X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
1999-03-10
2001-02-06
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S057000, C250S370070
Reexamination Certificate
active
06185274
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image detecting device for converting light into an electric signal and obtaining an image, in particular, to an image detecting device for directly converting light into electric charge and obtaining an image. In addition, the present invention relates to an image detecting device for use with a medical X-ray diagnosing system. Moreover, the present invention relates to a bi-plane type image detecting device having a plurality of image detecting planes.
2. Description of the Related Art
Image detecting devices with photoelectric elements as image detecting elements have been widely used for video cameras, digital still cameras, and so forth. In addition, such image detecting devices instead of conventional silver-halide films have been used for medical X-ray diagnosing system.
In recent years, databases for medical data of patients have been created so as to promptly and accurately treat the patients. There are also needs of databases for X-ray image data. Thus, it is desired to digitize detected X-ray images.
In medical X-ray diagnosing systems, diagnosis images have been photographed with silver-halide films. To digitize such images, after a photographed film is developed, the developed image should be scanned with a scanner. However, digitized images cannot be quickly and easily obtained. In addition, when the developed images are scanned, the image quality deteriorates.
Recently, a system that directly detects digital images with a CCD camera whose diameter is as small as for example one inch has been accomplished.
However, when the lungs of a patient are detected, it is necessary to detect an area of around 30 cm×30 cm. Thus, since an optical device that collects light is required, the size of the image detecting device becomes large.
As a system for solving such a problem, an image detecting device using thin film transistors (TFTs) composed of a-Si (amorphous silicon) as a semiconductor film has been proposed (for example U.S. Pat. No. 4,689,487).
FIG. 15
is a block diagram showing an example of the structure of such an image detecting device.
FIG. 16
is a schematic diagram showing an example of the structure of an image detecting device using a-Si TFTs.
An X-ray radiated from an X-ray source
51
penetrates an object
52
(a human body) and enters photoelectric elements of the a-Si TFT image detecting device
53
. The a-Si TFT image detecting device
53
converts the X-ray that has penetrated the object
52
into an analog signal corresponding to the dose of the X-ray. An A/D converting portion
57
converts the analog signal into a digital signal in time series. The digital signal is stored in an image memory
58
.
The image memory
58
can store image data for one to several pictures. The image data is successively stored to predetermined addresses corresponding to a control signal received from a controlling portion
63
. An arithmetic processing portion
59
extracts image data from the image memory
58
, calculates the extracted image data, and stores the calculated result to the image memory
58
. The calculated image data is supplied from the image memory
58
to a D/A converting portion
60
. The D/A converting portion
60
converts the digital signal into an analog signal. The analog signal is output to an external processing circuit such as an image monitor
61
through an interface. Then the X-ray transmitted images of the object
52
are displayed on the image monitor
61
.
In
FIG. 16
, a pixel e
1
,l that is an element of an image detecting area is composed of an a-Si thin film transistor
144
, a photoelectric film
140
, and a pixel capacitor
142
. The thin film transistor
144
is composed of a-Si as a semiconductor film. Pixels are arranged in a matrix array of 2000 (W) pixels×2000 (L) pixels (hereinafter referred to as thin film transistor array).
A bias voltage of a power supply
148
is applied to the photoelectric film
140
. The a-Si TFT
144
is connected to a signal line S
1
and a scan line G
1
. The a-Si TFT
144
is turned on/off corresponding to the voltage of the scanning signal applied from a scan line driving circuit
152
to the gate electrode through the scan line. A first end of the signal line S
1
is connected to an amplifier
145
such as a sense amplifier that detects a signal.
When light (for example, an X-ray or a soft X-ray) enters the photoelectric film, a current flows in the photoelectric film
140
. Thus, electric charge is stored in the pixel capacitor
142
. When the scan line driving circuit
152
drives the scan line and thereby all TFTs connected thereto are turned on, the stored electric charge flows to the amplifier
154
through the signal line S
1
. Due to the difference of the amount of electric charge corresponding to the dose of light entered into each pixel, the amplitude of an output signal of the amplifier
154
varies corresponding to the difference. In the device shown in
FIG. 16
, when an analog signal that is output form the amplifier
154
is converted into a digital signal, a digital image can be directly obtained.
The structure of the pixel area shown in
FIG. 16
is the same as the structure of a TFT-LCD that is used for a note type personal computer. The TFT-LCD is an active matrix type liquid crystal display of which thin film transistors are used as switching elements. Thus, an image detecting device that is thin and that has a large screen can be easily fabricated.
There are two types of image detecting devices that convert light such as an X-ray into electric charge.
The first type is referred to as indirect converting type. In the indirect converting type, an X-ray is converted into visible light by phosphor, scintillator or the like. The visible light is converted into electric charge by a photoelectric film. The second type is referred to as direct converting type. In the direct converting type, an X-ray is directly converted into electric charge by a photoelectric film.
In the indirect converting type, although an X-ray is converted into visible light by scintillator, since the light scatters in the scintillator, a sufficient resolution cannot be obtained. On the other hand, in the direct converting type, since an X-ray does not penetrate, an image with a high resolution can be obtained. An image with a high resolution is an essential condition for a medical image. Thus, a direct converting type image detecting device is becoming attractive.
However, in the direct converting type image detecting device, the photoelectric film should be thickened (for example, in the range from several 100 &mgr;m to several mm) so as to improve the efficiency for converting an X-ray into electric charge. Thus, to apply a proper electric field to the photoelectric film, a high voltage of several kilovolts should be applied to the photoelectric film. Thus, in the direct converting type image detecting device, it is necessary to take countermeasures against dielectric breakdown of the TFT array.
As one of the countermeasures against dielectric breakdown, a method for forming a dielectric layer on a photoelectric film has been proposed (as U.S. Pat. No. 5,319,206). However, in this method, electric charge stored in the dielectric layer cannot be quickly discharged. Thus, images cannot be successively detected.
To prevent dielectric breakdown from taking place in a TFT array, a method for disposing a protecting circuit in a pixel e (i, j) has been disclosed (for example, Japanese Patent Laid-Open Application Nos. 10-10237 and 10-170658).
FIG. 24
shows an example of an a-Si TFT image detecting device of which each pixel has a high voltage protecting diode.
In
FIG. 24
, each of pixels e (i, j) is composed of an electric charge reading a-Si TFT
201
, an electric charge capacitor
202
, a protecting diode
214
, and a photoelectric film
203
. The pixels e (i, j) are arranged in a matrix array of 2000 (W) pixels×2000 (L) pixels (hereinafter referred to as thin film transistor array). A several kV high v
Atsuta Masaki
Ikeda Mitsushi
Kinno Akira
Sakaguchi Takuya
Suzuki Kouhei
Bruce David V.
Hobden Pamela R.
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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