pH Estimation from CEST Data

One application of CEST Z-spectrum analysis is the estimation of pH using pH-responsive probes. Such probes may:

  • Have two labile proton frequencies, and the ratio of the saturation efficiencies at the two frequencies is pH-dependent.
  • Have a non-linear relationship between the saturation power and the saturation efficiency at a single offset frequency. The ratio of the saturation efficiency at the two RF powers is pH-dependent.

Using a calibration, the ratios above may be used to estimate pH in vivo.

To perform this type of analysis, select the pH Estimator tab:

Tab to perform pH estimation

You can then select whether you want to:

  • Button to estimate pH from two different offset frequencies. Select two input images of the CEST saturation efficiency at two different frequency offsets.
  • Button to estimate pH from two saturation pulse powers. Select two input images of the CEST saturation efficiency at two R.F. saturation powers.

These input images will have been produced either from asymmetry analysis, or Lorentzian fitting. If both cases, you will need to provide a calibration curve, in the form of a text file containing two columns of numerical data:

  • For dual frequency offsets, the first column (x-axis) contains the pH values and the second column (y-axis) contains the ratio of saturation efficiency at the two frequencies (RST): RST = SatEff2 / SatEff1
    where SatEff1 is the saturation efficiency at frequency offset 1, and SatEff2 is the saturation efficiency at frequency offset 2.
  • For dual R.F. saturation powers, the first column (x-axis) contains the pH values and the second column (y-axis) contains the ratio of saturation efficiency at the two R.F. saturation powers (RPM): RPM = SatEff2 / SatEff1
    where SatEff1 is the saturation efficiency at R.F. power 1, and SatEff2 is the saturation efficiency at R.F. power 2.

For these calibration curves, you need to ensure that the y-values (ratios) increase monotonically with increasing pH. If the ratio increases, and then decreases again, it is not possible to uniquely identify the pH from a particular ratio.

When set up, press Button to perform the analysis to estimate the pH. When complete, a new image is created showing the pH map. The image name will be taken from the first input image, with a suffix "_pH".