X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
1998-08-19
2001-06-12
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S112000
Reexamination Certificate
active
06246746
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an x-ray examination apparatus comprising an x-ray image sensor matrix for deriving an initial image signal from an x-ray image, a correction unit for deriving a corrected image signal from the initial image signal.
2. Description of Related Art
Such an x-ray examination apparatus is known from the European patent application EP 0 642 264.
The correction unit of the known x-ray examination apparatus counteracts disturbances in the initial image signal which are caused by delayed emission of charges from the x-ray image sensor matrix. Incident x-rays release electric charge carriers, i.e. photocharges, notably photoelectrons, in the x-ray image sensor matrix and these electric charges are detected. The signal levels of the initial image signal represent the detected charges. A part of the charge carriers can be trapped in a trap-state, be retained in such a trap-state, and can escape from the trap-state at a later stage and are detected as electric charges with a delay. If the initial image signal with the disturbances were applied to a monitor for displaying the image information, not only the image information of the instantaneous image would be reproduced, but at the same time also image information of a previously picked-up image. As a result after-images would be displayed together with the instantaneous image.
The correction unit of the known x-ray examination apparatus utilises an intricate mathematical model based on physical considerations for the trapping and subsequent release of charge carriers so as to correct disturbances due to charge carriers emitted in a delayed fashion. A drawback of the correction unit of the known x-ray examination apparatus is that the computations required to obtain the corrected image signal are rather complicated so that a powerful arithmetic unit is required which nevertheless requires a rather long computation time. Hence, the time between forming the x-ray image and displaying the image information is relatively long so that the known x-ray examination apparatus is not well suitable for imaging rapid dynamic processes in which variations occur within a period of time comparable to the required computation time. Moreover, the required programming of the arithmetic unit is complicated, so that highly skilled staff is needed to set up the correction unit.
SUMMARY OF THE INVENTION
An object of the invention is to provide an x-ray examination apparatus which requires, in comparison with the known x-ray examination apparatus, a shorter period of time to derive the corrected image signal from the initial image signal.
This object is achieved by means of an x-ray examination apparatus according to the invention which is characterised in that the correction unit includes a memory for storing correction values and an arithmetic unit for computing signal levels of the corrected image signal from signal levels of the initial image signal and at least some of said correction values.
The correction values are obtained from a separate calibration of the x-ray examination apparatus. This calibration involves irradiating the x-ray image sensor matrix with a predetermined calibration x-ray exposure. In particular, this predetermined calibration x-ray exposure is carried out by way of applying a pre-selected number of x-ray pulses with a pre-selected pulse-length, pulse-rate and x-ray dose per pulse. Subsequent to the calibration x-ray exposure the x-ray image sensor is read-out so as to obtain a calibration image signal. That is, the charge carriers in respective sensor elements of the x-ray sensor matrix after the predetermined calibration x-ray exposure are detected. The period of time clapsing between the calibration x-ray exposure and the generation of the calibration image signal is recorded or controlled. The calibration image signal represents the electric charges emitted during read-out after said period of time since the predetermined calibration x-ray exposure.
The physical process of trapping and releasing of electric charges in the x-ray image sensor matrix is represented by a mathematical model containing a small number of model parameters. This mathematical model has been disclosed in more detail in the article ‘
Measurements and simulation of the dynamic performance of an &agr;
-
Si:H image sensors
’ in the Journal of Non-crystalline solids Vol.164-166(1993)781-784. Values for the model parameters are derived from the calibration image signal, in particular by performing a best fit to the mathematical model. On the basis of the values of the model parameters there are computed correction values which represent delayed electric charges for separate x-ray exposure circumstances and for arbitrary periods of time lapsed since the latest x-ray exposure. This computation of the correction values can be performed separately from the x-ray examination apparatus, but the arithmetic unit of the x-ray examination apparatus itself can also be used. Sets of correction values are stored which relate to various x-ray exposure circumstances. In particular, sets of correction values are stored for separate values of the number of preceding x-ray pulses, the x-ray pulse rate, respective intensities of the preceding x-ray pulses. Each set includes correction values for several values of the time lapsed since the latest x-ray pulse.
Instead of calculating correction values for various periods of time lapsed since the latest x-ray exposure by way of the mathematical model, such correction values can be derived from a calibration signal sequence. The calibration signal sequence is formed by reading out the x-ray image sensor matrix at several instants after the latest x-ray exposure. The signal levels of the calibration signal sequence represent the decay of trapped charges as time proceeds. Hence, the correction values for separate periods of time lapsed since the latest x-ray exposure can be derived from the calibration signal sequence without calculations based on the mathematical model.
The arithmetic unit computes the signal levels of the corrected image signal from the signal levels of the initial image signal and the correction values. Only relatively simple computations are required such as subtracting the correction values from the signal levels of the initial image signal. Because the computations involving the mathematical model of the trapping and release of charges need only to be carried-out once for the calibration image signal, for correcting the initial image signal only simple calculations are required. The simplicity of the required calculations is achieved at the expense of storing a number of correction values which is substantially larger than the small number of model parameters.
Since retrieving correction values from the memory can be done very rapidly and because only simple calculations are required, it takes only a short time to derive the corrected image signal after the initial image signal has been read out. Moreover, electronic memories having a large storage capacity are commercially available and are relatively inexpensive. The x-ray examination apparatus according to the invention is suitable to handle x-ray images at a rate of 25-30 or even 60 images per second. Therefore, the x-ray examination apparatus according to the invention is especially suitable for real time x-ray imaging for instance in interventional procedures. In particular after images are suppressed when fluoroscopy is performed by continuously irradiating at low x-ray dose after one or a few brief x-ray exposures at high-dose have been carried-out.
A preferred embodiment of x-ray examination apparatus according to the invention is characterised in that the correction unit includes a selection unit for selecting correction values from the memory on the basis of exposure parameters.
The selection unit selects correction values which pertain to an x-ray exposure which has preceded the formation of the x-ray image. The preceding x-ray exposure has caused trapped charges which give ri
Conrads Norbert
Schiebel Ulrich
Weibrecht Martin
Wieczorek Herfried K.
Church Craig E.
U.S. Philips Corporation
Vodopia John F.
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