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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 (ΔR2b=0.412γ · 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 ΔR*2 enhancements, and 14 times the sensitivity of spin echo ΔR2 measurements. We believe this model can be used to facilitate the generation of quantitative maps of targeted cell populations. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.