Published online before print March 27, 2008, 10.1148/radiol.2472070856
(Radiology 2008;247:356-364.)
© RSNA, 2008
Right Heart: Split-Bolus Injection of Diluted Contrast Medium for Visualization at Coronary CT Angiography1
J. Matthias Kerl, MD,
James G. Ravenel, MD,
Shaun A. Nguyen, MD, MA,
Pal Suranyi, MD, PhD,
Christian Thilo, MD,
Philip Costello, MD,
Werner Bautz, MD, and
U. Joseph Schoepf, MD
1 From the Department of Radiology (J.M.K., J.G.R., S.A.N., P.S., C.T., P.C., U.J.S.) and Department of Medicine, Division of Cardiology (C.T., U.J.S.), Medical University of South Carolina, 169 Ashley Ave, Charleston, SC 29425; and Institute of Diagnostic Radiology, University Erlangen-Nuremberg, Erlangen, Germany (J.M.K., W.B.). Received May 15, 2007; revision requested July 23; revision received August 23; accepted September 26; final version accepted October 22. Supported by Medrad (Indianola, Pa). U.J.S. is a medical consultant to Bayer (Wayne, NJ), Bracco (Princeton, NJ), GE Healthcare (Princeton, NJ), Siemens (Malvern, Pa), and TeraRecon (San Mateo, Calif) and receives research support from Medrad, Bayer, Bracco, GE, and Siemens. P.C. is a medical consultant to Bracco and receives research support from Siemens. C.T. is a medical consultant to Medrad.
Address correspondence to U.J.S. (e-mail: schoepf{at}musc.edu).
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ABSTRACT
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Purpose: To retrospectively compare a split-bolus contrast medium injection protocol with a biphasic and a monophasic protocol in terms of visualization of the right and left heart, contrast medium–related streak artifacts, and level of attenuation in the cardiac chambers and coronary arteries at coronary computed tomographic (CT) angiography.
Materials and Methods: The human research committee approved this HIPAA-compliant study and waived informed consent. Seventy-five patients had undergone 64-section coronary CT angiography: 25 were injected by using a monophasic, contrast-medium-only protocol with a single-syringe injector; 25 were injected by using a biphasic protocol with a dual-syringe injector; and 25 were injected by using a split-bolus protocol with a dual-syringe injector and an initial bolus of contrast medium followed by 50 mL of a 70%:30% saline-to–contrast medium mixture and a 30-mL saline chaser. Two radiologists rated the visualization of right and left heart structures and the degree of streak artifacts. One observer performed attenuation measurements of the cardiac chambers and of the coronary arteries. Data were analyzed with one-way analysis of variance and Duncan post-hoc multiple comparison procedures.
Results: Data for 27 women and 48 men (mean age, 62 years) were included. Mean contrast medium attenuation in the right heart was significantly (P < .001) higher in the split-bolus group than in the biphasic injection group but was significantly (P < .001) lower than in the monophasic injection group. For the left heart and the coronary arteries, there were no significant differences among the three groups. Artifacts occurred less frequently (P < .001) in the biphasic and split-bolus groups than in the monophasic group. Visualization of right heart structures was rated significantly (P < .05) better in the split-bolus group than in the two other groups, while there was no difference for visualization of left heart structures.
Conclusion: Split-bolus injection provides sufficient attenuation for visualization of the right heart, while streak artifacts from high-attenuation contrast material can generally be avoided and arterial attenuation is maintained.
© RSNA, 2008
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INTRODUCTION
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Considerable effort is being directed at optimizing injection parameters for contrast material–enhanced computed tomographic (CT) applications in general (1–4) and for cardiac applications in particular (5–8) to ensure high, homogeneous, and consistent vascular attenuation while artifacts are minimized. The introduction of dual-syringe injectors with the ability to "chase" the main contrast medium bolus with saline (8–12) has been a pivotal development for achieving this objective. It has been demonstrated that this approach can result in suppression of streak artifacts from high-attenuation contrast medium in the superior vena cava (SVC) and the right heart (10) while high and consistent attenuation in the arterial system is maintained (6). However, in many patients, use of the saline chaser technique flushes all contrast material from the right heart so effectively (8) that right cardiac anatomy and potential disease can no longer be assessed.
Thus, the purpose of our study was to retrospectively compare a split-bolus contrast medium injection protocol with a biphasic and a monophasic protocol in terms of visualization of the right and left hearts, contrast medium–related streak artifacts, and level of attenuation in the cardiac chambers and coronary arteries.
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MATERIALS AND METHODS
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This study was supported by a research grant provided by Medrad (Indianola, Pa). One author (U.J.S.) is a medical consultant to Bracco (Princeton, NJ), Medrad, and Siemens (Forchheim, Germany); one (P.C.) is a medical consultant to Bracco; and one (C.T.) is a medical consultant to Medrad. The authors who are not consultants to the company providing support had control of the data and information submitted for publication.
Patient Groups
The human research committee of the Medical University of South Carolina approved this retrospective analysis and waived the need to obtain informed patient consent. This study was conducted in compliance with the Health Insurance Portability and Accountability Act.
We retrospectively analyzed data for three cohorts of patients who had undergone clinically indicated contrast-enhanced 64-section multi–detector row CT angiography of the heart at our institution between October 2004 and June 2006. Over this period, our injector systems and injection protocols had undergone two upgrades, from a single-syringe injector (Stellant Sx; Medrad) and monophasic injection to a dual-syringe system (Stellant D; Medrad) and biphasic injection; the latter system was eventually "dual-flow" enabled—that is, we had the ability to inject a contrast medium–saline mixture of a predetermined mixing ratio after a bolus of pure iodinated contrast medium (the so-called split-bolus technique). The benefits of this dual-flow split-bolus injection protocol, which were the basis for this retrospective analysis, were intuitively evident at clinical implementation, so this protocol has remained our clinical standard ever since. The last consecutive 25 patients who had undergone CT performed with single-syringe technology (monophasic protocol, group 1) and the last consecutive 25 patients who had undergone CT performed with dualsyringe technology (biphasic protocol, group 2) were compared with the first 25 consecutive patients who underwent CT performed with the split-bolus injection protocol (group 3).
Image Acquisition
CT scanning was performed with a 64-section multi–detector row CT scanner (Somatom Sensation 64 Cardiac; Siemens) by using retrospective electrocardiographic (ECG) gating. Contrast-enhanced coronary CT angiography was performed in each patient by using the following parameters: 32 detector rows, a section thickness of 0.6 mm, the z-flying focal spot technique, a rotation time of 0.33 second, a pitch of 0.2, a tube voltage of 120 kV, and a tube current of 900 mAs. Patients who had average heart rates greater than 65 beats per minute and who had no contraindications received up to three 5-mg intravenous injections (up to 15 mg total) of metoprolol tartrate (Lopressor; Novartis, East Hanover, NJ) immediately before the examination to achieve a heart rate of less than 65 beats per minute. Scanning was begun regardless of the eventual heart rate obtained after metoprolol injection, so no patients were excluded because of heart rate. In the absence of contraindications (hypotension, current treatment with nitrate medications, migraine sensitive to nitrates), patients were given a 0.4-mg nitroglycerin tablet (NitroQuick; Ethex, St Louis, Mo) sublingually 2 minutes before the examination. Scans were acquired in a craniocaudal direction, with simultaneous recording of the patient's ECG signal to enable retrospective registration of image reconstructions to the desired cardiac phase. The scan range extended from the level of the carina to just below the dome of the diaphragm.
All 75 examinations were performed by using the same nonionic, low-osmolar contrast medium (iopamidol, Isovue [370 mg iodine per milliliter]; Bracco). The delay time was determined with injection of a test bolus. The test bolus in group 1 (the monophasic group) consisted of 20 mL of contrast medium automatically injected at a flow rate of 5 mL/sec. In groups 2 (the biphasic group) and 3 (the split-bolus group), the 20-mL contrast material bolus was followed by 50 mL of saline injected by using the dual-syringe injector. Consecutive single-level CT sections at the level of the aortic root were obtained every 2 seconds during breath holding. The aortic time-resolved attenuation was then measured by using the time-attenuation evaluation program accessible on the scanner, and the time of peak attenuation was used as the delay time for the actual examination. In group 1, the actual enhancement was achieved with 50–75 mL of contrast material. In group 2, 50–75 mL of contrast material was also injected, followed by a 50-mL saline chaser bolus. In group 3, a split-bolus protocol was used, with injection of 50–75 mL of pure, undiluted iodinated contrast material followed by a constant volume of 50 mL of a 70%:30% saline-to–contrast medium mixture and 30 mL of pure saline, as empirically recommended by the manufacturer. The desired mixing ratio was achieved by moving both pistons of the dual-syringe injector simultaneously at different speeds.
In all three groups, the contrast medium and the saline were infused through an 18-gauge intravenous antecubital catheter at 5 mL/sec. In all three groups, the contrast medium volume for the first iodine phase of injection was individually computed according to the following formula: V = ST · 5, where V is volume in milliliters and ST is scanning time in seconds. With our CT scanners, the scanning time is calculated on the basis of the length of the anatomic scan coverage and the pitch and is displayed on the CT scanner user interface before scan acquisition.
Image reconstruction was performed by using single-segment reconstruction and retrospective ECG gating. Reconstruction intervals relative to the R-R interval (percentage R-R interval) with the least cardiac motion were determined on the basis of a preview series that consisted of 20 images reconstructed at 20 R-R interval positions in 5% increments (0% to 95% of the R-R interval) at the same z-position at the mid-level of the heart. Image reconstruction parameters comprised an individually adapted field of view encompassing the heart, a matrix size of 512 x 512 pixels, a medium soft-tissue convolution kernel (B25f), and a section thickness of 0.75 mm with an increment of 0.3 mm.
No episodes of adverse allergic reactions or extravasation occurred. Scanning and bolus timing procedures were successfully completed in all patients.
Image Analysis
One observer (J.M.K., with 1 year of experience reading cardiac CT scans) who was blinded to the injection technique performed attenuation measurements in regions of interest by using the transverse sections on a medical workstation (Syngo Multi-Modality Workplace; Siemens). In each patient and for each target structure, three regions of interest were prescribed on three consecutive transverse sections depicting the respective target structure. For the coronary arteries, these measurements were performed in the proximal and distal segments of the major coronary arteries (left anterior descending [LAD] artery, left circumflex [LCX] artery, and right coronary artery [RCA]) as the respective target structures. Region of interest size was adjusted to encompass the entire contrast-enhanced vessel lumen, avoiding vessel walls and atherosclerotic plaques. For the cardiac chambers, measurements were performed in the center of the right atrium (RA) and left atrium (LA) at the level of the aortic valve and in the center of the right ventricle (RV) and left ventricle (LV) at the level of the mitral valve. Regions of interest were as large as possible and were placed in such a fashion that the myocardium, valvular structures, and papillary muscles were avoided. The mean attenuation (in Hounsfield units) ± the standard deviation in the regions of interest on three consecutive sections was calculated for each target structure.
Two observers (U.J.S. and J.G.R., with 10 and 5 years of experience reading cardiac CT scans, respectively) who were blinded to the injection technique reviewed in consensus each scan in random order by using the Syngo workstation and freely adjustable window settings. Using a five-point scale (described below), the observers subjectively rated the visualization of right and left heart structures and the occurrence and severity of streak artifacts. In the left heart, we evaluated the papillary muscles, the aortic valve, the mitral valve, and the LV myocardium. In the right heart, we evaluated the papillary muscles, the moderator band, the tricuspid valve, and the pulmonary valve, as well as the RV myocardium.
Visualization of anatomic structures was classified as follows: A grade of 1 indicated that the structure in question was not visualized; a grade of 2, that the structure was faintly visualized; a grade of 3, that the structure was visualized but that delineation was limited; a grade of 4, that the structure was visualized and complete delineation was possible; and a score of 5, that there was excellent visualization and exquisite delineation of the structure. For example, for the mitral valve, a score of 1 reflects nonvisualization; a score of 2, visualization of minor portions of the mitral valve apparatus; a score of 3, visualization of most portions; a score of 4, visualization of all portions; and a score of 5, crisp delineation that included subtle anatomic features, such as the chordae tendineae.
Occurrence and severity of streak artifacts and their effect on image quality were assessed as follows: A score of 1 indicated that severe streak artifacts obscured portions of the surrounding extracavitary anatomy (eg, coronary artery segments), as well as intracavitary anatomy; a score of 2, that severe streak artifacts completely obscured only the intracavitary anatomy of the right heart; a score of 3, that moderate streaking partially obscured the intracavitary anatomy of the right heart; a score of 4, that there was mild streaking and no interference with the depiction of the intracavitary anatomy of the right heart; and a score of 5, that there were no streak artifacts.
Statistical Analysis
All analyses and graphs were performed with statistical software (Sample Power, version 2.0, Sigma Stat, version 3.0, and Sigma Plot, version 8.0; SPSS, Chicago, Ill). Categorical variables are presented as percentages, and continuous variables such as age, weight, image quality scores, and attenuation (in Hounsfield units) are presented as mean values ± standard errors of the mean. Significant differences among the three groups (monophasic, biphasic, and split bolus) were analyzed by using a one-way analysis of variance. Further analyses were performed by using post-hoc multiple comparison procedures (the Duncan method). Sample size was estimated on the basis of a one-tailed significance level of P < .05. A power analysis was performed by using a one-way analysis of variance. The sample size of 75 for the three groups (25 patients per group) was determined to have a power level of 100% for testing for significant differences in average attenuation, with a standard deviation of ± 37.1 for comparison between the three groups (monophasic, biphasic, and split bolus). For image quality scores, with a standard deviation of ± 0.756, the sample size provided an 87% power to detect mean differences between the three groups. P < .05 was considered to indicate a statistically significant difference for all statistical tests. F tests were performed with two and 74 degrees of freedom.
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RESULTS
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Patients
Data for a total of 75 patients (27 women, 48 men; mean age, 62 years; range, 32–85 years) were analyzed. There were no significant differences among the three groups in terms of patient demographic characteristics such as age (P = .105), weight (P = .214) (Table 1), and mean heart rate (P = .175).
Attenuation
The average attenuation in the RA was 301.84 HU ± 24.9 in the monophasic group (group 1), 143.05 HU ± 16.7 in the biphasic group (group 2), and 210.52 HU ± 11.1 in the split-bolus group (group 3). The average attenuation in the RV was 320.72 HU ± 22.9, 147.92 HU ± 9.65, and 225.04 HU ± 9.34 for groups 1, 2, and 3, respectively. One-way analysis of variance of these data revealed a significant difference between the three groups for the RA (F = 31.818, P < .001) and the RV (F = 18.697, P < .001) (Table 2). Analyses for both the RA and the RV indicated that the attenuation in the split-bolus group (group 3) was significantly higher than that in the biphasic group (group 2). The attenuation in the monophasic group (group 1) was significantly higher than that in groups 2 and 3 (P < .05, Duncan test) (Fig 1). The attenuation measurements in the LA (F = 1.469, P = .237) and LV (F = 0.180, P = .836) (Table 3), as well as those in the proximal and distal segments of the RCA, LAD artery, and LCX artery (Table 4), were not significantly different among the monophasic, biphasic, and split-bolus groups.
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Figure 1: Bar graph shows mean contrast material attenuation in RA and RV in patients injected by using monophasic (black bars), biphasic (light gray bars), and split-bolus (dark gray bars) protocols. Average attenuation was significantly higher (* = P < .05, Duncan test) in the split-bolus group than in the biphasic group, while the attenuation in the monophasic group was significantly higher (+ = P < .05, Duncan test) than that in the other two groups. Error bars = standard errors of the mean.
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Streak Artifacts
In the monophasic group, the prevalence of streak artifacts was 100% in the SVC and 94% in the RA (Fig 2). Streak artifacts in the SVC and RA, respectively, occurred in 34% and 59% of studies in the biphasic group and in 91% and 67% of studies in the split-bolus group. As compared with values in group 1, CT image quality in the SVC (F = 32.685, P < .001) and the RA (F = 28.212, P < .001) in groups 2 and 3 was significantly (P < .05, Duncan test) better and less affected by streak artifacts from high-attenuation contrast medium. Mean image quality scores in the SVC and the RA, respectively, were 1.56 ± 0.13 and 2.40 ± 0.19 in group 1, 3.56 ± 0.22 and 4.40 ± 0.18 in group 2, and 3.16 ± 0.19 and 3.76 ± 0.20 in group 3 (Fig 3). There was no significant difference (P > .05, Duncan test) in the occurrence and severity of streak artifacts between groups 2 and 3 (Fig 3).
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Figure 2a: (a, b) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using monophasic injection with contrast medium only are displayed as (a) transverse section and (b) volume rendering seen from right anterior oblique perspective. There is sufficient contrast material attenuation in all cardiac chambers. However, a streak artifact (arrow in a) emanating from high-attenuation contrast material in the RA overlies the RCA, the atrial septum, and the crista terminalis and causes artifactual stenosis (arrow in b) of the proximal portion of the RCA.
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Figure 2b: (a, b) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using monophasic injection with contrast medium only are displayed as (a) transverse section and (b) volume rendering seen from right anterior oblique perspective. There is sufficient contrast material attenuation in all cardiac chambers. However, a streak artifact (arrow in a) emanating from high-attenuation contrast material in the RA overlies the RCA, the atrial septum, and the crista terminalis and causes artifactual stenosis (arrow in b) of the proximal portion of the RCA.
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Figure 3: Bar graph shows mean subjective image quality scores for assessment of image distortion by streak artifacts from high-attenuation contrast medium in SVC and RA. Higher scores indicate less degradation due to streak artifacts. CT studies in patients injected by using biphasic (B) and split-bolus (SB) protocols were significantly (P < .001) less affected by streak artifacts, with higher average image quality scores than studies in patients injected by using the monophasic (M) protocol. There was no significant difference in the occurrence and severity of streak artifacts between the biphasic and split-bolus protocols. * = Statistically significant difference. Error bars = standard errors of the mean.
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Delineation of Cardiac Structures
The delineation of right heart structures (papillary muscles [F = 37.514, P < .001], moderator band [F = 35.564, P < .001], tricuspid valve [F = 37.719, P < .001], pulmonary valve [F = 13.456, P < .001], and RV myocardium [F = 31.762, P < .001], respectively) (Fig 4) received significantly lower scores (P < .05, Duncan test) in the monophasic group (2.36 ± 0.14, 2.64 ± 0.14, 1.6 ± 0.14, 2.48 ± 0.15, and 2.36 ± 0.18) (Fig 2) and the biphasic group (1.4 ± 0.15, 1.68 ± 0.16, 1.28 ± 0.09, 1.84 ± 0.16, and 1.6 ± 0.18) (Fig 5) than in the split-bolus group (3.48 ± 0.21, 3.68 ± 0.20, 2.84 ± 0.16, 3.08 ± 0.19, and 3.6 ± 0.17) (Fig 6).
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Figure 4: Bar graph shows mean subjective image quality scores for assessment of delineation of right heart structures. Visualization of papillary muscles (PM), moderator band (MB), tricuspid valve (TV), pulmonary valve (PV), and RV myocardium (MY) received significantly higher average scores (P < .05, Duncan test) in the split-bolus group (dark gray bars) than in the monophasic (black bars) and biphasic (light gray bars) groups.
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Figure 5a: (a–c) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using a dual-syringe injector with a biphasic bolus (saline chaser technique) are displayed as (a) transverse section, (b) multiplanar reformation in four-chamber view, and (c) curved multiplanar reformation. There is high and homogeneous contrast material attenuation in the LA and LV, which allows assessment of the mitral valve apparatus (black arrows in a and b). The coronary arteries (in c) show high attenuation throughout their course, from proximal to distal segments of the LAD artery, LCX artery, and RCA. However, the low attenuation in the RA and RV barely allows visualization of the moderator band (white arrow in a) in the RV.
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Figure 5b: (a–c) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using a dual-syringe injector with a biphasic bolus (saline chaser technique) are displayed as (a) transverse section, (b) multiplanar reformation in four-chamber view, and (c) curved multiplanar reformation. There is high and homogeneous contrast material attenuation in the LA and LV, which allows assessment of the mitral valve apparatus (black arrows in a and b). The coronary arteries (in c) show high attenuation throughout their course, from proximal to distal segments of the LAD artery, LCX artery, and RCA. However, the low attenuation in the RA and RV barely allows visualization of the moderator band (white arrow in a) in the RV.
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Figure 5c: (a–c) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using a dual-syringe injector with a biphasic bolus (saline chaser technique) are displayed as (a) transverse section, (b) multiplanar reformation in four-chamber view, and (c) curved multiplanar reformation. There is high and homogeneous contrast material attenuation in the LA and LV, which allows assessment of the mitral valve apparatus (black arrows in a and b). The coronary arteries (in c) show high attenuation throughout their course, from proximal to distal segments of the LAD artery, LCX artery, and RCA. However, the low attenuation in the RA and RV barely allows visualization of the moderator band (white arrow in a) in the RV.
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Figure 6a: (a–e) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using the dual-syringe injector with split bolus are displayed as (a) transverse section, (b) multiplanar reformation in four-chamber view, (c) oblique four-chamber view, and (d) and oblique long-axis view, as well as (e) curved multiplanar reformation. There is high and homogeneous contrast material attenuation in the LA and LV that allows assessment of the mitral valve apparatus (black arrows in a and b). The coronary arteries (white arrow in a) show high attenuation throughout their course, from proximal to distal segments (in e) of the LAD artery, first diagonal branch (D1), LCX artery, and RCA. The level of attenuation in the RA and RV is not high enough to cause streak artifacts but allows visualization of right heart structures, such as the moderator band (arrow in c) and the tricuspid valve apparatus (white arrows in b and d).
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Figure 6b: (a–e) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using the dual-syringe injector with split bolus are displayed as (a) transverse section, (b) multiplanar reformation in four-chamber view, (c) oblique four-chamber view, and (d) and oblique long-axis view, as well as (e) curved multiplanar reformation. There is high and homogeneous contrast material attenuation in the LA and LV that allows assessment of the mitral valve apparatus (black arrows in a and b). The coronary arteries (white arrow in a) show high attenuation throughout their course, from proximal to distal segments (in e) of the LAD artery, first diagonal branch (D1), LCX artery, and RCA. The level of attenuation in the RA and RV is not high enough to cause streak artifacts but allows visualization of right heart structures, such as the moderator band (arrow in c) and the tricuspid valve apparatus (white arrows in b and d).
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Figure 6c: (a–e) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using the dual-syringe injector with split bolus are displayed as (a) transverse section, (b) multiplanar reformation in four-chamber view, (c) oblique four-chamber view, and (d) and oblique long-axis view, as well as (e) curved multiplanar reformation. There is high and homogeneous contrast material attenuation in the LA and LV that allows assessment of the mitral valve apparatus (black arrows in a and b). The coronary arteries (white arrow in a) show high attenuation throughout their course, from proximal to distal segments (in e) of the LAD artery, first diagonal branch (D1), LCX artery, and RCA. The level of attenuation in the RA and RV is not high enough to cause streak artifacts but allows visualization of right heart structures, such as the moderator band (arrow in c) and the tricuspid valve apparatus (white arrows in b and d).
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Figure 6d: (a–e) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using the dual-syringe injector with split bolus are displayed as (a) transverse section, (b) multiplanar reformation in four-chamber view, (c) oblique four-chamber view, and (d) and oblique long-axis view, as well as (e) curved multiplanar reformation. There is high and homogeneous contrast material attenuation in the LA and LV that allows assessment of the mitral valve apparatus (black arrows in a and b). The coronary arteries (white arrow in a) show high attenuation throughout their course, from proximal to distal segments (in e) of the LAD artery, first diagonal branch (D1), LCX artery, and RCA. The level of attenuation in the RA and RV is not high enough to cause streak artifacts but allows visualization of right heart structures, such as the moderator band (arrow in c) and the tricuspid valve apparatus (white arrows in b and d).
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Figure 6e: (a–e) Contrast-enhanced retrospectively ECG-gated CT coronary angiograms obtained by using the dual-syringe injector with split bolus are displayed as (a) transverse section, (b) multiplanar reformation in four-chamber view, (c) oblique four-chamber view, and (d) and oblique long-axis view, as well as (e) curved multiplanar reformation. There is high and homogeneous contrast material attenuation in the LA and LV that allows assessment of the mitral valve apparatus (black arrows in a and b). The coronary arteries (white arrow in a) show high attenuation throughout their course, from proximal to distal segments (in e) of the LAD artery, first diagonal branch (D1), LCX artery, and RCA. The level of attenuation in the RA and RV is not high enough to cause streak artifacts but allows visualization of right heart structures, such as the moderator band (arrow in c) and the tricuspid valve apparatus (white arrows in b and d).
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The scores for diagnostic visualization of left heart structures (papillary muscles [F = 0.907, P = .408], mitral valve [F = 0.245, P = .783], aortic valve [F = 0.177, P = .838], and LV myocardium [F = 0.910, P = .407], respectively) were not different among group 1 (3.81 ± 0.18, 3.81 ± 0.18, 3.67 ± 0.21, and 3.96 ± 0.17), group 2 (4.16 ± 0.21, 4.0 ± 0.21, 3.84 ± 0.21, and 4.28 ± 0.17), and group 3 (3.88 ± 0.19, 3.88 ± 0.19, .3.76 ± 0.21, and 4.0 ± 0.18) (Fig 7).
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Figure 7: Bar graph shows mean subjective image quality scores for assessment of delineation of left heart structures. Visualization of papillary muscles (PM), mitral valve (MV), aortic valve (AV), and LV myocardium (MY) were not significantly different among the monophasic (black bars), biphasic (light gray bars), and split-bolus (dark gray bars) groups. Error bars = standard errors of the mean.
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DISCUSSION
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The results of our study suggest that a split-bolus injection protocol as investigated here may improve visualization of the right heart, while still suppressing streak artifacts from inflowing contrast material and maintaining high attenuation in the left heart and coronary arteries.
Uniform, high, and consistent vascular attenuation within the vessel lumen is desirable for successful coronary CT angiography. Adequate attenuation is a prerequisite for the evaluation of the luminal integrity of the coronary tree, assessment of vessel wall changes, visualization of small side branches (5), and the use of threshold-dependent two-dimensional and three-dimensional postprocessing techniques (13).
With the expanding clinical use of cardiac multi–detector row CT techniques, the optimization of intravenous contrast material injection protocols for appropriate attenuation of the heart has become a topic of intense investigation (5–8,12). The introduction of dual-syringe injectors facilitated the approach of "chasing" the main contrast medium bolus with saline (8–12), which had required careful techniques for layering contrast medium and saline in the same syringe in the era of single-barrel injectors. Flushing with saline solution only avoids pooling of contrast material in the injection system and in the arm veins, leading to better contrast material utilization (11) and improved bolus shaping, with more consistent and homogeneous attenuation of target vessels (6). Disturbing streak artifacts from inflowing contrast material in the SVC and the right cardiac cavities, which may result in diagnostic pitfalls, such as artifactual stenosis of the proximal RCA due to overlying streaks (12), can be effectively avoided (6,10,14).
However, with modern power injector systems, minimized contrast material volumes, and sophisticated scan timing techniques (15), the saline chasing approach has become so effective that the right cardiac chambers are almost entirely void of contrast material at the time of scan acquisition in many patients. Although this is partially desirable for suppressing streak artifacts (8), the absence of contrast material in the RA and RV prohibits diagnosis of right heart disease, such as cavitary thrombi or tumors. Right heart structures, such as the tricuspid valve, pulmonary valve, and papillary muscles, cannot be assessed. Pathologic thickening of the RV myocardium (eg, that secondary to pulmonary hypertension or fatty infiltration in patients with arrhythmogenic RV dysplasia) cannot be appropriately evaluated. Sufficient attenuation of the right heart is needed for more recent approaches to deriving parameters of RV function from CT scans (16,17). Last, a prolongation of contrast medium attenuation within the pulmonary arteries along with high enhancement in the aorta and coronary arteries is a prerequisite for the assessment of all thoracic vascular territories for approaches to assessing patients with acute chest pain with a single contrast-enhanced ECG-gated study (18).
These considerations prompted us to transition our clinical injection protocols from the biphasic saline chaser technique, which had served us well for the evaluation of the left heart and coronary arteries, to the described split-bolus protocol, as soon as the capability of moving both pistons of a dual-barrel injector simultaneously had been enabled on our injector systems. Although the split-bolus injection protocol may appear complicated in theory, in practice, it can be stored on the injector device and selected from the menu of stored protocols.
The results of our study suggest that the adoption of split-bolus injection protocols for CT angiography of the heart preserves the desired attenuation of the left heart and the coronary arteries and effectively suppresses streak artifacts but provides just enough attenuation of the right heart to allow visualization of right cardiac structures. Use of a similar split-bolus injection protocol has recently been shown to be beneficial for general thoracic and aortic imaging (14). Related alternative approaches comprising biphasic injection of contrast material with decelerated injection rates during the second phase of injection have been described for general vascular (19) and cardiac (18) applications. Centers that operate a dual-syringe injector but do not have the capability of injecting the contents of both barrels simultaneously at different rates may achieve similar results by manually mixing saline and contrast medium in a desired ratio in the saline barrel.
Although we used unselected, consecutive patient cohorts to analyze attenuation patterns during the evolution of our clinical injection protocols, our study was limited by its retrospective nature. We do believe that our results can be generalized overall, although we chose to assess subjective image quality by consensus reading rather than testing for interobserver variability. For this analysis, we were concerned only with the effect of different contrast medium injection protocols on the evaluation of cardiac structure; therefore, we were unable to determine actual diagnostic performance for detection of right heart disease. In general, on the basis of common clinical experience, the prevalence of incidental disease isolated to the right heart will be fairly low, although not entirely negligible.
Although the empirically determined mixing ratio of 30% contrast medium to 70% saline for the second phase of injection works well in our practice and delivers the desired clinical results, as shown by the results of our analysis, we did not systematically investigate other conceivable parameter constellations and whether they would yield even better results. Also, while the use of a saline chaser has been shown to generally reduce the required contrast material volume (6,10,11), we did not investigate whether our split-bolus protocol could be tailored so that the overall contrast material volume could be kept constant or reduced. Rather, this protocol delivers 15 mL more contrast material to the patient compared with our previous biphasic protocol. However, most may agree that this small volume of additional contrast material can be considered negligible and well invested, in view of the potential diagnostic benefit.
Arguably, similar enhancement patterns may be obtained with a biphasic protocol consisting of a contrast medium–only phase and a mixed phase, without the use of a pure saline phase, albeit with a very slight increase in the overall contrast material dose. Last, although we have shown that our split-bolus protocol improves visualization of the right heart while maintaining the benefits of the saline chaser technique in terms of visualization of the left heart, coronary attenuation, and artifact suppression, this protocol still represents a standardized approach. The contrast medium volume of the initial injection phase as a function of scan duration is the only variable. While this works well for clinical practice, future research ought to be directed at individualized, patient-based injection protocols to ensure that the minimum amount of contrast material is injected for achieving the desired contrast material attenuation throughout the target anatomic areas (2,20,21).
In conclusion, the split-bolus injection as described here provides sufficient attenuation for visualization of the right heart, while streak artifacts from high-attenuation contrast material can generally be avoided and arterial attenuation is maintained.
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ADVANCE IN KNOWLEDGE
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- Split-bolus injection protocols with diluted contrast medium during the second phase of injection performed by using a dual syringe injector suppress streak artifacts from high-attenuation contrast medium in the right heart and provide sufficient attenuation for visualization of right heart structures.
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IMPLICATION FOR PATIENT CARE
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- The use of split-bolus injection protocols such as the one evaluated here during routine performance of coronary CT angiography enables comprehensive evaluation of right and left heart structures while preserving attenuation in the left heart and coronary arteries.
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FOOTNOTES
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Abbreviations: ECG = electrocardiography • LA = left atrium • LAD = left anterior descending • LCX = left circumflex • LV = left ventricle • RA = right atrium • RCA = right coronary artery • RV = right ventricle • SVC = superior vena cava
Author contributions: Guarantors of integrity of entire study, J.M.K., C.T., U.J.S.; 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, J.M.K., C.T., U.J.S.; clinical studies, J.M.K., C.T., U.J.S.; experimental studies, J.M.K.; statistical analysis, J.M.K., S.A.N.; and manuscript editing, all authors
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References
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ABSTRACT
INTRODUCTION
