Fat-Correction

Traditionally, CEST contrast is calculated by the asymmetry analysis, but the presence of fat signals lead to wrong contrast quantification, hence to inaccurate pH measurements. We investigated four post-processing approaches to overcome fat signal influences that enable correct CEST contrast calculation and tumor pH measurements:

• positive method (#1) calculates the contrast considering only the positive part of the Z-spectrum
• linear method (#2) consists in removing the fat frequencies and replacing the missing range with a linear interpolation
• lorentzian method (#3) consists in replacing the negative part of the Z-spectrum by the water pool contribution upon Lorentzian fitting of the spectrum
• interpolation method (#4) corrects the calculated ratiometric values accordingly to the measured fat fraction levels, by interpolating the ratiometric values with cubic splines to correct for the proper pH values

Getting Started

The project was written in Matlab R2023b (The Math-Works, Inc., Natick, MA, USA). Lorentzian CEST curve fitting was implemented with the open-source Matlab-based code (https://github.com/cest-sources/CEST_EVAL).

In vitro analysis

The project for in vitro analysis consists of:

• FatCorrectionVitro.m: main script for in vitro analysis
• Utils: folder containing functions necessary for the main script
• CEST_EVAL/levmar_fit: Lorentzian CEST curve fitting (https://github.com/cest-sources/CEST_EVAL)

Two phantoms were prepared for in vitro validation. The phantoms consisted of a solution containing 30 mM iopamidol, one with the pH adjusted to 6.4 and the other with the pH adjusted to 6.9. Raw Bruker data are provided:

• 20200721_164409_dv_phantom_fat_iopa_3_1_3_pH_6_4.zip:

  • pH = 6.4
  • anatomical folder = 8
  • CEST pre folder = 10

• 20210324_094540_dv_iopa_fat_invitro_1_1_1_pH_6_9.zip:

  • pH = 6.9
  • anatomical folder = 9
  • CEST pre folder = 10

In vitro analysis can be carried out by running the script "FatCorrectionVitro.m". Inside the phantom folder, a results folder called "FF" is created which contains the Z-spectra and parametric maps resulting from the analysis of the asymmetric method and the 4 proposed methods.