Materials and methods
3.5 Integration-mode multi-channel range telescope characterization and perfor- perfor-manceperfor-mance
3.5.4 Further experimental considerations
The accurate retrieval of the phantomWETfrom2D-radiographies orRelative Stopping Power (RSP) from 3D-tomographic reconstructions rely on the uniform constitution of the RRD-channels over the whole active area, i.e., air or homogeneous material projections are expected to yield flat images. This condition, however, can be corrupted due to different factors, in-cluding PMMA-slabs inhomogeneities from the manufacturer, electric-field distortion due to environmental conditions (e.g. air temperature and pressure) or moisture effects within the air gap of thePPICs.
Radiographic image inhomogeneities were observed in air-projections, resulting from an un-expected channel-displacement effect at certain regions of the irradiatedFOVwhich was found in most of the detector channels. Clusters of severalRPswere recognized with theBPposition shifted one channel back than the expected location. For a clearer visualization of an exem-plary case, a color-saturated map assigning a contrast-color (green) to the wrongly assigned channel is shown in Figure 3.34a. A high-statistics air-projection (5⋅105 primaries perRP) is obtained with a 310.58 MeV/u 12C-ion beam. This dose-choice improves the signal-to-noise ratio, allowing to disentangle the study case from the signal quality. In the radiography, green colored RPsindicate wrongly allocated BPs (Channel 49), while the rest of the FOVreflects the expected BP location (Channel 50) according to the defined gray-scale. The correct BP
position is deduced from MC-simulations obtained with the same beam-energy. In the right panel, the reliability of single RP signals, in terms of charge accumulated perRP, is assessed by means of the reliability-index, previously introduced. The RI-map of the same irradiation shows an almost-uniform light-blue shade, which demonstrates no correlation between the per-formance of the trigger on collecting enough charge and the misplaced BPposition shown on the left.
Figure 3.34: Example of image-inhomogeneity effect due to uneven PMMAsabsorbers (a) and the reliability-map of the irradiation (b). A high-statistics air-projection (5⋅105 primaries perRP, to avoid signal disruption by noise) is obtained with a 310.58 MeV/u 12C-ion beam. All theBPs of the projection are expected to fall within the channel number 50, i.e., the pixels are expected to be colored in light-gray. However, theBPson the upper part of theFOV are allocated one channel before, which is saturated in green for easier identification.
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(a) Example of the effect of uneven material distribu-tion of the PMMA absorber(s) of the RRD, causing a discontinuousBPposition in theRPsof the upper part of the projection (wrongly assigned BPs are colored in green). In an air projection, all theRPsare expected to be assigned with the same channel/WETvalue. Dotted lines are drown as visual aid to determine the fraction of theFOVaffected.
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(b)Reliability index map of the air projection shown on the left. Thereliability factorgoes from 0 to 1. Higher values indicateRPsthat integrated more charge in rela-tion to the time that the beam remains on them.
Given the geometrical complexity of the phantoms investigated in this work, the transmitted BPs depth-distribution along the detector channels is equally-complex. Therefore, the impact of the revealed channel-shift in the overall image-accuracy is difficult to be assessed. Especially when the whole experimental setup consists of several components that could have an influence, i.e., the origin of the problem could be beam-line, beam-scanning or detector related. Thus, these variable factors had to be investigated separately.
Exhaustive measurements were conducted to deduce the main cause of the observed dis-placement. The results of these experiments aid to rule-out a scanning-pattern dependence, a beam-line components dependence,HV-supply influence, day-to-day variation and detection positioning. Temporal radiation-damage of the RRD was also rejected by comparing irra-diations performed years ago with actual acquisitions in the same experimental conditions.
Besides the signal-reliability (cf. Figure 3.34b), the signal-assessment methods presented in
3.5. Integration-mode multi-channel range telescope characterization and performance Section3.4.5helped to perceive the behavior of the BCsalong the shifted pattern (cf. Figure 3.35). Dose-independence is also supported by the fact that the same patterns appear when hyper-high doses are applied. Consequently, theBPdetermination in a previous channel than expected is a problem related to individual PPICs, the intrinsic RRD granularity and the maximum-signal identification criteria used in the first instance (without data post-processing (cf. Chapter4)), to assign the corresponding WETvalues.
Figure 3.35: SFR (a) and BP-steepness (b) signal-assessment maps to investigate the channel-shift effect shown in Figure3.34. Both maps show a clear correlation to the original deviation pattern.
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(b)BPSmap.
The beam energy is customized according to the phantom’s geometry and composition, so that it is guaranteed that all the BCs traversing different densities fall within the RRD and, if possible, avoiding under-responsive or noise-sensitive channels. However, given the single energy-beam used for imaging and the variability in material composition and/or thickness of the imaged phantoms, many BPs might be unfortunately located exactly in between two channels, leading to a channel-discretization issue sketched in Figure3.36 for three simulated pristine BCs. The stars designate the experimental data-points that discretize the ideal BC.
When theBPfalls in the frontal-edge of thePMMA-plate, it will be mislead. In case of broader BPs, resulting from range-mixing, the problem is less accentuated, i.e., the air or single-material projections, which lead to sharperBPs are more sensitive to this phenomenon. The examples depicted in Figure3.36vary in the skewness and the steepness of theBP fall-off. Thus, a 2D-BPS-map might indicate whether the maximum signal identification criteria is not providing the correct BP-position.
If the detector channels contain rough zones, it is likely that the channel-displacement of the BPsis enhanced according to the accumulated crossed-material by the moment the ion reaches its range.
An additional indirect way to infer the behavior of the BCs over the extended FOV is through signal-assessment metrics maps (cf. Section 3.4.5). Figure 3.35 shows the SFR-map
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Figure 3.36: Channel discretization sensitivity to maximum-signal identification criteria. By solely finding the maximum-signal, the channel determination can be classified in three main cases. In white dotted-lines three simulated pristineBPs are shown. The stars and colored lines indicate the experimental points in which they are discretized.
OtherBPsmight be shifted within a 1.5 mm range around these samples. ForBPslocated in a range from the middle towards the distal-edge of the channel (blue discrete curve), the data-point corresponding to the maximum-signal is higher, making the distal-drop of the curve steeper than in the case in which theBP is found at the entrance of thePMMA-slab (magenta discrete curve). For the latter case, the maximum signal will be registered by one channel before than actually should be.
(left panel) and the BP-steepness-map (right panel) suggesting a strong correlation between the channel-deviation and theBC profiles.
The SFR-map (cf. Figure 3.35a) indicates that the BPs in the upper zone of the FOV (in red, poor SFR) are more prone to be misplaced due to lower maximum signal amplitude detected, i.e., small peak-to-plateau difference and large noise fluctuations. This effect is also occurring when the BP is shifted towards the front of the PMMA in which is located (cf.
Figure 3.36). Moreover, a smooth transition from red to blue shades reveals that the BP-shift occurs gradually, giving the sign of a decreasing wedge-shaped material distribution from the top to the middle at this particularPMMA-slab or as cumulative effect from the previous ones.
Figure 3.35b, supports this hypothesis. Larger absolute values (yellow-orange shades) in the color scale mean a steeper descent, which according to Figure3.36refers toBPs falling on the second distal-half of the channel, i.e., that have crossed less material. In contrast, blue-green shaded regions are sign of a shallow spline derivative (cf. Figure 3.18) which, in turn, reflects a shifted BPs falling at the frontal-half of the absorber or one channel before. TheseBCsare likely to lead to a wrong WET value when only the maximum-signal identification criteria is used (cf. Figure 3.36). A particular blue-colored ring is distinguishable in the BP-steepness pattern, which may indicate a subtle layer of extra-material traversed by the beam. These are arguments which reassert the need of further post-processing methods able to increase the system intrinsic depth-resolution as the ones presented in next chapter.
The findings described above suggested a customized channel-wise rectification of the pro-totype RRD for future experimental acquisitions. A full energy calibration throughout the RRD is suggested to gather information of the FOV zones which are affected in each specific channel. This knowledge would allow to build channel correction-masks to be applied in data post-processing and posterior image formation, although at very-high post-processing cost.
Based on precise MC simulations, 88 beam energies were selected from the LIBC so that the BPis allocated in critical positions within thePMMA-slabs, such as the edges (cf. Figure3.36).
3.5. Integration-mode multi-channel range telescope characterization and performance The detector was scanned over a large extendedFOVof 20×15 cm2in the usual 1 mmRP-step, depositing 5⋅104 carbon-ions perRP. Examples of the shift-patterns appeared in two channels are shown in Figure 3.37. A clear trend is shared by the first 35 channels of the detector in which theBP-shift is located either on the top-left-corner or on the bottom-right-corner of the FOV. On the second half of the detector (after channel 35), the displacement pattern is more symmetrical along the vertical line of the FOV. The impact of this phenomena on the RRD experimental calibration is presented in thenext chapter.
Figure 3.37: Example of the channel-displacement effect shown in two different RRD channels. The projections follow the gray-scale tuned by the channel number and the color green is used to saturate theRPsfound in a wrong channel.
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(a) Air projection taken with a carbon-ion beam at 126.11 MeV/u. The BP is expected within the 10th channel according to preciseMCsimulations, while the green shaded region indicates all theBPs detected one channel before (channel 9). Some salt-and-pepper noise can be observed as well as a pick-up resonance affecting a horizontal sequence ofRPs.
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(b) Air projection taken with a carbon-ion beam at 252.40 MeV/u. TheBPis expected in the channel 35 ac-cording to preciseMCsimulations, while the green zone represent all theBPsdetected one channel before (chan-nel 34). On the chan(chan-nels belonging to the second-half of theRRD the deviation pattern remains mostly hori-zontal towards the lower part of theFOV, different from the shift patterns found in the first-half of theRRD(left panel), which are oblique.
Further MC-simulations were conducted to investigate the impact on the BC signals in the worse-case scenario, when an uniform ±5% absorber thickness variation is added to all the RRD PMMA absorber platesO. The channel-displacement effect is cumulative, therefore the BCstraversing air or light-density materials, falling in farther channels of the RRDare most affected. In such cases, the resulting deviation might be up to 2 channels. Additionally, the RRD discretization (cf. Figure 3.36) might affect differently the channel with the maximum signal according to the exact BP position relative to the adjacent PPIC. Systematically, all thePMMAplates seem thinner in the bottom-part and thicker in the top-part, within the 5%
precision limitation claimed by the manufacturer. These studies inquire an accurate quality
OThanks to Sebastian Meyer for performing this study and providing theMCsimulated data used for these detector investigations, which are experimentally unfeasible.
assurance on thickness and homogeneity of the PMMA-slabs considered for thenew RRD.