Composite and shaped pulses for efficient and robust pumping of disconnected eigenstates in magnetic resonance

Abstract

‡ 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;...

Long-lived nuclear spin states in rapidly rotating CH 2 D groups

“…The small chemical shift difference allows access to the long-lived singlet order of the CH 2 D protons, by using radiofrequency pulse techniques which operate in the nearequivalence regime [33][34][35][36]. In the current study, we used the SLIC (Spin-Lock Induced Crossing) method [35], as shown in Figure 2.…”

Singlet NMR methodology in two-spin-1/2 systems

“…In those cases, however, the coupling between singlet and triplet state is weak and therefore an ad-hoc methodology is needed to manipulate singlet order. The two methods proposed, magnetisation-to-singlet conversion (M2S) [37,38] and spin-lock induced crossing (SLIC) [40,41], are discussed below.…”

“…Three techniques have been proposed for manipulating singlet order in this regime: (i) magnetisation-to-singlet conversion (M2S) [37,38] (ii) its reverse, singlet-to-magnetisation (S2M); and (iii) spinlock induced crossing (SLC) [40,41], which works in both directions. The M2S and S2M pulse sequences can transform longitudinal into singlet order (and vice versa), as shown in Fig.…”

“…Spin-locking induced crossing (SLIC) [40,41] is an alternative method for manipulating singlet order in near magnetic equivalence regimes. The method converts transverse spin order into pSO (and vice versa) and consists in applying a radiofrequency pulse with a nutation frequency that is matched to ω J .…”

“…Since its discovery in the Levitt group [24,25,26] a number of research groups have investigated the field of singlet NMR over the past 12 years with the purpose of defining and characterising singlet order relaxation properties [3,27,28,29,30], and the conditions for its existence [31,32,33,34], and of developing the methodology to access singlet order in different magnetic regimes [24,26,35,36,37,38,39,9,40,11,41,42,43] and to demonstrate the existence of long-lived singlet order in multiple-spin systems [44,45,46,47,48,49] and in heteronuclear systems [50]. Significant studies on the chemical requirements for obtaining long-lived spin order have been also conducted [17,51,52,53,54] with a record lifetime of 70 min for a 13 C-pair [54], 26 min for a 15 N-pair [51], and 10 min for a 1 H-pair [17] demonstrated.…”