Potential of Contrast Agents Based on High-Z Elements for Contrast-Enhanced Photon-Counting Computed Tomography.
PURPOSE
In clinics, only iodine- and barium-based contrast agents are currently used for contrast-enhanced (CE) x-ray computed tomography (CT). Recently, the introduction of new photon-counting (PC) detectors increased the interest in developing new contrast agents based on heavier elements. These elements may provide more contrast and spectral information compared to iodine and barium thanks to their k-edges at higher energies. In this paper, the potential of high-Z elements in contrast-enhanced CT was evaluated for different patient sizes and x-ray spectra using a PC detector.


METHODS
An adult liver phantom with five high-Z element solutions (iodine, gadolinium, ytterbium, tungsten and bismuth) was scanned with a whole-body PCCT prototype. For each element, the contrast-to-noise ratio at unit concentration and at unit dose (CNRCD) was evaluated in low threshold images (T0 = 20 keV) as function of the tube voltage (80, 100, 120 and 140 kV) and in bin images (tube voltage = 120 kV) as function of the higher threshold (T0 = 20 keV and T1 2 [50; 90] keV). Simulations were performed for validation with measurements and to investigate more elements (cerium and gold), different patient sizes (infant, adult and obese) and spectrum filtration (with and without 0.4 mm tin filter). The dose reductions associated to the CNRCD improvements over iodine were quantified as well.


RESULTS
CNRCD improvements and dose reductions depend on the investigated scenario. For the infant phantom, dose reductions around 30% were reached using cerium or gadolinium in combination with the tin filter. For the adult and obese phantom, reductions around 50% were provided by gadolinium or ytterbium in combination with the tin filter. Independently of the high-Z element, the CNRCD of two optimally combined bin images was higher than the CNRCD of the low-threshold image. Good agreement was found between measurements and simulations.


CONCLUSIONS
Between the investigated elements, gadolinium resulted to have the highest potential as novel contrast agent in PCCT, providing significant dose reductions for all patient sizes. Compared to the other elements, the implementation of gadolinium as CT contrast agent may be facilitated since it is already deployed as contrast agents for magnetic resonance imaging.