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SE_MC sequence improves image quality of carotid arteries and atherosclerotic plaques

Introduction

Double-Inversion-Recovery (DIR) FSE is the standard MRI sequence used to acquire blood suppressed 2D high-resolution T1, PD and T2-weighted images for atherosclerotic plaque characterization in the carotid arteries.

Purpose

FSE images suffer from blurring because of non-uniform T2-weighting of k-space [Constable, Gore.MRM.1992.28(1):9-24], which is stronger on small structures with short T2 times, such as carotid arteries. To avoid this, we propose to generate T1-PD-T2-weighted images using Spin-Echo Multi-Contrast (SE_MC) sequence. FSE and SE_MC are both CPMG multi-echo sequences; however FSE acquires different lines of the same k-space at different TEs, whereas SE_MC samples multiple k-spaces (with constant TE) at different TEs.

Methods

The image formation process for SE and FSE sequences was simulated in MATLAB (MathWorks) applying the same parameters and k-space acquisition strategy used in-vivo (TE = 14 ms, ETL = 9). An average carotid artery (lumen-diameter = 7 mm, wall-thickness = 1 mm, T2 = 40 ms) was simulated by linear combination of Bessel functions in k-space. In the in-vivo study, 6 normal volunteers (6 m, 30 ± 5 years) and 12 atherosclerotic patients (10 m, 74 ± 9 years) were imaged at 3 T (TIM Trio, Siemens Medical Solutions) using surface coils (FOV = 150 × 150 mm, matrix = 320 × 320, slice = 2 mm). The ECG-gated DIR-FSE pulse sequence (ETL = 9) was used to acquire T1w (TE = 14 ms, TR = 1RR, Tacq <60s) PDw (TE = 14 ms, TR = 2RR, Tacq <120s) and T2w (TE = 89 ms, TR = 2RR, Tacq <120s) images. The ECG-gated parallel DIR-SE_MC sequence (partial k-space = 5/8, SENSE-iPat = 2) acquired 7 contrast images (TE = 25.8-103.2 ms) for TR = 1RR (Tacq <120s) and TR = 2RR (Tacq <240s). T2 maps were estimated by non-linear regression and synthetic T1-PD-T2-weighted images were generated at TE = 14 ms (T1w-PDw) and TE = 89 ms (T2w).

Results

In normal volunteers, FSE images show a significant blurring along the phase-encoding direction, consistent with the simulated images. T1w and PDw are more affected by blurring than T2w images because low spatial frequencies (along the phase-encoding direction) are sampled at short TEs. Horizontal and vertical image profiles highlight the difference between the phase-encoding and the frequency-encoding directions for both FSE and simulated images. SE_MC images do not suffer from blurring, thus arterial wall boundaries are sharper and better contrasted. Details of plaque morphology and composition are more clearly visible in SE_MC images, Figure 1.

Figure 1
figure1

Comparison between FSE and SE_MC dark-blood carotid images in computer simulations, normal volunteers and atherosclerotic patients.

Conclusion

This study suggests that replacing FSE with SE_MC in vascular MRI protocols could benefit the segmentation of vessel walls and atherosclerotic plaques with the only disadvantage of a slightly longer acquisition time. Phantom experiments will be soon performed to quantify the error on vessel wall measurements caused by FSE blurring.

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Correspondence to Luca Biasiolli.

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Open Access This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Biasiolli, L., Lindsay, A.C., Choudhury, R.P. et al. SE_MC sequence improves image quality of carotid arteries and atherosclerotic plaques. J Cardiovasc Magn Reson 12, P138 (2010). https://doi.org/10.1186/1532-429X-12-S1-P138

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Keywords

  • Carotid Artery
  • Atherosclerotic Plaque
  • Simulated Image
  • Plaque Characterization
  • Longe Acquisition Time