Examining fMRI time-series entropy as a marker for brain E/I balance with pharmacological neuromodulation in a non-human primate translational model.

Recently, there has been a lot of interest in the neuroimaging community in exploring fMRI time-series measures of local neuronal activity and excitation/inhibition (E/I) balance in the brain. In this preliminary study we probed the sensitivity of widely used sample entropy (SE) measure at multiple scales to controlled alteration of the brain's E/I balance in non-human primates (NHPs) with a well-characterized sub-anesthetic ketamine infusion fMRI model. We found that SE failed to detect the expected changes in E/I balance induced by ketamine. Subsequently, noticing that the complexity in the time series contributing SE could be dominated by non-neuronal noise in this experimental setting, we developed a new time-series measure called restricted sample entropy (RSE) by restricting SE estimations to regular portions of the fMRI time-series. RSE was able to adequately reflect the increased excitatory activity engendered by disinhibition of glutamergic neurons, through sub-anesthetic ketamine infusion. These results show that RSE is potentially a powerful tool for examining local neural activity, E/I balance, and alterations in brain state.

Chemokine receptor 4 targeted protein MRI contrast agent for early detection of liver metastases.

Liver metastases often progress from primary cancers including uveal melanoma (UM), breast, and colon cancer. Molecular biomarker imaging is a new non-invasive approach for detecting early stage tumors. Here, we report the elevated expression of chemokine receptor 4 (CXCR4) in liver metastases in UM patients and metastatic UM mouse models, and development of a CXCR4-targeted MRI contrast agent, ProCA32.CXCR4, for sensitive MRI detection of UM liver metastases. ProCA32.CXCR4 exhibits high relaxivities (r 1 = 30.9 mM-1 s-1, r 2 = 43.2 mM-1 s-1, 1.5 T; r 1 = 23.5 mM-1 s-1, r 2 = 98.6 mM-1 s-1, 7.0 T), strong CXCR4 binding (K d = 1.10 ± 0.18 µM), CXCR4 molecular imaging capability in metastatic and intrahepatic xenotransplantation UM mouse models. ProCA32.CXCR4 enables detecting UM liver metastases as small as 0.1 mm3. Further development of the CXCR4-targeted imaging agent should have strong translation potential for early detection, surveillance, and treatment stratification of liver metastases patients.

Quantifying exchange rates in chemical exchange saturation transfer agents using the saturation time and saturation power dependencies of the magnetization transfer effect on the magnetic resonance imaging signal (QUEST and QUESP): Ph calibration for poly-L-lysine and a starburst dendrimer.

The ability to measure proton exchange rates in tissue using MRI would be very useful for quantitative assessment of magnetization transfer properties, both in conventional MT imaging and in the more recent chemical exchange saturation transfer (CEST) approach. CEST is a new MR contrast mechanism that depends on several factors, including the exchange rate of labile protons in the agent in a pH-dependent manner. Two new methods to monitor local exchange rate based on CEST are introduced. The two MRI-compatible approaches to measure exchange are quantifying exchange using saturation time (QUEST) dependence and quantifying exchange using saturation power (QUESP) dependence. These techniques were applied to poly-L-lysine (PLL) and a generation-5 polyamidoamine dendrimer (SPD-5) to measure the pH dependence of amide proton exchange rates in the physiologic range. Data were fit both to an analytical expression and to numerical solutions to the Bloch equations. Results were validated by comparison with exchange rates determined by two established spectroscopic methods. The exchange rates determined using the four methods were pooled for the pH-calibration curve of the agents consisting of contributions from spontaneous (k0) acid catalyzed (ka), and base catalyzed (kb) exchange rate constants. These constants were k0 = 68.9 Hz, ka = 1.21 Hz, kb = 1.92 x 10(9) Hz, and k0 = 106.4 Hz, ka = 25.8 Hz, kb = 5.45 x 10(8) Hz for PLL and SPD-5, respectively, showing the expected predominance of base-catalyzed exchange for these amide protons.

In vivo oxygen detection using exogenous hemoglobin as a contrast agent in magnetic resonance microscopy.

In this work we show that exogenous molecular hemoglobin (Hb) is an effective indicator of relative local oxygen tension in magnetic resonance (MR) microscopy studies in vivo. This approach is more sensitive than other MRI oximetry methods; it can be used at higher resolutions and in specimens with no blood oxygen level-dependent (BOLD) effects. Using injection studies in flies, we show that Hb can permeate through relatively dense neural tissue, and that it is not obviously disruptive to physiology. Hb-injected flies show large changes in signal intensity (40-50%) when external O(2) levels are manipulated artificially from 0% to 21%. Oxygen-dependent contrast changes produced by exogenous Hb are detected in T(2)-weighted imaging experiments, and can be roughly calibrated if necessary. These studies demonstrate the feasibility of a contrast agent technique that may be useful for functional MRI (fMRI) studies of metabolism at tens of microns resolution.

In vivo magnetic resonance microscopy of brain structure in unanesthetized flies.

We present near-cellular-resolution magnetic resonance (MR) images of an unanesthetized animal, the blowfly Sarcophaga bullata. Immobilized flies were inserted into a home-built gradient probe in a 14.1-T magnet, and images of voxel size (20-40 microm)(3)--comparable to the diameter of many neuronal cell bodies in the fly's brain--were obtained in several hours. Use of applied field gradients on the order of 60 G/cm allowed minimally distorted images to be produced, despite significant susceptibility differences across the specimen. The images we obtained have exceptional contrast-to-noise levels; comparison with histology-based anatomical information shows that the MR microscopy faithfully represents patterns of nervous tissue and allows distinct brain regions to be clearly identified. Even at the highest resolutions we explored, morphological detail was pronounced in the apparent absence of instabilities or movement-related artifacts frequently observed during imaging of live animal specimens. This work demonstrates that the challenges of noninvasive in vivo MR microscopy can be overcome in a system amenable to studies of brain structure and physiology.