There continues to be increasing interest in developing molecular imaging approaches that enable traditional radiological imaging techniques to obtain a wide range of information about molecular and cellular processes that occur in normal and diseased tissue. A range of information is considered important such as the ability to monitor cell migration, the development of reporters that enable imaging of gene expression, the development of robust strategies to image receptors, and the development of environmentally sensitive agents that can be used to detect the presence of specific enzymes or monitor changes in ion status. The long term goals of this work are to develop strategies that enable MRI contrast that is sensitive to a wide range of molecular and cellular processes. This work builds on over 20 years of work where we have demonstrated the first MRI strategy for detecting gene expression, the first MRI approach for monitoring a surrogate of calcium influx, the first MRI approach for performing neuronal track tracing of newly born neurons, and the first MRI approach for monitoring the migration of single cells in vivo. These all represented initial reports by any radiological imaging technique which enabled these processes to be measured. These techniques are finding widespread application to imaging pre-clinical models of a broad range of diseases. Over the past year we have made progress in all of the specific aims.
Aim 1 : Develop iron oxide based contrast for labeling and imaging the migration of endogenous neural stem cells. Over the past few years we have demonstrated the unique advantages of micron sized iron oxide particles for MRI of specific cells. Single cells can be detected and indeed, single particles within single cells can be detected. The main paradigm for MRI of cell migration is to label cells ex vivo and monitor migration after transplantation into an animal. The ability to detect a single particle enables inefficient labeling strategies. In particular, over the past few years we have demonstrated that injection of particles into the ventricles of the rat brain enables particles to be taken up by neural precursors in the subventricular zone and MRI can monitor the migration of cells to the olfactory bulb. This year a publication was completed a demonstrated the effect of odor deprivation by naris occlusion on migration of new neurons into the olfactory bulb. There was a large decrease in the rate and number of cells. The number of cells migrating into the bulb decreased in proportion to the volume of the bulb indicating exquisite coupling between bulb anatomy and function. Removal of the occlusion leads to bulb growth and an increase in new neurons back to normal. Blocking new neurons prevents the bulb growth indicating new neurons are essential for the regrowth after naris occlusion. In the coming year we are determining the effects of exposure to a specific odor after naris occlusion on the sites of migration of these new neurons. A second major project images the entire brain to study immune brain interactions. We have attempted to label and study T cells and microglia both of which have challenges. over the next year we will look at other monocytes in particular populations that enter the brain and differentiate to macrophages as well as begin to determine if the flow pattern of macrophages in blood vessels can be altered by inflammation.
Aim 2 : Apply microfabrication techniques to manufacture unique metal structures that may be valuable for MRI contrast. Iron oxide particles commonly used for MRI are very potent contrast agents enabling detection of single micron sized particles. However, due to bulk phase manufacture of particles they are not very uniform and they do not contain very high content of metal. A solution to this problem is to use modern microfabrication techniques to manufacture metal based, micron sized contrast agents. Over the past few years we have shown that double doughnut, cylinders, and ellipsoid structures offer unique advantages for distinguishing particles and that these structures can be turned into sensors for pH. Over the past year we have continued to demonstrate the sensor properties with an aim to detecting force. We have also begun to use routinely simple microfabricated gold coated iron discs for cell tracing since they are eight times better than classical iron oxide particles.
Aim 3 : Develop novel delivery mechanisms to extend the applicability of manganese enhanced MRI. Over the past ten years we have demonstrated the remarkable utility of the manganese ion for MRI contrast. Manganese ion enters cells on ligand or voltage gated calcium channels and so can be used as an MRI agent to monitor calcium influx. Once inside of neurons, manganese will move in an anterograde direction and cross functional synapses enabling neuronal networks to be imaged with MRI. Finally, manganese given systemically gives cytoarchitectural information about the rodent brain. These successes have us interested in broadening the ways in which manganese ion can be delivered to cells. A major limitation of manganese enhanced MRI are the concentrations required. Over the past year we have completed an initial study that is submitted for publication demonstrating that manganese positron emitting isotopes will enable PET to obtain similar information that can be obtained with manganese enhanced MRI, including neural tracing and functional activation of tissue. We continue our collaboration with the human imaging groups s to test if an FDA approved agent that releases Mn might be useful for disease detection. Finally, in collaboration with Dorian McGavern we are pursuing novel approaches to add Mn2+ to the brain without disruption of the skull using the intricate vessel/marrow system of the skull.
Aim 4 : Develop strategies that enable cellular processes to alter the relaxivity of MRI contrast agents.
In specific aim 3 we demonstrated a way in which a normal biological process (endocytosis of transferrin-Mn or MnO particles) can alter the effectiveness of an MRI contrast agent. It would be very exciting to find ways in which this can occur which are sensitive to other biological processes. To this end we have begun to explore ways in which the microfabricated particles produced under Aim 2 can be modulated. Over the past few years we have completed a study that demonstrates that the microfabricated particles can be made into a pH sensor. This was accomplished by embedding a pH sensitive gel between the discs in our double disc microfabricated structure. Shrinking an s swelling of the gel changes the disc spacing which in turn leads to a large change in MRI properties. The strategy used is generalizable to sense many other processes and we will extend this over the coming year. We have completed publication of a study understanding the relaxivity of cerebral spinal fluid. The results indicate that glucose is a major determinant of CSF T2 relaxivity and that changes in glucose in CSF can be monitored without contrast agents in the brain. In the coming year we will explore the potential usefulness of this finding by seeking collaborations with our human imaging colleagues. A novel wireless MRI detector that shows much promise for use where it may be possible to implant an MRI detector ahs been developed. Experiments have demonstrated increased sensitivity and the ability to detect individual functional units of the kidney. We have completed a publication showing a new transistor based circuit that has advantages over our initial design. We have constructed a coil for high resolution imaging of the pituitary. This is being integrated into neurosurgical applications for identifying pituitary tumor with very high resolution MRI and if useful we will make this coil wireles
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