MR NAVIGATOR-ECHO MONITORING OF TEMPORAL CHANGES IN DIAPHRAGM POSITION - IMPLICATIONS FOR MR CORONARY ANGIOGRAPHY

Temporal changes in respiration could influence navigator-echo (NE)-ga ted MR coronary angiography (MRCA), but systematic investigation of th e effects of such variations sind how to limit them has not been perfo rmed, We addressed these issues by studying the influence of time in t he magnet on diaphragm position and respiratory patterns using NE diap hragm monitoring in volunteers and a phantom model, NE diaphragm monit oring was performed at .5 T in 10 subjects over a total period of 35 m inutes. The end-expiratory position was sustained for longer (1.1 vs . 4 seconds, P < .001) and with greater position stability (SD 1.9 vs 5. 9 mm, P = .01) than the end-inspiratory position, Drift of the end-exp iratory position occurred over time, causing a fall in scan efficiency (44-28%, P = .01), Up-drift of the end-expiratory position was most: common, Loss of scan efficiency was worse with up-drift because of los s of the end-expiratory pause from the NE window (up-drift 10% mm(-1), down-drift 7% mm(-1), both P = .03). Scan efficiency also was reduced during sleep (to a nadir of 0%), secondary to loss of the end-expirat ory pause, periodic breathing with oscillating end-expiratory position , and periods of apnea, The phantom model used actual diaphragm traces to evaluate the artifact resulting from diaphragm motion during acqui sition, Artifact was considerably reduced by NE adaptive motion correc tion compared with NE gating alone (ghosting ratio 2.0 vs 2.8, P < .01 ), Artifact also was significantly reduced with up-drift if scan effic iency was maintained above 35% (P = .05), For optimal NE-gated MRCA, t he following features are important: the NE window should be placed ar ound the end-expiratory position; subjects should not sleep; scan effi ciency should be monitored and the NE window should be repositioned if scan efficiency falls below 35%; and adaptive motion correction shoul d be used.