Usefulness of multislice spiral computed tomography coronary angiography in patients with acute chest pain in the emergency department

Journal of Cardiovascular Computed Tomography (2007) 1, 29 –37

Original Research Article

Usefulness of multislice spiral computed tomography coronary angiography in patients with acute chest pain in the emergency department Steffen Huber, MDa, Martin Huber, PhDd, Debra Dees, RNa, Frank A. Redmond, MD, PhDc, James M. Wilson, MDb, Scott D. Flamm, MDa,b* Departments of aRadiology and bCardiology, the Texas Heart Institute at St Luke’s Episcopal Hospital, and the c Department of Emergency Medicine, St. Luke’s Episcopal Hospital, Houston, TX, USA and dSiemens Medical Solutions USA, Inc, Malvern, PA, USA KEYWORDS: Acute chest pain; Coronary calcium scoring; Emergency department; Image quality; Multislice computed tomography; X-ray computed tomography

Abstract BACKGROUND: Despite reports that multislice spiral computed tomography (MSCT) has high sensitivity and specificity in preselected patient populations, the routine clinical feasibility and utility of MSCT coronary angiography in patients with acute chest pain in the emergency department remains uncertain. OBJECTIVES: We sought to determine whether 16-slice MSCT coronary angiography can provide diagnostically useful images in patients with acute chest pain in the emergency department. METHODS: Ninety-eight patients in the emergency department (41 men, 57 women; mean age ⫾ SD, 48.1 ⫾ 11.9 y) with acute chest pain underwent MSCT coronary angiography. Coronary calcium (Agatston) scoring was performed, followed by contrast-enhanced MSCT. Images were evaluated for mean image quality (MIQ) and for degree of stenosis. These data were correlated with body mass index (BMI; in kg/m2), heart rate, beat-to-beat variation, and calcium score to assess their influence on image quality. RESULTS: The 28 patients (29%) with nondiagnostic MIQs had significantly higher BMIs (mean ⫾ SD, 32.9 ⫾ 9.1 vs 28.9 ⫾ 6.7; P ⬍ 0.05) and heart rates (mean ⫾ SD, 71.0 ⫾ 11.9 beats/min vs 65.6 ⫾ 9.9 beats/min; P ⬍ 0.05) than patients with diagnostic MIQs. Forty-five patients (46%) had at least 1 nondiagnostic coronary segment. These patients had significantly higher heart rates (mean ⫾ SD, 70.5 ⫾ 10.3 vs 64.1 ⫾ 13.7; P ⬍ 0.05) than patients with only diagnostic-quality scans. Image quality correlated inversely and strongly with BMI and heart rate. CONCLUSIONS: Sixteen-slice MSCT coronary angiography cannot routinely provide diagnostically useful images in patients with acute chest pain in the emergency department. © 2007 Society of Cardiovascular Computed Tomography. All rights reserved.

Introduction When compared with selective coronary angiography, 4-slice,1–3 and 64-slice4 – 6 multislice spiral computed tomog-

Conflict of interest: Dr Flamm reports that he received grant or research support from Siemens Medical Solutions. Dr M. Huber is an employee of Siemens Medical Solutions. * Corresponding author. Address correspondence to Scott D. Flamm, MD, Cardiovascular Imaging Laboratory, Department of Radiology, Hb-6, Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195. E-mail address: [email protected] Submitted February 23, 2007. Accepted for publication April 30, 2007.

raphy (MSCT) coronary angiography have shown great promise for detecting coronary artery stenosis with high sensitivity and specificity. Enhanced sensitivity and specificity of 16-slice MSCT have been achieved in selected patient populations treated with ␤-blockers for heart rate reduction.2,3,7,8 High heart rate9 –15 and calcification7,9,16,17 are both known to decrease image quality. In one previous study, the ability of 16-slice MSCT to correctly diagnose coronary artery stenosis increased from 36% to 91% after thresholds for maximum calcification and heart rate were applied.18 In another study, sensitivity increased when only coronary segment images of excellent quality were evaluated19 or when adaptive multiseg-

1934-5925/$ -see front matter © 2007 Society of Cardiovascular Computed Tomography. All rights reserved. doi:10.1016/j.jcct.2007.04.013

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ment reconstructions were applied.12,20 Recently introduced 64-slice MSCT scanners have further improved sensitivity and specificity. However, 64-slice MSCT scanners are not routinely available to evaluate patients in the emergency department.4 – 6 Although 16-slice MSCT scanners are now widely available, their routine clinical feasibility and utility for evaluating patients with acute chest pain in the emergency department remain uncertain. The aims of the present study were to determine whether 16-slice MSCT coronary angiography can routinely provide diagnostically useful information for patients with acute chest pain in the emergency department and to assess the influence of body mass index (BMI; in kg/m2), heart rate, heart rate variability, and coronary calcium score on image quality.

Methods

Table 1

Patient characteristics

Total patients, n Age, y Sex, n Women Men Body mass index, kg/m2 Heart rate, beats/min TIMI risk score† Calcium score Pretest probability of CAD‡ Very low, n (%) Low, n (%) Intermediate, n (%) High, n (%) Not determined, n (%)

98 48.6 ⫾ 11.0 (26–80)*

30.3 67.9 1.8 51.8

57 41 ⫾ 7.7 (15.0–56.3)* ⫾ 10.8 (49–102)* ⫾ 1.3 ⫾ 269.4 (0.0–2092.3)*

8 (8.2) 22 (22.7) 57 (58.8) 4 (4.1) 6 (6.2)

*Values are mean ⫾ standard deviation (min–max). †Thrombolysis in myocardial infarction (TIMI) risk score for unstable angina or non-ST-elevation myocardial infarction.31 ‡Pretest probability of coronary artery disease (CAD).32

Study population One hundred consecutive, prospectively identified patients with acute chest pain who were admitted to our emergency department between January 2004 and March 2005 gave written informed consent to participate in this study, which had been approved by the hospital’s institutional review board and which complied with the Declaration of Helsinki. The study nurse screened 579 patients for enrollment in the study, of whom 262 met the study criteria and were enrolled. The emergency department or patient’s attending physician agreed with the enrollment 252 of 262 times. One hundred of the 262 patients consented to participate in the study. Two patients were excluded from the study after having allergic (nonanaphylactic) reactions to 20 mL contrast medium for a test bolus scan, leaving 98 patients who underwent MSCT coronary angiography (Table 1). Inclusion criteria were admission to the emergency department with acute chest pain, no history of coronary artery disease, and no contraindications to the administration of iodinated contrast media. Exclusion criteria were electrical or hemodynamic instability, confirmed diagnosis of acute myocardial infarction, urgent need for coronary angiography or admission to the critical care unit, renal insufficiency, previous kidney transplantation, confirmed or suspected pregnancy, arrhythmia, history of bronchospastic disease, and previous sensitivity to the ␤-blocker metoprolol. Patients received a standard clinical workup, including noninvasive testing. As a consequence, oral ␤-blocker therapy was administered only to 16 patients who were not scheduled for or expected to undergo nuclear stress testing.

Data acquisition All scans were performed with a 16-slice MSCT scanner (Sensation 16; Siemens Medical Solutions USA, Inc,

Malvern, PA). First, an electrocardiogram-gated scan was performed without contrast media to determine the total calcium burden of the coronary arteries. Before the contrastenhanced scan was performed, a 20-mL bolus of contrast medium (containing 320 mg/mL organically bound iodine) was injected intravenously at a rate of 4 mL/s to determine the optimal scan delay time. Then, a contrast-enhanced, retrospectively electrocardiogram-gated scan was performed to visualize the coronary arteries while an additional bolus of contrast medium (100 –120 mL) was administered intravenously at a rate of 4 mL/s. The scanning parameters were as follows: detector collimation, 16 ⫻ 0.75 mm; tube rotation time, 420 ms; table feed, 3 mm/rotation; tube voltage, 120 kV; tube current, 337– 444 mA; pitch, 0.28 – 0.31; and 500 – 610 effective mA. Prospectively triggered x-ray tube current modulation (electrocardiogram pulsing) was applied in all cases as a dose-saving technique. The beginning of the acquisition window with full dose was set to 65%. Each scan was acquired during a single breath-hold of approximately 20 seconds.

Image reconstruction Images were reconstructed with a soft tissue reconstruction kernel (B30f). To obtain motion-free images, the reconstruction window was initially set at 65%. Additional image reconstructions were performed as necessary to optimize coronary artery visualization. The mean number of reconstructed phases was 2.4. A reconstruction algorithm that used data obtained in half gantry rotation time was applied for patients with heart rates of 72 beats/min, resulting in an image acquisition window of 210 ms. A bisegmental reconstruction algorithm was applied in patients with a heart rate ⬎ 72 beats/min, resulting in an image acquisition window between 105 and 210 ms.

Huber et al Table 2

MSCT coronary angiography in emergency patients

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Characteristics of patients with diagnostic versus nondiagnostic mean image quality

2

BMI, kg/m HR, beats/min Mean BBV, ms SD BBV, ms Calcium score Age, y

Diagnostic (MIQ ⱕ 2) n ⫽ 70

Nondiagnostic (MIQ ⬎ 2) n ⫽ 28

Mean

SD

Mean

SD

P

Statistical test

28.9 65.6 24.0 34.6 37.5 47.2

6.7 9.9 28.8 59.7 204.0 11.3

32.9 71.0 18.5 24.5 18.2 50.0

9.1 11.9 13.2 26.5 52.1 9.8

0.03 0.005 0.42 0.69 0.90 0.54

t test Mann-Whitney Mann-Whitney Mann-Whitney Mann-Whitney t test

U U U U

BBV ⫽ beat-to-beat variation; BMI ⫽ body mass index; HR ⫽ heart rate; MIQ ⫽ mean image quality; SD ⫽ standard deviation.

Calcium scoring Coronary artery calcium scores were determined on an off-line workstation (Leonardo; Syngo Software version VD10B; Siemens Medical Solutions). The scoring system used was equivalent to the one originally developed by Agatston et al21 for quantifying coronary calcification by electron beam computed tomography.

Image evaluation Original axial slices obtained by MSCT were reformatted by trained technologists into standardized 5-mm maximum-intensity projections (left anterior oblique, right anterior oblique, and spider views). Two experienced readers jointly determined the diagnostic quality of each epicardial coronary segment according to American College of Cardiology and American Heart Association (ACC/AHA) criteria.22 Additional thin multiplanar reformation slices were obtained interactively in standard and nonstandard orientations as necessary to further define areas of suspected abnormality. Image quality was classified on a segment-bysegment basis with the following scale: 1 (excellent), no blurring or motion artifact; 2 (good), mild blurring or motion artifact; 3 (moderate), moderate blurring or motion artifact; and 4 (poor), severe blurring or motion artifact. Scans with a mean image quality (MIQ) of 1 or 2 were considered diagnostic; those with an MIQ of 3 or 4 were considered nondiagnostic.

Statistical analysis Simple correlation analyses were performed with Pearson product-moment coefficient to evaluate the relationships between MIQ and several potential predictors: BMI, heart rate, mean and standard deviation (SD) of beat-to-beat variations, and calcium score. Because heart rate variation during image acquisition can cause stepladder artifacts,8 heart rate variation was measured as both the mean and the SD of beat-to-beat variation. Mean beat-to-beat variation

was defined as the mean of the absolute differences between 2 consecutive interbeat (RR) intervals. The SD of beat-tobeat variation was defined as the SD of the differences between 2 consecutive RR intervals. Multiple linear regression analysis was used to assess the dependence of MIQ on heart rate, the mean and SD of beat-to-beat variations, and calcium score. Two models were evaluated: in the first model, all variables were entered, and in the second model, variables were selected with a stepwise technique that provided an optimum model by selecting a minimum number of variables that had less multicollinearity than the full set of variables. The threshold of significance, which was the criterion for entry into the model, was set at 0.05. For variables that were not distributed normally, the Mann-Whitney U test was used to test for differences. For normally distributed variables, Student t test was used to test for differences. Relations between categorical variables were assessed using the chi-square test. A P value ⬍ 0.05 was considered significant. The distribution of all variables was presented as the mean ⫾ SD.

Results Of a total of 1470 coronary artery segments scanned, 783 (53%) were visualized and evaluated for diagnostic quality, and 687 (47%) were excluded because they were ⬍2 mm in diameter or were not visualized at all. Of the 783 segments evaluated for image quality, 644 (82%) were of diagnostic quality (424 [54%] of image quality 1 and 220 [28%] of image quality 2), and 139 (18%) were of nondiagnostic quality (68 [9%] of image quality 3 and 71 [9%] of image quality 4). On average, 8.0 coronary segments were evaluated per patient. The MIQ of all coronary segments for all patients was 1.76 ⫾ 0.75. Twenty-eight patients (29%) had an MIQ ⬎ 2.0, which was considered nondiagnostic. These patients had a significantly higher BMI (mean ⫾ SD, 32.9 ⫾ 9.1 vs 28.9 ⫾ 6.7; P ⬍ 0.05) and heart rate (mean ⫾ SD, 71.0 ⫾ 11.9 beats/min vs 65.6 ⫾ 9.9 beats/min; P ⬍ 0.05) than did patients with a diagnostic MIQ (Table 2).

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Table 3

Characteristics of patients with all diagnostic segments versus patients with at least 1 nondiagnostic segment

HR, beats/min BMI, kg/m2 Mean BBV, ms SD BBV, ms Calcium score Age, y

All diagnostic (IQ ⱕ 2) n ⫽ 53

ⱖ1 Nondiagnostic (IQ ⬎ 2) n ⫽ 45

Mean

SD

Mean

SD

P

Statistical test

64.1 29.8 19.0 25.9 50.7 48.7

13.7 8.4 17.9 47.6 248.0 13.7

70.5 30.2 25.1 35.9 11.1 47.2

10.3 7.1 32.2 58.5 43.1 9.2

0.001 0.67 0.56 0.42 0.46 0.12

Mann-Whitney t test Mann-Whitney Mann-Whitney Mann-Whitney t test

U U U U

BBV ⫽ beat-to-beat variation; BMI ⫽ body mass index; HR ⫽ heart rate; IQ ⫽ image quality; SD ⫽ standard deviation.

Forty-five patients (46%) had at least 1 coronary segment that was of nondiagnostic image quality. These patients had a significantly higher heart rate (mean ⫾ SD, 70.5 ⫾ 10.3 vs 64.1 ⫾ 13.7; P ⬍ 0.05) than did patients whose scans were all of diagnostic quality. No significant differences were found in BMI, mean beat-to-beat variation, SD of beat-tobeat variation, or calcium score between these 2 groups of patients (Table 3). Patients with a heart rate ⬎ 65 beats/min had a lower proportion of patients whose coronary segments were all of diagnostic image quality (19 of 52; P ⬍ 0.05) than the patients with a heart rate ⬍ 65 beats/min (31 of 41; P ⬍ 0.05) (Table 4). The results of the simple correlation analyses using Pearson product-moment coefficient were as follows: 0.39 for MIQ vs heart rate (P ⫽ 0.009) (Fig. 1), 0.38 for MIQ vs BMI (P ⬍ 0.001) (Fig. 2), 0.30 for MIQ vs mean beat-tobeat variation (P ⫽ 0.005), 0.22 for MIQ vs SD of beat-tobeat variation (P ⫽ 0.029), and ⫺0.08 for MIQ vs calcium score (P ⫽ 0.261). Stepwise multiple linear regression analysis revealed significant correlations between BMI and MIQ and between heart rate and MIQ (y ⫽ ⫺0.759 ⫹ 0.019 heart rate ⫹ 0.041 BMI, r2 ⫽ 0.278) (Fig. 3). When all variables

Table 4

were entered into the model, the coefficient of determination (r2) was 0.280 — only 0.002 greater than the coefficient produced by the model that included only heart rate and BMI. Mean beat-to-beat variation, SD of beat-to-beat variation, and calcium score did not contribute significantly to any model. Mean maximum tube current was 362 mA, and mean average tube current was 260 mA. The consistent use of dose modulation thus resulted in a 28.1% dose savings. Without dose modulation, mean effective radiation dose would have been 10.7 mSv (8.3 mSv for men and 12.3 mSv for women). Estimated mean effective radiation dose for the MSCT examinations, therefore, was 7.6 mSv (5.8 mSv for men and 8.8 mSv for women).

Discussion The results of our study indicate that the proportion of nondiagnostic 16-slice MSCT coronary angiograms (using radiation dose-reduction strategies) is high in patients with

Characteristics of patients with heart rates ⬍ 65 beats/min vs ⬎ 65 beats/min HR ⬍ 65 n ⫽ 46

HR, beats/min BMI, kg/m2 Mean BBV, ms SD BBV, ms Calcium score Age, yrs MIQ

HR ⬎ 65 n ⫽ 52

⬎Mean

SD

Mean

SD

P

Statistical test

58.8 30.6 24.2 35.1 55.0 51.2 1.7

4.2 7.4 20.3 52.5 267.4 10.7 0.7

75.4 29.5 20.1 27.4 13.8 46.7 1.9

7.1 8.2 29.7 53.9 54.4 10.9 0.8

⬍0.001 0.330 0.002 0.013 0.194 0.045 0.023

Mann-Whitney t test Mann-Whitney Mann-Whitney Mann-Whitney t test Mann-Whitney

U U U U U

BBV ⫽ beat-to-beat variation; BMI ⫽ body mass index; HR ⫽ heart rate; IQ ⫽ image quality; MIQ ⫽ mean image quality; SD ⫽ standard deviation. For all diagnostic segments (IQ ⱕ 2), the frequency was 31 (67.4%) for patients with HR ⬍ 65 and 19 (36.5%) for patients with HR ⬎ 65 (P ⫽ 0.002, chi-square test).

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MSCT coronary angiography in emergency patients

Figure 1 Scatter plot between heart rate (HR; beats/min) and mean image quality (MIQ).

acute chest pain in the emergency department who are not treated with ␤-blockers to reduce heart rate. Further, diagnostic image quality is significantly adversely affected by high BMI and by heart rates ⬎ 66 beats/min (Fig. 3; Table 4), conditions routinely seen in patients with acute chest pain in the emergency department. As a result, 16-slice MSCT coronary angiography, when performed with current commercially available systems using single or bisegmental reconstruction algorithms and radiation dose-reduction strategies, cannot routinely provide diagnostically useful information to exclude or include the possibility of significant coronary artery disease in patients with acute chest pain in the emergency department. In general, it was not possible to consistently obtain images of sufficient quality from MSCT coronary angiography in patients in our emergency department. Forty-six percent had at least 1 coronary segment of nondiagnostic quality that could not be evaluated for stenosis. This finding contrasts with those of previous single-center studies, in

Figure 2 Scatter plot between body mass index (BMI; kg/m2) and mean image quality (MIQ).

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Figure 3 Mean image quality (MIQ) distribution relative to body mass index (BMI; kg/m2) and heart rate (HR; beats/min). The solid black horizontal line represents the mean BMI of all patients, and the dotted vertical black line represents the mean heart rate of all patients. Thus, each quadrant represents approximately 25% of the studied patient population. The lower-left quadrant representing BMI ⬍ 30 and heart rate ⬍ 66 has primarily excellent image quality (blue), whereas the upper-right quadrant representing BMI ⬎ 30 and heart rate ⬎ 66 has primarily moderate (green)-to-poor (red) image quality. The upper-left quadrant (BMI ⬎ 30 and heart rate ⬍ 66) has primarily moderate-to-poor image quality, reflecting the adverse impact of increased BMI. The lower-right quadrant (BMI ⬍ 30 and heart rate ⬎ 66) has primarily moderate image quality, although even with higher heart rates, good image quality may be achieved in those few patients with BMI ⬍ 18 (underweight).

which 16-slice MSCT coronary angiography showed high sensitivity and specificity compared with selective coronary angiography,2,3,9,18,23 but is in concordance with the recently published multicenter 16-slice MSCT study by Garcia et al.24 In this study of 238 patients across 11 centers, 38% of patients had at least 1 nonevaluable coronary segment. The lower performance of 16-slice MSCT in a multicenter study design is not unexpected, because multicenter studies frequently reflect a more typical clinical experience than do the focused efforts of single-center studies. It should be emphasized that our study population differed in several important ways from those of previous studies showing higher sensitivity and specificity.2,3,9,18,23 Our population consisted of consecutive patients with acute chest pain and without acute myocardial infarction in the emergency department. The normal workup was not altered for these patients, and, because most patients were scheduled for or expected to undergo nuclear stress testing, ␤-blockers could be administered to only a small minority (16 patients) to lower heart rate and facilitate MSCT. Compared with populations in previous single-center MSCT and multicenter coronary angiography studies, our population had a higher mean heart rate (66.5 beats/min vs 56 – 62 beats/min) and BMI (30.2 vs 27–28).3,17,18,24

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Figure 4 Coronary computed tomographic angiography examples: (A) A 56-year-old man with acute chest pain. Excellent image quality in a patient with low body mass index (BMI, 24.8 kg/m2) and low heart rate (58 beats/min). (B) A 39-year-old man with acute chest pain. Good image quality in a patient with high BMI (39.6) and low heart rate (60); the arrow highlights a “step” artifact that did not detract from diagnostic evaluation. (C) A 39-year-old woman with acute chest pain. Suboptimal image quality in a patient with low BMI (18.5) and high heart rate (75); the arrows denote multiple artifacts in the mid-right coronary artery that could not be resolved despite image reconstruction optimization. (D) A 46-year-old woman with acute chest pain. Poor image quality in a patient with high BMI (45.6) and low heart rate (64); the mid-to-distal right coronary artery (arrows) could not be resolved despite image reconstruction optimization.

The main influences on image quality in our study were heart rate and BMI, which were significantly higher in patients with a nondiagnostic MIQ (Fig. 4). Obesity (BMI ⱖ 30) is associated with risk factors for the development of atherosclerosis and with an increased incidence of noncardiac sources of chest discomfort,25,26 such as biliary disease,

gastroesophageal reflux disease, and esophageal dysmotility. As a result, the apparent overrepresentation of obesity in our consecutively acquired sample in comparison to the overall population is unlikely to be the result of selection bias.27 Rather, it is representative of the patient seeking medical evaluation of unexplained chest discomfort. In a

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MSCT coronary angiography in emergency patients

Greek study of acute coronary syndromes, the similar (although lower) mean BMI (28 ⫾ 5) of patients with a discharge diagnosis of unstable angina serves to emphasize that obesity is present in a significant proportion of patients referred for the evaluation of chest pain.28 Heart rate was also significantly higher in patients who had at least 1 nondiagnostic coronary segment than in patients whose coronary segments were all diagnostic. This finding is not surprising, because heart rate is known to have a substantial effect on image quality.2,11,14,18 Alternatively, the influence of BMI, although consistent with the known characteristics and limitations of the computed tomography technique, has not previously been documented in patients undergoing MSCT coronary angiography. This limitation is likely to be increasingly significant as the prevalence of overweight and obese patients increases in Western societies. Curiously, calcium score did not appear to correlate with MIQ. A high calcium score is known to influence diagnostic accuracy18,29 and probably diagnostic image quality. In two studies, higher sensitivities and specificities were achieved in patients with calcium scores ⬍ 1000. In these investigations, mean (⫾SD) calcium scores were 524.3 ⫾ 807.629 and 506 ⫾ 743,18 whereas the mean calcium score in our study was 51.8 ⫾ 269.4, and only 2 patients had calcium scores ⬎ 1000. Furthermore, the small number of patients with elevated coronary calcium scores limited the statistical power of our analysis, making significant differences in image quality difficult to detect. Our findings also suggest that MSCT image quality in patients with acute chest pain in the emergency department can be predicted on the basis of BMI and heart rate. Stepwise multiple linear regression analysis revealed that image quality correlated inversely with these 2 parameters. Conversely, beat-to-beat variation had no significant influence. Our MIQ data (Table 2) also suggest that patients in the emergency department who undergo 16-slice MSCT coronary angiography ideally should have a BMI ⬍ 29 and a heart rate ⬍ 66 beats/min. The BMI did not influence the number of segments that could be identified (Table 3) but did influence the overall image quality of the coronary segments (Table 2). These findings are similar to those of a recent 64-slice MSCT study in which improved sensitivity and specificity were found in patients with BMI ⬍ 30.6 Additional benefits may derive from the faster rotation time of newer 64-slice MSCT scanners, which improves temporal resolution and, potentially, image quality at higher heart rates, although at a penalty of a higher radiation dose. Alternative approaches include adaptive multisegment reconstructions, in which the influence of heart rate may be reduced. An additional parameter influencing our results was the effort to minimize patient radiation dose by consistently applying dose modulation in our MSCT examinations. The radiation dose in our patients (5.8 mSv for men and 8.8 mSv for women) was approximately half of that imparted in a recent study that used a newer 64-slice MSCT scanner (estimated effective radiation dose, 15 mSv for men and 20

35

mSv for women).5 Dose modulation (electrocardiogram pulsing) is known to decrease image quality whenever the optimal phase for motion-free visualization of individual coronary arteries lies outside the high radiation dose interval. In most previous studies showing better sensitivities and specificities,5–7,17,23 dose modulation was either not available or not used. The similar percentage of nonevaluable segments in our study (46%) and the study by Garcia et al24 (38%) is probably attributable to the incorporation of dose modulation in both study designs. A further advantage of some newer 64-slice MSCT scanners is the thinner slice collimation, which improves through-plane spatial resolution; nonetheless, the tradeoff again is an increased radiation dose. As might be anticipated, our study population of patients in the emergency department appeared to be at lower risk of coronary artery disease than other MSCT study populations from which patients scheduled for cardiac catheterization typically have been selected. This lower risk was reflected in the low mean (⫾ SD) calcium score (51.8 ⫾ 269.4) and relatively young age (48.1 ⫾ 11.9 y) of our patients. Only 15 patients had an Agatston score ⬎ 10, which probably explains the lack of correlation between calcium score and MIQ in our study. Patients with lower cardiac risk should be the patients most likely to benefit from noninvasive MSCT29; however, a high proportion of diagnostic studies is critical for successful clinical implementation. Schroeder et al30 reported that, in their study, only 15.4% of patient examinations were nondiagnostic, even though 86% of their patients were examined with a 4-slice MSCT scanner, and the remaining 14% were examined with a 16-slice MSCT scanner. In our study, 46% of examinations were nondiagnostic, probably because of our more rigorous definition of a diagnostic study, which excluded all patients with 1 or more nondiagnostic segments. Another potentially important difference is the lower mean heart rate (64.3 beats/min) in the patient population of Schroeder et al.30 Also, it is likely that our patients had a higher mean BMI; although Schroeder et al30 did not document their patients’ BMI, patients in previous studies—most were performed in Europe— had significantly lower BMIs than our patient population.3,17,18

Limitations This study was designed to evaluate the diagnostic image quality and utility of 16-slice MSCT coronary angiography in patients with acute chest pain in the emergency department. It was limited in not comparing 16-slice MSCT findings with findings from selective coronary angiography, the criterion standard for determining the severity of coronary artery stenosis. In addition, although multisegmental reconstructions, available on some MSCT scanners, have the potential to improve image quality at higher heart rates, most 16-slice MSCT scanners have not incorporated this technique (representatives of GE Healthcare, Philips Medical Systems, and Siemens Medical Solutions, personal

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communications, 2006). Finally, this study was performed with a 16-slice MSCT, rather than a current state-of-the-art 64-slice MSCT scanner. However, in 2006, there were more than twice as many 16-slice MSCT scanners capable of performing coronary artery angiographic examinations installed in the United States as there were 64-slice MSCT systems (representatives of GE Healthcare, Philips Medical Systems, and Siemens Medical Solutions, personal communications, 2006).

Conclusions Sixteen-slice MSCT coronary angiography with radiation dose reduction but without heart rate reduction techniques is not yet suitable for routine use in patients with acute chest pain in the emergency department. Image quality is sufficiently reduced by increased BMI and heart rate to render a substantial proportion of studies nondiagnostic. A low heart rate is needed, and excluding patients with high BMI is probably necessary to obtain consistent diagnostic image quality in patients with acute chest pain in the emergency department.10,13,15

Acknowledgments We thank Jorge Hernandez, RT, and Wanda ChesterArnold, RT, for their expert technical assistance in the performance of the MSCT scans, and we thank Jude Richard, ELS, Marianne Mallia, ELS, and Stephen N. Palmer, PhD, ELS, from the Department of Scientific Publications at the Texas Heart Institute for their help in preparing this article.

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