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, M 0 B ; the semi-solid transverse relaxation time, T 2 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. Semisolid 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. K E Y W O R D Schemical exchange saturation transfer, extended phase graphs, magnetization transfer, pulsed saturation, quantitative imaging

Chemical exchange saturation transfer MRI in central nervous system tumours on a 1.5 T MR-Linac

“…In vivo CEST was traditionally limited to MRI field strengths of 3T and 7T [17,18,25,26]. Recently, our group implemented CEST on a 1.5T clinical scanner using a pulsed saturation scheme [27]. Consistent with findings at higher field strengths [16][17][18], we showed associations between treatment response and CEST metrics measured during chemoradiation in GBM patients [28].…”

“…However, CEST has not been implemented on MR-guided radiotherapy systems, where it could be used directly to inform the adaptation of radiotherapy plans based on observed biological changes of the tumour. This study was the first to investigate the use of in vivo CEST on a hybrid MR-Linac system and extends previous work on diagnostic MR scanners at 1.5T [27,28] and 3T [7][8][9]. We demonstrate that CEST images can be successfully obtained within the clinical workflow of MR-Linac treatment in CNS patients.…”

“…CEST images were acquired on the third day of the five-day week [32] after dose delivery (i.e., during post-beam time immediately after the radiation treatment), between fractions 1 and 30. The CEST imaging pulse sequence was previously developed and validated for a Philips 1.5T Ingenia [27,28], with the source code recompiled for the MR-Linac. All MR-Linac imaging parameters, including CEST, B0, and B1 mapping scans, are included in Supplementary Figure S1.…”

Does the magnetization transfer effect bias chemical exchange saturation transfer effects? Quantifying chemical exchange saturation transfer in the presence of magnetization transfer

“…However, incorporating more pools may require re‐derivation of the model, which could prove difficult if several CEST effects are expected. Malik et al recently expanded extended phase graph theory to include an MT pool, and Chan et al adapted the extended phase graph model to quantify both CEST and MT effects. They found good agreement of their model with their phantom concentrations and were able to apply their methods in vivo.…”

“…They found good agreement of their model with their phantom concentrations and were able to apply their methods in vivo. Importantly, the methods presented here can be adapted to use Chan et al’s extended phase graph theory, which could further improve the model fittings. Comparing these methods, as well as expanding the analysis to incorporate multiple CEST saturation schemes in order to isolate CEST effects with different exchange rates, is the subject of future work.…”