1. -42 Impact of signal non-repeatability on spectral CT images
G. Shechter1
1Philips Medical Systems, Advanced Technology Center, P.O.B. 325, Haifa, Israel
Introduction
Projection domain decomposition[1] is used in spectral CT to reconstruct photo and scatter images from which virtual monochromatic (monoE) images are derived. We study the creation of band/ring artifacts in monoE images resulting from detector signal non-repeatability.
Method
Table 1. IQon simulation parameters
We model the detector signal non-repeatability as having two components – an average component and a fluctuating component with respect to the pixels belonging to the same detector module. For each of these two components we introduce a suitable functional that expresses the contribution of that component to the distortion of the de-composed line integrals. The signal non-repeatability captured in these functionals is measured by non-rotating scans.
In Fig. 2., we show the resulting monoE images obtained for each of the simulations. We used here an effective slice width of 5 mm, and a reconstruction filter that roles off from 2 to 13 cm-1. The window width is 40 HU. Each of the images show a faint level of band artifacts. Therefore, we take the tolerance thresholds for  as those given in table 1.
Band functional (average component)
40 keV
70 keV
120 keV
Definitions:  line integral (prep)  of detector pixel , of photo/scatter decomposed basis element at time   - signal  at energy bin
module number
Number of pixels per module
By a first order Taylor expansion we obtain the photo/scatter band functionals:
(1)
The monoE image is given as a linear combination of the reconstructed photo and scatter images with keV-dependent coefficients given by  and  , respectively The band functional is given by:
(2)
The ring level in the monoE image is proportional to given by Eq. (3). The ring functional is given by Eq. (4).
(3)
(4)
Figure 2. IQon monoE images corresponding to tolerance thresholds.
Band functionals of the PHILIPS IQon scanner
To find the band functionals, we used Eq. (2), where for   we used   given in table 1. To estimate the signal non-repeatability   we performed two identical non-rotating scans at 120 kVp, with a time gap of three days in between. A Delrin plate of 100 mm length was used as the attenuating object.
In Fig. 3 we show the resulting band functionals together with their conventional (single energy image) counterparts denoted by
The level of band artifacts in reconstructed CT images is proportional not only to the steps in    but also to the inverse square root of the distance of the band from the rotation center. For this reason, the gaps between the semi-continuous lines representing the tolerance thresholds for , are broadened off-center.
Ring functional (fluctuating component)
Tolerance thresholds for the PHILIPS IQon scanner
To determine the tolerance thresholds for this system we simulated noisy circular scans of a 150 diameter water phantom with 1.0 sec. rotation time and 400 mA. We simulated a signal non-repeatability with a relative signal change that alternates together for all energy bins across half of the detector arc, see Fig. 1. We also applied non-linear de-noising algorithms to the reconstructed images e.g. [2].
Figure 3. IQon band functionals.
Discussion
As reflected by Fig. 3, avoiding band artifacts induced by signal non-repeatability is more challenging for spectral images than for the conventional image. This is due to the different behavior of the non-repeatability that may arise at different energy bins. Philips’s dual detector layer IQon scanner meets this challenge.
References
[1] “Energy-selective reconstructions in X-ray computerized tomography”, R. E.
Alvarez and A. Macovski, Phys. Med. Biol., 21 (1976), p
[2] “Structure propagation restoration for spectral CT”, Liran Goshen, US patent
(A1), 2015.
Figure 1. Simulated pattern of the signal non-repeatability
In table 1. we address three simulations done to determine the tolerance thresholds of the monoE images at three different keV. On the left we give the signal non-repeatability peak to peak values (see Fig. 1.) used in the different simulations. In the middle we give the derivatives   calculated for the central pixel along the fan. On the right we give the resulting modulations of the band functional values obtained for these simulations according to Eq. (2).
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