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Intracoronary ultrasound (ICUS) provides highresolution tomographic images of selected segments of coronary arteries. Series of cross-sectional images are acquired with motorized pullback imaging catheters and used for quantitative analysis in intracoronary ultrasound studies (ICUS). IntroductionIntracoronary ultrasound (ICUS) gives insight into the composition and extent of atherosclerotic plaque [1]. Previous studies have shown that ICUS may visualize atherosclerotic plaques in angiographically normal coronary arteries. ICUS is used in clinical trials to evaluate the results of novel catheter-based interventional techniques as well as pharmaceutical treatments.Cross-sectional ICUS images are acquired during a continuous speed pullback of the catheter in the coronary artery. In quantitative analysis procedures vessel wall borders are traced in a consecutive number of images. To assess vessel wall morphology and to facilitate quantitative analysis, three-dimensional (3D) reconstruction is performed using quantitative coronary ultrasound (QCU) software that visualizes segments by longitudinal cut-planes (L-views). This procedure avoids the time-consuming manual tracing of a series of individual cross-sections. However, cyclic systolicdiastolic changes of vascular dimensions and catheter motion result in saw-tooth shaped image artifacts in the L-views that significantly hamper the quantitative analysis. (Semi-)automatic contour detection is interfered, the analysis process becomes time-consuming and the procedure may produce inaccurate results.Most QCU analysis software packages acquire ICUS images stored on videotape at a rate of 1 frame per two seconds, randomly within the cardiac cycle, resulting in 1 mm intervals between the frames (assuming the catheter is pulled back with 0.5 mm/s), for use in area measurement and subsequent calculation of volumetric quantitative parameters. However, it has been reported that longitudinal catheter motion of more than 5 mm may occur during the cardiac cycle.Previous studies have shown that an on-line ECGgated pullback procedure overcomes this motion problem and allows more accurate and reproducible measurements [2]. However, the technology requires expensive hardware, long setup times and considerably prolongs the acquisition procedure. Most laboratories still use non-gated acquisition and most existing image databases lack ECG-gated-data. Therefore, we developed the fully automated retrospective image-based gating method "Intelligate", that can select end-diastolic ICUS frames enabling fast and accurate analysis of ICUS studies. In this paper we describe this new method and its validation. The Intellgate methodThe Intelligate method selects ICUS images recorded in the end-diastolic phase. The rationale for selecting this phase is the mutual comparability of the images as the heart is relatively motionless here and blood flow has ceased. This means that forces originating from cardiac motion and the blood flow are no longer acting on the catheter. In the end-diastoli...