Shuning Huang

Characterization of cerebrovascular responses to hyperoxia and hypercapnia using MRI in rat

Understanding cerebrovascular responses to hyperoxia and hypercapnia is important for investigating exogenous regulation of cerebral hemodynamics. We characterized gas-induced vascular changes in the brains of anesthetized healthy rats using magnetic resonance imaging (MRI) while the rats inhaled 100% O(2) (hyperoxia) and 5% CO(2) (hypercapnia). We used echo planar imaging (EPI), arterial spin labeling (ASL), and intravascular superparamagnetic iron oxide nanoparticles (SPION) to quantify vascular responses as measured by blood oxygenation level dependence (BOLD), cerebral blood flow (CBF), cerebral blood volume (CBV), microvascular volume (MVV), and vessel size index (VSI) in multiple brain regions. Hyperoxia resulted in a statistically significant increase in BOLD-weighted MRI signal and significant decrease in CBF and CBV (P<0.05). During hypercapnia, we observed significant increases in BOLD signal, CBF, MVV, and CBV (P<0.05). Despite the regional variability, general trends of vasoconstriction and vasodilation were reflected in VSI changes during O(2) and CO(2) challenges. Interestingly, there was an evident spatial disparity between the O(2) and CO(2) stimuli-induced functional activation maps; that is, cortical and subcortical regions of the brain exhibited notable differences in response to the two gases. Hemodynamic parameters measured in the cortical regions showed greater reactivity to CO(2), whereas these same parameters measured in subcortical regions showed greater responsivity to O(2). Our results demonstrate significant changes of hemodynamic MRI parameters during systemic hypercapnia and hyperoxia in normal cerebral tissue. These gas-dependent changes are spatiotemporally distinctive, suggesting important feasibility for exogenously controlling local cerebral perfusion.

MR contrast probes that trace gene transcripts for cerebral ischemia in live animals

The aim of this research was to validate transcription magnetic resonance (MR) imaging (MRI) for gene transcript targeting in acute neurological disorders in live subjects. We delivered three MR probe variants with superparamagnetic iron oxide nanopar‐ticles (SPION’ a T2 susceptibility agent) linked to a phosphorothioate‐modified oligodeoxynucleotide (sODN) complementary to c‐fos mRNA (SPION‐cfos) or β‐actin mRNA (SPION‐β‐actin) and to sODN with random sequence (SPION‐Ran). Each probe (1 μgFe in 2 μl) was delivered via intracerebroventricular infusion to the left cerebral ventricle of male C57Black6 mice. We demonstrated SPION retention’ measured as decreased T2* signal or increased R2* value (R2* = 1/ T2*). Animals that received the SPION‐β‐actin probe exhibited the highest R2* values’ followed (in descending order) by SPION‐cfos and SPION‐Ran. SPION‐cfos retention was localiZed in brain regions where SPION‐cfos was present and where hybrids of SPION‐cfos and its target c‐fos mRNA were detected by in situ reverse transcription PCR. In animals that experienced cerebral ischemia, SPION‐cfos retention was significantly increased in locations where c‐fos mRNA increased in response to the ischemic insult;these elevations were not observed for SPION‐β‐actin and SPION‐Ran. This study should enable MR detection of mRNA alteration in disease models of the central nervous system.—Liu C. H., Huang, S., Cui, J., Kim, Y. R., Farrar, C. T., Moskowitz M. A., Rosen B. R., Liu P. K. MR contrast probes that trace gene transcripts for cerebral ischemia in live animals. FASEB J. 21, 3004–3015 (2007)

Manipulation of tissue contrast using contrast agents for enhanced MR microscopy in ex vivo mouse brain.

Detailed 3D mouse brain images may promote better understanding of phenotypical differences between normal and transgenic/mutant mouse models. Previously, a number of magnetic resonance microscopy (MRM) studies have successfully established brain atlases, revealing genotypic traits of several commonly used mouse strains. In such studies, MR contrast agents, mainly gadolinium (Gd) based, were often used to reduce acquisition time and improve signal-to-noise ratio (SNR). In this paper, we intended to extend the utility of contrast agents for MRM applications. Using Gd-DTPA and MnCl(2), we exploited the potential use of MR contrast agents to manipulate image contrast by drawing upon the multiple relaxation mechanisms and tissue-dependent staining properties characteristic of each contrast agent. We quantified r(1) and r(2) of Gd-DTPA and MnCl(2) in both aqueous solution and brain tissue and demonstrated the presence of divergent relaxation mechanisms between solution and tissue for each contrast agent. Further analyses using nuclear magnetic resonance dispersion (NMRD) of Mn(2+) in ex vivo tissue strongly suggested macromolecule binding of Mn(2+), leading to increased T(1) relaxation. Moreover, inductively coupled plasma (ICP) mass spectroscopy revealed that MnCl(2) had higher tissue affinity than Gd-DTPA. As a result, multiple regions of the brain stained by the two agents exhibited different image contrasts. Our results show that differential MRM staining can be achieved using multiple MR contrast agents, revealing detailed cytoarchitecture, and may ultimately offer a window for investigating new techniques by which to understand biophysical MR relaxation mechanisms and perhaps to visualize tissue anomalies even at the molecular level.

Forebrain Ischemia-Reperfusion Simulating Cardiac Arrest in Mice Induces Edema and DNA Fragmentation in the Brain:

Brain injury affects one-third of persons who survive after heart attack, even with restoration of spontaneous circulation by cardiopulmonary resuscitation. We studied brain injury resulting from transient bilateral carotid artery occlusion (BCAO) and reperfusion by simulating heart attack and restoration of circulation, respectively, in live C57Black6 mice. This model is known to induce neuronal death in the hippocampus, striatum, and cortex. We report the appearance of edema after transient BCAO of 60 minutes and 1 day of reperfusion. Hyperintensity in diffusion-weighted magnetic resonance imaging (MRI) was detectable in the striatum, thalamus, and cortex but not in the hippocampus. To determine whether damage to the hippocampus can be detected in live animals, we infused a T2 susceptibility magnetic resonance contrast agent (superparamagnetic iron oxide nanoparticles [SPIONs]) that was linked to single-stranded deoxyribonucleic acid (DNA) complementary in sequence to c-fos messenger ribonucleic acid (SPION-cfos); we acquired in vivo T2*-weighted MRI 3 days later. SPION retention was measured as T2* (milliseconds) signal reduction or R2* value (s−1) elevation. We found that animals treated with 60-minute BCAO and 7-day reperfusion exhibited significantly less SPION retention in the hippocampus and cortex than sham-operated animals. These findings suggest that brain injury induced by cardiac arrest can be detected in live animals.

In vivo quantification of transvascular water exchange during the acute phase of permanent stroke

Mechanisms that underlie early ischemic damages to the blood‐brain‐barrier (BBB) are not well understood. This study presents a novel magnetic resonance imaging (MRI) technique using a widely available pulse sequence and a long‐circulating intravascular contrast agent to quantify water movements across the BBB at early stages of stroke progression. We characterized the integrity of the BBB by measuring the flip angle dependence of the water exchange‐affected MRI signal intensity, to generate an efficient quantitative index of vascular permeability (WEI, or water exchange index). We performed in vivo MRI experiments to measure the transvascular WEI immediately after the permanent filament occlusion of the middle cerebral artery of mice (n = 5), in which we monitored changes in blood volume (Vb), apparent diffusion coefficient (ADC), and intra‐/extravascular WEI for 4 hours. Statistically significant elevations (P < 0.05) of WEI in the ischemic tissue were observed as early as 1 hour after ischemic onset. Initial reduction of the apparent blood volume (Vapp) in the infarct cortex was followed by a continuous increase of Vapp over time. Although the measured ADC in the ipsilesional cortex continuously decreased, the abnormally high intra‐/extravascular WEI remained constant at a significantly elevated level, indicating apparent BBB injury at this early stage of stroke. Magn Reson Med 60:813–821, 2008.

Using magnetic resonance microscopy to study the growth dynamics of a glioma spheroid in collagen I: A case study

BackgroundHighly malignant gliomas are characterized by rapid growth, extensive local tissue infiltration and the resulting overall dismal clinical outcome. Gaining any additional insights into the complex interaction between this aggressive brain tumor and its microenvironment is therefore critical. Currently, the standard imaging modalities to investigate the crucial interface between tumor growth and invasion in vitro are light and confocal laser scanning microscopy. While immensely useful in cell culture, integrating these modalities with this cancers clinical imaging method of choice, i.e. MRI, is a non-trivial endeavour. However, this integration is necessary, should advanced computational modeling be able to utilize these in vitro data to eventually predict growth behaviour in vivo. We therefore argue that employing the same imaging modality for both the experimental setting and the clinical situation it represents should have significant value from a data integration perspective. In this case study, we have investigated the feasibility of using a specific form of MRI, i.e. magnetic resonance microscopy or MRM, to study the expansion dynamics of a multicellular tumor spheroid in a collagen type I gel.MethodsAn U87mEGFR human giloblastoma multicellular spheroid (MTS) containing approximately 4·103 cells was generated and pipetted into a collagen I gel. The sample was then imaged using a T2-weighted 3D spoiled gradient echo pulse sequence on a 14T MRI scanner over a period of 12 hours with a temporal resolution of 3 hours at room temperature. Standard histopathology was performed on the MRM sample, as well as on control samples.ResultsWe were able to acquire three-dimensional MR images with a spatial resolution of 24 × 24 × 24 μm3. Our MRM data successfully documented the volumetric growth dynamics of an MTS in a collagen I gel over the 12-hour period. The histopathology results confirmed cell viability in the MRM sample, yet displayed distinct patterns of cell proliferation and invasion as compared to control.ConclusionIn this study, we demonstrate that a specific form of MRI, i.e. magnetic resonance microscopy or MRM, can be used to study the dynamic growth of a multicellular tumor spheroid (MTS) with a single cell scale spatial resolution that approaches the level of light microscopy. We argue that MRM can be employed as a complementary non-invasive tool to characterize microscopic MTS expansion, and thus, together with integrative computational modeling, may allow bridging of the experimental and clinical scales more readily.

Cerebral Blood Volume Affects Blood–Brain Barrier Integrity in an Acute Transient Stroke Model

Insufficient vascular reserve after an ischemic stroke may induce biochemical cascades that subsequently deteriorate the blood–brain barrier (BBB) function. However, the direct relationship between poor cerebral blood volume (CBV) restoration and BBB disruption has not been examined in acute stroke. To quantify BBB integrity at acute stages of transient stroke, in particular for cases in which extravasation of the standard contrast agent (Gd-DTPA) is not observed, we adopted the water exchange index (WEI), a novel magnetic resonance image-derived parameter to estimate the water permeability across the BBB. The apparent diffusion coefficient (ADC) and R2 relaxation rate constant were also measured for outlining the tissue abnormality, while fractional CBV and WEI were quantified for assessing vascular alterations. The significantly decreased ADC and R2 in the ischemic cortices did not correlate with the changes in CBV or WEI. In contrast, a strong negative correlation between the ipsilesional WEI and CBV was found, in which stroke mice were clustered into two groups: (1) high WEI and low CBV and (2) normal WEI and CBV. The low CBV observed for mice with a disrupted BBB, characterized by a high WEI, indicates the importance of CBV restoration for maintaining BBB stability in acute stroke.

Dynamic monitoring of blood-brain barrier integrity using water exchange index (WEI) during mannitol and CO2 challenges in mouse brain

The integrity of the blood–brain barrier (BBB) is critical to normal brain function. Traditional techniques for the assessment of BBB disruption rely heavily on the spatiotemporal analysis of extravasating contrast agents. However, such methods based on the leakage of relatively large molecules are not suitable for the detection of subtle BBB impairment or for the performance of repeated measurements in a short time frame. Quantification of the water exchange rate constant (WER) across the BBB using strictly intravascular contrast agents could provide a much more sensitive method for the quantification of the BBB integrity. To estimate WER, we have recently devised a powerful new method using a water exchange index (WEI) biomarker and demonstrated BBB disruption in an acute stroke model. Here, we confirm that WEI is sensitive to even very subtle changes in the integrity of the BBB caused by: (i) systemic hypercapnia and (ii) low doses of a hyperosmolar solution. In addition, we have examined the sensitivity and accuracy of WEI as a biomarker of WER using computer simulation. In particular, the dependence of the WEI–WER relation on changes in vascular blood volume, T1 relaxation of cellular magnetization and transcytolemmal water exchange was explored. Simulated WEI was found to vary linearly with WER for typically encountered exchange rate constants (1–4 Hz), regardless of the blood volume. However, for very high WER (>5 Hz), WEI became progressively more insensitive to increasing WER. The incorporation of transcytolemmal water exchange, using a three‐compartment tissue model, helped to extend the linear WEI regime to slightly higher WER, but had no significant effect for most physiologically important WERs (WER < 4 Hz). Variation in cellular T1 had no effect on WEI. Using both theoretical and experimental approaches, our study validates the utility of the WEI biomarker for the monitoring of BBB integrity. Copyright

Rapid and quantitative chemical exchange saturation transfer (CEST) imaging with magnetic resonance fingerprinting (MRF)

To develop a fast magnetic resonance fingerprinting (MRF) method for quantitative chemical exchange saturation transfer (CEST) imaging.

Role of eNOS in water exchange index maintenance-MRI studies

Stroke studies employ experimental models of cerebral ischemic and reperfusion injury in rodents. MRI provides valuable supravital data of cerebral blood flow and brain tissue damage. This paper presents MRI applications for cerebral blood flow research in mice lines with impaired nitric oxide production by endothelial nitric oxide synthase. Our data demonstrates that specific modifications of MRI methodology in transgenic mouse models help to evaluate the role of eNOS in the brain-blood barrier function.