UC San Diego

Background

In this study, we investigated the feasibility of quantitative ultrashort echo time (qUTE) magnetic resonance (MR) imaging techniques in the detection and quantification of iron oxide nanoparticle (IONP)-labeled stem cells.

Methods

A stem cell phantom containing multiple layers of unlabeled or labeled stem cells with different densities was prepared. The phantom was imaged with quantitative UTE (qUTE) MR techniques [i.e., UTE-T₁ mapping, UTE-T₂* mapping, and UTE-based quantitative susceptibility mapping (UTE-QSM)] as well as with a clinical T₂ mapping sequence on a 3T clinical MR system. For T₁ mapping, a variable flip angle (VFA) method based on actual flip angle imaging (AFI) technique was utilized. For T₂* mapping and UTE-QSM, multiple images with variable, interleaved echo times including UTE images and gradient recalled echo (GRE) images were used. For UTE-QSM, the phase information from the multi-echo images was utilized and processed using a QSM framework based on the morphology-enabled dipole inversion (MEDI) algorithm. The qUTE techniques were also evaluated in an ex vivo experiment with a mouse injected with IONP-labeled stem cells.

Results

In the phantom experiment, the parameters estimated with qUTE techniques showed high linearity with respect to the density of IONP-labeled stem cells (R²>0.99), while the clinical T₂ parameter showed impaired linearity (R²=0.87). In the ex vivo mouse experiment, UTE-T₂* mapping and UTE-QSM showed feasibility in the detection of injected stem cells with high contrast, whereas UTE-T₁ and UTE-T₂* showed limited detection. Overall, UTE-QSM demonstrated the best contrast of all, with other methods being subjected more to a confounding factor due to different magnetic susceptibilities of various types of neighboring tissues, which creates inhomogeneous contrast that behaves similar to IONP.

Conclusions

In this study, we evaluated the feasibility of a series of qUTE imaging techniques as well as conventional T₂ mapping for the detection of IONP-labeled stem cells in vitro and ex vivo. UTE-QSM performed superior amongst other qUTE techniques as well as conventional T₂ mapping in detecting stem cells with high contrast.