Metal artifact reduction and contrast agent performance as a function of source spectral shape

Objective. Empirically map the x-ray source of a Medipix All Resolution Scanner (MARS) photon counting CT (PCCT) with Cu and Sn pre-filters, assess metal artifact reduction (MAR) capabilities of these pre-filters, and measure pre-filtration impact on contrast performance (CP) of FDA approved iopromide and experimental tantalum oxide nanoparticles (TaOx NPs). Approach. The x-ray source of a MARS-PCCT system was empirically mapped with no pre-filtration, seven Cu filters (0.3-2.1 mm), and seven Sn filters (0.15-1.05 mm). A phantom with inserts containing water, lipid, iopromide, TaOx NPs, and metal was scanned with no pre-filtration and Cu and Sn pre-filters. Insert noise, signal, contrast-to-pooled-noise ratio (CPNR), and a fast Fourier transform artifact metric (FFTAM) were calculated for each filter to quantify MAR and CP. Main Results. Thick filters for Cu and Sn shifted mean energy of the unfiltered x-ray source (47.9 keV) by 19.2 and 23.4 keV, respectively. Thick filtration, 2.1 mm Cu and 0.6 mm Sn, greatly reduced noise (up to 74%) and FFTAM (up to 71%) for all inserts and energy bins. Thin filtration, 0.3 mm Cu and 0.15 mm Sn, also reduced noise (up to 47%) and FFTAM (up to 41%). In most cases, iopromide lost significant contrast (up to 50%). TaOx NPs also lost contrast, though to a lesser extent (up to 38%). Pre-filtration improved image efficacy (i.e. CPNR), especially for TaOx (up to 61%). Significance. By empirically mapping the source spectrum of a MARS-PCCT system with pre-filters, valuable information was gathered about photon flux distribution and detector artifacts; these findings will prove insightful for applications such as energy binning for effective material decomposition. Furthermore, this information will potentially guide clinical MAR development, most notably for the MARS Extremity 5X120 recently deployed for first-in-human trials. Lastly, TaOx NPs were shown to be more compatible than iopromide with spectral shaping.