X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
1999-04-26
2001-06-05
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S098700
Reexamination Certificate
active
06243440
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a radiographic apparatus for use in medical radiography or industrial nondestructive inspection, for example.
(2) Description of the Related Art
Conventionally, plain radiographic apparatus employing X-ray film and imaging plates have been used in mass medical examinations for tuberculosis and lung cancer. In recent years, apparatus have been developed which enable real-time reading by means of two-dimensional X-ray sensors (hereinafter referred to as flat panel sensors) (e.g. Japanese Patent Publication (Unexamined) S59-211263 and Japanese Patent Publication (Unexamined) H2-164067).
Flat panel sensors include the direct conversion type and indirect conversion type. A flat panel sensor of the direct conversion type has a plurality of switching elements (TFT switches) arranged in a matrix form on a flat panel, with a semiconductor layer superposed thereon. X-ray photons are converted into electric signals in the semiconductor layer to output a fluoroscopic image in electric digital signals. A flat panel sensor of the indirect conversion type has photodiodes and switching elements connected together and arranged in a matrix form, with a layer of scintillator (fluorescent substance) superposed on this photodiode array (semiconductor layer). X rays incident on the scintillator generate optical signals which are converted into electric signals in the photodiodes.
In these flat panel sensors, gate lines common to the respective columns are connected to the switching elements of pixels. A drive signal transmitted to the gate lines turns on the switching elements in the respective columns, whereby charge signals of the pixels are outputted from data lines common to the respective rows. The charge signals outputted are inputted to a signal reading circuit. After a charge-to-voltage transduction and amplification, the signals are put to an analog-to-digital conversion pixel by pixel, and then inputted to an image processing device or the like.
Conventionally, the plain radiographic apparatus employs a separate phototimer in order to avoid underexposure and overexposure. The phototimer has a semiconductor X-ray sensor disposed opposite a front surface of X-ray film (X-ray incidence surface). At an actual radiographing time, the phototimer monitors X-ray doses incident on the X-ray sensor, and a time for terminating the radiography is determined by checking whether an integrated value thereof has exceeded a predetermined value or not.
Such a method, however, poses a problem that the shadow of the phototimer falls on X-ray film. Where the phototimer is formed so thin as not to appear on radiographic images, a required level of X-ray sensitivity cannot be secured.
A proposal has been made recently to achieve an optimal radiography of the same purport with the flat panel sensor developed (e.g. Japanese Patent Publication (Unexamined) H7-72259). The gist of the invention described in this publication lies in frequently taking and adding readings from a predetermined monitor pixel during X-ray irradiation, and blocking X rays when the sum reaches a predetermined quantity.
However, the invention described in Patent Publication H7-72259 has a disadvantage that a reading operation eliminates all data relating to a column that shares a gate line with the monitor pixel. It is conceivable to store the data in a memory to use the data afterward as a radiograph. However, in an actual situation, there occurs a time drift or a lag in reading time, which results in a difference in image quality between the column including the monitor pixel from which data are read frequently, and the other columns of ordinary pixels.
SUMMARY OF THE INVENTION
This invention has been made having regard to the state of the art noted above, and its object is to provide a radiographic apparatus for acquiring radiographic images with a two-dimensional X-ray sensor, which is capable of performing radiography with optimal exposure and acquiring images of uniform quality at all times.
The above object is fulfilled, according to this invention, by a radiographic apparatus using a two-dimensional radiation sensor, comprising:
a radiation generator for irradiating an examinee;
the two-dimensional radiation sensor having detecting elements in a two-dimensional array for converting radiation transmitted through the examinee into charge signals, and switches connected to the detecting elements, respectively;
a gate driving circuit for successively driving, column by column, the switches connected to the detecting elements of the two-dimensional radiation sensor;
a signal reading circuit for transducing to voltages, amplifying and digitizing electric charges outputted from data lines of the two-dimensional radiation sensor;
a control circuit for controlling the gate driving circuit and the signal reading circuit;
a sequence controller for controlling a radiation generating sequence of the radiation generator to perform a monitoring radiography and a subsequent, production radiography with respective radiographic conditions; and
a radiographic condition computing unit for computing a radiographic condition for the production radiography based on a ratio between a quantity of charges collected from the two-dimensional radiation sensor through the signal reading circuit in time of the monitoring radiography and a desired quantity of charges in time of the production radiography, and applying the radiographic condition for the production radiography at least to the sequence controller.
The radiographic apparatus according to this invention determines a radiographic condition for production radiography based on a ratio between a quantity of charges collected from the two-dimensional radiation sensor in time of monitoring radiography and a desired quantity of charges in time of the production radiography. Thus, the production radiography is carried out constantly under an optimal radiographic condition. Moreover, in this invention, information (i.e. the charges collected) for determining a radiographic condition is collected through the monitoring radiography preceding the production photography. The invention does not execute a process in time of production radiography to acquire information from a particular detecting element to determine a radiographic condition. Thus, there occurs no lowering of a signal level with a column including the particular detecting element, thereby assuring an image of uniform quality.
The radiographic condition computing unit may compute an irradiating period or a tube current of the radiation generator as the radiographic condition for the production radiography.
Though not limitative, the following techniques may be adopted for determining the radiographic condition for the production radiography:
(1) Regarding, as the quantity of charges collected, an integrated value of charges of those of the detecting elements present in a region of interest among the charges of the detecting elements of the two-dimensional radiation sensor acquired through the monitoring radiography, and computing the irradiating period based on a ratio between the quantity of charges collected and an integrated value of a desired quantity of charges in the region of interest in time of the production radiography; and
(2) Regarding, as the quantity of charges collected, a charge of one of the detecting elements having a maximum value in a region of interest among the charges of the detecting elements of the two-dimensional radiation sensor acquired through the monitoring radiography, regarding, as the desired quantity of charges, a maximum charge within a range where the two-dimensional radiation sensor has linear input/output characteristics, and computing the irradiating period based on a ratio between the quantity of charges collected and the desired quantity of charges,
With the latter technique (2) in particular, all the pixels in the region of interest obtained through the production radiography are those present within the range where the two-dimens
Adachi Susumu
Oikawa Shiro
Takemoto Takayuki
Yamane Yasukuni
Arent Fox Kintner & Plotkin & Kahn, PLLC
Porta David P.
Shimadzu Corporation
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