X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
2000-07-28
2002-09-17
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S098200, C378S207000, C250S370090
Reexamination Certificate
active
06453008
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-215214, filed Jul. 29, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a noise reduction method for a radiation detector for converting radiation such as X-rays into an electrical signal in accordance with its intensity.
A planar type radiation detector has a plurality of pixels arrayed in the form of a matrix. Each pixel has a photoelectric conversion element and pixel electrode. When radiation is incident on the photoelectric conversion element, the photoelectric conversion element generates charge in an amount corresponding to the incident intensity. This charge is stored in a capacitor through the pixel electrode. The stored charge is read out from the capacitor through a readouting section.
FIG. 1
is a view showing a typical example of the arrangement of a conventional radiation detector. Referring to
FIG. 1
, a plurality of pixel electrodes
71
arranged in the form of a two-dimensional matrix acquire charge generated in photoelectric conversion films in accordance with the intensity of radiation that has passed through an object and struck on the films. A capacitor used as a charge storage element for storing the acquired charge (pixel charge) is connected to each pixel electrode
71
. The pixel charge stored in the each capacitor is read out through a thin-film transistor (TFT)
72
.
A gate line driver
74
selectively applies a gate voltage to a gate line
73
to turned on the gates of the TFTs
72
. A plurality of TFTs
72
connected to the selected gate line
73
are simultaneously turned on. Capacitor charges on the same row are read out as electrical signals (to be referred to as detection signals hereinafter) to amplifiers
76
through signal lines
75
. Thereafter, the amplified detection signals are sequentially sent to an A/D converter
78
through a multiplexer
77
.
Note that a layer on which the above gate lines
73
are laid to be parallel to each other and a layer on which the signal lines
75
are laid to be parallel to each other are overlaid in a direction perpendicular to the drawing surface of FIG.
1
through an insulating layer. That is, the gate lines
73
and signal lines
75
are formed on different layers so as not to be short-circuited.
In general, such radiation detectors digitize radiation images and hence are very advantageous in terms of, for example, transmission, storage, and search of radiation images as compared with conventional radiation photographic films. It is expected that these detectors will become more popular. The above radiation detector designed to directly digitize radiation has the merit of easily obtaining digital images as compared with a conventional film digitizer scheme and the like.
In the above conventional planar type radiation detector, noise components that become “disturbance factors” are superimposed on detection signals. In general, this makes it difficult to obtain accurate object information. In this case, as the “disturbance factors”, the following two factors are conceivable.
The first factor is associated with a “dark image”. It is known that in the photoelectric conversion film provided for the pixel electrode
71
, a current generally called a “dark current” is generated owing to, for example, the random thermal agitation of free electrons even when no radiation is incident. In addition, in general, an offset noise voltage is always observed in the amplifier
76
. These dark currents and offset noise voltages are finally constructed into images through the signal lines
75
and amplifiers
76
, thereby forming “dark images”.
For this reason, an image constructed on the basis of a detection signal having undergone no correction processing is the one obtained by superimposing the above dark image on a normal desired image. In order to obtain an accurate image, therefore, information associated with the dark image must be subtracted from the overall information.
Such an inconvenience has been recognized in the conventional scheme as well, and hence a method of acquiring correction information associated with a dark image in advance and subtracting it from detected information has been proposed. In general, however, the above dark current and offset noise voltage vary with temperatures, and the dark image changes accordingly. For this reason, the above method is not very effective. That is, since correction information is fixed, this method cannot cope with an actual condition (temperature) that incessantly changes over the operation time of the radiation detector and the like.
In addition, dark images change depending on the method of driving the gate lines
73
and TFTs
72
. This is because the TFT
72
is not an ideal switching element, which has finite resistances in both ON and OFF states. This characteristic poses the following problem. A current or charge information that should be obtained in accordance with an array driving sequence dissipates or unnecessary components are added thereto. Consider a general sequence as a driving sequence, in which the ith gate line
73
is driven (the TFTs
72
on this line are turned on) to extract charge information from the corresponding pixel electrodes
71
, and then the (i+1)th gate line
73
is driven at the same time when the driving of the ith gate line
73
is stopped (TFTS
72
are turned off). Current dissipation and addition may occur in the following two cases.
In the first case, a current dissipates as the TFTs
72
on the ith line are turned on and off. This is because the potential of the signal line
75
is equal to that of the pixel electrode
71
when the TFT
72
is turned on, but the potential of the pixel electrode
71
drops when the TFT
72
is turned off. This will be described in detail below. As shown in
FIG. 2A
, when the TFT
72
is ON, the charge given by
Q=Cgs·Von
is stored in a capacitor (capacitance Cgs) assumed to be formed on the source side of the TFT
72
. At this time, as obvious from
FIGS. 1 and 2A
, the node of a capacitor (capacitance Cpx) provided for the pixel electrode
71
and the capacitor Cgs is grounded (GND level). Considering that the node is disconnected from the GND level when the TFT
72
is turned off, and charge Q stored in the capacitor Cgs is distributed into the capacitors Cpx and Cgs, the following relation is established (refer to FIG.
2
B):
−
Q=−Q′+Q″
V
off−
Q′/Cgs−Q″/Cpx
=0
where Q′ is the charge of the capacitor Cgs, and Q″ is the charge of the capacitor Cpx. From the above three equations, the charge Q″ stored in the capacitor Cpx can be given by
Q″=−C
′·(
V
on−
V
off)
where C′=Cpx·Cgs/(Cpx+Cgs).
In this state, the potential of the capacitor Cpx, i.e., a potential V of the pixel electrode
71
, is given by
V=Q″/Cpx
=−(
C′/Cpx
)·(
V
on−
V
off)
and Von>Voff generally holds. Therefore, V<0. That is, when the TFT
72
is turned off, the potential drops.
As described above, when the potential of the pixel electrode
71
drops as the TFT
72
on the ith line is turned on/off, a voltage is applied between the source and drain of the TFT
72
. Obviously, a current is generated in this portion. As a consequence, excess charge is stored in the pixel electrode
71
. When the ith gate line
73
is driven again, this excess charge information is additionally read out. This makes it impossible to obtain accurate information. Note that such charge addition will be referred to as “first type offset noise” hereinafter for the sake of convenience.
In the second case, assuming that the (i+1)th gate line
73
is driven, all the remaining gate lines
73
, as well as the ith gate line
73
, are not driven. In this state, it is expected that all the charge information stored in the pixel
Nagai Seiichiro
Sakaguchi Takuya
Bruce David V.
Hobden Pamela R.
Kabushiki Kaisha Toshiba
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