Figure 2.

(a) Energy level diagram of a two spin–1/2 system I – S, assuming that ω_I > ω_S ≫ J > 0. Here |m_Im_S⟩ represent the eigenstates of Zeeman and heteronuclear J-coupling Hamiltonian in the high-field approximation, with m_N being the magnetic quantum number and ω_N being the Larmor frequency for each spin. The allowed values of m_N are labeled as α and β. The arrows connecting the energy levels (1 through 4) represent the observable transitions for each spin: red (1 → 2) and blue (3 → 4) for spin S, and green (1 → 3) and purple (2 → 4) for spin I. (b) Schematic ISSFP_xy sequence with the resulting spectra arising from each block for both spins, and with spectral colors associated with the transitions in panel (a). Spectra are referenced to the Larmor frequency of each spin. Density operators and describe the state of the spin ensemble at the time points marked by black arrows. (c) Simulations of the steady-state ¹³C transverse magnetization at the beginning of the first block (within ) of the indicated sequences as a function of repetition times τ_R and flip angle θ. Notice that separate plots are provided for each of the two observable S transitions (1 → 2 and 3 → 4). Spins I and S are chosen as ¹H and ¹³C, respectively, with a J-coupling of 200 Hz. Additional simulation details are given in the Supporting Information.