Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1998-08-24
2001-05-01
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370140
Reexamination Certificate
active
06225632
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image detection device for obtaining images by converting light signals into electrical signals and more particularly to an image detection device of a medical X-ray diagnostic system.
2. Discussion of Background
An image detection device using photoelectric converting elements as image detection elements is being widely used in video cameras, digital cameras and the like. It is also used in a medical X-ray diagnostic system instead of the conventional screen films.
The medical field is now trending toward making a data base of medical data of patients in order to be able to quickly and accurately give treatment. It is then required to make a data base of picture data obtained by X-ray photography and to digitize photographed X-ray pictures.
The conventional screen film has been used in the medical X-ray diagnostic system to take diagnostic pictures. However, it is necessary to develop the photographed film and then to scan the pictures by a scanner or the like to digitize the pictures, requiring considerable work and time. This method has had another problem that the image quality of the picture is lowered when scanned by the scanner.
A method of using a charge coupled device (CCD) camera whose size is as small as 1 inch square and of photographing directly to obtain digital images has been realized recently.
However, because it is necessary to photograph an area of about 30 cm×30 cm in photographing a human lung for example, it requires an optical apparatus for condensing light, causing a problem that the system is enlarged.
As a method for solving these problems, an image detection device using thin film transistors (TFT) using amorphous silicon (a-Si) as a semiconductor film has been proposed (e.g., U.S. Pat. No. 4,689,487).
There are two types of a-Si TFT image detection devices for converting X-rays into electric charges. One is called an indirect conversion image detection device which converts X-rays once into visible light by fluorescent substance or the like and then converts the visible light into electrical charges by means of a photoelectric converting film. The other is called a direct conversion image detection device which converts X-rays directly into electric charges by means of a photoelectric converting film.
While the indirect conversion image detection device allows to obtain pictures by converting X-rays into visible light by the fluorescent substance, it has a drawback that it is difficult to obtain enough spatial resolution because light scatters within the fluorescent substance.
Meanwhile, the direct conversion image detection device has merit in that it allows high spatial resolution image quality to be obtained because it requires no fluorescent substance which causes the deterioration of the spatial resolution. It is essential to be able to obtain high spatial resolution pictures for the purpose of medical use, so that the direct conversion image detection device is now drawing attention.
FIG. 9
is a schematic block diagram showing an overall system utilizing the image detection device. High voltage is supplied from a high voltage generating section
62
to an X-ray source
51
. X-rays irradiated from the X-ray source
51
penetrate through a specimen
52
and enter photoelectric converting elements of an a-Si TFT image detection device
53
. The a-Si TFT image detection device
53
converts the X-rays penetrated through the specimen
52
into an analog electrical signal corresponding to a dosage at the incident position of the photoelectric converting elements. The converted analog signal is then digitized by an A/D converter
57
to be stored in an image memory
58
in a time series manner.
The image memory
58
is capable of storing image data of one or several pictures sequentially at predetermined addresses under the control of control signals from a control section
63
. An arithmetic processing section
59
implements arithmetic processing on the image data by taking it out of the image memory
58
so that it can be displayed appropriately and stores the result thereof again in the image memory
58
. The processed image data in the image memory
58
is then converted into an analog signal by a D/A converter
60
. This analog signal is output to an external processing circuit such as an image monitor
61
via an interface. Accordingly, an X-ray penetration image of the specimen
52
is displayed, for example, on the image monitor
61
. The control section
63
also controls the high voltage generating section
62
.
FIG. 10
is a schematic diagram showing the structure of the image detection section of the direct conversion image detection device using the a-Si TFTs. As shown in
FIG. 10
, pixels e, each of which is a unit element composing an image detection area, are arrayed in a matrix of 2000 (H)×2000 (V) for example (hereinafter referred to as a thin film transistor array).
Bias voltage is applied to a photoelectric converting film
140
from a power source
148
. A drain of an a-Si TFT
144
is connected to a signal line
113
. A gate of the a-Si TFT
144
is connected to a scan line
118
. ON/OFF of the a-Si TFT
144
is controlled by potential of a scan signal applied from a scan line driving circuit
152
to a gate electrode via the scan line
118
. A terminal end of the signal line
113
is connected to an amplifier
154
such as a sense amplifier for detecting signals.
FIGS.
11
(
a
) and
11
(
b
) are schematic diagrams showing a basic structure of the direct conversion image detection device, wherein FIG.
11
(
a
) is a schematic section view of the image detection device and FIG.
11
(
b
) is a diagram schematically showing an equivalent circuit of the unit pixel of the image detection device. Each pixel e comprises the thin film transistor
144
composed of the semiconductor film of a-Si, the photoelectric converting film
140
and a signal storage capacitor element Cs.
When light (X-rays, soft X-rays or the like in this case) enters the photoelectric converting film
140
, a current flows through the photoelectric converting film
140
and electric charge is accumulated in a pixel capacitor Cs. When the scan line driving circuit
152
drives one scan line to turn ON all TFTs connected to the scan line, the accumulated charge is transferred to the amplifier
154
side via the signal line
113
. An output amplitude of the amplifier
154
also changes corresponding to a difference in amounts of charge caused by a difference in quantities of light entering the pixels. The method shown in
FIG. 10
allows a digital image to be obtained directly by converting the output signal of the amplifier
154
from analog to digital form.
The pixel shown in FIGS.
11
(
a
) and
11
(
b
) has a similar structure with an active matrix liquid crystal display using thin film transistors as switching elements (TFT-LCD), which is used for a notebook (laptop) personal computer and the like. Accordingly, a thin and large screen image detection device may be readily fabricated.
However, it is necessary to form the photoelectric converting film as thick as about several hundreds &mgr;m to several mm in the direct conversion image detection device in order to improve the efficiency of the conversion from X-rays to electric charges. Then, in order to apply an appropriate electric field to the photoelectric converting film, a high voltage of about several kV must be applied on the both sides of the photoelectric converting film.
As shown in FIGS.
11
(
a
) and
11
(
b
), the image detection area of the direct conversion image detection device includes the a-Si TFT
144
, a storage capacity electrode
202
, a pixel electrode
203
, an insulating layer
204
, the photoelectric converting film
140
and a voltage supplying wire
205
disposed on an insulated substrate
201
. Also provided are the signal line
113
and the scan line
118
as shown in FIG.
10
.
The signal storage capacitor element Cs is formed by pinching the insulating layer
204
between the
Kinno Akira
Suzuki Kouhei
Taira Kazuki
Gagliardi Albert
Hannaher Constantine
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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