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Cardiac Imaging |
1 From the Institute of Diagnostic Radiology (S.L., S.W., T.B., L.D., B.M., H.A.), the Clinic for Cardiovascular Surgery (A.P.), and the Cardiovascular Center (P.K., T.S., P.A.K.), University Hospital Zurich, Raemistrasse 100, CH-8091 Zurich, Switzerland; and the Center for Integrative Human Physiology, University of Zurich (P.A.K.). Received August 17, 2005; revision requested October 20; revision received December 29; final version accepted February 1, 2006. Supported by the National Center of Competence in Research, Computer Aided and Image Guided Medical Interventions of the Swiss National Science Foundation and by the Georg und Bertha Schwyzer-Winiker-Stiftung, Zurich, Switzerland. Address correspondence to H.A. (e-mail: hatem.alkadhi{at}usz.ch).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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Materials and Methods: The study protocol had local ethics committee approval; written informed consent was obtained. There were 125 patients (45 women, 80 men; mean age, 59.9 years ± 12.9 [standard deviation]; 79 receiving ß-blockers) who underwent 64-section CT coronary angiography with retrospective electrocardiographic gating. Data sets were reconstructed in 5% steps from 20% to 80% of R-R interval. Heart rate variability was calculated as 1 standard deviation from mean rate during scanning. Two observers rated image quality of each coronary segment at least 1.5-mm diameter (1 = no motion artifacts, 5 = not evaluative). Repeated analysis of variance measurements were performed to evaluate quantitative parameters. Pearson correlation analysis was performed to compare image quality in each patient with average heart rate and heart rate variability.
Results: Average heart rate was 63.3 beats per minute ± 13.1, with variability of 3.2 beats per minute ± 2.1. Diagnostic image quality (score 
3) was attained in 1821 of 1836 segments at the best reconstruction interval. There was no correlation between mean heart rate and image quality for all segments of the right coronary and left anterior descending arteries, but there was a significant correlation for left circumflex artery (r = 0.33, P < .05). Heart rate variability was correlated with image quality overall (r = 0.75, P < .001) and for each coronary artery. Heart rate was less variable and image quality was better (P < .05) in patients receiving ß-blockers. Best image quality was obtained in diastole with heart rate less than 80 beats per minute and in systole with faster heart rate.
Conclusion: Coronary angiography with 64-section CT provides diagnostic image quality within a wide range of heart rates. Reducing average heart rate and heart rate variability is beneficial for reducing artifacts.
© RSNA, 2006
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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The introduction of 64-section CT enables further improvement in temporal resolution and speed of volume coverage (9). The initial experience with this scanner type has shown a sensitivity of 94% and a specificity of 97% for the diagnosis of substantial (>50% of luminal diameter reduction) coronary artery stenoses (10), but the influence of heart rate on image quality was not assessed. Furthermore, variability in heart rate during scanning, which has been suggested to have an effect on image quality (3,7,11), has not yet been investigated with four–detector row, 16–detector row, or 64-section CT, to our knowledge. Thus, the purpose of our study was to evaluate prospectively the effect of average heart rate and variability in heart rate on image quality at 64-section CT coronary angiography.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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CT Data Acquisition and Postprocessing
All CT examinations were performed with a 64-section CT scanner (Somatom Sensation 64; Siemens Medical Solutions, Forchheim, Germany). The scanning range covered the entire heart from the level of the tracheal bifurcation to the diaphragm. An 80-mL bolus of iodixanol (Visipaque 320, 320 mg/mL; Amersham Health, Buckinghamshire, England) followed by 30 mL of saline solution was continuously injected into an antecubital vein through an 18–20-gauge catheter with an injection rate of 5 mL/sec. Contrast agent administration was controlled by means of bolus tracking. One radiologist (L.D., who had 2 years of experience in cardiovascular radiology) placed a region of interest (mean diameter, 12.5 mm ± 3.2; range, 8.0–16.5 mm) in the aortic root, and image acquisition started 5 seconds after the signal attenuation reached the predefined threshold of 140 HU.
Data were acquired in a craniocaudal direction with a detector collimation of 32 x 0.6 mm, a section collimation of 64 x 0.6 mm by means of a z-flying focal spot, a gantry rotation time of 370 msec, a pitch of 0.24, a tube voltage of 120 kV, and a tube current of 650–780 mAs. The electrocardiogram (ECG) was digitally recorded during data acquisition and was stored with the unprocessed CT data set. When automatic positioning of the R-wave indicators by the software failed, a radiologist (L.D.) manually repositioned the indicators to improve the quality of synchronization.
Thirteen CT data sets that were synchronized to the ECG data were retrospectively reconstructed from 20% to 80% of the R-R interval in 5% steps for each patient. The adaptive cardiac volume approach was used for image reconstruction; this approach automatically switches between one- and two-segment reconstruction depending on the patient's heart rate (12). Images were reconstructed with a section thickness of 0.75 mm, a reconstruction increment of 0.5 mm, and a medium soft-tissue convolution kernel (B30f). The field of view was adjusted to encompass the heart exactly (mean field of view, 154 mm ± 17; range, 129–180 mm). After patient and ECG information was removed, all reconstructed images were transferred to a dedicated workstation (Second Wizard with InSpace4D application; Siemens).
CT was successfully performed in all patients without complications. The CT protocol was well tolerated by all patients, and all were able to hold their breath during data acquisition (mean breath-hold duration, 11.8 seconds ± 0.6; median, 11.6 seconds; range, 10.2–12.8 seconds).
CT Data Analysis
For data analysis, coronary segments were defined according to American Heart Association guidelines (13), and all segments with a diameter of at least 1.5 mm at their origin were included. The cutoff point was set at 1.5 mm because the spatial resolution of 64-section CT was considered insufficient for accurate evaluation of smaller-diameter vessels. Diameters were measured with an electronic caliper tool. The right coronary artery (RCA) was defined to include segments 1–4, the left main (LM) and left anterior descending (LAD) arteries were defined to include segments 5–10, and the left circumflex (LCX) artery was defined to include segments 11–15.
All reconstructed images were evaluated and classified by two independent readers (S.L. and H.A., each with 3 years of experience in cardiovascular radiology). Images included transverse source images, multiplanar reformations, and thin-slab maximum intensity projections. For each coronary segment, both readers inspected all 13 data sets from 20% to 80% of the R-R interval, noting the presence of motion artifacts and assessing image quality semiquantitatively by using a previously described five-point ranking scale (14) in which a score of 1 indicated no motion artifacts and clear delineation of the segment; a score of 2, minor artifacts and mild blurring of the segment; a score of 3, moderate artifacts and moderate blurring without structure discontinuity; a score of 4, severe artifacts and doubling or discontinuity in the course of the segment; and a score of 5, image not evaluative and vessel structures not differentiable.
Examples of image quality scoring are shown in Figure 1. A score of 3 or lower was considered acceptable in terms of image quality for routine clinical diagnostic purposes. The optimal reconstruction window containing the fewest motion artifacts was individually defined for each coronary segment by the same two readers. For any disagreements in data analysis, consensus agreement was used.
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Statistical Analysis
Statistical analysis was performed with a commercially available statistical software (SPSS, version 11.5 for Windows; SPSS, Chicago, Ill). Quantitative variables were expressed as means ± standard deviations, and categorical variables were expressed as frequencies or percentages. We took into account the clustered nature of the data (ie, the fact that there were not 1836 independent vessel segments but instead clusters of segments in 125 patients). Interobserver agreement for image quality and best reconstruction interval readout were calculated with Cohen 
statistics (15) and interpreted according to the guidelines of Landis and Koch (16) by using the clustered data. Repeated-measures analysis of variance was used to compare effects of ß-receptor–blocking medication on average heart rate and heart rate variability with clustered data. The linear regression correlation between heart rate and the percentage of coronary segments with the best reconstruction interval in diastole or systole was calculated for each patient to identify the cutoff point, for that average heart rate in general, where shifting of best image quality from the diastolic to the systolic phase occurred. For each patient, Pearson correlation analysis was performed to compare the image quality score for all segments together and the scores for separate arteries (RCA, LM, LAD, and LCX) with the average heart rate and its standard deviation during CT scanning. A P value less than .05 indicated a statistically significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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Image Quality of Coronary Artery Segments
A total of 1836 segments with a diameter of at least 1.5 mm were evaluated in the 125 patients (12 segments were missing because of anatomic variations, and 27 segments had a diameter of less than 1.5 mm at their origin). When the best reconstruction interval was used, images without motion artifacts (score 1) were obtained in 960 of the 1836 coronary segments (52%); images with minor artifacts (score 2), in 607 segments (33%); and images with moderate artifacts (score 3), in 254 segments (14%). Severe artifacts (score 4) occurred in 0.8% of coronary segments (15 of 1836 segments, including six RCA segments, three LM and LAD artery segments, and six LCX artery segments), even at the best reconstruction interval between 20% and 80% of the R-R interval. No coronary segment had images rated as not evaluative (score 5) by both readers. Therefore, with the best reconstruction interval, image quality was diagnostic (score 3) in 99% of all segments (1821 of 1836). Interobserver agreement for image quality rating with clustered data was good ( = 0.70). Immediate agreement between both observers was achieved in 1320 of the coronary segments (72%). In the remaining 516 segments (28%) consensus reading was required to discriminate between image quality scores of 2 and 3 (398 of 516 segments [77%]) and between scores of 1 and 2 (118 of 516 segments [23%]).
Effect of Average Heart Rate and Variability of Heart Rate on Image Quality
No significant correlation was found between the average heart rate and the mean image quality scores for all coronary segments in each patient (r = 0.22, P = not significant) (Fig 2). There was also no significant correlation between the average heart rate and the image quality scores for the RCA (r = 0.15, P = not significant) and the LM and LAD arteries (r = 0.16, P = not significant), but there was a significant, though weak, correlation for the LCX artery (r = 0.33, P < .05).
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= 0.74). The average heart rate had an effect on the optimal reconstruction phase for imaging coronary arteries. At heart rates less than 85.5 beats per minute, the best image quality occurred in diastole (50%–80% of the R-R interval) in more than 50% of the coronary segments. With higher heart rates, the best reconstruction time shifted to end systole (25%–40% of the R-R interval) (Fig 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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Heart Rate and Image Quality
A significant inverse correlation between average heart rate and image quality has been observed with four–detector row CT (3,7,8,19) and 16–detector row CT (5,20). In most of these studies, image quality could not be systematically analyzed because evaluation was limited to the question of whether or not the vessels were visible (3,8,19,20). With four–detector row CT and a gantry rotation time of 500 msec, Hong et al (7) found that image quality was significantly decreased in each coronary artery with increasing average heart rate. Using 16–detector row CT and a 420-msec gantry rotation time and employing a combined score for the coronary arteries, Hoffmann et al (5) found a negative correlation between overall image quality and average heart rate. In our study, with 64-section CT and a gantry rotation time of 370 msec, the image quality for the LCX artery showed a weak dependence on the average heart rate, but no correlation was found for the RCA, the LM and LAD arteries, and all segments together. We individually rated each coronary segment because movement patterns are unequally distributed throughout the entire vessel tree (14). By selecting the optimal reconstruction time point for each vessel segment, we found no motion artifacts in 52% and minor to moderate motion artifacts in 47% of all coronary segments. Severe artifacts leading to nondiagnostic image quality were present in only 0.8% of segments and were evenly distributed throughout the coronary tree.
The motion of the LCX artery follows the motion of the left ventricle; therefore, the best reconstruction interval is in mid-diastole, at 50%–70% of the R-R interval, when the left ventricle is relatively quiescent (17). In contrast, motion of the RCA is mainly determined by the contraction of the right atrium, so the best reconstruction time interval is earlier in late systole and early diastole, at 30%–60% of the R-R interval, when the right atrium is relatively motionless (17). Because with higher heart rates the diastole shortens more than the systole (21), imaging of the RCA in late systole and early diastole is less prone to motion artifacts when shortening of the diastole occurs. The image quality of the LCX artery in mid-diastole is more impaired with higher heart rates and shortening of the diastolic phase. The LM and LAD arteries also follow left ventricular motion, but their velocity of motion is significantly lower than those of the RCA and the LCX artery (17), so high-quality images of the LM and LAD arteries could probably be obtained even at higher heart rates.
Discrepancies between our results and those from previous studies (5,7) could derive from differences in image reconstruction (ie, the adaptive cardiac volume approach used in our study vs the multicyclic reconstruction algorithm of Hoffmann et al [5]), differences in scanner types, or the use of multiple reconstruction intervals. The slightly improved temporal resolution of 64-section CT, the larger volume coverage of 32 x 0.6 mm per rotation with the decrease in total scanning time to approximately 12 seconds, and the improved longitudinal resolution (9) may explain why image quality with 64-section CT is less dependent on heart rate than the image quality achievable with earlier scanner generations.
The most important finding of our study is that the regularity of heart rate during scanning is a major determinant of image quality in coronary CT angiography. With intercycle variability in the heart rate, the commonly applied relative ECG-gated image reconstruction technique (ie, performing reconstructions at a certain percentage of the R-R interval) does not generate images in exactly corresponding cardiac phases. This is because the different functions within one cardiac cycle shorten or prolong nonproportionally with different heart rates (21). Previous studies with four– and 16–detector row CT in which the effect of the average heart rate was analyzed (5,7) did not take into account the effect of heart rate variability on image quality. The reported results regarding average heart rate may therefore have been confounded by additional heart rate variability.
Heart Rate and Optimal Reconstruction Window
Coronary motion has a biphasic pattern of rapid movement, with maximal motion during ventricular contraction at early to midsystole and during rapid filling in early diastole (18). At mid-diastole during isovolumetric relaxation and during mid- to late systole, coronary motion is relatively quiescent. With higher heart rates, the motion-free time in mid-diastole shortens more than that in mid- to late systole (21). Evaluation of coronary arteries should be performed in diastole when the vessel flow is maximized (22). After a study that involved four–detector row CT with a 500-msec gantry rotation time, Herzog et al (21) hypothesized that at heart rates higher than 65 beats per minute, only a systole-based reconstruction would lead to sufficient image quality. A more recent study involving 16–detector row CT and a 420-msec gantry rotation time revealed a shift from the diastolic to the systolic frame for optimal image reconstruction between 72 and 80 beats per minute (5). With 64-section CT and a 370-msec gantry rotation time, we observed this shift to occur at an average heart rate of 85.5 beats per minute. This increase in heart rate threshold could be explained by the increase in temporal resolution at 64-section CT, which allows diagnostic imaging in diastole even when this motion-free time interval is shortened. On the other hand, the difference in our findings may have been caused by our data analysis method, which enabled reconstructions in 5% steps during the R-R interval rather than preselected reconstruction windows only at 50% and 80% (5).
ß-Blockers and Image Quality
Reduction of heart rate with ß-blocker medication has been reported to improve the image quality of CT coronary angiography (14). However, to our knowledge, the effect of regularizing heart rate for CT coronary angiography with ß-blockers has not yet been investigated. In our study, heart rate variability was significantly lower, and image quality was therefore superior, in patients who were receiving ß-blockers than in those who were not. A possible implication of these results might be that patients ascertained to have irregular heart rhythm before 64-section CT examination may benefit from ß-blocking medication even when their heart rates are normal or only slightly elevated. In patients with higher heart rates but regular rhythm, 64-section CT coronary angiography can be performed and diagnostic image quality can be obtained without ß-blocking medication.
Limitations
We acknowledge the following limitations of our study. First, the image quality scoring system may have been influenced by a subjectivity bias. On the other hand, values of 0.72 and 0.80 indicate good interobserver agreement and may thus argue against such a bias. Second, our investigation did not analyze the influence of heart rate on diagnostic accuracy. We did not include coronary artery stenosis detection in our study, which may have been biased due to an incomplete data set. Third, no patients with heart rates above 102 beats per minute were included in our study; this may have limited our descriptive statistics. Finally, additional studies in larger patient populations are needed to prove the effect of ß-blockers on heart rate.
In conclusion, 64-section CT reliably provides diagnostic-quality images of the coronary arteries within a wide range of heart rates (38–102 beats per minute). Average heart rate has no negative effect on overall image quality, but variations in heart rate significantly degrade the image quality of the whole coronary tree. ß-Blocking medication administered before CT lowers and stabilizes the heart rate and thus is beneficial for image quality. By lowering the heart rate, ß-blocking medications enable optimal CT image quality in diastole, the phase in which coronary imaging should be performed.
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ADVANCES IN KNOWLEDGE |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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FOOTNOTES |
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Abbreviations: ECG = electrocardiogram • LAD = left anterior descending • LCX = left circumflex • LM = left main • RCA = right coronary artery
Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, S.W., T.B., B.M., H.A.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, S.L., H.A.; clinical studies, all authors; statistical analysis, H.A.; and manuscript editing, S.W., T.B., L.D., A.P., P.K., T.S., B.M., P.A.K., H.A.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEReferences |
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L. Husmann, S. Leschka, L. Desbiolles, T. Schepis, O. Gaemperli, B. Seifert, P. Cattin, T. Frauenfelder, T. G. Flohr, B. Marincek, et al. Coronary Artery Motion and Cardiac Phases: Dependency on Heart Rate Implications for CT Image Reconstruction Radiology, November 1, 2007; 245(2): 567 - 576. [Abstract] [Full Text] [PDF]
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D. Matt, H. Scheffel, S. Leschka, T. G. Flohr, B. Marincek, P. A. Kaufmann, and H. Alkadhi Dual-Source CT Coronary Angiography: Image Quality, Mean Heart Rate, and Heart Rate Variability Am. J. Roentgenol., September 1, 2007; 189(3): 567 - 573. [Abstract] [Full Text] [PDF]
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L. J. M. Kroft, A. de Roos, and J. Geleijns Artifacts in ECG-Synchronized MDCT Coronary Angiography Am. J. Roentgenol., September 1, 2007; 189(3): 581 - 591. [Abstract] [Full Text] [PDF]
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= outlier. (b) Box plot shows standard deviation of heart rate in patients receiving and those not receiving ß-blockers. Heart rate variability was significantly decreased in patients receiving ß-blockers (repeated-measures analysis of variance, P < .05). Box = 1st–3rd quartiles, bold line = median, whiskers = minimum and maximum values, 