Recently introduced fMRI techniques provide the ability to visualize elevated neuronal activity with high spatiotemporal specificity and resolution. Such a capability is essential for a system-level understanding of human brain function, which possesses unique attributes and can not be investigated by the invasive techniques available for animal model studies. However, fMRI signals can not all be quantitatively related to physiological parameters in a straightforward manner. Understanding the origin and limitations of signal changes detected by fMRI techniques is critical for their full utilization. The main emphasis of investigations in our lab relate to the following three critical issues in fMRI.
- Physiological basis of fMRI. The blood oxygenation level dependent (BOLD) effect is complex and depends on alterations in cerebral metabolic rate of oxygen, cerebral blood flow (CBF), and cerebral blood volume (CBV) in response to increased neuronal activity. Contribution of these metabolic and hemodynamic parameters to BOLD is expected to depend on vascular dimensions and geometry as well as experimental parameters such as static magnetic field and spatial resolution. Our understanding of these relationships remains largely qualitative, derives from modeling efforts, and requires additional experimental evaluation. Thus, it is critically important to study the spatiodynamics of vascular and metabolic changes which occur during neural stimulation in well-established animal models. We investigate the physiological sources of fMRI signals by augmenting BOLD studies with other methodologies including quantification of CBF, CBV, and arterial blood volume by MRI, measurements of oxygen tension and electrophysiology, and optical imaging.
- Spatial specificity of fMRI. Most fMRI studies have been performed by BOLD measurements with a spatial resolution of several millimeters to one centimeter. For further in-depth investigation of cortical information processing, the ability to map basic functional units is crucial. Neurons with common functional properties are often clustered into columns, which span the entire cortical plate. Individual functional columns in mammals are ~0.5 mm wide, and iso-functional columns often repeat about every millimeter. However, the spatial specificity of the conventional positive BOLD signal is limited by its point spread function, which extends about 2-3 millimeters beyond the neuronally active site. Therefore, the columnar structure responding to a single stimulus condition cannot be easily mapped by the conventional BOLD technique. We evaluate the feasibility of several different fMRI methods for resolving cortical columns; these studies include early negative BOLD responses (presumably induced by the early oxygen consumption increase), spin-echo BOLD, CBF, and CBV.
- Temporal resolution of fMRI. Since fMRI images can be acquired rapidly, very high temporal resolution can, in principle, be achieved. Temporal resolution is defined as the peak-to-peak interval between temporally distinguishable task-induced fMRI responses. However, the temporal resolution of fMRI is limited by an intrinsic hemodynamic response time constant(s) which may show inter-region and inter-subject variability, and a finite sensitivity. The assumption is that the relationship between neural activity and its fMRI response is linear. In a linear system, the response to an input function (e.g., a behavioral function, or more precisely neural activity) is additive and invariant with time. Two important consequences of linearity are that the functional MRI response can be predicted from the neural activity by convolution of a unit fMRI response function with a time-dependent stimulation parameter and, conversely, that the dynamic neural activity can be inferred from the dynamic fMRI data. This linearity assumption is examined by systematic measurements of field potentials, CBF responses, and BOLD fMRI signal changes.
Since the available methods to non-invasively investigate brain function rely on vascular or metabolic markers, it is extremely important to understand their relationship to neural activity. While NMR is particularly sensitive to vascular and metabolic markers but not neural activity directly, other methods are better suited to investigate the link between neural activity and their associated vascular and metabolic modulations. Recent advances in optical microscopy and fluorescent probe development, along with their relative ease to combine with traditional methods like electrophysiology, have placed these methods in the forefront for this purpose. Our laboratory focuses on optical imaging of intrinsic signal, laser speckle imaging, fluorescence microscopy, two-photon microscopy, oxygen microelectrodes and electrophysiology to investigate neuro-vascular and neuro-metabolic coupling.
- Neurovascular coupling. Two-photon microscopy is able to selectively measure light from under the brain surface with high spatial resolution (sub-micrometer resolution in-plane and micrometer resolution through-plane) and temporal resolution (sub-second) by exploiting the light excitation properties of fluorescent molecules. A fluorescent dye administered intra-venously, such as fluorescein or rhodamine-labeled dextran, can be used to image the cerebral vasculature, including the flow of red blood cells in individual capillaries as deep as 600 µm below the cortical surface. More importantly, one of the most significant technological advancements in brain imaging has been the use of calcium indicators (e.g. Oregon green BAPTA-1 acetoxymethyl ester or OGB1-AM) to image the function of a discrete population of neurons. In living cells, most depolarizing electrical signals (including neural activity) are associated with a calcium influx by the activation of voltage-gated calcium channels. In addition, calcium signals are essential in neuronal communication since they are involved in synaptic transmission. Calcium indicators are molecules that are able to bind calcium ions and, as a result, change their fluorescent properties. Our laboratory uses these two methods, intra-vascular dyes and calcium indicators, to investigate the relationship between neural activity and vascular function.
- Neurometabolic coupling. One of the traditional methods to measure tissue metabolism has been the imaging of intrinsic tissue auto-fluorescence. Much of the intrinsic fluorescence of living tissues stems from the reduced form of the coenzyme nicotinamide adenine dinucleotide (NADH) and the oxidized form of flavin adenine dinucleotide (FAD). Other cofactors in the same family also contribute to the intrinsic fluorescence of tissue, most notably its dinucleotide phosphate (NADPH) and their analogs, most of which also participate in metabolism. These proteins are directly involved in the TCA cycle as proton carriers for the electron transport chain where NADH and FADH2 are oxidized to NAD and FAD, respectively. While NADH is fluorescent, NAD+ is not, and the metabolic rate could be assessed by decreases in the fluorescence of NADH. Different from NADH, its analog FADH2 is not fluorescent, but its oxidized form (FAD, also called flavoprotein) is fluorescent; hence, increases in the metabolic rate can be assessed by increases in the fluorescence of FAD. Our laboratory has implemented flavoprotein autofluorescence imaging (FAI) and combined it with electrophysiology to investigate the relationship between oxidative metabolism and neural activity.
In vivo imaging of Neurovascular and Neurodegenerative Diseases
Neuroimaging methods are used to investigate neurovascular and neurodegenerative diseases such as stroke, hypertension, Alzheimer’s disease, Parkinson’s disease, glaucoma, etc. Our major interests are to examine early imaging markers, and to investigate in vivo anatomical, metabolic, physiological, and functional changes.
Since its discovery, the nuclear magnetic resonance (NMR) phenomenon has been utilized to extract an unprecedented level of chemical and biological information. Magnetic resonance imaging (MRI) has had a major impact in the clinical and research arenas, providing anatomical and pathological data, which are especially useful for disease diagnosis. Recent developments for the study of intact biological systems include functional MRI (fMRI) and physiological MRI methodologies. The primary focus of our research lab is to develop and understand the in vivo NMR techniques which provide information on function, physiology, and anatomy.
- Non-invasive Blood Flow and Blood Volume Measurement without contrast agent. The adult human brain is approximately 2% of the total body’s weight, but receives nearly 15% of the total cardiac output. The rate at which blood moves through the brain is characterized by cerebral blood flow (CBF), whereas the total capacity of blood is defined as cerebral blood volume (CBV). In addition, the brain vasculature can be divided into arterial and venous blood components. Arterial vessels including arteries, arterioles and pre-capillary small arterioles actively dilate and constrict in response to internal and external perturbations. In contrast, venous vessels, which include veins, venules and post-capillary small venules, respond passively. Vascular volume changes in the brain are important for regulation of blood flow under conditions of both normal and abnormal physiology. It is generally thought that dilation and constriction of arterial blood vessels, in response to perturbations caused by changes in CO2 and neural stimulation, is the major mechanism that maintains CBF within an auto-regulatory range. Thus, arterial CBV change is expected to be more sensitive than total CBV change in assessing cerebro-vascular regulation, as well as in identifying regions of abnormality. We have developed various non-invasive arterial CBV MR measurement techniques over last ten years, including arterial spin labeling with magnetization transfer effects or bipolar diffusion gradients, and BOLD fMRI with varied magnetization transfer effects. These approaches have been used to examine baseline CBF vs. arterial CBV, and functional changes in arterial vs. venous CBV.
- Chemical Exchange-Sensitive MRI. The chemical exchange (CE) effect has recently been exploited as a powerful method that is highly sensitive to detecting biological chemicals present in low concentrations. Measurement of the CE effect is possible due to the exchange of protons between specific chemicals and water, which results in a change in the detectable water signal. The detection of endogeous biochemical agents, such as metabolites, peptides and proteins, by CE-derived image contrast is particularly attractive because it can be used to probe the tissue microenvironment, which includes tissue pH, temperature and concentration of agents with exchangeable protons. In our laboratory, we have developed novel CE MRI approaches that can be specifically tuned to amide, amine and hydroxyl protons in proteins, peptides and metabolites, and is currently being applied to detect changes in the tissue microenvironment present in disease models. In this way, the development of CE MRI is important for early detection of disease and to determine the effectiveness and progress of treatments.