| Abstract | This work proposes the use of TurboSPI, a multi-echo single point imaging sequence, for the quantification of labeled cells containing moderate to high concentrations of iron oxide contrast agent. At each kspace location, TurboSPI acquires several hundred time points during a spin echo, permitting reliable relaxation rate mapping of large-R*₂ materials. An automatic calibration routine optimizes image quality by promoting coherent alignment of spin and stimulated echoes throughout the multi-echo train, and this calibration is sufficiently robust for in vivo applications. In vitro relaxation rate measurements of SPIO-loaded cervical cancer cells exhibit behavior consistent with theoretical predictions of the static dephasing regime in the spin echo case; the relaxivity measured with TurboSPI was 10.47 ± 2.3 s⁻¹/mG, comparable to the theoretical value of 10.78 s⁻¹/mG. Similar measurements of micron-sized iron oxide particles (0.96 µm and 1.63 µm diameter) show a reduced relaxivity of 8.06 ± 0.68 s⁻¹/mG and 7.13 ± 0.31 s⁻¹/mG respectively, indicating that the static dephasing criterion was not met. Nonetheless, accurate quantification of such particles is demonstrated up to R*₂ = 900 s⁻¹, with a potentially higher upper limit for loaded cells having a more favorable R'₂:R₂ ratio. Based on the cells used in this study, reliable quantification of cells loaded with 10 pg of iron per cell should be possible up to a density of 27 million cells/mL. Such quantification will be of crucial importance to the development of longitudinal monitoring for cellular therapy and other procedures using iron-labeled cells |
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