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"Based on power requirement at 1.5 and 4.0 Tesla, it had been expected that at least 1-2 Joules of energy would be required to produce maximal spin excitation at 8 Telsa. This is because current NMR theory predicts that the power required for spin excitation varies quadratically with field strength. This was, in a sense, the NMR equivalent of the ‘ultraviolet catastrophe’ (power µn2) first derived by Rayleigh and Jeans. The anticipated power requirements for spin excitation at ultra-high fields has been a major hurdle to the MRI community, since in this case, it is obvious that the sample, or more specifically the patient, cannot be arbitrarily heated. This consideration had provided ample ammunition to those in the scientific community who looked with suspicion on ultra-high field MRI...
...It is also interesting that the absorptive behavior at 8 Tesla cannot be reconciled with current NMR theory, which had predicted a quadratic increase in power. Nonetheless, this behavior has also been observed in our studies of mineral oil at 8 Telsa and our studies of water on high resolution liquid NMR spectrometers. As such, it is clear, in liquid samples and in the human head, that NMR does not behave in a Rayleigh-Jean like catastrophic manner (power µn2). Indeed, under these conditions, power in NMR follows a thermal nature which is more Planck like, whereby power increases initially with the square of the frequency. However, once a certain characteristic frequency is achieved, power starts to decrease."
P.-M. L. Robitaille et. al. NMR Biomedicine 11, 263-265 (1998)