Figure 5.

Images acquired after excitation by relaxation-matched pulses can be subtracted to highlight a specific range of T₂, but off-resonance precession occuring during the RF pulse can confound the subtraction. For short-T₂-specific UTE imaging, however, typical ranges of off-resonance expected in vivo—e.g., lipids precessing 3.5 ppm away from water at 3 T field-strength—are compatible with normal tip angles (a,b), and the difference between the magnetization excited by the pulses is relatively invariant with off-resonance. Additionally, when using a slightly higher tip angle with the ‘slow’ pulse, the improved robustness in differentiating short-T₂ signals from longer ones is retained across the normal range of off-resonance (c,d). Computing complex instead of magnitude subtractions—i.e. |1 − 2| instead of |1| − |2|—introduces significant positive sensitivity to off-resonance-induced phase accrual, which is not the desired contrast.