Water saturation shift referencing (WASSR) [28]

Water saturation shift referencing (WASSR) [28] buy Y-27632 is one of the most commonly used techniques to

correct for this shift; however, the method requires extra scans possibly before and after the CEST imaging. Using a model-based approach eliminates the additional scan(s) required because the shift can be determined directly from the collected spectrum as part of the model fitting [29]. Performing model-based quantitative analysis of the CEST effect for CW-CEST is simple and is generally achieved using the analytical solution to the Bloch–McConnell equations. However, CW-CEST is not feasible in clinical applications due to specific absorption rate (SAR) and hardware limitations, making pulsed-CEST the only viable irradiation scheme for clinical translation currently. Apitolisib clinical trial Finding the proton MR behavior in response to time varying RF power as present in the pulsed-CEST scheme for model-based analysis is time consuming because the solution to the Bloch–McConnell equations must be arrived at either using a numerical differential equation solver or discretizing the pulses into a series of short continuous RF segments. In the latter case, referred to here as the discretization method, the individual segments are solved using the simple analytical solution

for CW-CEST with the magnetization being propagated through each of the segments, the final values from one segment serving as the initial conditions for the next one [25] and [30]. Due to the combination of the repeated calculations required in the discretization method and the multiple iterations within the optimization used for model-based strategy, the analysis of pulsed-CEST is often much slower than its continuous counterpart. Hence, pulsed-CEST is often treated as CW-CEST by finding the equivalent

average field (AF) [31] and [32] or power (AP) [33] of the pulse train to perform the analysis using the faster solution to the Bloch–McConnell equations under continuous saturation. isothipendyl Recently, studies have shown that a continuous approximation (both AF and AP) produces narrower off-resonance excitations when compared with pulsed saturation [33] and that the CESTR is different for pulsed-CEST and CW-CEST when the exchange rate is more than 50 s−1[30]. These raise the issue whether pulsed-CEST can be analyzed via the equivalent CW-CEST or a discretization method must be used. In this study, the differences in the z-spectra from a pulsed-CEST experiment and the equivalent continuous (AF and AP) approximation are examined using simulations to determine the validity of the latter for the analysis of pulsed-CEST data. Additionally, model-based quantitative analysis of pulsed-CEST data from a tissue-like phantom using the continuous approximation and discretization methods are compared.

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