I've done MRI imaging of iron oxide nanoparticles in phantom gels. The images are showing decreased signal intensities upon increasing the iron concentration, this is due to iron oxide being a negative contrast agent. Up to this is perfect, but when I plot the curve between T2 relaxation and iron concentration, the slope is coming negative, which is opposite to the images!! Does anyone know why this is happening? I talked to someone and they said that I should take double exponential of the curve, how to take double exponential of a single exponential curve?
Could you be a bit more specific about your data? What exactly do you plot? The relaxation time T2 as a function of the concentration c? Or the relaxation rate R2? Or are you plotting signal intensities of T2-weighted (or T2*-weighted) acquisitions?
I agree that R2 should increase (approximately linearly) with the concentration c. Without seeing your data, it's difficult to guess what's going wrong. Some questions/suggestions:
- How do you determine R2? Do you acquire the signal at several TEs to perform exponential fitting of exp(−R2×TE)?
- Is this a spin-echo (or gradient-echo) (multi-echo?) pulse sequence? Is TR sufficiently long to avoid T1 weighting?
- What's the range of TEs? Do you obtain a reasonable signal decay (in the range between e.g. 10% and 95%) over all TEs?
- What's the range of concentrations? If the concentrations are too high, no signal will be left for fitting.
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