Influence of Acquisition Parameters on Silver Sulfide Nanoparticle Contrast in Photon-Counting Digital Mammography: A Phantom Study

Abstract

Photon-counting mammography is an emerging modality that allows for spectral imaging and provides a differentiation of material compositions. The development of photon-counting mammography-specific contrast agents has yet to be explored. In this study, the contrast, sensitivity, and organ dose between silver sulfide nanoparticles (Ag₂S-NPs) and a clinically approved iodinated agent (iopamidol) were investigated using a contrast-embedded gradient ramp phantom and a prototype scanner. For a given agent, the signal intensity increased with concentration, tube voltage (kV), and high bin fraction, while remaining constant with the tube current exposure time product (mAs). Moreover, Ag₂S-NPs produced significantly stronger contrast and improved sensitivity compared to iodine, especially when imaged at lower tube energies. Therefore, the use of photon-counting techniques and a silver-based contrast agent may markedly increase the contrast and contrast-to-noise ratios or reduce the radiation dose for contrast-enhanced mammography. Silver may be better suited than iodine for contrast agent development for spectral photon-counting mammography.

Keywords: mammography; nanoparticles; photon-counting; silver sulfide; spectral imaging.

Figure 1.

Characterization of ultrasmall Ag₂S-NPs. (A) Schematic depiction of Ag₂S-NP synthesis. (B) Electron micrograph, (C) EDX spectrum, (D) XRD pattern, and (E) in vitro biocompatibility of Ag₂S-NPs. * indicates p < 0.05 compared to control (no nanoparticle treatment).

Figure 2.

(A) Example phantom images of Ag₂S-NPs acquired using the following acquisition parameters: 38 kV, 20 mAs, and 0.3 HBF. The sum, LE, HE, and DE images are shown for each Ag₂S-NP concentration. Yellow boxes represent the ROIs used as part of image processing and analysis. (B) LE and HE signals as a function of concentration demonstrate the energy dependence of the contrast agents. Measurements are shown for a phantom imaged by using a technique of 38 kV, 20 mAs, and 0.3 HBF. DE signal intensity produced from Ag₂S-NPs as a function of (C) tube voltage (with HBF = 0.3) and (D) HBF (with 20 mAs) for a solution concentration of 30 mg silver per mL. Dashed lines represent the linear fit through the data.

Figure 3.

CNR as a function of HBF for (A) Ag₂S-NPs and (B) iodine contrast. Dashed lines represent the linear fit through the data. Data was analyzed from phantom images acquired using 30 mg/mL sample and 20 mAs.

Figure 4.

CNR and $\frac{\text{CNR}}{\sqrt{\text{dose}}}$ as a function of beam energy between (A,B) Ag₂S-NPs and (C,D) iodine agents. Phantom images were acquired using 20 mAs and 0.3 HBF. Note the difference in the ordinate scale. Organ dose as a function of (E) kV and (F) mAs for the Ag₂S-NP contrast agent. Phantom images were acquired using 0.3 HBF and 30 mg/mL sample. Dashed lines represent the linear fit through the data.

Scheme 1.

Schematic Depiction of the Effect of the Contrast Agent in Photon-Counting Mammography; (A) Incident Mammographic X-ray Energy Spectrum; (B) Schematic Depiction of the Attenuation of a K-Edge Material and Matrix (e.g., the Materials of the Phantom); (C) Transmitted Energy Spectrum after Attenuation by Contrast Materials with Detected Photons Being Discriminated via Different Energy Thresholds or High Bin Fractions; and (D) Estimates of the Corresponding LE and HE Signals Obtained in an Area Containing the Contrast Agent with Different Energy Bins or Thresholds