Magnetic resonance microscopy using magnetically labeled cells is an emerging discipline offering the potential for nondestructive studies targeting numerous cellular events in medical research. The present work develops a technique to quantify superparamagnetic iron-oxide (SPIO) loaded cells using fully balanced steady state free precession (b-SSFP) imaging. An analytic model based on phase cancellation was derived for a single particle and extended to predict mono-exponential decay versus echo time in the presence of multiple randomly distributed particles. Numerical models verified phase incoherence as the dominant contrast mechanism and evaluated the model using a full range of tissue decay rates, repetition times, and flip angles. Numerical simulations indicated a relaxation rate enhancement (⌬R 2b ␥214.0؍ ⅐ LMD) proportional to LMD, the local magnetic dose (the additional sample magnetization due to the SPIO particles), a quantity related to the concentration of contrast agent. Key words: steady state free precession; magnetic susceptibility; contrast agent; iron-oxide; relaxometry; magnetic resonance microscopy Magnetic resonance microscopy using magnetically labeled cells has been used to study a variety of cellular events in medical research including stem cell (1-4) and immune cell (5,6) migration, brain ischemia (7,8), and cancer (9 -11). One approach uses high resolution MRI to detect either single (12,13) or groups of cells labeled with paramagnetic contrast agents, such as superparamagnetic iron-oxide (SPIO). Cell labeling using this strategy has been accomplished through both in vitro and in vivo approaches. In vitro labeling involves incubation of a desired cell line in culture with SPIO prior to i.v. (1,5) or tissuespecific injection (2,3), while in vivo (or active) labeling is accomplished through i.v. injection of SPIO particles that are preferentially taken up by a target cell population (14,15). For the vast majority of cell types and with each labeling technique, SPIO agents have been shown to be minimally disruptive to cell function (16) with few exceptions (17), and show great promise for revealing cellular contributions to early stages of pathology (7), as well as the therapeutic effectiveness of stem cell homing in regenerative medicine (2,3).To date, the majority of imaging studies involve a qualitative assessment of the hypo-or hyper-intensities observed in images of tissues containing SPIO-labeled cells. Quantitative measures of cellular migration may allow assessment of the relative effectiveness of pharmacological interventions, stem cell homing, or changes in the intensity of inflammatory response that are typically associated with the progression of pathology. Detection of SPIO labeled cells has been accomplished through T 1 (9,18), T 2 (19,20) and TЈ 2 -weighted (21-23) acquisitions. TЈ 2 -weighted gradient echo acquisitions provide the greatest sensitivity to the presence of SPIO; however, these measurements are also sensitive to background field inhomogeneities induced by im...