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Pitfalls of 3D FLAIR Brain Imaging

Posted Sep 30, 2012 Updated Apr 10, 2023
By Shingo Kakeda MD , Yukunori Korogi MD , Yasuhiro Hiai PhD , Norihiro Ohnari MD , Toru Sato , Toshinori Hirai MD
10 min read

Rationale and Objectives

To prospectively compare the image contrast of various brain lesions on two-dimensional (2D) and three-dimensional (3D) fluid-attenuated inversion-recovery (FLAIR) images and to highlight the pitfalls of 3D FLAIR.

Materials and Methods

Institutional review board approval was obtained. We examined 94 brain lesions with 2D and 3D FLAIR at 3T. First, we optimized the repetition time and echo time of 3D FLAIR with a volunteer study. Then, we assessed the conspicuity and detection of the various lesions qualitatively, and the contrast ratio between the gray or white matter and lesions was calculated as a quantitative assessment. We also performed a phantom study to investigate the effects of different flow velocities on 2D and 3D FLAIR.

Results

With regard to the conspicuity and detection of most lesions (multiple sclerosis, ischemic lesions or infarction, brain tumors, or chronic trauma), 3D FLAIR was equal or superior to 2D FLAIR. For these lesions, the mean contrast ratios were higher on 3D FLAIR than on 2D FLAIR images. In terms of lesion conspicuity in the patients with hippocampal sclerosis and leptomeningeal metastasis, however, 3D FLAIR was equal or inferior to 2D FLAIR. The ivy sign in patients with moyamoya disease was frequently obscured on 3D FLAIR. The phantom study demonstrated that the signal–intensity ratio on 3D FLAIR decreased more rapidly with increasing velocity than that on 2D FLAIR.

Conclusion

Although 3D FLAIR may replace 2D FLAIR images for most patients, radiologists should keep in mind that 3D has some pitfalls.

Fluid-attenuated inversion-recovery (FLAIR) imaging is widely used for various intracranial diseases, such as subarachnoid hemorrhage, meningitis, acute infarction, hippocampal sclerosis, and multiple sclerosis (MS) . A recently developed three-dimensional (3D) FLAIR technique varies the flip angles of the refocusing radiofrequency (RF) pulses, which allows the echo train length to be significantly increased without incurring excessive blurring . Recent reports of 3D FLAIR in brain imaging demonstrated a reduction in pulsation artifacts and a better signal-to-noise ratio (SNR) compared with two-dimensional (2D) FLAIR imaging . Furthermore, because 3D FLAIR imaging yields 3D volume data with isotropic information, thinner slice images can be acquired in any plane; this minimizes the partial volume effect between small lesions and the surrounding tissue . A previous study reported that 3D FLAIR using 1.2-mm cubic voxels was superior to 2D FLAIR for the detection of MS lesions . During our work, however, we noticed that some leptomeningeal lesions were obscured on 3D FLAIR; this suggests that 3D FLAIR may have some pitfalls that radiologists should be aware of when using this technique in clinical practice. To our knowledge, no previous studies have directly compared 2D and 3D FLAIR to evaluate various brain lesions, except the previously mentioned MS lesion. Moreover, several studies have focused on searching for the clinical importance of 3D FLAIR. The purpose of our present study was to prospectively compare the image contrast of various brain lesions on 2D and 3D FLAIR images and to highlight the pitfalls of 3D FLAIR.

Subjects and methods

This study was approved by the institutional review board. All magnetic resonance (MR) examinations were performed with a 3T MR system (Signa EXCITE 3T; GE Healthcare, Milwaukee, WI) and were acquired in an axial plane for the 2D FLAIR sequence and in a sagittal plane for the 3D FLAIR sequence.

Healthy Volunteer Study: Optimization of TR and TE on 3D FLAIR

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Patient Study

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Table 1

Sequence Parameters of the 2D FLAIR and the 3D FLAIR

2D FLAIR 3D FLAIR (Cube) TR (ms) 12,000 10,000 TE (ms) 140 114 TI (ms) 2600 2507 ETL 30 190 BW (kHz) 25 62.5 FOV (cm) 22 22 ST (mm) 5 1.2 F-resol 256 224 P-resol 192 224 NEX 1 1 AF 1 2.86 AT 3:20 7:04

AF, acceleration factor; AT, acquisition time; BW, band width; ETL, echo train length; F-resol, frequency resolution; FLAIR, fluid-attenuated inversion-recovery; FOV, field of view; NEX, number of excitation; P-resol, phase resolution; TE, echo time; TI, inversion time; TR, repetition time; ST, slice thickness.

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Phantom Study

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Figure 1, A diagram depicting the experimental setup of our flow velocity study. A 5-mm-diameter tube filled with a blood-mimicking fluid was used. A continuous nonpulsatile flow (velocity 0, 0.1, 0.25, 0.5, 0.75, and 1.0 cm/s) was created in the tube with syringe pumps. A universal phantom and the tube were scanned simultaneously.

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Results

Healthy Volunteer Study: Optimization of TR and TE on 3D FLAIR

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Figure 2, Bar graphs showing the contrast-to-noise ratios (CNR) for three-dimensional fluid-attenuated inversion-recovery sequences with different repetition times (TR) of 6000, 8000, 10,000, and 12,000 ms. The CNR were calculated between the gray matter (GM), white matter (WM), and cerebral spinal fluid (CSF). The best contrast between the GM and WM was obtained at a TR of 12,000 ms; the best contrast between the CSF and GM or WM was obtained at a TR of 10,000 ms.

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Figure 3, Bar graphs showing the contrast-to-noise ratios (CNR) for three-dimensional (3D) fluid-attenuated inversion-recovery (FLAIR) sequences with different echo times (TE) of 83, 104, 114, 140, 154, and 183 ms. The contrast-to-noise ratios (CNR) were calculated between the gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF). The best contrast between the GM and WM was obtained at a TE of 114 ms, and the best contrast between the CSF and GM or WM was obtained at a TE of 80 ms. For the CNR between the GM and WM, the 3D FLAIR images obtained at a TE of 114 ms was slightly higher than 2D FLAIR.

Figure 4, A two-dimensional (2D) FLAIR image (a) and three-dimensional (3D) fluid-attenuated inversion-recovery (FLAIR) images obtained with the different echo times (TE) of 83 (b) , 104 (c) , 114 (d) , 140 (e) , 154 (f) , and 183 ms (g) . For the overall image quality, the 2D FLAIR image and 3D FLAIR images obtained at TE of 83, 104, and 114 ms were scored as grade 4 (excellent), whereas the 3D FLAIR images obtained at TE of 140, 154, and 183 ms were scored as grade 3 (adequate), grade 2 (inadequate), and grade 1 (poor), respectively. For the contrast between the GM and WM, the 3D FLAIR images obtained at TE of 104, 114, 140, and 154 ms were scored as grade 4, whereas the 2D FLAIR and 3D FLAIR images obtained at TE of 83 and 183 ms were scored as grade 3.

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Patient Study

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Table 2

Mean Contrast Ratios of Lesion-gray Matter and Lesion-white Matter: 2D FLAIR versus 3D FLAIR Images

Lesion-gray Matter Lesion-white Matter 2D FLAIR 3D FLAIR 2D FLAIR 3D FLAIR MS lesion (n = 4) 0.29 ± 0.09 0.19 ± 0.19 0.53 ± 0.10 0.80 ± 0.16 Acute or old infarction (n = 24) 0.38 ± 0.13 0.69 ± 0.23 ∗ 0.81 ± 0.16 1.09 ± 0.24 ∗ Ischemic lesion or infarction in moyamoya disease (n = 10) 0.38 ± 0.15 0.58 ± 0.24 ∗ 0.78 ± 0.28 0.97 ± 0.28 ∗ White matter lesion (n = 12) 0.26 ± 0.11 0.40 ± 0.96 ∗ 0.40 ± 0.10 0.68 ± 0.13 ∗ Glioma (n = 9) 0.28 ± 0.12 0.30 ± 0.08 0.83 ± 0.26 0.93 ± 0.41 Chronic trauma (n = 5) 0.26 ± 0.18 0.40 ± 0.13 0.59 ± 0.27 0.99 ± 0.25 Gray matter heterotopia (n = 2) 0.01 ± 0.04 0.11 ± 0.15 0.40 ± 0.07 0.53 ± 0.80

2D, two-dimensional; 3D, three-dimensional; FLAIR, fluid-attenuated inversion-recovery; MS, multiple sclerosis.

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Figure 5, Axial magnetic resonance (MR) images from a patient with MS. (a) A two-dimensional (2D) fluid-attenuated inversion-recovery (FLAIR) image. (b) A three-dimensional (3D) FLAIR image. The multiple sclerosis (MS) plaque ( arrows ) in the medulla oblongata is clearly visible on the 3D image, whereas the lesion may be missed on the 2D FLAIR image.

Figure 6, Axial magnetic resonance (MR) images from a patient with diffuse axonal injury. (a) A two-dimensional (2D) fluid-attenuated inversion-recovery (FLAIR) image. (b) A three-dimensional (3D) FLAIR image. The small lesion ( arrows ) is clearly visible on the 3D image, whereas the 2D FLAIR image does not depict the lesion.

Figure 7, Axial magnetic resonance (MR) images from a patient with cerebellopontine angle meningioma. (a) A two-dimensional (2D) fluid-attenuated inversion-recovery (FLAIR) image. (b) A three-dimensional (3D) FLAIR image. The 3D FLAIR image shows a very good delineation of the tumor ( arrow ), which may be missed because of cerebrospinal fluid pulsation artifacts on the 2D FLAIR image.

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Figure 8, Oblique coronal magnetic resonance (MR) images from a patient with left-sided hippocampal sclerosis. (a) A two-dimensional (2D) fluid-attenuated inversion-recovery (FLAIR) image. (b) A three-dimensional (3D) FLAIR image. The 2D FLAIR image shows increased signal intensity in the left hippocampus ( arrow ) with respect to the right hippocampus, whereas the 3D FLAIR image reveals subtle increased signal intensity.

Figure 9, Axial magnetic resonance (MR) images from a patient with leptomeningeal metastasis from breast cancer. (a,b) Two-dimensional (2D) fluid-attenuated inversion-recovery (FLAIR) images. (c,d) three-dimensional (3D) FLAIR images. The 2D FLAIR images demonstrate extensive areas of increased signal intensity ( arrows ) within the subarachnoid space, whereas the 3D FLAIR images show a smaller region of increased signal intensity compared with the 2D FLAIR images.

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Figure 10, A bar graph shows the ivy sign score according to four cortico-subcortical regions, with each color indicating a different grade, and represents the percentage of the score for each ivy sign score. ACA, anterior cerebral artery region; ant-MCA, anterior half of the MCA region; post-MCA, posterior half of the MCA region; PCA, posterior cerebral artery region.

Figure 11, Moyamoya disease in a 34-year-old female. (a,b) Two-dimensional (2D) fluid-attenuated inversion-recovery (FLAIR) images and (c,d) 3D FLAIR images obtained at the same level. The three-dimensional (3D) FLAIR images were reformatted into 5-mm-thick axial plane slices. The 2D FLAIR images show bilateral areas of high SI ( arrowheads ) along the leptomeninges, which are obscure on the 3D FLAIR images.

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Phantom Study

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Figure 12, A comparison of the signal intensity (SI) of blood-mimicking fluids relative to the universal phantom at various flow velocities on three-dimensional (3D) and two-dimensional (2D) fluid-attenuated inversion-recovery (FLAIR) images. In the 3D and 2D FLAIR images, the SI ratio gradually decreased as the flow velocity increased. At a flow velocity exceeding 0.25 cm/s, the SI ratio on 3D FLAIR was dramatically lower than that on the 2D FLAIR images.

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Discussion

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Academic Radiology, Volume 19, Issue 10
Journals General Radiology
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