The effect of heart rhythm on patient radiation dose with dual-source cardiac computed tomography

Journal of Cardiovascular Computed Tomography (2011) 5, 255–263

Original Research Article

The effect of heart rhythm on patient radiation dose with dual-source cardiac computed tomography Tust Techasith, BSa, Brian B. Ghoshhajra, MD, MBAa, Quynh A. Truong, MD, MPHa, Rodrigo Pale, MDb, Khurram Nasir, MD, MPHc, Michael A. Bolen, MDd, Udo Hoffmann, MD, MPHa, Ricardo C. Cury, MDa,e, Suhny Abbara, MDa, Thomas J. Brady, MDa, Ron Blankstein, MDa,f,* a

Cardiac MR PET CT Program, Department of Radiology and Division of Cardiology, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA; bCT Department, Instituto Nacional de Cardiologıa ‘‘Ignacio Chavez,’’ Mexico D.F., Mexico; cDepartment of Cardiology, Yale New Haven Hospital, New Haven, CT, USA; dImaging Institute, Cardiovascular Division, Cleveland Clinic, Cleveland, OH, USA; eBaptist Cardiac and Vascular Institute, Miami, FL, USA and fNoninvasive Cardiovascular Imaging Program, Department of Medicine and Radiology, Brigham and Women’s Hospital, 75 Francis St, Boston, MA 02115, USA KEYWORDS: Heart rhythm; Cardiac CT; Radiation dose

BACKGROUND: To lower the radiation exposure associated with cardiac CT, it is essential to identify all factors that influence radiation dose. OBJECTIVES: We explored the effect of heart rhythm during scan acquisition on radiation dose with a 64-slice dual-source cardiac CT. METHODS: Patient and scan data were collected prospectively in 302 consecutive patients referred for a clinical dual-source cardiac CT. Electrocardiograms recorded during acquisition were interpreted by a cardiologist and categorized as (1) normal sinus rhythm (NSR), (2) premature atrial contraction (PAC) or premature ventricular contraction (PVC), or (3) atrial fibrillation or flutter. RESULTS: Of the 302 patients, 227 (75.2%) were in NSR and had no ectopy, 55 (18.2%) had PAC/PVC, and 20 (6.6%) had atrial fibrillation or flutter during the scan. Patients with irregular rhythm (PAC/PVC and atrial fibrillation or flutter) were older than patients with regular rhythm (61.0 vs 54.8 years; P 5 0.006). Patients with NSR had the lowest estimated radiation dose, followed by PAC/PVC and atrial fibrillation/flutter (9.4, 14.5, 20.9 mSv; P , 0.001). The difference remained significant after adjustments for differences in examination type, tube current and voltage, scan length, pitch, and use of tube current modulation (9.8, 14.1, 17.9 mSv; P , 0.001). No significant association was observed between heart rhythm and subjective image quality although scans with regular rhythm and no ectopy had higher signal-to-noise and contrast-to-noise ratios (P , 0.01). CONCLUSION: Compared to patients with NSR, patients with atrial fibrillation/flutter had the highest radiation exposure, followed by those with PAC/PVC. Even after adjustment for factors associated with radiation exposure, a significant difference in radiation dose persisted. These findings can be used

Conflict of interest: The authors report no conflicts of interest. R.B., B.B.G., and Q.A.T. received support from the National Institutes of Health (grant 1T32 HL076136-02). * Corresponding author. E-mail address: [email protected] Submitted March 28, 2011. Accepted for publication May 17, 2011. 1934-5925/$ - see front matter Ó 2011 Society of Cardiovascular Computed Tomography. All rights reserved. doi:10.1016/j.jcct.2011.05.003

256

Journal of Cardiovascular Computed Tomography, Vol 5, No 4, July/August 2011 to identify patients who are more likely to receive higher radiation dose when undergoing cardiac CT and to develop future more-efficient scanner algorithms for use in patients with arrhythmias. Ó 2011 Society of Cardiovascular Computed Tomography. All rights reserved.

Introduction Cardiac computed tomography (CT) has been shown to have many potentially useful clinical applications.1–4 Nevertheless, radiation exposure is an important limitation of this increasingly used modality.5 Although many new techniques have recently been introduced for the reduction of radiation dose,6–8 a recent large international, multicenter observational study estimated that the median radiation dose exposure of patient undergoing a coronary CT angiography (CTA) with 64-slice CT was 12 mSv and that many currently available techniques to lower the radiation dose were underutilized.9 To achieve lower radiation exposure, a need exists to understand both patient-related and technical factors that contribute to increased radiation dose with cardiac CT. Factors that influence radiation dose in cardiac CT include scan length, tube current/voltage, and use of axial prospective electrocardiogram (ECG) triggering versus helical acquisition. However, heart rate and rhythm may also play a role as determinants of radiation exposure.10–14 In this study, we sought to identify the effect of heart rhythm during actual scan acquisition on patient radiation dose in a prospective patient cohort undergoing cardiac CT.

Methods Patient population This study was approved by our institutional review board. The study population included 304 consecutive patients referred for a clinical cardiac dual-source CT (DSCT) examination at our institution between December 2007 and May 2008. Two patients with pacemaker implant were excluded. Data on scan acquisition parameters, indication for examination, and heart rhythm during actual scan acquisition were all collected prospectively.

Image acquisition Images were acquired on the Siemens Definition dualsource 64-slice CT scanner (Siemens Medical Systems, Forchheim, Germany) with a gantry rotation time of 330 milliseconds and standard detector collimation of 0.6 mm. Alternating focal spots along the z-axis (z-Sharp technology; Siemens) were used to acquire 64 overlapping 0.6-mm slices with the use of two 32-detector rows. The temporal resolution of the scans was 83 milliseconds. Before each scan, a test bolus of 15–20 mL of contrast was administered at 5–6 mL/s and dynamic axial imaging

was performed. The timing for image acquisition was determined by adding 2 seconds to the time of peak contrast enhancement in the ascending aorta. During scan acquisition, contrast was administered at 5–6 mL/s. Sublingual nitroglycerin (dose of 0.6 mg) was administered to all patients to improve the visualization of the coronary artery lumen. b-blockers were administered to patients with heart rate .75 beats/min for a target heart rate %65 beats/min. Scan protocol was individualized for each patient on the basis of examination indication and body habitus. For each scan, a cardiovascular imaging specialist (radiologist or cardiologist) selected all scan acquisition parameters, including the tube current (mAs) and voltage (kV) on the basis of the following factors: (1) for patient height, weight, and calculated body mass index (BMI), the use of low tube voltage (100 kV) was suggested for patients with a BMI , 25kg/m2; (2) for assessment of the patient’s body habitus and chest wall attenuation, this was performed by both a physical inspection of the patient by the physician before the scan and by visualization of the anteroposterior scout image and axial test bolus images to determine whether excessive chest wall adiposity or soft tissue was present; (3) for clinical indication, physicians were advised to consider lower tube voltage for indications that did not require the highest possible contrast-to-noise ratio (CNR) levels (eg, suspected anomalous origins of the coronary arteries). In addition, physicians were advised to consider the use of axial acquisition with the use of prospective triggering (Sequential Scanning; Siemens) for younger patients (age , 50 years) who had low (,65 beats/min) and regular heart rate. All such axial acquisitions were obtained at 65% of the R-R interval. For scans that used helical acquisition with retrospective gating, tube current modulation pulse width (expressed as percentage of the R-R interval) was determined by the physician according to the clinical indication, patient’s heart rate, and probability of coronary artery disease. For patients with elevated or irregular heart rate, a window of 35%–75% of the R-R interval was selected. However, for patients with a lower heart rate, a window of 50%–75% of the R-R interval was selected. Automatic pitch adaptation was used for helical scans (ie, pitch between 0.2 and 0.5 selected by the scanner according to heart rate; a higher pitch was used with higher heart rates).

Image reconstruction and analysis For retrospectively gated scans, raw data from 5% to 95% of the cardiac cycle was used to reconstruct images at

Techasith et al

Effect of heart rhythm on patient radiation dose

10% intervals with slice thickness of 0.75 mm and overlap of 0.4 mm. For prospectively triggered scans, raw data at 65% of the cardiac cycle was used to reconstruct a single dataset. A medium smooth reconstruction kernel (Siemens B26f) was used for image reconstruction. Image analysis was performed with both axial and double-oblique images viewed in thin-slab maximal intensity projections and multiplanar reformation settings.

Radiation exposure CT dose index volume (CTDIvol) and dose-length product (DLP) were obtained from the scan console. Effective radiation dose was calculated by multiplying the DLP of the scan (excluding test bolus) by a constant (k 5 0.014 mSv/mGy/cm for cardiac scan).15 To study only the effect of ECG pulsing on the dose, the weighted CT dose index (CTDIw) was calculated for scans that used a helical acquisition as follows: CTDIw 5 CTDIvol ! pitch. Unlike the CTDIvol, the CTDIw provides an estimate of the radiation dose that is independent of pitch.16

257

Statistical analysis Data analysis was performed with Stata IC Version 11.0 (StataCorp LP, College Station, TX, USA). Continuous variables were expressed as mean 6 SD; categorical variables were expressed as a percentage. Differences in continuous variables were assessed with unpaired Student t tests. For dichotomous variables, differences in proportions were assessed with the chi-square test or Fisher exact test, as appropriate. The k statistic was used for interobserver agreement of image quality score. To estimate the adjusted mean dose in each group (ie, NSR with no ectopy, PAC/PVC, atrial fibrillation or flutter), we constructed a multivariable linear regression model to predict the effective radiation dose. The multivariable model included all variables that had a strong univariate association (P , 0.01) with radiation dose in the entire population studied.

Results

Heart rhythm

Baseline characteristics

Patient ECGs during scan acquisition were analyzed by a cardiologist to determine the heart rhythm. Heart rhythms were classified as (1) normal sinus rhythm (NSR) with no ectopy, (2) normal sinus rhythm with premature atrial contraction (PAC) or premature ventricular contraction (PVC), and (3) atrial fibrillation or atrial flutter.

Patient characteristics and indications for examination are described in detail in Table 1. Of the 302 patients included in the study, the average age was 56.4 6 13.7 years, and 190 (62.9%) were male. The average BMI was 29.4 6 5.4 kg/m2. Indications for examination were as follow: 182 patients (60.2%) were referred for the evaluation of the coronary arteries, 25 (8.3%) for coronary artery bypass graft, 54 (17.9%) for pulmonary veins, and the remaining 41 (13.6%) for other indications, including congenital heart disease, structural heart disease, and aortic diseases. Seventy-five patients (24.8%) had irregular rhythm during the examination (groups with PAC/PVCs and atrial fibrillation/flutter). Patients with irregular rhythm were significantly older than those with regular rhythm (61.0 6 13.6 vs 54.8 6 13.4; P 5 0.006), but there were no significant differences in sex and BMI between the 2 groups (P 5 0.38 and P 5 0.12, respectively). Patients who were referred for the evaluation of native coronary arteries were more likely to have regular heart rhythm than patients who underwent bypass graft, pulmonary vein, and congenital/structural heart disease evaluations (P 5 0.015).

Image quality Image quality was determined on the basis of subjective image quality (IQ) score or CNR and signal-to-noise ratio (SNR). Subjective IQ score was determined retrospectively with the use of a subjective 4-point scale as follows: 1, poor; 2, significantly reduced; 3, mildly reduced; and 4, excellent. Each study was independently reviewed by 2 blinded experienced readers (cardiologist and radiologist, both with .500 cardiac CT interpretations), and the average of the 2 scores was used. The interobserver agreement for IQ scores was excellent (k 5 0.87). Image noise was derived from the standard deviation of the density values (in Hounsfield units) within a large region of interest in the left ventricle. The SNR was defined as the ratio of the mean signal intensity divided by image noise. The CNR was defined as the difference between the mean density of the contrast-filled left ventricular chamber and the mean density of the left ventricular wall, which was divided by image noise. This method, which has been previously described,17 is relevant for a wide range of cardiac CT studies, regardless of whether evaluation of the coronary arteries was performed.

Scan acquisition and radiation dose in patients with normal versus abnormal rhythm Scan acquisition parameters are listed in full details in Table 2. A comparison between patients with regular and irregular heart rhythm showed no significant differences in tube voltage (P 5 0.11), tube current (P 5 0.50), or use of axial versus helical acquisition (P 5 0.39). However, the mean scan pitch was significantly lower for patients

258

Journal of Cardiovascular Computed Tomography, Vol 5, No 4, July/August 2011

Table 1

Patient characteristics and examination types

Characteristics Age, mean 6 SD (y) Sex Male, n (%) Female, n (%) Body mass index, mean 6 SD (kg/m2) Examination type Coronary, n (%) Bypass graft, n (%) Pulmonary vein, n (%) Other, n (%)

All (n 5 302)

Regular rhythm (n 5 227)

Irregular rhythm* (n 5 75)

P value

56.4 6 13.7

54.8 6 13.4

61.0 6 13.6

0.006

190 112 29.4 6 5.6

146 (64.3) 81 (35.7) 29.0 6 5.7

44 (58.7) 31 (41.3) 30.6 6 5.4

0.38 0.38 0.12 0.015

182 25 54 41

148 17 33 29

34 8 21 12

(60.2%) (8.3%) (17.9%) (13.6%)

(65.2%) (7.5%) (14.5%) (12.8%)

(45.3%) (10.7%) (28.0%) (16.0%)

*Irregular rhythm was defined as premature atrial contraction/premature ventricular contraction or atrial fibrillation/flutter.

with irregular heart rate (0.24 6 0.08 vs 0.31 6 0.07; P , 0.001). The scan length was also significantly longer for patients with irregular heart rate (20.4 6 6.5 vs 18.4 6 4.2 cm; P 5 0.003), reflecting that patients referred for evaluation of bypass grafts or evaluation of the pulmonary veins, both examinations that have a longer scan length compared with routine coronary CTA, were more likely to have abnormal heart rhythm during the examination. The CTDIvol (73.4 6 30.2 vs 57.8 6 24.8 mGy), DLP (1156.3 6 735.6 vs 669.9 6 386.6 mGy $ cm), and radiation dose (16.2 6 10.3 vs 9.4 6 5.4mSv) were all significantly higher for patients with irregular heart rhythm (all P , 0.001). The distribution of radiation dose in patients with regular (Fig. 1A) and irregular (Fig. 1B) heart rates is shown. Overall, the distribution of the cohort with irregular rhythm had more patients at the higher end of radiation dose. Among patients with irregular rhythm, those with atrial fibrillation/flutter received a higher dose than patients with PAC/PVC (Fig. 1B).

Differences in radiation dose versus severity of abnormal heart rhythm Patients with irregular rhythm were divided into 2 groups: minimally irregular rhythm (PAC/PVC) and a highly irregular rhythm (atrial fibrillation/flutter). As can be seen in Table 2 (right side columns), no difference was observed in tube voltage (P 5 0.59) between these 2 groups. No patients with atrial fibrillation or flutter underwent an axial acquisition scan with the use of prospective triggering. On average, patients with atrial fibrillation or flutter had higher tube current during retrospective acquisition (362 6 45.5 vs 325.8 6 70.8 mAs; P 5 0.04). A trend toward a lower pitch was observed in patients with atrial fibrillation or flutter (P 5 0.07), but no difference in scan length (P 5 0.91) was observed. The CTDIvol (85.4 6 31.5 vs 68.2 6 28.4 mGy), DLP (1494.2 6 472.5 vs 1033.4 6 778 mGy $ cm), and estimated effective radiation dos (20.9 6 6.6 vs 14.5 6 10.9 mSv)

were all significantly higher for patients with atrial fibrillation or flutter (all P , 0.05). With the use of linear regression analysis and adjusting for differences in examination type (eg, evaluation of native coronary arteries, bypass grafts, or pulmonary veins), tube current and voltage, scan length, pitch, and use of axial versus helical acquisition, the difference in the radiation dose among patients with regular rhythm, PAC/PVC, and atrial fibrillation/flutter remained highly significant (9.8 6 0.3 vs 14.1 6 0.6 vs 17.9 6 1.0 mSv; P , 0.001; Fig. 2). To further isolate the effect of pitch and pulsing efficiency related to tube current modulation, the CTDIw and CTDIvol, both parameters that are independent of scan length, were compared. The CTDIvol was approximately 30% higher for examinations with irregular heart rhythm (73.4 vs 57.8 mGy; P , 0.001), whereas the CTDIw (which incorporates changes in ECG pulsing efficiency but not pitch) was approximately 22% higher (14.5 vs 11.9 mGy). However, when these parameters were compared among examinations with atrial fibrillation or flutter (with the use of examinations with no ectopy as the baseline comparator), the CTDIvol and CTDIw were increased by 48% and 30%, respectively, suggesting that, although wider pulsing was the strongest contributor to a higher radiation dose in patients with irregular heart rhythms, there was a larger relative contribution to the higher radiation dose related to a lower pitch among examinations with atrial/fibrillation/flutter than examination with PACs or PVCs.

Image quality The subjective and objective image quality parameters for the cohort are shown in Table 3. The average image quality score did not show significant difference between regular and irregular rhythm groups (P 5 0.638). However, a comparison of objective measures, including image noise, CNR, and SNR, showed a small but significant difference, with the regular heart rhythm group having less noise and higher CNR and SNR (all P , 0.01).

Techasith et al

Scan parameters Irregular rhythm

Scan Parameters Tube voltage 80 kV, n (%) 100 kV, n (%) 120 kV, n (%) 140 kV, n (%) Tube current, mean 6 SD (mA)* Tube current, mean 6 SD (mAs)* Use of axial acquisition (eg, prospective triggering), n (%) Pitch, mean 6 SD CTDIvol, mean 6 SD (mGy) CTDIw, mean 6 SD (mGy)† Scan length, mean 6 SD (cm) DLP, mean 6 SD (mGy $ cm) Radiation dose, mean 6 SD (mSv) Adjusted radiation dose, mean 6 SD (mSv)

Regular rhythm (n 5 227)

Irregular rhythm (n 5 75)

1 (0.4) 54 (23.8) 165 (72.7) 7 (3.1) 199.4 6 41.9 325.5 6 74.7 33 (14.5)

2 (2.7) 22 (29.3) 51 (68.0) 0 (0.0) 210.1 6 30.2 336.8 6 66.0 8 (10.7)

P value

PAC/PVC (n 5 55)

Atrial fibrillation/flutter (n 5 20)

2 (3.6) 15 (27.3) 38 (69.0) 0 (0.0) 210.1 6 30.2 325.8 6 70.8 8 (14.5)

0 (0.0) 7 (35.0) 13 (65.0) 0 (0.0) — 362 6 45.5 0 (0)

0.11

0.31 57.8 11.9 18.4 669.9 9.4 9.8

6 6 6 6 6 6 6

0.07 24.8 4.5 4.2 386.6 5.4 0.3

0.24 73.4 14.5 20.4 1156.3 16.2 15.1

6 6 6 6 6 6

0.08 30.2 5.6 6.5 735.6 10.3

0.50 0.28 0.40 ,0.001 ,0.001 ,0.001 0.003 ,0.001 ,0.001 ,0.001

P value 0.59

0.26 68.2 14.1 20.3 1033.4 14.5 14.1

6 6 6 6 6 6 6

0.08 28.4 6.0 7.4 778 10.9 0.6

0.22 85.4 15.5 20.5 1494.2 20.9 17.9

6 6 6 6 6 6 6

0.06 31.5 4.8 3.0 472.5 6.6 1.0

— 0.04 0.07 0.07 0.03 0.34 0.91 0.015 0.015 ,0.001‡

Effect of heart rhythm on patient radiation dose

Table 2

PAC, premature atrial contraction; PVC, premature ventricular contraction; CTDIvol, CT dose index volume; CTDIw, CT dose index weighted; DLP, dose-length product. *For prospective electrocardiographic triggering, tube current (mAs) is calculated as (total mA ! exposure time), whereas for retrospective gating, tube current/rotation is calculated as (total mA ! gantry rotation time). † CTDIw (CTDIvol ! pitch) was calculated for all examinations that used helical acquisition (n 5 260). ‡ P values associated with adjusted radiation dose among regular heart rhythm, PAC/PVC, and atrial fibrillation/flutter.

259

260

Journal of Cardiovascular Computed Tomography, Vol 5, No 4, July/August 2011

Figure 1 Distribution of radiation exposure in patients. The patient cohort is divided into 2 groups: regular and irregular rhythm. Patients with regular rhythm (A) shows a bell-shaped distribution. Patients with irregular rhythm (B) shows a bimodal distribution; those with atrial fibrillation/flutter contribute more to the high end of the radiation dose.

Discussion In this study that evaluated the effect of heart rhythm during dual-source cardiac CT on radiation exposure, we observed that in comparison to patients with NSR and no ectopy, patients who had ectopic beats (PAC or PVC) or a highly irregular rhythm (atrial fibrillation/flutter) received a significantly higher radiation exposure. Furthermore, patients with atrial arrhythmias had a significantly higher radiation dose than patients who had sinus rhythm with ectopic beats. Notably, these differences were attenuated but remained highly significant, even after accounting for all other significant scan acquisition parameters known to affect radiation dose. Recent studies have shown that CTA can be accurately performed in the setting of atrial fibrillation, particularly if b-blockade is administered for rate control.18–21 However, the effect of heart rhythm during scan acquisition on radiation dose has not been previously examined. Although prior studies have attempted to correlate heart rate or heart rate variability to radiation exposure,16 this study extends prior observations through an examination of the relationship between heart rhythm and radiation exposure.

Figure 2 Estimated radiation exposure in cardiac CT. The patient cohort is divided into 3 groups: NSR, PAC/PVC, and atrial fibrillation/flutter. Patients with atrial fibrillation/flutter had the highest radiation exposure, followed by patients with PAC/PVC, then patients with NSR without ectopy. The difference decreases with statistical adjustment for other factors affecting radiation dose but remains significant.

In our cohort, patients with an abnormal heart rhythm or ectopy included older persons or patients being referred after bypass graft surgery or for evaluation of pulmonary veins (before or after ablation). These findings are consistent with the known increased prevalence of arrhythmias in older persons22,23 or in patients with prior bypass surgery.24 We observed longer scan length in patients with irregular rhythm. This difference in scan length can be explained by the increased prevalence of patients undergoing bypass graft and pulmonary vein evaluations among patients with irregular heart rhythm, both examinations which use a longer scan length. There are several noteworthy mechanisms underlying the increased radiation dose associated with an abnormal heart rhythm. First, in the DSCT, any major irregularities in the R-R interval cause the scanner to automatically select the lowest possible pitch (ie, 0.2) to ensure redundancy in the image data. Second, abnormalities in the R-R interval have a direct influence on the efficiency of the tube current modulation. When irregular rhythm is detected, the scanner widens the tube current modulation window width, thus increasing the interval in which the tube current is turned on at 100% capacity. In turn, this results in a higher radiation exposure that is unaccounted for by any scan acquisition parameters (Fig. 3). Although our multivariable analysis adjusted for scan pitch, we were unable to adjust for the efficiency of the tube current modulation. However, on the basis of our analysis of the CTDIw and CTDIvol, we determined that widening of the pulse window width during arrhythmia is probably the main mechanism behind the increase in radiation exposure observed in our study. Our analysis did not show any difference in subjective image quality between patients with regular and irregular heart rhythms. Possibly, this is related to the automatic compensation mechanism provided in the aforementioned discussion, features that also allow the subsequent use of ECG ‘‘editing’’ after acquisition to minimize image artifacts.25–27 In such a scenario, although radiation exposure was increased, the potential benefit obtained is preserved

Techasith et al Table 3

Effect of heart rhythm on patient radiation dose

261

Image quality

Image quality score 1. Poor, n (%) 2. Significantly reduced, n (%) 3. Mildly reduced, n (%) 4. Excellent, n (%) Measurements: Noise, mean 6 SD (HU) Contrast-to-noise ratio, mean 6 SD Signal-to-noise ratio, mean 6 SD

All (n 5 302)

Regular rhythm (n 5 226)*

Irregular rhythm (n 5 75)

11 50 144 96

9 37 112 68

2 (2.7) 13 (17.3) 32(42.7) 28 (37.3)

P value 0.64

(3.7) (16.6) (47.8) (31.9)

48.0 6 16.2 6.2 6 2.9 8.6 6 3.6

(4.0) (16.4) (49.6) (30.1)

46.2 6 15.1 6.4 6 3.0 8.9 6 3.8

53.3 6 18.2 5.4 6 2.4 7.7 6 2.8

,0.001 0.01 0.01

*Of 227 examinations, 226 were available for image quality analysis.

diagnostic image quality. This shows that the cardiac CT evaluation is often feasible in the setting of arrhythmia, although at the expense of increased radiation exposure. Similar findings, namely, preserved image quality but with increased radiation dose, have also been found by Weustink et al.16 In summary, arrhythmias lead to increased radiation; thus, other means of radiation dose reduction, such as minimization of scan length, should be implemented. For noncoronary examinations such as evaluating pulmonary veins, prospective triggering and perhaps the use of a nongated acquisition should be considered.28 This study is not without its limitations. Although the wide range of scan indications included in the study is a strength, it also is a limitation in that the heterogeneity makes it more difficult to isolate the effect of heart rhythm on radiation dose. To account for these differences, we used statistical adjustment with linear regression modeling to account for baseline differences in examination parameters that were associated with radiation dose. In addition, in our study, scan parameters used during image acquisition were not mandated by a fixed protocol. Instead, imaging parameters were individualized for each patient on the basis of the scan indication and the patient’s body habitus (as described

in ‘‘Methods’’). Nevertheless, our findings would not be expected to change even if a more-defined protocol was used to select the scan acquisition parameters. This was a single-center study that used DSCT; however, we expect that our findings are applicable to most other cardiac CT scanners, including single-source scanners (64-, 256-, and 320-detector row) and second-generation DSCT when helical acquisition is used. This is because the mechanisms that these scanners use to deal with arrhythmia (ie, increasing the redundancy of data through widening the pulse width window,29 decreasing the pitch, or use of multiple segment reconstruction30) are all associated with a higher radiation dose. In this prospective study that evaluated the effect of heart rhythm on radiation exposure of patients undergoing dual-source cardiac CT, we observed that heart rhythm during scan acquisition is a major determinant of radiation exposure, which is probably substantially under recognized. These findings can be used to identify patients who are more likely to receive higher radiation dose when undergoing cardiac CT as well as to develop future moreefficient scanner algorithms for use in patients with arrhythmias.

Figure 3 Examples of ECG tube current modulation pulse width in patients with various heart rhythms: (A) NSR with no ectopy, (B) PVC (black arrow), and (C) sinus arrhythmia. The pale-blue shading represents the peak tube current pulsing window. The pulsing window is narrowest for NSR. In patients with mildly irregular rhythm such as PVC, the pulse window widens slightly. In patients with a high degree of arrhythmia, the pulsing window is nearly full, and subsequently the CTDI and DLP are significantly higher. Scanrelated parameters are listed on the right of the ECG strip.

262

Journal of Cardiovascular Computed Tomography, Vol 5, No 4, July/August 2011

References 1. Greenland P, Bonow RO, Brundage BH, Budoff MJ, Eisenberg MJ, Grundy SM, Lauer MS, Post WS, Raggi P, Redberg RF, Rodgers GP, Shaw LJ, Taylor AJ, Weintraub WS, Harrington RA, Abrams J, Anderson JL, Bates ER, Grines CL, Hlatky MA, Lichtenberg RC, Lindner JR, Pohost GM, Schofield RS, Shubrooks SJ Jr., Stein JH, Tracy CM, Vogel RA, Wesley DJ: ACCF/AHA 2007 clinical expert consensus document on coronary artery calcium scoring by computed tomography in global cardiovascular risk assessment and in evaluation of patients with chest pain: a report of the American College of Cardiology Foundation Clinical Expert Consensus Task Force (ACCF/AHA Writing Committee to Update the 2000 Expert Consensus Document on Electron Beam Computed Tomography). Circulation. 2007;115:402–26. 2. Achenbach S, Giesler T, Ropers D, Ulzheimer S, Derlien H, Schulte C, Wenkel E, Moshage W, Bautz W, Daniel WG, Kalender WA, Baum U: Detection of coronary artery stenoses by contrast-enhanced, retrospectively electrocardiographically-gated, multislice spiral computed tomography. Circulation. 2001;103:2535–8. 3. Meijboom WB, Meijs MF, Schuijf JD, Cramer MJ, Mollet NR, van Mieghem CA, Nieman K, van Werkhoven JM, Pundziute G, Weustink AC, de Vos AM, Pugliese F, Rensing B, Jukema JW, Bax JJ, Prokop M, Doevendans PA, Hunink MG, Krestin GP, de Feyter PJ: Diagnostic accuracy of 64-slice computed tomography coronary angiography: a prospective, multicenter, multivendor study. J Am Coll Cardiol. 2008;52:2135–44. 4. Cury RC, Nieman K, Shapiro MD, Butler J, Nomura CH, Ferencik M, Hoffmann U, Abbara S, Jassal DS, Yasuda T, Gold HK, Jang IK, Brady TJ: Comprehensive assessment of myocardial perfusion defects, regional wall motion, and left ventricular function by using 64-section multidetector CT. Radiology. 2008;248:466–75. 5. Redberg RF: Cancer risks and radiation exposure from computed tomographic scans: how can we be sure that the benefits outweigh the risks? Arch Intern Med. 2009;169:2049–50. 6. Hausleiter J, Meyer T: Tips to minimize radiation exposure. J Cardiovasc Comput Tomogr. 2008;2:325–7. 7. Blankstein R, Shah A, Pale R, Abbara S, Bezerra H, Bolen M, Mamuya WS, Hoffmann U, Brady TJ, Cury RC: Radiation dose and image quality of prospective triggering with dual-source cardiac computed tomography. Am J Cardiol. 2009;103:1168–73. 8. Feuchtner GM, Jodocy D, Klauser A, Haberfellner B, Aglan I, Spoeck A, Hiehs S, Soegner P, Jaschke W: Radiation dose reduction by using 100-kV tube voltage in cardiac 64-slice computed tomography: a comparative study. Eur J Radiol. 2010;75:e51–6. 9. Hausleiter J, Meyer T, Hermann F, Hadamitzky M, Krebs M, Gerber TC, McCollough C, Martinoff S, Kastrati A, Schomig A, Achenbach S: Estimated radiation dose associated with cardiac CT angiography. JAMA. 2009;301:500–7. 10. Jakobs TF, Becker CR, Ohnesorge B, Flohr T, Suess C, Schoepf UJ, Reiser MF: Multislice helical CT of the heart with retrospective ECG gating: reduction of radiation exposure by ECG-controlled tube current modulation. Eur Radiol. 2002;12:1081–6. 11. Dikkers R, Greuter MJ, Kristanto W, van Ooijen PM, Sijens PE, Willems TP, Oudkerk M: Assessment of image quality of 64-row dual source versus single source CT coronary angiography on heart rate: a phantom study. Eur J Radiol. 2009;70:61–8. 12. Huang B, Law MW, Mak HK, Kwok SP, Khong PL: Pediatric 64-MDCT coronary angiography with ECG-modulated tube current: radiation dose and cancer risk. AJR Am J Roentgenol. 2009;193: 539–44. 13. Ketelsen D, Luetkhoff MH, Thomas C, Werner M, Buchgeister M, Tsiflikas I, Reimann A, Burgstahler C, Kopp AF, Claussen CD, Heuschmid M: Estimation of the radiation exposure of a chest pain protocol with ECG-gating in dual-source computed tomography. Eur Radiol. 2009;19:37–41.

14. Kirchhoff S, Herzog P, Johnson T, Bohm H, Nikolaou K, Reiser MF, Becker CH: Assessment of radiation exposure on a dual-source computed tomography-scanner performing coronary computed tomography-angiography. Eur J Radiol. 2010;74:e181–5. 15. McCollough C, Cody D, Edyvean S, Geise R, Keat N, Huda W, Judy P, Kalender W, McNitt-Gray M, Morin R, Payne T, Stern S, Rothenbert L, Shrimpton P, Timmer J, Wilson C: The measurement, reporting, and management of radiation dose in CT. College Park, MD: American Association of Physicists in Medicine; 2008. AAPM Tech Rep 96 , http://www.aapm.org/pubs/reports/rpt_96.pdf. Accessed July 20, 2009. 16. Weustink AC, Neefjes LA, Kyrzopoulos S, van Straten M, Neoh Eu R, Meijboom WB, van Mieghem CA, Capuano E, Dijkshoorn ML, Cademartiri F, Boersma E, de Feyter PJ, Krestin GP, Mollet NR: Impact of heart rate frequency and variability on radiation exposure, image quality, and diagnostic performance in dual-source spiral CT coronary angiography. Radiology. 2009;253:672–80. 17. Hausleiter J, Meyer T, Hadamitzky M, Huber E, Zankl M, Martinoff S, Kastrati A, Schomig A: Radiation dose estimates from cardiac multislice computed tomography in daily practice: impact of different scanning protocols on effective dose estimates. Circulation. 2006;113: 1305–10. 18. Yang L, Zhang Z, Fan Z, Xu C, Zhao L, Yu W, Yan Z: 64-MDCT coronary angiography of patients with atrial fibrillation: influence of heart rate on image quality and efficacy in evaluation of coronary artery disease. AJR Am J Roentgenol. 2009;193:795–801. 19. Marwan M, Pflederer T, Schepis T, Lang A, Muschiol G, Ropers D, Daniel WG, Achenbach S: Accuracy of dual-source computed tomography to identify significant coronary artery disease in patients with atrial fibrillation: comparison with coronary angiography. Eur Heart J. 2010;31:2230–7. 20. Pasricha SS, Nandurkar D, Seneviratne SK, Cameron JD, Crossett M, Schneider-Kolsky ME, Troupis JM: Image quality of coronary 320-MDCT in patients with atrial fibrillation: initial experience. AJR Am J Roentgenol. 2009;193:1514–21. 21. Xu L, Yang L, Fan Z, Yu W, Lv B, Zhang Z: Diagnostic performance of 320-detector CT coronary angiography in patients with atrial fibrillation: preliminary results. Eur Radiol. 2011;21:936–43. 22. Benjamin EJ, Levy D, Vaziri SM, D’Agostino RB, Belanger AJ, Wolf PA: Independent risk factors for atrial fibrillation in a population-based cohort. The Framingham Heart Study. JAMA. 1994;271:840–4. 23. Kannel WB, Wolf PA, Benjamin EJ, Levy D: Prevalence, incidence, prognosis, and predisposing conditions for atrial fibrillation: population-based estimates. Am J Cardiol. 1998;82:2N–9. 24. Pires LA, Wagshal AB, Lancey R, Huang SK: Arrhythmias and conduction disturbances after coronary artery bypass graft surgery: epidemiology, management, and prognosis. Am Heart J. 1995;129: 799–808. 25. Ohnesorge B, Flohr T, Becker C, Kopp AF, Schoepf UJ, Baum U, Knez A, Klingenbeck-Regn K, Reiser MF: Cardiac imaging by means of electrocardiographically gated multisection spiral CT: initial experience. Radiology. 2000;217:564–71. 26. Cademartiri F, Mollet NR, Runza G, Baks T, Midiri M, McFadden EP, Flohr TG, Ohnesorge B, de Feyter PJ, Krestin GP: Improving diagnostic accuracy of MDCT coronary angiography in patients with mild heart rhythm irregularities using ECG editing. AJR Am J Roentgenol. 2006;186:634–8. 27. Matsumoto H, Kondo T, Watanabe S, Kikumoto R, Shimada T, Hiraoka Y, Ueda K: ECG-edited middiastolic phase reconstruction improves image quality at 64-MDCT coronary angiography of patients with atrial fibrillation. AJR Am J Roentgenol. 2008;191: 1659–66. 28. Yang L, Xu L, Yan Z, Yu W, Fan Z, Lv B, Zhang Z. Low dose 320-row CT for left atrium and pulmonary veins imaging-the feasibility study [published online ahead of print March 8, 2011]. Eur J Radiol. doi: 10.1016/j.ejrad.2011.02.032.

Techasith et al

Effect of heart rhythm on patient radiation dose

29. Rybicki FJ, Otero HJ, Steigner ML, Vorobiof G, Nallamshetty L, Mitsouras D, Ersoy H, Mather RT, Judy PF, Cai T, Coyner K, Schultz K, Whitmore AG, Di Carli MF: Initial evaluation of coronary images from 320-detector row computed tomography. Int J Cardiovasc Imaging. 2008;24:535–46.

263

30. Einstein AJ, Elliston CD, Arai AE, Chen MY, Mather R, Pearson GD, Delapaz RL, Nickoloff E, Dutta A, Brenner DJ: Radiation dose from single-heartbeat coronary CT angiography performed with a 320-detector row volume scanner. Radiology. 2010;254: 698–706.