A fast field-cycling MRI relaxometer for physical contrasts design and pre-clinical studies in small animals

We present a fast field-cycling NMR relaxometer with added magnetic resonance imaging capabilities. The instrument operates at a maximum proton Larmor frequency of 5 MHz for a sample volume of 35 mL. The magnetic field homogeneity across the sample is 1400 ppm. The main field is generated with a notch-coil electromagnet of own design, fed with a current whose stability is 220 ppm. We show that images of reasonable quality can still be produced under such conditions. The machine is being designed for concept testing of the involved instrumentation and specific contrast agents aimed for field-cycling magnetic resonance imaging applications. The general performance of the prototype was tested through localized relaxometry experiments, T₁-dispersion weighted images, temperature maps and T₁-weighted images at different magnetic field intensities. We introduce the concept of positive and negative contrast depending on the use of pre-polarized or non-polarized sequences. The system is being improved for pre-clinical studies in small animals.