Individually tailored spatial-spectral pulsed CEST MRI for ratiometric mapping of myocardial energetic species at 3T.

PURPOSE

CEST MRI has been used to probe changes in cardiac metabolism via assessment of CEST contrast from Cr. However, B variation across the myocardium leads to spatially variable Cr CEST contrast in healthy myocardium.

METHODS

We developed a spatial-spectral (SPSP) saturation pulsed CEST protocol to compensate for B variation. Flip angle maps were used to individually tailor SPSP pulses comprised of a train of one-dimensional spatially selective subpulses selective along the principal B gradient dimension. Complete Z-spectra in the hearts of (n = 10) healthy individuals were acquired using conventional Gaussian saturation and SPSP schemes and supported by phantom studies.

RESULTS

In simulations, the use of SPSP pulses reduced the average SD of the effective saturation B values within the myocardium (n = 10) from 0.12 ± 0.02 μT to 0.05 ± 0.01 μT (p < 0.01) and reduced the average SD of Cr CEST contrast in vivo from 10.0 ± 4.3% to 6.1 ± 3.5% (p < 0.05). Results from the hearts of human subjects showed a significant reduction of CEST contrast distribution at 2 ppm, as well as amplitude, when using SPSP saturation. Corresponding phantom experiments revealed PCr-specific contrast generation at body temperature when SPSP saturation was used but combined PCr and Cr contrast generation when Gaussian saturation was used.

CONCLUSION

The use of SPSP saturation pulsed CEST resulted in PCr-specific contrast generation and enabled ratiometric mapping of PCr to total Cr CEST contrast in the human heart at 3T.