Ultra-low-field MRI: techniques and instrumentation for hybrid MEG-MRI

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Jaakko Nieminen Name of the doctoral dissertation Ultra-low-field MRI: techniques and instrumentation for hybrid MEG-MRI Publisher School of Science Unit Department of Biomedical Engineering and Computational Science Series Aalto University publication series DOCTORAL DISSERTATIONS 71/2012 Field of research Engineering Physics, Biomedical Engineering Manuscript submitted 28 February 2012 Manuscript revised 2 May 2012 Date of the defence 8 June 2012 Language English Monograph Article dissertation (summary + original articles) Abstract Magnetic resonance imaging (MRI) is a medical imaging modality that can noninvasively produce images of the human body with excellent soft-tissue contrast. Conventionally, MRI is performed in magnetic fields above 1 T. On the other hand, magnetoencephalography (MEG) is a tool for functional brain imaging. In modern MEG, an array of highly sensitive superconducting quantum interference device (SQUID) sensors is used to measure the weak magnetic field around the head produced by neuronal activity in the brain. It has been demonstrated that SQUID sensors can be used to measure also MR signals, if the amplitudes of the MRI fields are reduced and a prepolarization approach is applied. Ultra-low-field (ULF) MRI refers to MRI with signal detection in fields around 100 μT, or even lower, and typically utilizes SQUID sensors for signal readout. The use of the same sensors in MEG and MRI offers significant benefits and allows us to develop a single device capable of both MEG and MRI. This Thesis introduces several techniques for ULF MRI and its combination with MEG. It is shown that the origin of MR signals can be encoded by preparing the sample consecutively with spatially different polarizing fields. It is also demonstrated that by carefully designing the polarizing-field time course, contrast in ULF MRI can be improved. This Thesis provides also a general method to reconstruct images, when the magnetic fields within the imaging region are nonlinear. In addition, this Thesis describes how to design self-shielded polarizing coils with weak stray fields. Finally, a device for hybrid MEG-MRI was developed based on a commercial whole-head MEG system. The developed polarization-encoding method may ultimately enable MRI without phase encoding and become essential when developing new kinds of magnetic imaging. The contrast enhancement achieved with time-dependent polarizing fields may be useful when ULF MRI is applied for new purposes. Because in ULF MRI the encoding gradients are relatively strong, conventional reconstruction methods produce image artifacts, whereas the developed general reconstruction method performs much better. The self-shielded polarizing coils are essential when ULF MRI is performed inside magnetically shielded rooms, since otherwise strong eddy currents may be induced in the conductive shielding layers. The developed instrumentation for hybrid MEG-MRI has been successfully used for brain imaging and establishes a solid basis for future research.Magnetic resonance imaging (MRI) is a medical imaging modality that can noninvasively produce images of the human body with excellent soft-tissue contrast. Conventionally, MRI is performed in magnetic fields above 1 T. On the other hand, magnetoencephalography (MEG) is a tool for functional brain imaging. In modern MEG, an array of highly sensitive superconducting quantum interference device (SQUID) sensors is used to measure the weak magnetic field around the head produced by neuronal activity in the brain. It has been demonstrated that SQUID sensors can be used to measure also MR signals, if the amplitudes of the MRI fields are reduced and a prepolarization approach is applied. Ultra-low-field (ULF) MRI refers to MRI with signal detection in fields around 100 μT, or even lower, and typically utilizes SQUID sensors for signal readout. The use of the same sensors in MEG and MRI offers significant benefits and allows us to develop a single device capable of both MEG and MRI. This Thesis introduces several techniques for ULF MRI and its combination with MEG. It is shown that the origin of MR signals can be encoded by preparing the sample consecutively with spatially different polarizing fields. It is also demonstrated that by carefully designing the polarizing-field time course, contrast in ULF MRI can be improved. This Thesis provides also a general method to reconstruct images, when the magnetic fields within the imaging region are nonlinear. In addition, this Thesis describes how to design self-shielded polarizing coils with weak stray fields. Finally, a device for hybrid MEG-MRI was developed based on a commercial whole-head MEG system. The developed polarization-encoding method may ultimately enable MRI without phase encoding and become essential when developing new kinds of magnetic imaging. The contrast enhancement achieved with time-dependent polarizing fields may be useful when ULF MRI is applied for new purposes. Because in ULF MRI the encoding gradients are relatively strong, conventional reconstruction methods produce image artifacts, whereas the developed general reconstruction method performs much better. The self-shielded polarizing coils are essential when ULF MRI is performed inside magnetically shielded rooms, since otherwise strong eddy currents may be induced in the conductive shielding layers. The developed instrumentation for hybrid MEG-MRI has been successfully used for brain imaging and establishes a solid basis for future research. 1