B₁-Insensitive T₂ preparation for improved coronary magnetic resonance angiography at 3 T

At 3 T, the effective wavelength of the RF field is comparable to the dimension of the human body, resulting in B₁ standing wave effects and extra variations in phase. This effect is accompanied by an increase in B₀ field inhomogeneity compared to 1.5 T. This combination results in nonuniform magnetization preparation by the composite MLEV weighted T ₂ preparation (T₂ Prep) sequence used for coronary magnetic resonance angiography (MRA). A new adiabatic refocusing T₂ Prep sequence is presented in which the magnetization is tipped into the transverse plane with a hard RF pulse and refocused using a pair of adiabatic fast-passage RF pulses. The isochromats are subsequently returned to the longitudinal axis using a hard RF pulse. Numerical simulations predict an excellent suppression of artifacts originating from B₁ inhomogeneity while achieving good contrast enhancement between coronary arteries and surrounding tissue. This was confirmed by an in vivo study, in which coronary MR angiograms were obtained without a T₂ Prep, with an MLEV weighted T₂ Prep and the proposed adiabatic T₂ Prep. Improved quantitative and qualitative coronary MRA image measurement was achieved using the adiabatic T₂ Prep at 3 T.