Chapter 4
Results
48 20
40 60 80 100 120 140 160 180
1st 2nd 3rd 4th 6th 8th 9th 11th
Contrast Noise Ratio
months CNR - Isosenter (SPD=100cm)
1st 2nd 3rd 4th 6th 8th 9th 11th months
CNR - EPID (SPD=157cm)
EPID A EPID B
4 Results
4.1 Quality Control Analysis using the QC-3V phantom
The objective quantification of spatial resolution and contrast-to-noise ratio analysed with the QC-3V phantom has been used for routine quality assurance of portal imaging devices. The method claimed to be practical for the system performance determination and quality control analysis in time. Two Elekta iViewGt® a-Si flat panel detectors (EPID A and EPID B) were exhaustively tested under standard and non-standard acquisition conditions.
The CNR evaluation has shown variations of ~32% and ~8% (1SD) at isocenter and ~32%
and ~9% (1SD) at EPID positions, for EPID A and B respectively (Figure 4.1). CNR had shown to be higher for EPID B (153±8% (1SD)) compared to EPID A (121±32% (1SD)) at the isocenter. For EPID location (157cm) was found CNR values of 96±9% and 53±32% (1SD) for EPID B and EPID A respectively. Note that for EPID A the available data is restricted to six months, and different Lifetimes are shown (at the 6th month: 37months (EPID A) and 9 months (EPID B)). Also the setup was different in terms of dose rate and frame integration times, which are related to the combination of each Linear Accelerator and the respective imaging device.
The EPID response to different source to phantom distances (SPD) was tested to check the influence on CNR, noise and spatial resolution (Figure 4.2). By increasing the SPD, the CNR showed to increase from 100 to 130cm ~33% and a decrease for remain distance to the EPID touch guard surface (157cm) of about 57%. The noise behaves exactly in same opposite way, ~32% (100 to 130cm) and ~56% (130 to 157cm). This transversal test was useful to understand how noise detected by QC3 can influence CNR calculations, when changing the distance from the source and thus from the detector.
Figure 4.1 CNR measured in time with QC-3V phantom at isocenter position (100 cm) and EPID touch guard surface (157 cm).
Chapter 4
49 0
50 100 150 200 250
70 90 100 110 130 150 157
Value
Source Phantom Distance (cm) CNR vs Noise
CNR Noise
The SPD influence on the spatial resolution, was found to be linear with distance from the source (decrease distance from the detector) with high square correlation coefficients above 0.9.
The spatial resolution increase linearly from 70 to 150 cm, approximately 0.49%, 0.36% and 0.23%
for each centimetre, for RMTF(f30), RMTF(f40), RMTF(f50) respectively. For the SPD of 157cm the spatial resolution decreases slightly (Figure 4.3).
The Relative Modulation Transfer Function, detected by line pair gauge phantom (QC3) in Time, had slightly variations, <1% and ~2.3% (1SD) at Isocenter for EPID A and B respectively and;
~2.7% and ~4.7% (1SD) at EPID distance for EPID A and B respectively (Figure 4.4).
The RMTF(f50) for EPID A revealed better results than EPID B for phantom located in Isocenter position 0.364 ± 0.003 lp/mm and 0.349 ± 0.007 lp/mm, respectively. For phantom located in EPID position the spatial resolution was 0.424±0.011 lp/mm and 0.415± 0.019 lp/mm for EPID A and EPID B respectively. Weak correlations in time were found to be lower for EPID B, which could be due to higher sensitivity to small variations of linac output.
Figure 4.2 CNR versus Noise dependence on SPD
R² = 0,9456
R² = 0,9243
R² = 0,9272 0,2
0,3 0,4 0,5 0,6 0,7 0,8
60 80 100 120 140 160
Spatial Resolution (lp/mm)
Source Phantom Distance (cm)
Relative Modulation Transfer Function Source Phantom Distances
RMTF 30%
RMTF 40%
RMTF 50%
Figure 4.3 Variation of the amplitude modulation transfer of 30%, 40% and 50% with different Source Phantom Distance to the source, from 70cm to 157 cm (EPID touch guard surface).
50 R² = 0,1042
R² = 0,0958
R² = 0,2252
R² = 0,1244 0,32
0,34 0,36 0,38 0,4 0,42 0,44 0,46
1st 2nd 3rd 4th 6th 8th 9th 11th
Spatial Resolution (lp/mm)
months
RMTF 50% - EPID vs ISOCENTER
EPID A - EPID EPID B - EPID EPID A - ISO EPID B - ISO
0 20 40 60 80 100 120 140 160 180
5 10 15 20 25 30 35
C N R
Lifetime (month) Quality Assurance : CNR
IN TIME
B_ISO B_EPID A_ISO A_EPID
0,3 0,32 0,34 0,36 0,38 0,4 0,42 0,44 0,46
5 10 15 20 25 30 35
lp/mm
Lifetime (month) Quality Assurance : RMTF(f50)
IN TIME
A_EPID A_ISO B_EPID B_ISO
Standard acquisition conditions for both Linacs were processed together, to find out a relation to the clinical usability. Since the CNR is highly influenced by detectable photons in the sensitive matrix area, the reduction on EPIDs sensitivity with ageing may decrease the global CNR for both phantom positions (Figure 4.5a). For standard acquisition conditions, the global RMTF had shown weak correlations in time and better results for the older EPID A (figure 4.5b).
By using the same procedures and Rajapakshe algorithms, new tests were performed to analyse different panel regions, by placing the phantom at the same relative position to the source and moving the EPID in (x, y) directions.
Figure 4.4 RMTF (f50) measurements on EPID and isocenter positions, following the mean time
Figure 4.5 In time Quality assurance tests performed with QC-3V phantom for CNR (a) and RMTF(f50) (b) on EPID and isocenter positions
a) b)
EPID A EPID A
EPID B
EPID B
Chapter 4
51 0,4
0,42 0,44 0,46
UL LL Center UR LR
RMTF f50 (lp/mm)
Orthogonal position
Std. rot 90º rot
0,32 0,34 0,36 0,38
UL LL Center UR LR
RMTF f50 (lp/mm)
Orthogonal Postion
Std. rot 90º rot
By rotating the phantom 90 degrees, it was possible to measure the influence of the anisotropic focal spot size on the system spatial resolution. The influence of different source phantom position at isocenter (100 cm) and EPID surface touch guard (157 cm) were also tested in combination to the other two parameters (phantom rotation and EPID matrix location), in figure 4.6. These tests were performed once in the fourth month investigation for EPID A and B.
By using QC3 in different EPID locations: upper Left (UL); lower left (LL); Center; upper right (UR); and lower right (LR), the EPID panel matrix spatial response was tested in five regions, in which the measurement describes significant variations. The RMTF response had shown, at the Isocenter SPD, a slightly better spatial resolution at matrix: Center, and right sided positions (UR and LR) for both EPIDs.
a) EPID A – EPID location
0,4 0,42 0,44 0,46
UL LL Center UR LR
RMTF f50 (lp/mm)
Orthogonal position
Std. rot 90º rot
0,32 0,34 0,36 0,38
UL LL Center UR LR
RMTF f50 (lp/mm)
Orthogonal position
Std. rot 90º rot (d) EPID B – Isocenter
b) EPID B – EPID location
0,0 5,0 10,0
UL LL Center UR LR
ratio (%)
Orthogonalposition RMTF 50% - Isocenter
ratio 90rot/ Std.rot
EPID A EPID B
Figure 4.6 RMTF f50 values for QC3 phantom analysis, both standard (circles) and 90degree rotation (diamonds), placed on different orthogonal (x,y) panel matrix locations. The RMTFs were tested for EPID A and B for both, source phantom distances, at EPID (a), (b) and Isocenter (c), (d).
Figure 4.7 Ratio RMTF f50 between QC3 standard and 90 degree rotations at isocenter position (c) EPID A – Isocenter
52 Comparing both EPIDs at different SPDs for different EPID matrix locations, the imaging system became more sensitive to variations on spatial resolution, when increasing the distance to the flat panel detector (lower SPDs). By measuring the spatial resolution (RMTF) of the system at the isocenter, EPID A had shown a higher variation between standard and 90º degrees phantom positions. Therefore this variation also varies depending on the phantom position in the matrix, 6.5±0.95% and 2.15±1.5% for EPID A and EPID B respectively (Figure 4.7).
4.2 Spatial resolution: by an Edge Response technique
The spatial resolution technique applied in this experiment used a spatial domain technique for a description of how the pixels response and influence the ability for resolving an input edge.
In this test were determined that for different directions, EPIDs shown slightly variations on resolution, which could be due to differences on focal spot size for each Linac. The method detected also an off-axis variation on Edge response.
More determinant aspect for this approach is that a noisy edge spread function (ESF) influence the edge response (10% to 90% distance) and the result of the tests (figure 4.8).
Figure 4.8 Input edge (a) generate a steep pixel gradient from the attenuated to the non attenuated region (b). Part of this response is result from pixel sample aperture (pitch=0,4mm), which limits the maximum achievable spatial resolution (c). For noisy edge spread functions the spatial resolution is likely to be reduced (d).
a)
b)
c)
d)
Chapter 4
53 As the same results derive from QC3 based on 1cm2 this results demonstrates that for different places in the EPID, exploring different directions, we can obtain different values for spatial resolution. This could be a tightening for the usability of QC3 phantom, which in a standard way makes use of small measurement areas near the EPID Caxis.
A normal ESF curve is describe by to homogeneous tails that reach the 0 and 1 normalized function. Increasing noise, by means the presence of artefacts due to statistical noise or the presence of bad pixels (defected, damaged or less sensitive), the edge response from 10% to 90%
will vary because the bias level increase, which may change considerably the system spatial resolution in some matrix areas.
Horizontal edges, for EPID A showed a noise increase for subpanels in the (x) index numbers above 512, which is the EPID area located in the outside extremity of the detector panel matrix, nearby the source (Figure 4.9).
Both EPIDs describe a 2nd degree polynomial tendency, where the central value is situated by next to the CAXIS. The variations across the edge, from the 76 to 935 (x) row direction show an edge response in respective orthogonal direction (y). For EPID A left side position (x <512) were found an edge response (ER) of 3,92 ± 0.28 mm and for right side (x>512) 3.97 ± 0.20 mm. EPID B revealed 3.87 ± 0.32 mm and 3.75 ± 0.26 mm, for left and right sides respectively.
Figure 4.9 Edge response curves for testing the spatial resolution in the (y) direction for both Linacs, by measuring along the (x) index matrix direction.
2,5 3 3,5 4 4,5 5
76 101 126 142 167 192 217 242 257 282 307 332 357 382 398 423 448 473 498 513 538 563 588 613 638 654 679 704 729 754 769 794 819 844 869 894 910 935
distance (mm)
(x) index numbers
Horizontal Edge response
Upper - EPID B Upper - EPID A CAXIS - EPID B CAXIS - EPID A Lower - EPID B Lower - EPID A
Left subpanels Right subpanels
54 2,5
3,5 4,5
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16
Distance (mm)
Subpanels Subpanel Edge Response
upper / lower
EPID A EPID B
2,5 3,5 4,5
S1 - Caxis - S9 S2 - Caxis - S10
S3 - Caxis - S11
S4 - Caxis - S12
S5- Caxis - S13
S6 - Caxis - S14
S7 - Caxis - S15
S8 - Caxis - S16
Distance (mm)
Subpanels Subpanel Edge Response
Horizontal - CAXIS
EPID A EPID B
A more close analysis was performed to understand how the subpanels in the matrix are able to identify the edge discontinuity (Figures 4.10 and 4.11).
The second order polynomial demonstrates a variation in system response along the edge.
The results suggest a decrease on spatial resolution demonstrated by the increasing edge response distance, from the left to the right side of the displayed images, which is probably related to the proximity to the beam isocenter.
upper subpanels lower subpanels
Figure 4.10 Horizontal Edge responses for upper and lower subpanels. Inside red ellipses are the subpanels that shown increased edge response distances, by comparing data from the both EPIDs, which differ by more than 2.5%. The difference in the edge response between EPID subpanels (>2.5%) were found for subpanels closest to the beam source S6, S7, S8, S13, S14, S15, S16.
Figure 4.11 Horizontal Edge responses for CAXIS. The difference in the edge response between EPID subpanels (∆ > 2.5%) were found for the edge between the following subpanels:
S4/S12, S5/S13, S6/S14, S7/S15, S8/ S16 (red ellipses). The 2nd order polynomial fit represents the difference on dose response characteristics of imaging system.
∆ 3.96%
∆ 10.07%
∆ 8.05%
∆ 2.69%
∆ 4.85%
∆ 4.5%
∆ 3.09%
∆ 4.12% ∆ 4.36%
∆ 5.72%
∆ 10.86%
∆ 12.74%
Chapter 4
55 The edge response distance along the step function of different subpanel positions, proved to be a method susceptible to small pixel variations, which can introduce a bias error for spatial resolution analysis.
The variation of the (y) edge response along the (x) direction was measured and compared with analogue subpanel for both EPDIs. Variations above 2.5% between analogue subpanels were detected and marked (red ellipses) mainly for right sided subpanels (nearby the beam source) and more pronounced for the CAXIS edge analysis, as can be seen in Figures 4.10 and 4.11. By looking the edge response itself, was found better spatial resolution (lower ER values) results for EPID B comparing to the analogue subpanels of EPID A. Lower subpanels (S9-S16) were found with worse response comparing to upper analogue subpanels (S1-S8) for both EPIDs
The variations across the Vertical edge, from the 76 to 937 (y) row directions show an edge response in respective orthogonal direction (x). For EPID A upper side position (y <512) were found an edge response (ER) of 3.96 ± 0.48 mm and for lower side (y>512) 4 ± 0.39 mm. EPID B revealed 3.88 ± 0.38 mm and 3.85 ± 0.35 mm, for upper and lower sides respectively (Figure 4.12 and 4.13).
The spatial resolution for EPID A was highly influenced by pixel-by-pixel intensity variation across the edge spread function (ESF) in the read out direction (x), which leads to higher variations in the edge response (red lines in figure 4.12). The vertical edge response for EPID B appears to be highly dependent to the Linac photon beam output and the imaging system response (Figure 4.13). A detailed analysis was performed to understand how the analogue subpanels of both EPIDs can be related (Figures 4.14 and 4.15).
Figure 4.12 Edge response curves for testing the spatial resolution in the (x) direction for EPID A, by measuring along the (y) index matrix direction.
2 3 4 5 6
76 101 126 151 176 201 226 251 276 301 326 351 376 401 426 451 476 501 512 537 562 587 612 637 662 687 712 737 762 787 812 837 862 887 912 937
distance (mm)
(y) index numbers
Vertical Edge response
EPID A
EPID A - E1EPID A - E2 EPID A - E3 EPID A - V-CAXIS EPID A - E5 EPID A - E6 EPID A - E7
upper subpanels lower subpanels
56 3
3,5 4 4,5 5
S2 S3 S4 S4 -
Caxis - S5
S5 S6 S7 S10 S11 S12 S12 -
Caxis - S13
S13 S14 S15
distance (mm)
Subpanels Vertical Edge Response
subpanels
EPID A EPID B
The global vertical edge response for upper and lower subpanels were found similar and the 2nd order polynomial, unlike the horizontal edge test, cannot be seen since is taken out from each single subpanel in the x direction, for a total of two for each vertical edge (i.e. S2-S10, S3-S11, etc.).
As like in the horizontal edge, we found higher percent variation for the right sided subpanels which can probably be relate to the image degradation of EPID A. Pixel-by-pixel variation can be seen in figure 4.15 for above marked subpanels in figure 4.14.
Figure 4.14 Vertical Edge responses for upper and lower subpanels. Inside red ellipses are the subpanels that shown increased edge response distances, by comparing data from the both EPIDs, which differ by more than 2.5%. The difference in the edge response between EPID subpanels (>2.5%) were found for subpanels closest to the beam source S5, S6, S7, S12-CAXIS-S13, S14, S15.
Figure 4.13 Edge response curves for testing the spatial resolution in the (x) direction for EPID B, by measuring along the (y) index matrix direction.
2 3 4 5 6
76 101 126 151 176 201 226 251 276 301 326 351 376 401 426 451 476 501 512 537 562 587 612 637 662 687 712 737 762 787 812 837 862 887 912 937
distance (mm)
(y) index numbers Vertical Edge response
EPID B EPID B - E1
EPID B - E2 EPID B - E3 EPID B - V-CAXIS EPID B - E5 EPID B - E6 EPID B - E7
upper subpanels lower subpanels
upper subpanels lower subpanels
∆ 9.29%
∆ 4.49%
∆ 5.55%
∆ 11.15%
∆ 8.11% ∆ 8.58%
Chapter 4
57 2
3 4 5
distance (mm)
(y) index subpanel 5
EPID A EPID B
4.3 Image Noise: by a Subpanel Image Noise Technique
The SNR analysed by the subpanel image noise (SIN) technique Image performed in time, intent to give a measure of how the system responds to different attenuated fields, and if this variation is affected by the number of frames/dose per image.
By increasing copper attenuation, we found that the SNR decrease by decreasing the relationship between the pixel average value and the standard deviation, which means that for a higher attenuation the mean pixel value for each row of each subpanel decrease and a better visualization of the less responding pixels by the non-linear SD become possible.
Measurements with/without attenuation combined with different integrated frames were performed to test the higher discrimination of noisy areas as the EPID gets old. In the figure 4.16, open fields were combined with 10 and 100 frs/img. For the last month measurement the increased frames per image revealed an increase on the global SIN values of ~28% and ~55% for EPID A and EPID B respectively.
Figure 4.15 Show the difference on subpanel ERs at (x) direction measured along the (y) index matrix direction, for marked noisy subpanels of EPID A (squares) comparing to the analogue subpanel positions of EPID B (triangles).
2 3 4 5
distance (mm)
(y) index
Subpanel S12-CAXIS-S14
2 3 4 5
distance (mm)
(Y) index subpanel 6
2 3 4 5
distance (mm)
(y) index
subpanel 14
2 3 4 5
distance (mm)
(y) index subpanel 7
2 3 4 5
distance (mm)
(y) index subpanel 15
58 0
200 400 600 800 1000
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID A
10frs
33rd month 34th month 35th month 37th month
0 500 1000 1500 2000
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID A
100frs
35th month 37th month
0 200 400 600 800 1000
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID B
10frs
5th month 6th month 7th month 9th month 12th month 14th month 15th month
The higher the SNR value, the better is the recognition of the read out signal. For both EPIDs the number of frames per image increases the SNR, by increasing the number of detected photons in the panel detection layer which leads to an increase of mean value and a reduction on statistical noise. When reducing the influence of random noise, data can be more accurately relate to the EPID structural noise. By reducing the amount of photons that impinge the detector from different attenuations and frames per image, EPIDs showed noisier and thus a reduction on the SNR can be observed (figure 4.17 and 4.18).
0 500 1000 1500 2000
sub3 sub4 sub5 sub6 sub11sub12 sub13 sub14
subpanels EPID B
100frs
7th month 9th month 12th month 14th month 15th month
Figure 4.16 Subpanel image noise by SNR measured in a homogeneous field without attenuation combined with 10 and 100 frs/image, for a 15 x 15 cm2 field size at the isocenter.
Chapter 4
59 0
50 100 150 200 250
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14 subpanels
EPID B - 7th month Cu attenuation 15 x15
5cm_5frs 9cm_5frs 5cm_10frs 9cm_10frs 5cm_100frs 9cm_100frs
0 50 100 150 200 250
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14 subpanels
EPID A - 35th month Cu attenuation 15 x15
5cm_5frs 9cm_5frs 5cm_10frs 9cm_10frs 5cm_100frs 9cm_100frs
0 50 100 150 200 250 300 350
sub2 sub3 sub4 sub5 sub6 sub7 sub10sub11sub12sub13sub14sub15
EPID subpanels EPID A - 35th month
Polystyrene 16cm
20 x 20 L2_5frs
L2_10frs L2_100frs
This transversal study had shown that for CAXIS subpanels (sub: 4, 5, 12, 13) the SNR is highly dependent on attenuation (copper or polystyrene) and integrated frames during the acquisition. For both EPIDs the Off-axis subpanels (sub2, sub3, sub6, sub7, sub10, sub11, sub14 and sub15) SNR remains below ~110.
0 50 100 150 200 250 300 350
sub2 sub3 sub4 sub5 sub6 sub7 sub10sub11sub12sub13sub14sub15
EPID subpanels EPID B - 7th month
Polystyrene 16cm
20 x 20 L2_5frs
L2_10frs L2_100frs
Figure 4.18 SIN by SNR calculation using 16cm polystyrene attenuation, with 5, 10 and 100 frs/img. both for EPID A (left) and EPID B (right) based on a 20x20 cm2 analysis. Increasing the off- axis distance, decrease the system response to the different number of integrated frames (sub2, sub3, sub6, sub7, sub10, sub11, sub14 and sub15). The response variation in subpanel CAXIS (sub:
4, 5 12, 13) is lower for EPID A comparing to EPID B.
Figure 4.17 Subpanel image noise (SIN) by SNR calculation when applying 5 and 9 cm of copper attenuation with 5, 10 and 100 integrated frames, both for EPID A (left) and EPID B (right) based on a 15x15 cm2 analysis. EPID B was found to be more sensitive to changes in attenuation and frames per image
a) b)
a) b)
60 0
20 40 60 80 100
sub2 sub3 sub6 sub7 sub10 sub11 sub14 sub15 subpanels
EPID B - 7th month Copper: 4cm 20 x 20
SPD91_10frs SPD91_100frs TRAY_10frs TRAY_100frs By ageing the panel (EPID A) the sensitivity seems to be decreased which was demonstrated by the SNR reduction of CAXIS subpanels (figures 4.17a and 4.18a). For higher attenuation levels (5 and 9cm Cu), the reduction on pixel matrix sensitivity was easily recognized, figure 4.17a and 4.17b. With polystyrene the response for larger areas are similar and have always an increased value near the CAXIS.
The SNR assessment for larger area 20x20 cm2 was done by using 4 cm copper attenuation, where subpanels 4, 5, 12 and 13 were skipped, due to the central slab line artefact, caused by the attenuation junction. The test performed with larger copper attenuation, intended to measure the influence of the attenuator position related to the source (Tray=67cm and SPD=91cm) and the pixel response to the SNR (Figure 4.19).
Using the large slabs and skipping the central area, the remaining subpanels in evaluation didn’t show an evident difference on SIN response when increasing frames per image from 10 to 100 and changing the physical position of the 4 cm copper slabs. Moreover, for a 5 cm copper attenuation irradiated with 10 frs/img the ageing process only affect the central subpanels which is seen also for higher attenuations and frames per image (Figure 4.20).
Figure 4.19 SIN by SNR calculation using 4cm copper attenuation, with 10 and 100 frs/img.
based on a 20x20 cm2 analysis. The response variation for all subpanels have only slightly changes, even when the distance to the attenuator changes and integrated frames per image.
0 20 40 60 80 100
sub2 sub3 sub6 sub7 sub10 sub11 sub14 sub15 subpanels
EPID A - 35th month Copper: 4cm 20 x 20
SPD91_10frs SPD91_100frs TRAY_10frs TRAY_100frs
a) b)
Chapter 4
61 0
20 40 60 80 100 120 140 160
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID B
9cm Cu / 10frs 5th month
6th month 7th month 9th month 12th month 14th month 15th month 0
20 40 60 80 100 120 140 160 180 200
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14 EPID subpanels
EPID A inTime SNR
5cm Cu - 10frs 33rd month
34th month 35th month
0 20 40 60 80 100 120 140 160 180 200
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14 EPID subpanels
EPID B inTime SNR 5cm Cu /10frs
5th month 6th month 7th month
The evaluation of SIN in time by the SNR measurement was performed in time for the four available EPIDs, each one with different ages. EPID D and B, the young panels, showed a SNR response for CAXIS subpanels above ~100, for the tested months, and off-axis subpanels between 60 and 40, see Figure 4.21a and 4.21b.
The older panels, had shown SNR below 100 and 70 for all measurements with EPID C and A respectively (Figure 4.21c and 4.21d).
0 20 40 60 80 100 120 140 160
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID D
9cm Cu / 10 frs 2nd month
4th month 5th month
Figure 4.20 SIN by SNR calculation when applying 5cm copper with 10 frs/img based on a 15x15 cm2 analysis. CAXIS subpanels (sub: 4, 5, 12, and 13) has revealed in 3 month tested a SNR of 81 ± 19 and 164 ± 13 (1SD) for EPID A and EPID B respectively. The SNR for off-axis subpanels (sub: 3, 6, 11 and 14) remain stable in time within SNR variation of 4.3 and 2.5 (1SD) for EPID A and EPID B respectively.
a) b)
Fig. 4.21a Fig. 4.21b
62 0
20 40 60 80 100 120 140 160
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID A
9cm Cu / 10 frs 33rd month
34th month 35th month 37th month
In Time the four EPIDs reveal a decrease for almost all subpanels which was more relevant for central subpanels which experiment higher percent of reduction comparing to those off-axis subpanels (sub: 3, 6, 11, 14). Increasing the number of frames per image (100) allows a higher discrimination between the subpanels by increase the number of detectable photons, which increase the mean value and decrease the standard deviation for the younger EPIDs (Figure 4.22a and 4.22b). For EPID A and C the SIN values suggested that in time the SNR for the CAXIS may decrease and be very similar to the off-axis subpanels SNR values. For EPID A (Figure 4.22d) the SNR for all subpanels can be found beneath the ~50.
The table 4-I summarize the SIN variation of CAXIS subpanels for each EPID Lifetime, by setting up 5 and 9 cm copper attenuation with 10 and 100 frs/img.
5cm 10frs/img 9cm 10frs/img 9cm 100frs/img LIFETIME Research (month)
SNR: Mean ± SD SNR: Mean ± SD SNR: Mean ± SD
EPID D - 103 ± 26 140 ± 41 2nd – 5th
EPID B 164 ± 13 115 ± 9 155 ± 22 5th – 15th
EPID C - 48 ± 19 59 ± 31 26th – 28th
EPID A 81 ± 19 37 ± 9 30 ± 12 33rd – 37th
0 20 40 60 80 100 120 140 160
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID C
9cmCu / 10 frs 26th month
28th month
Figure 4.21 SIN by SNR calculation when applying 9cm copper with 10 frs/img based on a 15x15 cm2 analysis. For (a) and (b) the SNR values can be found above ~100 for CAXIS and between 60 and 40 for the off-axis subpanels. The older EPIDs in (c) and (d) revealed a reduction for less than 70 at all subpanels for EPID C and EPID A. (values far from the tendency are considered outliers – red circle)
c) d)
Table 4-I SIN evaluation for CAXIS subpanels with different setup conditions.
Analysis in Time for EPID A, B, C and D
Chapter 4
63 0
50 100 150 200 250
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID B
9cmCu / 100frs
7th month 9th month 12th month 14th month 15th month
0 50 100 150 200 250
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID C
9cmCu / 100 frs
26th month 28th month 0
50 100 150 200 250
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID D
9cmCu / 100 frs
2nd month 4th month 5th month
0 50 100 150 200 250
sub3 sub4 sub5 sub6 sub11 sub12 sub13 sub14
subpanels EPID A
9cmCu / 100 frs
35th month 37th month
Figure 4.22 SIN by SNR calculation when applying 9cm copper with 100 frs/img based on a 15x15 cm2 analysis. For (a) and (b) the SNR values can be found above ~115 for CAXIS and bellow ~65 for the off-axis subpanels. The older EPIDs in (c) and (d) revealed a reduction for less than ~75 and below ~50 at all subpanels for EPID C and EPID A respectively. (values far from the tendency are considered outliers – red ellipses and circles)
a) b)
c) d)
64 0
50 100 150
0 5 10 15 20 25 30 35 40
EPIDs Lifetime (months) Global SNR 9cm / 100 frs/img
EPID A EPID B EPID C EPID D 0
50 100 150
0 5 10 15 20 25 30 35 40
EPIDs Lifetime (months) Global SNR 9cm /10 frs/img
EPID A EPID B EPID C EPID D
Figure 4.23 Global evaluations in Time for four EPIDs in tests, when applied 9 cm of copper attenuation with 10frs/img (a) and 100frs/img (b). The images in between (a) and (b) are displayed according the time trends from the left to the right. The global EPIDs SNR was increased by 10% to 35 % from 10 to 100 frs/img. For older EPIDs (C and A) the SNR values were fixed below ~50, and more specifically below ~30 for EPID A. The red circle in EPID C, for both the 10 and 100 frs/img was considered as a study “outliers” caused by unknown setup parameters when imaging system was tested. For younger EPIDs the frames per image become an important parameter to setup limits for a decrease in the SNR, i.e. for EPID B was found a global SNR of ~85 and ~105 for 10 and 100 frs/img respectively. For EPID D we found values of ~95 and ~105 for 10 and 100 frs/img respectively.
a)
b)
EPID A EPID C
EPID B EPID D
Chapter 4
65 The SIN by the SNR measurement was tested for all four EPIDs with 10 and 100 frs/img acquisition mode (Figure 4.23a and 4.23b). This method was exhaustively calculated for all lines of each attenuated subpanel on the matrix when applying a 15x15cm2 beam. The variation in time is much likely to occur in the central axis subpanels. By increasing the integration frame number the SNR becomes higher for all measured subpanels in the matrix.
Combining results from the four EPIDs and with both frames per image, the reduction of global SIN in time showed high square correlation coefficients, higher than 0.83 (Figure 4.24). With ageing the EPID response become less dependent on the number of integrated frames per image, which could be found to be very similar after ~34 months.
R² = 0,8318 R² = 0,8451
0 10 20 30 40 50 60 70 80 90 100 110
5 10 15 20 25 30 35
SNR (%)
Lifetime (month) Subpanel Image Noise : SNR
IN TIME
SIN 100 SIN 10
Figure 4.24 Global SIN evaluation combining the four EPIDs responses with integrated 10 and 100 frs/img (normalized to 100% (maximum value of SIN 100)).
66 4.4 System sensitivity by Non Average Pixel response
The variation of NAPs in time is shown in the figure 4.24a, where the EPID A has revealed an increase of about 50% (2932 NAPs) and 51% (2145 NAPs) from the total NAPs, detected for 2,5 and 3 SD limits respectively, counted between the 33th and 37th month of lifetime (1st to 5th research time) for 10 frs/img.
Increasing the integrated frames per image gives a little increase on NAPs count which suggests an increase of sensitivity of the test for detection of small variations of matrix fixed noise pattern (Figure 4.25b).
For EPID B, when applying 10frs/img, the NAPs remains below the 0.5% of the total pixels in the matrix, for both statistical limits, 2.5 and 3 SD. For an integration of 100frs/img the NAPs ranged from 0.22% to 0.72% NAPs/matrix, with variation of 0.45±0.23% for 2.5SD statistical limits and; 0.1% to 0.28% NAPs/matrix with 0.21±0.08% for 3SD statistical limits.
Absolute and relative NAP values are summarised in the Table 4-I.
Figure 4.25 Describe the in Time variation of NAPs for EPID A and B, by setup 9cm copper attenuation with 10 and 100 integrated frames per image, (a) and (b) respectively. The standard deviations of 2.5 and 3 were set as statistical limits for the non average pixels count, related to a 500 x 500 matrix dimension (2,5x105 pixels).
0,00 0,50 1,00 1,50 2,00 2,50
1 2 3 5 8 10 11
% NAPs
Time (research) NAPs
10 frs/img (9cm copper)
EPID A (2.5 SD) EPID B (2.5 SD) EPID A (3 SD) EPID B (3 SD)
0,00 0,50 1,00 1,50 2,00 2,50
3 5 8 10 11
% NAPs
Time (research) NAPs
100 frs/img (9cm copper)
EPID A (2.5 SD) EPID B (2.5 SD) EPID A (3 SD) EPID B (3 SD)
a) b)
Chapter 4
67 The NAPs shown in EPID B are homogeneously distributed in the EPID analysis area, which could be related to a lower percentage of NAPs in the matrix, see Figure 4.24b. The clustered NAPs in the matrix present in the EPID A suggest a fixed noise pattern, located near the edges of each subpanel, Figure 4.26a. The histograms of each image in the figure 4.26 represent the variation of pixels intensity in the 500 x 500 pixel matrix around the mean value normalized to “1”.
Figure 4.26 Show the 500x500 matrix for NAPs analysis for EPID A, a) and B, b) where the fixed pattern can be seen in the junctions of the amplifiers subpanels. The EPID A begins to show some artefacts, and in EPID B is already installed a fixed noise pattern, represented by junction artefacts, that may affect considerably the treatment verification. The histograms are displayed in the logarithmic scale where the variable (y) indicates frequency of the pixels in the matrix. The abscissas indicate the intensity pixel value of the processed images with mean value normalized to
“1”.
a) b)
Pixel intensity Pixel intensity
Frequency (log) Frequency (log)
68 The Table 4-II is an overview of the optimal detection in time, by maximizing the NAPs count with 9cm copper with 2.5 SD limits for a 500x500 pixel matrix.
The NAPs in time when applying 2.5 SD limits and 9 cm copper attenuation on a 500 x 500 pixel matrix, demonstrated strong correlations above 0.9 for both 10 and 100 frs/img, see table 4-II and figure 4.27.
2.5 SD - 9cm copper
EPID A (33rd - 37th month) EPID B (5th – 15th month) EPID D (2nd – 5th month) Frames/image
month 10 100 10 100 10 100
1st 2961 1.18% 113 0.05%
2nd 5299 2.12% 795 0.32%
3rd 5377 2.15% 5416 2.17% 799 0.32% 1304 0.52%
5th 5893 2.36% 6025 2.4% 765 0.31% 561 0.22% 9 0.004% 2 0.001%
8th 1076 0.43% 1352 0.54% 255 0.102% 8 0.003%
10th 644 0.26% 599 0.24% 9 0.004% 6 0.002%
11th 1016 0.41% 1790 0.72% 6 0.002% 8
0.003%
Table 4-II NAPs percentage variation in a 500 x 500 matrix analysis. Relative percentage of NAPs in the entire panel matrix, appear to increase in a time trend matrix analysis for 9cm Cu attenuation with 10 and 100 frs/image.
R² = 0,9475
R² = 0,9064
0,0001 0,001 0,01 0,1 1
5 10 15 20 25 30 35
NAPs (%)
Lifetime (month) Non Average Pixel response
IN TIME
NAP 100 NAP 10
Figure 4.27 NAPs in the time trend, for 9 cm copper attenuation acquired with 10 and 100 frs/img, and calculated for 500 x 500 pixels based on 2.5 SD limits (the logarithm scale was applied to include all data points).
Chapter 4
69 0
1 2 3 4
2std 2,5std 3std
% NAPs
Stat. limits NAPs
5 / 9 cm Cu : 10 / 100frs/img 500x500
A_5_10 B_5_10 A_5_100 B_5_100 A_9_10 B_9_10 A_9_100 B_9_100 The influence of attenuation on non average pixels is shown in the Figure 4.28, where the setup of 5/9 cm of copper attenuation associated with 10/100 frames per image, shown only a small deviation in the percentage of NAPs present in the matrix for 2.5 and 3 SD. When applying 2SD limits, homogeneous NAPs spread may be found in the matrix for EPID B, See Figure 4.28d.
Different attenuations and frames per image seem to have influence on the NAPs evaluation when 2 SD was applied to the EPID B, which reveal a greater sensitivity to random fluctuations.
Figure 4.28 Demonstrates the influence of attenuation and frames per image in the NAPs count, when applying combinations of 5 and 9 cm of copper field attenuation with 10 and 100 frs/img for both EPIDs. The variation of NAPs across seems to vary for EPID B where the noise seems to have more influence of homogeneous and random noise pixel intensity distribution.
This becomes more pronounced when applying a higher integration number of frames. The variation of standard deviation analysis was set by 2SD for a) and d); 2,5SD for b) and e) and 3SD for c) and f), which are representations of 9cm copper attenuated fields with 10 integrated frames per image.
a) b) c)
d) e) f)
70 0,0
1,0 2,0 3,0 4,0 5,0
2std 2,5std 3std
% NAPs
NAPs 4 cm Cu (660x660)
EPID A_10frs EPID A_100frs EPID B_10frs EPID B_100frs By using 16 cm of polystyrene (PS) and 4cm copper attenuation, the variation of NAPs in the 660x660 were tested. The EPIDs that have been irradiated with a less attenuation levels (16cm and 4cm Cu) revealed higher sensitivity for integration frames variation.
The NAPs for EPID A and EPID B increase with PS attenuation when applying 100frs/img (Figure 4.29a). For 4cm Copper attenuation the influence of integrated frames per image is more pronounced for EPID B Figure 4.29b. By using smaller attenuations the NAPs count become more subjective for evaluation. As higher the difference between different integration frames, the higher is the statistical noise dependence.
For EPID A the effect of frame integration becomes less statistical dependent, which can be related to a fixed noise pattern in the images.
0,0 1,0 2,0 3,0 4,0 5,0
2std 2,5std 3std
% NAPs
NAPs
Polystyrene (16cm) (660x660)
EPID A_10frs EPID A_100frs EPID B_10frs EPID B_100frs
Figure 4.29 The NAPs measured in a 660 x 660 matrix, for 16 cm of polystyrene a) and 4 cm copper attenuation b) had shown to have a greater influence from different integration frames.
a) b)
EPID A
EPID B
Chapter 4
71 The central artefact in the 4cm Cu attenuation test is present in NAPs when the Cu slabs junction artefact is inside the statistical SD limits. The evaluation of NAPs in the 660 x 660 panel matrix was always affected by this mid line artefact which may induce false positive NAPs by pixel intensity variation along the line, even when replace the slab line artefact junction by the mean pixels values of the matrix.
The large area analysis for EPID B were tested in time, which demonstrates a strong correlation of the sample in time (R2=0,981), when applying a two standard deviation limits (Figure 4.30). For this analysis with 2SD, the result demonstrate an increase of NAPs from 13049 (~3 %) to 17339 (~4 %) in a 660x660 pixels matrix. Increasing the statistical limits by 2.5 and 3, the NAPs in time, cannot be correlated to degradation of the panel (4cm Cu), which was suggested by the results with higher attenuation levels like 5 and 9cm copper.
4.5 Subjective clinical evaluation
Clinical images were acquired, and displayed for quality appreciation for both pelvic and thorax locations. The clinical evaluation of portal images performed by clinical specialists is summarized in Figure 4.31 and 4.32. The analysis consisted in the association of questions 1.1, 1.2 and 1.5 which represent the anatomical edge detection and delineation and the overall image quality, this association was performed due to the ability of the human eye to find edges which is related to signal, noise and spatial resolution that are related to the overall image quality.
The question 1.3 just can be only applied to specific treatment sites and verification procedures.
The applicability of fiducial markers (small radiopaque seeds inserted in the prostate by the urologist), is justified by the organ motion during the treatment. As the number of images that contain the fiducials wasn´t enough to constitute a significant sample size, they have been removed from the initial data.
The question concerning to artefacts/noise in the images (1.4) was performed to achieve more specific understanding about defected pixels in the clinical images.
The clinical evaluation of anatomical edge detection had shown small differences from edge detection to artefacts detection (Figure 4.31a and 4.31b). Data had shown that for the first 30
R² = 0,9819
0,0 1,0 2,0 3,0 4,0 5,0
9 12 14 15
%NAPs
EPID Lifetime EPID B 660 x 660 4cm CU / 100frs/img
2 SD 2.5 SD 3 SD
Figure 4.30 Shows in time variation of NAPs on a 660x660 pixel matrix for EPID B. For 2 SD statistical limits the NAPs increase from 3% to 4%.
72 months, 75% of the results lay between score 5 (strongly agree) and 3 (undecided). For pelvic location a reduction on the image quality was observed for the 34 and 36 months (EPID A), where about 50% of the values can be found below “undecided” and 75% between score 4 (agree) and 1 (strongly disagree).
In terms of qualitative analysis the scores were reorganized in Table 4-IIa and Table 4-IIIa to determine negative (scores 1 and 2), null (score 3) and positive (scores 4 and 5) image quality by the clinical subjective appreciation.
For clinical anatomical edge detection evaluation on pelvic location (Table 4-IIa), EPID B (younger) had shown for the period of 15 months a score of 3.9 ± 0.4, with 81 ± 14% of positive against 14 ± 8% of negative answers. EPID D had initial positive value of 93.3 % which decrease about 35% in the second evaluation. EPID C during a 6 month period showed a score of 3.5 ± 0.3%, with 66 ± 10% positive and 18 ± 10% negative answers. EPID A (older) had a reduction of image quality from a score of ~4 to 2.8 during 6 month of evaluation. This result came together with a final measurement (before the end of clinical usability) of about 35% of positive answers in opposition to ~45% of negative answers.
When analysing the artefacts in the clinical images the results demonstrate that 75% of values can be found beneath score 4 (agree). For the last three lifetime months (34th to 36th) at least 75% of the values were found below “undecided”, with 50% less than “disagree”.
Qualitative analysis of artefact detection (Table 4-IIb) showed for EPID B a score of 3.7 ± 0.44, with 74 ± 18% of positive against 22 ± 14 % of negative answers, during a 15 month evaluation. Looking at EPID D, the first measurement was scored positively for 80% of the cases which decreases about 45% after 3 months. EPID C with 45 % of negative cases increased approximately 30% in 6 months (27th to 33rd), with a score of 2.8 ± 0.25 during this time.
Figure 4.31 Quantitative clinical evaluations of edge detection/image quality (a) and artefacts detection (b) for EPIDs (identified in the (x) scale line) aged from 2 to 36 months of clinical usability at pelvic location. Images were scored from 1 (strongly disagree - less clear) to 5 (strongly agree – more clear).
a) b)
Table 4-II Qualitative clinical detection (b) at pelvic location
positive (score 4 and 5) was measure for lifetime evaluation.
images for the respective EPIDs During the 6 months (30th to 36
from 45% to ~75%, with a main score decrease from ~3 to ~2.
Edge Detection / Image Quality (qualitative): PELVIS EPID A
Month* Mean Score (%)
Month*
neg nul pos
30 3,7 12,5 10,7 76,8 2
33 4,3 - 3,3 96,7 5
34 2,7 44,6 17,9 37,5 6
36 2,8 41,7 25 33,3 7
11 14 17
EPID A EPID
Month* Mean Score (%)
Month* Mean
neg nul pos
30 3,1 45 5 50 2 3,9
33 2,6 50 20 30 5 4,3
34 2,0 80 20 - 6 3,8
36 2,2 70 20 10 7 4,1
11 3,2
14 3,8
17 3,1
36th month 33th month
11th month 6th month
a)
b)
clinical evaluations of edge detection/image quality (a)
location. The percentage of negative (score 1 and 2), null (score 3) and positive (score 4 and 5) was measure for lifetime evaluation. Images represent portal localization images for the respective EPIDs
to 36th) of evaluation EPID A experiment an increase of negative cases to ~75%, with a main score decrease from ~3 to ~2.
Edge Detection / Image Quality (qualitative): PELVIS
EPID B EPID C
Mean Score (%)
Month* Mean Score (%)
Month*
neg nul pos neg nul pos
4,3 1,7 8,6 89,7 27 3,5 18,3 21,7 60
4,4 - 3,3 96,7 30 3,8 8,3 15 76,7
3,8 17,9 12,5 69,6 33 3,3 27,8 12,2 60
4,3 - 3,3 96,7
3,5 21,7 18,3 60
3,7 10 16,7 73,3
3,6 20 - 80
Artefacts Detection (qualitative): PELVIS
EPID B EPID C
Mean
Score (%)
Month* Mean
Score (%)
neg nul pos neg nul pos
3,9 5,9 11,8 82,4 27 2,8 45 25 30
4,3 - 12,5 87,5 30 3,0 40 25 35
3,8 - 25 75 33 2,5 76,7 6,7 16,7
4,1 - 5 95
3,2 30 20 50
3,8 - 20 80
3,1 30 20 50
month month
30th month (AP/Lat.)
Chapter 4
73 edge detection/image quality (a) and artefacts . The percentage of negative (score 1 and 2), null (score 3) and Images represent portal localization xperiment an increase of negative cases
EPID D
Month* Mean Score (%)
neg nul pos
4 4,1 6,7 - 93,3
7 3,4 27,8 14,4 57,8
EPID D
Month* Mean
Score (%)
neg nul pos
4 3,7 10 10 80
7 2,8 44,8 20,7 34,5
4h month (AP/Lat.)
