ABSTRACT
Few experimental techniques can claim an evolutionary period as long as magnetic resonance (MR). Discovered over 50 years ago [1,2], MR is still œnding new applications. šis period has been characterized by a steady trend of increasing MR sensitivity. še signal in MR depends on the product of the number of spins and the fractional nuclear spin polarization, which is only 10-6-10-5 for typical high-œeld (~1.5 T) equilibria. šis severely limits the use of MR in biomedicine, especially for nuclei with low gyromagnetic ratios (γ) such as carbon-13 (13C) and nitrogen-15 (15N). Nuclear spin polarization scales linearly with magnetic œeld, and this relationship between magnetic œeld strength and sensitivity has in turn perpetuated the development and widespread availability of human and preclinical MR systems up to 7 and 12 T, respectively. From standard clinical 1.5-T human instruments to the currently available 7-T systems, nuclear spin polarization has increased by 4.7-fold. While other factors (radiofrequency (RF) coil sensitivity, T1/T2 relaxation, and pulse sequences) contribute to the overall MR sensitivity, the lone impact of the nuclear spin polarization theoretically decreases acquisition time by 4.72 or 21.8-fold to achieve the same signal-to-noise ratio (SNR) at 7 T compared with that at 1.5 T. šese advances have enabled the penetration of new layers of diagnosis and have driven the emergence of new applications.
