Figure 3 demonstrates the visual effects of our background subtraction technique showing the effectiveness of our technique. However, the question then becomes at what point are true counts being artificially subtracted out of the image as we increase the background value. Table 3 allows us to better understand the magnitude of these background. The background values range from 9.4% ± 4.9 % to 23.5% ± 11.6% of the max count value in the image, showing we are subtracting a substantial count value compared to the largest value in the image. Additionally, Table 3 shows that although most counts within this background contour are small, you can see that there are still large values within this area, just furthering the point that this background subtraction is necessary considering that in theory, there should be no counts found within this contour. By subtracting these low count values uniformly across the image we are decreasing the amount of noise and low dose in the image.
Table 3 Statistics for the background contour for each patient based on SPECT counts
Patient no.
|
Mean background count
|
Background count max
|
Background count standard deviation
|
1
|
5.0
|
76.8
|
4.8
|
2
|
5.1
|
27.4
|
3.6
|
3
|
1.6
|
15.4
|
1.5
|
4
|
6.5
|
58.3
|
5.6
|
5
|
7.7
|
47.0
|
5.8
|
6
|
3.9
|
20.9
|
2.7
|
7
|
6.6
|
40.1
|
5.0
|
8
|
4.7
|
38.0
|
4.1
|
9
|
6.5
|
32.2
|
4.3
|
10
|
5.1
|
85.0
|
3.8
|
As expected, Table 4 shows that as the subtracted background value increases, the max dose value increases for both dose calculation methods. This trend further demonstrates that a statistically defined background subtraction is needed. No background correction can predict too low of a dose delivered while too high of a background subtraction will predict too high of a dose delivered.
Table 4 Effect of background subtraction on maximum dose value
|
Ratio of max dose values between background subtracted images and original images
|
|
WFBH Bkgrd:
WFBH
|
WFBH Bkgrd + SD:
WFBH
|
WFBH Bkgrd + 2SD:
WFBH
|
LDM Bkgrd:
LDM
|
LDM Bkgrd + SD:
LDM
|
LDM Bkgrd + 2SD:
LDM
|
Average
|
6.692
|
10.503
|
15.864
|
4.805
|
7.516
|
11.270
|
Standard Deviation
|
2.351
|
4.056
|
7.483
|
2.012
|
3.439
|
6.005
|
Ratio of max dose value obtained after background subtraction compared to its respective original image max dose value averaged over all patients
The dose line profiles in Figure 4a demonstrate that absolute dose is effected by background subtraction, the maximum dose increases as the background subtraction value increases. When the dose line profiles are normalized to the maximum point in each respective method, we see that dose distributions are not spatially being effected for a given background level (Figure 4b). Also note in Figure 4b the low dose region shrinking as the background subtraction value increases. This demonstrates the potential of removing low dose delivered regions that could important to consider in future patient care. Through these dose profiles we see an agreement between LDM and WFBH dose calculation methods and Table 5 shows the agreement of the resulting maximum dose values. LDM yielded higher maximum dose values on average without background consideration but upon background subtraction the two methods give comparable ratios.
Table 5 Comparison of LDM and WFBH maximum dose values
|
Ratio of max dose values obtain via LDM and WFBH methods
|
|
LDM:
WFBH
|
LDM Bkgrd:
WFBH Bkgrd
|
LDM Bkgrd + SD:
WFBH Bkgrd + SD
|
LDM Bkgrd + 2SD:
WFBH Bkgrd + 2SD
|
Average
|
1.378
|
0.970
|
0.967
|
0.966
|
Standard Deviation
|
0.205
|
0.106
|
0.105
|
0.106
|
Ratio of max dose values obtained comparing LDM to WFBH dose calculation methods, averaged over all patients
Figure 5 shows representative DVH curves for the listed liver dose distributions. We found that DVH curves for WFBH and LDM methods were not statistically different in 39 out of 44 curves through two tailed p-values (statistical significance deemed to be less than 0.05). In these 5 curves that were deemed statistically different, 3 were from the same patient and were for the resulting DVH curves after each background subtraction value. Figure 6 shows the DVH curves for this patient and as we see the LDM method is shifted below the WFBH method in these three cases. The other 2 statistically different curves were for just one set of curves in each patient. Across all three patients their treatment area is right at the edge of the liver contour.
In Figure 5-7 WFBH Bkgrd and LDM Bkgrd show an expected shape change for DVH curves due to the decrease in low dose and increase in high dose as well as healthy tissue sparing. Figure 7 shows DVH curves for the treated region, defined to be the region that is up to 20% of the max activity. These curves demonstrate better dose coverage with Bkgrd subtraction value compared to no correction. At the same time, Bkgrd + SD and Bkgrd + 2SD DVH curves show that these background values are unphysically skewing the delivered dose distribution to less coverage at low dose values than no background correction hence an unphysical altering of dose distribution at these levels. With Bkgrd subtraction level we do not see this effect and it is deemed the largest value we can subtract without creating unphysical changes in the DVH curves.