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Cardiac Imaging |
1 From the Departments of Radiology (F.C., K.N., A.v.d.L., R.H.R., N.M., P.M.T.P., P.J.d.F., G.P.K.) and Cardiology–Thoraxcentrum (K.N., N.M., P.J.d.F.), Erasmus Medical Center, Dr Molenwaterplein 40, 3015GD Rotterdam, the Netherlands. Received April 28, 2003; revision requested July 8; final revision received March 16, 2004; accepted April 1. Address correspondence to F.C. (e-mail: filippocademartiri@hotmail.com).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: This study had institutional review board approval, and patients gave informed consent. Thirty-eight patients (mean age, 60 years; three women) were randomized into two groups according to bolus timing technique: group 1 (20-mL test bolus with 100-mL main bolus) and group 2 (bolus tracking with 100-mL main bolus). All patients underwent electrocardiography-gated 16–detector row CT coronary angiography with 12 detectors (collimation, 0.75 mm; rotation time, 420 msec). In group 1, test bolus peak attenuation was used as a delay, while in group 2, a +100-HU threshold in ascending aorta triggered angiographic acquisition, with an additional 4-second delay for patient instruction. Attenuation was measured in the longitudinal direction throughout the examination in three main vessels: ascending aorta (region of interest [ROI] 1), descending aorta (ROI 2), and main pulmonary artery (ROI 3). Mean attenuation and slope of bolus geometry curve were calculated in each patient and ROI. Attenuation at origin of coronary arteries was measured. Student t test was used to compare results.
RESULTS: Mean scan delay was 6 seconds longer in group 2 (P < .05). Average attenuation values were 306.6 HU ± 44.0 (standard deviation) and 328.2 HU ± 58.6 (P > .05) in ROI 1, 291.6 HU ± 45.1 and 326.4 HU ± 62.6 (P > .05) in ROI 2, and 354.7 HU ± 78.0 and 305.3 HU ± 71.4 (P < .05) in ROI 3 for groups 1 and 2, respectively. Average slope values were 5.8 and –0.8 (P < .05) in ROI 1, 7.7 and 0.7 (P < .05) in ROI 2, and –1.0 and –13.3 (P < .05) in ROI 3 for groups 1 and 2, respectively. Average attenuation values in left main, left anterior descending, and left circumflex arteries were higher in group 2 (P < .05); there were no differences (P > .05) between groups in right coronary artery.
CONCLUSION: Bolus-tracking yields more homogeneous enhancement than does the test bolus technique.
© RSNA, 2004
Index terms: Aorta, CT, 562.12112, 563.12112 • Computed tomography (CT), contrast enhancement, 50.12112 • Coronary vessels, CT, 54.12112 • Pulmonary arteries, CT, 564.12112
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Several manufacturers have recently introduced a new generation of multi–detector row CT scanners with 16 rows of detectors and increased spatial and temporal resolution (4–6).Owing to the increased number of detector rows and faster gantry rotation, the time needed to image the entire heart has been reduced to approximately 20 seconds (6). Early experiences indicate improved results in the detection of significant stenosis in coronary arteries (6–8).
A prerequisite for successful CT angiography is optimal synchronization between the arterial passage of contrast material and CT data acquisition (9). However, the reduction in scanning time raises questions concerning contrast material bolus optimization (10,11).
In general, for helical CT or multi–detector row helical CT, the most frequently used bolus timing techniques are (a) a fixed delay technique, (b) determining the transit time by using a test bolus injection, and (c) bolus tracking. Although the bolus-tracking technique has previously been proposed as an effective tool for better synchronizing scanning with contrast enhancement (12–16), controversy still exists as to whether or not bolus-timing techniques result in actual advantages in terms of optimal vascular attenuation (17).
Nevertheless, for cardiac and coronary multi–detector row CT imaging, only fixed delay and test bolus techniques have been used until now (1–3,18,19). The relatively long scanning time in four–detector row CT may require hyperventilation or administration of oxygen before the start of data acquisition. The large respiratory movements in hyperventilation do not allow a reliable monitoring sequence for bolus tracking at the level of the ascending aorta. The shorter scanning time in 16–detector row CT allows the use of the bolus-tracking technique at coronary CT angiography.
Thus, the purpose of our study was to compare the test bolus and bolus-tracking techniques for intravenous contrast material administration at coronary angiography performed with a 16–detector row CT scanner.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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After enrollment, each patient was randomly placed into one of two groups by using a random number table. In group 1 (n = 19), the test bolus technique was used, while in group 2 (n = 19), the bolus-tracking technique was used. In each group, patient age, sex, body weight, and heart rate during the examination were recorded.
Scanning Protocol
Examinations were performed with a 16–detector row CT scanner (Sensation 16; Siemens Medical Solutions, Forchheim, Germany). Before the examination, the heart rate was measured. Patients with a preexamination heart rate of 65 beats per minute or higher were given 100 mg of metoprolol (Selokeen; AstraZeneca, United Kingdom) by mouth 1 hour before the examination. Patients were given thorough instructions with respect to the examination and the breath-hold procedure. They were required to perform a deep inspiration and to continue to hold their breath without pushing (ie, the Valsalva maneuver). During this trial, the operator observed the patient for compliance and the electrocardiogram for important anomalies.
The contrast material—iodixanol (320 mg of iodine per milliliter) (Visipaque; Amersham Health, Little Chalfont, England)—was injected through a 180-gauge needle in the antecubital vein by using a power injector (EnVision; MedRAD, Pittsburgh, Pa). Contrast material volume and injection rate, respectively, were 20 mL and 4 mL/sec for the test bolus and 100 mL and 4 mL/sec for the main angiographic bolus (total injection time, 25 seconds). For retrospective electrocardiographically gated scanning of the coronary arteries, the 12 central detector rows were used.
The main scanning parameters were as follows: number of detectors, 12 (because the retrospectively electrocardiographically gated protocol did not allow the use of all 16 rows); individual detector width, 0.75 mm; gantry rotation time, 420 msec; 120 kV; 400 mAs; table feed per rotation, 2.8 mm; table feed per second, 6.7; scanning direction, craniocaudal. For the purposes of the present study, two data sets were reconstructed, both with retrospective electrocardiographic gating with a time window starting at 400 msec before the next R wave on the electrocardiogram, a field of view of 200 mm, and a medium-smooth convolution filter (B30f, part of Sensation 16 scanner). Two data sets were reconstructed with effective section widths and reconstruction intervals, respectively, of 1 and 3 mm and 0.6 and 3 mm. The images were transferred to a stand-alone workstation and evaluated by using dedicated analysis software (Leonardo; Siemens Medical Solutions). Bolus timing procedures and main acquisitions were successfully completed in all patients. No clinically important adverse reactions to contrast material were observed. All patients were able to hold their breath during the examination.
Test bolus technique.—This technique is based on the intravenous injection of a small amount of contrast material (20 mL [generally 15%–20% of the main bolus]) during the acquisition of a series of dynamic low-dose (120 kV, 20 mAs) monitoring scans at the level of the vessel of interest.
In the present study, dynamic monitoring scans were positioned at the level of the ascending aorta. The delay between each monitoring scan acquisition was 1.25 seconds. Acquisition of the dynamic monitoring scans started 10 seconds after the beginning of the injection of intravenous contrast material (20 mL of contrast material injected at 4 mL/sec). A region of interest (ROI) as large as the aortic root was drawn inside the lumen by one operator (R.H.R.) to generate an enhancement curve (generated by using DynEVA software; Siemens Medical Solutions), which showed the time needed to reach the peak of maximum enhancement for the test bolus. The time to peak enhancement in the ascending aorta for the test bolus was the delay applied for angiographic scanning, as previously described (1,3,19,20).
Bolus-tracking technique.—This technique (performed with CARE bolus software; Siemens Medical Solutions) is based on real-time monitoring of the main bolus during injection with the acquisition of a series of dynamic low-dose (120-kV, 20-mAs) monitoring scans at the level of the vessel of interest. It is possible to start main scanning manually or automatically with a trigger threshold. In the present study, monitoring scanning was performed at the level of the ascending aorta. An ROI as large as the aortic root was drawn by one operator (R.H.R.). Dynamic monitoring scanning began 10 seconds after the beginning of the intravenous contrast material injection. The trigger threshold inside the ROI was set at +100 HU above the baseline (approximately 140–160 HU in absolute value). The delay between each monitoring scan acquisition was 1.25 seconds. As soon as the threshold was reached, the table moved to the cranial start position while the patient was instructed to take a deep breath and hold it. During this interval (4 seconds, which was necessary to give breath-hold instructions to the patient safely), the contrast material concentration increased to the desired level of enhancement. The rationale for the choice of a rather low threshold was based on the extra 4 seconds needed before scanning could begin.
Data Collection
One radiologist (F.C.) with 3 years of experience in cardiac CT performed and collected all the measurements at the workstation.
Two main sets of measurements were performed: (a) bolus geometry in the great vessels (ie, the variation in the attenuation inside a vessel over time after the administration of intravenous contrast material) and (b) the attenuation in the coronary arteries.
Bolus geometry in the great vessels.—To generate a plot of bolus geometry during the main angiographic scanning protocol, the attenuation values were extracted from the data sets as follows: The Digital Imaging and Communications in Medicine, or DICOM, layout of the images enabled us to read the exact time (down to hundreds of a second) of the data acquisition in each reconstructed section. The attenuation values (in Hounsfield units) were assessed on transverse images in the data set that had effective section widths of 3 mm and reconstruction intervals of 3 mm. At intervals of 1 second, in each section, an ROI was drawn throughout the entire data set into three main regions (Figs 1, 2): (a) the ascending aorta (ROI 1), (b) the descending aorta (ROI 2), and (c) the main pulmonary artery (ROI 3). The ROIs were drawn as large as the anatomic configuration of the area allowed in the transverse section.
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Attenuation in the coronary arteries.—To yield information regarding the effect of the contrast material protocol on coronary artery enhancement, the attenuation was measured at the origin of four coronary artery branches. The data set with an effective section width of 1 mm and a reconstruction interval of 0.6 mm was used. This data set had, in fact, a higher spatial resolution in the z-axis, allowing us to measure structures with diameters of 3–5 mm more accurately. An ROI that was as large as the vessel lumen was plotted on a transverse image in which the lumen was easily identified. Calcifications of the coronary wall and soft plaques were carefully avoided. Four vessels (and four ROIs) were considered: the left main (LM) and left anterior descending (LAD) coronary arteries, the left circumflex (LCX) artery, and the right coronary artery (RCA).
Data and Statistical Analysis
To rule out significant differences between the two sample populations, an analysis of variance was applied to the following parameters: age, weight, and mean heart rate during the examination.
The scan delays were derived from the test bolus dynamic series in group 1 and from the bolus-tracking dynamic series in group 2.
Bolus geometry in the great vessels.—To analyze the geometry of the bolus, time-attenuation curves were generated. In each of the three main ROIs and in each patient, the start of the main scanning protocol was synchronized as time 0 for both groups. Average time-attenuation curves for each ROI in each group were generated and expressed as means ± standard deviations.
The evaluation of the efficacy of the synchronization protocols (test bolus vs bolus tracking) was performed by using several parameters, including the average attenuation (the mean attenuation calculated in each sample during scanning) and the slope of the time-attenuation curve; both were calculated in each ROI in each patient. The average attenuation represents an assessment of the amount of contrast material that was present inside the vessel during scanning, while the slope represents an assessment of the position of scanning in relation to bolus geometry. For instance, a positive slope (>0) means that scanning was performed during increasing attenuation (eg, early in bolus geometry), a negative slope (<0) means that scanning was performed during decreasing attenuation (eg, late in bolus geometry), and a slope around 0 means that scanning was performed during the plateau or the peak of attenuation (eg, in the middle of bolus geometry). Additional parameters calculated are as follows: (a) the attenuation value at time 0, (b) the maximum enhancement value (MEV) in each ROI, and the time to the MEV (tMEV) in each ROI. Differences between groups were assessed with the Student t test, and P < .05 was considered to indicate a statistically significant difference.
Attenuation in the coronary arteries.—The attenuation recorded at the origins of the LM, LAD, and LCX arteries and the RCA were measured (as mean values ± standard deviations). Differences between groups were assessed with the Student t test, and P < .05 was considered to indicate a statistically significant difference.
Interdependency between measurements.—To address the problem of interdependency of the measurements between different vessels, the following analysis was performed. The paired parameters used for the description of the curves (average attenuation, slope, time 0, MEV, and tMEV) were subtracted in the two groups, and the differences obtained were compared by using a paired t test. The analysis was performed for each great vessel and for each coronary vessel. P < .05 was considered to indicate low interdependency, while P < .01 was considered to indicate no interdependency.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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In the main pulmonary artery (Table 2, Fig 3), the average attenuation was significantly lower (P < .05) in group 2. The slope was significantly higher in group 1 (P < .05). Attenuation values at time 0 were 356.8 HU ± 79.8 and 361.9 HU ± 90.6 in groups 1 and 2, respectively (P > .05). The average MEV was 426.9 HU ± 80.5 in group 1 and 394.5 HU ± 85.6 in group 2 (P > .05). The average tMEV was 4.1 seconds in group 1 and 2.1 seconds in group 2 (P < .05).
Coronary Arteries
In the coronary arteries (Table 3, Fig 3), the average attenuation was significantly higher (P < .05) in group 2 in the LM, LAD, and LCX arteries, while in the RCA, although attenuation was higher in group 2, this difference was not significant (P > .05).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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A few techniques—namely, the fixed delay technique (on the basis of experience and related to angiographic data), the test bolus technique, and the bolus-tracking technique—enable the synchronization of contrast material administration and the start of data acquisition in clinical practice (9). Although the synchronization of contrast material administration is becoming increasingly important owing to the introduction of faster spiral CT scanners, controversies still remain regarding the opportunity of using these techniques. In a study by Macari et al (17), a fixed delay of 25 seconds was used to image the abdominal aorta with the intravenous administration of 150 mL of contrast material. The attenuation, measured throughout the data set in the aorta and the iliac arteries, was greater than 200 HU—considered adequate by the authors for angiographic evaluation—in 98% of the measurements.
In the present study, test bolus and bolus-tracking techniques in CT angiography of the coronary arteries were compared. Enhancement was measured in the ascending aorta, the descending aorta, and the pulmonary artery. Enhancement in the ascending aorta represents the contrast material that drains into the coronary arteries; therefore, the attenuation achieved at this level plays a major role in the optimal enhancement of the coronary arteries. The descending aorta represents a monitoring vessel for arterial bolus geometry because it runs parallel to the z-axis (ie, the longitudinal or temporal axis). Pulmonary artery evaluation can be useful for monitoring the pooling of contrast material in the right side of the heart.
In addition, to measure the effect of the two synchronization protocols on the attenuation in the coronary arteries, the attenuation at the origins of the LM, LAD, and LCX arteries and the RCA was assessed.
The bolus-tracking group had better synchronization between scanning and contrast material administration. In fact, the bolus-tracking group had more homogeneous and steady enhancement compared with the test bolus group, with less pooling of contrast material in the pulmonary vessels and right side of the heart. The amount of contrast material used was also reduced by 20%. This resulted in higher attenuation at the level of the coronary arteries. In particular, the left coronary artery (ie, the LM artery) and its main branches (the LAD and LCX arteries) showed significantly higher attenuation in the bolus-tracking group. Also, the attenuation was higher, although not significantly different, in the RCA in the bolus-tracking group. The calculated scan delay was 6 seconds later in the bolus-tracking group than in the test bolus group. It is reasonable to state that only because of this difference the geometry of contrast material attenuation would be different in the two groups.
In fact, even though the average attenuation was not different in the ascending and descending aortas, the slopes were significantly different: In the bolus-tracking group, the slope was approximately 0, meaning that scanning was performed during the plateau of enhancement of bolus geometry at the level of the thoracic aorta. Conversely, the slope in the test bolus group was greater than 0, meaning that scanning was performed during the upslope of attenuation, when the contrast is still increasing. The mirror image of this behavior is displayed in the pulmonary artery, where in the test bolus group, the average attenuation was significantly higher (ie, more contrast material was still present in the right side of the pulmonary circulation) and the slope was significantly lower and negative (the trend was toward decreasing attenuation in the right side of the pulmonary circulation) in comparison to these parameters in the bolus-tracking group. In simple words, by using the bolus-tracking protocol, scanning is performed during the plateau of attenuation and there is less pooling of contrast material in the right side of the heart.
Confirming this observation, the attenuation values at time 0 (the start of the examination) were significantly lower in the test bolus group in the ascending and descending aortas because the geometry was still in the upslope. The MEV was not significantly different among all three locations, meaning that the peak of attenuation was within the scanning duration in both groups but at different moments: The tMEV was significantly different in all three locations. The MEV in the test bolus group was 3 seconds later than that in the bolus-tracking group in the ascending and descending aortas. This discrepancy between the difference in the start delay (6 seconds) and the difference in tMEV can be explained by the increased intrathoracic pressure caused by the apnea that occurred 6 seconds earlier in the test bolus group. In fact, the Valsalva maneuver reduces the incoming flow of contrast material through the innominate veins. This phenomenon could also explain the slightly (although not significantly) lower MEV in the test bolus group.
The mean scan delay was 6 seconds longer in group 2, meaning that the protocol for the test bolus needed a time factor (ie, a constant delay) to be added to the delay calculated from the peak of maximum enhancement in the test bolus sequence. The time factor could probably be approximately 6 seconds, but more accurate studies are needed to optimize this approach.
A limitation of this study was that, for practical reasons, bolus geometry during the main scanning protocol was assessed during table movement. This means that scanning was not performed at the same level during the passage of contrast material.
Another limitation was related to the evaluation of coronary arteries. Ideally, the best way to assess the efficacy of a contrast material protocol would be to measure the length of coronary arteries and the number of side branches that can be visualized. We preferred to assess the origin of the main coronary arteries quantitatively because the group of patients was too small to account for the many variables that can affect coronary artery visualization: A high heart rate, a small vessel size, a variable anatomy, a heavily calcified vessel wall, and the presence of stenosis or occluded vessels can heavily affect the visualization of coronary arteries, regardless of the attenuation of the vessel.
Fixed delay and test bolus techniques have been used successfully until now in multi–detector row CT coronary angiography. More optimal synchronization of contrast material passage and data acquisition is possible by using the bolus-tracking technique and a 16–detector row CT scanner, resulting in consistently high and homogeneous contrast enhancement at coronary angiography. Also, 20% less contrast material volume is administered with the bolus-tracking technique. We believe that the bolus-tracking technique should be integrated into the routine protocol for 16–detector row CT coronary angiography.
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FOOTNOTES |
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Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, F.C., G.P.K.; study concepts and design, F.C., A.v.d.L.; literature research, F.C., A.v.d.L.; clinical studies, F.C., K.N.; data acquisition and analysis/interpretation, F.C., K.N., A.v.d.L., R.H.R.; statistical analysis, F.C.; manuscript preparation, F.C.; manuscript definition of intellectual content, F.C., A.v.d.L., P.M.T.P., P.J.d.F., G.P.K.; manuscript editing, F.C., A.v.d.L.; manuscript revision/review, F.C., K.N., N.M., A.v.d.L.; manuscript final version approval, F.C., K.N., A.v.d.L., R.H.R., P.M.T.P., P.J.d.F., G.P.K.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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