Scintillation detectors have previously been used for detecting gamma radiation and X-ray radiation, particularly in CT systems and dual-energy CT systems. In these detectors, the incident radiation is detected indirectly by the excitation of electrons and the conversion thereof into photons. Additionally, counting detector systems are being developed, in which individual photons of the incident radiation can be counted, and so there is direct detection of the radiation. In the process, an electrical pulse is generated, the area of which—and, approximately, the height of which as well—is proportional to the amount of charge and, thus, to the energy of the absorbed photon. Correct scanning and digitizing of the electrical pulses generated thereby, in particular resolving the number and height of the occurring pulses, is non-trivial.
However, a basic problem of using these direct-conversion detectors in CT systems lies in the handling of the high photon fluxes that have to be processed. These are generated in the successive signal pulses that, with a high probability, are incident almost simultaneously. This phenomenon, known as “pile-up”, leads to saturation or paralysis of the detector. Moreover, the response of the comparator changes in the case of high fluxes. The comparator, for example in the form of a continuously operating pulse-height discriminator, compares the input signal to a predetermined energy threshold and emits a corresponding output signal. In the process, there can be a rate-dependent shift in the effective energy threshold used to register the signals in the detector.
If the detector can be paralyzed as a result of its design, the measured signals can no longer be unambiguously assigned to the photon rate because fluxes that are too high are measured with a lower detection efficiency due to the higher degree of paralysis. This leads to a count result that is too low and the result can also be achieved by lower photon fluxes at a higher detection efficiency of the detector. Hence, the measurement result is ambiguous.
There already are a number of approaches for resolving this ambiguity problem. The laid-open application DE 10 2007 034 982 A1 illustrates an option for increasing the robustness in comparison with a variation in the pulse width in the case of a clocked signal scan by setting the scanning rate so high that, as a result of this, the scanning time-interval is less than the average expected pulse width. However, this method has not solved the problem of an optimum energy resolution in the case of low photon fluxes on the detector.
Moreover, a shortening of the pulse processing time, that is to say the charge collection time and the pulse conditioning time, and a reduction of the photon stream can reduce the probability of the “pile up” effect. This may possibly eliminate the ambiguity problem entirely. This approach is usually insufficient due to material-specific limitations in the detector material and application-dependent requirements.
Likewise, the ambiguity of the signal registration can be resolved by simultaneously measuring the signal flux using an integrating channel. However, this is accompanied by increased circuit complexity and increased amounts of data. This approach is described in, for example, the European patent document EP 1 231 485 A3.
The patent application with the application number DE 10 2008 005 373.2, which does not have a prior publication date, supplies a further approach for resolving the ambiguity. This application describes a method for determining a radiation intensity using a direct-conversion detector, wherein a continuously counting pulse-height discriminator and a pulse-height discriminator counting in a clocked fashion are connected in parallel. The incident signal pulses are amplified by a preamplifier. The continuously counting pulse-height discriminator then generates a count result every time an amplified signal pulse exceeds a set energy threshold.
The advantage of this measure is that the height of the signals can be registered very precisely by varying one or more threshold values and the count rate in the process can be determined independently of a varying pulse width. This type of registration assumes a relatively low photon flux, that is to say non-overlapping signal pulses. In the case of very high fluxes, this concept supplies much underestimated count rates, right up to a paralyzing behavior of the detector. It is for this reason that the clocked pulse-height discriminator is operated in parallel thereto. Like in the case of the continuously operating pulse-height discriminator, the correct number of amplified signals is underestimated when there are high photon fluxes, but the detector no longer exhibits paralyzing behavior. Here, the count rate depends directly on the average pulse width for low fluxes, and so these variations can be problematic.
A combination logic circuit is arranged between the counter and the two pulse-height discriminators in order to avoid the continuous double counting (and hence incorrect counting) of signal pulses. This is advantageous in that, at low fluxes, the continuously operating pulse-height discriminator correctly discriminates the signal pulses in terms of energy while, at high fluxes, the clocked pulse-height discriminator avoids too great an underestimation of the count rate.
Thus, preamplified signals are counted by the clocked pulse-height discriminator, connected in parallel, during phases brought about by the “pile-up”, i.e. phases with high fluxes, when the energy threshold in the continuously operating pulse-height discriminator is exceeded for a relatively long time, the clock rate of said clocked pulse-height discriminator having been adapted to the inverse of the pulse conditioning time. This obtains a monotonically increasing count result with increasing photon flux or increasing pulse frequency. In the case of high fluxes with a great number of multiple hits, and thus a great proportion of signal pulses registered in a clocked fashion in the count result, the energy then only infrequently drops below the threshold. Thus, the detector is in a saturation region with dominant activity by the clocked pulse-height discriminator. In the process, the ability to distinguish between measurements with similar high-frequency signal fluxes is reduced. The disadvantage of this approach, or rather of the amplification of the signal pulses by the preamplifier, is that the noise of the input signal is also increased at the same time. Hence, this does not improve the signal-to-noise ratio. The noise is likewise registered by the detector and it may worsen the count result.