X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
2001-03-27
2002-08-06
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S095000
Reexamination Certificate
active
06430258
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for operating a radiation examination device, in particular an X-ray-examination device, which includes a radiation source and a detector device for the acquisition of radiation images, the imaging dose and/or dose rate that is incident on a detector of the detector device being measured and a control valve for controlling the radiation source being determined while using said measured dose and/or dose rate. The invention also relates to a corresponding detector device as well as to a radiation examination device with a radiation source and a corresponding detector device for carrying out the method.
2. Description of Related Art
For operation of such radiation examination devices it is desirable that the imaging dose or dose rate that is incident on the detector during the irradiation is known as exactly as possible so as to enable the radiation source to be controlled in such a manner that an optimum amount of radiation for the relevant examination is emitted by the radiation source. This is important notably for medical radiation examination devices such as, for example X-ray diagnostic devices. The patient to be examined therein should be exposed to the minimum necessary X-ray dose only.
In the context of the present application the terms “radiation source” and “X-ray source” are to be understood to means the entire equipment emitting radiation used for the examination. The terms “dose” and “dose rate” are to be understood to mean the input dose or dose rate incident on the detector behind the object to be examined, for example the patient.
The measurement of the incident radiation during a radiation pulse is problematic notably when use is made of flat dynamic X-ray detectors (Dynamic Flat Panel X-ray Detectors). The information concerning the X-rays incident during an image can customarily be derived only from a preceding image, unless additional devices are used for measuring the dose or dose rate in an “on-line” fashion (that is, during the X-ray pulse).
For example, U.S. Pat. No. 5,194,736 discloses an X-ray examination apparatus which includes a sensor matrix where the residual currents occurring because of stray capacitances around the switching transistor of the relevant matrix element are used to measure the radiation dose. This measurement can be performed at option via the read-out line, utilizing signal amplifiers that are present any way, or via special amplifiers in the counter electrode. At least the radiation duration or the radiation intensity is controlled by means of a control unit in dependence on the radiation measurement thus performed. This method has a drawback in that the measuring zone is fixed in a sense that either complete columns of the detector matrix or predetermined, specially wired regions must be read out, because for reasons of cost it would not be efficient to associate a respective amplifier with each individual sensor element. Such measuring zones, however, usually do not correspond exactly to the relevant region of interest (ROI) during the respective examination.
Generally speaking, the term ROI is used to indicate the region within the image that is of special interest to the relevant examination. For example, in the case of an X-ray examination of a patient this is the image region in which the relevant organ to be examined is reproduced.
Also known are methods in which an ionization chamber is arranged in front of the detector itself, which ionization chamber is used to measure the dose rate. This does not offer an optimum possibility either for taking into account the specific ROI during the measurement, because the ionization chamber limits the ROI functionality.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved method of the kind set forth and corresponding devices for carrying out this method, enabling a simple, economical and effective control of the radiation source such that each individual image is formed as exactly as possible while utilizing the optimum radiation dose for the selected ROI,
This object is achieved by a method of the kind set forth which is characterized in that for each image of a measuring sequence of successive images acquired by the detector device an image correction value is determined in dependence on a selected image region of the detector device and an adaptive correction value is determined while using said image correction value and the image correction values of the preceding images in the measuring sequence, the control value for controlling the radiation source being derived from the measured dose and/or dose rate while utilizing said adaptive correction value.
The additional adaptive correction value compensates the measuring errors of the device for measuring the dose or the dose power to a high degree; the dependency on a selected image region thus enables the ROI to be taken into account for the correction so that the ROI is taken up in the control value for controlling the radiation source. Because of the adaptive method, all image correction values of all preceding images are used within a measuring sequence. Compensation is thus made for the fact that the image correction value can be determined only after the formation of an image and hence becomes available for subsequent images only, so that it is not possible to determine the image correction value during the irradiation for direct control of the radiation source. Furthermore, compensation takes place in that the adaptive correction value is combined with the instantaneously measured dose or dose rate.
The method is particularly suitable for use in conjunction with dynamic flat panel X-ray detectors in which it is necessary to utilize said special devices for determining the dose or the dose rate. The invention, however, can also be used in principle in any other detector such as Static Flat Panel X-ray Detectors or imaging systems based on image intensifiers/TV chains in which, for example, information concerning the radiation intensity can be acquired via the photosensor during the X-ray exposure.
In a particularly advantageous embodiment for each image acquired first a working point of the detector device is determined from the ratio of a mean image output signal within the selected image region to a maximum image output signal of the detector device. This image working point of the ROI is indicated in relation to the maximum image output signal value. The image correction value is determined while utilizing said working point.
The ratio of the working point to the dose incident on the entrance face of the detector is determined by the so-called transfer function of the detector device. This ratio of working point to incident dose is also dependent on the spectrum because of the spectral dependency of the detector system. During a calibration procedure, carried out by means of a defined calibration radiation spectrum, therefore, the dose value is determined for a so-called “nominal working point”. This calibration dose value is the so-called “dose nominal value”, that is, for this dose nominal value the nominal working point is obtained automatically on the detector, or within the ROI of the detector, during an exposure in conformity with the calibration spectrum. When real objects, or patients, to be examined are present in the path of the X-ray beam, however, it is to be assumed that the X-ray spectrum incident on the detector deviates from said special calibration spectrum and that, consequently, the dose actually incident on the detector deviates from the dose determined by means of the working point derived from the image.
Preferably, the working point determined for the relevant image is first multiplied by a nominal scaling factor in order to form a normalized working point. This nominal scaling factor is formed by the quotient of the nominal dose value and a selected dose value, that is, a dose value adjusted by the operating staff. Subsequently, the quotient of a nominal working point value an
Kim Robert H.
Koninklijke Philips Electronics , N.V.
Vodopia John
Yun Jurie
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