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. 2015 May;41(5):1440-6.
doi: 10.1002/jmri.24679. Epub 2014 Jun 30.

Reduced scan time three-dimensional FLAIR using modulated inversion and repetition time

Neville D Gai  1 , John A Butman
Affiliations

Reduced scan time three-dimensional FLAIR using modulated inversion and repetition time

Neville D Gai et al. J Magn Reson Imaging. 2015 May.

Abstract

Background: The purpose of this study is to design and evaluate a new reduced scan time three-dimensional (3D) FLuid Attenuated Inversion Recovery (FLAIR) sequence.

Methods: The 3D FLAIR sequence was modified so that the repetition time was modulated in a predetermined smooth manner (3D mFLAIR). Inversion times were adjusted accordingly to maintain cerebrospinal fluid (CSF) suppression. Simulations were performed to determine SNR for gray matter (GM), white matter (WM), and CSF. Fourteen volunteers were imaged using the modified and product sequence. SNR measurements were performed in GM, WM, and CSF. Mean value and the 95% confidence interval ([CI]) were assessed. Scan time for the 3D FLAIR and 3D mFLAIR sequences was measured.

Results: There was no statistically significant difference in the SNR measured in GM (P value = 0.5; mean SNR = 42.8 [CI]: 38.2-45.5 versus 42.2 [CI]: 38.3-46.1 for 3D FLAIR and 3D mFLAIR, respectively) and WM (P value = 0.25; mean SNR = 32.1 [CI]: 30.3-33.8 versus 32.9 [CI]: 31.1-34.7). Scan time reduction greater than 30% was achieved for the given parameter set with the 3D mFLAIR sequence.

Conclusion: Scan time for 3D FLAIR can be effectively reduced by modulating repetition and inversion time in a predetermined manner while maintaining the SNR and CNR of a constant TR sequence.

Keywords: 3D FLAIR; modulated inversion time; modulated repetition time; scan time reduction; variable repetition time.

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Figures

Figure 1
Figure 1
k-space data (absolute value) of a 321 slice acquisition of a human brain using a 3D FLAIR sequence. (Data along ky was MIPed to project the data in two dimensions.) kx has 256 steps while kz has 321 overcontiguous steps. Embedded gray scale bar shows that the window was adjusted to accomodate the entire range of data values (≥ 0).
Figure 2
Figure 2
Sequence diagram of the 3D FLAIR sequence. A preparation scheme introduces T2 weighting which is followed by inversion and the multi spin-echo low flip angle readout train. An optional fat saturation sequence can be introduced prior to the readout train.
Figure 3
Figure 3
TR and TI are varied in a smooth manner based on the Blackman-Harris window. While TR varies from 2.9 s to 8 s, TI varies from 984 ms to 2357 ms.
Figure 4
Figure 4
Change in slab profile for (a) gray matter (b) white matter and (c) CSF obtained with the two different 3D FLAIR implementations using full TR = 8s and variable TR and TI. Figure 4(d) shows the simulated case of a hyperintense (20% higher) signal in a single pixel (pixel 33) in white matter along the z direction. Equally good conspicuity for 3D FLAIR and 3D mFLAIR sequence can be seen. The original profile is also shown.
Figure 5
Figure 5
Native sagittal slices and reformatted axial and coronal images for the full TR 3D FLAIR sequence (left) and the varying TR and TI, 3D mFLAIR sequence (right). Scan time for 3D mFLAIR sequence (2:50) was 33% lower than 3D FLAIR (4:16). Window/level was the same for all images.
Figure 6
Figure 6
Mean SNR (and std) measured in GM, WM and CSF for (a) full TR = 8s and (b) 3D mFLAIR.
Figure 7
Figure 7
Three subjects exhibited benign white matter lesions. One comparison image from 3D FLAIR and 3D mFLAIR is shown. No perceptible difference in lesion conspicuity was noticed between the two implementations although scan time for 3D mFLAIR was 2:56 compared with 4:24 for 3D FLAIR resulting in a 33% reduction in scan time.
See this image and copyright information in PMC

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