JP4146113B2 - X-ray diagnostic equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray diagnostic apparatus capable of acquiring a good image by appropriately correcting the output of an X-ray detector even when a temporary decrease in sensitivity or a temporary increase in offset occurs. .
[0002]
[Prior art]
Conventionally, an X-ray fluoroscopic apparatus equipped with an image intensifier (II) -TV camera system has been used as an X-ray imaging means. This I.I. I. As shown in the schematic cross-sectional view of FIG. 12, the input fluorescent screen 81 that converts incident X-rays into a visible light image and the light intensity distribution of the visible light image are converted into a photoelectron emission density distribution. In addition, a photoelectric conversion film 82 to which a cathode potential is applied, an anode 83 that provides an accelerating electric field that accelerates an electron beam emitted from the photoelectric conversion film, a focusing electrode 84 that focuses the electron beam on an output phosphor screen, and accelerated electrons. And an output phosphor screen 85 that is incident upon the beam and converted into an optical image again. The optical image formed on the output phosphor screen is amplified to a brightness several thousand times that of the optical image on the input phosphor screen. The luminance-amplified image is projected on a monitor device through a television camera or recorded on an image recording device.
[0003]
On the other hand, in recent years, this I.D. I. An X-ray detector using a semiconductor for the X-ray detection portion has been proposed from the viewpoint of being smaller and thinner.
[0004]
As a configuration of an X-ray detector using this semiconductor, an indirect conversion type X-ray detector (US Pat. No. 4,689,487), a direct conversion type X-ray detector (US Pat. No. 5,319,206) and the like have been proposed.
[0005]
An indirect conversion X-ray detector converts X-rays into light via a chemical substance such as an intensifying screen or cesium iodide (CsI) crystal, and converts the intensity of this light into a charge by the photoelectric conversion action of a photodiode. Then, this electric charge is accumulated in the capacity of each pixel. Then, charges accumulated by switching means such as a thin film transistor (hereinafter abbreviated as “TFT”) matrix are sequentially read out and converted into a voltage by a charge amplifier (also referred to as a first-stage integration amplifier). An image signal is obtained.
[0006]
On the other hand, in the direct conversion X-ray detector, as shown in the schematic cross-sectional view of FIG. 13, X-rays incident on a semiconductor such as selenium (Se) under a high electric field directly contribute to charge generation by the photoelectric effect. Charges are accumulated in the signal storage capacitor for each pixel. As in the indirect conversion type, the accumulated charges are sequentially read out by switching the TFTs, converted into voltages by a charge amplifier (not shown), and A / D converted to obtain a digital image signal.
[0007]
Further, when diagnosis is performed using the X-ray detector as described above, the sensitivities of a plurality of detection elements provided in the X-ray detector vary. In order to correct this variation in sensitivity, conventionally, correction using an offset correction table 102 and a gain (sensitivity) correction table 103 is performed as shown in FIG.
[0008]
That is, the gain correction table 103 is a table in which sensitivity characteristics in the X-ray detector are measured in advance and different sensitivity correction coefficients are stored for each pixel (each detection element). The gain correction table 103 outputs a detection value corrected by multiplying the output of the X-ray detector. The offset correction table 102 measures offset characteristics in the X-ray detector in advance and stores different offset correction coefficients for each pixel. The offset correction table 102 outputs a detection value corrected by subtracting from the output of the X-ray detector 101.
[0009]
However, the present inventors have found that X-ray detectors, particularly X-ray detectors using semiconductors, are temporarily used in accordance with the intensity of X-rays when X-rays having a predetermined value or more are incident. We found a phenomenon in which the sensitivity decreased and the offset increased temporarily, and these phenomena recovered with time. This means that the sensitivity or offset varies with time in each pixel.
[0010]
This phenomenon remarkably appears when the X-ray intensity condition, that is, the X-ray dose, which is the product of the X-ray tube current and the exposure time, differs greatly. For example, in fluoroscopy in which relatively weak X-rays are continuously exposed, the exposure time is long. In radiography in which relatively strong X-rays are intermittently exposed, strong X-rays are exposed. This is the case. Note that there may be a difference of several hundred times in the maximum amount of charge accumulated in one pixel between fluoroscopy and photographing.
[0011]
This temporary decrease in sensitivity and increase in offset cause, for example, superimposition of an afterimage based on the X-ray detected last time called a ghost on a regular image on an image acquired by fluoroscopy or imaging.
[0012]
[Problems to be solved by the invention]
Therefore, the present invention solves the above problems, and even when a temporary decrease in sensitivity or a temporary increase in offset occurs, a good image can be obtained by appropriately correcting the output of the X-ray detector. An object is to provide a possible X-ray diagnostic apparatus.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, the present invention takes the following measures.
[0014]
The invention according to claim 1 estimates a change in sensitivity characteristic of the X-ray detector using an X-ray detector that detects incident X-rays and a signal value detected by the X-ray detector in the past. A computing unit to
A first correction device for correcting the signal value output from the X-ray detector to cancel the change in the estimated sensitivity characteristic, and a past signal value detected by the X-ray detector; An increase amount of the offset is estimated based on an elapsed time from the time point to the present time and the signal value detected at the past time point, and a signal value detected by the X-ray detector is calculated using the estimated increase amount. An X-ray diagnostic apparatus comprising: a second correction device that performs correction.
According to a fifth aspect of the present invention, there is provided an X-ray detector in which a plurality of semiconductor elements that detect incident X-rays and generate electrical information are arranged in a two-dimensional matrix, and signals detected by the semiconductor elements in the past. An arithmetic unit that estimates a change in sensitivity characteristic of each semiconductor element, and a correction device that corrects the signal output from each semiconductor element to cancel the estimated change in each characteristic; Based on the elapsed time from the past time point when the signal value was detected by each semiconductor element to the present time and the signal value detected at the past time point, the increase amount of the offset is estimated, and the estimated increase amount is And a second correction device for correcting a signal value detected by the X-ray detector.
The invention according to claim 8 is an X-ray detector in which a plurality of semiconductor elements are arranged in a two-dimensional matrix, and detects image data based on electrical information generated by the plurality of semiconductor elements;
A memory for storing a correlation between a change in offset characteristic and a change in sensitivity characteristic of each of the semiconductor elements, and the X based on electrical information generated by the plurality of semiconductor elements due to past X-ray exposure. A first computing unit that estimates a change in the offset characteristics of each semiconductor element using a first offset image generated by the line detector and a second offset image not caused by past X-ray exposure; A second arithmetic unit that estimates a change in sensitivity characteristic of each semiconductor element from a change in the estimated offset characteristic based on the correlation, and a signal output from each semiconductor element, An X-ray diagnostic apparatus comprising: a correction device that performs correction to cancel the estimated change in each sensitivity characteristic.
[0017]
According to such a configuration, even when a temporary decrease in sensitivity or a temporary increase in offset occurs, an X image capable of acquiring a good image can be obtained by appropriately correcting the output of the X-ray detector. A line diagnostic apparatus can be realized.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, first and second embodiments of the present invention will be described with reference to the drawings. In the following description, components having substantially the same function and configuration are denoted by the same reference numerals, and redundant description will be given only when necessary.
[0019]
(First embodiment)
FIG. 1 is a block diagram of an X-ray diagnostic apparatus 1 according to the first embodiment. In general, the X-ray diagnostic apparatus 1 includes a direct type that directly converts incident X-rays into electric signals, and an indirect type that converts incident X-rays into optical signals once and then converts them into electric signals. For simplicity of explanation, the X-ray diagnostic apparatus 1 according to this embodiment is a direct type. However, the idea of the present invention can also be applied to an indirect X-ray diagnostic apparatus.
[0020]
  The invention according to claim 1 estimates a change in sensitivity characteristic of the X-ray detector using an X-ray detector that detects incident X-rays and a signal value detected by the X-ray detector in the past. A computing unit to
  A first correction device for correcting the signal value output from the X-ray detector to cancel the change in the estimated sensitivity characteristic, and a past signal value detected by the X-ray detector; An increase amount of the offset is estimated based on an elapsed time from the time point to the present time and the signal value detected at the past time point, and a signal value detected by the X-ray detector is calculated using the estimated increase amount. An X-ray diagnostic apparatus comprising: a second correction device that performs correction.
  According to a fifth aspect of the present invention, there is provided an X-ray detector in which a plurality of semiconductor elements that detect incident X-rays and generate electrical information are arranged in a two-dimensional matrix, and signals detected by the semiconductor elements in the past. An arithmetic unit that estimates a change in sensitivity characteristic of each semiconductor element, and a correction device that corrects the signal output from each semiconductor element to cancel the estimated change in each characteristic; Based on the elapsed time from the past time point when the signal value was detected by each semiconductor element to the present time and the signal value detected at the past time point, the increase amount of the offset is estimated, and the estimated increase amount is And a second correction device for correcting a signal value detected by the X-ray detector.
  The invention according to claim 8 is an X-ray detector in which a plurality of semiconductor elements are arranged in a two-dimensional matrix, and detects image data based on electrical information generated by the plurality of semiconductor elements, and each of the semiconductors A memory for storing a correlation between a change in the offset characteristic of the element and a change in the sensitivity characteristic; and the X-ray detector based on electrical information generated by the plurality of semiconductor elements due to past X-ray exposure. A first computing unit that estimates a change in an offset characteristic of each semiconductor element using a first offset image that is generated and a second offset image that is not caused by past X-ray exposure; and the correlation. A second computing unit that estimates a change in sensitivity characteristic of each semiconductor element from the estimated change in offset characteristic and a signal output from each semiconductor element. A correction device for correcting that offset the change in the sensitivity characteristics,Based on the elapsed time from the past time point when the signal value was detected by each semiconductor element to the present time and the signal value detected at the past time point, the increase amount of the offset is estimated, and the estimated increase amount is A second correction device for correcting a signal value detected by the X-ray detector,An X-ray diagnostic apparatus comprising:
[0021]
The X-ray solid state flat detector 37 is provided corresponding to each of a plurality of X-ray conversion elements 43 corresponding to each pixel arranged in a matrix of 1000 × 1000, for example, and each X-ray conversion element 43. A plurality of TFTs 41 as readout switches, a bias electrode 95 that applies a bias voltage to each X-ray conversion element 43 in common, a gate driver 45 that sends a drive signal to the gates of the TFTs 41 in each column, and the drains of the TFTs 41 in each row Are connected in common, a multiplexer 61 for time-division multiplexing the outputs of the respective first-stage integration amplifiers 71, an amplifier 63 for amplifying the output of the multiplexer 61, and an output of the amplifier 63 for analog / digital conversion. And an analog / digital converter (ADC) 65 for outputting to the signal processing unit 21.
[0022]
For the detection film of the X-ray conversion element 43, for example, an amorphous-selenium film (hereinafter abbreviated as a-Se) is used. The X-ray conversion element 43 may be a direct conversion type that converts X-rays directly into electric charges, or converts X-rays into visible light by a phosphor (not shown) formed on the X-ray incident surface. An indirect conversion type that converts the intensity distribution of visible light into electric charges may be used.
[0023]
The first-stage integrating amplifier 71 includes a differential amplifier, a capacitor, and a semiconductor switch such as a bilateral gate using a complementary MOS-FET that is electronically opened and closed.
[0024]
The signal processing unit 21 includes a data correction unit 21a and an image display signal conversion unit 21b.
[0025]
The data correction unit 21a performs a predetermined correction process on the input data in order to remove the influence of a decrease in sensitivity of each pixel or a temporary increase in offset. The image display signal converter 21b converts the corrected data into an image display signal and outputs it to the CRT 25.
[0026]
FIG. 2 is a block diagram of the data correction unit 21a. As shown in FIG. 2, the data correction unit 21 a includes an electron number estimation unit 51, an XP conversion unit 52, a trap number calculation unit 53, a sensitivity correction coefficient calculation unit 54, a multiplier 55, and a memory 56. .
[0027]
The electron number estimation unit 51 determines the number of electrons X in one pixel based on the output signal of the ADC 65._iIs estimated. This estimation is performed based on the collection conditions stored in the memory 56. Here, the collection conditions are the X-ray intensity input in one pixel and the number of electrons X generated in one pixel._iIt shows the relationship. This collection condition can be calculated from, for example, the material of the detection film, a gain factor such as a voltage applied to the X-ray conversion element 43, and the like. The XP conversion unit 52 has the number of electrons X estimated by the number of electrons estimation unit 51_iBased on the above, the generation probability P of the electron trap described later_iIs calculated. The number-of-traps calculation unit 53 generates an electron trap generation probability P obtained by the XP conversion unit 52._iNumber of traps Y_iAsk for. The sensitivity correction coefficient calculation unit 54 determines the number of traps Y_iOn the basis of the sensitivity correction coefficient 1 / (1-Y_i/ N) is calculated. Here, N means the number of divided areas of the detection film having a length of 1 pixel, for example, which is used as a reference for obtaining the sensitivity correction coefficient. The multiplier 55 calculates the calculated sensitivity correction coefficient 1 / (1-Y_i/ N) is multiplied by each pixel data to calculate a corrected data value. The memory 56 includes various parameters used in each process of the electron number estimation unit 51, and the number of electrons X in one pixel._iAnd the number of traps Y in the XP conversion unit_iAn LUT (Look Up Table) used in the calculation process is stored.
[0028]
The configuration of the LUT, its creation example, and the data correction process executed by the data correction unit 21a will be described in detail later.
[0029]
A series of operation examples in the diagnosis of the X-ray diagnostic apparatus 1 will be described as follows. That is, when X-ray imaging or fluoroscopy is performed, a menu screen is first displayed on the CRT 25. According to this menu, X-ray exposure conditions such as tube voltage, tube current, and exposure time are selected from the KB 23 by an X-ray exposure condition selection unit. 19 is input. The X-ray exposure condition selection unit 19 outputs a control signal corresponding to the input X-ray exposure condition to the X-ray exposure control unit 13.
[0030]
Next, when X-rays are emitted from the X-ray tube 11 according to the output of the X-ray exposure control unit 13, the X-ray exposure control unit 13 passes through a subject (not shown) and enters each pixel of the X-ray solid flat detector 37. The intensity of the X-rays converted into charges by the X-ray conversion elements 43, and the amount of charge accumulated in each X-ray conversion element 43 is converted to a voltage by the first stage integration amplifier 71 by a switch of the TFT 41 provided for each pixel. Converted.
[0031]
The output of the first-stage integrating amplifier 71 is amplified by the amplifier 63 via the multiplexer 61, then input to the ADC 65, subjected to analog / digital conversion, and then written to the signal processing unit 21 for data correction and image display signal. Conversion is performed and an X-ray image is displayed on the CRT 25.
[0032]
(Electronic trap)
In an X-ray detector, particularly an X-ray detector using a semiconductor, when X-rays having a strength higher than a predetermined value are incident, the sensitivity is temporarily reduced or temporarily depending on the X-ray intensity. A phenomenon occurs in which the offset increases. The occurrence of this phenomenon is thought to be affected by electron traps. This electron trap means that electrons (or holes) generated by external energy such as X-rays are trapped at a certain energy level. Hereinafter, the mechanism by which this trap causes a temporary decrease in sensitivity at a predetermined portion of the pixel will be quantitatively described with reference to FIG. For the sake of concrete explanation, it is assumed that the X-ray detector is a direct conversion type.
[0033]
FIG. 3 is a conceptual cross-sectional view of a part of the detection film of the X-ray detector. Here, a cross-sectional view of one pixel is shown for ease of explanation. The hatched portion in FIG. 3 is a region that may cause a trap when the detection film having a length of 1 pixel is divided into N minute regions (hereinafter, referred to as “trappable region”). Now, the maximum number of trappable areas is M₀Individual. Therefore, N-M₀No traps have occurred in these areas.
[0034]
In FIG. 3, when the first X-ray is irradiated with a predetermined intensity, X in one pixel.₀Electrons (or holes) are generated.
[0035]
Within one pixel, maximum M₀There are trappable areas. The probability that this trappable area is the area where the trap was actually generated (hereinafter referred to as “trap generation area”) is P₀And At this time, the number of all trap generation regions (hereinafter referred to as “the number of traps”) is M₀P₀It is a piece. The probability P that the trappable area is the trap generation area₀Generally depends on the number of electrons generated.
[0036]
Therefore, M₀P₀When there are multiple trap generation regions, the number of output electrons from the detection film is Z₀X₀(1-M₀P₀/ N).
[0037]
By the way, in general, the number of traps M₀P₀Is thought to decay over time due to some action. This attenuation is the curve M_t = M₀P₀Assume that exp (-at) is obeyed. Where M_tRepresents the number of traps after the elapse of time t (that is, the number of trap generation regions). Therefore, the number of trap generation regions at time t is M_tThe number of trappable areas that are traps and do not generate traps is M₀-Mt.
[0038]
Following the first X-ray irradiation, X₀X instead of pieces₁The second X-ray is irradiated with the intensity to generate individual electrons. P₁Then, from the trappable area where the trap is not generated, (M₀-Mt) P₁New trap generation areas will be generated.
[0039]
Therefore, X₁Cumulative (M₀-Mt) P₁There are + Mt trap generation regions.
[0040]
In this embodiment, for the sake of simplicity of explanation and from the viewpoint of practicality, for example, t is a unit time, that is, t = 1, and A = exp (−a). At this time, the number of traps Mt after the elapse of time t is Mt = M₀P₀A. Therefore, the cumulative trap generation area number Y combined with the previous X-ray irradiation₁Y₁= M₀P₁+ (1-P₁) M₀P₀A.
[0041]
As a result, when the second X-ray irradiation is completed, the number of electrons blocked by the electron trap is X₁Y₁/ N, the number of electrons Z output from the detection film₁Z₁= X₁{1-Y₁/ N}.
[0042]
Thereafter, the third and subsequent X-ray irradiations are executed, and a total of i times of X-ray irradiations are executed. FIG. 4 shows the relationship between the number of electrons generated in one pixel in each X-ray irradiation, trap generation probability, number of traps, number of electrons output, and detector sensitivity. As is clear from this, the number of electrons generated by the i-th X-ray irradiation at a predetermined time t is expressed as X_iThis X_iP is the trap generation probability according to_iThen trap number Y_iY_i= M₀P_i+ (1-P_i) Y_i-1It can be expressed as A. Also, the number of traps Y shown in FIG._i= M₀P₁  + (1-P_i) Y_i-1A is a regression type operation.
[0043]
Number of traps Y above_i= M₀P_i+ (1-P_i) Y_i-1In A, P_iIs previously determined as a collection condition as described, and stored in the memory 56. Therefore, in order to estimate the number of traps quantitatively, the maximum number of trappable areas M₀, Parameters A and a need to be determined. These can be determined, for example, as follows.
[0044]
That is, first, relatively strong X-rays are continuously irradiated with the same intensity from the X-ray tube 11 to the X-ray solid flat detector 37 in the absence of the subject. The number of electrons Xi generated in one pixel can be estimated from the X-ray exposure conditions (tube current, tube voltage, etc.) at this time and the above-described collection conditions.
[0045]
However, the number of electrons Z output by the electron trap_iIs not Xi but Z as described above_i= Xi (1-Yi / N), so if N = 1000, these Z_i, Xi, and N, Yi = N (1-Z_iYi can be obtained as / Xi). Z_iCan be calculated from the output of the X-ray detector. Note that N may be determined in consideration of necessary accuracy, calculation amount, and the like. When N is set to a large value, the amount of calculation increases, but more accurate correction is possible.
[0046]
Further, if the X-ray exposure is continued, a trap saturation state occurs in which the number of electrons output at a certain value, that is, the output value of the detector becomes the lower limit value. This shows a case where the maximum number of electron traps is generated, that is, a state where Pi is almost 1. FIG. 7 illustrates the relationship between time and output value when X-rays are continuously exposed as a graph.
[0047]
In this state, the number of traps Y_sIs Y from Pi = 1_s= M₀Because M₀= Y_s= N (1-Z_s/ X_i), And if N is an appropriate value, for example, N = 1000, then M₀Can be calculated. Z_sIs the number of electrons output from the detection film in a saturated state.
[0048]
Next, in this state, continuous X-ray exposure is stopped, and X-rays having the same intensity are intermittently exposed at relatively long intervals. When the relationship between the time and the output value at this time is measured, a graph as shown in FIG. 8 is obtained as an example. The white circles displayed intermittently are actually measured values representing the output during X-ray irradiation, and the solid line represents an interpolation curve.
[0049]
Here, of irradiation performed intermittently, three irradiations are considered. The number of traps in these three times is Y_j-1, Y_j, Y_{j + 1}Then, as shown in FIG._j= M₀P_j+ (1-P_j) Y_j-1exp (-at₁) And Y_{j + 1}= M₀P_{j + 1}+ (1-P_{j + 1}) Y_j  exp (-at₂). In the case of X-rays having the same intensity, since the number of generated electrons is the same, the trap generation probability is also the same, P_j= P_{j + 1}It becomes. T₁Is the first and second time, that is, j-1 and j X-ray irradiation interval time, t₂Indicates the second and third times, i.e., j and j + 1 X-ray irradiation interval times.
[0050]
Number of traps Y_j-1To Y_{j + 1}Is the number of electrons Z output as described above_j-1Thru Z_{j + 1}Can be obtained from Therefore, Y_j= M₀P_j+ (1-P_j) Y_j-1exp (-at₁) And Y_{j + 1}= M₀P_j+ (1-P_j) Y_jexp (-at₂A) can be obtained from the two equations.
[0051]
In the present embodiment, M₀And a were measured in one experiment, but naturally M₀And a can be measured separately. Regarding a in particular, as long as the degree of recovery can be measured, it is not necessary to expose the X-rays to the saturation value.
[0052]
(Data correction by the signal processor)
As mentioned above, the number of traps Y_i= M₀P_i+ (1-P_i) Y_i-1From A, the number of electrons Z output from the detection film_iZ_i= X_i{1-Y_i/ N}. That is, the signal value output from each pixel is a coefficient {1-Y_i/ N} only. Therefore, a correction that cancels this coefficient, that is, the output value from each pixel is 1 / {1-Y_i/ N} can eliminate the effects of electron traps. Thus, in order to remove the influence of the electron trap, a coefficient integrated with the output value from each pixel is called a sensitivity correction coefficient. In the above example, the sensitivity correction factor is 1 / {1-Y_i/ N}.
[0053]
As a method of acquiring the sensitivity correction coefficient, there are a method based on sensitivity characteristics and a method based on offset characteristics. As described above, the method based on the sensitivity characteristic is a method for directly obtaining the sensitivity correction coefficient from the number of input electrons and the number of output electrons affected by the electron trap. On the other hand, the method based on the offset characteristic is a method for indirectly obtaining the sensitivity correction coefficient from the correlation between the offset characteristic and the electron trap.
[0054]
Hereinafter, in the present embodiment, data correction by a method based on sensitivity characteristics will be described with reference to FIGS. 5 and 6. A method based on the offset characteristic will be described in the second embodiment.
[0055]
FIG. 5 is a flowchart for explaining data correction by a method based on sensitivity characteristics. In FIG. 5, first, X-ray irradiation according to a predetermined sequence is executed, and an X-ray signal is detected (step S1).
[0056]
FIG. 6 shows a timing chart of the X-ray irradiation in step S1 and the output of the X-ray solid flat detector. In the figure, X-ray exposure is given irregularly, and the output of the X-ray solid flat detector is not synchronized with X-ray exposure.
[0057]
For example, as an output from the X-ray detector 1, X-ray addition applied during a predetermined time interval, for example, an interval of 10 seconds to 1 minute, or an average is output. As a method of obtaining an output by addition or averaging, the gate driver 45 and the first stage integration amplifier 71 are controlled.
[0058]
Next, in FIG. 5, the data correction unit 21a estimates the number of electrons Xi generated in one pixel of the detection film 22 from the signal output from the ADC 65 and the collection conditions stored in the memory 56 (step S2). ).
[0059]
Here, X at the time of imaging of the subject_iThe estimation of the number of electrons output from the detection film is not performed from the X-ray exposure condition as in the case where there is no subject._iSensitivity correction coefficient 1 / (1-Y in the previous data_i-1Based on the product of the inverse of / N). In other words, the number of generated electrons X_iZ_i(1-Y_i-1/ N). This is because, unlike the case where there is no subject, X-rays are absorbed in the body of the subject and it is difficult to know how much X-rays reach the X-ray detector. is there.
[0060]
Next, the XP conversion unit 52 uses the predetermined look-up table stored in the memory 56 to estimate the number of electrons X_iThe trap occurrence probability Pi corresponding to is obtained (step S3). Note that the lookup table contains X_iIf there is no value identical to, X on the lookup table_iPi may be obtained by approximation with the closest value. Further, Pi may be obtained by performing linear interpolation or the like from the preceding and following values. The creation of the lookup table will be described later.
[0061]
Next, in the trap number calculation unit 53, as described above, Yi = M₀Pi + (1-Pi) Y_i-1The current number of traps Yi is obtained by performing a regression type calculation of A (step S4). In this calculation, the parameter M stored in the memory 56 is used.₀, A, and Y already calculated during the previous sampling_i-1Is used.
[0062]
Next, the sensitivity correction coefficient calculation unit 54 calculates 1 / (1-Yi / N) as the sensitivity correction coefficient (step S5). The method for calculating the sensitivity correction coefficient is as described above.
[0063]
Finally, the multiplier 55 multiplies the original output of the X-ray detector by the sensitivity correction coefficient to perform sensitivity correction (step S6).
[0064]
Hereinafter, the sensitivity-corrected data signal is converted into an image display signal by the image display signal converter 21b. The CRT 25 displays an X-ray image based on the input display signal.
[0065]
(LUT creation example)
The LUT stored in advance in the memory 56 and used in the data correction process will be described in detail. Specifically, the lookup table used in the data correction process is X_iIs described as follows, for example.
[0066]
First, M₀As in the case of obtaining the above, the number of electrons Xi generated in one pixel is estimated from the X-ray exposure conditions and the like in the absence of the subject.
[0067]
For the number of traps Yi, Yi = M₀Pi + (1-Pi) Y_i-1A, but when the sensitivity has almost recovered after a lapse of time, the trapped state disappears, that is, the number of traps Y_i-1= 0, Yi = M₀Pi. Therefore, M that has already been sought₀And Z obtained from the actual output value_iY obtained from_iTo P_i= Y_i/ M₀As P_iCan be requested.
[0068]
In this way, each time the sensitivity is almost recovered, the X-ray intensity is changed and the output of the X-ray detector is measured a plurality of times, thereby generating the number of electrons X_iAnd the trap generation probability Pi can be obtained, and a record of this is a lookup table.
[0069]
The above is the information stored in the memory 56 before imaging the subject, and the imaging of the subject is performed using these pieces of information. The information stored here may be obtained by the operator himself or may be stored in advance in a storage medium such as a hard disk, CD-R, or DVD when the apparatus is shipped. It should be noted that more accurate correction is possible by periodically inspecting and updating these stored information.
[0070]
(Modification)
Next, a first modification of the first embodiment will be described. In the first embodiment, as an example of the property of the detection film 22, the number of traps simply decreases with time. On the other hand, in this modification, an example is shown in which electrons trapped in the course of this phenomenon are emitted. The model in which the trapped electrons are emitted with time can be considered as a model that takes into account the increase in offset.
[0071]
In this modification, the electrons emitted from the detection film 22 are components proportional to the number of traps. In other words, the number of electrons output from one pixel of the detection film is the number of traps Y at the previous time._i-1Number of electrons proportional to, BY_i-1Added, X_i(1-Y_i/ N) + BY_i-1The sensitivity may be calculated as Here, B ≧ 0.
[0072]
This method of obtaining B uses a state in which the sensitivity of the detector used in the first embodiment is saturated. Specifically, as in the first embodiment, as shown in FIG. 6, the number of electrons Z output from the detector by continuously performing X-ray exposure._iAsk for. This Z_iZ as described above_i= X_i(1-Y_i/ N) + BY_i-1If the sensitivity of the detector is saturated, Y_i≒ Y_i-1And Y_i-1= M₀Z_i= X_i(1-M₀/ N) + BM₀It is.
[0073]
In this way, the X-ray exposure is then stopped while the detector sensitivity is saturated. Then, since there is no X-ray exposure, the number of electrons is not generated in the detection film._i= 0. Electrons are output from the detector at this time by a slight offset, and this number of electrons Z ′_iIs M₀B. That is, Z '_i= M₀B.
[0074]
Therefore, this is_i= X_i(1-M₀/ N) + BM₀Substituting for M₀= N {1- (Z_i-Z '_i) / X_i} And M₀B = Z '_i/ M₀B can be obtained from
[0075]
The obtained B is stored in the memory 56 and used as in the first embodiment.
[0076]
In the present modification, trapped electrons are emitted with time, and it is possible to perform more precise correction that takes into account the increase in offset.
[0077]
Many other modifications can be considered, for example, a model in which the number of emitted electrons is not proportional to the number of traps but is proportional to the change in the number of traps, that is, Z_i= X_i(1-Y_i/ N) + B '(Y_i-1-Y_i) Is also possible. The selection of these models may be determined by experiment, or may be determined in consideration of the amount of calculation, circuit complexity, and the like.
[0078]
For example, when comparing the first embodiment with this modification, in this modification, it can be considered that the amount of calculation is as much as the amount B is used, but on the other hand, the correction with the offset is added. Is possible.
[0079]
Also, among models, those that perform sensitivity correction or offset correction based on the output of the regression type calculation as in the first embodiment and the modification have a very wide application range, and the sensitivity, It is possible to correct offset deterioration.
[0080]
In the first embodiment and the modification, the number of traps in the initial state is 0, that is, Y₀= M₀P₀In addition to this, the last Y 'that was done before_iYou may set to the value of.
[0081]
However, in this case, Y 'based on the elapsed time T between the previously calculated time and the current time_iIt is better to change the value of.
[0082]
This change is possible by measuring the properties of the detection film in advance and measuring how the number of traps changes according to the state of energization.
[0083]
For example, if the measurement shows that the number of traps naturally decreases by 1% per hour, the trap number is expected to decrease by exp (-0.01T) at the elapsed time T (hours). Y₀= M₀P₀+ exp (-0.01t) Y '_iThe calculation can be started as
[0084]
In particular, among the X-ray detectors, what is generally called an X-ray flat panel detector has been described. However, the present invention is not particularly limited to the arrangement of detection elements.
[0085]
Further, in the first embodiment and the modification, the case where the X-ray exposure and the detector output are not synchronized has been described, but they may be synchronized.
[0086]
In addition, regarding the properties of the detection film, the two cases of the first embodiment and the modification have been shown. However, the properties of the detection film are not particularly limited, and the model described above is slightly changed. In some cases, it may be effective.
[0087]
Further, in each of the embodiments and the modified examples, the method of performing sensitivity correction by hardware has been described. However, all or a part of the method may be performed on software.
[0088]
  Next, it is obtained by the X-ray diagnostic apparatus 1 configured as described above.About effectSome examples will be described.
[0089]
First, the effect obtained when the abdomen is seen through after imaging the neck is described.
[0090]
In a conventional X-ray diagnostic apparatus, a strong X-ray is irradiated and a cervical image as shown in FIG. 9 is taken. At this time, regions A and B are regions imaged based on X-rays directly exposed without passing through the subject. Therefore, compared to the region C where X-rays are transmitted through the subject, the regions A and B are irradiated with intense X-rays. As a result, in the areas A and B, the sensitivity temporarily decreases compared to the area C, and the offset increases.
[0091]
In a state where such a decrease in sensitivity and an increase in offset have occurred, when a fluoroscopic image of the abdomen as shown in FIG. 10 is obtained by irradiating weak X-rays, the fluoroscopic image includes the previous time as shown in FIG. May be superimposed as a ghost G. This is because the pixels existing in the regions A and B are particularly affected by the signal detected in the previous photographing.
[0092]
On the other hand, in the present X-ray diagnostic apparatus 1, since the data correction for removing the influence of the previously detected signal is performed on the output value of each pixel, the ghost G as shown in FIG. 10 can be removed.
[0093]
Next, effects obtained when performing fluoroscopy for a long time will be described.
[0094]
Since fluoroscopy uses relatively weak X-rays, the contrast difference is small compared to imaging. Therefore, for example, as shown in FIG. 10, the area A and the area C, or the area B and the area C form an image with different luminance, for example, as shown in FIG. It may be significantly affected, and an appropriate image may not be displayed. In this example, a case where a catheter (shown by a solid line) that may be used for fluoroscopy is further inserted into the subject is shown. In addition, this catheter shows the case where it exists across the area | region A and the area | region C of a subject, and the area | region A of a catheter is displayed darker than the area | region C. FIG.
[0095]
Even in such a case, since the present X-ray diagnostic apparatus 1 performs data correction for removing the influence of the signal detected last time on the output value of each pixel, the change in luminance as shown in FIG. 10 is removed. be able to.
[0096]
In addition to the case where fluoroscopy and imaging are performed continuously, in addition, when imaging is performed using, for example, a collimator, the region where X-rays are blocked by the collimator on the X-ray detector is blocked. The sensitivity of the non-existing region will be significantly different, and when the collimator is replaced, an appropriate image may not be displayed as described above.
[0097]
In addition to continuous imaging and fluoroscopy, or when changing the collimator, the X-ray transmittance of muscles and bones may be different due to the difference in the imaging position, and similar problems may occur. There is.
[0098]
As described in detail above, according to the present X-ray diagnostic apparatus, at least one of the sensitivity and offset of the detector can be corrected in consideration of temporal changes, and
Measurement can be performed while reducing the influence of the variation in degree. Therefore, it is possible to provide a good image.
[0099]
Even when the sensitivity of the detector changes with time, it is possible to provide a good image by performing correction according to this sensitivity.
[0100]
(Second Embodiment)
In the second embodiment, an X-ray diagnostic apparatus that acquires a sensitivity correction coefficient from a correlation with an offset characteristic and corrects data using this sensitivity coefficient will be described. In the following description, correction for removing a ghost image generated by an electronic trap is referred to as “ghost correction”.
[0101]
FIG. 11 is a block diagram of the data correction unit 21a in the second embodiment. As shown in FIG. 11, the data correction unit 21a includes an offset image holding unit 131, an initial offset holding unit 132, a subtracter 133, a subtracter 134, a ghost correction coefficient calculation unit 135, an initial sensitivity correction coefficient holding unit 136, and a multiplier. 137 and a multiplier 138.
[0102]
The offset image holding unit 131 holds offset image data (hereinafter referred to as a first offset image) immediately before X-ray exposure. Here, the offset image is an image detected by the X-ray flat detector 37 without X-ray exposure. That is, in the X-ray flat detector 37, detection is performed in a state in which X-rays are not exposed due to dark current of each pixel, offset of each integration amplifier, dark current X-ray from the X-ray generation system, and the like. It is an image based on the background of an electrical signal. This offset image includes an increase in the offset component generated by the previous X-ray exposure.
[0103]
The initial offset image holding unit 132 holds offset image (hereinafter referred to as a second offset image) data in a state that does not include an increase in offset component due to X-ray exposure.
[0104]
The first subtracter 133 performs offset correction by subtracting the offset image data stored immediately before the X-ray exposure held in the offset image holding unit 131 from the X-ray image data output from the ADC 65. .
[0105]
The second subtracter 134 subtracts the second offset image from the first offset image to obtain a difference between the images. This difference is an increase in the offset component caused by the previous X-ray exposure.
[0106]
The ghost correction coefficient calculation unit 135 stores a sensitivity correction coefficient (hereinafter referred to as “first sensitivity correction coefficient”) calculated based on a correlation between an increase in offset component and a sensitivity decrease that are obtained in advance. Specifically, the ghost correction coefficient calculation unit 135 is, for example, an LUT (Look Up Table). The ghost correction coefficient calculation unit 135 calculates and outputs a first sensitivity correction coefficient for correcting the sensitivity decrease from the input offset component increase based on the correlation between the stored increase in offset component and the sensitivity decrease. .
[0107]
The initial sensitivity correction coefficient holding unit 136 holds a sensitivity correction coefficient (hereinafter referred to as “second sensitivity correction coefficient”) in a state that does not include a decrease in sensitivity due to X-ray exposure. By integrating the second sensitivity correction coefficient to the X-ray image data, it is possible to correct a sensitivity change not caused by X-ray exposure.
[0108]
The first multiplier 137 multiplies the second sensitivity correction coefficient from the initial sensitivity correction coefficient holding unit 136 and the first sensitivity correction coefficient from the ghost correction coefficient calculation unit 135.
[0109]
The second multiplier is the product of the second sensitivity correction coefficient input from the first multiplier 137 and the first sensitivity correction coefficient for the X-ray image data input from the first subtracter 133. Multiply
[0110]
Next, data correction processing executed by the data correction unit 21a having the above configuration will be described.
[0111]
First, the X-ray image data output from the ADC 65 is offset-corrected by the first subtracter 133 by the offset image data held in the offset image holding unit 131 and collected immediately before the X-ray exposure.
[0112]
Subsequently, the second multiplier 138 multiplies the product value of the second sensitivity correction coefficient input from the first multiplier 137 and the first sensitivity correction coefficient, thereby correcting the initial sensitivity and ghosting. Correction is applied.
[0113]
The X-ray image data corrected by the second multiplier 138 is converted into an image display signal and displayed on the CRT 25 as an X-ray image.
[0114]
Even with such a configuration, the same effects as those of the first embodiment can be obtained.
[0115]
Note that each modification described in the first embodiment is also applicable to the X-ray diagnostic apparatus according to the second embodiment.
[0116]
Although the present invention has been described based on the embodiments, those skilled in the art can come up with various changes and modifications within the scope of the idea of the present invention. It is understood that it belongs to the scope of the present invention.
[0117]
Further, the embodiments may be combined as appropriate as possible, and in that case, the combined effect can be obtained. Furthermore, the above embodiments include inventions at various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. For example, even if some constituent requirements are deleted from all the constituent requirements shown in the embodiment, the problem described in the column of the problem to be solved by the invention can be solved, and the effect described in the column of the effect of the invention If at least one of the following is obtained, a configuration in which this configuration requirement is deleted can be extracted as an invention.
[0118]
【The invention's effect】
As described above, according to the present invention, an X-ray capable of acquiring a good image by appropriately correcting the output of the X-ray detector even when a temporary decrease in sensitivity or a temporary increase in offset occurs. A diagnostic device can be realized.
[Brief description of the drawings]
FIG. 1 is a block diagram of an X-ray diagnostic apparatus 1 according to a first embodiment.
FIG. 2 is a block diagram of a data correction unit 21a.
FIG. 3 is a conceptual cross-sectional view of a part of a detection film of an X-ray detector.
FIG. 4 shows the relationship between the number of electrons generated in one pixel, the trap generation probability, the number of traps, the number of electrons to be output, and the sensitivity of the detector in total x i-ray irradiations.
FIG. 5 is a flowchart for explaining data correction by a method based on sensitivity characteristics;
FIG. 6 shows a timing chart of the X-ray irradiation and the output of the X-ray solid flat detector.
FIG. 7 is a graph showing a relationship between an X-ray exposure time and an output value from a pixel.
FIG. 8 is a graph showing a relationship between a recovery time and an output value from a pixel.
FIG. 9 is a diagram for explaining the effect of the X-ray diagnostic apparatus 1 according to the embodiment.
FIG. 10 is a diagram for explaining the effect of the X-ray diagnostic apparatus 1 according to the embodiment.
FIG. 11 is a block diagram of a data correction unit 21a according to a second embodiment.
FIG. 12 is a schematic cross-sectional view of an image intensifier.
FIG. 13 shows a partial block diagram of a conventional X-ray diagnostic apparatus.
FIG. 14 is a cross-sectional view of a direct conversion type X-ray flat panel detector.
[Explanation of symbols]
1 ... X-ray diagnostic equipment
11 ... X-ray tube
13 ... X-ray exposure control unit
19 ... X-ray exposure condition selection unit
21 ... Signal processing unit
21a: Data correction unit
21b ... Image display signal converter
22 ... Detection membrane
23 ... Keyboard
25 ... CRT
37 ... X-ray flat panel detector
41 ... TFT
43 ... X-ray conversion element
45 ... Gate driver
51 ... Number of electrons estimation unit
52 ... P converter
53 ... Trap number calculation unit
54 ... Sensitivity correction coefficient calculation unit
55 ... Multiplier
56 ... Memory
61 ... Multiplexer
63 ... Amplifier
65. Analog to digital converter
71. First stage integrating amplifier
131: Offset image holding unit
132: Initial offset image holding unit
133, 134 ... subtractor
135: Ghost correction coefficient calculation unit
136 ... Initial sensitivity correction coefficient holding unit
137, 138 ... multiplier

Claims (8)

An X-ray detector for detecting incident X-rays;
An arithmetic unit that estimates a change in sensitivity characteristics of the X-ray detector using signal values detected by the X-ray detector in the past;
A first correction device that performs correction to cancel the estimated change in sensitivity characteristic with respect to the signal value output from the X-ray detector;
Based on the elapsed time from the past time point when the signal value is detected by the X-ray detector to the present time and the signal value detected at the past time point, the increase amount of the offset is estimated, and the estimated increase amount A second correction device for correcting the signal value detected by the X-ray detector using
An X-ray diagnostic apparatus comprising:   The X-ray diagnostic apparatus according to claim 1, wherein the X-ray detector is an X-ray detector using a semiconductor for an X-ray detection portion.   3. The X-ray diagnostic apparatus according to claim 2, wherein the X-ray detector is a direct conversion type detector.   The X-ray diagnostic apparatus according to claim 3, wherein the semiconductor detector includes selenium. An X-ray detector in which a plurality of semiconductor elements that detect incident X-rays and generate electrical information are arranged in a two-dimensional matrix;
An arithmetic unit that estimates a change in sensitivity characteristics of each semiconductor element using signals detected by each semiconductor element in the past;
A correction device that performs correction to cancel the estimated change in each characteristic with respect to the signal output from each semiconductor element;
Based on the elapsed time from the past time point when the signal value was detected by each semiconductor element to the present time and the signal value detected at the past time point, the increase amount of the offset is estimated, and the estimated increase amount is A second correction device for correcting a signal value detected by the X-ray detector,
An X-ray diagnostic apparatus comprising:   6. The X-ray diagnostic apparatus according to claim 5, wherein the plurality of semiconductor elements are direct conversion type elements that directly convert incident X-rays into electrical signals.   6. The X-ray diagnosis apparatus according to claim 5, wherein each of the semiconductor elements has a detection film using selenium. A plurality of semiconductor elements arranged in a two-dimensional matrix, and an X-ray detector for detecting image data based on electrical information generated by the plurality of semiconductor elements;
A memory for storing a correlation between a change in offset characteristic and a change in sensitivity characteristic of each of the semiconductor elements;
A first offset image generated by the X-ray detector based on electrical information generated by the plurality of semiconductor elements due to past X-ray exposure, and a second not caused by past X-ray exposure. A first computing unit that estimates a change in an offset characteristic of each semiconductor element using an offset image;
A second computing unit that estimates a change in sensitivity characteristic of each of the semiconductor elements from the estimated change in offset characteristic based on the correlation;
A correction device that performs correction to cancel the estimated change in each sensitivity characteristic with respect to the signal output from each semiconductor element;
Based on the elapsed time from the past time point when the signal value was detected by each semiconductor element to the present time and the signal value detected at the past time point, the increase amount of the offset is estimated, and the estimated increase amount is A second correction device for correcting a signal value detected by the X-ray detector,
An X-ray diagnostic apparatus comprising:

Images