Quantification of superparamagnetic iron oxide with large dynamic range using TurboSPI James A. Rioux a,b , Kimberly D. Brewer c , Steven D. Beyea a,b , Chris V. Bowen a,b,⇑ a Institute for Biodiagnostics (Atlantic), National Research Council, 1796 Summer Street, Suite 3900, Halifax, Nova Scotia, Canada B3H 3A7 b Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4R2 c Immunovaccine Inc., 1344 Summer Street, Suite 412, Halifax, Nova Scotia, Canada B3H 0A8 article info Article history: Received 14 October 2011 Revised 24 January 2012 Available online 4 February 2012 Keywords: Quantification Relaxometry Single point imaging Iron oxide Static dephasing abstract This work proposes the use of TurboSPI, a multi-echo single point imaging sequence, for the quantifica- tion 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 lm and 1.63 lm 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 ¼ 900 s 1 , with a potentially higher upper limit for loaded cells having a more favorable R 0 2 : R 2 ratio. Based on the cells used in this study, reli- able quantification of cells loaded with 10 pg of iron per cell should be possible up to a density of 27 mil- lion 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. Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved. 1. Introduction The use of superparamagnetic oxide (SPIO) particles as an MRI contrast agent has permitted many advances in the non-invasive imaging and tracking of labeled cells [1,2]. While cells can be la- beled in vivo by uptake of injected SPIO [3], it is more common for in vitro labeling and subsequent implantation, a procedure use- ful for imaging both stem cells [4,5] and immune cells [6,7], among others. For these applications, the ultimate aim is quantitative lon- gitudinal monitoring of the distribution and fate of the implanted cells. Such studies require a technique capable of detecting and quantifying iron-loaded cells across a large dynamic range, from the high initial concentration after implantation to the lower con- centrations resulting from dispersal and repeated division of cells. Many MRI techniques used to detect iron-loaded cells have con- trast based on the relaxation rate R  2 ¼ 1=T  2 , which combines con- tributions from both irreversible (R 2 = 1/T 2 ) and reversible R 0 2 ¼ 1=T 0 2   relaxation processes as R  2 ¼ R 2 þ R 0 2 . Under suitable conditions these techniques are capable of detecting even single iron-loaded cells [8,9]. However, many techniques which easily de- tect labeled cells cannot be used for quantification of large iron concentrations due to near-complete loss of signal in the voxels of interest. Though this can be addressed by a variety of positive contrast techniques [10–12] which create hyperintense signal in the vicinity of magnetic perturbers, the hyperintense voxels are distributed around the source of the field distortion, making accu- rate localization difficult. While attempts to quantify iron concen- tration based on the volume of the ‘‘blooming’’ artefact (on negative-contrast images) or the volume of the hyperintense re- gion (on positive-contrast images) have found some success [13], these are indirect measures of iron concentration. An alternative approach is relaxation rate mapping, generally using multiple gradient echo readouts to sample a number of points along the FID and/or a spin echo. Cellular density can then be calculated assuming a linear relationship between R 0 2 (or R  2 ) and iron concentration. Sequences used for this procedure include 1090-7807/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2012.01.017 ⇑ Corresponding author at: Institute for Biodiagnostics (Atlantic), National Research Council, 1796 Summer Street, Suite 3900, Halifax, Nova Scotia, Canada B3H 3A7. Fax: +1 902 473 1851. E-mail addresses: James.Rioux@nrc-cnrc.gc.ca (J.A. Rioux), kbrewer@imvaccine. com (K.D. Brewer), Steven.Beyea@nrc-cnrc.gc.ca (S.D. Beyea), Chris.Bowen@nrc -cnrc.gc.ca (C.V. Bowen). Journal of Magnetic Resonance 216 (2012) 152–160 Contents lists available at SciVerse ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr