5.2 Qualitative approach to image reconstruction using the scattering and
5.2.2.1 Image quality
Figures5.6,5.7,5.8and5.9show the reconstructed images of the RSP, of the scattering, of the cumulative scattering as well as of the transmission and linear transmission respectively. In addition, the phantom in RSP values is shown on Figure5.6(a).
5.2. QUALITATIVE APPROACH TO IMAGE RECONSTRUCTION USING THE SCATTERING AND TRANSMISSION RATE
For easier inter-comparison, Figure 5.10 shows images reconstructed using the different observables using 1000 protons/mm2/projection.
Visual assessment
Visual assessment of the images makes it possible to determine the following: the RSP images (Figure5.6) seem to present a much higher quality than the images reconstructed using the other data. This is not surprising, as it was shown that proton imaging (of the RSP) makes it possible to reach a satisfactory density and spatial resolution to be used in a clinical setting. The effect of the 3-σ cuts on the data is not clearly visible, nor is the effect of the increase in the statistics. Indeed, as the statistical uncertainty on the measured energy (that is translated to noise in the projections and therefore in the images) is very low, using 1000 protons per square millimetre of projection is not necessary.
Nevertheless, the images reconstructed using the other observables all show some anatomical information.
In the case of the reconstructions using the scattering (Figure 5.7), it can be seen that the cuts help distinguish the tumours in the case of 100 protons/mm2. The improvement in terms of reduction of noise in the image when using 1000 protons/mm2is considerable.
The lesions can be clearly seen on the image of 1000 protons/mm2 with cuts.
The images reconstructed using the cumulative scattering (Figure5.8) exhibit interesting visual properties. Increasing the number of protons treated from 100 to 1000/mm2allows a much easier differentiation of the internal structures of the head. Moreover, the border of the two lesions appears clearly, in bright, allowing for a good detection. The images reconstructed using the transmission rate (Figures5.9(a)and5.9(b)) show an important level of noise, that is reduced with the increase in the number of particles studied. It can be seen that the images reconstructed using the linear transmission (Figures 5.9(c)and 5.9(d)) show a much greater contrast between the soft tissues and the bones than any other. However, for all the transmission images, the tumours can not be distinguished.
Signal to noise ratio
The signal to noise ratio (SNR) of the reconstructed images was evaluated. A number of N = 10 regions of interest (ROI) of 200 mm3 each were drawn inside an area made of skeletal muscle (arrows in Figure 5.6(a)). The SNR was defined as:
SNR = N1
Results are presented in Table5.4. As expected from the results shown in Table5.3, the statistical uncertainty on the transmission rate and scattering induces a much lower SNR in the images reconstructed from these observables than from the RSP. Indeed, in the cases of the cumulative scattering and of the (linear) transmission, the SNR is an order of magnitude lower than for the images of the RSP.
(a)
100 protons/mm2 1000 protons/mm2
nocuts
(b) (c)
0 1.5
3-σcuts
(d) (e)
Figure 5.6: Transverse slices of(a)the RSP phantom, and of FBP reconstructions of the RSP for (b) 100 proton/mm2 and (c) 1000 protons/mm2 as well as reconstructions of the RSP with 3-σ cuts on the data for(d)100 proton/mm2 and (e)1000 protons/mm2. Images with [0:1.5] colour range window.
On(a), the colour circles indicate the ROI inside the carcinoma (bottom, red) and inside the brain tissue (top, green). The two arrows on the image of the phantom designate the areas of skeletal muscle in which the SNR (Equation5.3) was calculated.
5.2. QUALITATIVE APPROACH TO IMAGE RECONSTRUCTION USING THE SCATTERING AND TRANSMISSION RATE
100 protons/mm2 1000 protons/mm2
nocuts
(a) (b) 0
6⋅10-4
3-σcuts
(c) (d) 0
5⋅10-4
Figure 5.7: Transverse slices of FBP reconstructions of the scattering for (a) 100 proton/mm2 and (b) 1000 protons/mm2 with a colour window range of [0:6·10−4].
Reconstructions of the scattering with 3-σ cuts on the data for(c)100 proton/mm2 and (d)1000 protons/mm2 with a colour window range of [0:5·10−4].
100 protons/mm2 1000 protons/mm2
nocuts
(a) (b)
0 0.045
3-σcuts
(c) (d)
Figure 5.8: Transverse slices of FBP reconstructions of the cumulative scattering on the MLP for (a) 100 proton/mm2 and (b) 1000 protons/mm2 as well as reconstructions of the cumulative scattering with 3-σ cuts on the data for (c) 100 proton/mm2 and (d) 1000 protons/mm2. Images with a colour window range of [0:0.045].
5.2. QUALITATIVE APPROACH TO IMAGE RECONSTRUCTION USING THE SCATTERING AND TRANSMISSION RATE
100 protons/mm2 1000 protons/mm2
transmission
(a) (b) 0
2.5⋅10-3
lineartransmission
(c) (d) 0
0.01
Figure 5.9: Transverse slices of FBP reconstructions using the transmission rate for(a) 100 proton/mm2 and(b) 1000 protons/mm2 using a colour window range of [0:0.0025].
Reconstructions using the linear transmission rate for(c)100 proton/mm2 and(d)1000 protons/mm2 using a colour range of [0:0.01].
(a)
(b) (c)
(d) (e)
Figure 5.10: Transverse slices of the reconstructed images shown in the previous figures, with 1000 protons/mm2/projection: (a) RSP image (Figure 5.6(e)); (b) image reconstructed from the scattering (Figure 5.7(d)); (c) image reconstructed from the cumulative scattering (Figure 5.8(d)); (d) image reconstructed from the transmission (Figure5.9(b)); (e)image reconstructed from the linear transmission (Figure 5.9(d)).
5.2. QUALITATIVE APPROACH TO IMAGE RECONSTRUCTION USING THE SCATTERING AND TRANSMISSION RATE
Table 5.4: SNR of the different reconstructed images for 100 and 1000 protons/mm2/projection
Image 100 protons/mm2 1000 protons/mm2
RSP 298 318
RSP - with cuts 300 320
Angular scattering 47 51
Angular scattering - with cuts 55 49
Cumulated scattering 19 19
Cumulated scattering - with cuts 22 20
Transmission 27 64
Linear transmission 23 20
Region of interest study
Three ROI were defined in the reconstructed images as follows: one inside each carcinoma and one in the brain tissue. Each 3D cylindrical ROI has a volume of 406 mm3. They are represented by the circles on Figure5.6(a). The study was performed on the images reconstructed using 1000 protons/mm2.
It is possible to evaluate the accuracy of the reconstructed images in the case of the RSP and in the case of the transmission.
For the RSP, the expected values were computed analytically using the Bethe formula and knowing the stoichiometric composition of the materials simulated. Figure 5.11 shows the reconstructed values inside the three ROI, as well as the expected value for the RSP images. It can be seen that the values reconstructed in the ROI, for the images with and without the 3-σ cuts on the data, are quite close to the expected values.
Figure 5.12 shows the reconstructed values as well as the expected values for the transmission image. The expected values were calculated using the information on the transmission rate of protons inside an homogeneous cube of the materials (Section 5.1.1.2). While the reconstructed values are really close to the expected ones, the uncertainty on the average, represented on the histogram by the error bars, are significant.
The values in the reconstructed images using the scattering, cumulative scattering and linear transmission rate is difficult to interpret because they depend on multiple physics processes. As a consequence, no expected values could be determined for these images. Figures 5.13,5.14 and 5.15 show the reconstructed values in the three ROI for the images produced from the scattering, cumulative scattering and linear transmission rate, respectively. It can be seen from Figure 5.13 that some difference between the three ROI can be distinguished. For both images, with and without the 3-σ cuts, the values in the carcinoma ROI can be set apart from the values in the brain ROI. As far as the cumulative scattering goes (Figure 5.14), the noise in the regions of interest results in the fact that the different ROI can not be well differentiated. Finally, in the case of
0 0.2 0.4 0.6 0.8 1 1.2
Right carcinoma Left carcinoma Brain
reconstructed RSP
expected valueRSP with cuts
Figure 5.11: Values in the three ROI for the RSP reconstructions without and with cuts.
0.0e+00 2.0e-04 4.0e-04 6.0e-04 8.0e-04 1.0e-03 1.2e-03 1.4e-03
Right carcinoma Left carcinoma Brain
reconstructed attenuation coefficient (1/mm)
expected valueTransmission
Figure 5.12: Values in the three ROI for the transmission image.
the image reconstructed from the linear transmission (Figure5.15), it seems that some difference between the two carcinoma appears, but the noise may make them hardly distinguishable from the brain.