Study design―This was a retrospective, single-center and observational study to compare the cross-sectional data measured by IVUS and OCT at the same observational point of the reverberation signal for patients with calcification in Nanjing First Hospital. From Apr 2014 to Oct 2020, reverberation phenomena were found by IVUS imaging detection in a total of fifty lesions in forty-nine patients with severe calcifications, and OCT detection in the corresponding segment was also performed at the same time. Patients were excluded if they did not receive both IVUS and OCT detection at the same time, IVUS did not find a reverberation phenomenon, or the OCT imaging quality was not high enough for measurement of the calcification thickness at the corresponding segment. If both IVUS and OCT imaging met the requirements simultaneously (IVUS found reverberation and OCT could measure the thickness of the calcification precisely), reverberation measurements were repeated every 1 mm, and the calcification thickness at the same level and direction was measured at the same site. Ultimately, 428 valid points with reverberation in 49 patients met the criteria of our study and were enrolled in the analysis. The study was approved by the institutional review board, and written informed consent was obtained from all patients.
IVUS and OCT images acquisition―Both IVUS and OCT images were acquired after nitroglycerin intracoronary injection for the same target vessel. IVUS (iLAB, Boston Scientific Corporation, Marlborough, Massachusetts) detection was performed by pulling back automatically at a speed of 0.5 mm/sec with a 40-MHz, 2.6F imaging catheter (Atlantis™ SR Pro, Galaxy®, Boston Scientific, Natick, MA). For OCT image checking, both ILUMIEN OPTIS and C7-XR (Lightlab Imaging Incorporated, Westford, MA) could be applied by a 2.7F (Dragonfly OPTIS or Dragonfly Duo imaging catheter, Westford, MA) catheter automatic pullback at a speed of 36 mm/sec with continuous contrast injection (3–4 ml/s).
IVUS and OCT images analysis―Off-line IVUS image data were analyzed by planimetry EchoPlaque 4.0 software (Index Medical Systems, Santa Clara, CA). Coronary calcification in IVUS was defined as a region with a hyperechoic leading edge compared to the adventitia with acoustic shadowing, which was categorized as superficial calcification when the leading edge appeared near the intima of the lumen or deep calcification appeared near the adventitia or mixed calcification, which included superficial and deep calcifications.(5, 10) Reverberation in IVUS imaging is considered one type of artifact represented by secondary, false echoes of the same structure and caused by a smooth leading edge of calcification,(5, 10, 11) which is usually found post RA. Off-line OCT images were analyzed using Lightlab OPTIS, E. 4 software (Lightlab Imaging Incorporated, Westford, MA). Calcification in OCT was defined as a signal-poor and heterogeneous region with sharply delineated near and far boundaries. (4, 7, 12, 13) The calcification thickness of each observational point was measured in the case of consistent OCT and IVUS observational points.
IVUS and OCT images were analyzed after manual coregistration by fiduciary side branch (position and direction as the marker of calibration) and known pullback speeds (the observational point was calculated by counting the slice number by the frame interval to the marker of calibration).(5, 7) Valid observational points were defined as follows: Radial positioning of observation points: in accordance with the direction indicated by the marker of calibration (referring to the direction of the side branch emission), the other three observation points in the same plane (i.e., there might be four observation points in each plane) were defined in the clockwise direction of the cross every 90 degrees. If there was a reverberation signal at this point, the observational point was enrolled in the study; otherwise, the point was excluded. Longitudinal positioning of the observation point was performed according to the calibration indicator point (with the center point of the side branch ostium as reference), accurately positioning the observation point of IVUS and OCT by the frame-counting method (Fig. 1).
The measurement indices of IVUS for each enrolled point were defined as follows: the number of layers of reverberation signal at each observational point: counting the number of layers of the radial reverberation signal; the interval between two adjacent reverberation signals in the radial direction (interval1: the distance between the inner reverberation signal and the second reverberation signal; interval2: the distance between the second reverberation signal and the third reverberation signal; interval3: the distance between the third reverberation signal and the fourth reverberation signal; and interval4, interval5 and so on in a similar fashion, Fig. 1); and Di: the distance between the IVUS catheter center and the inner surface of the calcification (inner reverberation signal, Fig. 1). The measurement index of OCT for each enrolled point was the thickness of the calcification in the corresponding site.
All imaging data (IVUS and OCT measurement indices post manual coregistration) that met the requirements were analyzed offline by two independent professional technicians who were blinded to the clinical information. If these two technicians’ judgments diverged, another experienced technician worked with them until a consensus was reached. Intra- and interobserver variabilities of the image analysis were assessed by measuring 40 enrolled points randomly for both IVUS and OCT data. The intra- and interobserver reproducibilities of image analyses were assessed by Kappa statistics for categorical variables or intraclass correlation coefficients (ICCs) for continuous variables. There was very good intra- and interobserver consistency for the number of layers of reverberation by IVUS detection (Kappa: 0.992, 0.996), the distance between two layers of reverberation signals (ICC: 0.993, 0.998), the distance between the IVUS catheter center and the inner reverberation signal (ICC: 0.924, 0.997) and the calcification thickness by OCT measurement at the same point and the direction relative to the IVUS data (ICC: 0.990, 0.927). Finally, IVUS and OCT data were compared point-by-point to confirm the correlation between the reverberation signal and the calcification thickness.
Statistical analysis―Categorical variables are expressed as frequencies and counts, whereas continuous variables are expressed as means ± standard deviations or as medians with interquartile ranges (IQR), as appropriate. Categorical variables were compared by the chi-square test, and the normality of continuous variables was analyzed by the Shapiro–Wilk test. Nonnormally distributed continuous variables are shown as medians and first and third quartiles and were compared by the Mann–Whitney U or Kruskal–Wallis test with post hoc analysis by the Dunn-Bonferroni test. To study the correlation between reverberation and calcification thickness, a univariable Cox regression was performed for all variables, including the number of layers of reverberation signal, the interval between two adjacent reverberation signals and Di. The independent association of several variables with reverberation was evaluated using forward stepwise Cox regression analysis if possible. All statistical tests were 2-tailed, and a p value < 0.05 was considered to indicate statistical significance. Statistical analysis was performed with SPSS software, version 18.0 (SPSS 18, Inc., Chicago, Illinois) and Windows version R 4.0.5 software (https://www.r-project.org/).