title: Quantification of superparamagnetic iron oxide with large dynamic range using TurboSPI.
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 k-space 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)('):R(2) 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.
authors.affiliation: Institute for Biodiagnostics (Atlantic), National Research Council, 1796 Summer Street, Suite 3900, Halifax, Nova Scotia, Canada B3H 3A7. James.Rioux@nrc-cnrc.gc.ca
authors.forename: James A
authors.initials: JA
authors.lastname: Rioux
authors.forename: Kimberly D
authors.initials: KD
authors.lastname: Brewer
authors.forename: Steven D
authors.initials: SD
authors.lastname: Beyea
authors.forename: Chris V
authors.initials: CV
authors.lastname: Bowen
mesh_terms: D000465:Algorithms; D000818:Animals; D016477:Artifacts; D002138:Calibration; D045744:Cell Line, Tumor; D002468:Cell Physiological Phenomena; D004574:Electromagnetic Fields; D004578:Electron Spin Resonance Spectroscopy; D005260:Female; D005290:Ferric Compounds / Q000737:chemistry*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D008297:Male; D017711:Nonlinear Dynamics; D010316:Particle Size; D019047:Phantoms, Imaging; D051381:Rats; D020318:Rats, Long-Evans; D013056:Spectrophotometry, Ultraviolet; D013113:Spin Labels
chemical_list: D005290:Ferric Compounds; D013113:Spin Labels; C000499:ferric oxide