R2* Quantification of High Iron Concentrations for Cellular Therapy Applications with TurboSPI

Introduction: Many MRI techniques are now capable of imaging cells that have been labelled with super-paramagnetic iron oxide (SPIO) [1], and these techniques will be of great benefit to the developing field of cellular therapy. However, the majority of these techniques are not capable of providing quantitative information about cellular density at high iron concentrations, which is critical to monitoring the progress of cellular therapies and assessing the effectiveness of different treatments. Cellular density can be related to the local magnetic dose (LMD) of iron oxide, which affects the relaxation rate R2* = 1/T2* [2]. To quantify cellular density, an MRI technique must therefore provide spatially resolved measurements of R2* over a large dynamic range. We are exploring the use of an accelerated single point imaging method, TurboSPI [3], to acquire such quantitative R2* maps in samples containing SPIO-loaded cells. TurboSPI (Figure 1) uses a train of spin echoes to continually refocus the signal, and samples the rise and decay of each echo with high temporal resolution (2 μs). This sequence is optimal for imaging materials with large R2* (> 200 s ) but small R2 (< 50 s ), and it is a prime candidate for imaging cells containing high concentrations of SPIO because, when compartmentalized in cells, SPIO greatly increases R2* but has a comparatively small effect on R2. In this work, we evaluate TurboSPI’s ability to quantify R2* for the model system of micron-sized particles of iron oxide (MPIO) suspended in gelatin, which offer more consistent results than labelled cell populations while retaining a favourable R2*/R2 ratio.