JP4812503B2 - X-ray equipment - Google Patents

Description

  In an X-ray imaging apparatus used in X-ray diagnosis or the like, the input / output characteristics of an X-ray detector are matched with a subject to prevent erroneous imaging.

In the field of X-ray diagnostic apparatuses, an X-ray detector called an X-ray FPD (Flat Panel Detector) has recently been used. The X-ray FPD has a scintillator arranged on the input surface of the image sensor, and is detected by the image sensor after the X-ray image is converted into a light image by the scintillator. A TFT (Thin Film Transistor) sensor is used as the image sensor. A TFT sensor is one in which photodiodes and TFT switches are formed on a glass substrate mainly using an amorphous silicon process, and has an advantage that a thin and large area can be produced at low cost. However, since the X-ray FPD does not have a signal amplification function like the X-ray II (Image Intensifier), the S / N in the low-dose region is worse than the conventional X-ray detector combining the X-ray II and the TV camera. There is a drawback.
In order to improve the S / N of the X-ray FPD, an active pixel X-ray FPD in which an amplifier is built in each pixel of the TFT sensor has been studied (for example, Non-Patent Document 1).

Larry E. Antonuk et al., “Investigation of Strategies to Achieve Optimal DQE Performance from Indirect Detection, Active Matrix Flat-Panel Imagers (AMFPIs) through Novel Pixel Amplification Architectures”, Proc. SPIE, Vol. 5745, pp18-31, 2005

  Non-Patent Document 1 discloses a method for detecting a current output of an active pixel X-ray FPD. The circuit configuration used in this method is shown in FIG. In FIG. 6, each pixel 103 includes a photodiode PD, a pixel switch M1, a pixel amplifier M2, and a PD reset switch M3. A supply potential Vdd is applied to one end of the pixel amplifier M2 and one end of the PD reset switch through a line (not shown). A PD reference potential Vss is applied to one end of the photodiode PD through a line (not shown). The other end of the photodiode PD is connected to the PD reset switch M3. When M3 is turned on, a reverse bias voltage of Vdd-Vss is applied to both ends of the photodiode PD. When the photodiode PD detects light, a photocurrent is generated, and the gate voltage Vo of the pixel amplifier M2 changes from Vdd to Vss. Therefore, the gate voltage Vo of the pixel amplifier M2 changes according to the amount of light incident on the photodiode PD. When the pixel switch M1 is turned on through the pixel selection line 102, a drain current Id flows through the pixel amplifier M2 by the gate voltage Vo. The drain current is integrated by the charge amplifier CSA through the data line 100. In this way, the signal detected at each pixel is output in the form of a drain current Id. Here, the drain current Id can be expressed by the following equation.

  However, Vo is the gate voltage of M2, Vt2 is the threshold voltage of M2, and Vs is the reference potential of CSA. R and k are respectively expressed by the following equations.

  Here, μ, Co, W, and L represent carrier mobility, gate film oxidation capacity, gate width, and gate length, respectively, and subscripts 1 and 2 correspond to M1 and M2 TFTs, respectively. Furthermore, Vp is the gate voltage of M1. From Equation 1, it can be seen that the relationship between the gate voltage Vo of M2 and the drain current Id is non-linear. This corresponds to the fact that the input / output relationship becomes nonlinear in this circuit system. As an example, FIG. 7 shows the result of calculating the input / output relationship. Such nonlinearity of the input / output characteristics causes a decrease in the quantitativeness of the X-ray measurement. In particular, cone beam CT measurement using an X-ray FPD has a problem that the nonlinearity of the input / output characteristic significantly reduces the accuracy of the CT value. On the other hand, FIG. 7 shows that in this circuit system, the smaller the input signal Vdd-Vo, the larger the amount of change in the output, and the slower the change in output as the input signal becomes larger. Since such input / output characteristics expand the dynamic range of the detector, there is an advantageous aspect for X-ray measurement. For example, in chest imaging in an X-ray diagnostic apparatus, a part with a low signal level such as the mediastinum part and a part with a high signal level such as a lung field part are mixed. For this reason, in order to prevent halation from occurring in the lung field at the same time without impairing the contrast of the mediastinum, the input / output characteristics as in this circuit system are advantageous. However, such input / output characteristics are not always effective, and it is desirable to be able to select appropriate input / output characteristics according to the object to be imaged. However, until now, it has been difficult to change the input / output characteristics of the X-ray FPD, so that such an attempt has not been made.

  An object of the present invention is to provide a technique for changing desired input / output characteristics of an X-ray FPD in accordance with an object to be imaged so as to select desired characteristics. Another object of the present invention is to provide a technique for enabling highly quantitative X-ray measurement even for input characteristics of a non-linear X-ray FPD.

The following is a brief description of an outline of typical inventions among the inventions disclosed in the present application.
(Means 1)
In an X-ray imaging apparatus having means for generating X-rays and X-ray detection means for detecting an X-ray transmission image of a subject, individual pixels constituting the imaging surface of the X-ray detection means are photodiodes and photodiodes. An amplifier for amplifying a detection signal; supply voltage setting means for setting a voltage to be supplied to the photodiode and the amplifier; and an input / output for correcting input / output characteristics of the X-ray detector in accordance with the supply voltage An X-ray imaging apparatus including a characteristic correction unit. This corresponds to changing the range in which the gate voltage Vo of the pixel amplifier M2 changes in Equation 1. Thereby, various input / output characteristics of the detector can be changed. In addition, since any measured input / output characteristic can be converted into linear input / output characteristic data, X-ray measurement can be performed without impairing quantitativeness.
(Means 2)
The X-ray imaging apparatus according to means 1, comprising reset means for resetting the voltage across the photodiode and voltage holding means for holding the voltage across the photodiode at the time of reset constant regardless of the supply voltage. An X-ray imaging apparatus characterized by the above. Thereby, even if the supply voltage is changed, stable measurement can be performed without changing the characteristics of the photodiode (leakage current, junction capacitance, etc.).
(Means 3)
The X-ray imaging apparatus according to any one of the means 1 and 2, wherein the set value of the supply voltage can be designated in advance according to a subject to be processed. Thereby, the user can select in advance appropriate input / output characteristics for each photographing target. As a result, it is possible to save the trouble of setting appropriate input / output characteristics each time according to the object to be photographed.
(Means 4)
In an X-ray imaging apparatus having means for generating X-rays and X-ray detection means for detecting an X-ray transmission image of a subject, individual pixels constituting the imaging surface of the X-ray detection means are photodiodes and photodiodes. An amplifier for amplifying a detection signal; output voltage setting means for setting an output voltage of the amplifier; and input / output characteristic correction means for correcting input / output characteristics of the X-ray detector in accordance with the output voltage. It is an X-ray imaging apparatus characterized by having. This corresponds to changing the reference potential Vs of the charge amplifier CSA in Equation 1. Thereby, the effect similar to the means 1 can be acquired.
(Means 5)
4. The X-ray imaging apparatus according to claim 4, wherein a set value of the output voltage can be designated in advance according to a subject to be processed. Thereby, the effect similar to the means 3 can be acquired.
(Means 6)
The X-ray imaging apparatus according to any one of Items 1 to 5, wherein the input / output characteristic correction unit corrects the input / output characteristic of the X-ray detector based on an input / output characteristic correction table prepared in advance. X-ray apparatus. Thereby, the input / output characteristics can be corrected at high speed.

The following is a brief description of the effects obtained by the typical inventions among the inventions disclosed in the present application.
Various input / output characteristics of the X-ray detector can be changed, and a desired input / output characteristic can be selected in accordance with the object to be imaged. Thereby, the dynamic range of an X-ray detector can be expanded and diagnostic performance can be improved.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows the configuration of the X-ray imaging apparatus according to the present embodiment. The X-ray imaging apparatus includes an X-ray tube 1, a collimator 2, a support 3, an X-ray detector 4, a bed 6, a console 10, an imaging control unit 11, a frame memory 12, a signal processing unit 13, and an input / output characteristic conversion table 14. , Input / output characteristic setting table 15, image display means 16, and the like. In addition to a known instruction mechanism for instructing X-ray imaging and a known setting mechanism for setting imaging conditions, the console 10 has a voltage that defines the imaging target and input / output characteristics of the X-ray detector 4. A mechanism for instructing the setting is provided.

  Hereinafter, a configuration including the X-ray tube 1, the collimator 2, and the X-ray detector 4 is referred to as an imaging system. A known device is used for each component of the photographing system. The collimator 2 is fixed to the front surface of the X-ray tube 1 and has a known function for limiting the X-ray irradiation range to the subject 5. The entire photographing system is supported by the support 3.

  A typical example of the distance between the X-ray generation point of the X-ray tube 1 and the input surface of the X-ray detector 4 is 110 [cm]. An active pixel type FPD, which will be described later, is used for the X-ray detector 4. A typical example of the specification of the X-ray detector 4 is an input surface size of 307.2 [mm] × 307.2 [mm] and the number of pixels of 2048 [pixels] × 2048 [pixels]. The X-ray detector 4 has an imaging mode and a fluoroscopic mode, and a typical example of the frame rate in the fluoroscopic mode is 30 [frame / second]. The above specifications are not limited to these values and can be variously changed according to the configuration of the X-ray imaging apparatus.

  Next, the operation procedure of the X-ray imaging apparatus according to this embodiment will be described. First, the examiner designates a region to be imaged (head, chest, abdomen, etc.) through the console 10 prior to photographing the subject 5. The console 10 refers to the input / output characteristic setting table 15 according to the designation of the imaging target region, and sets the voltage setting (Vdd, Vss, Vs) for realizing the predetermined input / output characteristic of the X-ray detector 4. Are set in the X-ray detector 4. Next, the console 10 gives an instruction to start photographing to the photographing control means 11. The imaging control means 11 gives a sequence to the X-ray tube 1 and the X-ray detector 4 to start imaging or fluoroscopy. At this time, the X-ray transmission image of the subject 5 is detected by the X-ray detector 4, and the acquired image is written in the frame memory 12. Next, the signal processing means 13 refers to the corresponding input / output characteristic conversion table 14 based on the set value of the input / output characteristic instructed from the console 10 and converts the pixel value of the image written in the frame memory 12. Note that the conversion of the pixel value is performed by correcting the distortion of the input / output characteristics of the X-ray detector 4 and converting it to linear characteristics. Finally, the signal processing unit 13 outputs the converted image to the image display unit 16, and the image is displayed. Note that the above-described series of processing is repeatedly performed at a predetermined frame rate during fluoroscopy.

  FIG. 2 is a diagram for explaining the configuration of the X-ray detector 4. The X-ray detector 4 is obtained by forming pixels 103 in an array on a glass substrate 20 using a polysilicon and / or amorphous silicon process. Note that the internal configuration of the pixel 103 is the same as that shown in FIG. Pixel reset and signal readout pixel selection are performed via a reset line 101 and a pixel selection line 102, respectively. The drain current Id output from the selected pixel is read out via the data line 100. In addition to this, a shift register 21 and a signal readout circuit 22 are arranged on the glass substrate 20. These are both individual chips formed on a silicon wafer, and are later bonded and fixed on a glass substrate. The shift register 21 sequentially switches the voltage applied to the reset line 101 and the pixel selection line 102 in the vertical direction, and selects the pixel readout position in this direction. The signal readout circuit 22 is composed of a well-known charge amplifier (not shown), a CDS (Correlated Double Sampling) circuit, a multiplexer, an AD converter, and the like. The readout signal is switched in the horizontal direction on the paper and each signal is converted into digital data. And converted to the frame memory 12. The photographing control by the photographing control means 11 is performed by giving a sequence to the shift register 21 and the signal readout circuit 22. The console 10 controls the supply potential Vdd, PD reference potential Vss, and CSA reference potential Vs through data lines (not shown).

  FIG. 3 shows the configuration of the input / output characteristic setting table 15. In this table, recommended settings and user settings are provided for each part to be imaged. The examiner can input user settings, select an imaging target region, and select recommended settings and user settings via the console 10. At this time, the examiner may directly input the voltage setting (Vdd, Vss, Vs), or a desired one from a sample of input / output characteristics prepared in advance (input / output characteristics 1, 2,...). You may select and set.

  FIG. 4 is a graph showing the relationship between the supply potential Vdd and the input / output characteristics. Here, the CSA reference potentials Vs are all 0 [V]. Vdd-Vo on the horizontal axis is a voltage proportional to the amount of light incident on the photodiode. As can be seen from FIG. 4, the linearity of the input / output characteristics improves as Vdd increases. On the other hand, when Vdd is low, linearity decreases as the amount of incident light increases, and the change in Id is compressed. When AD conversion is performed on a signal having such input / output characteristics, there is an advantage that the density resolution can be improved particularly in a region where the amount of incident light is small. Note that the table of conversion from Id to Vdd-Vo based on the input / output characteristics shown in FIG. If the input / output characteristic conversion table 14 is used, nonlinear input / output characteristics can be converted into linear ones, so that it is possible to cope with measurements such as cone beam CT that require linearity in particular. In FIG. 4, the set value of the PD reference potential Vss is not shown, but Vss is a parameter that defines the variable range of Vo to Vss to Vdd. However, it is necessary to determine the value of Vss so that Vo changes within the range in which the pixel amplifier M2 operates. In addition, characteristics such as photodiode leakage current and junction capacitance change when the value of Vdd-Vss changes, so Vss is automatically set according to changes in Vdd while keeping the value of Vdd-Vss constant. Anyway.

  FIG. 5 shows the relationship between the CSA reference potential Vs and the input / output characteristics. Here, all the supply potentials Vdd are set to 3 [V]. Referring to FIG. 5, by changing Vs, the same input / output characteristics as in FIG. 4 can be realized.

  Although the embodiment of the present invention has been described above, the present invention is not limited to this embodiment, and various modifications can be made without departing from the scope of the present invention. For example, it goes without saying that the present invention can be applied to, for example, an industrial X-ray imaging apparatus.

The structure of the X-ray imaging apparatus which concerns on this Embodiment. Configuration of the X-ray detector 4. Configuration of input / output characteristic setting table 15. Relationship between supply potential Vdd and input / output characteristics. Relationship between CSA reference potential Vs and input / output characteristics. Configuration of an active pixel X-ray FPD circuit. An example of input / output characteristics of an active pixel X-ray FPD.

Explanation of symbols

1 ... X-ray tube,
2 ... Collimator,
3 ... posts,
4 ... X-ray detector,
5 ... Subject,
6 ... Sleeper,
20 ... Glass substrate,
21... Shift register,
22... Signal readout circuit,
100 ... data line,
101 ... reset line,
102 ... Pixel selection line,
103: Pixel.

Claims (4)

  In an X-ray imaging apparatus having means for generating X-rays and X-ray detection means for detecting an X-ray transmission image of a subject, individual pixels constituting the imaging surface of the X-ray detection means are photodiodes and photodiodes. An amplifier for amplifying a detection signal; a voltage setting means for setting a voltage supplied to the photodiode and the amplifier or an output voltage of the amplifier; and an input of the X-ray detector according to the set voltage. An X-ray imaging apparatus comprising input / output characteristic correction means for correcting output characteristics. 2. The X-ray imaging apparatus according to claim 1, wherein a reset means for changing a voltage across the photodiode to a predetermined set value and a difference value between the voltages across the photodiode upon resetting by the reset means are supplied. An X-ray imaging apparatus comprising voltage holding means for holding constant regardless of voltage.   3. The X-ray imaging apparatus according to claim 1, wherein the set value of the supply voltage can be designated in advance according to a target subject.   4. The X-ray imaging apparatus according to claim 1, wherein the input / output characteristic correction unit corrects the input / output characteristic of the X-ray detector based on an input / output characteristic correction table prepared in advance. X-ray imaging device.

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