Synthesis, characterization, and computed tomography imaging of a tantalum oxide nanoparticle imaging agent.

Water-soluble ≤6 nm tantalum oxide nanoparticles have been synthesized and characterized in solution using HPLC-ICP, DLS, and multinuclear NMR. Nanoparticle formulation permitted intravenous injection, in vivo imaging, and subsequent renal clearance. A clinical CT scanner provided excellent resolution following agent injection, and distribution to the arterial system was visualized. In vitro CT imaging studies indicated that at equal molar concentration of tantalum and iodine, tantalum produced greater image contrast than iodine across the diagnostic X-ray spectrum with contrast benefit increasing with peak X-ray energy.

Estimating amounts of iron oxide from gradient echo images.

Rat legs directly injected with superparamagnetic iron oxide (SPIO) were studied by dual-echo, gradient-echo imaging. The amount of iron injected was estimated using a point dipole model for the SPIO injection site. Saturation magnetization of 6:1 PEG/amino modified silane-coated iron oxide particles with 5- to 6-nm core and 20-25 hydrodynamic diameter was approximately 110 emu/g of iron. Estimates of the amount of iron injected made from signal void volumes surrounding SPIO centers yielded erroneous results varying with sample orientation in the scanner and echo time (TE). For example, a 10 microL, 3-microg iron injection produced signal void volumes of 80 and 210 microL at TE of 9.8 and 25 ms, respectively, giving apparent iron contents of 6 +/- 1 and 10 +/- 2 microg respectively. A more effective approach uses the phase difference between two gradient recalled echo images. To estimate iron content, this approach fits the expected (3 cos(2)theta - 1)/(/r/3) spatial phase distribution to the observed phase differences. Extraneous phase effects made fitting phase at a single TE ineffective. With the dual echo method, 18 independent estimates were 2.48 +/- 0.26 microg std, independently of sample orientation. Estimates in empty control regions were -90 and -140 ng. A 1-microg injection indicated 0.5, 1.2, and 1.2 microg.

Ergodic and non-ergodic phase transitions in globular protein suspensions.

The equilibrium and nonequilibrium phase behavior of a protein suspension is investigated as a function of strength of interparticle attraction and protein concentration. The equilibrium phase behavior suggests that the range of the particle attractions is a small fraction of their diameter. At volume fractions and strengths of attractions smaller than those characterizing the spinodal of a metastable fluid-fluid transition, the suspensions gel before they crystallize. At higher volume fractions and lower strengths of attraction, gels are formed first from which crystals nucleate over a period of time. However, at higher strength of attraction, crystals form first at lower volume fractions while gels are observed at higher volume fractions. We hypothesize that this behavior results from the competition between the rates of gelation and crystal nucleation. The location of the gel line is well predicted by mode coupling theories adapted for low volume fractions and square well fluids. At low strength of attraction, the gel line occurs at small supersaturations suggesting gels will be seen before nucleation. However, at higher strengths of attraction the gel line occurs at greatly increased supersaturations such that crystal nucleation can occur before the gel line is crossed. That mode coupling theories should predict gelation when crystallization does not intervene is tested by investigating the gel dynamics.