Combined ultra-low-field MRI and MEG: instrumentation and applications

Magnetic resonance imaging (MRI) is a noninvasive method that allows the study of the interior structure of matter. Today, MRI is widely used in medical diagnosis and research, thanks to its versatile contrast and the lack of ionizing radiation. Conventionally, the signal-to-noise ratio of an MRI measurement scales with the strength of the applied magnetic field. This has driven the development of MRI scanners towards fields of 3 T and above. Ultra-low-field (ULF) MRI is an emerging technology that uses microtesla-range magnetic fields for image formation. The low signal-to-noise ratio is partly compensated for by prepolarizing the sample in a field of 1 – 200 mT and using superconducting quantum interference devices (SQUIDs) for signal detection. Advantages of ULF MRI include unique low-field contrast mechanisms, flexibility in the sequence design, and the possibility to construct a silent scanner with an open geometry. ULF MRI is also compatible with magnetoencephalography (MEG), which uses SQUIDs to record the magnetic field produced by neuronal activity. With a hybrid scanner combining MEG and MRI, both the structure and function of the human brain can be studied with a single device. In this Thesis, a hybrid MEG-MRI device was designed, constructed, and tested. The system is based on a commercial whole-head MEG device that was modified to accommodate ULF-MRI functionality. In particular, the effects of the various magnetic fields applied inside a magnetically shielded room were studied. To prevent the harmful effects of the eddy currents caused by changing magnetic fields, a self-shielded polarizing coil was designed and constructed. Moreover, the conventional SQUID design was modified in order to develop sensor modules that tolerate the relatively strong polarizing field. Finally, the device was used to measure MEG data and ULF-MR images of the human brain. In addition to the instrumentation development, several applications of ULF MRI were investigated. A method for imaging electric current density was presented. The technique takes advantage of the flexibility of ULF MRI by encoding the signal in zero magnetic field. Furthermore, the temperature dependence of the MRI relaxation times was studied. Drastic variations were found as a function of the field strength. The results were used to reconstruct temperature maps using ULF MRI. The results presented in this Thesis demonstrate that upgrading MRI functionality into an existing commercial MEG device is a feasible concept. Such a device has the potential to enable new methods and paradigms for neuroscientific research. The possibility of taking advantage of the unique low-field contrast is an interesting subject for further research.