An early investigation of ytterbium nanocolloids for selective and quantitative "multicolor" spectral CT imaging

Abstract

We report a novel molecular imaging agent based on ytterbium designed for use with spectral "multicolor" computed tomography (CT). Spectral CT or multicolored CT provides all of the benefits of traditional CT, such as rapid tomographic X-ray imaging, but in addition, it simultaneously discriminates metal-rich contrast agents based on the element's unique X-ray K-edge energy signature. Our synthetic approach involved the use of organically soluble Yb(III) complex to produce nanocolloids of Yb of noncrystalline nature incorporating a high density of Yb (>500K/nanoparticle) into a stable metal particle. The resultant particles are constrained to vasculature (∼200 nm) and are highly selective for binding fibrin in the ruptured atherosclerotic plaque. Nanoparticles exhibited excellent signal sensitivity, and the spectral CT technique uniquely discriminates the K-edge signal (60 keV) of Yb from calcium (bones). Bioelimination and preliminary biodistribution reflected the overall safety and defined clearance of these particles in a rodent model.

Figure 1

Synthesis and physico-chemical characterization of self-assembled Ytterbium nanocolloids (YbNC). Schematic describing the preparation of Yb-enriched YbNC: (i) Suspension of Yb(III)-2,4-pentanedionate in Polyoxyethylene (20) sorbitan monooleate, vigorously vortex and mixing, filter using cotton bed, vortex; (ii) preparation of phospholipids thin film comprised of egg lecithin PC, (iii) resuspension of the thin film in water (0.2 μM); (iv) microfluidization at 4°C, 20,000 psi (141 MPa), 4 min, dialysis (cellulosic membrane, MWCO 20K).; characterization table for a representative preparation of YbNC.; (a) Number-averaged hydrodynamic diameter distribution of YbNC; (b) TEM images of the lipid-encapsulated nanocolloids; (c) AFM image of YbNC drop deposited over glass grid; (d) Physico-chemical characterization chart.

Figure 2

While conventional CT renders an image providing information about the overall attenuation, commonly represented in Hounsfield units (a), spectral CT is capable of separating the K-edge information and selectively image Ytterbium (b). The Yb k-edge signal (red-yellow) were overlaid on the traditional CT image of YbNC dilutions, a water and CaCl₂ sample (gray). The signal intensity from serial dilutions of YbNC shows a linear correlation to Yb concentrations obtained by ICP-OES (c) which makes spectral CT a quantitative imaging technique.

Figure 3

Blood pool imaging in mouse after bolus application of nontargeted Yb nanocolloids (6ml/kg). (a) Pseudo-conventional CT image composed from spectral measurements, slice through heart (dashed line). Statistical image reconstruction of Yb signal after 1 (b) and 20 iterations. (c) The volume rendered conventional CT image with super positioned Yb signal (green) shows accumulation of Yb in the heart and the clear separation from bone (d).

Figure 4

In vivo pharmacokinetics, biodistribution and clearance of YbNC in mouse. (a) pharmacokinetic profile of nontargeted YbNC with a biexponential fit [y=0.0456*exp(−0.0391*x)+0.1022*exp(−0.0018*x)). (b) Organ distribution of YbNC based on ytterbium estimation of major organs by ICP-OES at 2, 24h and 7d following intravenous injection of YbNC (1 mg/mL). (c) whole body clearance of YbNC showing only ca.11% remaining after 7days post YbNC administration.