Effects of varying the molecular weight of PEG expressed on Gd-lipid nanoparticles on Gd relaxivity constant and the particle diameter.

aGd-DTPA lipid nanoparticles containing 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC; 9 part), and 1,2- distearoyl-sn-Glycero-3-phophoethanolamine-N-DTPA (DSPE-DTPA; 1 part), and N-(Carbonyl-methoxypolethyleneglycol-polymer of listed molecular weight)-1,2-distearoyl-sn-glycero-3-phospoethanolamine (mPEG-DSPE, 1 part) in suspension were mixed with gadolinium (III) chloride hexahydrate (Gd3+, at 1∶1 DTPA:Gd mole ratio) for 20 minutes. For comparison, water soluble commercial agents such as Omniscan (Gd-DTPA-BMA) and Magnevist (GD-DTPA) were also included. The lipid particle diameter was expressed at mean ±a SD.bThe relaxation time T1 was measured using the standard spin-echo sequence on a 3T MR scanner. These concentration dependent T1 values were plotted versus Gd3+ concentration from which the rising curve was fitted by linear regression to estimate apparent molar relaxivity constant r1 and r2. The covariant of r1 and r2 data were 15% or lower.