T1 measurements in cell cultures: a new tool for characterizing contrast agents at 1.5T.

The objective of this work was to assess the feasibility and accuracy of T1 and relaxivity measurements in cell cultures using 1.5T magnetic resonance imaging (MRI) with the long-term goal to develop a tool for evaluation of novel paramagnetic agents in a realistic macromolecular environment. This initial study was carried out using MCF-7 cells treated with independently determined concentrations of Gd-DTPA. Two cell culture systems were evaluated: cell pellets and single layers of cells grown on microporous inserts. High-resolution T1 measurements of cell cultures were acquired with two dimensional Inversion Recovery Fast Spin Echo (2D-IR-FSE), three dimensional Inversion Recovery Fast Spin Echo (3D-IR-FSE), and 3D-SPGR sequences. The T1 and relaxivity accuracy of these sequences was confirmed with aqueous Gd-DTPA samples of known concentration. Relaxivities of 1.71 +/- 0.15 [mM(-1)second(-1)] and 1.55 +/- 0.50 [mM(-1)second(-1)] were measured in the cell pellets and cell monolayers, respectively, and were different from the value of 4.3 [mM(-1)second(-1)] for Gd-DTPA in water. Both cell pellets and monolayers are suitable for initial assessment of novel MR contrast agents.

Paramagnetic oligonucleotides: contrast agents for magnetic resonance imaging with proton relaxation enhancement effects.

An antisense paramagnetic oligonucleotide analogue targeted to a model macromolecular receptor (5S rRNA) was prepared. The paramagnetic agent's relaxivity (dependence of the relaxation rate on paramagnetic agent concentration) in the presence and absence of the macromolecular receptor was measured at 1.5 and 6.3 T. The relaxivity of the targeted agent increased specifically in the presence of the macromolecular receptor (16% at 6.3 T and 15% at 1.5 T). This effect was specific for a paramagnetic oligonucleotide targeted to the receptor and was larger than the relaxivity enhancement due simply to receptor-induced viscosity differences. Maximizing this relaxivity enhancement of tumor targeted paramagnetic oligonucleotides will aid in contrast agent development for magnetic resonance imaging.