Investigating hydroxyl chemical exchange using a variable saturation power chemical exchange saturation transfer (vCEST) method at 3 T

Effectiveness of fat suppression using a water-selective binomial-pulse excitation in chemical exchange saturation transfer (CEST) magnetic resonance imaging

PURPOSE

The purpose of this study was to characterize the individual contribution of multiple fat peaks to the measured chemical exchange saturation transfer (CEST) signal when using water-selective binomial-pulse excitation and to determine the effects of multiple fat peaks in the presence of B₀ inhomogeneity.

METHODS

The excitation profiles of multiple binomial pulses were simulated. A CEST sequence with binomial-pulse excitation and modified point-resolved spectroscopy localization was then applied to the in vivo lumbar spinal vertebrae to determine the signal contributions of three distinct groups of lipid resonances. These confounding signal contributions were measured as a function of the irradiation frequency offset to determine the effect of the multi-peak nature of the fat signal on CEST imaging of exchange sites (at 1.0, 2.0 and 3.5 ppm) and robustness in the presence of B₀ inhomogeneity.

RESULTS

Numerical simulations and in vivo experiments showed that water excitation (WE) using a 1-3-3-1 (WE-4) pulse provided the broadest signal suppression, which provided partial robustness against B₀ inhomogeneity effects. Confounding fat signal contributions to the CEST contrasts at 1.0, 2.0 and 3.5 ppm were unavoidable due to the multi-peak nature of the fat signal. However, these CEST sites only suffer from small lipid artifacts with ∆B₀ spanning roughly from  - 50 to 50 Hz. Especially for the CEST site at 3.5 ppm, the lipid artifacts are smaller than 1% with ∆B₀ in this range.

CONCLUSION

In WE-4-based CEST magnetic resonance imaging, B₀ inhomogeneity is the limiting factor for fat suppression. The CEST sites at 1.0, 2.0 ppm and 3.5 ppm unavoidably suffer from lipid artifacts. However, when the ∆B₀ is confined to a limited range, these CEST sites are only affected by small lipid artifacts, which may be ignorable in some cases of clinical applications.

Myoinositol CEST signal in animals with increased Iba-1 levels in response to an inflammatory challenge-Preliminary findings

Neuroinflammation plays an important role in the pathogenesis of a range of brain disorders. Non-invasive imaging of neuroinflammation is critical to help improve our understanding of the underlying disease mechanisms, monitor therapies and guide drug development. Generally, MRI lacks specificity to molecular imaging biomarkers, but molecular MR imaging based on chemical exchange saturation transfer (CEST) can potentially detect changes of myoinositol, a putative glial marker that may index neuroinflammation. In this pilot study we aimed to investigate, through validation with immunohistochemistry and in vivo magnetic resonance spectroscopy (MRS), whether CEST imaging can reflect the microglial response to a mild inflammatory challenge with lipopolysaccharide (LPS), in the APPSwe/ PS1 mouse model of Alzheimer’s disease and wild type controls. The response to the immune challenge was variable and did not align with genotype. Animals with a strong response to LPS (Iba1+, n = 6) showed an increase in CEST contrast compared with those who did not (Iba1-, n = 6). Changes of myoinositol levels after LPS were not significant. We discuss the difficulties of this mild inflammatory model, the role of myoinositol as a glial biomarker, and the technical challenges of CEST imaging at 0.6ppm.

Real-time simultaneous shim and motion measurement and correction in glycoCEST MRI using double volumetric navigators (DvNavs)

PURPOSE

CEST MRI allows for indirect detection of molecules with exchangeable protons, measured as a reduction in water signal because of continuous transfer of saturated protons. CEST requires saturation pulses on the order of a second, as well as repeated acquisitions at different offset frequencies. The resulting extended scan time makes CEST susceptible to subject motion, which introduces field inhomogeneity, shifting offset frequencies and causing distortions in CEST spectra that resemble true CEST effects. This is a particular problem for molecules that resonate close to water, such as hydroxyl group in glycogen. To address this, a technique for real-time measurement and correction of motion and field inhomogeneity is proposed.

METHODS

A CEST sequence was modified to include double volumetric navigators (DvNavs) for real-time simultaneous motion and shim correction. Phantom tests were conducted to investigate the effects of motion and shim changes on CEST quantification and to validate the accuracy of DvNav motion and shim estimates. To evaluate DvNav shim and motion correction in vivo, acquisitions including 5 experimental conditions were performed in the calf muscle of 2 volunteers.

RESULTS

Phantom data show that DvNav-CEST accurately estimates frequency and linear gradient changes because of motion and corrects resulting image distortions. In addition, DvNav-CEST improves CEST quantification in vivo in the presence of motion.

CONCLUSION

The proposed technique allows for real-time simultaneous motion and shim correction with no additional scanning time, enabling accurate CEST quantification even in the presence of motion and field inhomogeneity.

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

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, ω 1 , 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 ω 1 , m a x ( δ ) 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.

Non-invasive Amide Proton Transfer Imaging and ZOOM Diffusion-Weighted Imaging in Differentiating Benign and Malignant Thyroid Micronodules

Background: Pre-operative non-invasive differentiation of benign and malignant thyroid nodules is difficult for doctors. This study aims to determine whether amide proton transfer (APT) imaging and zonally oblique multi-slice (ZOOM) diffusion-weighted imaging (DWI) can provide increased accuracy in differentiating benign and malignant thyroid nodules. Methods: This retrospective study was approved by the institutional review board and included 60 thyroid nodules in 50 patients. All of the nodules were classified as malignant (n = 21) or benign (n = 39) based on pathology. It was meaningful to analyze the APT and apparent diffusion coefficient (ADC) values of the two groups by independent t-test to identify the benign and malignant thyroid nodules. The relationship between APT and ZOOM DWI was explored through Pearson correlation analysis. The diagnostic efficacy of APT and ZOOM DWI in determining if thyroid nodules were benign or malignant was compared using receiver operating characteristic (ROC) curve analysis. Results: The mean APTw value of the benign nodules was 2.99 ± 0.79, while that of the malignant nodules was 2.14 ± 0.73. Additionally, there was a significant difference in the APTw values of the two groups (P < 0.05). The mean ADC value of the benign nodules was 1.84 ± 0.41, and was significantly different from that of the malignant nodules, which was 1.21 ± 0.19 (P < 0.05). Scatter point and Pearson test showed a moderate positive correlation between the APT and ADC values (P < 0.05). The ROC curve showed that the area under the curve (AUC) value of ZOOM DWI (AUC = 0.937) was greater than that of APT (AUC = 0.783) (P = 0.028). Conclusion: APT and ZOOM DWI imaging improved the accuracy of distinguishing between benign and malignant thyroid nodules. ZOOM DWI is superior to APTw imaging (Z = 2.198, P < 0.05).