Published online before print August 2, 2002, 10.1148/radiol.2243010691
(Radiology 2002;224:913-918.)
© RSNA, 2002
Coronary Artery Bypass Graft Flow: Qualitative Evaluation with Cine Single–Detector Row CT and Comparison with Findings at Angiography1
Richard Tello, MD, MSME, MPH,
George G. Hartnell, MD, FRCR,
Philip Costello, MD and
Christian P. Ecker, MD
1 From the Department of Diagnostic Radiology, Beth Israel-Deaconess Hospital, Harvard Medical School, Boston, Mass (R.T., G.G.H., P.C., C.P.E.); and Department of Radiology, Atrium-2, Boston University School of Medicine, 88 Newton St, Boston, MA 02218 (R.T.). From the 1992 RSNA scientific assembly. Received March 28, 2001; revision requested May 21; final revision received March 25, 2002; accepted March 28. Supported by Public Health Service grant RR 05591. Address correspondence to R.T. (e-mail: tello@alum.mit.edu).
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ABSTRACT
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A four-point ordinal-scale qualitative flow index was used for assessment of patency of 75 coronary artery bypass grafts in 26 patients examined with spiral computed tomography (CT). CT findings were compared with selective graft angiographic findings. Of 54 open grafts, 52 were patent at initial selective graft angiography and 50 were patent at spiral CT; accuracy rates were 97% (73 of 75) and 95% (71 of 75), respectively. Spiral CT flow index agreed with angiographically determined flow in 85% (95% CI: 0.77, 0.93) of grafts. The 
statistic demonstrated very good to excellent intermodality (0.75) and interobserver (0.89) agreement. Spiral CT may be a feasible means of assessing quality of flow in bypass grafts.
Supplemental material: radiology.rsnajnls.org/cgi/content/full/2243010691/DC1
© RSNA, 2002
Index terms: Angiography, technology, 54.1242 • Computed tomography (CT), helical, 54.12115 • Coronary vessels, bypass graft • Coronary vessels, CT, 54.12115, 54.12117 • Grafts
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INTRODUCTION
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The use of computed tomography (CT) in the assessment of coronary artery bypass graft (CABG) patency was discussed more than 20 years ago (1). Subsequently, the visualization of CABGs with CT by using either sequential imaging of the heart or dynamic CT has been widely reported, with varying success and a sensitivity range of 45%–100% and with contrast material volumes of 25–150 mL (2–5). Findings in many studies indicated increased sensitivity as the number of levels imaged was increased (6–8). In particular, since the left anterior descending artery graft often arches upward over the main pulmonary artery, it may be missed if an appropriate level is not chosen. Thus, this missing of grafts at imaging contributes to the false-positive rate observed with CT (9). Although it should follow that there would be greater sensitivity for any potential volumetric imaging technique, this has generally been precluded by the large amounts of contrast agent that would be required with use of previous CT technology. In each study, CT results were directly correlated with angiographic assessment.
Spiral CT involves continuous scanning while the patient is simultaneously advanced through the gantry during a single breath hold. Acquired data can be reconstructed in the axial plane at varying section positions at any point in the scanning cycle, with no apparent difference in spatial resolution between conventional and volumetric scans (10). As the intravenously administered contrast material travels through the graft segments, good image detail is achieved, and this achievement of detail allows visualization of entire coronary artery graft segments during one injection. Earlier work has demonstrated the comparable accuracy of spiral CT and selective graft angiography in establishing patency rates (11,12). The purpose of this article was to report our experience and technique with spiral CT in the assessment of a qualitative coronary artery bypass graft flow index and the purpose of this study was to determine its accuracy compared with that of selective graft angiography.
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Materials and Methods
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Patients
Institutional review board approval was obtained, and patients were consecutively selected from the population of cardiac catheterization patients at Beth Israel-Deaconess Hospital. During this period, daily cardiac catheterization schedules were reviewed by one (R.T.) of us to identify patients who were undergoing selective graft angiography, who did not have an elevated creatinine level (<1.3 mg/dL [<114.9 µmol/L]), who did not undergo harvesting of the right internal mammary artery for graft placement, and who were not participating in any other study. Of 23 patients who met these criteria, 20 signed the institutional review board–approved consent form, and 17 of these patients underwent a spiral CT examination.
An additional nine patients were directly referred for clinical investigation to clarify ambiguous anatomy. All 26 patients underwent selective graft angiography. Nine (35%) patients were women and 17 were men (65%), nine were diabetic (35%), and the mean age was 67 years (range, 53–82 years) at the time of cardiac catheterization. Mean weight was 79 kg ± 15. Mean graft age was 6 years (range, 0–17 years) at the time of examination with spiral CT. The mean time between selective graft angiography and spiral CT was 2.5 days (range, 0–66 days), and 65% (17 of 26) of spiral CT studies were performed within 24 hours after selective graft angiography.
CT Technique
With a minimum of a 20-gauge Angiocath (Deseret Medical, Sandy, Utah) in an antecubital vein for vascular access, the patient was placed on the table of the CT unit (Somatom Plus-S; Siemens Medical Systems, Erlangen, Germany) in the supine position with the arms next to the sides. A safety strap was secured around the middle of the thorax to support the arms. The catheter was then connected to a power injector (Mark IV; Medrad Systems, Pittsburgh, Pa) that was preloaded with 100 mL of contrast material (iopamidol, Isovue 300; Squibb, New Brunswick, NJ). An 18-mL bolus (ie, administered at a rate of 3 mL/sec for 6 seconds) was injected, and during injection a single-level multiple scanning acquisition was performed. The specific time delay in the patient was calculated as was reported previously (11).
Depending on the cephalocaudal extent of the heart in the patient, either a 5 mm/sec or an 8 mm/sec table feed for 24–32 seconds with a mean beam width of 5 or 8 mm, respectively, was used. During spiral CT with the single-breath-hold technique (without cardiac gating), the patient received a bolus injection of intravenous contrast material administered at a rate of 3 mL/sec for 23 seconds (69 mL), with image acquisition commencing after the calculated time delay (11,13). The images were reconstructed at 2-mm intervals, and the initial three-dimensional (3D) reconstruction image was obtained after an appropriate threshold (usually 50–100 HU) was selected by the radiologist after reviewing all transverse images (55–92 images per study). The lower threshold was determined according to the ability to separate graft from myocardium.
A graft was defined as being open if it was seen to opacify over more than 1.2 cm on the transverse images (six sequential images), as has been previously validated (11). Graft patency was evaluated with visual assessment of graft opacification on multiple transverse images and 3D reconstruction images by two reviewers (P.C., G.G.H.) who were blinded to the catheterization results in each patient and to each other’s evaluation.
Image Analysis
Three-dimensional flow images were generated by a technologist who selected a 3D projection by using a shaded-surface display that optimally demonstrated the grafts by using the previously mentioned threshold and then generating 10 more 3D images in the same projection, each at a threshold 10 HU higher than the previous image. These frames were then reviewed in a cine-loop mode by a radiologist (R.T. or G.G.H.), and open grafts were classified by using an ordinal score. Flow index evaluation was determined according to the following scale: grade 0, none; grade 1, poor; grade 2, good; or grade 3, excellent. This evaluation was performed without knowledge of the results of selective graft angiography, with independent readings conducted by blinded observers (R.T., G.G.H.). This selective determination of thresholds generates images at differing points along the distance-density curve and yields information regarding the leading edge of the contrast bolus in a graft segment. Acquisition time, image generation time, and room time were recorded, as was clip artifact, breath-holding ability, and the patient-specific time delay between injection and scan acquisition.
Selective Graft Angiography
In all cases, selective graft angiography was performed through a femoral approach by using selective catheterization of grafts or graft stumps. In cases in which grafts were known to have been inserted but could not be opacified, an ascending aortogram was obtained (typically with 50 mL of contrast material administered at a rate of 20–25 mL/sec), all angiograms were reviewed by a cardiac radiologist (G.G.H.) who was unaware of CT results, and the TIMI II A flow criteria were used to rate graft flow (14).
Statistical Analysis
All results were collated at the end of the study, and patency rates with sensitivity and specificity were computed. The quality of flow (flow index) was correlated with the statistic, which was calculated to measure agreement between raters and between the ratings at angiography and at CT (15). Results were tabulated and compared with those at angiography, and the McNemar test of marginal homogeneity for asymmetry between the tests was performed (16). The Wilcoxon signed rank test was performed to compare the ordinal grading of flow determined at selective graft angiography and at cine spiral CT (17). Analysis was performed with a statistical software package (version 5.0; Stata, College Park, Tex).
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Results
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Patients
At spiral CT, 14 patients were examined at a table feed of 8 mm/sec and 12 were examined at a table feed of 5 mm/sec. One patient was examined at both table feeds. In the latter patient, there was no difference in image quality, number of grafts identified, correlation with angiographic findings, quality of 3D reconstruction, or cine-loop visualization. The mean dose of contrast material was 86 mL ± 14 (SD). Figure 1 demonstrates cine 3D flow reconstruction images obtained in this patient at the examination at a table feed of 8 mm/sec (See also Movie; radiology.rsnajnls.org/cgi/content/full/2243010691/DC1). The reconstruction images were generated in a right anterior oblique view with caudal angulation, with three frames at thresholds of 150, 130, and 110 HU. In none of these patients did surgical clips produce beam-hardening artifacts that complicated image analysis. All patients successfully held their breath for the 24–32-second scan acquisition. Mean time delay for contrast material arrival at the aortic root at spiral CT in these cases was 20.7 seconds ± 3.6 (range, 16–29 seconds).
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Figure 1a. Frames from sequential cine angiographic 3D reconstruction image from spiral CT scan obtained in same 71-year-old white man 1 year after four-vessel CABG was performed show patent right (curved arrow) saphenous vein grafts in right anterior oblique projection. Note retained external pacer leads (straight arrow) on the pericardial surface are observed as thicker structures because of high CT attenuation and partial-volume artifacts compounded by cardiac motion. Images were obtained by using a transverse acquisition and have been reconstructed in 30° right anterior oblique view with slight caudal angulation. (Movie, radiology.rsnajnls.org/cgi/content/full/2243010691/DC1.) (a) Reconstruction image obtained with threshold at 150 HU. (b) Reconstruction image obtained with threshold at 130 HU. (c) Reconstruction image obtained with threshold at 110 HU.
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Figure 1b. Frames from sequential cine angiographic 3D reconstruction image from spiral CT scan obtained in same 71-year-old white man 1 year after four-vessel CABG was performed show patent right (curved arrow) saphenous vein grafts in right anterior oblique projection. Note retained external pacer leads (straight arrow) on the pericardial surface are observed as thicker structures because of high CT attenuation and partial-volume artifacts compounded by cardiac motion. Images were obtained by using a transverse acquisition and have been reconstructed in 30° right anterior oblique view with slight caudal angulation. (Movie, radiology.rsnajnls.org/cgi/content/full/2243010691/DC1.) (a) Reconstruction image obtained with threshold at 150 HU. (b) Reconstruction image obtained with threshold at 130 HU. (c) Reconstruction image obtained with threshold at 110 HU.
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Figure 1c. Frames from sequential cine angiographic 3D reconstruction image from spiral CT scan obtained in same 71-year-old white man 1 year after four-vessel CABG was performed show patent right (curved arrow) saphenous vein grafts in right anterior oblique projection. Note retained external pacer leads (straight arrow) on the pericardial surface are observed as thicker structures because of high CT attenuation and partial-volume artifacts compounded by cardiac motion. Images were obtained by using a transverse acquisition and have been reconstructed in 30° right anterior oblique view with slight caudal angulation. (Movie, radiology.rsnajnls.org/cgi/content/full/2243010691/DC1.) (a) Reconstruction image obtained with threshold at 150 HU. (b) Reconstruction image obtained with threshold at 130 HU. (c) Reconstruction image obtained with threshold at 110 HU.
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A representative distance-density curve and the determination of the arrival of the contrast material bolus at 13.7 seconds in a left saphenous vein graft and in the aorta after peripheral injection are demonstrated in Figure 2. Of a total of 75 original grafts, 54 were patent. Fifty-two (sensitivity, 96%; specificity, 100%) were patent at initial selective graft angiography, and 50 (sensitivity, 93%; specificity, 100%) were patent at spiral CT, and these data yielded accuracy rates of 97% and 95%, respectively. In the grafts that were missed at coronary angiography originally, a cannula could not be inserted during angiography, and these grafts could not be seen at aortography. After spiral CT demonstration of patency, repeat selective graft angiography demonstrated that these grafts were patent. In addition, examination at repeat bypass surgery confirmed the spiral CT results in the one case in which surgery was performed.
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Figure 2. Graph of CABG distance-density curves. Hounsfield units versus length along coronary bypass graft. DA = descending aorta, LSVG = left vein graft, RIMA = right internal mammary artery, RSVG = Right vein graft. Time is horizontal axis and represents time at which contrast level was measured during spiral CT.
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Conceptually, as a graft with rapid flow is imaged, the contrast material will arrive rapidly and plateau rapidly. Hence, selective determination of a threshold will generate an image of a graft of long length on most frames. However, a graft with poor flow will have a slower uptake of contrast material, and selective determination of a threshold will generate an image of a graft of varying length on all frames. This is demonstrated in principle in Figure 3.
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Figure 3. Depiction of time-attenuation curves and lengths of grafts visualized. As two relative time-attenuation curves are generated for spiral CT to assess flow in two vessels, a vessel with fast flow and a more rapid upstroke in the time-attenuation curve and a vessel with slower flow and a more delayed time to peak, the visualized length of a vessel at a given Hounsfield unit level will be longer earlier in the vessel with faster flow (white rectangle) than in the vessel with slower flow (black rectangle). This conceptually elucidates the premise behind the flow index derived from a CT acquisition, because higher flows will correlate with longer lengths of grafts visualized. As selective threshold determination is used, the vessels with greater flow will have a faster upstroke and thus will have a longer vessel length visualized at lower thresholds.
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Twenty-five patent vein grafts coursed to the right coronary artery, and 17 were patent. Thirty-three patent vein grafts coursed to the left (main and circumflex arteries), and 24 were patent at selective graft angiography and spiral CT. Seventeen left internal mammary artery grafts were found, and 13 were patent. Flow index evaluation was performed according to the following scale: grade 0, none; grade 1, poor; grade 2, good; or grade 3, excellent. Results are tabulated in Table 1, and these results demonstrate an 85% agreement (95% CI: 0.77, 0.93) between quality of flow determined at cine spiral CT and at selective graft angiography. Correlation of graft flow quality determined at cine spiral CT by two independent observers (G.G.H., R.T.) was not statistically different on the basis of the Wilcoxon signed rank test.
Comparison of spiral CT and selective graft angiography by using the McNemar test of marginal homogeneity shows that there is no significant (P = .39) asymmetry between the two techniques in establishing quality of flow.
Observer Agreement
The statistic analysis for interobserver agreement was 0.75 (95% CI: 0.61, 0.88) for both spiral CT readings (Table 2), with intramodality agreement of 85% (95% CI: 0.77, 0.93) (Table 1). The statistic for interobserver agreement was 0.89 (95% CI: 0.80, 0.98) at the first reading and 0.90 (95% CI: 0.81, 0.97) at the second reading. For observers 1 and 2, the analysis yielded intraobserver agreement of 0.80 (95% CI: 0.60, 0.92) and 0.89 (95% CI: 0.80, 0.98), respectively. This indicates that agreement between observers and methods was very good to excellent.
Time
All CT acquisitions were performed with a 32-second scanning time, no patient required more than 30 minutes of scanning room time, and time for all image generation was less than 30 minutes in all cases.
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Discussion
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Spiral CT has created numerous opportunities for the assessment of coronary vascular disease with low-contrast volumes (10,18). The ability to effectively ascertain the mean transit time (19,20) in each patient with the use of tailored timing (21) to improve the quality of contrast enhancement at CT has been crucial in establishing the ability of spiral CT to optimize static graft opacification by tailoring the image acquisition in each patient on the basis of a dynamic multiscanning technique (11). It is a natural extension to attempt to evaluate flow within actual graft segments once the data are acquired.
The use of multiple frames generates images that reflect the leading edge of the contrast material bolus within CABG segments. Thus, segments with better flow will have longer segments opacified at a point in time after initial bolus administration. Though it may not be ultimately desirable to determine an individual patient’s circulation time with dynamic multiscanning and the requisite use of ionizing radiation and iodinated contrast material, this determination precludes the need for additional equipment and separate methods (21) and is simple. In addition, this method can be applied to CT angiographic evaluation of other vascular structures, such as renal arteries (22), carotid arteries (23), and venous structures (24).
The potential role that coronary stents would play in this technique is yet to be evaluated; however, in light of the ability for intrastent patency to be evaluated in renal arteries, (25) it is anticipated that similar results would apply to evaluation in the heart. In the setting of similar techniques with contrast material–enhanced magnetic resonance (MR) imaging, the results for patency evaluation are similar (26); however, no measures of flow assessment were made, though future work could be performed in a manner similar to ours. The use of MR imaging may be limited in the context of pacemakers, and, as such, CT techniques should always have a place in CABG evaluation.
This article outlines a technique that allows the presentation of a graft flow index by using spiral CT in a way similar to that of selective graft angiography without demanding extensive computation or graph generation that requires extensive training for interpretation, as would be required in any approach based on distance-density curves (27). With the use of a cine-loop display of the 3D images obtained, an additional dimension of information can be delineated (28). Thus, by analogy, the cine-loop display of 3D images allows the addition of temporal flow information to be extracted.
The discordance in flow index assessment may be caused by deficiencies in the cine spiral CT technique, may reflect the difference between the true physiologic flow determined by peripheral myocardial demands at differing points in time, or may reflect differences in flow assessment between a noninvasive administration of contrast material, as in spiral CT, and a forceful delivery of contrast material at the origin of graft segments, as in coronary angiography. However, further studies regarding these possible explanations, with a comparative assessment that includes myocardial scintigraphic assessment, to attempt to quantitate myocardial perfusion effects should be performed.
It is important to keep in mind that although graft patency and quality of flow through individual grafts can, at this time, be determined with spiral CT, a few caveats should be mentioned. Because of the temporal nature of a 32-second volumetric acquisition, the actual registration of graft positions from section to section may not be accurate. In addition, anastamosis sites and areas of stenosis may not be adequately evaluated compared with selective graft angiography. However, the potential ability to evaluate patients’ grafts for change and to stratify patients into groups with increasing probability of graft dysfunction would appear to have clinical utility. In addition, as electron-beam and spiral technology evolves to a common ground, the technique elaborated in this article may actually allow graft flow evaluation without these limitations.
Findings of this study confirm that evaluation of CABG flow indices with spiral CT is feasible, rapidly performed (ie, less than 30 minutes is required for each examination in total, and up to 32 seconds is required for the actual volumetric acquisition), and relatively noninvasive. Work in the integration of the volumetric acquisition, in such a way that a more accurate numeric measure of graft flow and detailed graft anatomy with 3D display of cardiac structures could be performed, would be a natural next goal. Furthermore, additional calculation indicates that a multiinstitutional study of graft flow performed by using the technique described herein would be the next step to demonstrate a 5% difference in establishing quality of flow indices with a power of .8. As such, the potential application toward graft flow assessment and stress flow reserve with spiral CT may be feasible.
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ACKNOWLEDGMENTS
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We thank CT technologists Betty Scholz, RRT, Cathy Hoffman, RRT, Carol Zemski, RRT, and Stan Dudak, RRT, for their patience and assistance throughout this study.
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FOOTNOTES
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Abbreviations: CABG = coronary artery bypass graft,
3D = three-dimensional
Author contributions: Guarantor of integrity of entire study, R.T.; study concepts and design, R.T., G.G.H.; literature research, R.T., C.P.E., P.C.; clinical and experimental studies, R.T., C.P.E., G.G.H., P.C.; data acquisition, R.T., C.P.E., G.G.H.; data analysis/interpretation, R.T., C.P.E., P.C., G.G.H.; statistical analysis, R.T.; manuscript preparation, G.G.H., R.T., P.C.; manuscript definition of intellectual content and editing, R.T.; manuscript revision/review, R.T., G.G.H.; manuscript final version approval, R.T.
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ABSTRACT
INTRODUCTION
