Photon-counting Computed Tomography of Degradable Bone Cement Loaded With Gadolinium Nanoparticles

This study aims to improve the radiopacity of absorbable bone cements through the addition of gadolinium nanoparticles (GdNP). We also aim to determine whether photon-counting CT (PCCT) provides superior contrast-to-noise ratio (CNR) between GdNP-loaded bone cement and vertebral bone when compared with energy-integrating CT (EID-CT), and to evaluate the accuracy of PCCT material decomposition for quantifying gadolinium concentration in solution and the GdNP-loaded cements.

GdNPs were synthesized using a one-pot thermal decomposition method and characterized using transmission electron microscopy and dynamic light scattering. Hydroxyapatite-based bone cement was loaded with varying mass fractions of GdNPs (0% to 10% w/w), and the CNR between the GdNP-loaded cement and vertebral bone was evaluated using preclinical micro-EID-CT and micro-PCCT scanners. Gadolinium material decomposition images were used to measure the amount of gadolinium present in each of the cements. In addition, gadolinium standards (0 to 20 mg/mL) were imaged with a preclinical micro-PCCT, and the concentration of gadolinium in the vials was estimated using gadolinium material decomposition images.

The synthesized GdNPs had a mean diameter of 15.42±1.82 nm. Signal intensity increased with increasing mass fractions of GdNPs for both EID-CT and PCCT. In EID-CT images, cements with ≥4% GdNP loading had higher CNRs relative to bone than the cement with no GdNP loading ( P <0.05). The CNR between the 8% and 10% GdNP-loaded bone cement significantly differed from than the bone cement with no GdNP loading for all PCCT energy bins ( P <0.05). The 42-51 keV energy bin yielded the largest CNRs overall when compared with the CNRs of other energy bins. Overall, the CNRs obtained from PCCT images were larger than the EID-CT CNRs. The concentration of gadolinium in the cements measured using the PCCT material decomposition images was correlated with the mass fractions of GdNP ( r =0.9753). Estimated gadolinium concentrations were highly correlated with the nominal concentration of the gadolinium standards ( r =0.999) and the PCCT was able to accurately quantify gadolinium concentrations with a root mean square error of 1.60 mg/mL.

The use of GdNPs led to a higher cement-vertebra CNR for both EID-CT and PCCT. Overall, PCCT demonstrated higher CNRs than EID-CT. Material decomposition successfully quantified the concentration of gadolinium in vials and allowed for improved visual differentiation of the GdNP - loaded bone cement from the calcium-based vertebral bodies. Thus, the incorporation of radiopaque GdNPs and imaging with PCCT improved visualization of the bone cement. These methods could be used to improve monitoring of implanted bone cements. In addition, PCCT material decomposition enabled accurate quantification of gadolinium in solution.