Novel Detection Methods for Chemical Exchange and Application to Breast Cancer Imaging

Chemical exchange saturation transfer (CEST) is a novel contrast mechanism that is based on the chemical exchange processes between the protons in water and solutes. CEST can indirectly detect the low concentrated solutes, which are not observable with conventional MRI and indicate the quantitative environmental parameters such as pH. Therefore, many promising applications of CEST are explored. The aim of my projects is to develop new CEST imaging techniques and apply these techniques in human studies at 3 T. The first project is to develop a fast and quantitative imaging method based on the balanced steady-state free precession sequence as an alternative way for chemical exchange detection (bSSFPX). The feasibility of bSSFPX for chemical exchange detection was proved both theoretically through Bloch-McConnell Equations simulations and experimentally by phantoms studies. Analytical models for bSSFPX were developed for quantitative measurements of T1ρ and exchange rate. In a first in vivo experiment, bSSFPX was applied in the human brain to detect the chemical exchange possibly from fast exchanging metabolites with resonance frequencies close to water that would be challenging at 3 T for standard CEST imaging methods. As a new CEST data acquisition method, the bSSFPX experiment holds high promise for fast, quantitative and 3D CEST imaging. The second project is to develop a CEST-Dixon sequence for fat free CEST imaging and apply it to breast cancer imaging. The influence of non-exchanging fat on CEST imaging was studied by simulation, in phantoms, and in vivo at different fat fractions and echo times. The CEST-Dixon method has been proved to eliminate lipid contamination robustly in breast CEST imaging. In the breast cancer study, higher CEST effects were observed in the more aggressive cancer group than the less aggressive cancer, benign and normal groups in all three frequencies ranges (hydroxyl, amine and amide) explored, while no significant differences were observed between the less aggressive cancer, benign and normal groups. In addition, significant correlation between MTRasym and Ki-67 was observed for cancer groups. While the study is preliminary, the results indicate that the CEST-Dixon method may differentiate between more aggressive and less aggressive breast cancer.