Experiment 21: Understanding differences in phases of a 4DCT using uniform quantisation.

Aim: In experiment 18, all phases of a 4DCT were examined. It was obvious that there were differences in the phases. The aim of this experiment was to understand the differences in phases of the 4DCT when the structures were analysed with uniform quantisation. I wanted to understand what effect tumour movement may have on the elephant plot. Because the 4DCT is divided into phases, I felt this was a predictable way of investigating this, although this was less likely to affect post treatment imaging, as most images are taken as a single phase in inspiration breath hold (if the patient can tolerate it).

Method: The following phases were chosen: CT0 (end inspiration), CT20 (mid-expiration), CT50 (end expiration) and CT70 (mid inspiration) were chosen, as well as both composite volumes (CTAVIP and CTMIP).

Results and Discussion: Figure 19 aids hypothesis generation. The most obvious finding is that all of the texture plots have a high entropy low density region except the AVIP of the contra- lateral lung in patients 5 and 8. The data plot for patient 17 has a much thinner high density low entropy region in the contra-lateral lung when compared to the ipsilateral lung containing tumour. The obvious difference between the ipsilateral and contra-lateral plots in the AVIP phase for patients 5 and 8 suggest it would be relatively simple to detect whether or not there is tumour in the plot, but this does not explain why there is a high density low entropy region visible in all phases of the 4DCT of the contra-lateral lung, i.e. in the lung that does not contain tumour.

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Figure 19: comparison of CT0, CT20, CT50, CT70, CTAVIP and CTMIP between ipsilateral lung containing tumour and contra-lateral lung, for 3 patients (pt5, pt 8 and pt 17) using uniform quantisation levels.

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Figure 20: region of interest analysis of high density low entropy region of the AVIP phase of the lung contra-lateral to tumour from the 4DCT radiotherapy planning scan for pt 17.

The second finding is the shape of the high density high entropy region. The AVIP plots are ‘averages’ based on phases CT0-CT90. For phases CT0, CT20, CT50 and CT70 there is a visible extra region, which is not visible in the AVIP plots, this is most obvious in patient 8. This is illustrated in figure 21. The region of interest analysis shows this section relates to rib being included in the volume. The volumes for this experiment were generated using an auto- contouring tool in eclipse from the AVIP volume. In patient 8, there appeared to be more movement and therefore the amount of rib moving in and out of the volume would be greater. As it is different parts of the rib, this is ‘smoothed’ out in the AVIP data plot.

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Figure 21: region of interest analysis of high density low entropy region of CT0 for contralateral (image a) and ipsilateral lung (image b). The high density low entropy region in plot a) relates to contrast in a hilar blood vessel. In plot b) the same region of interest in the data plot relates to both a central blood vessel and the tumour.

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Figure 22: Region of interest analysis of high entropy high density region of ipsilateral lung to tumour of CT0 for pt 8. The CT and texture map appearance can be seen and show that the high density high entropy region relates to rib being included in the volume.

The choice of the AVIP for further analysis, is beneficial because it includes all planning information and the contoured structures. Secondly that from examining pt 5, 8 and 17, the AVIP excludes the high density low entropy region for 2 of the 3 patients analysed in the contralateral lung, which are seen in all other phases of the 4DCT analysed and illustrated in figure 18. The high density low entropy region in the AVIP phase of the contra-lateral lung to tumour for patient 17, is less obvious than for the ipsilateral lung from the same scan. Figure 20 shows that this is related to IV contrast.

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This difference in morphology between the AVIP and other series is likely to be related to 2 factors, firstly the auto-contoured structures are generated from the AVIP. It appears that rib being included in the lung volume is an artefact of the auto-contouring algorithm. The movement in different phases of the 4DCT means that different areas of rib and contrast appear in different geographical regions in different phases, meaning they do not appear as an average structure in AVIP.

Figure 23: comparing anatomical position of high density voxels from different phases of the same slice of a 4DCT in the lung contra-lateral to tumour, in patient 5. a) = CT0, b) = CT20, c) = CT50, d) = CTMIP.

Figure 23 shows 4 different phases of the 4DCT, which contained a high density low entropy region on the data plot. CT0 shows a large blood vessel with contrast, CT20 and CT 50 shows the same blood vessel has been excluded from the volume by the auto-contouring algorithm in eclipse. CT20 shows some high density, low entropy data points on the rim of the medial side of the lung. These are not visible on CT0 or CT50. The areas of uptake are different in different phases of the original 4DCT. The CTMIP is a composite image, showing maximal movement and as a result shows uptake at both the rim of the lung and contrast related overlap in the large blood vessel.

The AVIP phase of the contra-lateral lung does not have a high density low entropy region in the data plot. As the AVIP is a composite of the phases, it takes an average of all the phases, it appears that as the anatomical position of the high density low entropy region is different in each phase, this region of the plot gets smoothed out. As a result it is not visible in the AVIP.

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The advantage of the absence of the high density low entropy region in the contra-lung suggests it could help guide whether or not there is tumour in a lung. To see if this was a consistent finding more AVIP phases were compared in the next experiment.

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5.1.11 Experiment 22: Does excluding more of the hilum in the initial volume analysed in a