Catheters usually are used for calibration purposes in quantitative coronary angiography (QCA). The systematic and random errors in these calibration factors (CFs) are dependent on the size and quality of the catheters and limited by out-of-plane magnification (OPM). Theoretically, a guide wire with evenly spaced marker bands would solve many of these potential problems. For this reason, we tested the Cordis Stabilizer marker wire, featuring 10 radiopaque platinum marker bands 15 mm apart, in in vitro and in vivo studies. To assess the effect of foreshortening, wires were positioned in a tube phantom; a centimeter grid was used as the gold standard. Radiographic images were acquired at 5-inch and 7-inch image-intensifier sizes, 512(2) and 1,024(2) matrix sizes and angulations from 0 degrees to 70 degrees in steps of 10 degrees. It was concluded that the relative errors in CFs are less than 7% if the foreshortening angles remain less than 20 degrees. In DICOM images of 15 patients, 65 measurements were taken after calibration on an 8F catheter and on a guide wire positioned in the coronary lesion. In all but two cases, the wire CFs were larger than the catheter CFs (relative difference, 24.7 +/- 19.6%). The measurements were divided into four groups: (I) no apparent OPM or foreshortening (n = 7), (II) only OPM (n = 4), (III) only foreshortening (n = 10), and (IV) the combination of both (n = 44). In group I (no OPM or foreshortening) the QCA results were similar using the guide wire or catheter as the calibration device (relative CF difference, 2.9% only). In group III the diameters were overestimated using the guide wire (obstruction diameter difference, 0.22 +/- 0.11 mm; reference diameter difference, 0.35 +/- 0.06 mm). For only OPM (group II) and the combination of OPM and foreshortening (group IV), the lesion length was underestimated on average by 2.4 mm using the catheter instead of the guide wire. In conclusion, if accurate assessment of the lesion length is important, the marker wire should be used for calibration purposes. For vessel diameter measurements, the conventional catheter calibration approach is the method of choice.
To assess the magnitude of differences in QCA outcomes between two cooperating core laboratories in a single trial, we have carried out an inter-core laboratory variability study. Two QCA experts at the Montreal Heart Institute and Heart Core Leiden both analyzed 32 lesions (pre- and post-intervention) in accordance with previously agreed upon standard operating procedures. One of the experts analyzed the whole image set twice to determine the intraobserver variability. The inter-core laboratory differences in the acute gain (n = 31 pairs) are non-significant. The systematic errors of the individual measurements (n = 63 analyses) show an excellent intraclass correlation coefficient of reliability (>75%), except for the stent length (67.7%). The corresponding random errors are small. In general, the intra-observer systematic and random errors are both slightly smaller than those for the inter-core laboratory study. QCA analyses in clinical trials can be carried out in core laboratories at two different locations if and only if highly standardized conditions are maintained.
Compression ratios of 10:1 and 16:1 affected the QCA results negatively and therefore should not be used in clinical research studies.
With the accepted use of (lossy) data compression at low compression factors (2, 3 and 4 on the Philips DCI), the question was posed whether higher lossy compression ratios can also be used without statistically affecting the results of quantitative coronary arteriography. In this study the influence of two data compression schemes (LOT and JPEG) at three different compression factors (5, 8 and 12) on coronary measurements was assessed with the Automated Coronary Analysis (ACA) package. A series of 30 original acquired digital images were compressed and decompressed at the different factors, and together with the original non-compressed images processed using the ACA package. In these images a total of 37 obstructed coronary segments were analyzed twice to assess the intra-observer variabilities in the obstruction and reference diameters and in the percent diameter stenosis. The results of the first and second measurements in each image were averaged, and from the differences in corresponding images with different compression ratios, the inter-compression variability was obtained. The results show that the intra-observer systematic errors in the absolute diameters are all small (< 0.07 mm), and statistically not significantly different. The intra-observer random errors for the compressed/decompressed series, however, were all larger (up to 0.21 mm) than for the original series (< 0.13 mm). Statistically significant differences in the intra-observer random errors were found for the JPEG compression scheme at a compression ratio of 5 and for the LOT scheme at a compression ratio of 12. The inter-compression systematic errors in the absolute diameter measurements were also small (< 0.07 mm) and not significant, while the random errors were found to be high in the range between 0.23 mm and 0.31 mm. Given the higher intra-observer variabilities for the compressed/decompressed image series as compared to original images, and the fact that all inter-compression variabilities were found to be so high, we must conclude that the higher compression ratios affect the results of QCA in a negative sense. In conclusion, the use of lossy data compression with JPEG or LOT compression schemes at ratios 5, 8 and 12 must be discouraged for QCA.
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