New magnetic resonance techniques for in vivo free radical detection
Naturally occurring free radicals are involved in normal metabolism and in a number of conditions such as myocardial ischaemia, inflammation, ageing and cancer. Stable free radicals can be introduced into animals and used as "tracers" to study renal function, integrity of the blood-brain barrier, subcutaneous neoplasms, extra vascular viscosity in tumour tissues, oxygen concentration in cells and tissues. They have also been studied as contrast enhancing systems in Magnetic Resonance Imaging (MRI). At present, two closely-related magnetic resonance techniques exist which are capable of imaging free radicals, namely Proton-Electron Double-Resonance Imaging (PEDRI) and Electron Paramagnetic Resonance Imaging (EPRI). These techniques are at the beginning of their development and a substantial improvement of their performances is crucial for future applications. The aim of this thesis was to investigate new methods for improving PEDRI and EPRI techniques at radio frequencies. In summary, the main topics can be grouped as follows: (i) A pulsed dynamic nuclear polarisation (DNP) technique at 10 mT was developed and a theoretical model for calculation of the DNP enhancement with a train of EPR pulses was proposed. It was shown experimentally that the use of pulsed DNP techniques at 10 mT can increase the enhancement and/or decrease the required average applied RF power to samples containing free radicals in solution. (ii) A simultaneous multiple CW EPR irradiation technique for PEDRI at 10 mT was introduced. A novel EPR resonator and a PEDRI apparatus suitable for multiple-frequency EPR irradiation experiments were designed and tested. It was shown that simultaneous multiple CW EPR irradiation increases the PEDRI sensitivity (up to a factor of three with nitroxides in solution). (iii) Novel radio frequency pulsed EPR instrumentation was developed. This apparatus has a submicrosecond dead time and was developed for fast direct detection of free radicals in large samples. However, its field of applicability is very broad and, for example, it can be used for pulsed NMR imaging of solids.