Background: Aggressive dose reduction strategies for cardiac CT require the prospective selection of limited\r\ncardiac phases. At lower heart rates, the period of mid-diastole is typically selected for image acquisition. We aimed\r\nto identify the effect of heart rate on the optimal CT acquisition phase within the period of mid-diastole.\r\nMethods: We utilized high temporal resolution tissue Doppler to precisely measure coronary motion within diastole.\r\nTissue-Doppler waveforms of the myocardium corresponding to the location of the circumflex artery (100 patients) and\r\nmid-right coronary arteries (50 patients) and the duration and timing of coronary motion were measured. Using\r\nregression analysis an equation was derived for the timing of the period of minimal coronary motion within the RR\r\ninterval. In a validation set of 50 clinical cardiac CT examinations, we assessed coronary motion artifact and the effect of\r\nusing a mid-diastolic imaging target that was adjusted according to heart rate vs a fixed 75% phase target.\r\nResults: Tissue Doppler analysis shows the period of minimal cardiac motion suitable for CT imaging decreases almost\r\nlinearly as the RR interval decreases, becoming extinguished at an average heart rate of 91 bpm for the circumflex\r\n(LCX) and 78 bpm for the right coronary artery (RCA). The optimal imaging phase has a strong linear relationship with\r\nRR duration (R2 = 0.92 LCX, 0.89 RCA). The optimal phase predicted by regression analysis of the tissue-Doppler\r\nwaveforms increases from 74% at a heart rate of 55 bpm to 77% at 75 bpm. In the clinical CT validation set, the optimal\r\nCT acquisition phase similarly occurred later with increasing heart rate. When the selected cardiac phase was adjusted\r\naccording to heart rate the result was closer to the optimal phase than using a fixed 75% phase. While this effect was\r\nstatistically significant (p < 0.01 RCA/LCx), the mean effect of heart-rate adjustment was minor relative to typical\r\nbeat-to-beat variability and available precision of clinical phase selection.\r\nConclusion: High temporal resolution imaging of coronary motion can be used to predict the optimal acquisition\r\nphase in cardiac CT. The optimal phase for cardiac CT imaging within mid-diastole increases with increasing heart rate\r\nalthough the magnitude of change is small.
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