X-ray detector having a large dynamic range

Radiant energy – Invisible radiant energy responsive electric signalling

Reexamination Certificate

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C250S370110

Reexamination Certificate

active

06759658

ABSTRACT:

The invention relates to an X-ray detector which includes at least one conversion unit, at least one evaluation unit for the counting and integration of absorbed X-ray quanta, and also at least one data processing unit. The invention also relates to a method of evaluating the absorption signals of an X-ray detector which is preferably arranged so as to face an X-ray source in a computed tomography apparatus, and to an X-ray examination apparatus which includes an X-ray source for the emission of X-rays and an X-ray detector.
Absorbed X-ray quanta are converted into electric charge signals in X-ray detectors and X-ray imaging systems. X-ray detectors of this kind are used, for example, in computed tomography (CT) in the medical field. Further applications of X-ray detectors can be found in the industrial field (for example, for the testing of materials) or in the field of security. Normally speaking, (dynamic) X-ray detectors then consist of:
a) a conversion unit or stage which absorbs a large part of the incident X-ray quanta so as to convert such quanta into electric charge signals,
b) an evaluation unit or stage in which the signals from the conversion unit are amplified and further processed, as well as
c) a data processing unit or stage which includes means for data acquisition, for control and for output of the acquired signals.
Two different processing concepts are customarily utilized in the evaluation unit. For example, on the one hand there are integrating evaluation units of the kind disclosed, for example, in EP 434 154 or EP 440 282. The charge signals which are prepared by these conversion units are integrated over a predetermined period of time, that is, the so-termed integration period. In the ideal case the result of this integration represents the energy deposited by the absorbed X-ray quanta during the integration period in the conversion unit.
There is also the concept of counting evaluation units (see P. Fischer et al., “A counting pixel readout chip for imaging applications”, Nucl. Instr. and Meth. A 405 (1998), pp. 53 to 59). Therein, the charge pulses produced by individual X-ray quanta in the conversion unit are individually acquired and counted. To this end, use is customarily made of a signal-forming amplifier which is succeeded by a comparator whose digital output signals are counted by a counter. In the ideal case the number of the charge pulses counted within a counting period thus corresponds exactly to the number of X-ray quanta absorbed in the conversion unit during this time. The advantage of the counting method resides in the fact that a very large counting range having in principle an ideal linearity can thus be covered. The counting range in principle is limited only by the depth of the counter used. This method, however, imposes a problem in that the count rates (that is, the number of counting pulses per unit of time) that can be reached are limited due to various idle time effects, in which no signals can be detected, and also by a limited bandwidth of the analog electronic circuitry.
In contrast therewith, the integration method also enables operation in the case of very high quantum flows. However, this method is rather limited in respect of usable dynamic range and linearity. For the integration method it is also known that, because of various afterimage effects, residues of preceding X-ray images may still be present in subsequent images. An example in this respect is the afterglow that is known to occur in many scintillators. Generally speaking, counting detectors are less susceptible to such afterimage effects, because the afterimage effects normally do not give rise to significant short-time charge pulses and hence do not falsify the counting results.
Furthermore, U.S. Pat. No. 4,591,984 discloses an X-ray detector in which the charge signals from a conversion unit can be subjected alternatively to counting or to integration. For counting the pulses are applied to a pulse height analyzer whose output is activated only when the charge signal exceeds a threshold value. The frequency of activation of the output can then be counted by a counter. Alternatively, however, the charge signals can also be applied to a low-pass filter in an integration circuit, the charges then being accumulated over a selectable period of time in a capacitor. When the integration result is to be evaluated, the capacitor is discharged with a constant current. For as long as the voltage of the capacitor remains higher than a reference voltage, the pulses of an external clock signal are applied to the previously mentioned counter in which they are counted. The larger the charge accumulated in the capacitor, the longer the discharge operation will last and the longer the voltage of the capacitor will remain higher than the reference voltage. As a result, a correspondingly larger number of pulses of the clock signal will be counted by the counter.
Either the output pulses of the pulse height analyzer or the conducted pulses of the external clock signal can be applied to the counter by switching over a switch in the X-ray detector which is known from U.S. Pat. No. 4,591,984. Depending on the position of the switch, therefore, the absorbed X-ray quanta are counted or the absorbed radiation energy is measured. Because the counting method is more exact in the case of small quanta flows while the integration method is more exact for higher quanta flows, the relevant better measuring method can be applied each time by choosing the appropriate switch position. The user then has to guess in advance which quanta flows are to be expected, so that the user can choose the appropriate switch position. However, this is a drawback since advance knowledge about the best method to be used is necessary. When the switch is set to the wrong position, the result of an X-ray operation cannot be evaluated by means of the optimum method. In given circumstances it may then be necessary to repeat the X-ray operation; this should be avoided at all costs in particular in the case of medical applications, because the patient is then exposed to an additional radiation load.
Considering the foregoing it is an object of the present invention to provide an improved X-ray detector, a method of evaluating the signals of an X-ray detector and an X-ray examination apparatus with an X-ray detector which enable optimum measurement in a wide dynamic range without necessitating presetting by a user.
This object is achieved by means of an X-ray detector as disclosed in the characterizing part of claim
1
as well as by means of a method as disclosed in the characterizing part of claim
8
. Advantageous further embodiments are disclosed in the dependent claims.
The X-ray detector in accordance with the invention includes the following elements:
a) at least one conversion unit in which X-ray quanta are absorbed so as to generate an electric charge signal whose magnitude corresponds to the absorbed energy,
b) at least one evaluation unit in which said charge signal from the conversion unit is processed in parallel in a counting channel and in an integrator channel, the counting output of the counting channel presenting a measure of the number of charge signals detected since the beginning of measurement, and the integrator output of the integrator channel presenting a measure of the overall charge detected since the beginning of measurement, which beginning of measurement for the counting channel is preferably being identical to the beginning of measurement for the integrator channel, be it that this is not absolutely necessary,
c) at least one data processing unit that processes the signals from the counter output and from the integrator output in combination so as to determine the absorbed quantity of X-rays.
The evaluation unit in the X-ray detector thus performs a counting method and an integration method in parallel for the charge signals generated; the results of these methods are used together in the data processing unit so as to determine an overall result for the absorbed quantity of X-rays. The a

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