To estimate the exchange rate of creatine (Cr) CEST and to evaluate the pH sensitivity of guanidinium (Guan) CEST in the mouse brain.

The exchange rate of creatine CEST in mouse brain

To extract guanidinium (Guan) and amide CEST on the human brain at 3 T MRI with the high spectral resolution (HSR) CEST combined with the polynomial Lorentzian line-shape fitting (PLOF).Continuous wave (cw) turbo spin-echo (TSE) CEST was implemented to obtain the optimum saturation parameters. Both Guan and amide CEST peaks were extracted and quantified using the PLOF method. The NMR spectra on the egg white phantoms were acquired to reveal the fitting range and the contributions to the amide and GuanCEST. Two types of CEST approaches, including cw gradient- and spin-echo (cwGRASE) and steady state EPI (ssEPI), were implemented to acquire multi-slice HSR-CEST.GuanCEST can be extracted with the PLOF method at 3 T, and the optimum B1=0.6μT$$ {\mathrm{B}}_1=0.6\kern0.2em \upmu \mathrm{T} $$ was determined for GuanCEST in white matter (WM) and 1.0 μT in gray matter (GM). The optimum B1 = 0.8-1 μT was found for amideCEST. AmideCEST is lower in both WM and GM collected with ssEPI compared to those by cwGRASE (ssEPI = [1.27-1.63]%; cwGRASE = [2.19-2.25]%). The coefficients of variation (COV) of the amide and Guan CEST in both WM and GM for ssEPI (COV: 28.6-33.4%) are significantly higher than those of cwGRASE (COV: 8.6-18.8%). Completely different WM/GM contrasts for Guan and amide CEST were observed between ssEPI and cwGRASE. The amideCEST was found to have originated from the unstructured amide protons as suggested by the NMR spectrum of the unfolded proteins in egg white.Guan and amide CEST mapping can be achieved by the HSR-CEST at 3 T combing with the PLOF method. 

Guanidinium and amide CEST mapping of human brain by high spectral resolution CEST at 3 T

PURPOSE
To assess the feasibility of CEST-based creatine (Cr) mapping in brain at 3T using the guanidino (Guan) proton resonance.


METHODS
Wild type and knockout mice with guanidinoacetate N-methyltransferase deficiency and low Cr and phosphocreatine (PCr) concentrations in the brain were used to assign the Cr and protein-based arginine contributions to the GuanCEST signal at 2.0 ppm. To quantify the Cr proton exchange rate, two-step Bloch-McConnell fitting was used to fit the extracted CrCEST line-shape and multi-B1 Z-spectral data. The pH response of GuanCEST was simulated to demonstrate its potential for pH mapping.


RESULTS
Brain Z-spectra of wild type and guanidinoacetate N-methyltransferase deficiency mice show a clear Guan proton peak at 2.0 ppm at 3T. The CrCEST signal contributes ∼23% to the GuanCEST signal at B1  = 0.8 μT, where a maximum CrCEST effect of 0.007 was detected. An exchange rate range of 200-300 s-1 was estimated for the Cr Guan protons. As revealed by the simulation, an elevated GuanCEST in the brain is observed when B1 is less than 0.4 μT at 3T, when intracellular pH reduces by 0.2. Conversely, the GuanCEST decreases when B1 is greater than 0.4 μT with the same pH drop.


CONCLUSIONS
CrCEST mapping is possible at 3T, which has potential for detecting intracellular pH and Cr concentration in brain.

Creatine mapping of the brain at 3T by CEST MRI.

PURPOSE
To confirm that CrCEST in muscle exhibits a slow-exchanging process, and to obtain high-resolution amide, creatine (Cr), and phosphocreatine (PCr) maps of skeletal muscle using a POlynomial and Lorentzian Line-shape Fitting (PLOF) CEST at 3T.


METHODS
We used dynamic changes in PCr/CrCEST of mouse hindlimb before and after euthanasia to assign the Cr and PCr CEST peaks in the Z-spectrum at 3T and to obtain the optimum saturation parameters. Segmented 3D EPI was employed to obtain multi-slice amide, PCr, and Cr CEST maps of human skeletal muscle. Subsequently, the PCrCEST maps were calibrated using the PCr concentrations determined by 31 P MRS.


RESULTS
A comparison of the Z-spectra in mouse hindlimb before and after euthanasia indicated that CrCEST is a slow-exchanging process in muscle (<150.7 s-1 ). This allowed us to simultaneously extract PCr/CrCEST signals at 3T using the PLOF method. We determined optimal B1 values ranging from 0.3 to 0.6 μT for CrCEST in muscle and 0.3-1.2 μT for PCrCEST. For the study on human calf muscle, we determined an optimum saturation time of 2 s for both PCr/CrCEST (B1  = 0.6 μT). The PCr/CrCEST using 3D EPI were found to be comparable to those obtained using turbo spin echo (TSE). (3D EPI/TSE PCr: (2.6 ± 0.3) %/(2.3 ± 0.1) %; Cr: (1.3 ± 0.1) %/(1.4 ± 0.07) %).


CONCLUSIONS
Our study showed that in vivo CrCEST is a slow-exchanging process. Hence, amide, Cr, and PCr CEST in the skeletal muscle can be mapped simultaneously at 3T by PLOF CEST.

Simultaneous creatine and phosphocreatine mapping of skeletal muscle by CEST MRI at 3T.

pH enhanced (pHenh ) CEST imaging combines the pH sensitivity from amide and guanidino signals, but the saturation parameters have not been optimized. We propose pHdual as a variant of pHenh that suppresses background signal variations, while enhancing pH sensitivity and potential for imaging ischemic brain injury of stroke.Simulation and in vivo rodent stroke experiments of pHenh MRI were performed with varied RF saturation powers for both amide and guanidino protons to optimize the contrast between lesion/normal tissues, while simultaneously minimizing signal variations across different types of normal tissues. In acute stroke, contrast and volume ratio measured by pHdual imaging were compared with an amide-CEST approach, and perfusion and diffusion MRI.Simulation experiments indicated that amide and guanidino CEST signals exhibit unique sensitivities across different pH ranges, with pHenh producing greater sensitivity over a broader pH regime. The pHenh data of rodent stroke brain demonstrated that the lesion/normal tissue contrast was maximized for an RF saturation power pair of 0.5 μT at 2.0 ppm and 1.0 μT at 3.6 ppm, whereas an optimal contrast-to-variation ratio (CVR) was obtained with a 0.7 μT saturation at 2.0 ppm and 0.8 μT at 3.6 ppm. In acute stroke, CVR optimized pHenh (i.e., pHdual ) achieved a higher sensitivity than the three-point amide-CEST approach, and distinct patterns of lesion tissue compared to diffusion and perfusion MRI.pHdual MRI improves the sensitivity of pH-weighted imaging and will be a valuable tool for assessing tissue viability in stroke. 

Dual contrast CEST MRI for pH-weighted imaging in stroke.

Purpose To develop a pipeline for measuring the exchange rates and concentrations of in vivo excgangeable protons, and to demonstrate this for the amide and arginine (Arg) guanidinium (Guan) protons in mobile proteins in the mouse brain. Methods An ultra-short echo (UTE) CEST sequence with a continuous wave presaturation (preRadCEST) was applied to acquire Z-spectra with robustness to motion and physiological fluctuations. AmideCEST and Arginine guanCEST (ArgCEST) were extracted and their proton concentrations and exchange rates obtained using a two-step multi-B1 Bloch fitting approach that included the semisolid macromolecular background. To minimize contamination from the amine protons from creatine and phosphocreatine, ArgCEST measurements were performed on the Guanidinoacetate N-methyltransferase deficiency (GAMT-/-) mouse characterized by low creatine and phosphocreatine concentrations in the brain. Results For the amideCEST proton pool, the exchange rate and concentrations were found to be 59.6 ± 9.0 s-1 and 41.7 ± 7.0 mM, respectively, with the maximum signal observed at B1 = 0.8 μT. For the ArgCEST proton, the guanidinium exchange these were 70.1 ± 5.5 s-1 and 10.1 ± 1.3 mM, respectively, with the maximum effect observed at B1 = 0.9 μT. The current study suggests that the inverse pH dependence in GuanCEST of brain is led by the CrCEST component, not ArgCEST. Conclusion The current pipeline is expected to have general use for in vivo CEST quantitation and optimization of visible CEST resonances.

/pdf/the-exchange-rates-of-amide-and-arginine-guanidinium-cest-in-2x2246t7.pdf

The exchange rates of amide and arginine guanidinium CEST in the mouse brain

Cerebral small vessel disease (cSVD) is a disease defined by clinical symptoms and neuroimaging, which often causes a series of pathophysiological changes, blood-brain barrier destruction, brain tissue ischemia and involves cerebral arterioles, capillaries and venules. The exact pathogenesis of cSVD is unclear and there is no specific prevention and treatment for this potentially high disability rate disease. This article reviewed the latest research progress of neuroimaging of cSVD in order to improve our understanding of cSVD's manifestation and potential mechanism. We introduced the neuroimaging markers which can be accurately identified by diffusion tensor imaging, including recent subcortical infarction, white matter lesions, brain atrophy, lacunar infarction, cerebral microhaemorrhage and other cSVD neuroimaging markers. Besides, we also interpreted the total load score of cSVD, which described a wide range of clinical, pathological and neuroimaging features, reflecting the acute and chronic damage of the whole brain. Combined with the neuroimaging methods, capturing the imaging features of early cSVD can improve the diagnostic ability of cSVD and provide strong support for the longitudinal study.

Review of neuroimaging research progress of cerebral small vessel disease.

Subjective cognitive decline (SCD) is considered to be the preclinical stage of Alzheimer's disease (AD) and has the potential for the early diagnosis and intervention of AD. It was implicated that CSF-tau, which increases very early in the disease process in AD, has a high sensitivity and specificity to differentiate AD from normal aging, and the highly connected brain regions behaved more tau burden in patients with AD. Thus, a highly connected state measured by dynamic functional connectivity may serve as the early changes of AD. In this study, forty-five normal controls (NC), thirty-six individuals with SCD, and thirty-five patients with AD were enrolled to obtain the resting-state functional magnetic resonance imaging scanning. Sliding windows, Pearson correlation, and clustering analysis were combined to investigate the different levels of information transformation states. Three states, namely, the low state, the middle state, and the high state, were characterized based on the strength of functional connectivity between each pair of brain regions. For the global dynamic functional connectivity analysis, statistically significant differences were found among groups in the three states, and the functional connectivity in the middle state was positively correlated with cognitive scales. Furthermore, the whole brain was parcellated into four networks, namely, default mode network (DMN), cognitive control network (CCN), sensorimotor network (SMN), and occipital-cerebellum network (OCN). For the local network analysis, statistically significant differences in CCN for low state and SMN for middle state and high state were found in normal controls and patients with AD. Meanwhile, the differences were also found in normal controls and individuals with SCD. In addition, the functional connectivity in SMN for high state was positively correlated with cognitive scales. Converging results showed the changes in dynamic functional states in individuals with SCD and patients with AD. In addition, the changes were mainly in the high strength of the functional connectivity state.

/pdf/abnormal-dynamic-functional-networks-in-subjective-cognitive-2mz5dlqs.pdf

Abnormal Dynamic Functional Networks in Subjective Cognitive Decline and Alzheimer's Disease

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The exchange rate of creatine CEST in mouse brain

The exchange rates of amide and arginine guanidinium CEST in the mouse brain

Review of neuroimaging research progress of cerebral small vessel disease.

Abnormal Dynamic Functional Networks in Subjective Cognitive Decline and Alzheimer's Disease