The present invention relates to x-ray detectors and more particularly to methods for monitoring changes in the detective quantum efficiency of x-ray detectors.
X-ray systems have been used for some time for imaging and measuring objects, such as medical patients. X-ray beams projected from a source pass through the object or patient and are detected by an x-ray detector and then converted into a visible light image. High resolution solid state x-ray detectors are currently in use and are beneficial to the analysis of the patient. The digital x-ray detectors typically utilize two-dimensional arrays of photodiode detector elements which produce electrical signals corresponding to the brightness of a picture element in the x-ray image projected onto it. The signals from the detector elements are read out individually and digitized for further imaging processing, storage and display.
In order to insure consistent and accurate measurements from the x-ray detectors, it is necessary to periodically monitor the detectors in order to evaluate potential detector degradations. X-ray imaging detectors may change over time and it is necessary to measure and track changes in the detectors for maintenance and/or replacement when necessary. The detective quantum efficiency (DQE) is recognized as one of the important objective measures of the performance of an x-ray imaging detector. DQE is the measure of the ability of the detector to transfer signal-to-noise ratio from its input to its output. Currently, the ability to measure or calculate the quantities and factors necessary for a determination of DQE is complicated and time-consuming. Typically, measurements of DQE can only be satisfactorily performed by trained and skilled physicists.
There thus is a need for an easier and less complicated methodology or system for measuring or tracking changes in the detective quantum efficiency of an x-ray imaging detector.
The present invention provides an improved methodology for determining changes in the detective quantum efficiency of an x-ray imaging detector. The present invention also automatically measure changes in detective quantum efficiency within an x-ray detector without relying upon complicated sets of measurements.
With the present invention, a mathematical equation and relationship has been developed which meets the above-stated objectives and overcomes the problems currently existing with present DQE measurements. With the invention, the calculation of relative DQE changes is limited to the calculation and measurement of only two quantities, both of which can be achieved in an automatic fashion on a digital x-ray system. One of the quantities, namely the modulation transfer function (MTF), is obtained using an edge phantom made from a piece of x-ray absorbing material, or a linepair bar pattern that is positioned in front of the image detector. The measurement of the second quantity, namely the noise power spectrum (NPS), is secured from the use of flat field images and may be calculated from data obtained during system calibration.
The possible degradation of the detector and the potential need for field replacement can be determined by monitoring a DQE ratio as a function of time. This DQE ratio is obtained from ratios of MTF and normalized NPS. A computer stores the MTF and NPS quantities when they are measured at system calibration and over time. When the MTF and NPS data are acquired, the computer then directly calculates the DQE change at certain frequencies. If the DQE falls below a specification limit, a warning is displayed to the operator or field engineer.
Other objects, benefits, and features of the present invention will become apparent after review and analysis of the following description of the invention, when taken in accordance with the attached drawings and appended claims.