Relaxometry model of strong dipolar perturbers for balanced-SSFP: application to quantification of SPIO loaded cells

Magnetic resonance microscopy using magnetically labeled cells is an emerging discipline offering the potential for non-destructive 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 (DeltaR(2b)=0.412 gamma . 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. A phantom model of SPIO loaded cells showed excellent agreement with simulations, demonstrated comparable sensitivity to gradient echo DeltaR(*) (2) enhancements, and 14 times the sensitivity of spin echo DeltaR(2) measurements. We believe this model can be used to facilitate the generation of quantitative maps of targeted cell populations.