A new approach to Z‐spectrum acquisition: prospective baseline enhancement (PROBE) for CEST/Nuclear Overhauser Effect

Abstract

Purpose To develop a prospective baseline enhancement that compensates for intermingled background effects in Z‐spectra to achieve sensitivity enhancement of peaks related to CEST and nuclear Overhauser effect. Methods An MRI sequence–specific compensation of background effects is achieved through variation of the pulsed saturation power, ω1,max, with the chemical shift, δ. After a “scout acquisition” of a standard Z‐spectrum, the background is modeled through an appropriate spin system. Subsequently, an optimization procedure yields ω1,maxfalse(δfalse) values that compensate for background contributions yielding a flat baseline. Contributions from metabolites not considered in the optimization procedure are enhanced as distinct perturbations to the baseline. For experimental verification, mapping of the lactate concentration in the presence of cross‐linked bovine serum albumin was performed in phantoms at 7 T. As proof of concept, explorative experiments were performed in healthy human subjects at 3 T. Results Nuisance contributions from direct water saturation, macromolecular magnetization transfer, and exchanging background protons were successfully removed from the Z‐spectrum in phantoms and in brain tissue. The lactate methyl, methine, and hydroxyl peaks were readily observable in vitro. The peak areas correlated linearly with known concentrations. Improvement of the detection limit was achieved by a sparse distribution of saturation frequencies, allowing for more efficient signal averaging. Conclusion An optimization framework for high‐resolution metabolite mapping by means of CEST/nuclear Overhauser effect was developed. It offers full flexibility to select spin‐pool moieties, whose influence on the Z‐spectrum will be compensated. Deviations from this background model will provide a contrast at the respective offset frequencies.

Radiation damping at clinical field strength: Characterization and compensation in quantitative measurements

“…Our simulations indicate only small errors for $$$ {\tau}_p\le 1 $$$ ms. Significantly longer pulses of potentially high amplitude are often used in MT imaging, 37,67,68 or CEST 58,69 . Integration of RD suppression approaches may, hence, be beneficial in such experiments.…”

“…The algorithm included an ordinary differential equation solver (ode45) based on explicit Runge–Kutta formulas of orders 4 and 5. The (digitized) RF pulse shapes were directly obtained as played out during runtime from the scanner's sequence programming software (IDEA; Siemens Healthineers) 38,57,58 . The NLLS fitting used the function nlinfit (or also lsqnonlin).…”

“…The (digitized) RF pulse shapes were directly obtained as played out during runtime from the scanner's sequence programming software (IDEA; Siemens Healthineers). 38,57,58 The NLLS fitting used the function nlinfit (or also lsqnonlin). Confidence intervals were calculated using the nlparci algorithms of Matlab's Statistics and Machine Learning Toolbox (R2020).…”

High-Resolution Magnetization-Transfer Imaging of Post-Mortem Marmoset Brain: Comparisons with Relaxometry and Histology

“…For better efficiency, polynomial interpolation was employed to calculate the matrix exponentials without bias (D.K. Müller et al ., 2013; Lenich et al ., 2019). The separately recorded B 0 map was used for voxel-by-voxel correction of all offset frequencies.…”

Chemical Exchange Saturation Transfer for Lactate-Weighted Imaging at 3 T MRI: Comprehensive In Silico, In Vitro, In Situ, and In Vivo Evaluations

“…Chemical exchange saturation transfer (CEST) imaging is increasingly used as an extension of MRS. CEST combines the advantages of MRS and MRI [ 26 ]. During CEST imaging, exogenous or endogenous molecules with exchangeable protons, such as hydroxyl protons (-OH), amine protons (-NH 2 ), and amide protons (-NH), are selectively saturated, and the exchanges with water protons are detected [ 27 ]. Furthermore, due to a large number of multiple proton exchanges during the saturation time, CEST is characterized by higher sensitivity than MRS [ 28 ].…”