Image Quality and Radiation Exposure in Pediatric Cardiovascular CT Angiography From Different Injection Sites

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1 Cardiopulmonary Imaging Original Research Pediatric Cardiovascular CT Angiography Cardiopulmonary Imaging Original Research Ming Yang 1 Xu-Ming Mo 2 Ji-Yang Jin 1 Jian Zhang 1 Bin Liu 1 Min Wu 1 Gao-Jun Teng 1 Yang M, Mo XM, Jin JY, et al. Keywords: congenital heart disease, CT angiography, CT image quality, CT radiation exposure DOI: /AJR Received January 28, 2010; accepted after revision June 29, The study was supported by the Scientific Research Foundation of Graduate School of Southeast University (grant YBJJ0729) and Scientific Research Foundation of Southeast University (grant KJ ). 1 Department of Radiology, Zhong-Da Hospital, Southeast University, 87 Dingjiaqiao Rd., Nanjing , China. Address correspondence to G. J. Teng (gjteng@vip.sina.com). 2 Department of Cardiothoracic Surgery, Nanjing Children s Hospital, Nanjing Medical University, Nanjing , China. Address correspondence to X. M. Mo (mohsuming@hotmail.com). WEB This is a Web exclusive article. AJR 2011; 196:W117 W X/11/1962 W117 American Roentgen Ray Society Image Quality and Radiation Exposure in Pediatric Cardiovascular CT Angiography From Different Injection Sites OBJECTIVE. The purpose of this article is to evaluate the effect of different injection sites (i.e., head, arm, or leg vein) on image quality and radiation exposure in pediatric cardiovascular CT angiography (CTA) with 64-MDCT. MATERIALS AND METHODS. CTA was performed in 61 children with suspected extracardiac abnormalities. Patients were assigned to three groups according to the different injection sites: head, arm, or leg vein. Enhancement of heart chamber and great vessels and background noise were quantified. Signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), dose length product (DLP), and effective dose (ED) were calculated. Subjective image quality was assessed by two radiologists in consensus. RESULTS. There was no significant difference among all groups in the mean attenuation of the heart chamber, pulmonary artery (PA), and aorta. There was also no significant difference in their mean attenuation, background noise, SNR, and CNR. However, there were significant differences among the three groups for aorta image quality (p = 0.006), despite the nonsignificant differences in heart chamber and PA image quality. There also were significant differences among the three groups for total DLP and ED (p = 0.01 for both), with prescanning DLPs of 17.6%, 20.2%, and 24.5%, respectively, of the total DLP for each group. CONCLUSION. Although all injection sites can yield diagnostic-quality images with a low radiation dose in pediatric cardiovascular CTA, the injection site has a slight impact on the image quality of different targeted areas with a significantly different radiation dose. The optimization of a prescanning protocol may open an avenue to reduce the radiation dose associated with cardiovascular CTA. T he introduction of helical CT in the late 1980s revolutionized diagnostic medical imaging and resulted in a considerable increase in the number of CT examinations performed and in the average scanned volume obtained per examination. In a study by Mettler et al. [1], 11% of the CT examinations were performed in the pediatric population, which was higher than the previously estimated value. The increase in population radiation exposure from CT, particularly in children, has been of great concern. There is wide agreement that the benefits of an indicated CT scan far outweigh the risks [2]. It is essential to balance image quality and radiation dose delivered when performing CT to reduce radiation exposure in patients, especially in children. In August 2001, members of the As Low As Reasonably Achievable conference of the Society for Pediatric Radiology questioned whether it is possible to reduce the dose delivered to chil- dren by reducing the kilovoltage [3]. Radiologists and medical physicists must be attentive to their responsibility to maintain an appropriate balance between diagnostic image quality and radiation dose [4, 5]. Currently, many efforts are occurring internationally to minimize radiation dose and to ensure appropriate utilization [6 8]. Many protocols have been done to reduce ionizing radiation in CT, such as low kilovoltage, automatic anatomic tube current modulation, and shielding [9 13]. Congenital heart disease (CHD) is a common condition, with a varying incidence of four to six cases per 1,000 live births for complex forms and up to 75 cases per 1,000 live births if trivial cases, such as small muscular ventricular septal defects, are included [14]. Although echocardiography is the firstline diagnostic technique for patients with suspected CHD, this technique is limited in some cases because of its deficiency in delineating extracardiac anomalies. MRI has ex- AJR:196, February 2011 W117

2 cellent anatomic and functional assessment capabilities, but it is often limited for the seriously ill, uncooperative patients, and patients with pacemakers [15]. Moreover, MRI studies are also time consuming and require sedation of patients. As is well known, catheterbased cardiovascular angiography has been the reference standard for evaluation of CHD. However, it has the drawbacks of ionizing radiation and the risks associated with iodinated contrast material. In addition, this method is expensive, invasive, and may cause some unpredictable complications. CT also has the drawbacks of iodinated contrast material and exposure to ionizing radiation, but it is able to delineate extracardiac morphology and evaluate airway simultaneously with a good temporal and contrast resolution. Therefore, considering all diagnostic techniques mentioned above, MDCT with a low-dose scanning protocol has become a widely used imaging technique in the diagnosis of CHD [16, 17]. Because of their small body size, different peripheral vein injection sites are selected for cardiovascular CT angiography (CTA) of pediatric patients with CHD. In our center, a nurse selects the largest peripheral vein in the head, arm, or leg that is expected to obtain satisfactory contrast images. In accordance with the literature, we have adopted low kilovoltage and automatic tube current modulation to reduce radiation exposure [9, 18]. In this article, a retrospective study was used to evaluate the effect of different injection sites on image quality and radiation exposure of cardiovascular CTA, which, to our knowledge, has not been reported before. Materials and Methods Patient Groups The patients in this study were 4 36 months old and were admitted to our hospital between May 2008 and October The indication for performing CTA was based on known and suspected positive extracardiac results of echocardiography. Patients with known allergy to contrast material or with renal insufficiency and those whose parents refused to provide informed consent for MDCT were excluded from the study group. Written informed consent for the CT procedure and for the research protocol was given by each parent after the nature of the examination had been fully explained. The study was approved by the ethics committee of our hospital. In total, 61 patients entered the study. They were assigned to patient group A (head vein), patient group B (arm vein), or patient group C (leg vein) according to the different injection sites. The characteristics of the three patient groups are shown in Table 1. Cardiovascular CTA All scans were performed with a commercially available CT scanner (Sensation 64 Cardiac, Siemens Healthcare). Chloral hydrate was orally administrated for all patients (50 mg/kg). Patients were examined in the supine position with elevated arms. Scanning protocols were as follows: an unenhanced topogram view was acquired at 80 kvp and 39 ma, a premonitoring scan was performed at 80 kvp and 17 mas at the level of the carina, and a bolus-tracking monitoring scan (Care Bolus, Siemens Healthcare) was performed using the manufacturer s default protocol with a region of interest (ROI) placed in the descending aorta (DA). A monitoring image started at 5 seconds after the injection of contrast medium and then was achieved every 0.99 second. After an additional delay of 9 seconds with a trigger level of 80 H, a cardiovascular angiography scan was performed in the caudocranial direction and the scan was performed from the upper margin of the shoulder to just below the diaphragm. No ECG gating or breath-holding was applied. A standard collimation of mm was used, with a gantry rotation speed of 0.33 second and a pitch factor of 1.1. A weight-based low-dose CT protocol (80 kvp) was used with automatic tube current modulation software (CAREDose4D, Siemens Healthcare) [9, 18]. The nurse observed the peripheral vein diameters in the head, arm, and leg before injection and chose the largest vein to insert a 24-gauge IV catheter. Patients received a dose of ml/kg of nonionic contrast medium (iohexol, iodine content 350 mg/ml; Omnipaque, GE Healthcare) at a rate of ml/s followed by a same-volume IV saline flush with the same injection rate. The injection rate was based on our clinical experience and was related to body weight. Injections were performed automatically by using a commercially available injector ( A OptiVantage DH, Liebel Flarsheim). For further postprocessing, thin-section reconstruction was performed with a section thickness of 1 mm, an increment of 0.7 mm, and a smooth reconstruction kernel (B25f). Final image analysis was performed with transverse images (section thickness, 3 mm) and volume-rendered images. Volume-rendered images were created on a workstation (Leonardo, Siemens Healthcare) using In- Space software (Siemens Healthcare). Assessment of Image Parameters Attenuation measurements were determined on a workstation (Leonardo, Siemens Healthcare), including 10 different levels: left atrium (LA), left ventricle (LV), right atrium (RA), right ventricle (RV), the main pulmonary artery (MPA), right pulmonary artery (RPA), left pulmonary artery (LPA), ascending aorta (AA), and DA at the level of the bifurcation and arch. Averaged values of LA, LV, RA, and RV were used for the final calculation of heart chamber attenuation. Averaged values of MPA, RPA, and LPA were used for the final calculation of pulmonary artery (PA) attenuation. Averaged values of AA, DA, and arch were used for the final calculation of aorta attenuation. The ROIs used for these measurements were chosen to be as large as the vessels and heart chamber. The measurement of background noise was based on the assessment of attenuation (in Hounsfield units) within the surrounding air at three ROIs in front of the patient (central, left, and right) with a size of 1 cm 2 ; averaged values were used for the final calculation of background noise. In addition, attenuation of the central parts of pectoral muscles and the deep paraspinal muscles were measured on both sides and averaged (muscle attenuation) [12]. On the basis of these measurements, signal-to-noise ratio (SNR) and contrastto-noise ratio (CNR) were calculated according to the following equations: SNR = mean attenuation / TABLE 1: Patient Characteristics and Scanning Parameters Characteristic or Parameter Group A (Head Vein) Group B (Arm Vein) Group C (Leg Vein) p Patient characteristic Age (mos) 7.8 ± ± ± Body weight (kg) 6.9 ± ± ± No. of male subjects:no. of female subjects Scanning parameter Perpendicular distance from skin to anterior ribs (cm) No. of extracardiac malformations No. of intracardiac malformations 12:13 13:9 8: ± ± ± Contrast injection rate (ml/s) 1.1 ± ± ± Note Except where noted otherwise, data are mean ± SD. W118 AJR:196, February 2011

3 Pediatric Cardiovascular CT Angiography background noise, and CNR = (mean attenuation muscle attenuation) / background noise, where mean attenuation is the total mean attenuation of the heart chamber, PA, and aorta. Subjective image quality with regard to the heart chamber, PA, and aorta was assessed by two readers with 14 and 18 years of experience in chest CT in consensus by using volume-rendered and axial 3-mm CT maximum-intensity-projection images together with original 3-mm reconstructed images. The two observers discussed the image and came to a common conclusion in cases of disagreement for image quality. The images were assessed by using a 5-point scale based on artifacts and enhancement, as follows: 1, unacceptable; 2, suboptimal; 3, adequate; 4, good; and 5, excellent diagnostic quality. Diagnostic quality was considered achieved when the score was 3 or higher. Assessment of image data for the presence of intracardiac and extracardiac abnormalities at cardiovascular CTA was performed for the three groups. Effective Radiation Dose Evaluation The amount of extrathoracic fat and soft tissue was individually estimated by measuring and averaging the perpendicular distance between the skin and the anterior margins of the ribs on both Fig month-old boy with pulmonary artery atresia, ventricular septal defect, and patent ductus arteriosus (aorta score, 3). Contrast medium injection site was head vein. A, Anterior view from CT volume-rendered image shows streak artifacts of ascending aorta. B, Axial CT maximum-intensity-projection image shows contrast medium in superior vena cava causes image blurring to ascending aorta. Fig month-old girl with tetralogy of Fallot, stenosis of main pulmonary artery, and right aortic arch with mirror image branching (aorta score, 3). Contrast medium injection site was arm vein. A, Anterior view from CT volume-rendered image shows streak artifacts of ascending aorta. B, Axial CT maximum-intensity-projection image shows contrast medium in superior vena cava causes image blurring to ascending aorta. sides at the level of the aortic arch (as an imagebased quantification tool for obesity) [12]. Dose length products (DLPs) before scanning (including premonitoring and monitoring scanning) and for cardiovascular scanning, which were provided by the scanner system, were recorded. Effective radiation dose (ED) was calculated by multiplying the total DLP by a constant (k = msv/ mgy/cm) that is based on age- and region-specific pediatric DLP conversion coefficients [19] in ac- TABLE 2: Measurements of Attenuation and Objective and Subjective Image Quality Attenuation (H) Parameter Group A (Head Vein) Group B (Arm Vein) Group C (Leg Vein) p Heart chamber ± ± ± Pulmonary artery ± ± ± Aorta ± ± ± Muscle 12.7 ± ± ± Background noise 7.4 ± ± ± Signal-to-noise ratio 61.9 ± ± ± Contrast-to-noise ratio 60.2 ± ± ± Image quality score Heart chamber 4.08 ± ± ± Pulmonary artery 4.12 ± ± ± Aorta 3.76 ± ± ± Note Except for p values, data are mean ± SD. A A B B AJR:196, February 2011 W119

4 cordance with our patients with CHD in the study. All measurements were performed by an operator with 3 years of experience in chest CT. Statistical Analysis Results of attenuation measurements, SNR, CNR, subjective image quality, DLP, ED, patient age, body weight, contrast injection rate, and perpendicular distance from skin to anterior ribs are expressed as mean ± SDs. Normality of data distribution was assessed by using the Kolmogorov-Smirnov test. Characteristics of the three patient groups (age, sex, body weight, and distance between thoracic skin and anterior ribs) were analyzed, and results for all three patient groups were compared with regard to attenuation measurements, SNR, CNR, DLP, ED, and subjective image quality. The comparison of continuous variables was performed by using a one-way analysis of variance, and that of categoric variables was done by Kruskal-Wallis test and chi-square test. If there was a significant effect between variables, the Scheffe test or Kruskal-Wallis test, respectively, was performed to further specify the effects. A p value of 0.05 or less was considered to indicate a statistically significant difference for all statistical tests. All calculations were performed with a standard PC by using software (SPSS 15.0 for Windows, SPSS). Results Patient Characteristics Comparison of the three patient groups did not reveal significant differences with regard to patient age, sex, body weight, contrast injection rate, amount of extrathoracic soft tissue, or incidence of intracardiac and extracardiac malformations at cardiovascular CTA (Table 1). Thus, further analysis and comparison of attenuation measurements and radiation exposure were considered valid and feasible. A Image Parameters Measurements of attenuation and objective and subjective image quality are shown in Ta- B ble 2. Application of the Kolmogorov-Smirnov test showed that all continuous variables satisfied normality. Mean heart chamber, PA, and aorta attenuation in group B were higher than those in groups A and C, but the difference was not significant. Similarly, there was no significant difference among the three patient groups for mean attenuation of paraspinal and pectoral muscles, background noise, SNR, and CNR. Assessment of subjective image quality with volume-rendered and transverse images revealed that no examination was rated as nondiagnostic (grades 1 and 2) (Figs. 1 3). The statistical analysis showed no significant difference in subjective image quality among the three groups for heart chamber and PA image quality, but significant differences were found for aorta image quality among the three groups. The Kruskal-Wallis test further revealed significant differences for aorta image quality between groups A and C (p = 0.009), as well as between groups B and C (p = 0.002). However, there was no significant difference between groups A and B (p = 0.71). Radiation Exposure DLP and ED values for the different groups are shown as Table 3. Among all three groups, there was no significant difference for DLP of cardiovascular scanning, but there were significant differences for monitoring DLP, total DLP, and ED. The Scheffe test revealed significant differences between groups A and C for monitoring DLP (p = 0.001), total DLP (p = 0.003), and ED (p = 0.003) but no significant difference between groups A and B (p = 0.13 for monitoring DLP, p = 0.08 for total DLP, and p = 0.08 for ED) and groups B and C (p = for monitoring DLP, p = 0.14 for total DLP, and p = 0.14 for ED). The DLP before scanning was 17.6%, 20.2%, and 24.5% of the total DLP in groups A, B, and C, respectively. Discussion In groups A (head vein), B (arm vein), and C (leg vein) in this study, no significant difference was found in heart chamber (p = 0.57), PA (p = 0.17), and aorta attenuation (p = 0.76), as well as in mean attenuation of paraspinal and pectoral muscles (p = 0.52) and in background noise (p = 0.56). The best criteria for objectively assessing image quality, the SNR and CNR, also showed no significant difference among three groups (p = 0.52 for both) (Table 2). The scores for heart chamber, PA, and aorta were more than 3, which mean all CTA images were of diagnostic quality. There was no significant difference for subjective aorta image quality between groups A and B TABLE 3: Dose Length Product (DLP) and Effective Dose (ED) of Patient Groups Radiation Dose, Protocol Group A (Head Vein) Group B (Arm Vein) Group C (Leg Vein) p DLP (mgy cm) Fig month-old boy with ventricular septal defect and aneurysmal dilatation of main pulmonary artery (aorta score, 4). Contrast medium injection site was leg vein. A, Anterior view from CT volume-rendered image shows edge of aorta is smooth. B, Axial CT maximum-intensity-projection image shows aorta is not affected by contrast medium in superior vena cava. Premonitoring NA Monitoring 2.95 ± ± ± Cardiovascular scanning 16.6 ± ± ± Total 20.1 ± ± ± ED (msv), total 0.52 ± ± ± Note Except for premonitoring DLP and p values, data are mean ± SD. NA = not available. W120 AJR:196, February 2011

5 Pediatric Cardiovascular CT Angiography (p = 0.71), but significant differences existed between groups A and C (p = 0.009) and between groups B and C (p = 0.002). The discrepancy between objective and subjective aorta image quality may be the result of the different methods in assessment. The objective image qualities were based on cardiovascular attenuation, whereas the subjective image qualities were based on visual-sensory reception. The injection of contrast medium from the head or arm caused image blurring of the AA because of a high concentration in the superior vena cava, even though contrast medium was immediately followed by a saline flush. This resulted in a low subjective image quality. On the other hand, AA attenuation was not much affected by this image blurring, so the objective image quality did not reduce significantly. The injection of contrast medium from the leg did not cause image blurring to the AA; therefore, the highest subjective score was obtained in group C. The results suggest that the leg should be selected as the injection site if the AA was the major item being evaluated. ECGgated cardiovascular CTA has been suggested as a possible technique in evaluating AA for its avoiding motion blurring (i.e., supravalvular aortic stenosis) [20]. It is worth noting that the scanning protocol in ECG-gated cardiovascular CTA would cause much more radiation exposure than non-ecg-gated [21] and should be used cautiously. Much attention has been paid to reduce radiation exposure in our studies, because the increase in population radiation exposure from CT, particularly for children, has been of concern to radiologists and medical physicists. As proposed by the as-low-as-reasonably-achievable principle, the selection of appropriate scanning parameters focuses on the optimization of the image quality while delivering the lowest possible radiation dose and shifting the risk benefit balance toward benefit [4]. Low kilovoltage and automatic tube current modulation have been proposed for effectively decreasing radiation exposure without deteriorating image quality during CT examinations [18, 22 27]. These measures have been applied in some medical centers and have attracted more and more attention, especially in pediatric radiology departments [28 31]. Hence, the low kilovoltage (80 kvp) and automatic tube current modulation were also used to reduce radiation exposure in this study. As a result, the ED of ionizing radiation varied from 0.52 to 0.63 msv. Because ED is currently deemed the best available dose descriptor for quantifying stochastic risks in diagnostic radiology [32], it can be concluded that a low radiation exposure was achieved here. This level was lower than the average annual ED from background radiation of about 3 msv [33] and also far lower than that of the catheter-directed cardiovascular angiography, which has the potential to impart high radiation doses because of extended fluoroscopic and cine evaluation [34, 35]. For example, Bacher et al. [34] reported median EDs of 6.0 msv for therapeutic pediatric cardiac catheterizations and 4.6 msv for diagnostic cardiac catheterizations. Rassow et al. [35] found high EDs in newborns (18.0 msv [90th percentile] and 6.5 msv [50th percentile]) resulting from cardiac catheterizations. The low ED of ionizing radiation in this study indicates that a proper set of scanning parameters can effectively reduce radiation exposure. From this point of view, we propose that cardiovascular CTA is a very effective and safe technique in diagnosing CHD under the proper scanning parameters. In this study, we found out that the level of radiation exposure depended on injection site, although all cardiovascular CTAs are associated with a low radiation dose. The injection site from the leg caused the highest radiation dose (Table 3). The significant difference came from the prescanning (mainly in monitoring scanning) rather than from the formal cardiovascular scanning (p = and p = 0.25, respectively). This finding suggests that prescanning plays a major role in the radiation dose. It is easy to understand that the longest route from the injection site to the heart and most venous valves in group C required the longest trigger time, thus causing the highest radiation dose in monitoring scanning. The monitoring image was designed in default protocol by the manufacturer to start at 5 seconds after the injection of contrast medium and then be achieved every 0.99 second. It resulted in the maximum number of monitoring images and also the highest radiation dose in group C. Even though the radiation dose caused by prescanning has been ignored and not evaluated along with the total radiation dose in previous studies [9, 11, 12], we now find it necessary to include it in evaluating total cardiovascular CTA radiation dose and to consider it carefully. With the development of low radiation dose scanning protocol, the radiation dose from pre scanning accounts for a considerable proportion of the total radiation. In this study, prescanning DLP accounted for 17.6%, 20.2%, and 24.5% of the total DLP in groups A, B, and C, respectively. Reducing the prescanning radiation dose will open an avenue to reduce CT radiation dose in total cardiovascular angiography. We have attempted to revise the manufacturer s default protocol into a different one whereby the monitoring image was started at 8 seconds after injecting contrast medium and then was achieved every 1.98 seconds. As a result, a great decrease in prescanning DLP was realized. A further study is in progress and will be reported later. There are some limitations to our study. First, the three groups have different congenital cardiovascular abnormalities, which could cause different homodynamic circulations and different attenuation of the heart chamber and great vessels. Because this was a retrospective study and clinical cases were variable, it is impossible to control the type of CHD. Fortunately, the extracardiac and intracardiac malformations in the three groups were not significantly different (p = 0.99 and p = 0.94, respectively), which means that the three groups were comparable. Second, the findings on CTA were not compared with findings at surgery. However, the aim of the study was not to compare diagnostic accuracy of cardiovascular CTA but to evaluate the effect of different injection sites on image quality and radiation exposure. Third, topogram was not included in DLP and ED. This was because the contribution of the pilot scan to the total radiation dose of a CT examination was typically considered negligible [10] and the radiation dose was not provided by the manufacturer. Finally, we did not compare body mass index but perpendicular distance from skin to anterior ribs as a measure of body configuration. Because it is a retrospective study, only body weight and not body height were recorded. We thus concluded that extrathoracic fat and soft tissue can represent patients body configuration and can be related directly to radiation dose. In conclusion, the effect of different injection sites on image quality and radiation exposure has been studied. The results show that all injection sites can acquire a diagnosticquality image with a low radiation dose in pediatric cardiovascular CTA. If the evaluation of the AA is important, the injection site from the leg should be selected, but the radiation dose was highest in this case. The radiation dose caused by prescanning should be taken into account, and an optimization of prescanning protocol will open an avenue to reduce radiation dose of cardiovascular CTA. Acknowledgments We thank Dr. Hong-Bo Huang (Jiangsu Testing and Assessment Research Institute, Nanjing, China) for his help in data process- AJR:196, February 2011 W121

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