment, the operations of steps 122 and 123 of FIG.
- the first and second inverse logarithmic converters 226 and 227 of the decompressor perform step 222 as one step.
- Configurations and operations other than the above-described configuration of the X-ray CT apparatus according to the second embodiment are the same as those of the X-ray CT apparatus according to the first embodiment, and thus description thereof is omitted.
- the signal processing device 124 determines whether or not the imaging condition received by the input device 121 from the operator is a predetermined low-dose imaging condition, and the determination result is the low-dose imaging condition. In this case, the projection data a stored in the storage device 123 is restored by the restoring unit 24, and system noise is reduced.
- the successive approximation image reconstruction method or bias correction by the user's selection has been described, but in view of the fact that the successive approximation image reconstruction method or bias correction is necessary at the time of low-dose imaging,
- a determination device 141 that performs this determination is arranged in the signal processing device 124 as shown in FIG.
- the determiner 141 instructs the restorer 24 to restore the projection data a, and causes the image reconstruction to be performed by successive approximation image reconstruction or bias correction.
- Which of the successive approximation image reconstruction method and the bias correction is performed may be set in advance, or may be configured to accept an operator's selection.
- the irradiation dose is less than the threshold
- the tube voltage is less than the threshold
- the rotation speed of the rotating body 115 is less than the threshold
- the area of the projection value in any view of the projection data a is greater than the threshold
- at least one of the measurement data x If any one of the conditions that the signal value of one element is equal to or less than the threshold value or a predetermined combination of two or more is satisfied, the determiner 141 determines that the low-dose imaging condition is satisfied.
- the threshold of the low-dose imaging condition may be input by an operator, or a preset value may be used.
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Abstract
Description
実施形態1のX線CT装置は、図3のように信号処理装置124が、正数変換器11と、対数変換器18とを備えている。正数変換器11は、予め定めた正数変換関数により計測データの負数データを正数データに変換する。対数変換器18は、正数変換器11で変換後の正数データに対数変換を施して投影データを生成する。正数変換器11の正数変換関数は、所定の範囲の負数について単調増加関数である。正数変換関数として単調増加関数を用いることにより、単調増加関数の逆関数により投影データから所定の範囲の0以下の信号値の計測データを復元することができる。
また、φ(x)はs≧xの範囲において一定の値、例えばLの値となる。
この算出方法については後述する。
計測データxが0以下の値をとる要因は、システムノイズである。システムノイズは、一般的に正規分布によってモデル化することができ、平均値0かつデータ収集装置108に依存した分散値σをとる。
第二の逆変換器27は、閾値Lより大きくtより小さい正数データyを受け取り、第二の変換器16で用いた関数φ(x)の逆関数により変換して計測データxとして出力する。これにより、正数のみならず、s<x≦0の範囲においても計測データxは復元される(ステップ123)。
実施形態2のX線CT装置について説明する。
実施形態4のX線CT装置について説明する。実施形態4のX線CT装置は、入力装置121が操作者から受け付けた撮影条件が、予め定めた低線量撮影条件かどうかを信号処理装置124が判定し、判定結果が低線量撮影条件である場合には、記憶装置123に格納された投影データaを復元器24によって復元し、システムノイズを低減する。
Claims (9)
- X線を被検体に照射するX線発生装置と、被検体を通過した前記X線を検出するデータ収集装置と、前記データ収集装置の出力信号を処理して、0以下の信号値を含む計測データを得て、対数関数を含む予め定めた関数により前記計測データを変換処理して投影データを生成する信号処理装置と、前記投影データを再構成処理して画像を生成する再構成演算装置とを有し、
前記予め定めた関数は、所定の負数以上の値に対して逆関数が存在する関数であり、前記逆関数を前記投影データに適用することにより、所定の範囲の0以下の信号値を含む前記計測データが前記投影データから復元されることを特徴とするX線CT装置。 - 請求項1に記載のX線CT装置において、
前記信号処理装置は、予め定めた正数変換関数により前記計測データの0以下の信号値を正数データに変換する正数変換器と、前記正数変換器で変換後の前記正数データに対数変換を施して前記投影データを生成する対数変換器とを含み、
前記正数変換器の前記正数変換関数は、前記所定の範囲の負数について単調増加関数であることを特徴とするX線CT装置。 - 請求項1に記載のX線CT装置において、前記投影データを格納する記憶装置をさらに有し、
前記信号処理装置は、前記記憶装置に格納された投影データを読み出して、前記所定の範囲の0以下の信号値を含む前記計測データを復元する復元器を含むことを特徴とするX線CT装置。 - 請求項3に記載のX線CT装置において、前記信号処理装置は、前記復元器により復元された計測データに補正を施す補正装置を含むことを特徴とするX線CT装置。
- 請求項3に記載のX線CT装置において、前記再構成演算装置は、前記復元器が復元した前記計測データに逐次近似画像再構成を施して画像を生成する逐次近似再構成部を有することを特徴とするX線CT装置。
- 請求項3に記載のX線CT装置において、操作者から、通常の画像再構成、または、前記復元器によって復元された計測データを用いた画像再構成の選択を受け付ける入力装置をさらに有し、
前記信号処理装置は、前記入力装置が受け付けた選択が、前記復元器によって復元された計測データを用いた画像再構成である場合、前記記憶装置に格納された投影データを前記復元器によって復元することを特徴とするX線CT装置。 - 請求項3に記載のX線CT装置において、操作者から撮影条件の設定を受け付ける入力装置をさらに有し、
前記信号処理装置は、前記入力装置が受け付けた撮影条件が、予め定めた低線量撮影条件かどうかを判定し、判定結果が前記低線量撮影条件である場合には、前記記憶装置に格納された投影データを前記復元器によって復元することを特徴とするX線CT装置。 - 被検体を通過したX線を検出した信号を処理して、0以下の信号値を含む計測データを得て、前記計測データを予め定めた関数を用いて対数変換して投影データを生成する信号処理装置と、前記投影データを再構成処理して画像を生成する再構成演算装置とを有し、
前記予め定めた関数は、所定の負数以上の値に対して逆関数が存在する関数であり、前記逆関数を前記投影データに適用することにより、所定の範囲の0以下の信号値を含む前記計測データが復元されることを特徴とするX線CT装置用画像演算装置。 - X線を被検体に照射するX線発生装置と、被検体を通過した前記X線を検出するデータ収集装置と、前記データ収集装置の出力信号を処理して、0以下の信号値を含む計測データを得て、対数関数を含む予め定めた関数により前記計測データを変換処理して投影データを生成する信号処理装置と、前記投影データを再構成処理して画像を生成する再構成演算装置と、前記投影データを格納する記憶装置を有し、
前記信号処理装置は、前記記憶装置に格納された投影データを読み出して、所定の範囲の0以下の信号値を含む前記計測データを復元する復元器を含むことを特徴とするX線CT装置。
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