Preliminary demonstration of in vivo quasi‐steady‐state CEST postprocessing—Correction of saturation time and relaxation delay for robust quantification of tumor MT and APT effects

Abstract

Chemical exchange saturation transfer (CEST) MRI is versatile for measuring the dilute labile protons and microenvironment properties. However, the use of insufficiently long RF saturation duration (Ts) and relaxation delay (Td) may underestimate the CEST measurement. This study proposed a quasi-steady-state (QUASS) CEST analysis for robust CEST quantification. Methods: The CEST signal evolution was modeled as a function of the longitudinal relaxation rate during Td and spin-lock relaxation rate during Ts, from which the QUASS-CEST effect is derived. Numerical simulation and in vivo rat glioma MRI experiments were conducted at 11.7 T to compare the apparent and QUASS-CEST results obtained under different Ts/Td of 2 seconds/2 seconds and 4 seconds/4 seconds. Magnetization transfer and amide proton transfer effects were resolved using a multipool Lorentzian fitting and evaluated in contralateral normal tissue and tumor regions. Results: The simulation showed the dependence of the apparent CEST effect on Ts and Td, and such reliance was mitigated with the QUASS algorithm. Animal experiment results showed that the apparent magnetization transfer and amide proton transfer effects and their contrast between contralateral normal tissue and tumor regions increased substantially with Ts and Td. In comparison, the QUASS magnetization transfer and amide proton transfer effects and their difference between contralateral normal tissue and tumor exhibited little dependence on Ts and Td. In addition, the apparent magnetization transfer and amide proton transfer were significantly smaller than the corresponding QUASS indices (P < .05). |ZHANG et Al. | INTRODUCTIONChemical exchange saturation transfer MRI has emerged as a sensitive method for detecting dilute labile protons and microenvironment properties such as pH and temperature. [1][2][3][4] It has shown promise in measuring endogenous metabolites and compounds (eg, glucose, glycogen, amide protons) and exogenous diamagnetic/paramagnetic CEST agents, offering a novel means to image a host of disorders, including acute stroke, tumor, and epilepsy. [5][6][7][8][9][10] Although CEST MRI is versatile, its contrast is quite complicated. The measurable CEST effect depends not only on the labile proton concentration and exchange rate, but also on experimental parameters, 11,12 such as the duration of RF irradiation and the relaxation delay (Td) between RF saturation. [13][14][15][16] Indeed, the CEST effect reflects two competing processes, namely, the signal reduction due to saturation transfer from irradiated labile protons and the signal recovery through relaxation. The CEST-MRI effect has a complicated dependence on bulk water T 1 . 17 This is further confounded in the presence of T 1 changes in diseased tissues. 18 Recently, Tanoue et al demonstrated the effect of RF saturation duration (Ts) on the CEST effect at 11.7 T. 19 It takes prolonged saturation and recovery time to reach the steady state. A trade-off has to be made between the magnitudes of the CEST effect and scan tim...

Demonstration of fast and equilibrium human muscle creatine CEST imaging at 3 T

“…29 The QUASS Z-spectra (Z QUASS ) were calculated as described. 26 Briefly, the CEST signal was modeled as…”

Fast and equilibrium CEST imaging of brain tumor patients at 3T

“…However, the SNR per unit time shall be compared to account for the scan time difference which favors a relatively short Ts/Td ( Jiang et al, 2016 ). As such, the QUASS algorithm allows the scans to be optimized under the peak SNR efficiency condition while reconstructing the equilibrium CEST effect as validated previously ( Zhang et al, 2021 ), promising to advance APT imaging within a shortened scan time and improved SNR efficiency, such as tumor grading and characterization ( Jiang et al, 2017 , Meissner et al, 2019 , Ohno et al, 2016 , Park et al, 2016a , Togao et al, 2014 , Wu et al, 2019 ) and pH quantification ( Harston et al, 2015 , McVicar et al, 2014 , Wang et al, 2019a ). Also, the QUASS algorithm provides a practical postprocessing approach to standardize the CEST measurements among studies by unifying results obtained from different experimental protocols.…”

Molecular Imaging of Brain Tumors and Drug Delivery Using CEST MRI: Promises and Challenges

“…CEST contrast is dependent on acquisitions and post-processing methods. In brain tumors, careful interpretation is needed since other contributions, such as MT and T1, could be quite different from normal brain tissues [10,21,25,34,89,90,96,99,100,104,168,169]. In this section, we will explain the principle of CEST acquisition, the common methods in analyzing the Z-spectrum, and recent developments in using deep-learning to assist CEST post-processing.…”