Double inversion recovery: impact of incidental magnetic transfer effects on optimal inversion times

Doube inversion recovery (DIR) is increasingly used in clinical studies on multiple sclerosis, as it selectively images gray matter and lesions. The inversion times used in literature differ significantly from theoretical predictions. One explanation for this are incidental magnetic transfer (iMT) effects, which are caused by the high density of pulses. In this work, the influence of iMT on DIR was analyzed and compared with theoretical assumptions and inversion times found in literature.

Three subjects were measured with 2D DIR sequences. The number of slices measured was varied. Optimum inversion times for white matter (WM) suppression were determined for 1 and for 24 slices by increasing TI2. The impact of slice-to-slice cross talk was evaluated with phantom measurements.

For constant inversion times TI1 = 3400 ms and TI2 = 325 ms signal intensity of WM decreased with increasing number of slices. The effective longitudinal relaxation time T1sat of WM was with 24 slices (strong iMT effect) 28% lower than at 1 slice (almost no iMT effect). The determined T1sat for 24 slices was 540 ms, compared with 750 ms with 1 slice.

Incidental magnetic transfer effects have a huge impact on 2D DIR sequences. The number of slices measured affect strongly the severity of the iMT effect. This can lead to a strong decrease of T1sat for WM depending on the interleaving scheme. Results from different studies and at different field strengths are therefore not easily comparable, without exact knowledge of the sequence design.