Magnetization prepared 3D-FLAIR Imaging at 7.0 Tesla

Purpose: To develop a sub millimeter 3D FLAIR sequence with minimal T1 weighting and high T2 contrast at 7 Tesla. Aims: At 7T, a fundamental problem for FLAIR imaging is the lengthening of the T1 relaxation times of grey and white matter (GM and WM) while the T1 of CSF is less field dependent. This has two detrimental effects. First, the desired T2 contrast is severely compromised, as the prolonged T1 causes significantly increased T1-weighting in the FLAIR images. Second, the gain in SNR at high field is limited as part of the gain in magnetization obtained by the increased field strength is lost by the reduced magnetization recovery at the moment of excitation (CSF nulling).

Materials and Methods: To reduce T1 weighting in gray and white matter, a magnetization preparation (MP) pulse was applied prior to the inversion pulse. The magnetization preparation consists of a T2 preparation pulse which saturates tissues with short T2, like GM and WM, while it has minimal effect on the magnetization of CSF, which has a long T2. Thus, in principle, CSF experiences inversion recovery, while GM and WM experience saturation recovery. This yields less T1 weighting for GM and WM during data sampling, around the moment of zero crossing of CSF. Twelve subjects were imaged at 7T (Philips Healthcare), of which 8 subjects were also imaged at 1.5T for comparison. 3D FLAIR images were acquired with and without MP. Parameters 7T: TR/TI/TE 8000/2400/303, SENSE 2.5×2.5 (APxRL), refocusing train length 125, refocusing angle 70°, FOV 250×250×180, acquired resolution 0.8×0.8×0.8mm³. Parameters at 1.5T were similar, except for SENSE (2.0×2.0 (APxRL)) and the acquired resolution (1.0×1.0×1.0mm³). The duration of the MP sequence was 100ms.

Results: Fig 2 illustrates the improved contrast and SNR in 7T 3D FLAIR images with magnetization preparation. The contrast between GM and WM, defined as SNR_GM/SNR_WM, was significantly improved in MP FLAIR at 7T (1.81±0.25) compared with the contrast in FLAIR without MP (1.39±0.27). With MP, the increase in SNR from 1.5T to 7T is higher for both GM and WM, indicating less SNR loss from the prolonged T1 (Table 1). Besides, the increase in SNR is similar for GM and WM with MP, indicating less contamination from T1 contrast.

Conclusions: Magnetization preparation allows 3D FLAIR with reduced T1 weighting and high T2 contrast at 7T.