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Coronary MR Imaging

Rationale and Objectives

The aim of this study was to investigate whether a respiratory biofeedback system could increase navigator efficiency and maintain image quality compared to conventional respiratory-gated magnetic resonance coronary angiography (MRCA).

Materials and Methods

Eighteen healthy volunteers underwent MRCA using three different respiratory-gating protocols. A conventional expiratory free-breathing (FB) sequence was compared to two approaches using navigator echo biofeedback (NEB), a midinspiratory approach (NEBin) and an expiratory approach (NEBex). Navigator data reflecting the position of the diaphragm relative to a 3-mm gating window were made available to the subject using a video projector in combination with a Plexiglas screen and mirror goggles. Image quality was graded by two radiologists in consensus using a visual score ranging from 1 (not visible) to 4 (excellent vessel depiction).

Results

The NEB approaches improved navigator efficiency (71.1% with NEBex and 68.0% with NEBin vs 42.2% with FB), thus reducing total imaging time. This difference was statistically significant ( P NEBin = .007; P NEBex = .001). Image quality in the NEBex group was comparable to that in the FB group (median score, 2.44 vs 2.52), but it proved to be significantly lower (median score, 1.94 vs 2.52) for the right coronary artery and the left anterior descending coronary artery in the NEBin group.

Conclusion

NEB maintains image quality and significantly increases navigator efficiency, thereby decreasing total imaging time by about 40% compared to a conventional FB acquisition strategy.

The noninvasive nature of magnetic resonance coronary angiography (MRCA) offers great potential for assessing the morphologic changes in coronary artery disease (CAD) . The benefits of excluding CAD must be balanced against the risks and adverse effects of highly invasive procedures such as conventional catheter angiography, especially in patients with low pretest probability of disease.

The two main challenges that limit the introduction of MRCA into clinical routine are cardiac and respiratory motion that give rise to severe artifacts and blurring . Various strategies have been used to compensate for motion artifacts, including cardiac gating , respiratory gating , and navigator echoes with or without section correction. To reduce or eliminate respiratory motion artifacts, two distinct approaches are used in modern MRCA: using a navigator echo to measure the superior-inferior position of the diaphragm and accepting data acquired only in a certain range while the subject breathes freely . This is usually done at the most likely position of the diaphragm (ie, end-expiration).

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Materials and methods

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Figure 1, Sample screenshots of the different navigator protocols: conventional expiratory free-breathing sequence (a) , expiratory navigator echo biofeedback (NEB) (b) , and midinspiratory NEB (c) . The white line above the white bars represents the lung-liver interface in a coronal projection, and therefore inspiration results in a downward movement of the marker.

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Results

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Figure 2, Navigator efficiency, image quality, and heart rate (HR) data of the three study groups. NEBex, expiratory navigator echo biofeedback; NEBin, expiratory navigator echo biofeedback. ∗ P < .05.

Table 1

Navigator Efficiency, Image Quality, and Heart Rate Data of the Three Study Groups

Navigator Technique_n_ Navigator Efficiency (%) Image Quality (Score 1–4) Average HR (beats/min) HR Variability (beats/min) Free breathing 18 42.2 ± 10.1 2.52 ± 1.1 64.4 ± 10.8 2.8 ± 1.5 NEBin 18 68.0 ∗ ± 12.7 1.94 ∗ ± 0.8 71.0 ± 12.9 3.8 ± 1.8 NEBex 8 71.1 ∗ ± 7.2 2.44 ± 1.3 68.7 ± 14.6 3.1 ± 1.4

HR, heart rate; NEBex, expiratory navigator echo biofeedback; NEBin, expiratory navigator echo biofeedback.

Data are expressed as mean ± standard deviation.

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Figure 3, Multiplanar reconstructions (short axis) of a steady-state free-precession whole-heart sequence: proximal right coronary artery (white arrows) of a 25-year-old healthy volunteer. The right ventricle is marked by asterisks . Images were acquired using a conventional respiratory navigator (a) , an expiratory biofeedback navigator (b) , and an inspiratory biofeedback navigator (c) . Note the increased motion blurring in the inspiratory group (c) , whereas (a) and (b) are of comparable image quality.

Figure 4, Curved multiplanar reconstructions of a steady-state free-precession whole-heart sequence: proximal right coronary artery (white arrows) of a 23-year-old healthy volunteer. Expiratory (a) versus inspiratory navigator echo biofeedback (b) . Severe motion blurring reduced the quality of the inspiratory navigator echo biofeedback image. The right ventricle is marked by asterisks .

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Discussion

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