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Coronary MRA at 3 T using 3d multi-interleaved multi-echo acquisition with varpro fat-water separation

Introduction

Coronary MR Angiography is a valuable tool for non-invasive assessment of coronary arteries. Presently, contrast-enhanced, fat-saturated, ECG-triggered and navigator-gated 3D spoiled gradient-echo sequence is employed for whole-heart Coronary MRA at 3 T[1]. However, large static field variations at 3 T frequently result in non-uniform fat-suppression over the field-of-view (FOV), obscuring the delineation of coronary arteries. Multi-echo Dixon approaches utilizing iterative decomposition have been shown to provide robust fat-water separation even in the presence of large field inhomogeneities. In this study, an ECG-triggered navigator-gated 3D spoiled gradient-echo multi-interleaved multi-echo (GRE-MEMI) pulse sequence is introduced which utilizes VARPRO[2] fat-water separation to achieve reliable fat-suppression and provides enhanced visualization of coronary arteries.

Methods

A 3D GRE-MEMI sequence (Fig. 1) was implemented on a 3 T whole-body MR scanner (MAGNETOM Trio, Siemens AG) with support for navigator-gating and ECG-triggering. Water-only and fat-only images were reconstructed using VARPRO. Four healthy volunteers were imaged pre- and during contrast agent administration targeting right coronary artery (RCA). Typical imaging parameters for pre-contrast GRE-MEMI scan are listed in Table 1. Additionally, a conventional single-echo fat-saturated GRE scan was acquired for comparison. Thereafter, 0.2 mmol/kg Gd-DTPA (Magnevist®, Bayer Healthcare) was slowly injected at a rate of 0.3 ml/s followed by 20 ml of saline solution injected at the same rate. GRE-MEMI acquisition with inversion preparation (TI = 300 ms) was started 30 s after injection.

Table 1 Typical imaging parameters for conventional 3D fat-saturated GRE and 3D GRE-MEMI measurements.
Figure 1
figure1

Pule Sequence Diagram for 3D ECG-triggered, navigator gated, spoiled gradient echo sequence with multi-echo multi-interleave readout (GRE-MEMI). Multiple echoes are used during reconstruction by VARPRO for iterative water-fat decomposition. No fat-saturation prepulse is applied separately. Multi-interleaved scheme achieves shorter echo time increments between multiple echoes, which improves the fat-water separation.

Results

Targeted RCA images were successfully acquired in all volunteers with effective fat-water separation. The average total imaging time was 8.93 ± 1.2 min with navigator efficiency of 33.8 ± 4.6%. Fig. 2 shows pre-contrast coronary artery images from a healthy volunteer. Conventional fat-saturation yields suboptimal fat-suppression whereas robust fat-suppression is evident in water-only images which provide clear depiction of coronary artery. Fig. 3 illustrates enhanced contrast-to-noise with the use of contrast agent.

Figure 2
figure2

Conventional fat saturation (chemical selective saturation) image (a) and fat-water separated images (b, c) from a targeted right coronary artery (RCA) measurement at 3 T in a healthy volunteer without any contrast agent administration. Conventional fat saturation yields suboptimal results in some areas (a - red arrows), however, robust fat suppression is achieved over the entire FOV using the proposed technique. Moreover, the use of multiple echoes increases the signal-to-noise ratio (SNR). Water-only (d) and fat-only (e) pre-contrast images from another health subject demonstrate excellent fat suppression and clearly depict RCA (red arrows).

Figure 3
figure3

Water-only (a) and fat-only (b) images acquired from a healthy subject during slow infusion of contrast media. Note that water and fat signals are effectively separated and the RCA (a - red arrows) is sharply delineated. Compared to pre-contrast GRE measurements, use of contrast agent increases contrast-to-noise ratio between blood and background tissues.

Conclusion

3D GRE-MEMI sequence was successfully utilized for targeted fat-water separated coronary artery imaging in healthy volunteers. VARPRO fat-water separation provides reliable fat-suppression at 3 T and improves the delineation of coronary arteries. Moreover, without the use of a fat-saturation prepulse, readout duration within a heartbeat can be extended to cover the entire quiescent period without any degradation in fat-suppression. Multi-echo acquisition results in increased acquisition time, however, the resulting water-only image provides the benefit of increased SNR due to intrinsic averaging effect of fat-water separation. Further improvement in acquisition speed using higher parallel imaging factors is required to achieve 3D whole-heart coverage.

References

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Correspondence to Saurabh Shah.

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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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Shah, S., Bi, X., Hernando, D. et al. Coronary MRA at 3 T using 3d multi-interleaved multi-echo acquisition with varpro fat-water separation. J Cardiovasc Magn Reson 12, P42 (2010). https://doi.org/10.1186/1532-429X-12-S1-P42

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Keywords

  • Right Coronary Artery
  • Contrast Agent Administration
  • Coronary Artery Image
  • Navigator Efficiency
  • Parallel Imaging Factor