Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-03-19
2001-11-27
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370070
Reexamination Certificate
active
06323490
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray semiconductor detector suitable for a medical X-ray diagnostic apparatus.
In recent years, in the medical fields, medical data of patients have been formed into a database for quick, proper treatment. The patient often uses a plurality of medical institutes. In this case, the patient may not receive proper treatment without data of other medical institutes. For example, the patient may suffer affective reaction with drugs administered by other medical institutes. The patient must be properly treated in consideration of drugs administered by other medical institutes.
Demands have arisen for a database of X-ray photographing image data and digital X-ray photographing images. A conventional medical X-ray diagnostic apparatus uses a silver chloride film. To digitize an X-ray image formed on the silver chloride film, the image on the developed film must be converted into an electrical signal by the scanner. This is very cumbersome and time-consuming.
Recently, digital image data can be obtained by directly photographing an X-ray image with a CCD camera about one inch. However, when, e.g., lungs are to be photographed, an optical device for focusing light is required to photograph an area about 40 cm×40 cm, which makes the apparatus large. Further, a resolution is lowered by a conversion of a optical system.
To eliminate the time-consuming, cumbersome processing, downsize the apparatus and improve a resolution, an X-ray semiconductor detector using an amorphous silicon thin-film transistor (a-Si TFT) is proposed (U.S. Pat. No. 4,689,487).
FIG. 1
shows an example of the arrangement of this X-ray semiconductor detector.
In
FIGS. 1 and 2
, a pixel e
1,1
is made up of an a-Si TFT
9105
, a photosensitive film (photoconductor film)
9101
, and an storage capacitor
9103
. Pixels e are laid out in an array (to be referred to as a TFT array hereinafter) made up of several hundred to thousand pixels on each of line and column sides.
The photosensitive film
9101
receives a bias voltage from a power supply
9109
. The a-Si TFT
9105
is connected to a signal read line S
1
and a gate line Gl, and turned on/off under the control of a gate electrode driver
9113
. The terminal end of the signal read line Si is connected to a signal detection amplifier
9115
via a signal read TFT
9107
.
When light is incident, a current flows through the photosensitive film
9101
to storage charges in the storage capacitor
9103
. The gate electrode driver
9113
drives the gate line to turn on all TFTs connected to one gate line, and then the stored charges are transferred toward the amplifier
9115
via the signal read line S
1
. The signal read TFT
9107
inputs the charges to the amplifier
9115
in units of pixels, and the amplifier
9115
converts the charges into dot-sequential signals so as to be displayed on the CRT or the like. The charge amount changes depending on the light quantity incident on pixels, and the output amplitude of the amplifier
9115
changes.
In the detector shown in
FIGS. 1 and 2
, an output signal from the amplifier
9115
can be directly A/D-converted into a digital image. The pixel area shown in
FIGS. 1 and 2
has the same structure as in a TFT-LCD (Thin-Film Transistor Liquid Crystal Display) adopted in a notebook personal computer, and can be easily formed into a low profile, small thickness, large-screen display.
The above description concerns an X-ray semiconductor detector of an indirect conversion type in which an incident X-ray is converted into a visible light by a phosphor or the like, and the converted light is converted into charges by the photoconductor or photosensitive film of each pixel. In addition, there is an X-ray semiconductor detector of a direct conversion type in which an X-ray incident on pixels is directly converted into charges.
The X-ray semiconductor detector of this direct conversion type is different from that of the indirect conversion type in the magnitude of a bias voltage and the method of applying it to the charge conversion film. In indirect conversion, a negative bias of several V is applied to only the photosensitive film, and when light is incident on the photosensitive film, charges are stored in the storage capacitor arranged parallel to the photosensitive film and a capacitance Csi of the photosensitive film itself in each pixel. In this case, the maximum voltage applied to the storage capacitor is the several-V bias applied to the photosensitive film. To the contrary, in direct conversion-type, the X-ray/charge conversion film and storage capacitor are series-connected to each other, and receive a high bias of several kV. When an X-ray is incident on pixels, charges generated in the X-ray/charge conversion film are stored in the storage capacitor. If the quantity of incident X-ray is excessively large, charges stored in the storage capacitor increase to apply a voltage of several kV at maximum to the storage capacitor, causing dielectric breakdown of a TFT formed as a pixel switch or the storage capacitor.
For this reason, direct conversion must adopt any measure to protect the storage capacitor from an excessive voltage. For example, as shown in
FIGS. 3 and 4
(Denny L. Lee etc., SPIE, Vol. 2,432, p. 237, 1995), a dielectric layer (insulating layer) is formed on the X-ray/charge conversion film to series-connect three capacitors (dielectric layer Cd, X-ray/charge conversion film Cse, and storage capacitor), and charges generated in the X-ray/charge conversion film are partially stored in the capacitance formed by this dielectric layer to prevent dielectric breakdown of the TFT. As shown in
FIG. 5
, when an excessive quantity of X-ray is incident on pixels, a necessary amount of generated charges is stored in the storage capacitor, and the remaining charges are removed outside the pixel via a protective diode formed on each pixel, thereby preventing dielectric breakdown of the TFT.
In the example of
FIGS. 3 and 4
, no electrode layers for discharging charges are inserted between the X-ray/charge conversion film Cse and the dielectric layer Cd. Accordingly, resetting of Cd after receiving an image spends a relatively long time, so no moving picture can be obtained.
In the example of
FIG. 5
, since capacitors are not formed in series, unlike the example of
FIGS. 3 and 4
, the reset time of Cd is relatively short, and a fluoroscopy mode can be realized. However, when, e.g., a TFT is used as a protective diode, if the drain electrode on the opposite side to the pixel out of the terminals of the protective diode is set to a potential of 0V, i.e., connected to the electrode of the storage capacitor, the pixel potential (threshold voltage) at which removal of charges from the pixel starts becomes small (0 to 4V), the leakage current becomes large, and the TFT cannot be used as a protective diode. This problem can be solved by supplying a positive potential to the drain electrode. However, signal noise may increase or the yield of the TFT array may decrease depending on the layout of the power supply line (bias line) for applying the voltage.
As the number of TFTs used as protective diodes increases, the number of power supply lines increases, and the yield seriously decreases.
At the same time, since the occupation rate of the TFT and power supply line in the pixel increases, a pixel electrode serving as an effective area for the sensor of one pixel and the capacitance of the pixel are difficult to ensure.
In photographing a patient or the like, the X-ray intensity must be set as low as possible. To ensure a large dynamic range, even a weak signal is preferably detected.
The lower limit of this weak signal is determined by the OFF current of the protective diode, a signal shift by the stray capacitance, noise of the operational amplifier, and the like. Since another noise can be reduced by another appropriate means, a change in pixel potential by the leakage current of the protective diode finally determines the lowest detectable signal level of the wea
Atsuta Masaki
Ikeda Mitsushi
Kamiura Norihiko
Kinno Akira
Suzuki Kohei
Gabor Otilia
Hannaher Constantine
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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