UNIVERSIT . . AT BONN Physikalisches Institut - Prof. Dr. Norbert ...

3. CIX 0.2 detector
3.1 Concept
The simultaneously counting and integrating X-ray detector CIX was designed as a merger
of two distinct signal processing concepts that are commonly used in X-ray imaging. The
first concept is the photon counting or pulse mode approach. In this case detectors register
individual photons by means of one or several thresholds. Depending on the number of
thresholds per pixel, these devices permit tissue identification or contrast agent imaging
without a variation of the X-ray beam energy. Regardless of this advantage, current ASIC 1
generations only allow single pulse counting up to rates of approximately 50 Mcps/mm 2 [4]
because of pulse pile-up. Therefore current counting systems are not suited for high flux
applications like computed tomography where the photon rates can exceed 1 Gcps/mm 2 .
In high flux conditions integrating systems are used. These detectors register the total
amount of charge that is deposited inside the sensor instead of identifying individual photons.
Hence, integrators can cope with very large photon fluxes but are on the other hand
limited by electronic noise at low fluxes.
The CIX concept combines the individual advantages of the counting and integrating concepts
in every single pixel cell. By using the photon counter in the low flux regime and by
relying on the integrator data under high flux conditions, CIX offers an increased dynamic
range compared to counting only or integrating only systems. The second advantage of a
simultaneously counting and integrating detector is the ability to reconstruct the average
energy E of the registered photons. This value is obtained by dividing the signal current
I, measured with the integrator, by the number of photons N registered in the counter:
I · Ee/h E =
N · t −1
(3.1)
F rame · e
In this equation Ee/h denotes the electron-hole pair generation energy of the sensor material,
tF rame the measurement duration and e represents the elementary charge. Medical
imaging could profit from this information in terms of an improved image quality as the
average photon energy gives a direct measure of the beam hardening, which, if uncorrected,
leads to unwanted image artifacts. Apart from the large dynamic range and the
measurement of the average photon energy, the CIX detector can also be used for dual
energy imaging, i.e. the reconstruction of the photoelectric and the Compton components
of the total absorption coefficient (see section 2.6). As has been demonstrated in [50], the
noise correlations between the counter and the integrator measurements reduce the image
noise compared to dual energy imaging devices that lack this correlation, e.g. tube energy
switching or dual layer systems.
3.2 CIX 0.2 ASIC specifications
At the moment of writing, the CIX chip is available in its second major revision. The
design of CIX 0.2 introduced a number of changes compared to the predecessor generation
1 ”Application specific integrated circuit”