Near-resonance spin-lock contrast

Spin-lock and spin-tip excitations are the two magnetization components created by the preparatory RF pulse of an MRI contrast enhancement sequence. Only spin-lock is inherently adiabatic, preserving spin alignment so that tissue-specific relaxation can generate desired saturation contrasts. Spin-tip is the rotating-frame oscillating excitation, and generally causes nonadiabatic loss of all detectable magnetization. Relative levels of spin-lock and spin-tip are important to understand as a function of the preparatory B1Δ amplitude, resonance frequency offset, Δ, and the pulse waveform. These MR responses can be accurately analyzed theoretically and numerically by using Torreyʹs tipped coordinates to formulate Blochʹs equations. At near-resonance offsets, (Δ/γB1) less than 2.0, spin-lock contrast (SLC) depends strongly on T2, due to the nature of spin-lock T1p relaxation in the RF pulse interval. The relaxation rates 1T1p and 1T2p apply for active B1Δ, but remain linear combinations of ordinary (1T1) and (1T2) for motionally narrowed MR. The SLC increases rapidly as Δ decreases below 2000 Hz; carefully chosen B1Δ rise times avoid spin-tip losses down to 150 Hz or less. The SL saturation enhances or multiplies any other indirect saturation effects that may be also present, such as magnetization transfer. A strong near-resonance SLC multiplier is advantageous for clinically practical MRI sequences that use short B1Δ pulses and fast SE multislice scan modes. Simulations based upon spin-lock/spin-tip theory and measured (T1, T2) yield excellent agreement with real MRI results for clinically practical fast multislice scans.