Photon-counting x-ray CT apparatus

Abstract

To extend a preservable period of projection data in a console in a photon-counting type x-ray CT apparatus.SOLUTION: A console 110 comprises a data management unit 1110 which converts raw data stored as projection data to compressed data at predetermined timing, stores the compressed data, and deletes the raw data. The raw data is detection value data indicating the number of photons per a plurality of energy bins of a photon measurement type detection element. The compressed data is ratio data indicating the ratio between detection value data on all energy areas indicating the total sum of the detection value data per the plurality of energy bins and detection value data per the plurality of energy bins to the detection value data of all the energy areas.SELECTED DRAWING: Figure 11

Description

Technical Field

[0001] The present disclosure relates to a photon counting X-ray CT apparatus.

Background Art

[0002] In a photon counting type X-ray computed tomography (CT) apparatus, compared with an energy integrating type X-ray CT apparatus, the output of each element (detection element) of the detector is divided into energy regions (bins), and the size of the detection element becomes smaller and thinner in both the channel direction and the slice direction, so that the number of elements of the detector increases. As a result, the size of the projection data generated by the photon counting type X-ray CT apparatus becomes several tens of times larger than that of the energy integrating type X-ray CT apparatus.

[0003] Patent Document 1 proposes a data irreversible compression technique for achieving both shortening of the data transfer time from the detector to the console and maintaining the image quality in a photon counting type X-ray CT apparatus.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The inventors focused on the size of the projection data (raw data) that serves as the basis for image reconstruction and is stored in the console. Generally, in an X-ray CT apparatus, the projection data is not immediately deleted but is stored in the console for a certain period of time. This is to meet the demand for ensuring diagnosis during the reading of inspection images or for performing image reconstruction with changed conditions for other reasons. The storage period of the raw data in the console depends on the storage capacity of the console and the storage capacity required by the X-ray CT apparatus for one day's inspections. In a photon-counting type X-ray CT apparatus, since the size of the projection data is several tens of times that of an energy-integrating type X-ray CT apparatus as described above, if the storage capacity of the apparatus is the same, the storage period must be significantly shortened, resulting in a decline in the usability of the apparatus.

[0006] An object of the present invention is to provide a compression method in a photon-counting type X-ray CT apparatus that can obtain an image equivalent to that of an energy-integrating type X-ray CT apparatus and can utilize energy information, thereby reducing the capacity of the raw data stored in the console and extending the storage period of the projection data in the console.

Means for Solving the Problems

[0007] A photon-counting X-ray CT apparatus according to an embodiment of the present invention includes an X-ray tube, a detector including a photon measurement type detection element that detects X-rays emitted from the X-ray tube and transmitted through a subject, a gantry to which the X-ray tube and the detector are attached so as to face each other with the subject therebetween, and a console that stores projection data acquired while rotating the X-ray tube and the detector around the subject by the gantry. The console includes a data management unit that converts the raw data stored as projection data into compressed data at a predetermined timing, stores the compressed data, and deletes the raw data. The raw data is detection value data indicating the number of photons for each of a plurality of energy bins of the photon measurement type detection element, and the compressed data is detection value data in the total energy region indicating the sum of the detection value data for each of the plurality of energy bins and ratio data indicating the ratio of the detection value data for each of the plurality of energy bins to the detection value data in the total energy region.

Advantages of the Invention

[0008] By storing the projection data of the photon-counting X-ray CT apparatus in the console by a compression method capable of obtaining an image equivalent to that of an energy-integrating X-ray CT apparatus and utilizing energy information, the usability is improved while taking advantage of the features of the apparatus. Other problems and novel features will become apparent from the description of this specification and the accompanying drawings.

Brief Description of the Drawings

[0009]

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Figure 10A

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Figure 12

Embodiments for Carrying Out the Invention

[0010] FIG. 1 shows an overall configuration diagram of a photon counting X-ray CT apparatus 101. Here, the horizontal direction of the paper surface is defined as the X-axis, the vertical direction as the Y-axis, and the direction orthogonal to the XY plane as the Z-axis. The X-ray CT apparatus 101 mainly includes a gantry 102, an X-ray tube 103, a bowtie filter 105, a couch 106, a detector 108, and a console 110.

[0011] The subject 107 is placed on the couch 106 and disposed in an opening 109 provided in the gantry 102. The X-ray 104 emitted from the X-ray tube 103 is shaped into a beam shape suitable for the size of the subject 107 by the bowtie filter 105 and then irradiated onto the subject 107. After passing through the subject 107, it is detected by the detector 108. The X-ray tube 103 and the detector 108 are attached to the gantry 102 so as to face each other with the subject 107 interposed therebetween, and are rotated around the subject 107 by a rotation drive unit of the gantry 102. By repeating the X-ray irradiation from the X-ray tube 103 and the X-ray measurement by the detector 108 along with the rotation by the rotation drive unit, projection data at various projection angles are acquired.

[0012] The console 110 includes an arithmetic unit 111, a display device 112, and an input device 113. The acquired projection data is subjected to image reconstruction processing by the arithmetic unit 111, thereby generating a tomographic image of the subject 107, which is then displayed on the display device 112. Further, when projection data is acquired while the couch 106 on which the subject 107 is placed and the gantry 102 relatively move in the Z-axis direction, a volume image of the subject 107 is generated. Note that the X-ray dose irradiated from the X-ray tube 103, the rotation speed of the gantry 102, and the relative movement speed between the gantry 102 and the couch 106 are set based on the scan conditions input by the operator via the input device 113.

[0013] Detector 108 is composed of a plurality of detection elements arranged in an arc shape centered on the X-ray focal point of X-ray tube 103. The detection elements are photon counting type detection elements that measure the X-ray energy, which is the energy of incident X-ray photons, and output a detection value corresponding to the X-ray energy. In the X-ray CT apparatus 101 equipped with such a photon counting type detector, since the X-ray energy spectrum regarding the projection data of the subject 107 can be acquired, medical images in which substances with different compositions are discriminated or medical images divided into a plurality of energy components can be generated.

[0014] Moreover, the arithmetic unit 111 has a hardware configuration similar to that of a general computer device, and includes a CPU (Central Processing Unit), a memory, a non-volatile storage device such as an HDD (Hard Disk Drive), etc., and performs control of each part and correction processing on projection data and the like.

[0015] First, an outline of the compression method of the projection data (raw data) stored in the photon counting type X-ray CT apparatus of this embodiment will be described. In the X-ray CT apparatus, as candidate raw data to be stored in the console, two types are conceivable: the detection value data output from the detector and the projection value data calculated based on the detection value data. Here, the detection value data is the data of the number of photons counted by energy region in the photon counting type X-ray CT apparatus, and is the data of the intensity of photons in the entire energy region in the energy integration type X-ray CT apparatus. The projection value data is represented by the logarithmic value of the reciprocal of the X-ray transmittance calculated based on the detection value, and means the line integral value of the linear attenuation coefficient.

[0016] Figure 2 summarizes the relationship between the detected values and the projection values in a photon-counting type X-ray CT apparatus. Here, it is assumed that the energy of the X-ray photons to be detected is divided into five energy bins E1 to E5. It is assumed that X-rays of the same intensity are irradiated onto the air 202 and the subject 204, and the X-rays transmitted through the air 202 are detected by the detector 201, and the X-rays transmitted through the subject 204 are detected by the detector 203. Table 205 shows the relationship between the detected values (photon numbers) and the projection values in each energy bin Ei (i = 1 to 5) in this case. Further, Figure 3 shows the relationship between the linear attenuation coefficient and the energy. In this figure, the energy bins E1 to E5 are superimposed and shown.

[0017] In an energy-integrating type X-ray CT apparatus, it is common to save the projection value data as raw data in the console. In contrast, in a photon-counting type X-ray CT apparatus, it is desirable to save the detected value data as raw data in the console. Generating inspection images by utilizing information for each energy region, such as material discrimination, improvement of contrast using the low energy region and K absorption edge, and reduction of beam hardening artifacts due to metal using the projection values in the high energy region, is most expected for a photon-counting type X-ray CT apparatus. For this reason, it is desirable to leave the original detected value before being converted into the projection value representing the attenuation coefficient.

[0018] Also, in the case of the detected value data, it is advantageous from the viewpoint of the amount of information. The sum of the detected values in each energy region (energy bin) detected by a photon-counting type X-ray CT apparatus is equivalent to the detected value detected by an energy-integrating type X-ray CT apparatus. Therefore, if this information is saved, an image equivalent to that of an energy-integrating type X-ray CT apparatus can be obtained. Furthermore, by saving the detected value of each energy region not as the value itself but as a ratio to the sum, the storage capacity required for storing the raw data can be reduced.

[0019] That is, in this embodiment, the raw data saved in the console of the photon-counting type X-ray CT apparatus is the detected value data, and the compressed data is the sum of the detected values in each energy region (ΣIEi )), that is, the detected value I in the total energy region Eall , and the ratio of the detected value in each energy region to the detected value in the total energy region (I Ei / I Eall ) are saved in such a form that the capacity of the raw data is reduced while retaining the energy information.

[0020] Hereinafter, a specific compression method for the detected value data will be described. In the following examples, it is assumed that all are divided into five energy regions of energy bins E1 to E5.

Example

[0021] As described above, the detected value is the number of photons, which is on the order of 100 megacounts / second per 1 mm 2 . Therefore, the detected value data treated as raw data is not the detected value (number of photons) output from the detector itself, but a value obtained by converting the logarithmic value of the detected value into an integer value (hereinafter referred to as an integerized logarithm). FIG. 4 shows a numerical example of the detected value of a one-channel detector. Note that this numerical value is a numerical example for explaining the compression and restoration of raw data.

[0022] The detected value 402 represents the detected value I for each energy bin 401 output by the detector Ei and represents the number of photons. The detected value (logarithm) 403 is the logarithm Log_I Ei of the detected value 402, and the detected value (integerized logarithm) 404 is the integerized logarithm Raw_Log_I Ei of the detected value (logarithm) 403. The detected value (integerized logarithm) 404 corresponds to the raw data saved in the console. The significant figures for integerizing the logarithmic value are determined in consideration of the accuracy of the inspection image and the storage capacity required to save the detected value (integerized logarithm) 404. In this example, it is integerized with 5 significant figures.

[0023] When the detected value (integerized logarithm) 404 is represented by 16 bits, the possible values are 0 to 65,535 (2 16-1). If the detected values (integerized logarithms) 404 for the five divided energy regions are to be saved to the console, an area of 80 (= 16 * 5) bits per channel is required.

[0024] In contrast, in Example 1, it is saved as compressed data as described below. First, the sum of the detected values of each energy region (ΣI Ei ), that is, the detected value data I Eall of all energy regions can be saved in a 16-bit area as an integerized logarithm, similar to the detected values (integerized logarithms) 404 for each energy region. Next, the ratio data (I Ei / I Eall ) between the detected value data of each energy region and the detected value data of all energy regions. However, the number of bits to save this requires an appropriate decision because it is a trade-off between the compression rate and the accuracy of the energy information. Here, it is assumed that the ratio is saved in 10 bits.

[0025] In making this decision, the calculation accuracy of the CT value was referred to. The CT value is a value representing the projection value (linear attenuation coefficient) of the subject with reference to the projection value (linear attenuation coefficient) of water, and is shown as an integer value obtained by multiplying by 10 3 . In this case, the ratio can be regarded as being shown with an accuracy of 1 / 10 3 ≒ 2 -10 (third decimal place). Following this, in Example 1, the base constant of the ratio is determined to be 1 / (2 10 - 1), and the ratio data (I Ei / I Eall ) is shown as its multiple, so that it can be saved in a 10-bit area. Therefore, in Example 1, the compressed data per channel can be saved in an area of 66 (16 + 10 * 5) bits. The compression rate in this case is 17.5% (= (1 - 66 / 80) * 100). Hereinafter, for the numerical example in FIG. 4, the specific procedures for compressing and restoring the raw data will be described.

[0026] Compression process: (1) Calculation of the integerized logarithm (Raw_Log_I Eall ) of the detected value I Eall of all energy regions The detected value (integerized logarithm) 404 of each energy bin stored as raw data is inversely calculated by (Equation 1). Here, i = 1 to 5. Raw_I Ei = 10 ^ (Raw_Log_ Ei / 10,000) ···(Equation 1) The detected value Raw_I calculated for each energy bin Ei is shown as the detected value (inverse calculation) 405 in Figure 4. Subsequently, the detected value of the entire energy region is calculated as the sum of the detected values Raw_I Ei of each energy bin, and the logarithm of ΣRaw_I Ei is converted to an integerized logarithm detected value Raw_Log_I by integerizing it to 5 significant digits in the same way as the detected values of each energy bin. Eall is calculated.

[0027] In this numerical example, ΣRaw_I Ei = 225,920 (see Figure 4), and the detected value data Raw_Log_I Eall of the entire energy region is 53,540.

[0028] (2) Calculation of the 10 - bit representation (Raw_R Ei / Raw_I Eall ) of the ratio Ei ) To represent the ratio (Raw_I Ei / Raw_I Eall ) in 10 - bit representation, the ratio is expressed as a multiple of the ratio base constant C (1 / (2 10 - 1) = 1 / 1,023) by (Equation 2). As a result, the ratio Raw_R Ei is represented as an integer value in the range of 0 to 1,023. Raw_R Ei = (Raw_I Ei / Raw_I Eall ) / C ···(Equation 2) For the numerical example in Figure 4, the ratio data Raw_R EiThe calculated values are shown in Fig. 5. In the detected value (integerized logarithm) 502, the detected values (integerized logarithms) of each energy bin Ei (i = 1 to 5) are the same as the detected values (integerized logarithms) 404 in Fig. 4, and the detected value (integerized logarithm) of the entire energy region Eall is the calculated value in the compression method (1). In the detected value (inverse operation) 503, the detected values (inverse operations) of each energy bin Ei (i = 1 to 5) are the same as the detected values (inverse operations) 405 in Fig. 4, and the detected value (inverse operation) of the entire energy region Eall is the value obtained by inverse-operating Raw_Log_I Eall (=53,540) (see Equation (1)). By substituting the value of the detected value (inverse operation) 503 into Equation (2), the ratio data Raw_R Ei (i = 1 to 5) is calculated.

[0029] The compressed data calculated by the above compression processes (1) and (2) is saved. In Fig. 5, the data saved as the compressed data of the raw data is shown surrounded by a black frame.

[0030] Restoration process: By restoring the compressed data using Equation (3), the detected value (restored) Re_I Ei is obtained. Here, C is the base constant of the same ratio as in Equation (2). Re_I Ei = Raw_I Eall × Raw_R Ei / C ··· (Equation (3)) Fig. 6 shows the restoration result. The compressed data 602 is the ratio data Raw_R Ei saved in the console and the detected value data Raw_Log_I Eall of the entire energy region, and the detected value (restored) 603 shows the detected value Re_I Ei restored for each energy bin 601.

[0031] This restored detected value is compared with the detected value (inverse operation) when the raw data (the range within the black frame in Fig. 4) was saved using 80 bits. The detected value when the raw data was saved is the detected value (inverse operation) 405, and the restoration rate of the restored detected value (= 1 - |Re_I Ei - Raw_I Ei| / Raw_I Ei ) is shown as a restoration rate of 604 in percentage display.

[0032] Also shown is the result of evaluating the deviation compared to FIG. 7. The ratio (raw data) 702 is the result of calculating the ratio for each energy bin 701 based on the detected value (inverse calculation) 405 (see FIGS. 4 and 6), and the ratio (restored data) 703 is the result of calculating the ratio for each energy bin 701 based on the detected value (restored) 603 (see FIG. 6). As shown by the deviation 704, almost no deviation occurs in any energy bin.

Example

[0033] In Example 1, ratio data Raw_R is saved for all energy bins Ei (i = 1 to 5), while in Example 2, the saving of ratio data for any one of the energy bins is omitted. For example, if the saving of the ratio data for energy bin E5 is omitted, then from the detected value Raw_I of the entire saved energy region Eall Eall and the restored data of each energy bin E1 to E4, the detected value (restored) Re_I E5 = Raw_I Eall - ΣRe_I Ei (i = 1 to 4) can be calculated.

[0034] In this case, the compressed data per channel can be saved in a 56 (16 + 10 * 4)-bit region, and the compression rate in this case is 30% (=(1 - 56 / 80) * 100).

Example

[0035] In the numerical example of FIG. 4, the maximum value of the ratio data Re_R Ei which is the compressed data, is 307 (see FIG. 5), and this can be represented in 9 bits (0 to 512 (= 2 9 - 1)). Further, in Example 2, if the saving of the ratio data for energy bin E3 which shows the maximum value of the ratio data Re_R Ei is omitted, then the maximum value of the ratio data Re_R Ei to be saved becomes 246, and this is 8 bits (0 to 255 (= 28 -1) can be expressed as

[0036] Therefore, in the third embodiment, the ratio data Re_R, which is expressed in 10 bits, Ei The value of is stored in the number of bits according to the actual value, and the ratio data Re_R Ei The number of bits to store the value of is called the multiple bit number. The multiple bit number is expressed as the difference between the number of bits of the ratio base constant C and the multiple bit number (called the difference bit number). The difference bit number is stored as compressed data, and the ratio data Re_R Ei The value of is expressed in multiple bits, which allows for even greater compression.

[0037] When the bit number of the ratio base constant C is 10, the difference bit number is 2 bits (0 to 3 (=2 2 -1)) to save the multiple bit number in the range of 7 to 10 bits, and the ratio data Re_R Ei It is possible to save

[0038] An example of compressing the compressed data 602 in FIG. 6 by the method of the third embodiment will be described. In this case, Re_R E1 is 7 bits, Re_R E2 ,Re_R E4 ,Re_R E5 is 8 bits, Re_R E3 can be expressed by a multiple of 9 bits, and by applying the concept of the second embodiment, the ratio data Re_R E3 Other than the four ratio data Re_R Ei If you want to save the ratio data Re_R Ei Since the data can be stored in 8 bits, the compressed data per channel can be stored in a 50 (16+8*4+2) bit area. In this case, the compression ratio improves to 37.5% (=(1-50 / 80)*100). EXAMPLES

[0039] In the above examples, the ratio base constant C is set to 1 / (2 10 -1) and the ratio data for compression is Raw_R Eiis represented in 10-bit format. However, depending on the form of processing that utilizes energy information, there may be cases where the accuracy of the ratio data Raw_R Ei can be reduced. In Example 4, the compression rate is further improved by reducing the number of bits of the ratio base constant C.

[0040] For the numerical example in FIG. 4, FIG. 8 shows the compressed data and the restored data when the number of bits of the ratio base constant C is varied in the range of 10 to 6 bits. In FIG. 8, together with the restored data, the restoration rate and the ratio deviation are shown as the accuracy information.

[0041] Thus, when making the number of bits of the ratio base constant C variable, it is necessary to retain this information in some form. However, since this information does not need to be retained for each channel, for example, it may be saved as part of the header information of the compressed data. Also, in the case of the variable range shown in the table of FIG. 8, if the variable range is expressed as the difference between the basic 10 bits and the number of bits of the ratio base constant C to be applied (referred to as the variable number of bits), then up to 7 bits of the number of bits of the ratio base constant C, the variable number of bits is 2 bits (0 to 3), and if the number of bits of the ratio base constant C is made 6 bits, it can be expressed up to a variable number of bits of 3 bits (0 to 7). This is a size that can be ignored in terms of the overall capacity.

[0042] When the concepts of Examples 1 to 4 are used in combination, the following compression rates are expected for the compressed data per channel. Compression rate when the number of bits of the ratio base constant C is 9 bits: 16 bits + 7 bits × 4 + 2 bits = 46 bits, (1 - 46 / 80) × 100 = 42.5% Compression rate when the number of bits of the ratio base constant C is 8 bits: 16 bits + 6 bits × 4 + 2 bits = 42 bits, (1 - 42 / 80) × 100 = 47.5% Compression rate when the number of bits of the ratio base constant C is 7 bits: 16 bits + 5 bits × 4 + 2 bits = 38 bits, (1 - 38 / 80) × 100 = 52.5% Compression ratio when the number of bits of the ratio base constant C is 6 bits: 16 bits + 4 bits × 4 + 2 bits = 34 bits, (1 - 34 / 80) × 100 = 57.5% In each case, the differential number of bits is calculated as the difference between the number of bits of the applied ratio base constant C and the multiple number of bits.

Example

[0043] In Example 5, it is shown that energy information can be compressed in the same way even for the data that increases due to high definition, which is another feature of the photon counting type X-ray CT apparatus.

[0044] FIG. 9 shows an example of a detection element of a photon counting type X-ray CT apparatus. The detection element 901 divides one channel equivalent (macro pixel) of an energy integrating multi-type X-ray CT apparatus into 3×3 sub-pixels S1 to S9, and information that is subdivided in both the channel (CH) direction and the slice (Z) direction can be obtained. In a photon counting type X-ray CT apparatus including such a detection element 901, in the normal mode, the number of photons is output in units of macro pixels, and in the high definition mode, the number of photons is output for each sub-pixel. Since the number of photons incident on the macro pixel is divided into sub-pixels, the detection value obtained from the macro pixel can be regarded as equivalent to the sum of the detection values of the sub-pixels. Therefore, in the normal mode, the compression process may be performed by the method described in Examples 1 to 4. In the high definition mode, compression is performed by bundling the energy information of the sub-pixels into macro pixels.

[0045] A method for creating compressed data to be stored in the console in the case of the high definition mode will be described with reference to FIGS. 10A and 10B. First, the compression and restoration procedures corresponding to Example 1 in the case of the high definition mode will be described.

[0046] Compression process: (1) Detection value I of the entire energy region for each sub-pixel SkEall Integer logarithm of (Raw_Log_I SkEall ) calculation (k = 1 to 9) One channel in high-definition mode corresponds to one sub-pixel. Therefore, the detection value per channel in Example 1 is replaced with the detection value per sub-pixel, and the same operation is performed. The calculation method of the detection value (integerized logarithm) in the compression process (1) will be described with reference to FIG. 10A.

[0047] In high-definition mode, the detection value (integerized logarithm) of each energy bin for each sub-pixel is stored as raw data. Region 1001 shows the detection value (inverse operation) Raw_I SkEi (k = 1 to 9, i = 1 to 5). Region 1002 shows Raw_I SkEall obtained as the sum of the detection values (inverse functions) of each energy bin for each sub-pixel (k = 1 to 9), and Region 1003 shows the detection value Raw_Log_I SkEall obtained by converting these to integerized logarithms (k = 1 to 9). The detection value data within the region 1003 surrounded by the black frame is the detection value data to be stored as compressed data in high-definition mode.

[0048] (2) Calculation of the ratio (Raw_I McEi / Raw_I McEall ) The ratio is calculated in units of macro-pixels. The calculation method of the ratio in the compression process (2) will be described with reference to FIG. 10B.

[0049] In the detection value (inverse operation) 1011, the detection value of each energy bin Ei (i = 1 to 5) is the sum of the detection values for each energy bin Ei in Region 1001 (FIG. 10A) bundled in units of macro-pixels, and the detection value of the entire energy region Eall is the further sum of these. The detection value (integerized logarithm) 1012 is calculated by integerizing the logarithm of the detection value (inverse operation) 1011. Here, "Mc" is used as a symbol to indicate that the detection values are bundled in units of macro-pixels. The detection value (inverse operation) 1013 is calculated by performing an inverse operation on the detection value (integerized logarithm) 1012. Since these operations are the same as those described in Example 1, duplicate explanations are omitted.

[0050] Ratio 1014 is calculated as Raw_I using the detected value (inverse calculation) 1013. McEi / Raw_I McEall Although omitted in Fig. 10B, similar to Example 1, ratio Raw_R McEi is expressed as a multiple of the ratio base constant C (= 1 / 1023) and stored in 10-bit representation. The ratio data to be stored as compressed data in high-definition mode is shown enclosed in a black frame.

[0051] Through the above compression process, the information amount that originally required 16 bits × (3 × 3) × 5 = 720 bits can be stored in 16 bits × (3 × 3) + 10 bits × 5 = 194 bits. The compression rate at this time is 73.1% (=(1 - 194 / 720) * 100).

[0052] Restoration process: By restoring the compressed data according to (Equation 4), the detected value (restored) Re_I in sub-pixel units SkEi is obtained. Here, C is the ratio base constant used in the compression process. Re_I SkEi = Raw_I SkEall × Raw_R McEi / C ···(Equation 4) Here, Raw_I SkEall is obtained by inversely calculating the detected value data Raw_Log_I SkEall which is the compressed data.

[0053] The above describes the compression and restoration processes corresponding to Example 1 for the high-definition mode. Similarly, the processes corresponding to Examples 2 to 4 can be applied to the compression and restoration processes for the high-definition mode.

[0054] As described above, the compression and restoration processes of the raw data specifically stored in the console have been explained as Examples 1 to 5. Hereinafter, in the photon counting X-ray CT apparatus 101, a mechanism for implementing the operation of the compression and restoration processes of the raw data will be explained. FIG. 11 shows a functional block diagram for the console 110 to manage the storage of projection data. The management of the storage of projection data is implemented as a function of the arithmetic unit 111, and the projection data storage management function is implemented, for example, as a program of the arithmetic unit 111. As described above, the arithmetic unit 111 has a hardware configuration similar to that of a general computer device, and the CPU of the arithmetic unit 111 functions as a functional unit (functional block) that provides a predetermined function by executing processing according to a program loaded into the main memory. The storage device of the arithmetic unit 111 stores data and programs used by the functional units.

[0055] The data management unit 1110 is a functional unit that manages the projection data stored in the storage device 1120, and includes a compression rate setting unit 1111, a compression / restoration unit 1112, and a compression operation unit 1113. The projection data captured by the device is stored in the storage device 1120. The projection data includes raw data 1121 and compressed data 1122. The raw data is, for example, the data shown as the detection value (integerized logarithm) 404 in FIG. 4, and the compressed data is, for example, the data surrounded by a black frame in FIG. 5. The data management unit 1110 manages the projection data so as to convert the raw data into compressed data and store it at an appropriate timing, delete the original raw data, and store the projection data in the storage device 1120 for a longer period.

[0056] The compression / restoration unit 1112 is a functional unit that executes the compression and restoration processes described as Examples 1 to 5. Here, as Example 4 of the compression and restoration processes, an aspect of reducing the number of bits of the ratio base constant C has been explained. Since there is a trade-off between the compression rate and the accuracy of the energy information, it is desirable to provide information for the operator to determine what compression rate is acceptable. FIG. 12 shows the determination flow of the number of bits of the ratio base constant C by the compression rate setting unit 1111.

[0057] The compression rate setting unit 1111 receives the specification of the number of bits of the base constant C of the ratio by the operator (S01), compresses and restores the raw data using the base constant C of the ratio with the specified number of bits in the compression / restoration unit 1112, and obtains restored data (S02, S03). Based on the obtained restored data, the operator is made to perform image reconstruction processing using energy information desired by the operator (for example, virtual monochromatic X-ray image, effective atomic number image, electron density image, iodine image, etc.), and the obtained image and evaluation value are presented to the operator (S04). For example, it is desirable to use numerical information such as the average pixel value and SD value within the ROI (Region of Interest) as the evaluation value.

[0058] When the operator re-specifies a different number of bits (Yes in S05), the processing from step S01 and subsequent steps is executed again. Based on the image and evaluation value presented in step S04, the operator determines the number of bits to be used for the compression process (S06), and the compression rate setting unit 1111 stores the determined number of bits.

[0059] Note that it is desirable to specify the slice position at the time of the scan plan or to attach markers and ROIs to the cross-section of the slice of interest when checking the reconstructed image generated from the raw data. Thereby, in the compression rate setting unit 1111, in order to evaluate the restored data, an image of the position specified in advance as being of interest to the operator can be generated with the energy information of the specified compression rate (S04), and the confirmation burden on the operator can be reduced.

[0060] The compression operation unit 1113 performs operations of converting raw data into compressed data and deleting the original raw data to secure the storage capacity of the storage device 1120, and operations of deleting the compressed data at a predetermined timing. At this time, instead of immediately deleting the compressed data, the raw data is first converted into compressed data using the base constant of the ratio of bit C1, and then converted into compressed data using the base constant of the ratio of bit C2 (C1 > C2) at the next timing. It is also possible to operate in such a way as to gradually reduce the size of the compressed data. The timing of converting the raw data into compressed data or the timing of deleting the compressed data may be determined after the elapse of a predetermined time, or may be determined based on the storage capacity of the storage device 1120.

[0061] The present invention is not limited to the above-described embodiments, and includes various modifications. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Also, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, it is possible to add, delete, or replace other configurations for a part of the configuration of each embodiment.

Explanation of Reference Numerals

[0062] 101: Photon-counting X-ray CT apparatus, 102: Gantry, 103: X-ray tube, 104: X-ray, 105: Bowtie filter, 106: Bed, 107: Subject, 108: Detector, 109: Aperture, 110: Console, 111: Arithmetic unit, 112: Display device, 113: Input device, 201, 203: Detector, 202: Air, 204: Subject, 205: Table, 901: Detection element, 1110: Data management unit, 1111: Compression rate setting unit, 1112: Compression / restoration unit, 1113: Compression operation unit, 1120: Storage device, 1121: Raw data, 1122: Compressed data.

Claims

1. An X-ray tube, A detector including a photon measurement type detection element that detects X-rays emitted from the X-ray tube and transmitted through a subject, A gantry to which the X-ray tube and the detector are attached so as to be opposed to each other with the subject therebetween, A console that stores projection data acquired while rotating the X-ray tube and the detector around the subject by the gantry, The console includes a data management unit that converts raw data stored as the projection data into compressed data at a predetermined timing, stores the compressed data, and deletes the raw data, The raw data is detection value data indicating the number of photons for each of a plurality of energy bins of the photon measurement type detection element, The compressed data is detection value data in an all energy region indicating the sum of the detection value data for each of the plurality of energy bins, and ratio data indicating the ratio of the detection value data for each of the plurality of energy bins to the detection value data in the all energy region. A photon counting X-ray CT apparatus.

2. In claim 1, The ratio data is expressed as a multiple of a ratio base constant of the ratio. A photon counting X-ray CT apparatus.

3. In claim 2, The ratio data does not include the ratio of any one of the detection value data of the plurality of energy bins to the detection value data in the all energy region. A photon counting X-ray CT apparatus.

4. In claim 3, The ratio data does not include the ratio of the maximum detection value data among the plurality of energy bins to the detection value data in the all energy region. A photon counting X-ray CT apparatus.

5. In any one of claims 2 to 4, The ratio of the ratio data is expressed by a multiple bit number which is the number of bits required to represent the maximum ratio among the ratios included in the ratio data. The ratio data, together with the ratio, includes a differential bit number indicating the difference between the number of bits of the base constant of the ratio and the multiple bit number, in a photon counting X-ray CT apparatus.

6. In any one of Claims 2 to 4, A photon counting X-ray CT apparatus in which the number of bits of the base constant of the ratio is variable.

7. In Claim 6, The ratio of the ratio data is expressed by a multiple bit number which is the number of bits required to represent the maximum ratio among the ratios included in the ratio data. The ratio data, together with the ratio, includes a differential bit number indicating the difference between the number of bits of the base constant of the ratio and the multiple bit number, in a photon counting X-ray CT apparatus.

8. In Claim 1, The photon measurement type detection element is divided into a plurality of sub-pixels, The raw data is detection value data indicating the number of photons for each of the plurality of energy bins for each of the plurality of sub-pixels, The compressed data includes detection value data in the total energy region indicating the sum of the detection value data for each of the plurality of energy bins for each of the plurality of sub-pixels, and ratio data indicating the ratio of the detection value data for each of the plurality of energy bins to the detection value data in the total energy region for a macro-pixel formed by bundling the plurality of sub-pixels, in a photon counting X-ray CT apparatus.

9. In Claim 1, The data management unit, includes a compression / restoration unit that compresses the raw data into the compressed data and restores the compressed data, and a compression rate setting unit that sets the compression rate of the compressed data by the compression / restoration unit by the data management unit. The compression / decompression unit expresses the ratio in the ratio data as a multiple of the base constant of the ratio. A photon counting X-ray CT apparatus that can present for comparison an image reconstructed from data compressed and decompressed by changing the number of bits of the base constant of the ratio, and its evaluation value, and an image reconstructed from the raw data and its evaluation value.

10. In claim 1, The data management unit includes a compression / decompression unit that compresses the raw data into the compressed data and decompresses the compressed data, and a compression operation unit that manages the storage capacity stored in the console, the compression / decompression unit expresses the ratio in the ratio data as a multiple of the base constant of the ratio, The compression operation unit is configured to convert the raw data into first compressed data generated with the number of bits of the base constant of the ratio as a first number of bits at the predetermined timing, store the first compressed data, delete the raw data, and then convert the first compressed data into second compressed data generated with the number of bits of the base constant of the ratio as a second number of bits smaller than the first number of bits at a subsequent predetermined timing, store the second compressed data, and delete the first compressed data.

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