Neurophysiology Imaging Facility Core: Functional and Structural MRI

National Institute Of Mental Health

Investigators

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Abstract

Structural and functional imaging have become a central part of brain research, with scanning performed on several dozen scanners around the world. For a wide range of experiments, the 3D map of structures and/or activity patterns throughout the brain greatly aid research, including its efficiency and accuracy. The inherent connection with human MRI methods also makes many of the results obtained directly applicable and translatable to humans. The NIF facility at the NIH strives to make structural and functional imaging available and straightforward for laboratories on the NIH campus wishing to applying MRI methods. Thus one of our main goals is to lower the many conceptual and practical barriers involved in the scanning itself so that researchers can pursue combinatorial methods. Our staff assists in any of these scans, and further works to help scientists gain autonomy in conducting their own experiments, thus after a stint of research in the NIF facility many trainees know the basic principles of MRI and are able to operate the scanner. Functional MRI (fMRI) allows researchers to visualize activity patterns within the brain of an awake human or animal. This approach to neuroscience often involves mapping the responses for one type of sensory stimulus relative to that for another. There are many analytic steps between the acquisition of raw MR signals and the scientific interpretation of the measured neural signals. This is particularly true for functional MRI (fMRI), where activity maps are generated based upon the evaluation of time varying intensity values throughout the brain from a series of MR volumes. Most neuroscience researchers are not experts in the physics or engineering aspects of MRI and thus rely heavily on experts in these domains to develop and maintain the best scanning environment possible. Thus, MRI experiments are typically done in the context of a core imaging facility. In animal studies, the challenges of MRI are compounded by technical issues, such as the production of specialized radiofrequency (RF) coils and the need to obtain practical skills in surgery and animal training. Animal scanning is typically combined with other procedures such as pharmacological manipulation or simultaneous electrophysiological recording, often further complicating the imaging procedure. Overcoming these obstacles is of enormous value, since fMRI uniquely allows one to map activity over the entire brain and combine this method with other manipulations. This combination of a Siemens 3T scanner and a Bruker 4.7T scanner allow for a spectrum of different scanning possibilities for researchers at the NIH, ranging from routine anatomical scans to intricate, multimodal fMRI projects. These scanners serve all the NIH community and play an increasingly important role for biomedical and disease research. The new 3T scanner provides needed additional scanning time for our users and is serving to centralize much of the anatomical scanning for the community. It also provides a longer-term stability for the transition to replace the 4.7T vertical scanner with a cutting-edge system in the future. Six staff members including Dr. Leopold, each from a different scientific background and with different skills, aim to provide the most efficient functional scanning services possible for a broad range of investigators. Many users of the facility focus only on structural scanning, for which the staff takes over most of the procedure and the scientist provides information about the target sites and basic scanning requirements. This approach is widely used to identify electrophysiological target sites and the position of indwelling microelectrodes, and to evaluate the experimental precision of a brain manipulation such as an injection. One particularly valuable use of structural imaging is the direct comparison of electrical recording sites with foci of fMRI responses in the context of a cognitive task. There are a range of contrast options, including diffusion weighted scans that can identify features in the white matter, or provide the basis for tractography. We have also recently purchased a computerized tomography (CT) machine to reside in the facility, and to serve as part of a pipeline to further improve surgical accuracy for a wide range of users. For functional scanning, much of the work is done by scientists in individual laboratories, which the NIF staff train to become largely autonomous in their experiments. The Intramural Research Program at the NIH is one of the very few sites around the world in which monkeys can routinely participate in both fMRI and electrophysiological studies. The fMRI studies go beyond mapping functional specialization in the brain. Experiments within the facility typically combine fMRI with other procedures, such as microelectrode recordings or pharmacological inactivation, or cortical ablation. The fMRI experiments produce large data files that must be processed to evaluate the functional activity patterns across the brain. The facility provides storage of these data, guidance in the initial processing steps, and server machines for full data analysis. The NIF staff spends a relatively small fraction of its time carrying out research related to MRI itself. In the past several years, we have focused on completing studies related to diffusion tractography. We have been working in a highly collaborative effort with other groups inside and outside the NIH, we are continuing to study (1) the neuroanatomical basis of diffusion imaging, and (2) comparative fiber pathways across species. In addition, we have recently completed studies on the role of the basal forebrain in resting state spontaneous fMRI signals, as well as collaborative work involved in atlases, templates, and data sharing. At present, research in the facility is focused on the design and testing of implanted radiofrequency coils, with the hope that this method can become routine for users seeking to obtain higher signal-to-noise images. Other research lines in the facility involve the development of scanning with newly available contrast agents.