Maximizing microcalcification detectability in low-dose dedicated cone-beam breast CT: parallel cascades-based theoretical analysis

Abstract

Purpose: We aim to determine the combination of X-ray spectrum and detector scintillator thickness that maximizes the detectability of microcalcification clusters in dedicated cone-beam breast CT.

Approach: A cascaded linear system analysis was implemented in the spatial frequency domain and was used to determine the detectability index using numerical observers for the imaging task of detecting a microcalcification cluster with 0.17 mm diameter calcium carbonate spheres. The analysis considered a thallium-doped cesium iodide scintillator coupled to a complementary metal-oxide semiconductor detector and an analytical filtered-back-projection reconstruction algorithm. Independent system parameters considered were the scintillator thickness, applied X-ray tube voltage, and X-ray beam filtration. The combination of these parameters that maximized the detectability index was considered optimal.

Results: Prewhitening, nonprewhitening, and nonprewhitening with eye filter numerical observers indicate that the combination of 0.525 to 0.6 mm thick scintillator, 70 kV, and 0.25 to 0.4 mm added copper filtration maximized the detectability index at a mean glandular dose (MGD) of 4.5 mGy.

Conclusion: Using parallel cascade systems' analysis, the combination of parameters that could maximize the detection of microcalcifications was identified. The analysis indicates that a harder beam than that used in current practice may be beneficial for the task of detecting microcalcifications at an MGD suitable for breast cancer screening.

Keywords: breast CT; breast cancer; cascaded systems; microcalcifications; numerical observers.

Fig. 1

X-ray spectra (normalized to unit area) incident on the breast with varying thicknesses of copper filtration.

Fig. 2

Imaging task considered was the detection of a cluster of microcalcifications. (a) Orientation of the cluster in the spatial domain. Each microcalcification was modeled as a calcium carbonate sphere of $170\mu \mathrm{m}$ diameter and the spacing between the spheres was 2 mm. (b) The task function in the spatial frequency domain.

Fig. 3

Validation of the CSA model with empirical measurements of (a) MTF and (b) NPS. Good agreement is seen between the experimental results and the CSA model. The acquisition and system parameters used in the CSA model matched those of the BCT system.

Fig. 4

System MTF and the factors that contribute to the system MTF. Parameters are for 70 kV, $525\mu \mathrm{m}$ scintillator thickness, 4.5 mGy MGD, and 0.25 mm thick Cu filter. Nyquist frequency for the system is $4.7687\mathrm{cy}/\mathrm{mm}$.

Fig. 5

The system MTF at (a) 50 kV and (b) 75 kV for varying scintillator thickness. In order to show the kV dependence, the (c) 50% MTF and (d) 10% MTF are shown. The thickness of the Cu filter is fixed at 0.25 mm and the MGD at 4.5 mGy.

Fig. 6

Projection space DQE for various scintillator thickness. X-ray spectrum was fixed at 70 kV and 0.25 mm Cu filtration.

Fig. 7

Zero-frequency DQE versus applied tube voltage (kV). Copper filter thickness is fixed at 0.25 mm.

Fig. 8

Noise power spectra (NPS) for various combinations of applied tube voltage (kV) and scintillator thickness. The filter thickness and the MGD are fixed at 0.25 mm Cu and 4.5 mGy, respectively. Top two rows show the radial (cross-sectional) NPS and bottom two rows show the longitudinal NPS.

Fig. 9

Detectability indices as a function of applied tube voltage (kV) for (a) PW numerical observer with 0.4 mm thick Cu X-ray beam filter, (b) prewhitening (NPW) numerical observer with 0.4 mm thick Cu X-ray beam filter, and (c) NPWE filter numerical observer with 0.25 mm thick Cu X-ray beam filter. For each numerical observer, the Cu filter thickness corresponds to the maxima of the detectability index for that observer model and is shown in Table 1.

Fig. 10

Detectability index for PW numerical observer as a function of Cu filter thickness (a) 50 kV, (b) 65 kV, and (c) 75 kV. The vertical dotted line represents the X-ray tube/generator limit.

Fig. 11

Detectability index for NPWE numerical observer as a function of Cu filter thickness (a) 50 kV, (b) 65 kV, and (c) 75 kV. The vertical dotted line represents the X-ray tube/generator limit.