IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 1, FEBRUARY

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 1, FEBRUARY 2004 225 Analysis of the Heart Rate and Its Variation Affecting Image Quality and Optimized Reconstruction Window in Retrospective ECG-Gated Coronary Angiography Using Multidetector Row CT Sang Ho Lee, Byoung Wook Choi, *Hee-Joung Kim, Member, IEEE, Haijo Jung, Hye-Kyung Son, Won-Suk Kang, Sun Kook Yoo, Kyu Ok Choe, and Hyung Sik Yoo Abstract It is clinically important to examine the effect of the heart rate and its variation on the image quality and selection of the optimized window in coronary angiography using multidetector row CT (MDCT). This study performed contrast-enhanced coronary angiography using MDCT on 83 patients. Fifty-two cases with information on the heart rate available were enrolled in this study. The effect of heart rate and its variation were systemically analyzed. Two radiologists rated the image quality as follows: 4 excellent; 3 good; 2 fair; 1 bad. Cardiac cycle windows at 40% and 70% were routinely selected for image reconstruction. The optimized window was rated as 1 when a 40% reconstruction had a better quality than the 70% reconstruction, as 2 when the 40% reconstruction was the same as the 70% reconstruction, and as 3 when the 70% reconstruction was better than the 40% reconstruction. The image quality was more affected by a variation of the heart rate than by the high heart rate. The selection of the optimized reconstruction window for a good image quality was mostly affected by the heart rate and there was a tendency for the 40% phase reconstruction to have a better image quality than the 70% reconstruction at higher heart rates. Index Terms Beats per minute (BPM), coronary angiography, ECG-correlated CT, image quality, multidetector row CT (MDCT), optimized reconstruction window, variation of heart rate. I. INTRODUCTION X-RAY coronary angiography is widely used as the diagnostic gold standard for coronary artery disease. It is an invasive procedure and the catheterization involves some discom- Manuscript received December 18, 2002; revised October 29, 2003. This work was supported in part by a Grant from the Brain Korea (BK) 21 Project for Medical Sciences and in part by the Basic Research Program of the Korea Science and Engineering Foundation (KOSEF) under Grant R01-2002-000-00205-0 (2002), Yonsei University. Asterisk indicates corresponding author. S. H. Lee, *H.-J. Kim, H.-K. Son, and W.-S. Kang are with the BK21 Project for Medical Sciences, Research Institute of Radiological Science, Yonsei University College of Medicine, Seoul 120-752, Korea (e-mail: shlee1@yumc.yonsei.ac.kr; hjkim@yumc.yonsei.ac.kr; hkson@yumc.yonsei. ac.kr; wskang@yumc.yonsei.ac.kr). B. W. Choi, H. Jung, H. S. Yoo, and K. O. Choe are with the Research Institute of Radiological Science, Department of Radiology, Yonsei University College of Medicine, Seoul 120-752, Korea (e-mail: bchoi@yumc.yonsei.ac.kr; hjjung1@yumc.yonsei.ac.kr; hsyoo@yumc.yonsei.ac.kr; kochoe@yumc. yonsei.ac.kr). S. K. Yoo is with the Department of Medical Engineering, Yonsei University College of Medicine, Seoul 120-752, Korea (e-mail: sunkyoo@yumc.yonsei.ac.kr). Digital Object Identifier 10.1109/TNS.2004.825973 fort to the patient. Therefore, conventional angiography should be undertaken only after strict clinical indications. Cross-sectional computed tomography (CT) imaging has been used continuously to visualize the coronary arteries in radiology. At the early stage, the use of conventional nongated helical CT has limitations in accurately displaying the coronary arteries because of the small caliber of the coronary vessels, their complicated shape, the tortuous course and motion artifacts resulting from a cardiac contraction [1]. For artifact-free cardiac imaging, a scanning time less than 50 ms was suggested by Ritchie et al. [2], [3]. Electron beam CT (EBCT) [4] [11], with its 100 ms scanning time, synchronized to the cardiac cycle by using prospective electrocardiographic (ECG) triggering, was proposed as a noninvasive imaging modality for diagnosing coronary artery disease by Achenbach et al. [4]. It is mainly used for the CT angiography of the coronary arteries, for quantifying the coronary artery calcium level and evaluating the cardiac morphology and movement. Reconstruction of the images is possible at only one point of the cardiac cycle, which is determined by the prescribed trigger delay time at which the minimum cardiac motion is expected. The results can be optimal for only one of the three major coronary arteries because each has a different motion pattern [12] [14]. Moreover, the vascular sections are often displayed at an undesirable cutting angle or are affected by partial volume effects as a result of their rapid movements [15]. Actually, this method is also known to cause a high rate of motion artifacts, resulting in 25% of the vessel segments being unanalyzable [4], [5]. The effects of cardiac motion can be reduced by cardiac gating of CT [16], [17]. Gating of the cardiac images to the ECG test can be retrospectively executed by helical CT. A multidetector row CT (MDCT), with a 250 ms image acquisition time, enables a higher signal-to-noise ratio (SNR) and a spatial resolution than EBCT. Moreover, when compared with conventional single-section CT, not only is the acquisition time remarkably decreased, but the z-plane and the in-plane resolution are also improved [1], [18]. Recently, because image quality is highly dependent on the heart rate, it has been advised to make an evaluation at heart rates of less than 65 beats per minutes (BPM) using beta-blocker to achieve best image quality [19]. However, the effect of the 0018-9499/04$20.00 2004 IEEE

226 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 1, FEBRUARY 2004 heart rate and its variation on the image quality has not had a systematic study. In addition, it has been reported that an image reconstruction window for CT angiography of the coronary arteries should be adapted to each coronary artery [20]. With retrospective ECG gating, multiple selection of the image reconstruction window is possible for MDCT coronary angiography. The length of the cardiac cycle is continuously changing depending on the patient s condition in a single breath hold time. As the heart rate increases, the difference between the image acquisition time and the minimum cardiac motion time increases. As this variation increases, an unreliable cardiac gating of the CT data is caused by the cardiac cycles that appear at irregular intervals. Therefore, the aim of this study was to evaluate the influence of the heart rate and its variation on the image quality and optimized window for a good quality reconstruction in retrospective ECG gated coronary angiography using MDCT. II. MATERIALS AND METHODS A. Materials A total of 83 consecutive patients (60 male, 23 female; mean age, 60 years ; range, 35 to 74) underwent contrast-enhanced coronary angiography using MDCT. Fifty-two cases with the available information on the heart rate were enrolled in this study. All the patients were informed of the total scanning duration and were instructed to hyperventilate and hold their breath at the near-maximum lung capacity for each examination. A sinus rhythm was presented at the time of the investigation. The range of BPM variation (mean, ; range, 2 to 65) and the median BPM value (mean, ; range, 50 to 100) were recorded for each patient. Helical scanning (120 kv and 300 ma) was performed within a single breath hold from 1 cm below the carina for the native coronary artery or from a point on the ascending aorta 1 cm above the uppermost graft if it was present on the aorta to the base of the heart, in 30 to 35 s (mean, 33.2 s), depending on the dimensions of the patient. B. MDCT A LightSpeed Plus (GE medical systems, Milwaukee, WI) scanner was used for MDCT. Generations of MDCT scanners operate at a rotation rate of 500 ms and produce up to four slices of 1.25 mm collimation simultaneously, based on spiral algorithm with a constant time resolution of 250 ms per section [21]. A multisector reconstruction algorithm was not available (i.e., the data from only one heart cycle was used to reconstruct an image). C. Scan Protocol After a low-dose pre-contrast helical localization (collimation 2.5 mm, pitch 1.5, 142 kv, 60 ma, rotation time 500 ms), a test bolus of 15 ml of the contrast medium was injected through an 18-gauge catheter into an the antecubital vein in order to determine the circulation time. For the contrast-enhanced scan (collimation 1.25 mm, 120 kv, 300 ma, rotation time 500 ms), 150 ml of the contrast agent (Iopamiro 370 mg/ml, Bracco s.p.a., Milano, Italy) was injected at 4 ml/s during the first half of the total scan time plus a delayed period after injecting the contrast material and at 2 ml/s during the second half. The start Fig. 1. Retrospectively ECG gated four-section spiral CT reconstruction with a temporal resolution of 250 ms. Each rectangular region represents the image slices that can be reconstructed using the corresponding single cardiac cycles. The dashed lines show how the four detector rows travel along the z-axis at a fixed speed. Continuous volume coverage is possible in multiple phases. In this study, 40% and 70% reconstructions of the R-R interval were employed. of the contrast-enhanced scan was adapted to the calculated circulation time. D. ECG Gating Retrospective gating, which allows post-scan acquisition window selection and optimum gating was used in this study [17]. This approach improves the image quality and reduces the sensitivity to arrhythmia and ECG noise. Even if ECG irregularities occur, the retrospective ECG gating cannot be adjusted until the scanning was finished because fixed delays were used for each heartbeat, based on time period of the average heartbeat. E. Image Reconstruction In order to set the best delay suited to obtaining a high image quality, the sections at every given z position were reconstructed at 40% and 70% of the R-R interval, which represents the systolic and diastolic phases, respectively (Fig. 1). This is because the optimal delay is at 40% of the R-R interval for the RCA and 70% of the R-R interval for LAD and LCx [20]. The reconstructed image data was transferred to a computer workstation (Advanced Workstation 4.0, GE Medical Systems) for three-dimensional volume-rendered post-processing using a standardized procedure. F. Evaluation of Image Quality The image quality of the coronary arteries was evaluated in 2-D images with a 40% and 70% window reconstruction by reading the proximal and mid coronary arteries, which were segments of the major coronary arteries (RCA, LAD, and LCx) and are defined according to the American Heart Association (AHA) (Fig. 2). The image quality was rated to each pair of the reconstruction windows by the consensus of two radiologists who worked together to determine a single score for each case, as excellent when all the proximal to mid coronary arteries were assessable, good when two proximal to mid coronary arteries were assessable, fair when all the proximal coronary arteries were assessable, and bad when only one or two proximal coronary arteries can be assessed. In addition, the image quality between the

LEE et al.: HEART RATE AND ITS VARIATION USING MULTIDETECTOR ROW CT 227 Fig. 2. (a) Three-dimensional segmental anatomy of major coronaries obtained after ECG-gated multidetector row CT. Segments were defined according to the American Heart Association (AHA). (b) Conventional angiography for the right coronary artery (RCA) (c) Conventional angiography for the left anterior descending (LAD) and the left circumflex artery (LCx). 40% and 70% reconstruction images was compared. The optimized window was rated as 1 when the 40% reconstruction was a better quality than the 70% reconstruction, as 2 when the 40% reconstruction was the same as the 70% reconstruction, and as 3 when the 70% reconstruction was better than the 40% reconstruction. G. Statistics Linear regression analysis was performed in order to test the correlation between the median BPM value and the BPM variation. Analysis of variance (ANOVA) and post hoc least significant difference (LSD) analysis was used to perform multiple comparisons between group means of median BPM value and BPM variation, and to test the hypothesis that the median BPM value or the BPM variation has an effect on the image quality or the optimized window rating. A two-tailed P value was considered statistically significant. All the statistical analyses were conducted using the SPSS 9.0 statistics software package (SPSS Inc., Chicago, IL). III. RESULTS Fig. 3 shows that the correlation between median BPM value and BPM variation is statistically significant, which generally indicates that BPM variation is narrow at low heart rates and is wide at high heart rates. Table I shows the multiple comparisons between the group means of the median BPM value and the BPM variation on image quality rating. The differences between the group means of both the median BPM value and the BPM variation had statistically significant effects for the image quality rating, and the effect of the BPM variation was relatively higher than that of the median BPM value (ANOVA and, respectively). Table II represents the multiple comparisons between the group means of the median BPM value and BPM variation on the optimized window rating. Although the differences between the group means of both the median BPM value and the BPM variation had no significant effects on the optimized window Fig. 3. Linear regression curve estimation between median BPM value and BPM variation. Their correlation is statistically significant (r = 0:301; p = 0:030). rating, the effect of the median BPM value was relatively higher than that of the BPM variation (ANOVA and, respectively). The box plots shown in Fig. 4(a) and (b) represent the relationship between the image quality and the heart rate and that between image quality and variation of heart rate, respectively. The box plots the 25th percentile, the median (the 50th percentile), the 75th percentile, and outlying or extreme values. The boundaries of the box indicate the 25th percentile and the 75th percentile. The variability can be determined from the length of the box. The horizontal line inside the box represents the median. Lines are drawn from the ends of the box to the largest and smallest values and are called whiskers. The case numbers are used to label the outliers and extremes. The outliers (o) are cases with the values between 1.5 and 3 box-lengths from the 75th or 25th percentile. The extreme values (*) are the cases with values more than 3 box-lengths from the 75th percentile or 25th percentile. Based on the 50th percentile in the box plots, the widest variation of the heart rate was represented at 1 (bad

228 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 1, FEBRUARY 2004 TABLE I MULTIPLE COMPARISONS BETWEEN GROUP MEANS OF THE MEDIAN BPM VALUE AND THE BPM VARIATION ON THE IMAGE QUALITY RATING TABLE II MULTIPLE COMPARISONS BETWEEN THE GROUP MEANS OF THE MEDIAN BPM VALUE AND THE BPM VARIATION ON THE OPTIMIZED WINDOW RATING Fig. 4. Box plots representing the relationship between (a) the image quality and heart rate and (b) the image quality and BPM variation. Based on the 50th percentile, the widest variation of the heart rate is represented at bad score, whereas the highest heart rate is represented at fair score rather than at bad score. The image quality is better in the lower BPM variation. score), whereas the highest heart rate was represented at 2 (fair score) rather than at 1 (bad score). The box plots shown in Fig. 5(a) and (b) represent the relation between the better reconstruction window and the median BPM value, and the relation between better reconstruction window and BPM variation, respectively. Based on the 50th percentile in the box plots, the median BPM value was reduced at a fixed rate from the 1 to 3 on optimized window rating, whereas the BPM variation was narrower than the rest at 2 (40% 70%). Fig. 6 compares the 70% and 40% reconstructed images in a patient with a median heart rate of 54 BPM and a variation of 16 that was scored as bad. The right coronary artery is well visualized only in the 70% reconstructed image because the median BPM value is relatively low. Fig. 7 compares the 40% and 70% reconstructed 3-D images in a patient with a median heart rate of 81 BPM. The 40% reconstruction image shows fewer artifacts than the 70% reconstruction because the median BPM value is relatively high.

LEE et al.: HEART RATE AND ITS VARIATION USING MULTIDETECTOR ROW CT 229 Fig. 5. Box plots representing the relationship between (a) the better-optimized window and heart rate and (b) the better-optimized window and BPM variation. Based on the 50th percentile, the median BPM value is reduced at a fixed rate according to the optimized window rating, whereas the BPM variation is narrower than the rest when 40% reconstruction is the same as the 70% reconstruction. Reconstruction in the 40% phase produces better quality than the 70% reconstruction at the higher heart rates. Fig. 6. Comparison of (a) the 70% and (b) the 40% reconstructed images in a patient with a median heart rate of 54 BPM and a variation of 16 that was rated as bad. The right coronary artery (arrow) is well visualized only in the 70% image. Fig. 8. 70% reconstructed 3-D image in a 69 years old female patient with a median heart rate of 96 BPM and variation of 7. Note severe mismatching of phase slice by slice probably because of high heart rate. Fig. 7. Comparison of (a) 40% and (b) 70% reconstructed 3-D images in a patient with a median heart rate of 81 BPM. The 40% reconstruction image shows fewer artifacts than the 70% reconstruction. Fig. 8 shows the 70% reconstruction 3-D image in a 69 years old female patient with a high median heart rate of 96 BPM and variation of 7. Severe mismatching of phase appears slice by slice even in the lower variation BPM because of the higher heart rate. IV. DISCUSSION From the viewpoint of high heart rate, two elements are essential for reducing the number of cardiac motion artifacts. First, the acquisition time was reduced in order to provide a sufficiently short temporal resolution. In this study, the temporal resolution could not be improved to as low as 125 ms, being the time required to use the image data of two consecutive heart cycles, because a multisector reconstruction algorithm was unavailable. Secondly, the heart rate needs to be reduced. In contrast, heart rate acceleration increases the systolic component of the cardiac cycle relative to the diastolic, a decrease in heart rate in particular to fewer than 75 BPM, reduces it [22]. Therefore, lowering the heart rate will increase the diastolic component and consequently improve the image quality. Although beta receptor-blocking agents can lower the heart rate, their use is associated with the side effects such as a heart failure in patients with heart disease, headache, sleeping disorder, diarrhea, etc.

230 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 1, FEBRUARY 2004 Therefore, a more promising area of research is the development of scanning technologies to provide a high temporal resolution against a high heart rate. From the viewpoint of the variation in the heart rate, the core for the optimal image quality is the precise selection of the cardiac cycle phase. The maintenance of the ECG normality while coping effectively with a wider variation of the heart rate will result in a significant improvement of the image quality. In this study, we determined through linear regression analysis that heart rate and variation of heart rate were positively well correlated with each other (Fig. 3) and that both of them could simultaneously affect image quality and optimized reconstruction window. The further experimental results showed that both the heart rate and the variation in the heart rate made independently significant contributions to the image quality through ANOVA analysis (Table I), whereas there were no significant contributions from the selection of the optimized window (Table II). Moreover, the results suggest that the heart rate as well as the variation in the heart rate mainly affected the image quality and the optimized reconstruction window, respectively. In Fig. 4(a), the median BPM value corresponding to the bad score suggests that the variation of the heart rate at even relatively lower heart rates can cause a further degradation of the image quality. On the other hand, in Fig. 5(b), which shows a wider BPM variation than the rest, when the 40% reconstruction has a better quality than the 70% reconstruction, confirms that the heart rate acceleration increases the systolic component of the cardiac cycle relative to the diastolic, and that the effect of the variation of the heart rate in the systolic phase is relatively less serious than in the diastolic phase at the higher heart rates. A limitation of this study is that special attention was focused only on the proximal and mid coronary arteries, and no distal coronary arteries were examined. The incidence of motion artifacts was high on the images of the distal segments. Because the distal parts were scanned last, the patients cannot hold their breath in the end and a wide variation in the heart rate was created. Although distal parts were not covered and more detailed correlations could not be made, the point in the MDCT with a time resolution of 250 ms was whether or not the clinically important proximal and middle segments can be observed. Several improvement plans for minimizing the image degradation as a result of the high heart and the variation in the heart rate can be suggested from these results. First of all, the temporal resolution of 250 ms in this study is suitable at low heart rates. Multisector reconstruction algorithms should be applied to improve the temporal resolution at high heart rates. However, because the benefit varies considerably throughout the acquisition depending on the instantaneous heart rate, significant improvement in spatial resolution are not warranted, and is will only be achieved as a result of increasing radiation exposure. Therefore, administering beta-blocking agents is necessary to improve image quality in patients of mainly high heart rate under the present circumstances, despite their side effects [23]. Furthermore, our retrospective ECG gating equipment did not allow an adjustment in response to variations in the heart rate during scanning. In order to achieve the selection of a more precisely optimized reconstruction window against a variation in the cardiac cycle, such an adjustment is essential. Finally, as the currently required breath hold time of approximately 35 s was too long, MDCT scanners equipped with a larger number of thinner detector rows and lower rotation time should be used. V. CONCLUSION This study demonstrated the priority among the factors that affect the image quality and optimized the reconstruction window in retrospective ECG gated coronary angiography using MDCT. First, the image quality is more affected by variations in the heart rate than by a high heart rate. Secondly, the selection of an optimized reconstruction window for an enhanced image quality is mainly affected by the heart rate and there is a tendency for an earlier phase reconstruction to have a superior image quality than the later phase reconstruction at higher heart rates. More advanced techniques that can essentially reduce the degree of image degradation, by dealing effectively with a high heart rate and a wide variation in the heart rate during cardiac scanning and post-processing, will be needed to expand the amount of image information and improve the clinical applications. REFERENCES [1] C. Hong, C. R. Becker, A. Huber, U. J. Schoepf, B. Ohnesorge, A. Knez, R. Brüning, and M. F. Reiser, ECG-gated reconstructed multi-detector row CT coronary angiography: Effect of varying trigger delay on image quality, Radiol., vol. 220, pp. 712 717, 2001. [2] C. J. Riechie, J. D. Godwin, C. R. Crawford, W. Stanford, H. Anno, and Y. Kim, Minimum scan-speeds for suppression of motion artifacts in CT, Radiol., vol. 185, pp. 37 42, 1992. [3] R. J. Alfidi, W. J. Maclntyre, and J. R. Hagga, The effects of biological motion on CT resolution, AJR Amer. J. Roentgenol., vol. 127, pp. 11 15, 1976. [4] S. Achenbach, W. Moshage, D. Ropers, J. Nossen, and W. G. Daniel, Value of electron-beam computed tomography for the noninvasive detection of high-grade coronary-artery stenoses and occlusions, N. Engl. J. Med., vol. 339, pp. 1964 1971, 1998. [5] D. Ropers, W. Moshage, W. G. Daniel, J. Jessl, M. Gottwik, and S. Achenbach, Visualization of coronary artery anomalies and their anatomic course by contrast-enhanced electron beam tomography and three-dimensional reconstruction, Amer. J. Cardiol., vol. 87, no. 2, pp. 193 197, 2001. [6] W. Moshage, S. Achenbach, B. Seese, K. Bachmann, and M. Kirchgeorg, Coronary artery stenoses: Three-dimensional imaging with electrocardiographically triggered, contrast agent-enhanced, electron-beam CT, Radiol., vol. 196, pp. 707 714, 1995. [7] G. P. Reddy, D. M. Chernoff, J. R. Adams, and C. B. Higgins, Coronary artery stenoses: Assessment with contrast-enhanced electron beam CT and axial reconstructions, Radiol., vol. 209, pp. 167 172, 1998. [8] B. J. Rensing, A. Bongaerts, R. J. van Geuns, P. van Ooijen, M. Oudkerk, and P. J. de Feyter, Intravenous coronary angiography by electron-beam computed tomography: A clinical evaluation, Circulat., vol. 98, pp. 2509 2512, 1989. [9] A. Schmermund, B. J. Rensing, P. F. Sheedy, M. R. Bell, and J. A. Rumberger, Intravenous electron-beam computed tomographic coronary angiography for segmental analysis of coronary artery stenoses, J. Amer. Coll. Cardiol., vol. 31, pp. 1547 1554, 1998. [10] D. M. Chernoff, C. J. Ritchie, and C. B. Higgins, Evaluation of electron-beam CT coronary angiography in healthy subjects, AJR Amer. J. Roentgenol., vol. 169, pp. 93 99, 1997. [11] T. Nakanishi, K. Ito, M. Imazu, and M. Yamakido, Evaluation of coronary artery stenoses using electron-beam CT and multiplanar reformation, J. Comput. Assist. Tomogr., vol. 21, pp. 121 127, 1997. [12] S. Paulin, Coronary angiography: A technical, anatomic, and clinical study, Acta. Radiol., vol. 233S, pp. 1 215, 1964. suppl..

LEE et al.: HEART RATE AND ITS VARIATION USING MULTIDETECTOR ROW CT 231 [13] M. J. Potel, J. Rubin, S. A. Mackay, A. Aisen, J. Al-Sadir, and R. E. Sayre, Methods for evaluating cardiac wall motion in three dimensions using bifurcation points of the coronary arterial tree, Invest. Radiol., vol. 18, pp. 47 57, 1983. [14] S. Achenbach, D. Ropers, J. Holle, G. Muschiol, W. Daniel, and W. Moshage, In-plane coronary arterial motion velocity: Measurement with electron-beam CT, Radiol., vol. 216, pp. 457 463, 2000. [15] S. Achenbach, W. Moshage, D. Ropers, and K. Bachmann, Comparison of vessel diameters in electron beam tomography and quantitative coronary angiography, Int. J. Cardiol. Imaging, vol. 14, pp. 1 7, 1998. [16] K. M. Baskin, W. Stanford, B. H. Thompsom, J. Tajik, S. D. Heery, and E. A. Hoffman, Helical versus electron-beam CT in assessment of coronary artery calcification (abstr.), Radiol., vol. 197(P), p. 182, 1995. [17] B. Ohnesorge, T. Flohr, C. Becker, A. F. Kopp, U. J. Schoepf, U. Baum, A. Knez, K. Klingenbeck-Regn, and M. F. Reiser, Cardiac imaging by means of electrocardiographically gated multisection spiral CT: Initial experience, Radiol., vol. 217, pp. 564 571, 2000. [18] M. Kachelriess, S. Ulzheimer, and W. A. Kalender, ECG-correlated image reconstruction from subsecond multi-slice spiral CT scans of the heart, Med. Phys., vol. 27, no. 8, pp. 1881 1902, 2000. [19] S. Schroeder, A. F. Kopp, A. Kuettner, C. Burgstahler, C. Herdeg, M. Heuschmid, A. Baumbach, C. D. Claussen, K. R. Karsch, and L. Seipel, Influence of heart rate on vessel visibility in noninvasive coronary angiography using new multislice computed tomography: Experience in 94 patients, Clinic. Imaging, vol. 26, no. 2, pp. 106 111, 2002. [20] A. F. Kopp, S. Schroeder, A. Kuettner, M. Heuschmid, C. Georg, B. Ohnesorge, R. Kuzo, and C. D. Claussen, Coronary arteries: Retrospectively ECG-gated multi-detector row CT angiography with selective optimization of the image reconstruction window, Radiol., vol. 221, pp. 683 688, 2001. [21] K. Klingenbeck-Regn, S. Schaller, T. Flohr, B. Ohnesorge, A. F. Kopp, and U. Baum, Subsecond multi-slice computed tomography: Basics and applications, Eur. J. Radiol., vol. 31, pp. 110 124, 1999. [22] H. Boudoulas, S. E. Rittgers, R. P. Lewis, C. V. Leier, and A. M. Weissler, Changes in diastolic time with various pharmacologic agents: Implications for myocardial perfusion, Circulat., vol. 60, pp. 164 169, 1979. [23] C. R. Becker, B. M. Ohnesorge, U. J. Schoepf, and M. F. Reiser, Current development of cardiac imaging with multidetector-row CT, Eur. J. Radiol., vol. 36, pp. 97 103, 2000.