Effect of Concentration of SH U 555A Labeled Human Melanoma Cells on MR Spin Echo and Gradient Echo Signal Decay at 0.2, 1.5, and 3T

In the current study the effect of increasing concentrations of superparamagnetic iron oxide labeled cells on the MRI signal decay at magnetic field strengths of 0.2, 1.5, and 3 T was evaluated. The spin echo and gradient echo cellular transverse relaxivity was systematically studied for various concentrations (N = 1, 5, 10, 20, 40, and 80 cells/μl_gel) of homogeneously suspended SH U 555A labeled SK-Mel28 human melanoma cells. For all field strengths investigated a linear relationship between cellular transverse relaxation enhancement and cell concentration was found. In the spin echo case, the cellular relaxivities i.e., d(ΔR ₂)/dN] were determined to 0.12 s⁻¹ (cell/μl)⁻¹ at 0.2 T, 0.16 s⁻¹ (cell/μl)⁻¹ at 1.5 T, and 0.17 s⁻¹ (cell/μl) at 3 T. In the gradient echo case, the calculated cellular relaxivities (i.e., d(ΔR ₂ ^* )/dN) were 0.51 s⁻¹ (cell/μl)⁻¹ at 0.2 T, 0.69 s⁻¹ (cell/μl)⁻¹ at 1.5 T, and 0.71 s⁻¹ (cell/μl)⁻¹ at 3 T. The proposed preparation technique has proven to be a simple and reliable approach to quantify effects of magnetically labeled cells in vitro. On the basis of this quantification well suited tissue specific models can be derived.