Radiant energy – Source with recording detector
Reexamination Certificate
2001-07-03
2003-01-07
Hannaher, Constantine (Department: 2878)
Radiant energy
Source with recording detector
C250S591000, C378S028000
Reexamination Certificate
active
06504166
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for recording and/or reading image information by using a photoconductor which generates electric charges when exposed to an electromagnetic wave such as a radiation and light.
2. Description of the Related Art
Conventionally, a number of systems are proposed for recording and/or reading image information by employing a photoconductor realized by an organic or inorganic amorphous semiconductor material, and utilizing the property of exhibiting electric conductivity (i.e., generating pairs of electric charges in the photoconductor) when the photoconductor is exposed to electromagnetic waves. For example, the above systems are disclosed in the coassigned U.S. Ser. No. 09/792,035 corresponding to Japanese Patent Application No. 2000-50201, the coassigned U.S. Ser. No. 09/534,204 corresponding to Japanese Patent Application Nos. 2000-50202, 2000-50203, 2000-50204, 2000-50205, and 11(1999)-79984, the coassigned U.S. Ser. No. 09/136,739, now U.S. Pat. No. 6,268,614, corresponding to Japanese Unexamined Patent Publication No. 2000-105297, the coassigned U.S. Ser. No. 09/539,412 corresponding to Japanese Unexamined Patent Publication No. 2000-284056, the coassigned U.S. Ser. No. 09/538,479 corresponding Japanese Unexamined Patent Publication No. 2000-284057, the U.S. Pat. Nos. 5,648,660, 5,661,309, and 4,535,468, Japanese Unexamined Patent Publication No. 9(1997)-206293, and Medical Physics, Vol. 16, No.1, January/February 1989, pp.105-109.
The recording systems for recording image information have a laminated structure in which a photoconductor is sandwiched between two electrodes, and a charge storing portion is provided for storing charges generated in the photoconductor. When the photoconductor is exposed to electromagnetic waves (or recording light) carrying image information while applying a voltage between the two electrodes so as to produce an electric field in the photoconductor, pairs of charges are generated in the photoconductor, and latent-image charges in the generated pairs are stored in the charge storing portion. Thus, the image information is recorded as a latent image.
On the other hand, in the reading systems for reading information, charges are generated in a photoconductor when the photoconductor is exposed to electromagnetic waves carrying image information while applying an electric field to the photoconductor, and the image information is read by detecting the generated charges, i.e., detecting currents produced by the generated charges. The electromagnetic wave are, for example, X rays which have penetrated through an object, or accelerated phosphorescence light emitted from a stimulable phosphor sheet used as an image recording medium.
However, in the case where an amorphous material such as a-Se (amorphous selenium) is used in each of the above photoconductors, charges are directly injected from the electrodes located on both sides of the photoconductor into the photoconductor from the beginning of application of a voltage (which is generally high) between the electrodes until short-circuiting of the electrodes. A portion of the injected charges is trapped as space charges in the photoconductor or at the interfaces between the photoconductor and the electrodes, and the other portion of the injected charges is not trapped, and output from the photoconductor as a leakage current. Thus, a dark current flows in the photoconductor.
In the recording systems, unnecessary charges caused by the dark current are accumulated in the charge storing portion. Therefore, a dark latent image, which is produced by the unnecessary charges, and does not carry true image information, is superimposed on a true latent image corresponding to the true image information. Thus, when the charges stored in the charge storing portion is read after the recording operation, the dark latent image produced during the recording operation appears as dark latent image noise in a regenerated image.
On the other hand, in the reading systems, the dark current flowing in the photoconductor is superimposed on a true current component carrying the true image information. Therefore, the dark current flowing in the photoconductor during the reading operation also appears as dark latent image noise in a regenerated image.
In particular, since the quantum efficiency of the photoconductor with respect to X rays is low, the amount of charges generated by direct exposure to X rays which have penetrated through an object is very small. In addition, since the accelerated phosphorescence light is very weak, the amount of charges generated by exposure to the accelerated phosphorescence light is also very small. Therefore, in these cases, when the dark current is large, the S/N ratio decreases seriously.
If the dark current can be reduced, the influence of the dark latent image noise is also reduced, and the decrease in the S/N ratio can be prevented. However, in order to reduce the dark current, the dark resistance must be increased. For example, in the case where a detector which includes an a-Se photoconductor having a thickness of 500 micrometers is exposed to a 10 mR dose of radiation having energy of 80 keV for one second, the magnitude of the dark current must be reduced to 10 pA/cm
2
or less in order to reduce the influence of the dark latent image to an ignorable degree. In order to achieve such reduction of the dark current, the dark resistance must be increased to a very great value as much as 10
15
&OHgr;.cm or more when an electric field of 10 V/&mgr;m is applied to the photoconductor.
Although a-Se is usable under the dark resistance of 10
15
&OHgr;.cm in the electric field of 10 V/&mgr;m, the dark resistance of 10
15
&OHgr;.cm is insufficient to achieve a satisfactory S/N ratio in the regenerated image in the case where recording or reading is performed by direct exposure of the photoconductor to X rays or exposure of the photoconductor to accelerated phosphorescence light. Therefore, a higher dark resistance is required. Conventionally, increase of the dark resistance by appropriate selection of a material for the electrodes is proposed, for example, by R. E. Johanson et al. (“Metallic Electrical Contacts to Stabilized Amorphous Selenium for Use in X-ray Image Detectors,” Journal of Non-Crystalline Solids, Vol. 227-230 (1998) pp. 1359-1362). In addition, increase of the dark resistance by arrangement of an appropriate blocking layer between the photoconductor and the electrodes (which is made of, for example, a-Se) is proposed, for example, by B. Polischuk et al., (“Selenium Direct Converter Structure for Static and Dynamic X-ray Detection in Medical Imaging Application,” Proceedings of the SPIE Conference on Physics of Medical Imaging, February 1998, SPIE Vol. 3336, Paper #: 3336-51).
Conventionally, the strength of the electric field which is most often applied to the photoconductor is 10 V/&mgr;m. However, when the strength of the electric field of the photoconductor is increased beyond 10 V/&mgr;m in order to increase the quantum efficiency and sensitivity by causing the avalanche amplification, the dark current often increases more than the true signal component, and therefore the SIN ratio decreases.
On the other hand, the dark current has a characteristic that a very large, momentary charging current first flows at the beginning of the application of an electric field. Thereafter, a transient current (absorption current) flows, where the absorption current gradually decreases with time to a constant leakage current. In other words, the dark resistance at the beginning of the application of the electric field is smaller than the dark resistance in a high resistance state, in which the stabilized low leakage current flows. The higher the application voltage is, the more pronounced the above phenomenon is. As disclosed in the R. E. Johanson reference, it takes a relatively long time to reach a stable, high resistance state after voltage application. For example, it takes usu
Fuji Photo Film Co. , Ltd.
Hannaher Constantine
Moran Timothy
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