Page 67 - Cellular Imaging in Regenerative Medicine, Cancer and Osteoarthritis

Discussion
Mono-exponential signal decay
Several theoretical works and experimental studies (20–23) report deviation from mono-exponential signal decay in the presence of magnetic particles. In the case of SPIO labeled cells, a very fast initial signal decay followed by a slower decay was expected, where the components originated from the extra- and the intracellular compartments (21). Furthermore, Jensen et al. (26) predict nonexponential behavior of the signal decay, where the deviation from exponential depends on the MR acquisition scheme, diffusion and the shape of magnetic disturbers.
However, these predictions were not observed in our measurements. We found good agreement with mono-exponential signal decay in every investigated sample. This can be explained in part by the relatively large value of the minimum echo times (10 ms in spin-echo, 4 ms in gradient echo experiments), that prohibit the observation of the fast initial signal decay.
Theoretical considerations
The theoretical works of Brown (27) and Yablonskiy and Haacke (28) describe relaxation rates in the presence of magnetic inhomogeneities. These models are valid under certain assumptions, most importantly the static dephasing and dilute disturbers regimes (hereafter SD). From the model it follows that the relaxation rate is proportional to the local magnetization dose (LMD), the product of the volume fraction of the magnetic compartments, f (in our case, SPIO labeled cells) and the internal magnetization of these compartments, M:
R2' = γ 2π . f . M (1) 9√3
With one further assumption, that the internal magnetization of the labeled cells (M) is proportional to the number of incorporated SPIOs, this predicts the following: rearrangement of SPIO particles between cells leaves R2' unchanged, if cell number (NL), total number of SPIOs and total volume (V) are constant.
Quantification of iron labeled cell
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