Quantification of pulsed saturation transfer at 1.5T and 3T

Abstract

Purpose: To compare magnetization transfer (MT) and CEST effects between 1.5T and 3T in phantom and in vivo experiments.

Methods: A pulsed saturation scheme using block-shaped pulses separated by gaps was used to overcome the single RF amplifier duty cycle limitations of a clinical 1.5T scanner. Modeling was performed by incorporating the extended phase graph formalism into a Bloch-McConnell simulation. Two saturation pulse types (with long and short pulses) were used. Estimated parameters for MT (the semi-solid pool fraction, M₀^B ; the semi-solid transverse relaxation time, T₂^B ) and CEST (asymmetry; areas) were compared between 1.5T and 3T in phantoms and in the healthy brain.

Results: Improved fits were shown after inclusion of extended phase graphs. Semi-solid pool fractions in phantom (for agar with ammonium chloride) were higher for short compared to long pulses at 3T (by 19% over all concentrations) and higher at 1.5T compared to 3T (by 5%) using short pulses. In the in vivo experiments, differentiation of white and gray matter was seen in the brain at both field strengths with improved white-gray matter contrast at 3T. In white matter, the mean semi-solid fractions were 18 ± 2% at 3T and 15 ± 2% at 1.5T. The CEST asymmetry in white matter was negative (-4.9 ± 0.4%) at 3T and zero (0.0 ± 0.3%) at 1.5T.

Conclusions: The pulsed saturation method with short pulses, using the extended phase graph formalism in the Bloch McConnell simulations, led to improved model fits to the data, when compared to those without extended phase graphs.

Keywords: chemical exchange saturation transfer; extended phase graphs; magnetization transfer; pulsed saturation; quantitative imaging.