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Cardiac Imaging |
1 From the Institute of Clinical Radiology (C.H., C.R.B., A.H., U.J.S., R.B., M.F.R.) and Division of Cardiology (A.K.), Klinikum Grosshadern, University of Munich, Marchioninistrasse 15, 81377 Munich, Germany; Siemens Medical Systems, Forchheim, Germany (B.O.); and the Department of Radiology, Tongji Medical University, Tongji, China (C.H.). From the 2000 RSNA scientific assembly. Received November 30, 2000; revision requested January 11, 2001; revision received March 12; accepted March 22. Address correspondence to C.R.B. (e-mail: christoph.becker@ikra.med.uni-muenchen.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Sixty-five patients with different heart rates underwent coronary CT angiography. Raw helical CT data and ECG tracings were combined to retrospectively reconstruct at the defined consecutive z position with a temporal resolution of 250 msec per section. The starting points of the reconstruction were chosen between 30% and 80% of the R-R intervals. The relationships between heart rate, trigger delay, and image quality were analyzed.
RESULTS: Optimal image quality was achieved with a 50% trigger delay for the right coronary artery and 60% for the left circumflex coronary artery. Optimal image quality for the left anterior descending coronary artery was equally obtained at 50% and 60% triggering. A significant negative correlation was observed between heart rate and image quality (P < .05). The best image quality was achieved when the heart rate was less than 74.5 beats per minute.
CONCLUSION: To achieve high image quality, the heart rate should be sufficiently slow. Selection of appropriate trigger delays and a decreasing heart rate are effective to reduce cardiac motion artifacts.
Index terms: Coronary vessels, CT, 54.12111, 54.12115, 54.12116 • Heart, CT, 54.12111, 54.12115, 54.12116
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Earlier investigators (1,2) have suggested that for artifact-free cardiac imaging, scanning times should be less than 50 msec. Electron-beam CT, with its 100-msec scanning time, synchronized to the cardiac cycle by using prospective electrocardiographic (ECG) triggering, has been used to detect and quantify coronary artery calcification (3–7). However, electron-beam CT is not widely available and possesses some image quality trade-off that makes it function best as a dedicated tool for detecting coronary artery calcification.
Coronary calcium can also be detected with helical CT, although some investigators have dismissed helical CT as too unreliable for quantification because of a lack of sensitivity, accuracy, and reproducibility (8). Becker et al (9,10) have recently pursued the application of helical CT using ECG triggering to quantify coronary artery calcification with scoring results, with good correlation to electron-beam CT results.
Cardiac gating of CT data is one possible way to decrease the effects of cardiac motion (8,11). With helical CT, gating of the cardiac images to the ECG examination can be accomplished retrospectively. The essence of the technique is reconstruction of images at the point in the cardiac cycle at which minimal cardiac motion is present. Multi–detector row CT, with four simultaneously scanned sections and half-second rotation time, provides a new opportunity for cardiac imaging. Partial view acquisition and ECG-gated helical reconstruction, both of which are feasible with this new type of scanner, allow for a 250-msec image acquisition in the slow-motion phase of the cardiac cycle (11). This permits both detection and measurement of coronary calcium (12), as well as CT angiography (13,14). Despite a still slightly longer exposure time, as compared with electron-beam CT, multi–detector row CT promises to be superior to electron-beam CT, with a higher signal-to-noise ratio and spatial resolution. The purpose of this study was to evaluate the effectiveness of ECG-gated retrospective image reconstruction for multi–detector row CT coronary angiography in reducing cardiac motion artifacts and to evaluate the influence of heart rate on cardiac image quality.
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MATERIALS AND METHODS |
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CT angiography was performed with a multi–detector row scanner (Somatom Volume Zoom; Siemens Medical Systems, Forchheim, Germany) with commercially available cardiac reconstruction software (HeartView; Siemens Medical Systems). Contrast material was administered with an 18-gauge needle in the cubital vein. The scanning delay was determined by injecting a 20-mL test bolus, with a 3-mL/sec flow rate and scanning every 2 seconds in the ascending aorta at the level of the main pulmonary artery. The scanning delay was then determined as the interval from the start of injection to peak enhancement in the ascending aorta, plus 3 seconds. The additional 3-second delay was chosen to ensure passage of the contrast material into the coronary vessels before the onset of scanning, especially in patients with reduced cardiac output caused by atrial fibrillation.
Coronary vessel enhancement was achieved with 140 mL of nonionic contrast material (Ultravist 300; Schering, Berlin, Germany) injected at a flow rate of 3 mL/sec. After the predetermined delay, helical CT scanning (120 kV and 300 mAs) was begun with simultaneous acquisition of four 1-mm collimated sections at a 3.6-mm/sec table speed and a 500-msec rotation time. During helical scanning, the ECG tracing was digitally recorded and stored on a personal computer workstation. Helical scanning was performed within a single breath hold from 1 cm below the carina to the base of the heart, covering 12 cm in 30–34 seconds (mean, 32 seconds), depending on the dimensions of the patient. All patients were informed of the total scanning duration and instructed to hyperventilate and hold their breath at near-maximal lung capacity for each examination.
Raw helical CT data and the digital ECG trace were used to retrospectively reconstruct the transverse images, with a constant temporal resolution of 250 msec per section. To find the delay best suited to obtaining high image quality, sections at every given z position were reconstructed beginning at 30%, 40%, 50%, 60%, 70%, and 80% of the R-R interval after the previous R wave. The field of view was adjusted to encompass only the heart. The effective section thickness and reconstruction increment were 1.25 and 0.50 mm, respectively, resulting in 210–240 transverse sections for the entire volume.
Multi–detector row CT angiography was successfully completed in all 65 patients. The imaging protocol was well tolerated by all patients. No reactions to contrast material or adverse events occurred. All subjects were able to hold their breath for the required duration. The total room time required for each study was 20 minutes or less, including all preparations for scanning.
Monitor reading in scroll-through mode was used for the independent interpretation of the transverse multi–detector row CT angiographic images by two radiologists (C.R.B., R.B.) experienced in cardiac imaging (with 4 and 3 years of experience in cardiac CT, respectively). Both readers were blinded to the ECG-gating parameter. The coronary tree was segmentally analyzed on the basis of a model suggested by the American Heart Association, which included the right coronary artery (RCA), left anterior descending (LAD) coronary artery, and left circumflex (LCx) coronary artery (15). The results were compared on the basis of image quality for sharp delineation of the three major coronary arteries reconstructed at each retrospective ECG trigger delay tested.
A five-point scale was used, with which the image quality in each coronary segment reconstructed at each trigger delay was interpreted as: 5, no motion artifacts (clear delineation); 4, minor artifacts (slight blurring in some segments of the major vessels); 3, moderate artifacts (double-imaged structures in more than half the course of the major vessels); 2, severe artifacts (doubling and blurring of the whole course of the major vessels with structure discontinuity); or 1, nondiagnostic (vessel structures not differentiable). The scores of image quality for each major coronary artery were rated independently by using this scale; a score of 4 or higher was considered acceptable in terms of image quality.
The mean values for all patients from the two observers were correlated with delay. Moreover, the correlation between heart rate and image quality was calculated for all patients by using the Spearman rank correlation coefficient. The linear regression correlation between heart rate and image quality resulting from the optimal trigger delay was calculated to determine the limit of heart rate suitable for obtaining high image quality.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The linear regression equation (Fig 4) shows that the most suitable heart rate for obtaining high-quality images (score of 4 or 5) for the three major coronary arteries with an optimized trigger delay of 50% of the R-R interval was less than 74.5 beats per minute.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Electron-beam CT with prospective ECG triggering (3) is currently established as an effective noninvasive imaging modality for detecting coronary artery calcification. For prospective ECG triggering, the next R-R interval is estimated on the basis of the median of the last seven R-R intervals. Prospective ECG triggering fails with rapid changes in heart rate, as is the case in patients with cardiac arrhythmias (16). Retrospective ECG gating does not rely on estimation of the presumed next R-R interval. Reconstruction data intervals can be accurately measured on the basis of the actual R-R intervals (17) that occurred during scanning. With retrospective ECG gating, the final reconstruction is based only on images obtained during a portion of middiastole, with the reconstruction trigger selected according to the actual heart rate. This greatly reduces cardiac motion artifacts.
A major problem causing the reduced sensitivity of electron-beam CT angiography in previous studies (18–20) has been the inability to assess the RCA and LCx artery in a large proportion of patients. Schmermund et al (21) found that in 26% of patients, not all proximal and middle segments of the RCA and LCx artery could be visualized because of motion artifact caused by systolic contraction of the atria, arrhythmias, small vessel size, poor opacification, or poor signal-to-noise ratio. In addition, similar reasons plus severe calcification of the coronary arteries are responsible for a large number of false-positive results in CT angiography of the coronary arteries.
Clinically, there is therefore great interest in the assessment of heart rate and trigger delays as important factors that influence cardiac CT image quality. To our knowledge, few systematic evaluations of the influence of heart rate on cardiac image quality (22–24) have been published. The current study focused on heart rate and the reconstruction trigger delay as predictors of CT image quality for each coronary artery. The differences in resolution between images acquired across a spectrum of R-R interval trigger delays described in our study suggests that delaying the reconstruction trigger in diastole improves image quality. However, paradoxically, a very long trigger delay, such as a trigger of 80% of the cardiac cycle, can also degrade the image because atrial contraction during end diastole causes rapid movement of the base of the heart (25).
Essential for the reduction of cardiac motion artifacts is shortening of acquisition time. Although the current electron-beam CT technique with a shortened temporal resolution of 100 msec has advantages over multi–detector row CT, it too has a strong dependence on the time of image acquisition during the cardiac cycle (24). Acquisitions at the end of systole or at early diastole (beginning at 10%–30% of the cardiac cycle) and before atrial contraction (at approximately 80% of the cardiac cycle) result in more artifacts than do those at middiastole. As compared with multi–detector row CT, with a 250-msec acquisition time, electron-beam CT more easily enables completion of acquisitions during short cardiac cycles in patients with higher heart rates. Authors of a recent study (26) in which electron-beam CT angiography was performed also have shown that the fewest motion artifacts occur at 40%–60% of the R-R interval. Similar to our results, the movement of each coronary artery varies individually. The current standard protocol of triggering at 80% of the R-R interval might not be optimal for imaging of the coronary segments near the right or left atrium. In the future, an individualized reconstruction protocol allowing each coronary artery its own optimal trigger point would be likely to further decrease motion artifacts.
Retrospective ECG gating can be also applied to multi–detector row helical CT reconstruction techniques. Multi–detector row CT allows reconstruction of images with arbitrary (ie, overlapping) increments in any phase of the heart cycle. Continuous retrospectively ECG-gated multi–detector row CT volume scanning allows three-dimensional reconstruction of the heart volume in different cardiac motion phases. This allows assessment of cardiac motion in a "four-dimensional" fashion (cine-display, animated three-dimensional reconstruction with the added temporal dimension of myocardial motion) (11). In all phases of the heart cycle, imaging data are available for reconstruction of the entire heart volume without gaps. Such a four-dimensional image for true functional volume imaging of the moving heart can be generated from various three-dimensional images with incrementally changed settings of the delay parameters within the cardiac cycle. New cardiac CT applications need to be investigated that make use of functional imaging capabilities, such as functional evaluation of specific cardiac anatomic structures (eg, the pulmonary or mitral valve) or determination of ventricular function parameters on the basis of reformations of long- and short-axis views. Three-dimensional volumes obtained with 250-msec temporal resolution in phases of high motion (ie, the systolic phase) may be of adequate quality for evaluating larger structures.
We have shown an inverse relationship between heart rate and image quality. This relationship is apparent in reconstruction of data in patients with higher heart rates, irregular heart rates, or arrhythmias. This relationship is also true for myocardial infarction, which often results in a shorter diastole, as well as ECG abnormalities that may prevent appropriate trigger selection. Our correlation was better in patients with a lower heart rate, and this may be attributable to a longer period of diastole. Whereas heart rate acceleration increases the systolic component of the cardiac cycle relative to diastole, a decrease in heart rate reduces it, particularly a decrease to fewer than 75 beats per minute (27). Thus, decreasing heart rate (eg, with pharmacologic treatment) would appear to increase diastole and thus secondarily improve image quality.
As the heart rate changes during investigation as a result of the Valsalva maneuver, which is inevitably performed with deep inspiration (16), rapid adaptation of the ECG trigger to the heart rate is essential, especially in arrhythmias. The trigger, retrospectively chosen and expressed as a percentage of the cardiac cycle, will be much more suitable for continuously keeping the reconstruction positions in the least-motion phase than will the prospective trigger as a fixed time.
Multi–detector row CT technique is currently limited by the temporal resolution per section (250 msec). To achieve high image quality, the heart rate of the patient needs to be sufficiently slow. We therefore anticipate the potential benefit of heart rate–lowering drugs, such as the ß-blockers that we often administer orally 1 hour prior to investigation. Even if the relationships between heart rate reduction, trigger selection, and image quality are complex, there is a strong possibility that ß-adrenergic blockade would improve cardiac CT image quality.
In conclusion, the results of our study have demonstrated the dependence of image quality on trigger delay when retrospectively reconstructing ECG-gated helical CT coronary angiograms. Selecting appropriate trigger delays and decreasing heart rate are effective for reducing cardiac motion artifacts. Although cardiac arrhythmias can still interfere with retrospective gating, this method is expected to continue to improve with the introduction of new techniques for finding the optimum trigger. Some progress can be achieved with reconstruction algorithms that provide improved temporal resolution with sampling data from consecutive heartbeats. However, general application of these algorithms is limited because they rely on regular sinus rhythms and heartbeat; improved temporal resolution is often related, with reduced spatial resolution. In the long term, hardware improvements, in terms of faster rotation speeds to reduce exposure times, may favorably overcome the limitations in temporal resolution of the current CT scanners.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, C.H., C.R.B., M.F.R.; study concepts, C.R.B., C.H., A.H.; study design, C.H., C.R.B., U.J.S.; literature research, C.H., R.B.; clinical studies, C.R.B., U.J.S., A.K.; data acquisition, C.H., C.R.B., R.B.; data analysis/interpretation, C.H., A.H., C.R.B., B.O.; statistical analysis, A.H., C.R.B.; manuscript preparation and definition of intellectual content, C.H., C.R.B., B.O.; manuscript editing, C.H., C.R.B.; manuscript revision/review and final version approval, C.H., C.R.B., M.F.R.
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REFERENCES |
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J. Horiguchi, H. Yamamoto, Y. Akiyama, K. Marukawa, N. Hi |
