Department of Diagnostic and Interventional Neuroradiology, University Hospital Tübingen, Tübingen, Germany.
Invest Radiol. 2010 Apr;45(4):196-201. doi: 10.1097/RLI.0b013e3181d2ff0e.
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.
双反转恢复（DIR）越来越多地用于多发性硬化症的临床研究，因为它可以选择性地对灰质和病变进行成像。文献中使用的反转时间与理论预测有很大差异。造成这种情况的一个原因是偶然的磁传递（iMT）效应，这是由脉冲密度引起的。在这项工作中，分析了 iMT 对 DIR 的影响，并将其与理论假设和文献中发现的反转时间进行了比较。
对 3 名受试者进行了 2D DIR 序列测量。测量的切片数量有所不同。通过增加 TI2，确定了用于 1 个和 24 个切片的白质（WM）抑制的最佳反转时间。通过体模测量评估了切片间串扰的影响。
对于恒定的反转时间 TI1 = 3400 ms 和 TI2 = 325 ms，随着切片数量的增加，WM 的信号强度降低。WM 的有效纵向弛豫时间 T1sat 在 24 个切片（强烈的 iMT 效应）时比在 1 个切片（几乎没有 iMT 效应）时低 28%。确定的 24 个切片的 T1sat 为 540 ms，而 1 个切片的 T1sat 为 750 ms。
偶然的磁传递效应对 2D DIR 序列有很大的影响。测量的切片数量强烈影响 iMT 效应的严重程度。这可能导致 WM 的 T1sat 随着交错方案的不同而大幅降低。如果没有确切了解序列设计，来自不同研究和不同场强的结果就不容易进行比较。
