Relationship between motion of coronary arteries and diaphragm during freebreathing: Lessons from real-time MR imaging

OBJECTIVE. Diaphragmatic navigators are frequently used in free-breathing c oronary MR angiography, either to gate or prospectively correct slice posit ion or both. For such approaches, a constant relationship between coronary and diaphragmatic displacement throughout the respiratory cycle is assumed. The purpose of this study was to evaluate the relationship between diaphra gmatic and coronary artery motion during free breathing. SUBJECTS AND METHODS. A real-time echoplanar MR imaging sequence was used i n 12 healthy volunteers to obtain 30 successive images each (one per cardia c cycle) that included the left main coronary artery and the domes of both hemidiaphragms, The coronary artery and diaphragm positions (relative to is ocenter) were determined and analyzed for effective diaphragmatic gating wi ndows of 3, 5, and 7 mm (diaphragmatic excursions of 0-3, 0-5, and 0-7 mm f rom the end-expiratory position, respectively). RESULTS. Although the mean slope correlating the displacement of the right diaphragm and the left main coronary artery was approximately 0.6 for all d iaphragmatic gating windows, we also found great variability among individu al volunteers. Linear regression slopes varied from 0.17 to 0.93, and r(2) values varied from .04 to .87. CONCLUSION. Wide individual variability exists in the relationship between coronary and diaphragmatic respiratory motion during free breathing. Accord ingly, coronary MR angiographic approaches that use diaphragmatic navigator position for prospective slice correction may benefit from patient-specifi c correction factors. Alternatively, coronary MR angiography may benefit fr om a more direct assessment of the respiratory displacement of the heart an d coronary arteries, using left ventricular navigators.