Dataset Information

Quasi-steady-state CEST (QUASS CEST) solution improves the accuracy of CEST quantification: QUASS CEST MRI-based omega plot analysis.

ABSTRACT:

Purpose

CEST MRI omega plot quantifies the labile proton fraction ratio (f_r ) and exchange rate (k_sw ), yet it assumes long RF saturation time (Ts) and relaxation delay (Td). Our study aimed to test if a quasi-steady-state (QUASS) CEST analysis that accounts for the effect of finite Ts and Td could improve the accuracy of CEST MRI quantification.

Methods

We modeled the MRI signal evolution using a typical CEST EPI sequence. The signal relaxes toward its thermal equilibrium following the bulk water relaxation rate during Td, and then toward its CEST steady state following the spin-lock relaxation rate during Ts from which the QUASS CEST effect is derived. Both f_r and k_sw were solved from simulated conventional apparent CEST and QUASS CEST MRI. We also performed MRI experiments from a Cr-gel phantom under serially varied Ts and Td times from 1.5 to 7.5 s.

Results

Simulation showed that, although k_sw could be slightly overestimated (3%-15%) for the range of Ts and Td, f_r could be substantially underestimated by as much as 67%. In contrast, the QUASS solution provided accurate k_sw and f_r determination within 2%. The CEST MRI experiments confirmed that the QUASS solution enabled robust quantification of k_sw and f_r , superior over the omega plot analysis based on the conventional apparent CEST MRI measurements.

Conclusions

The QUASS CEST MRI algorithm corrects the effect of finite Ts and Td times on CEST measurements, thereby allowing robust and accurate CEST quantification.

SUBMITTER: Sun PZ

PROVIDER: S-EPMC8076066 | biostudies-literature | 2021 Aug

REPOSITORIES: biostudies-literature

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Publications

Quasi-steady-state CEST (QUASS CEST) solution improves the accuracy of CEST quantification: QUASS CEST MRI-based omega plot analysis.

Sun Phillip Zhe PZ

Magnetic resonance in medicine 20210310 2

<h4>Purpose</h4>CEST MRI omega plot quantifies the labile proton fraction ratio (f<sub>r</sub> ) and exchange rate (k<sub>sw</sub> ), yet it assumes long RF saturation time (Ts) and relaxation delay (Td). Our study aimed to test if a quasi-steady-state (QUASS) CEST analysis that accounts for the effect of finite Ts and Td could improve the accuracy of CEST MRI quantification.<h4>Methods</h4>We modeled the MRI signal evolution using a typical CEST EPI sequence. The signal relaxes toward its therm ...[more]

PMID: 33749052

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Project description:PurposeTo quantify chemical exchange saturation transfer contrast in upper extremities of participants with lymphedema before and after standardized lymphatic mobilization therapy using correction procedures for B0 and B1 heterogeneity, and T1 relaxation.MethodsFemales with (n = 12) and without (n = 17) breast cancer treatment-related lymphedema (BCRL) matched for age and body mass index were scanned at 3.0T MRI. B1 efficiency and T1 were calculated in series with chemical exchange saturation transfer in bilateral axilla (B1 amplitude = 2µT, Δω = ±5.5 ppm, slices = 9, spatial resolution = 1.8 × 1.47 × 5.5 mm3 ). B1 dispersion measurements (B1 = 1-3 µT; increment = 0.5 µT) were performed in controls (n = 6 arms in 3 subjects). BCRL participants were scanned pre- and post-manual lymphatic drainage (MLD) therapy. Chemical exchange saturation transfer amide proton transfer (APT) and nuclear Overhauser effect (NOE) metrics corrected for B1 efficiency were calculated, including proton transfer ratio (PTR'), magnetization transfer ratio asymmetry (MTRasymmetry') , and apparent exchange-dependent relaxation (AREX'). Nonparametric tests were used to evaluate relationships between metrics in BCRL participants pre- versus post-MLD (two-sided P < 0.05 required for significance).ResultsB1 dispersion experiments showed nonlinear dependence of Z-values on B1 efficiency in the upper extremities; PTR' showed < 1% mean fractional difference between subject-specific and group-level correction procedures. PTR'APT significantly correlated with T1 (Spearman's rho = 0.57, P < 0.001) and body mass index (Spearman's rho = -0.37, P = 0.029) in controls and with lymphedema stage (Spearman's rho = 0.48, P = 0.017) in BCRL participants. Following MLD therapy, PTR'APT significantly increased in the affected arm of BCRL participants (pre- vs. post-MLD: 0.41 ± 0.05 vs. 0.43 ± 0.03, P = 0.02), consistent with treatment effects from mobilized lymphatic fluid.ConclusionChemical exchange saturation transfer metrics, following appropriate correction procedures, respond to lymphatic mobilization therapies and may have potential for evaluating treatments in participants with secondary lymphedema.

Project description:Accurate quantification of chemical exchange saturation transfer (CEST) effects, including dipole-dipole mediated relayed nuclear Overhauser enhancement (rNOE) saturation transfer, is important for applications and studies of molecular concentration and transfer rate (and thereby pH or temperature). Although several quantification methods, such as Lorentzian difference (LD) analysis, multiple-pool Lorentzian fits, and the three-point method, have been extensively used in several preclinical and clinical applications, the accuracy of these methods has not been evaluated. Here we simulated multiple-pool Z spectra containing the pools that contribute to the main CEST and rNOE saturation transfer signals in the brain, numerically fit them using the different methods, and then compared their derived CEST metrics with the known solute concentrations and exchange rates. Our results show that the LD analysis overestimates contributions from amide proton transfer (APT) and intermediate exchanging amine protons; the three-point method significantly underestimates both APT and rNOE saturation transfer at -3.5 ppm (NOE(-3.5)). The multiple-pool Lorentzian fit is more accurate than the other two methods, but only at lower irradiation powers (?1 ?T at 9.4 T) within the range of our simulations. At higher irradiation powers, this method is also inaccurate because of the presence of a fast exchanging CEST signal that has a non-Lorentzian lineshape. Quantitative parameters derived from in vivo images of rodent brain tumor obtained using an irradiation power of 1 ?T were also compared. Our results demonstrate that all three quantification methods show similar contrasts between tumor and contralateral normal tissue for both APT and the NOE(-3.5). However, the quantified values of the three methods are significantly different. Our work provides insight into the fitting accuracy obtainable in a complex tissue model and provides guidelines for evaluating other newly developed quantification methods.

Dataset Information

Quasi-steady-state CEST (QUASS CEST) solution improves the accuracy of CEST quantification: QUASS CEST MRI-based omega plot analysis.

Purpose

Methods

Results

Conclusions

Publications

Quasi-steady-state CEST (QUASS CEST) solution improves the accuracy of CEST quantification: QUASS CEST MRI-based omega plot analysis.

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