Modeling Siemens Lac&Ace Phantom Spectra and Quantification

Analysis / Modeling
1

Hi everyone,

I am conducting a test-retest validation using a Siemens phantom. The phantom contains 1 mol/L of sodium acetate and 1 mol/L of lithium lactate. I measured the phantom six times at different times using the sLASER sequence with 26 ms TE at 7T, which includes 64 water-suppressed spectra and 8 water-unsuppressed spectra. After processing the data with MRspa for ECC, frequency, phase correction, and averaging, I fitted the water and lactate using the Gannet fitting functions’ Water and Lac models. Additionally, I used the Water fitting model to fit the acetate by modifying the fitting frequency range. The fitting results are as follows. An example of a spectrum fitting is shown in the figure below.

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I would like to know why the Lac/Ace ratio is not 1:1. Is it related to the T1 and T2 relaxation times of the two substances?
Additionally, the instability of the WaterArea leads to the instability of the water-scaled concentrations of Lac and Ace. Could this be because the 8 water-unsuppressed scans are not sufficient?

2
  1. Yes, different relaxation times can be a contributor.
  2. The lineshapes are not perfectly Lorentzian (looking at the water and Ace singlets in particular, you have a substantial B0 distribution convolving in), so your simplified Lorentzian fitting models are going to give biased integral estimates.
  3. It also seems like your model doesn’t get the Lac splitting perfectly right (but probably close enough). You might consider a basis-set-based approach (extra work of course)
  4. How precisely do you think Siemens calibrated the phantom? I personally wouldn’t trust myself to mix a phantom with target concentrations to the second significant digit (maybe they can).
  5. Do lactate or acetate dissociate in aqueous solution? (I do not have a precise answer to that TBH)
3

Thank you very much for your insights！