Magnetic Resonance Imaging Technology Development

National Institute Of Neurological Disorders And Stroke

Investigators

Linked publications & trials

Abstract

During reporting year 2010-2011, the magnet for a new ultra-high field human MRI scanner, operating at 11.7T, was delivered. This is the strongest MRI magnet currently available. This magnet is currently prepared for activation (ramping of the magnetic field) and integrated with electronics to become a functional MRI system for studying the brain. Scientists in the AMRI section and in the core section of LFMI are designing and developing part of the radio-frequency electronics for signal generation and reception. In addition, IRB and FDA approval is being requested to perform brain imaging studies of human subjects. When completed, the 11.7T MRI system is expected to generate images of brain anatomy and function with the highest rsolution available. In addition novel MRI applications are expected due to the altered contrast at this field strength. One of the technical difficulties in performing MRI at fields as high as 11.7T is the fact that the RF fields used for MRI become inhomogeneous due to wavelength effects. These effects can be quite strong as the 500MHz RF frequency used at 11.7T has a wavelength of about 8cm. This means that both electrical and magnetic components of the RF field become dependent on the position in the head, leading to variations in sensitivity, contrast, and local tissue heating. The latter may severely restrict the range of applications available to study the brain. The spatial variation in the RF field can to some extent be mitigated by sophisticated design of the RF antenna (i.e transmit and receive coils), combined with dedicated MRI pulse sequences and post-processing methods. AMRI has been working on all these aspects, and has modeled the severity of wavelength effects, and has designed and built improved RF coils. In addition, in early 2011, a new 7T system has been delivered to the Functional MRI Facility at NIH and became in March of that year. AMRI has had significant involvement in the planning and development of this system, and has developed two new technologies to control variations in RF amplitude that may be detrimental to high field MRI. The first technology is based on an improved method to measure B1 distributions across the brain, and the second technology concerns the mitigation of B1 variations and the resulting effects on sensitivity and contrast. The latter technology is under review for patenting. Both methods are expected to facilate the use of high field MRI for the study of the human brain. Finally, methods were developed that allow improved imaging of the brain's structure based on magnetic susceptibility contrast. The first method improved the study of white matte fiber orientation by exploiting the orientation dependence of T2* relaxation. This was evaluated in brain samples, which allow easy rotation in the magnetic field. A theory was developed to model the angular dependence of T2* relaxation and was evaluated by fitting to experimental data. The results suggested that the angular dependence contains contributions from anisotropic susceptibility, and including this contribution allows accurate description of the angular dependence. The second improves the detection of resonance frequency shifts originating from susceptibility variations and makes it possible to observe some of the subtle anatomical that has been seen at 7T at the lower field of 3T. This method is based on a rapid MRI scan technique known as steady-state free precession (SSFP), which is very sensitive to subtle variations in frequency shift. It was demonstrated that in limited areas of the brain, contrast-to-noise ration gaind of 2-3 can be obtained with this technique as compare with conventional susceptibility-weighted scan techniques.