Unconventional MRI scanner technology and intelligent dynamics

Medical imaging provides valuable information about the internal structure and function of the body. In magnetic resonance imaging (MRI), a scanner device communicates with atomic nuclei in the tissues via magnetic fields to obtain data suitable for image reconstruction. Another imaging technology, magnetoencephalography (MEG), detects electrical brain activity via the weak magnetic fields it generates. In this Thesis, unconventional MRI theory, methods, instrumentation and software were developed for applications that have requirements such as compatibility with other technology, portability, silent operation, open geometry, high spatial accuracy or low cost. In particular, a full-scale human brain scanner was designed and constructed that is capable of both MEG and MRI. MRI theory is expanded to allow spatial and temporal variations in the magnetic field direction. This enables, for instance, precise analysis and flexible use of an MRI approach where the main magnetic field is separated into two parts: a polarizing field and a readout field. Such MRI relaxes constraints related to field strength and homogeneity. The use of a lower readout field can be combined with detection by highly sensitive superconducting quantum interference devices (SQUIDs), also used in MEG. Arrays of different types of SQUID sensors are studied regarding their MRI performance as well as their suitability for hybrid MEG-MRI. Superconductivity – the quantum phenomenon where a material completely loses its electrical resistivity below a critical temperature – is used not only in the SQUID sensors but also in pulsed polarizing magnets. Besides the theory of supercurrents, also current flow in normal conducting structures is considered, and the results are used for field coil designs and to analyze thermal magnetic noise as well as eddy currents induced by MRI magnetic fields. This is further applied to the design and construction of a magnetically shielded room and of low-noise thermal superinsulation. Low noise is also important in the developed MRI electronics, such as an ultra-low-noise amplifier for the readout field and gradients. A method is introduced for achieving high spatial accuracy by exploiting the multichannel acquisition and highly accurate signal model at low fields. Another method, zero-field-encoded current density imaging, allows the full 3D measurement of electric currents in volume using MRI. Rotary scanning acquisition significantly simplifies the hardware requirements for MRI. Finally, a versatile approach for solving problems is introduced: Dynamical coupling for additional dimensions (DynaCAN), or pulse-waveform coupling, uses specifically designed pulse waveforms to couple with the dynamics of a target system such as a device component. Several different use cases are described, with a detailed experimental demonstation in the case of suppressing eddy currents. Initial results from simulations also suggest that the approach could be used for neuromodulation or brain stimulation.