JP2010087563A - Afterimage correction method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an afterimage correction apparatus for applying afterimage correction on a radiation image signal which is read from a radiation image detector used repeatedly for photographing radiological images, capable of performing prior reading in a short time and performing afterimage correction with high accuracy, regardless of the variance in the charge storage time of the photographing, or the difference in the time elapsed from radioactive ray irradiation at the previous photographing. <P>SOLUTION: Electron charge is accumulated in the radiological image detector 20 for a fixed time duration where irradiation of radiation after a first photographing is not carried out; the accumulated electron charge is read as a prior reading image signal; whereas radiation-photographing conditions contain at least one of the storage time of electron charge generated in the radiation image detector 20 by a second photographing, after the reading of the previous reading image signal and the time elapsed from the first photographing. Based on the previous reading image signal and the compensation factor according to the radiation photographing conditions, afterimage correction is applied on a radiological image signal read by the second photographing. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an afterimage correction method and apparatus for performing afterimage correction on a radiation image signal read from a radiation image detector repeatedly used for radiographic image capturing.

  2. Description of the Related Art Conventionally, in the medical field and the like, various radiological image detectors that record a radiographic image related to a subject by irradiation with radiation that has passed through the subject have been proposed and put into practical use.

  As the radiation image detector, for example, there is a radiation image detector using amorphous selenium that generates an electric charge when irradiated with radiation. As such a radiation image detector, a so-called optical reading type or TFT (thin film) is used. An electric reading type using a transistor, a thin film transistor (CCD), a charge coupled device (CCD), a complementary metal oxide semiconductor (CMOS) sensor, or the like has been proposed.

  As an optical reading type radiographic image detector, for example, a first electrode layer that transmits radiation carrying a radiographic image, and a recording light that generates an electric charge when irradiated with radiation transmitted through the first electrode layer Charge transport layer, reading light which acts as an insulator for charges of one polarity among the charges generated in the conductive layer and the photoconductive layer for recording, and acts as a conductor for charges of the other polarity A photoconductive layer for reading that generates an electric charge by receiving irradiation, and a second electrode layer composed of a transparent linear electrode that transmits the reading light and a light shielding linear electrode that blocks the reading light are stacked in this order. What has been proposed.

  Moreover, as a radiographic image detector using a TFT in an electric reading system, for example, an upper electrode to which a bias voltage is applied, a semiconductor layer that generates charges upon irradiation with radiation, and charges generated in the semiconductor layers An active matrix substrate in which a plurality of pixels having two-dimensionally arranged pixels each having a pixel electrode for collecting the charge, a storage capacitor for storing the charge collected by the pixel electrode, and a TFT switch for reading the charge stored in the storage capacitor Layered ones have been proposed.

  The radiological image detector as described above can repeatedly perform recording of a radiographic image and reading of an image signal representing the radiographic image, and can be used for radiographic image capturing repeatedly. It is not possible to read out all the charges generated in the radiation image detector due to the irradiation of radiation, and the charges remain in the radiation image detector even after the reading, and this residual charge is used when the next radiation image is taken. There is a problem that becomes an afterimage.

Therefore, in Patent Document 1 to Patent Document 3, so-called pre-reading is performed in which a correction image signal is read from the radiation image detector before the radiographic image is actually captured, and the radiographic image signal acquired by the actual imaging is subjected to the previous imaging. There has been proposed a method of performing afterimage correction by subtracting a correction image signal obtained by reading.
JP 2000-214297 A JP 2002-369077 A JP 2006-334085 A

  However, in the afterimage correction methods described in Patent Document 1 to Patent Document 3, in order to perform accurate afterimage correction, the charge accumulation time corresponding to the image signal for correction to the radiation image detector and the charge in the main imaging are corrected. Although it is necessary to match the accumulation time, especially in the case of a radiographic imaging device for breasts, the charge accumulation time in the main imaging, that is, the radiation irradiation time is determined by AEC (automatic exposure control), so a correction image signal is acquired. In this case, the charge accumulation time cannot be grasped, and the charge accumulation time corresponding to the image signal for correction cannot be matched with the charge accumulation time in the actual photographing.

  In addition, the accuracy of afterimage correction can be improved by performing pre-reading immediately before the radiation of the main imaging, but if the pre-reading time is increased in accordance with the radiation irradiation time of the main imaging, the compression time of the breast This increases the burden on patients.

  Furthermore, since the afterimage changes depending on the elapsed time from radiation irradiation in the previous main imaging, there is a problem that the afterimage correction is likely to be shifted.

  In view of the above circumstances, the present invention is an afterimage correction method and apparatus for performing afterimage correction on a radiographic image signal read from a radiographic image detector repeatedly used for radiographic image capturing, and is a charge storage for main imaging An afterimage correction method and apparatus capable of performing pre-reading in a short time and performing high-accuracy afterimage correction regardless of changes in time or differences in elapsed time from radiation exposure in the last main imaging. The purpose is to provide.

  The afterimage correction method of the present invention is a radiation image detector that generates charges upon receiving radiation transmitted through a subject, outputs a radiation image signal representing a radiation image of the subject, and is used repeatedly for radiographic imaging. An afterimage correction method for performing afterimage correction on a radiographic image signal read from the radiographic image detector, which is accumulated in the radiographic image detector for a certain period of time without radiation irradiation after the first imaging. The charge is read out as a pre-read image signal, the read-out pre-read image signal, (i) the accumulation time of the charge generated in the radiation image detector by the second imaging after reading out the pre-read image signal, and (ii) the above Radiation read out by the second imaging based on the correction coefficient corresponding to the radiographic conditions including at least one of the elapsed time from the first imaging And characterized by applying residual image correction on the image signal.

  An afterimage correction apparatus according to the present invention is a radiation image detector that generates charges upon receiving radiation that has passed through a subject, outputs a radiation image signal representing a radiation image of the subject, and is used repeatedly to capture a radiation image. An afterimage correction apparatus that performs afterimage correction on a radiographic image signal read from the radiographic image detector, and is accumulated in the radiographic image detector for a certain period of time without radiation irradiation after the first imaging. A pre-reading image signal acquisition unit that reads and acquires charges as a pre-reading image signal; a pre-reading image signal acquired by the pre-reading image signal acquisition unit; and (i) a second imaging after reading the pre-reading image signal. Correction according to the radiographic conditions including at least one of the accumulation time of the charges generated in the radiographic image detector and (ii) the elapsed time from the first imaging Based on the number, characterized in that a residual image correction unit that performs residual image correction on the read radiographic image signal by said second shooting.

  In the afterimage correction apparatus of the present invention, information on the temperature of the radiation image detector at the time of the second imaging can be included in the radiation imaging conditions.

  Further, the time required in the step of acquiring the pre-read image signal and / or the time required in the step of acquiring the radiographic image signal by the second imaging after the imaging of the second imaging is set constant for each imaging. be able to.

  Also, a first time after the first shooting and before the accumulation of the charge of the pre-read image signal, and a second time after the pre-read image signal is read and before the second shooting. And a light irradiation unit that irradiates light to the radiation image detector at at least one time point among the third time points after reading out the radiation image signal by the second imaging.

  In addition, the light irradiating unit may set the amount of light irradiated to the radiation image detector at the second time point to be greater than the light amount irradiated to the radiation image detector at the first time point and / or the third time point. It can be made smaller.

  According to the afterimage correction method and apparatus of the present invention, after the first imaging, the electric charge accumulated in the radiation image detector for a certain period of time without radiation is read as a preread image signal, and the read preread Radiation imaging conditions including at least one of an image signal, an accumulation time of charges generated in the radiation image detector by the second imaging after reading the preread image signal, and an elapsed time from the first imaging Since the afterimage correction is performed on the radiographic image signal read out by the second imaging based on the corresponding correction coefficient, the change in the charge accumulation time of the second imaging or the first imaging is performed. Regardless of the difference in the elapsed time from the time of reading, pre-reading can be performed in a short time, and afterimage correction with high accuracy can be performed.

  In the afterimage correction apparatus of the present invention, when information about the temperature of the radiation image detector at the time of the second imaging is included in the radiation imaging conditions, the afterimage change due to a temperature change is also supported. Thus, afterimage correction can be performed, and more accurate afterimage correction can be performed.

  Further, the time required in the step of acquiring the pre-read image signal and / or the time required in the step of acquiring the radiographic image signal by the second imaging after the imaging of the second imaging is set constant for each imaging. In such a case, since it is possible to perform afterimage correction using the same correction coefficient for each shooting, it is possible to reduce the trouble of obtaining various correction coefficients and to store the correction coefficients in advance. The capacity can be reduced, and the cost can be reduced.

  Also, a first time after the first shooting and before the accumulation of the charge of the pre-read image signal, and a second time after the pre-read image signal is read and before the second shooting. When the radiation image detector is irradiated with light at at least one time point among the third time points after reading out the radiation image signal by the second imaging, an afterimage is further generated by the light irradiation. Can be erased, so that afterimage correction can be performed with higher accuracy.

  In addition, the amount of light irradiated to the radiation image detector at the second time point is made smaller than the light amount irradiated to the radiation image detector at the first time point and / or the third time point. In this case, the amount of light after acquisition of the pre-reading image signal is reduced, so that an error between the amount of afterimage at the time of acquiring the pre-reading image signal and the amount of afterimage included in the radiographic image signal by the second imaging. Can be made smaller, so that afterimage correction with higher accuracy can be performed.

  Hereinafter, a breast image photographing display system using an embodiment of an afterimage correction apparatus of the present invention will be described with reference to the drawings. First, a schematic configuration of the whole breast image photographing display system will be described. FIG. 1 is a diagram showing a schematic configuration of the breast image photographing display system.

  The breast image capturing and displaying system displays a breast radiation image based on a breast image capturing apparatus 10 that captures a radiation image of a breast and a radiation image signal representing the radiation image of the breast captured by the breast image capturing apparatus 10. And a breast image display device 15.

  The mammography apparatus 10 includes a radiation irradiation unit 2 that houses the radiation source 1 therein, an imaging table 3 on which a breast M that is a subject is installed, and the radiation irradiation unit 2 and the imaging table 3 so as to face each other. And an arm 4 attached to a base 5 so as to be rotatable about an axis C. The arm 4 is provided with a compression plate 6 that compresses the breast M installed on the imaging table 3.

  In the imaging table 3, a radiation image detector 20 that two-dimensionally detects radiation emitted from the radiation source 1 and transmitted through the breast M, and light irradiation that irradiates the radiation image detector 20 with erasing light or the like. A unit 30, a radiation amount detection unit 70 that detects a radiation amount of radiation that has passed through the breast M, and a temperature detection unit 80 that detects an environmental temperature in the vicinity of the radiation image detector 20 are installed.

  FIG. 2 shows a schematic diagram of the breast image photographing apparatus 10 and a schematic configuration diagram inside the breast image display apparatus 15 connected to the breast image photographing apparatus 10.

  As shown in FIG. 2, the breast image display device 15 acquires a radiological image signal representing a radiographic image of the breast M output from the radiographic image detector 20, and performs radiation before the radiographing of the radiographic image of the breast M. An image signal acquisition unit 40 that acquires a pre-read image signal detected by the image detector 20, and a pre-read image signal and a preset correction coefficient for the radiation image signal acquired by the image signal acquisition unit 40. An image processing unit 50 that performs afterimage correction based on the image, a display unit 60 that displays a radiographic image of the breast M based on the radiographic image signal that has undergone afterimage correction in the image processing unit 50, and a radiation dose detection unit 70. A predetermined control signal is output based on the information on the amount of radiation and the temperature information detected by the temperature detection unit 80, and the radiation source 1, the radiation image detector 20, and the light irradiation unit And a control unit 90 for controlling the operation of such 0.

  The radiation source 1 emits radiation toward the breast M as described above, and is so-called AEC (automatic exposure control). Specifically, the radiation source 1 starts emitting radiation based on the irradiation start signal output from the control unit 90. And the radiation which permeate | transmitted the breast M from the injection | emission start time is detected by the radiation dose detection part 70, and when the radiation dose detected by the radiation dose detection part 70 becomes more than a predetermined threshold value, the signal which notifies that is sent. The radiation amount detection unit 70 outputs the control unit 90. Then, the control unit 90 outputs an irradiation stop signal to the radiation source 1 based on the signal, and the radiation source 1 stops the emission of radiation based on the irradiation stop signal.

  The radiation image detector 20 may be any device as long as it generates charges upon receiving the radiation transmitted through the breast M and outputs a radiation image signal representing the radiation image of the subject. A so-called optical reading type radiation image detector is used. FIG. 3 is a perspective view showing a schematic configuration of the radiation image detector 20, and FIG. 4 is a sectional view taken along line 2-2 of the radiation image detector 20 shown in FIG.

  Specifically, as shown in FIG. 3 and FIG. 4, the radiation image detector 20 includes a first electrode layer 21 that transmits radiation carrying a radiation image, and irradiation of radiation that has passed through the first electrode layer 21. The recording photoconductive layer 22 that generates a charge by receiving the charge, acts as an insulator for the charge of one polarity among the charges generated in the recording photoconductive layer 22, and for the charge of the other polarity In this case, a charge transport layer 23 acting as a conductor, a reading photoconductive layer 24 that generates charges when irradiated with reading light, and a second electrode layer 25 are laminated in this order. In the vicinity of the interface between the recording photoconductive layer 22 and the charge transport layer 23, a power storage unit 26 that accumulates charges generated in the recording photoconductive layer 22 is formed.

The first electrode layer 21 may be any material as long as it transmits radiation. For example, the first electrode layer 21 may be a nesa film (SnO ₂ ), ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), or an amorphous light-transmitting oxide film. A certain IDIXO (Idemitsu Indium X-metal Oxide; Idemitsu Kosan Co., Ltd.) can be used with a thickness of 50 to 200 nm, and Al or Au with a thickness of 100 nm can also be used.

  The recording photoconductive layer 22 only needs to generate a charge when irradiated with radiation, and is excellent in that it has a relatively high quantum efficiency with respect to radiation and a high dark resistance. A material mainly composed of Se is used. Moreover, when using the photoconductive substance which has a-Se as a main component, it uses with what has a-Se as a main component containing alkali metals, such as Na, K, Li, and content of an alkali metal is 0.01- It is preferable to set it as 5000 ppm. The thickness is suitably 100 μm or more and 2000 μm or less. In particular, when it is used for mammography, it is preferably 150 μm or more and 250 μm or less, and when used for general photographing, it is preferably 500 μm or more and 1200 μm or less.

As the charge transport layer 23, for example, the larger the difference between the mobility of charges charged in the first electrode layer 21 at the time of recording a radiographic image and the mobility of charges having the opposite polarity, the better (for example, 10 ^2). or more, preferably 10 ³ or higher), such as poly N- vinylcarbazole (PVK), N, N'-diphenyl -N, N'-bis (3-methylphenyl) - [1,1'-biphenyl] -4 , 4′-diamine (TPD), organic compounds such as discotic liquid crystal, or TPD polymer (polycarbonate, polystyrene, PVK) dispersion, semiconductors such as a-Se, As ₂ Se ₃ doped with 10 to 200 ppm of Cl The substance is appropriate. A thickness of about 0.2 to 2 μm is appropriate.

  The reading photoconductive layer 24 may be any material that exhibits conductivity when irradiated with reading light and erasing light. For example, a-Se, Se-Te, Se-As-Te, metal-free phthalocyanine, A photoconductive substance mainly containing at least one of metal phthalocyanine, MgPc (Magnesium phtalocyanine), VoPc (phase II of Vanadyl phthalocyanine), CuPc (Cupper phtalocyanine) and the like is preferable. A thickness of about 5 to 20 μm is appropriate.

  The second electrode layer 25 includes a plurality of transparent linear electrodes 25a that transmit reading light and a plurality of light shielding linear electrodes 25b that shield reading light. The transparent linear electrodes 25a and the light-shielding linear electrodes 25b are alternately arranged in parallel at predetermined intervals as shown in FIGS.

  The transparent linear electrode 25a transmits reading light and is made of a conductive material. Any material may be used as long as it is as described above. For example, as with the first electrode layer 21, ITO, IZO, or IDIXO can be used. Alternatively, a metal such as Al or Cr may be used to form a thickness that allows the reading light to pass (for example, about 10 nm).

  The light shielding linear electrode 25b shields the reading light and is made of a conductive material. Any material may be used as long as it is as described above. For example, there are Cr, Mo, and W having a thickness of 100 to 300 nm. Alternatively, a light shielding layer made of a resist material may be patterned in a stripe shape in advance, and the same material as that of the transparent linear electrode may be patterned thereon in a stripe shape to function as a light shielded electrode.

  The light irradiation unit 30 irradiates light to the radiation image detector 20 two-dimensionally. Specifically, the light irradiation unit 30 may use a plurality of fluorescent tubes arranged or may emit light such as an LED. A device in which a number of elements are two-dimensionally arranged may be used.

  The image signal acquisition unit 40 acquires a radiation image signal and a pre-read image signal, and includes a memory for storing these signals.

  In the image processing unit 50, correction coefficients corresponding to the charge accumulation time (described in detail later) in the main shooting and the elapsed time from the previous main shooting are set in advance as a table and output from the control unit 90. A correction coefficient is obtained based on the charge accumulation time and the elapsed time, and afterimage correction is performed on the radiographic image signal of the main imaging based on the correction coefficient and the pre-read image signal. A method for acquiring a preset correction coefficient will be described in detail later.

  As the radiation dose detection unit 70, any configuration may be adopted as long as it outputs a signal when the dose of incident radiation exceeds a predetermined threshold.

  The temperature detector 80 indirectly detects the temperature of the radiation image detector 20 itself by detecting the temperature in the vicinity of the radiation image detector 20.

  Although not shown in FIGS. 1 and 2, linear reading light extending in a direction orthogonal to the extending direction of the linear electrodes of the second electrode layer 25 is emitted into the imaging table 3. A linear reading light source is provided.

  Next, the operation of the breast image radiographing display system will be described with reference to the timing chart shown in FIG. 5 and the flowchart shown in FIG. In this mammography / display system, the control sequence of the entire system and the afterimage correction and breast image display are performed in parallel. Therefore, the flowchart of FIG. 6 shows these sequences performed in parallel. Yes.

  First, when the subject stands beside the mammography apparatus 10, the imaging table 3 moves to a height corresponding to the height of the subject, and the arm 4 corresponds to the breast size and shape of the subject. Rotate. After the subject's breast is placed on the imaging table 3, the compression plate 6 moves so that the breast has a thickness suitable for imaging, and the breast is compressed.

  Then, after the compression of the breast is completed, the front light is emitted from the light irradiation unit 30 based on the control signal from the control unit 90, and is irradiated from the second electrode layer 25 side of the radiation image detector 20 (S10). ). Charges are generated in the radiation image detector 20 by the irradiation of the front light, and the charges and the charges remaining in the radiation image detector 20 are combined to erase a part of the afterimage existing in the radiation image detector 20. Is done.

  Next, in a state where no radiation is emitted from the radiation source 1, a negative high voltage (hereinafter referred to as pre-high voltage) is set in advance for a predetermined time on the first electrode layer 21 of the radiation image detector 20. Applied (S12). A part of the charge (negative charge) remaining in the recording photoconductive layer 22 by the application of the previous high voltage is accumulated in the power storage unit 26.

  Next, a linear reading light source (not shown) that emits linear reading light extending in a direction orthogonal to the extending direction of the linear electrodes of the second electrode layer 25 while the first electrode 1 is grounded. ) Is scanned in the extending direction of the linear electrode to perform pre-reading (S14). More specifically, the reading light passes through the transparent linear electrode 25a and is applied to the reading photoconductive layer 24. The positive charge generated in the reading photoconductive layer 24 by the irradiation of the reading light is combined with a part of the remaining charge in the power storage unit 26, and the negative charge is read by the transparent linear electrode 25a and the light shielding linear electrode 25b. Then, it is detected as a pre-read image signal by a charge amplifier (not shown) connected to the light shielding linear electrode 25b. The preread image signal is input to the image signal acquisition unit 40 and temporarily stored (S30).

  Then, before the actual photographing, the main light is emitted from the light irradiation unit 30 and irradiated from the second electrode layer 25 side of the radiation image detector 20 (S16). Charges are generated in the radiation image detector 20 by the irradiation of the front light, and a part of the afterimage existing in the radiation image detector 20 is combined by combining the charges and the charges remaining in the radiation image detector 20. Erased. The light irradiation unit 30 is controlled by the control unit 90 so that the light amount of the main light irradiated to the radiation image detector 20 at this time is smaller than the light amount of the above-described front light and erasing light described later. The

  And after irradiation of this front light is complete | finished, while applying the negative high voltage (henceforth this high voltage) to the 1st electrode layer 21 of the radiographic image detector 20, from the control part 90 to the radiation source 1 An irradiation start signal is output and radiation irradiation by the radiation source 1 is started (S18). Note that the high pressure applied to the radiation image detector 20 at this time is the same size as the pre-high pressure described above.

  The radiation emitted from the radiation source 1 passes through the breast M, is detected two-dimensionally by the radiation image detector 20, and the dose is detected by the radiation dose detector 70.

  Charge irradiation is generated in the recording photoconductive layer 22 of the radiation image detector 20 by the irradiation of radiation as described above, and the positive charge is combined with the negative charge charged in the first electrode layer 21 and disappears. The negative charges are accumulated as latent image charges in the power storage unit 26 formed at the interface between the recording photoconductive layer 22 and the charge transport layer 23, and a radiographic image of the breast M is recorded.

  On the other hand, in the radiation dose detection unit 70, it is determined whether or not the detected radiation dose exceeds a predetermined threshold value (S20), and a signal to that effect is displayed when the detected radiation dose exceeds the predetermined threshold value. 70 to the control unit 90. Then, the control unit 90 outputs an irradiation stop signal to the radiation source 1 based on the signal, and the radiation source 1 stops the emission of radiation based on the irradiation stop signal. In addition, the control unit 90 stops the application of the high voltage to the first electrode layer 21 of the radiation image detector 20 simultaneously with the stop of radiation emission.

  Next, a linear reading light source (not shown) that emits linear reading light extending in a direction orthogonal to the extending direction of the linear electrodes of the second electrode layer 25 while the first electrode 1 is grounded. ) Is scanned in the extending direction of the linear electrode to perform the main reading (S22). More specifically, the reading light passes through the transparent linear electrode 25a and is applied to the reading photoconductive layer 24. The positive charge generated in the reading photoconductive layer 24 by the irradiation of the reading light is combined with the latent image charge in the power storage unit 26, and the negative charge is read by the transparent linear electrode 25a and the light shielding linear electrode 25b. It is detected as a radiographic image signal representing a radiographic image of a subject by a charge amplifier (not shown) connected to the light shielding linear electrode 25b. Then, the radiation image signal is input to the image signal acquisition unit 40 and temporarily stored (S32).

  Next, erasing light is emitted from the light irradiation unit 30 based on a control signal from the control unit 90, and is irradiated from the second electrode layer 25 side of the radiation image detector 20 (S24). A charge is generated in the radiation image detector 20 by the irradiation of the erasing light, and a part of the residual image remaining in the radiation image detector 20 by combining the charge and the charge left unread in the radiation image detector 20. Is erased.

  On the other hand, the pre-read image signal stored in the image signal acquisition unit 40 and the radiographic image signal of the main imaging are output to the image processing unit 50. On the other hand, the time during which the high voltage is applied to the first electrode layer 21 in S18 to S20 (the time during which radiation is applied to the radiation image detector 20) is acquired by the control unit 90 as the charge accumulation time. The charge accumulation time is input to the image processing unit 50. Further, in the control unit 90, the elapsed time from the radiation irradiation start time in the previous radiographic image capturing, that is, from the time when the irradiation start signal is output to the time of starting the main reading in S22 of the current radiographic image capturing. The measured time is also output to the image processing unit 50. The temperature information detected by the temperature detection unit 80 at the time of the main reading in S22 is also acquired by the control unit 90, and this temperature information is also input to the image processing unit 50.

  Then, the image processing unit 50 acquires a correction coefficient based on the input charge accumulation time, elapsed time, and temperature information, multiplies the correction coefficient by the pre-read image signal, and the result of the multiplication is a radiation image signal. The afterimage correction of the radiation image signal is performed by subtracting from the image, and the afterimage corrected radiation image signal is output to the display unit 60 (S26). The correction coefficient will be described in detail later.

  Then, the display unit 60 displays a radiation image of the breast based on the input afterimage-corrected radiation image signal.

  The steps from S10 to S28 described above are steps performed by taking a single radiographic image. As shown in FIG. 5, when the pre-reading process is from the start of irradiation of the pre-light at S10 to immediately before the application of the main high voltage at S18, and the main reading process is from immediately after the application of the high voltage at S18 to the end of the main reading at S22 The time required for the pre-reading step and the time required for the main reading step are preferably constant for each photographing. Further, in the case where the irradiation of the front light and the main front light is not performed, the pre-reading process is performed from the start of applying the pre-high voltage in S12 to immediately before the main high-voltage application in S18. It is desirable that the time required for the recording is constant for each photographing.

Here, as described above, correction coefficients corresponding to the charge accumulation time and elapsed time are preset in the image processing unit 50 as a table as shown in Table 1 below. And the table shown in the following table 1 is preset for every predetermined temperature. The image processing unit 50 selects one of the tables set for each temperature based on the input temperature information, and refers to the selected table to input charge accumulation time and elapsed time. Get the corresponding correction factor.

  If there is no table corresponding to the same temperature as the temperature detected by the temperature detector 80, a table corresponding to a closer temperature may be selected, or two temperatures sandwiching the detected temperature may be selected. It is also possible to select a table to be created and interpolate these two tables to create a new table.

  When the same charge storage time as the charge storage time input to the image processing unit 50 does not exist in the table, a correction coefficient corresponding to a closer charge storage time may be acquired or input. A correction coefficient corresponding to two charge accumulation times sandwiching the charge accumulation time may be acquired, and the two correction coefficients may be interpolated to newly obtain a correction coefficient.

  Similarly, when the elapsed time that is the same as the elapsed time input to the image processing unit 50 does not exist in the table, a correction coefficient corresponding to a closer elapsed time may be acquired or input. A correction coefficient corresponding to two elapsed times sandwiching the elapsed time may be acquired, and the two correction coefficients may be interpolated to newly calculate a correction coefficient.

  Here, the correction coefficient described above is acquired using the present breast image radiographing display system and preset in the image processing unit 50. The acquisition method will be described below.

  First, the first electrode layer 21 of the radiographic image detector 20 is irradiated without irradiating radiation in a state where a sufficient time has elapsed since the last radiographic image capturing and no afterimage remains in the radiographic image detector 20. After a high voltage is applied for a predetermined application time, a dark image signal is read from the radiation image detector 20 by irradiating with reading light. Note that the application time of the high voltage at this time is set to one of the assumed charge accumulation times of the main photographing.

  Next, in a state where a radiation absorbing member such as an Al plate is installed on a part of the surface of the radiation image detector 20 on the radiation source 1 side and a high voltage is applied to the first electrode layer 21 of the radiation image detector 20. A predetermined dose of radiation is emitted from the radiation source 1. Then, the radiation image corresponding to the predetermined dose by this irradiation and the radiation image corresponding to the dose attenuated from the predetermined dose by passing through the Al plate are recorded in the radiation image detector 20. Here, a radiation of 1R dose is emitted from the radiation source 1, a radiation image corresponding to the dose of 1R is recorded on a part of the surface of the radiation image detector 20, and an Al plate is formed on a surface other than the part of the surface. A radiographic image corresponding to the dose of 7 mR attenuated by is recorded. Note that the high voltage application time and radiation irradiation time at this time are set to one of the assumed charge accumulation times of the main imaging, similarly to the application time set when the dark image signal is acquired. . In addition, a part of the radiographic image by the irradiation of the 1R radiation becomes an afterimage of the radiographic image captured later.

  The radiation image recorded in the radiation image detector 20 as described above is read out by grounding the first electrode layer 21 and irradiating the reading light.

  Then, after a predetermined time has elapsed after the reading, the above-described pre-reading steps from S10 to S16 are performed, and the dark image signal acquired above is subtracted from the image signal acquired by the pre-reading step. Thus, a pre-read image signal for afterimage correction is acquired. Note that the pre-high voltage application time in the pre-reading process at this time is set to the same time as the pre-high voltage application time in the pre-reading process performed at the time of actual radiographic image capturing.

  Next, in a state where a predetermined high voltage is applied to the first electrode layer 21, a radiation having a dose equivalent to the radiation dose after passing through the Al plate described above is emitted from the radiation source 1, and the radiation The entire surface of the image detector 20 is irradiated. That is, here, a radiation with a dose of 7 mR is emitted from the radiation source 1 and applied to the entire surface of the radiation image detector 20. Note that the application time of high voltage and the irradiation time of radiation at this time are also set to one of the above-described assumed charge accumulation times of actual imaging.

  Then, by grounding the first electrode layer 21 and irradiating the reading light, the radiation image of 7 mR recorded in the radiation image detector 20 as described above is read out. Note that the 7 mR radiographic image includes an afterimage of the radiographic image recorded in the radiographic image detector 20 by the irradiation of the 1R radiation described above.

  Then, an afterimage correction image signal is obtained by subtracting the dark image signal obtained above from the read radiation image signal representing the 7 mR radiation image.

  Then, the difference between the signal corresponding to the Al plate installation range and the signal corresponding to the range other than the Al plate installation range in the pre-read image signal for afterimage correction acquired as described above is obtained, and the Al plate installation in the image signal for afterimage correction is obtained. The difference between the signal corresponding to the range and the signal corresponding to the range other than the Al plate installation range is obtained, and the ratio of the signal difference in the afterimage correction image signal to the signal difference in the afterimage correction preread image signal is obtained.

  FIG. 7 is a diagram schematically showing the afterimage correction preread image signal and the afterimage correction image signal. As shown in FIG. 7, for example, the difference between the signal corresponding to the Al plate installation range (7 mR range) and the signal corresponding to a range other than the Al plate installation range (1R range) in the afterimage correction pre-read image signal. Is 50, and the difference between the signal corresponding to the Al plate installation range (7 mR range) and the signal corresponding to the range other than the Al plate installation range (1R range) in the afterimage correction image signal is 75. The correction coefficient is 75/50 = 1.5.

  In this way, a correction coefficient corresponding to one of the assumed charge accumulation times of the main photographing described above is acquired.

  Then, a high voltage application time when acquiring the dark image signal, a high voltage application time when acquiring the afterimage correction pre-read image signal, and a high voltage application time when acquiring the afterimage correction image signal are assumed. The correction coefficient for each charge accumulation time can be acquired by setting the charge accumulation time for the main photographing and performing the same process as described above.

Also, a high voltage application time when acquiring a dark image signal, a high voltage application time when acquiring a pre-read image signal for afterimage correction, and a high voltage application time when acquiring an afterimage correction image signal are assumed. One of the charge accumulation times of the main imaging is set, and the elapsed time from the start of irradiation of the 1R radiation described above to the start of readout of the radiographic image signal representing the 7 mR radiographic image is assumed to be the same as that of the assumed previous imaging. By setting each of the elapsed times from the radiation irradiation start time, a correction coefficient for each elapsed time can be acquired.

  A table as shown in Table 1 can be created as described above.

  As for the table for each temperature, the correction coefficient table for each temperature can be acquired by forcibly changing the temperature detected by the temperature detection unit 80 and acquiring the correction coefficient.

  Note that the breast image radiographing display system according to the present embodiment substantially calculates a correction coefficient corresponding to the afterimage level that changes in accordance with the charge accumulation time and the time elapsed since the previous radiography as shown in FIG. Will be.

  Here, an afterimage correction property evaluation method and an evaluation result of afterimage correction using the correction coefficient acquired as described above will be described below.

  First, the first electrode layer 21 of the radiographic image detector 20 is irradiated without irradiating radiation in a state where a sufficient time has elapsed since the last radiographic image capturing and no afterimage remains in the radiographic image detector 20. After a high voltage is applied for a predetermined application time, a dark image signal is read from the radiation image detector 20 by irradiating with reading light. Note that the application time of the high voltage at this time is set to one of the assumed charge accumulation times of the main photographing.

  Next, a plurality of types of radiation absorbing members such as an Al plate and a lead plate are installed on a part of the surface of the radiation image detector 20 on the radiation source 1 side, and high pressure is applied to the first electrode layer 21 of the radiation image detector 20. In the applied state, a predetermined dose of radiation is emitted from the radiation source 1. Then, the radiation image corresponding to the predetermined dose by this irradiation and the radiographic image corresponding to the dose attenuated from the predetermined dose by transmitting through the Al plate and the lead plate are recorded in the radiation image detector 20. To do. Here, a radiation absorbing member having three types of radiation transmittance is arranged using an Al plate, a lead plate, or the like, a radiation of 2R dose is emitted from the radiation source 1, and 2R of radiation is applied to a part of the surface of the radiation image detector 20. A radiographic image corresponding to the dose is recorded, and a radiographic image corresponding to a dose of 7 mR, a radiographic image corresponding to a dose of 500 mR, and a radiographic image corresponding to a dose of 1R are recorded on a surface other than the above-mentioned part. To do. Note that the high voltage application time and radiation irradiation time at this time are set to one of the assumed charge accumulation times of the main imaging, similarly to the application time set when the dark image signal is acquired. . Further, a part of the radiographic image by the irradiation of the 2R radiation becomes an afterimage of the radiographic image to be captured later.

  The radiation image recorded in the radiation image detector 20 as described above is read out by grounding the first electrode layer 21 and irradiating the reading light.

  Then, after a predetermined time has elapsed after the reading, the above-described pre-reading steps from S10 to S16 are performed, and the dark image signal acquired above is subtracted from the image signal acquired by the pre-reading step. To obtain a pre-read image signal for afterimage correction evaluation. Note that the pre-high voltage application time in the pre-reading process at this time is set to the same time as the pre-high voltage application time in the pre-reading process performed at the time of actual radiographic image capturing.

  Next, in a state where a predetermined high voltage is applied to the first electrode layer 21, a radiation with a dose of 7 mR is emitted from the radiation source 1 to irradiate the entire surface of the radiation image detector 20. Note that the application time of high voltage and the irradiation time of radiation at this time are also set to one of the above-described assumed charge accumulation times of actual imaging.

  Then, by grounding the first electrode layer 21 and irradiating the reading light, the radiation image of 7 mR recorded in the radiation image detector 20 as described above is read out. Note that the 7 mR radiographic image includes an afterimage of the radiographic image recorded in the radiographic image detector 20 by the irradiation of the 2R radiation described above.

  Then, an afterimage correction evaluation image signal is acquired by subtracting the acquired dark image signal from the read radiographic image signal representing the 7 mR radiographic image.

  Then, the pre-read image signal for afterimage correction evaluation acquired above is multiplied by the correction coefficient corresponding to the assumed charge accumulation time of the main photographing described above, and the multiplied value is subtracted from the image signal for afterimage correction evaluation. An afterimage corrected image signal is acquired.

  Then, a result obtained by measuring the maximum value of the density difference in the afterimage-corrected image signal under the condition that γ (gradient of density change with respect to dose) is five times is acquired as an evaluation result.

  In this way, an evaluation result corresponding to one of the assumed charge accumulation times of the main photographing described above is obtained.

  FIG. 9 schematically shows an afterimage correction evaluation image signal, an afterimage correction evaluation preread image signal, and an afterimage corrected image signal. Note that k is a correction coefficient. Further, in the afterimage correction evaluation image signal and the afterimage correction evaluation preread image signal, a range in which a 2R radiation image is recorded, a range in which a 7 mR radiation image is recorded, and a 500 mR radiation image are recorded, respectively. The range and the range where 1R radiographic images were recorded are shown.

  As shown in FIG. 9, when the correction coefficient is set appropriately, there is almost no density difference in the afterimage-corrected image signal, and the evaluation result is also a sufficiently small value.

  The high voltage application time when acquiring the dark image signal, the high voltage application time when acquiring the afterimage correction pre-read image signal, and the high voltage application time when acquiring the afterimage correction evaluation image signal are assumed. It is possible to obtain an evaluation result for each charge accumulation time by performing the same process as described above by setting each of the charge accumulation times for the actual photographing.

  Also, a high voltage application time when acquiring a dark image signal, a high voltage application time when acquiring a pre-read image signal for afterimage correction, and a high voltage application time when acquiring an afterimage correction evaluation image signal are assumed. One of the charge accumulation times of the main imaging, and the elapsed time from the start of irradiation of the 2R radiation described above to the start of readout of the radiation image signal representing the 7 mR radiation image is assumed to be the previous imaging. By setting each of the elapsed times from the radiation irradiation start time, the evaluation result for each elapsed time can be acquired.

Table 2 below summarizes the evaluation results corresponding to the assumed charge accumulation time in actual photographing and the elapsed time from the previous photographing. When the evaluation result is less than 0.06, it means that the afterimage is almost invisible, and when it is 0.06 or more and less than 0.09, the afterimage can be visually recognized but the image can be diagnosed. If it is 0.09 or more, it means that the afterimage is at a level causing a problem in image diagnosis.

  As shown in Table 2 above, all the evaluation results are less than 0.06, and it can be seen that afterimage correction of the present embodiment is at a level where the afterimage is hardly visually recognized.

In the above evaluation method, a 7 mR radiographic image is recorded in order to acquire an afterimage correction evaluation image signal. However, a 14 mR radiographic image is recorded and evaluated in the same manner as described above. Table 3 shows.

  As shown in Table 3 above, even when a radiographic image of 14 mR is recorded, all the evaluation results are less than 0.05, and it can be seen that the afterimage correction of this embodiment is at a level where the afterimage is hardly visible. . Further, it can be seen that the afterimage becomes less conspicuous as the radiation dose is larger than when a 7 mR radiation image is recorded, and the evaluation result is improved.

  In the description of the above embodiment, the radiation image detector 20 is irradiated with the front light, the main front light, and the erasing light in order to erase the afterimage, but it is not always necessary to irradiate all of these. You may make it irradiate at least any one.

Further, all of the front light, the main front light and the erasing light may not be irradiated. Table 4 below shows the correction coefficients obtained in the same manner as above without irradiating the front light, the main front light, and the erasing light, and the results of evaluating the afterimage correction properties using the correction coefficients are shown below. Table 5 shows. The afterimage correction property evaluation method is the same as that described above except that no pre-light is irradiated in the pre-reading step, and a 7 mR radiation image is recorded in order to acquire an afterimage correction evaluation image signal.

  As shown in Table 5 above, since all of the front light, the main front light, and the erasing light are not irradiated, the afterimage correction performance is slightly worse compared to the evaluation results of Table 2 where these are irradiated. I understand. However, it is 0.07 at the maximum, and it can be seen that it can be at a level that is not particularly problematic for image diagnosis.

  Further, in the breast image radiographing display system of the above embodiment, an optical reading type radiographic image detector is used as the radiographic image detector 20, but the invention is not limited to this, and an electric reading type radiographic image detector is used. May be. For example, an upper electrode to which a bias voltage is applied, a semiconductor layer that generates charges when irradiated with radiation, a pixel electrode that collects charges generated in the semiconductor layer, and a storage capacitor that accumulates charges collected by the pixel electrodes And a radiation image detector using TFTs in which an active matrix substrate in which a large number of pixels each having a TFT switch for reading out charges accumulated in a storage capacitor are two-dimensionally arranged is laminated. Also good.

  In the case of using the radiation image detector using the TFT as described above, a bias voltage is always applied to the upper electrode, and charges are always read out by the active matrix substrate at a predetermined clock during standby. The pre-read image signal for afterimage correction in this case is an image signal read at a predetermined timing immediately before the main photographing. The charge accumulation time is the time from the pre-reading to the main reading after the main photographing.

  In the above embodiment, a so-called direct conversion type radiation image detector that converts radiation directly into electric charge is used. However, radiation is once converted into light by a phosphor and the light is converted into electric charge. A so-called indirect conversion type radiation image detector may be used.

  In the above description, an embodiment of the afterimage correction apparatus according to the present invention has been described for a mammography imaging display system. However, the imaging system is not limited to imaging a radiographic image of a breast. It can also be used for a general radiographic imaging display system that captures radiographic images of hands and feet.

1 is a schematic configuration diagram of a breast image photographing display system using an embodiment of an afterimage correcting apparatus of the present invention. Block diagram showing schematic configuration of breast image display device The perspective view which shows schematic structure of the radiographic image detector in the mammography imaging display system shown in FIG. 2-2 sectional view of the radiation image detector shown in FIG. Timing chart for explaining the operation of the breast image photographing display system Flow chart for explaining the operation of the breast image photographing display system The figure which showed typically the after-reading image signal for afterimage correction, and the image signal for afterimage correction Diagram showing the relationship between afterimage level, charge accumulation time and elapsed time Schematic diagram of afterimage correction evaluation image signal, afterimage correction evaluation preread image signal, and afterimage corrected image signal

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Radiation source 2 Radiation irradiation part 3 Imaging stand 4 Arm 5 Base 6 Compression board 10 Breast imaging device 15 Breast image display device 20 Radiation image detector 30 Light irradiation part 40 Image signal acquisition part 50 Image processing part 60 Display part 70 Radiation dose detection unit 80 Temperature detection unit 90 Control unit

Claims (6)

In a radiological image detector that generates a charge upon receiving irradiation of radiation that has passed through a subject, outputs a radiological image signal representing a radiographic image of the subject, and is repeatedly used for radiographic imaging, the radiographic image detector An afterimage correction method for applying afterimage correction to a read radiation image signal,
After the first imaging, the charge accumulated in the radiation image detector for a certain period of time without reading the radiation is read as a pre-read image signal,
The read pre-read image signal;
(I) at least one of a charge accumulation time generated in the radiation image detector by the second imaging after reading the pre-read image signal and (ii) an elapsed time from the first imaging. An afterimage correction method, wherein afterimage correction is performed on a radiation image signal read out by the second imaging based on a correction coefficient according to a radiation imaging condition. In a radiological image detector that generates a charge upon receiving irradiation of radiation that has passed through a subject, outputs a radiological image signal representing a radiographic image of the subject, and is repeatedly used for radiographic imaging, the radiographic image detector An afterimage correction apparatus that performs afterimage correction on a read radiation image signal,
A pre-read image signal acquisition unit that reads and acquires the charge accumulated in the radiological image detector for a certain period of time without performing the irradiation of radiation as a pre-read image signal after the first imaging;
A preread image signal acquired by the preread image signal acquisition unit;
(I) at least one of accumulation time of charge generated in the radiation image detector by the second imaging after reading the pre-read image signal and (ii) elapsed time from the first imaging An afterimage correction apparatus, comprising: an afterimage correction unit that performs afterimage correction on a radiation image signal read out by the second imaging based on a correction coefficient according to a radiation imaging condition.   The afterimage correction apparatus according to claim 2, wherein the radiation imaging condition includes information related to a temperature of the radiation image detector at the time of the second imaging.   The time required in the step of acquiring the pre-read image signal and / or the time required in the step of acquiring the radiographic image signal by the second imaging after the imaging of the second imaging is set constant for each imaging. The afterimage correction apparatus according to claim 2 or 3, wherein   A first time after the first photographing and before accumulation of charges of the pre-read image signal, and a second time after the pre-read image signal is read and before the second photographing And a light irradiation unit for irradiating the radiation image detector with light at at least one of the third time points after reading out the radiation image signal by the second imaging. The afterimage correction apparatus according to any one of 2 to 4.   The light irradiation unit determines the amount of light irradiated to the radiation image detector at the second time point from the light amount irradiated to the radiation image detector at the first time point and / or the third time point. 6. The afterimage correcting apparatus according to claim 5, wherein the afterimage correcting apparatus is also made smaller.