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.
1. We have acquired data at several TEs and then curve fit it with exponential expression to determine R2.
2. We do multiple echo and we don't know if it is long enough to avoid T1 weighting, how much should be the TR to avoid T1 weighting?
3. The range of TEs was from 4-120 ms. Yes we got a reasonable decay for all the concentration in the given range.
4. We have taken concentration range from 0.5-70 micro gram /ml but after 10 micro gram/ml we are not getting a linear curve with positive slope 📷
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