Figure 1
(A) QC phantom fixed at the breast position on the patient support. The upper portion is uniformly filled with water, while the radiographic details are visible in the lower portion. (B) Arrangement of the details at the bottom of the QC phantom. The rods are made of polyethylene (PE), nylon, polyoxymethylene (POM), polytetrafluoroethylene (PTFE), and BR12, a plastic material mimicking the attenuation properties of breast tissue. The structure of the low contrast PMMA insert, exhibiting the five holes of varying diameter, is clearly perceivable.
Figure 2
Example of reconstructed water image (left panel) and detail image (right panel), acquired at 38 keV and high dose.
Table 1
Parameters of the linear regressions between measured CT numbers and tabulated attenuation coefficients
| Energy (keV) | Dose | R2 | slope (cm–1) | intercept (cm–1) |
|---|---|---|---|---|
| 32 | high | 0.9996 | 175 ± 4 | 0.019 ± 0.009 |
| low | 0.9996 | 170 ± 4 | 0.025 ± 0.009 | |
| 35 | high | 0.9997 | 176 ± 4 | 0.018 ± 0.008 |
| low | 0.9996 | 173 ± 4 | 0.017 ± 0.009 | |
| 38 | high | 0.9998 | 177 ± 3 | 0.016 ± 0.005 |
| low | 0.9992 | 175 ± 3 | 0.013 ± 0.006 | |
| 40 | high | 0.9992 | 174 ± 7 | 0.021 ± 0.012 |
| low | 0.9991 | 172 ± 7 | 0.019 ± 0.012 |
Figure 3
Examples of linear regressions, performed on high dose images acquired at 32, 35, 38 and 40 keV, from top left panel to bottom right panel. R2 coefficients, slopes and intercepts are given as well.
Table 2
Accuracies of reconstructed attenuation coefficients for the plastic materials and water, acquired at 38 keV and high dose. Expected values, measured values and relative percentual differences are given for μ coefficients, absolute differences for the corresponding HU deviations
| Material | expected μ (cm–1) | measured μ (cm–1) | accuracyμ (%) | accuracyHU |
|---|---|---|---|---|
| PE | 0.216 | 0.221 ± 0.002 | 3.1 ± 1.3 | 24 ± 10 |
| BR12 | 0.260 | 0.249 ± 0.004 | 4.0 ± 1.6 | 37 ± 14 |
| Nylon | 0.271 | 0.273 ± 0.003 | 0.9 ± 1.0 | 8 ± 9 |
| PMMA | 0.290 | 0.291 ± 0.003 | 0.4 ± 1.2 | 4 ± 12 |
| POM | 0.359 | 0.360 ± 0.003 | 0.2 ± 0.8 | 3 ± 11 |
| PTFE | 0.622 | 0.633 ± 0.003 | 0.1 ± 0.5 | 3 ± 11 |
| Water | 0.282 | 0.278 ± 0.003 | 1.5 ± 0.9 | 14 ± 9 |
Table 3
Uniformity of reconstructed attenuation coefficient for water. Relative percentual values are given for μ coefficients, absolute values for the corresponding HU deviations
| Energy (keV) | Dose | uniformityμ (%) | uniformityHU |
|---|---|---|---|
| 32 | high | 1.79 | 17 |
| low | 3.19 | 32 | |
| 35 | high | 1.45 | 12 |
| low | 1.47 | 13 | |
| 38 | high | 2.58 | 26 |
| low | 1.35 | 13 | |
| 40 | high | 2.54 | 24 |
| low | 1.07 | 10 |
Table 4
Noise measurement of the reconstructed attenuation coefficient for water. Relative percentual results are given for μ coefficients, absolute results for the corresponding HU values
| Energy (keV) | Dose | noiseμ (%) | noiseHU |
|---|---|---|---|
| 32 | high | 4.4 | 60 |
| low | 8.5 | 103 | |
| 35 | high | 3.8 | 83 |
| low | 7.3 | 114 | |
| 38 | high | 3.7 | 66 |
| low | 6.5 | 109 | |
| 40 | high | 3.5 | 66 |
| low | 6.2 | 111 |
Tabel 5
Low contrast measurements. Relative percentual contrasts are given for μ coefficients, absolute contrasts for the corresponding Hounsfield unit (HU)
| Energy (keV) | Detail (mm) | μ contrast (%) | HU contrast | ||
|---|---|---|---|---|---|
| high dose | low dose | high dose | low dose | ||
| 32 | 10 | 0.81 | 0.71 | 7 | 5 |
| 8 | 0.96 | 0.87 | 9 | 7 | |
| 6 | 1.10 | 0.53 | 10 | 4 | |
| 4 | 1.08 | 1.12 | 10 | 10 | |
| 2 | 0.52 | 0.93 | 5 | 8 | |
| 35 | 10 | 1.80 | 1.85 | 15 | 15 |
| 8 | 1.68 | 1.73 | 14 | 14 | |
| 6 | 1.56 | 1.64 | 13 | 13 | |
| 4 | 1.21 | 0.87 | 10 | 8 | |
| 2 | 0.90 | 0.91 | 8 | 8 | |
| 38 | 10 | 4.14 | 3.86 | 41 | 39 |
| 8 | 3.99 | 3.41 | 40 | 34 | |
| 6 | 3.66 | 4.02 | 36 | 41 | |
| 4 | 3.91 | 3.45 | 39 | 35 | |
| 2 | 3.17 | 3.79 | 31 | 38 | |
| 40 | 10 | 5.35 | 5.49 | 54 | 55 |
| 8 | 5.09 | 5.43 | 52 | 55 | |
| 6 | 5.12 | 4.85 | 52 | 49 | |
| 4 | 5.31 | 3.41 | 54 | 34 | |
| 2 | 5.73 | 3.79 | 59 | 38 | |