‡ These authors contributed equally.Long-lived (symmetry protected) hyperpolarized spin states offer important new opportunities (for example, in clinical MR imaging), but existing methods for producing these states are limited by either excess energy dissipation or high sensitivity to inhomogeneities. We extend recent work on continuouswave irradiation of nearly-equivalent spins (spin-lock induced crossing) by designing composite pulse and adiabatic shaped-pulse excitations which overcome the limitations. These composite and adiabatic pulses differ drastically from the traditional solutions in two-level systems. We also show this works in chemically equivalent spin pairs, which has the advantage of allowing for polarization transfer from and to remote spins. The approach is broadly applicable to systems where varying excitation strength induces an avoided crossing to a dark state, and thus to many other spectroscopic regimes.Hyperpolarization methods produce nuclear magnetization many orders of magnitude larger than what is available at thermal equilibrium, and are particularly promising in clinical and preclinical applications of magnetic resonance imaging [1][2][3]. However, a fundamental challenge is the nuclear spin-lattice relaxation time T 1 , which typically is too short in solution or tissue (tens of seconds for carbon-13) to monitor many meaningful biological processes. For this reason, symmetry-protected nuclear spin states (such as the singlet, which is a "dark state" with no dipole allowed transitions) have drawn considerable attention [4][5][6][7][8][9][10][11][12][13][14][15][16][17].The first demonstrations [6,7] used inequivalent spins to convert population from the normally accessible triplet stateinto the singlet, then strong spin locking or translation to a low field to preserve the singlet state. More recent work has shown that chemically equivalent [4,[18][19][20] [8,10,19,20]. Specifically, the so-called "M2S" sequence, consisting of precisely spaced π pulses, can interconvert magnetization and singlet-state polarization. It has become clear that multiple families of biologically compatible molecules exist that can bear protected singlet states with lifetimes of many minutes to hours, giving this approach transformative potential.However, serious obstacles remain to using such reagents in MRI. The most important challenge is that in clinical applications, allowable energy deposition is limited. Recently, DeVience et. al. [21,22] introduced a new approach for pumping singlet states in nearly-equivalent spins, called spin-lock induced crossing (SLIC), which drastically decreases power dissipation but is not robust to the inevitable rf or static field inhomogeneities in MRI. Here we extend their approach to the equivalent-spin case, and create an energy-efficient and robust method for population transfer using novel composite and shaped pulses. Composite pulses have been used for decades to improve robustness in magnetic resonance [23,24] and laser [25,26] applications, as have shaped pulses;...

In this paper we elucidate, theoretically and experimentally, molecular motifs which permit Long-Lived Polarization Protected by Symmetry (LOLIPOPS). The basic assembly principle starts from a pair of chemically equivalent nuclei supporting a long-lived singlet state and is completed by coupling to additional pairs of spins. LOLIPOPS can be created in various sizes; here we review four-spin systems, introduce a group theory analysis of six-spin systems, and explore eight-spin systems by simulation. The focus is on AA'XnX'n spin systems, where typically the A spins are (15)N or (13)C and X spins are protons. We describe the symmetry of the accessed states, we detail the pulse sequences used to access these states, we quantify the fraction of polarization that can be stored as LOLIPOPS, we elucidate how to access the protected states from A or from X polarization and we examine the behavior of these spin systems upon introduction of a small chemical shift difference.