Adjustment of rotation and saturation effects (AROSE) for CEST imaging.

PURPOSE: Endogenous CEST signal usually has low specificity due to contaminations from the magnetization transfer contrast (MTC) and other labile protons with overlapping or close Larmor frequencies. We propose to improve CEST signal specificity with adjustment of rotation and saturation effects (AROSE). METHODS: The AROSE approach measures the difference between CEST signals acquired with the same average irradiation power but largely different duty cycles, for example, a continuous wave or a high duty cycle pulse train versus a low duty cycle pulse train with a flip angle φ. Simulation, phantom, and in vivo rodent studies were performed to evaluate the characteristics of the AROSEφ signal. RESULTS: Simulation and experimental results show that AROSE2π is a low-pass filter that can suppress fast exchanging processes (e.g., >3000 s-1 ), whereas AROSEπ is a band-pass filter suppressing both fast and slow exchange (e.g., <30 s-1 ) rates. For other φ angles, the sensitivity and the exchange-rate filtering effect of AROSEφ falls between AROSEπ and AROSE2π . AROSE can also minimize MTC and improve the Larmor frequency selectivity of the CEST signal. The linewidth of the AROSE1.5π spectrum is about 60% to 65% when compared to the CEST spectrum measured by continuous wave. Depending on the needs of an application, the sensitivity, exchange-rate filtering, and Larmor frequency selectivity can be adjusted by varying the flip angle, duty cycle, and average irradiation power. CONCLUSION: Compared to conventional CEST signals, AROSE can minimize MTC and improve exchange rate filtering and Larmor frequency specificity.

Improving standardization and accuracy of in vivo omega plot exchange parameter determination using rotating-frame model-based fitting of quasi-steady-state Z-spectra.

PURPOSE: Although Ω-plot-driven quantification of in vivo amide exchange properties has been demonstrated, differences in scan parameters may complicate the fidelity of determination. This work systematically evaluated the use of quasi-steady-state (QUASS) Z-spectra reconstruction to standardize in vivo amide exchange quantification across acquisition conditions and further determined it in vivo. METHODS: Simulation and in vivo rodent brain chemical exchange saturation transfer (CEST) data at 4.7 T were fit with and without QUASS reconstruction using both multi-Lorentzian and model-based fitting approaches. pH modulation was accomplished both in simulation and in vivo by inducing global ischemia via cardiac arrest. Amide parameters were determined via Ω-plots and compared across methods. RESULTS: Simulation showed that Ω-plots using multi-Lorentzian fitting could underestimate the exchange rate, with error increasing as conditions diverged from the steady state. In comparison, model-based fitting using QUASS estimated the same exchange rate within 2%. These results aligned with in vivo findings where multi-Lorentzian fitting of native Z-spectra resulted in an exchange rate of 64 ± 13 s-1 (38 ± 16 s-1 after cardiac arrest), whereas model-based fitting of QUASS Z-spectra yielded an exchange rate of 126 ± 25 s-1 (49 ± 13 s-1). CONCLUSION: The model-based fitting of QUASS CEST Z-spectra enables consistent and accurate quantification of exchange parameters through Ω-plot construction by reducing error due to signal overlap and nonequilibrium CEST effects.

Average saturation efficiency filter ASEF-CEST MRI of stroke rodents.

PURPOSE: The average saturation efficiency filter (ASEF) is a novel method of improving the specificity of CEST; however, there is a mismatch between the magnetization transfer (MT) effect under high-duty cycle and low-duty cycle pulse trains. We explore measures of mitigation and the sensitivity and potential of ASEF imaging in phantoms and stroke rats. METHODS: Simulation and nicotinamide phantoms in denatured protein were used to investigate the effect of different average saturation powers and MT pool parameters on matching coefficients used for correction as well as the ASEF ratio signal and baseline. Then, in vivo studies were performed in stroke rodents to further investigate the sensitivity and fidelity of ASEF ratio spectra. RESULTS: Simulation and studies of nicotinamide phantoms show that the matching coefficient needed to correct the baseline MT mismatch is strongly dependent on the average saturation power. In vivo studies in stroke rodents show that the matching coefficient required to correct the baseline MT mismatch is different for normal versus ischemic tissue. Thus, a baseline correction was performed to further suppress the residue MT mismatch. After correction of the mismatch, ASEF ratio achieved comparable contrast at 3.6 ppm between normal and ischemic tissue when compared to the apparent amide proton transfer (APT*) approach. Moreover, contrasts for 2.0 and 2.6 ppm were also ascertainable from the same spectra. CONCLUSION: ASEF can improve the CEST signal specificity of slow exchange labile protons such as amide and guanidyl, with small loss to sensitivity. It has strong potential in the CEST imaging of various diseases.

Chemical exchange saturation transfer imaging of creatine, phosphocreatine, and protein arginine residue in tissues.

Chemical exchange saturation transfer (CEST) MRI has become a promising technique to assay target proteins and metabolites through their exchangeable protons, noninvasively. The ubiquity of creatine (Cr) and phosphocreatine (PCr) due to their pivotal roles in energy homeostasis through the creatine phosphate pathway has made them prime targets for CEST in the diagnosis and monitoring of disease pathologies, particularly in tissues heavily dependent on the maintenance of rich energy reserves. Guanidinium CEST from protein arginine residues (i.e. arginine CEST) can also provide information about the protein profile in tissue. However, numerous obfuscating factors stand as obstacles to the specificity of arginine, Cr, and PCr imaging through CEST, such as semisolid magnetization transfer, fast chemical exchanges such as primary amines, and the effects of nuclear Overhauser enhancement from aromatic and amide protons. In this review, the specific exchange properties of protein arginine residues, Cr, and PCr, along with their validation, are discussed, including the considerations necessary to target and tune their signal effects through CEST imaging. Additionally, strategies that have been employed to enhance the specificity of these exchanges in CEST imaging are described, along with how they have opened up possible applications of protein arginine residues, Cr and PCr CEST imaging in the study and diagnosis of pathology. A clear understanding of the capabilities and caveats of using CEST to image these vital metabolites and mitigation strategies is crucial to expanding the possibilities of this promising technology.

Average saturation efficiency filter (ASEF) for CEST imaging.

PURPOSE: Endogenous CEST signal usually has low specificity due to contamination from the magnetization transfer effect and from fast exchanging labile protons with close Larmor frequencies. We propose to improve CEST signal specificity with an average saturation efficiency filter (ASEF). METHODS: ASEF measures the difference between CEST signals acquired with similar average irradiation power but largely different duty cycles (DC), for example, a continuous wave or a high DC pulse train versus a low DC one. Simulation and Cr phantom studies were performed to evaluate the characteristics of ASEF for CEST. RESULTS: Theoretical and simulation studies show that ASEF can suppress fast exchanging processes, with only a small loss of chemical exchange contrast for slow-to-intermediate exchange rates if the difference in DC is large. In the RF offset range of 2 to 5 ppm with an averaged saturation power of 0.8 and 1.6 microteslas, there is a mismatch of ∼0.1% to 2% in the magnetization transfer signal between saturation by continuous wave and a pulse train with DC = 15% and pulse duration of 24 ms, respectively. This mismatch can be minimized by careful selection of saturation power, pulse duration, and DC differences or by applying a small fudge factor between the 2 irradiation powers. Phantom studies of Cr confirmed that ASEF can minimize the magnetization transfer effect and reduce sensitivity to fast exchange processes. CONCLUSION: ASEF can improve the specificity of slow-to-intermediate exchanging CEST signal with a relatively small loss of sensitivity.

Low duty cycle pulse trains for exchange rate insensitive chemical exchange saturation transfer MRI.

PURPOSE: To introduce and validate a pulse scheme that uses low duty cycle trains of π-pulses to achieve saturation that is relatively insensitive to exchange rate yet linearly dependent on labile proton concentration. METHODS: Simulations were performed to explore the exchange rate sensitivity of π-pulse trains and continuous wave chemical exchange saturation transfer (CEST) signals. Creatine phantoms with varying pH and varying concentrations were imaged to demonstrate pH insensitivity and concentration dependence of low duty cycle π-pulse saturation. RESULTS: Simulations show decreasing the duty cycle of π-pulse saturation decreases peak sensitivity to exchange rate, and this range of insensitivity can be tuned to different exchange rates through average B1 power. The range of insensitivity is unaffected by changes in relaxation and magnetization transfer, while the sensitivity of CEST signal maintains linear dependence on labile proton concentration. Under B1,avg = 0.48 µT, 30 mM creatine with pHs ranging between 6.36 and 8.21 exhibited CEST contrast ranging between ~6 and 11% under continuous wave and ~4% across all pHs using 10% duty cycle π-pulses. Imaging these phantoms using duty cycles of 5, 10, 25, and 50% showed decreasing pH sensitivity with decreased duty cycle. Creatine phantoms with varied concentrations and pHs reveal that π-pulse train saturation exhibited stricter correlation to concentration at lower DCs. CONCLUSION: Low DC π-pulse train is an easy-to-implement way of providing labile proton concentration-dependent CEST MRI signal that is insensitive to exchange rate. This approach can be useful in studies where a change of chemical exchange rate may interfere with accurate assessments of physiology or pathology.

Chemical exchange sensitive MRI of glucose uptake using xylose as a contrast agent.

PURPOSE: Glucose and its analogs can be detected by CEST and chemical exchange spin-lock (CESL) MRI techniques, but sensitivity is still a bottleneck for human applications. Here, CESL and CEST sensitivity and the effect of injection on baseline physiology were evaluated for a glucose analog, xylose. METHODS: The CEST and CESL sensitivity were evaluated at 9.4 T in phantoms and by in vivo rat experiments with 0.5 and 1 g/kg xylose injections. Arterial blood glucose level was sampled before and after 1 g/kg xylose injection. The effect of injection on baseline neuronal activity was measured by electrophysiology data during injections of saline, xylose, and 2-deoxy-D-glucose. RESULTS: In phantoms, xylose shows similar chemical exchange sensitivity and pH-dependence with that of glucose. In rat experiments with a bolus injection, CESL shows higher sensitivity in the detection of xylose than CEST, and the sensitivity of xylose is much higher than glucose. Injection of xylose does not significantly affect blood glucose level and baseline neural activity for 1-g/kg and 0.6-g/kg doses, respectively. CONCLUSION: Due to its relatively high sensitivity and safety, xylose is a promising contrast agent for the study of glucose uptake.

Chemical exchange saturation transfer imaging of phosphocreatine in the muscle.

PURPOSE: To determine the exchange parameters for the CEST of phosphocreatine (PCrCEST) in phantoms and to characterize PCrCEST in vivo in the muscle at different saturation powers and magnetic fields. METHODS: Exchange parameters were measured in PCr solutions using varying saturation power at 15.2 T. Z-spectra were analyzed using multipool Lorentzian fitting in the hindlimb using various powers at 2 different fields: 9.4 T and 15.2 T. Modulation of PCr signal in PCrCEST and phosphorus MRS was observed in the mouse hindlimb before and after euthanasia. RESULTS: The exchange rate of PCr at physiological pH in phantoms was confirmed to be in a much slower exchange regime compared with Cr: kex at pH 7.3 and below was less than 400 s-1 . There was insufficient signal for detection of PCrCEST in the brain, but PCrCEST in the hindlimb was measured to be 2.98% ± 0.43 at a B1 of 0.47 µT at 15.2 T, which is 29% higher than 9.4T values. The value of PCrCEST at a B1 of 0.71 µT was not significantly different than that measured at a B1 of 0.47 µT. After euthanasia, PCrCEST signal dropped by 82.3% compared with an 85% decrease in PCr in phosphorus MRS, whereas CrCEST signal increased by 90.6%. CONCLUSION: The PCrCEST technique has viable sensitivity in the muscle at high fields and shows promise for the study of metabolic dysfunction and cardiac systems.

Chemical-exchange-sensitive MRI of amide, amine and NOE at 9.4 T versus 15.2 T.

Chemical exchange (CE)-sensitive MRI benefits greatly from stronger magnetic fields; however, field effects on CE-sensitive imaging have not yet been studied well in vivo. We have compared CE-sensitive Z-spectra and maps obtained at the fields of 9.4 T and 15.2 T in phantoms and rats with off-resonance chemical-exchange-sensitive spin lock (CESL), which is similar to conventional chemical exchange saturation transfer. At higher fields, the background peak at water resonance has less spread and the exchange rate relative to chemical shift decreases, thus CESL intensity is dependent on B0 . For the in vivo amide and nuclear Overhauser enhancement (NOE) composite resonances of rat brains, intensities were similar for both magnetic fields, but effective amide proton transfer and NOE values obtained with three-point quantification or a curve fitting method were larger at 15.2 T due to the reduced spread of attenuation at the direct water resonance. When using intermediate exchange-sensitive irradiation parameters, the amine proton signal was 65% higher at 15.2 T than at 9.4 T due to a reduced ratio of exchange rate to chemical shift. In summary, increasing magnetic field provides enhancements to CE-sensitive signals in the intermediate exchange regime and reduces contamination from background signals in the slow exchange regime. Consequently, ultrahigh magnetic field is advantageous for CE-sensitive MRI, especially for amine and hydroxyl protons.

Spatiotemporal microstructural white matter changes in diffusion tensor imaging after transient focal ischemic stroke in rats.

Structural reorganization in white matter (WM) after stroke is a potential contributor to substitute or to newly establish the functional field on the injured brain in nature. Diffusion tensor imaging (DTI) is an imaging modality that can be used to evaluate damage and recovery within the brain. This method of imaging allows for in vivo assessment of the restricted movements of water molecules in WM and provides a detailed look at structural connectivity in the brain. For longitudinal DTI studies after a stroke, the conventional region of interest method and voxel-based analysis are highly dependent on the user-hypothesis and parameter settings for implementation. In contrast, tract-based spatial statistics (TBSS) allows for reliable voxel-wise analysis via the projection of diffusion-derived parameters onto an alignment-invariant WM skeleton. In this study, spatiotemporal WM changes were examined with DTI-derived parameters (fractional anisotropy, FA; mean diffusivity, MD; axial diffusivity, DA; radial diffusivity, RD) using TBSS 2 h to 6 weeks after experimental focal ischemic stroke in rats (N = 6). FA values remained unchanged 2-4 h after the stroke, followed by a continuous decrease in the ipsilesional hemisphere from 24 h to 2 weeks post-stroke and gradual recovery from the ipsilesional corpus callosum to the external capsule until 6 weeks post-stroke. In particular, the fibers in these areas were extended toward the striatum of the ischemic boundary region at 6 weeks on tractography. The alterations of the other parameters in the ipsilesional hemisphere showed patterns of a decrease at the early stage, a subsequent pseudo-normalization of MD and DA, a rapid reduction of RD, and a progressive increase in MD, DA and RD with a decreased extent in the injured area at later stages. The findings of this study may reflect the ongoing processes on tissue damage and spontaneous recovery after stroke.

Neuroplasticity for spontaneous functional recovery after neonatal hypoxic ischemic brain injury in rats observed by functional MRI and diffusion tensor imaging.

For infants and children, an incredible resilience from injury is often observed. There is growing evidence that functional recovery after brain injury might well be a consequence of the reorganization of the neural network as a process of neuroplasticity. We demonstrate the presence of neuroplasticity at work in spontaneous recovery after neonatal hypoxic ischemic (HI) injury, by elucidating a precise picture in which such reorganization takes place using functional MRI techniques. For all 12 siblings, 6 rats were subjected to severe HI brain injury and 6 rats underwent sham operation only. Severe HI brain injury was induced to postnatal day 7 (p7) Sprague-Dawley rats according to the Rice-Vannucci model (right carotid artery occlusion followed by 150min of hypoxia with 8% O2 and 92% of N2). Brain activation maps along with anatomical and functional connectivity maps related to the sensory motor function were obtained at adult (p63) using blood oxygen level dependent (BOLD)-functional MRI (fMRI), resting state-functional MRI (rs-fMRI) and diffusion tensor imaging (DTI); each of these MRI data was related to sensory motor functional outcome. In-depth investigation of the functional MRI data revealed: 1) intra-hemispheric expansion of BOLD signal activation in the contralesional undamaged hemisphere for ipsilesional forepaw stimuli to include the M2 and Cg1 in addition to the S1 and M1 wide spreading in the anterior and posterior directions, 2) inter-hemispheric transfer of BOLD signal activation for contralesional forepaw stimuli, normally routed to the injured hemisphere, to analogous sites in the contralesional undamaged hemisphere, localized newly to the M1 and M2 with a reduced portion of the S1, 3) inter-hemispheric axonal disconnection and axonal rewiring within the undamaged hemisphere as shown through DTI, and 4) increased functional interactions within the cingulate gyrus in the HI injured rats as shown through rs-fMRI. The BOLD signal amplitudes as well as DTI and rs-fMRI data well correlate with behavioral tests (tape to remove). We found that function normally utilizing what would be the injured hemisphere is transferred to the uninjured hemisphere, and functionality of the uninjured hemisphere remains not untouched but is also rewired in an expansion corresponding to the newly formed sensorimotor function from both the contralesional and the ipsilesional sides. The conclusion drawn from the data in our current study is that enhanced motor function in the contralesional hemisphere governs both the normal and damaged sides, indicating that active plasticity with brain laterality was spontaneously generated to overcome functional loss and established autonomously through normal experience via modification of neural circuitry for neonatal HI injured brain.

Model-Based Chemical Exchange Saturation Transfer MRI for Robust z-Spectrum Analysis.

This paper introduces a novel, model-based chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI), in which asymmetric spectra of interest are directly estimated from complete or incomplete measurements by incorporating subspace-based spectral signal decomposition into the measurement model of CEST MRI for a robust z-spectrum analysis. Spectral signals are decomposed into symmetric and asymmetric components. The symmetric component, which varies smoothly, is delineated by the linear superposition of a finite set of vectors in a basis trained from the simulated (Lorentzian) signal vectors augmented with data-driven signal vectors, while the asymmetric component is to be inherently lower than or equal to zero due to saturation transfer phenomena. Spectral decomposition is performed directly on the measured spectral data by solving a constrained optimization problem that employs the linearized spectral decomposition model for the symmetric component and the weighted Frobenius norm regularization for the asymmetric component while utilizing additional spatial sparsity and low-rank priors. The simulations and in vivo experiments were performed to demonstrate the feasibility of the proposed method as a reliable molecular MRI.

Improvement of orthotopic lung cancer mouse model via thoracotomy and orotracheal intubation enabling in vivo imaging studies.

Investigation of molecular mechanisms and the efficiency of novel therapeutics for the treatment and prevention of a disease require accurate and accessible preclinical models. Recent developments in personalized medicine employing molecular medicine concepts have favored mice because their genetic make-up is well known and easy to manipulate. For lung cancer, however, orthotopic models in mice are difficult to create due to their narrow glottis openings which act as obstacles to intubation. In the present study, we develop an orotracheal intubation device which gives a clearer view of the narrow mouse glottis and increases the success rate of intubation. We achieved anesthetization via orotracheal intubation using this novel device and then performed a thoracotomy by making an incision between the fourth and fifth intercostal ribs on the right side of the chest. Lung tumor cells were then inoculated at this site. Tumor formation was monitored through bioluminescence optical and magnetic resonance (MR) imagings, which was confirmed by histological analysis. Temperature drop (<35) and/or loss of body weight (>30% of the initial body weight) observed during any procedure were used as interruption criteria. This method exhibited high tumorigenicity (100%) and a low mortality rate (8%) at specific sites making it ideal for creating orthotopic lung tumor models and making it particularly useful for sequential follow-up studies using in vivo image analysis.

Magnetic resonance imaging for monitoring therapeutic response in a transgenic mouse model of Alzheimer's disease using voxel-based analysis of amyloid plaques.

In this study, we have shown the potential of a voxel-based analysis for imaging amyloid plaques and its utility in monitoring therapeutic response in Alzheimer's disease (AD) mice using manganese oxide nanoparticles conjugated with an antibody of Aß1-40 peptide (HMON-abAß40). T1-weighted MR brain images of a drug-treated AD group (n=7), a nontreated AD group (n=7), and a wild-type group (n=7) were acquired using a 7.0 T MRI system before (D-1), 24-h (D+1) after, and 72-h (D+3) after injection with an HMON-abAß40 contrast agent. For the treatment of AD mice, DAPT was injected intramuscularly into AD transgenic mice (50 mg/kg of body weight). For voxel-based analysis, the skull-stripped mouse brain images were spatially normalized, and these voxels' intensities were corrected to reduce voxel intensity differences across scans in different mice. Statistical analysis showed higher normalized MR signal intensity in the frontal cortex and hippocampus of AD mice over wild-type mice on D+1 and D+3 (P<0.01, uncorrected for multiple comparisons). After the treatment of AD mice, the normalized MR signal intensity in the frontal cortex and hippocampus decreased significantly in comparison with nontreated AD mice on D+1 and D+3 (P<0.01, uncorrected for multiple comparisons). These results were confirmed by histological analysis using a thioflavin staining. This unique strategy allows us to detect brain regions that are subjected to amyloid plaque deposition and has the potential for human applications in monitoring therapeutic response for drug development in AD.

Dynamic contrast-enhanced MRI for assessing therapeutic response of choroidal neovascularization in a rat model.

PURPOSE: We evaluated the potential of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) as a noninvasive biomarker of choroidal neovascularization (CNV) and its utility as a tool for monitoring therapeutic response in laser-induced rat CNV models. METHODS: CNV was induced in the right eyes of 14 rats using a laser. Rats (n = 7) were treated daily for 14 days with a candidate drug (KR-31831, 50 mg/kg of body weight) having antiangiogenic effects, whereas control rats (n = 7) were treated with the vehicle alone (10% cremophor, 10% absolute ethyl alcohol, and 80% saline). DCE-MRI examinations were performed on the day before surgery (D - 1), and 3, 7, and 14 days after surgery (D + 3, D + 7, and D + 14), from which pharmacokinetic parameters (K(trans), v(e), v(p)) were calculated. Angiography was performed to visualize CNV using FITC-labeled high molecular weight dextran after MRI on D + 14. The paired Wilcoxon test and Mann-Whitney U test were performed for statistical analysis. RESULTS: The K(trans) and v(e) values of the CNV-induced right eyes were significantly higher than those of the intact eyes in control rats at D + 14 (P < 0.05). In the CNV-induced eyes, the relative K(trans) and v(e) values of the KR-31831-treated group were significantly lower than those of the nontreated group at D + 14 (P < 0.05). The angiography showed that decreased CNV was observed in rats treated with KR-31831. CONCLUSIONS: Quantitative DCE-MRI produces noninvasive biomarker of CNV, thus allowing monitoring of therapeutic response of antiangiogenic drugs in neovascular age-related macular degeneration (AMD).