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3D TOF MRA of Intracranial Aneurysms at 1.5 T and 3 T

Posted Apr 30, 2008 Updated Apr 7, 2023
By Yasuhiro Hiai RT , Shingo Kakeda MD , Toru Sato RT , Norihiro Ohnari MD , Junji Moriya MD , Mika Kitajima MD , Toshinori Hirai MD , Yasuyuki Yamashita MD , Yukunori Korogi MD
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Rationale and Objectives

A 3-T magnetic resonance imaging system provides a better signal-to-noise ratio and inflow effect than 1.5 T in three-dimensional time-of-flight (3D TOF) magnetic resonance angiography (MRA). The purpose of this study is to analyze the influence of matrix, parallel imaging, and acquisition time on image quality of 3D TOF MRA at 1.5 and 3 T, and to illustrate whether the combination of larger matrixes with parallel imaging technique is feasible, by evaluating the visualization of simulated intracranial aneurysms and aneurysmal blebs using a vascular phantom with pulsatile flow.

Materials and Methods

An anthropomorphic vascular phantom was designed to simulate the various intracranial aneurysms with aneurysmal bleb. The vascular phantom was connected to an electromagnetic flow pump with pulsatile flow, and we obtained 1.5- and 3-T MRAs altering the parameters of 3D TOF sequences, including acquisition time. Two radiologists evaluated the depiction of simulated aneurysms and aneurysmal blebs.

Results

The aneurysmal blebs were not sufficiently visualized on the high-spatial resolution 1.5-T MRA (matrix size of 384 × 256 or 512 × 256), even with longer acquisition time (9 or 18 min). At 3 T with acquisition time of 4.5 min using parallel imaging technique, however, the depiction of aneurysmal blebs was significantly better for the high-spatial resolution sequence than for the standard resolution sequence. For the high-spatial resolution sequence, the longer acquisition times did not improve the depiction of aneurysmal blebs in comparison with 4.5 min at 3 T.

Conclusions

For 3D TOF MRA, the combination of the large matrix with parallel imaging technique is feasible at 3 T, but not at 1.5 T.

Three-dimensional time-of-flight (3D TOF) magnetic resonance angiography (MRA) is a noninvasive imaging modality now readily accepted as a first-line diagnostic tool in magnetic resonance examination of several cerebrovascular diseases ( ). Concerning TOF MRA, the 3-T system offers some potential advantages compared to the 1.5-T system. The approximate doubling of signal-to-noise ratio from 1.5 to 3 T can provide the higher spatial resolution ( ) and the increased T1 relaxation time at higher magnetic field strength yields improvement of vessel–tissue contrast at 3-T imaging ( ). Several previous studies have reported that the high-spatial resolution 3-T MRA allows better visualization of small vessel segments and vascular disease, including intracranial aneurysms and intracranial stenoses and obstructions ( ). The various parameters of the 3D TOF MR angiograms, such as the matrix size, reduction factor in parallel imaging, and acquisition time, however, have not been compared between 1.5 and 3 T.

The purpose of this study is to analyze the influence of matrix, parallel imaging, and acquisition time on image quality of 3D TOF MRA at 1.5 and 3 T, and to illustrate whether the combination of larger matrixes with the parallel imaging technique is feasible, by evaluating the visualization of simulated intracranial aneurysms and aneurysmal blebs using a vascular phantom with pulsatile flow.

Materials and methods

Phantom Design

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Figure 1, Photograph (a) and schematic drawing (b) of the anthropomorphic vascular phantom used in this study. The phantom was designed to simulate the intracranial arteries with a total of 32 aneurysms. Of 32 aneurysms, 15 had an aneurysmal bleb with diameter of 2 mm.

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Image Acquisition

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Table 1

Scanning Parameters for the 3D Time-of-Flight MRA and Results in Evaluation for the Depiction of Simulated Lesions

Sequence No. TR TE FA BW FOV F-E mtx ST RF NEX AT Mean Score for Image Quality (mean ± standard error) Aneurysm Bleb 1.5T 1 30 6.3 20 31.25 180 mm 128 × 128 1.0 mm 2 1 time 2 min, 18 sec 3.10 ± 0.515 2.20 ± 1.095 2 30 6.3 20 31.25 180 mm 192 × 192 1.0 mm 2 1 time 3 min, 25 sec 3.00 ± 0.426 2.00 ± 1.000 3 30 6.3 20 31.25 180 mm 256 × 256 1.0 mm 2 1 time 4 min, 32 sec 2.58 ± 0.515 1.20 ± 0.447 4 30 6.3 20 31.25 180 mm 256 × 256 1.0 mm 1.3 1 time 6 min, 47 sec 3.30 ± 0.778 2.80 ± 0.837 5 30 6.3 20 31.25 180 mm 256 × 256 1.0 mm NA 1 time 9 min, 1 sec 3.89 ± 0.798 4.00 ± 1.000 6 30 6.3 20 31.25 180 mm 256 × 256 1.0 mm NA 2 times 17 min, 59 sec 4.08 ± 0.669 4.20 ± 0.837 7 30 6.3 20 31.25 180 mm 384 × 256 1.0 mm 2 1 time 4 min, 32 sec 2.25 ± 0.754 1.00 ± 0.000 8 30 6.3 20 31.25 180 mm 384 × 256 1.0 mm 1.3 1 time 6 min, 47 sec 3.20 ± 0.718 1.80 ± 1.095 9 30 6.3 20 31.25 180 mm 384 × 256 1.0 mm NA 1 time 9 min, 1 sec 3.17 ± 0.492 2.20 ± 0.837 10 30 6.3 20 31.25 180 mm 384 × 256 1.0 mm NA 2 times 17 min, 59 sec 3.70 ± 0.492 3.40 ± 0.548 11 30 6.3 20 31.25 180 mm 512 × 256 1.0 mm 2 1 time 4 min, 32 sec 2.08 ± 0.669 1.00 ± 0.000 12 30 6.3 20 31.25 180 mm 512 × 256 1.0 mm NA 2 times 17 min, 59 sec 3.80 ± 0.389 2.80 ± 0.447 3T 1 30 6.3 20 31.25 180 mm 128 × 128 1.0 mm 2 1 time 2 min, 18 sec 3.60 ± 0.515 3.25 ± 0.837 2 30 6.3 20 31.25 180 mm 192 × 192 1.0 mm 2 1 time 3 min, 25 sec 3.70 ± 0.452 3.40 ± 0.548 3 30 6.3 20 31.25 180 mm 256 × 256 1.0 mm 2 1 time 4 min, 32 sec 4.00 ± 0.739 4.20 ± 0.837 4 30 6.3 20 31.25 180 mm 384 × 256 1.0 mm 2 1 time 4 min, 32 sec 4.40 ± 0.515 4.20 ± 0.837 5 30 6.3 20 31.25 180 mm 384 × 256 1.0 mm 1.25 1 time 7 min, 16 sec 4.80 ± 0.389 4.40 ± 0.894 6 30 6.3 20 31.25 180 mm 384 × 256 1.0 mm NA 1 time 9 min, 1 sec 4.80 ± 0.389 4.60 ± 0.894 7 30 6.3 20 31.25 180 mm 512 × 256 1.0 mm 2 1 time 4 min, 32 sec 4.80 ± 0.389 4.40 ± 0.548

TR: repetition time; TE: echo time; FA: flip angle; BW: bandwidth; FOV: field of view; F-E mtx: frequency-encoded matrix; ST: section thickness; RF: reduction factor; NEX: number of excitation; AT: acquisition time; NA: not applicable.

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Image Analysis of MRA

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Statistical Analysis

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Results

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Relationship Between Matrix Size and Image Quality of MRAs with Use of Parallel Imaging (Reduction Factor = 2)

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Figure 2, Source images of three-dimensional time-of-flight magnetic resonance angiography (MRA) at 1.5 and 3T obtained with different matrix sizes using the reduction factor of 2: the matrix size of 128 × 128 (a) , 192 × 192 (b) , 256 × 256 (c) , 384 × 256 (d) , and 512 × 256 (e) at 1.5 T, and 128 × 128 (f) , 192 × 192 (g) , 256 × 256 (h) , 384 × 256 (i) , and 512 × 256 (j) at 3 T. At 1.5 T, the higher matrix sizes lead to the decrease in signal-to-noise ratio in vessel lumen. At 3 T, the image quality regarding the depiction of aneurysmal blebs becomes gradually better as the matrix size increases (arrows), and the image quality of 3T MRA is not influenced by the decrease in signal-to-noise ratio because of higher matrix sizes.

Figure 3, Volume-rendered (VR) images (anterior projection) of three-dimensional time-of-flight magnetic resonance angiography (MRA) at 1.5 and 3T obtained with various matrix sizes using the reduction factor of 2: the matrix size of 192 × 192 (a) and 384 × 256 (b) at 1.5 T, and 192 × 192 (c) , 384 × 256 (d) , and 512 × 256 (e) at 3 T. The aneurysmal blebs are insufficiently visualized with the matrix size of 192 × 192 (a) or not visualized with the matrix size of 384 × 256 (b) at 1.5-T MRA. At 3T (c–e) , the image quality regarding the depiction of aneurysmal blebs becomes gradually better as the matrix size increases (arrows).

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Relationship Between Acquisition Time (Reduction Factor) and Image Quality of MRAs

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Figure 4, Comparison of the average reader ratings regarding the depiction of aneurysmal blebs at 1.5 and 3 T obtained with various acquisition times (reduction factors). With the matrix size of 256 × 256 or 384 × 256 at 1.5 T, the reduction factor of 2 shows a degradation of image quality compared to the reduction factor of 1 (no use of parallel imaging) and 1.3. The image quality of 3-T magnetic resonance angiography with the matrix size of 384 × 256 is not influenced by the reduction factor.

Figure 5, Volume-rendered (VR) images (anterior projection) of three-dimensional time-of-flight magnetic resonance angiography at 1.5 T (matrix size of 256 × 256) and 3 T (matrix size of 384 × 256) obtained with various reduction factors: the reduction factor of 2 (a) , 1.3 (b) , and 1 (no use of parallel imaging) (c) at 1.5 T, and the reduction factor of 2 (d) , 1.25 (e) , and 2 (f) at 3 T. Although the image quality is not influenced by the reduction factor at 3 T (d–f) , the image quality is degraded as increasing reduction factor at 1.5 T (a–c) .

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Figure 6, Volume-rendered (VR) images (anterior projection) of three-dimensional time-of-flight magnetic resonance angiography (MRA) at 1.5 T (matrix size of 256 × 256) obtained with an acquisition time of 9 min (a) and 18 min (number of excitations 2) (b) , and at 3T (matrix size of 384 × 256) obtained with an acquisition time of 4.5 min (reduction factor 2). Regarding the depiction of aneurysmal blebs, the 3-T MRA (c: 4.5 min) is superior to the 1.5-T MRA (a: 9 min), and almost equivalent to the 1.5T MRA (b: 18 min).

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Interobserver Agreement

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Discussion

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Academic Radiology, Volume 15, Issue 5
Journals General Radiology
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