[Field and temperature dependence of contrast in magnetic resonance imaging]

Whereas a change of temperature within the physiological range does not lead to significant contrast alterations in MRI, changing magnetic field strength can lead to drastic alterations in tissue contrast caused by incalculable changes of T1 relaxation. At medium and high fields, longitudinal relaxation is mainly influenced by water content of tissues, at low and ultra-low fields macromolecular structures gain additional influence. Combination of relaxometric measurements with a field cycling device (between 0.0002 and 1.2 T, i.e. 10 kHz and 50 MHz) and imaging experiments at ultra-low, low, and medium fields (between 0.02 and 0.6 T) showed field depending inherent contrast changes with a T1 contrast maximum at medium fields. Induced contrast alterations by application of contrast changing substances in medium and high fields are unpredictable because there is no simple interrelationship between field strength and T1. Furthermore, it could be shown that T1 of pathologically altered tissue is not higher than T1 of normal tissue in any case but can be lower or equal below a certain field strength what contradicts the hypothesis that pathological tissue has a higher T1 than normal tissue.