Pediatric 16-slice CT Protocols:

Transcription

1 Pediatric 16-slice CT Protocols: Radiation Dose and Image Quality 1 Dong Hyun Yang, M.D., Hyun Woo Goo, M.D. Purpose: To assess radiation dose and image quality of our pediatric 16-slice CT protocols and to compare them with published standards. Materials and Methods: For 540 weight-based pediatric 16-slice CT examinations in six anatomic regions, CTDI vol, DLP, effective dose, and image noise were determined. Two radiologists evaluated the visual quality of CT images by consensus. We analyzed the relationship of CTDI vol and image noise with body diameter. Our results were compared with published data. Results: The average CTDI vol (mgy), DLP (mgy cm), effective dose (msv), and image noise (HU) were as follows: 4.1/125.5/1.6/16.2 for chest CT, 3.3/54.2/1.2/13.7 for heart CT, 5.8/256.6/3.8/13.0 for abdomen-pelvis CT, 6.8/318.7/5.9/12.0 for dynamic abdomen CT, 3.5/86.2/0.35/7.9 for neck CT and 25.4/368.0/1.6/3.7 for brain CT, respectively. All CT images were diagnostic upon visual analysis. The CTDI vol and image noise were proportional to body diameter. Our dose parameters were comparable to the first quartile of the cited German survey, whereas image noise in our study was similar to published data. Conclusion: Our pediatric CT dose is at the lower end of published standards and our image noise can be used as a target noise for each protocol in developing better pediatric multi-slice CT protocols. Index words : Child Tomography, X-Ray Computed Quality assurance, health care Radiation dosage Radiation exposure has a greater potential risk in a pediatric CT than an adult CT because children have greater radio-sensitivity and a longer life expectancy (1). 1 Department of Radiology and Research Institute of Radiology, Asan Medical Center, University of Ulsan College of Medicine, Korea. Received June 30, 2008 ; Accepted September 1, 2008 Address reprint requests to : Hyun Woo Goo, M.D., Department of Radiology and Research Institute of Radiology, Asan Medical Center, University of Ulsan College of Medicine, Poongnap-2 dong, Songpa-gu, Seoul , Korea Tel Fax hwgoo@amc.seoul.kr 333 Thus, particular emphasis has been posed on the reduction of radiation dose for pediatric CT (2) by means of various strategies as follow (3 9). Adaptation of the tube current corresponding to the patient age (3) or body weight (4) is central to dose-saving for the CT protocol. Moreover, the use of low tube potential and tube current modulation has recently been recommended for further optimization of CT dose (5 9). To optimize a CT protocol, a radiologist should balance dose reduction and image quality because adjust-

2 ing each parameter represents a trade-off. Thus, one should be familiar with the parameters of radiation dose and image quality and their relationships. Although recent dose surveys have described current dose levels in pediatric CT imaging, no evaluations of image quality parameters, (e.g. image noise) have been made (10, 11). For pediatric CT imaging, only a small number of phantom studies have simultaneously considered radiation dose and image quality (2, 12, 13). Since 2000, we have developed our own pediatric low-dose CT protocol to which a combination of body weight-based tube current adaptation, low tube potential, and tube current modulation were applied (6, 13). We carried out the present study to assess radiation dose and image quality of our pediatric multi-slice CT protocols and to compare them with published standards. Materials and Methods Our Institutional Review Board approved this retrospective study and informed consent was waived. Study group During the six months from July 2006 to December 2006, 1142 pediatric CT examinations were performed using eight CT scanners following the low-dose protocol at our department. The majority of scans (n = 619, 54%) were performed with two 16-slice CT scanners (Somatom Sensation 16 with version VB10B; Siemens, Forchheim, Germany; gantry rotation time of s). The other six CT scanners included: a single-slice CT (n = 1, HiSpeed; General Electric, Milwaukee, U.S.A.), four-slice CT (n = 2, LightSpeed; General Electric), 16- slice CT (n = 1, LightSpeed 16; General Electric), 16-slice CT (n = 1, Somatom Sensation 16; Siemens; gantry rotation time of 0.5 s), and a dual source CT (n = 1, Somatom Definition; Siemens). Among them, 64 CT examinations, including upper abdomen CT images, high-resolution chest CT images, unenhanced body CT images and electrocardiography (ECG)-synchronized heart CT images were excluded because the number of examinations was too small to be analyzed. Thus, the six examination protocols included in our study included chest, non-ecgsynchronized heart, abdomen-pelvis, dynamic abdomen, neck, and brain CT images (Table 1). Further, 15 examinations in which the dose report (CTDI vol and DLP) was not available were excluded from the study. Finally, a total of 540 CT examinations from 319 children (198 boys, 121 girls; mean age, 5.8 years; range, 1 day 15 years) were enrolled in our study. The reasons for the CT examination included: malignancies (n = 280), congenital anomalies (n = 124), infections (n = 59), inflammatory diseases other than infections (n = 34), benign tumors (n = 17) and others (n = 26). CT imaging In our CT protocol, tube current and tube potential were adjusted for six body-weight groups (Table 1). All CT examinations other than brain CT imaging were performed with a spiral scan, followed by the application of tube current modulation (CARE Dose 4D; Siemens, Forchheim, Germany). For brain CT imaging, axial scans were used with a gantry rotation time of 1.0 second to avoid image blurring and artifacts related to multi-slice spiral scans. A gantry rotation time of seconds was chosen for all spiral scans to minimize motion artifacts. Beam collimation, reconstruction section thickness, reconstruction kernel, and scan range are summarized in Table 2. In all spiral scans, a beam pitch of 1.0 was used. In addition, iodinated contrast agent (Iomeron 300 or 400; Bracco Imaging SpA, Milan, Italy) was administered intravenously ( ml/kg), followed by a saline flush using a dual power injector (Optivantage DH, Tyco Health/Mallinckrodt, St Louis, U.S.A.). For Table 1. Adapted Values of Tube Potential and Tube Current for Six Body Weight Groups and Six Anatomic Regions Body Weight Chest Heart Abdomen-pelvis Dynamic Abdomen Neck Brain (n=202) (n=75) * (n=160) (n=41) (n=26) (n=36) < 5 kg 080/40 (3) 00080/50 (25) 0080/65 (2) 0080/80 (0) 0100/40 (1) 100/120 (4) kg 080/50 (2) 00080/65 (30) 0080/80 (6) 00080/90 (15) 0100/50 (1) 100/140 (8) kg 0080/65 (75) 00080/90 (11) 0080/105 (67) 080/120 (7) 0100/60 (7) 100/190 (5) kg 0080/90 (65) 080/120 (9) 0080/130 (48) 00100/85 (16) 0100/70 (7) 0120/160 (13) kg 0100/65 (47) 0100/85 (0) 00100/95 (32) 100/100 (3) 0100/80 (7) 120/170 (5) kg 100/100 (10) 100/110 (0) 100/130 (5) 100/160 (0) 100/100 (3) 120/200 (1) Note - Data are presented as tube potential (kv) / effective tube current-time product (mas). * Heart CT was performed without electrocardiography gating. The numbers in parentheses indicate the number of examinations. 334

3 heart and dynamic abdomen CT images in which vascular opacification is of paramount importance, iodinated contrast agent containing 400 mg I/mL (Iomeron 400) was used, whereas iodinated contrast agent containing 300 mg I/mL (Iomeron 300) was used for all other CT examinations. An injection rate ranging from 0.3 to 3.0 ml/sec was determined according to the gauge (24 18) of the peripheral venous catheter. Further, a scan delay time was determined using a bolus tracking method in chest CT, heart CT, and the arterial phase of dynamic abdomen CT. A fixed delay time was employed in abdomen-pelvis CT (50 65 sec), the portal venous phase of dynamic abdomen CT (50 65 sec) and neck CT (40 60 sec). Conforming to the As Low As Reasonably Achievable (ALARA) principle, an unenhanced CT was not performed other than for a brain CT (unenhanced scan only, n = 22; unenhanced and enhanced scans, n = 14). Non-ECG-synchronized heart CT imaging was performed to assess the anatomy of the heart and thoracic great vessels in patients with congenital heart disease. Dynamic CT imaging of the abdomen, which consists of arterial and portal venous phases, was performed to assess hepatic vessels for liver transplantation or enhancement patterns of focal hepatic lesions. In the two examination protocols requiring multiplanar and three-dimensional reformations, a thinner collimation with a relatively greater radiation dose was used, compared to chest and abdomen-pelvis CT images requiring only axial images (Tables 1, 2). Radiation dose parameters A volume CT dose index (CTDI vol ; mgy), dose length product (DLP; mgy cm), and effective dose (msv) were described in our study as the three important radiation dose parameters. CTDI vol and DLP were automatically generated and saved as a dose report after each CT examination. The dosimetric measurement of these parameters was not performed. The CTDI vol is a phantombased dose parameter and is based on the unit volume. In a Sensation 16 CT, the phantom size used to calculate CTDI vol is predetermined by a specific scan mode (e.g., 16-cm phantom for head mode, 32-cm phantom for body mode) irrespective of patient size. In pediatric CT imaging, even in body scan mode, 16-cm phantombased dose parameters have been commonly used because these values provide more realistic estimates of radiation dose for children than 32-cm phantom-based dose parameters (10, 11). Moreover, the effective dose in pediatric CT images has been estimated from 16-cm phantom-based values (10, 11). In a Sensation 16 CT scanner, the radiation dose based on a 16-cm phantom is known to be approximately two times higher than the value based on a 32-cm phantom (15); consequently, we converted 32-cm phantom-based dose parameters for the CT protocols (chest, heart, abdomen-pelvis, and dynamic abdomen) performed in body scan mode to 16- cm phantom-based ones by multiplying the 32-cm dose parameters by a factor of two. In order to determine the effective dose, we applied a formula which had been used in a German pediatric CT Table 2. CT Imaging Parameters and Scan Ranges in CT Examination Protocols Chest Heart Abdomen-pelvis Dynamic Abdomen * Neck Brain Beam Collimation (mm) or or Reconstructed Section Thickness (mm) Reconstruction Kernel B30f B20f or B30f B30f B30f H30f H40s Scan Range Upper Limit Lung apex Top of aortic arch Liver dome Liver dome Orbit floor Vertex Lower Limit left kidney Cardiac Symphysis Inferior liver Sternoclavicular Skull base hilum apex pubis angle (arterial) junction left kidney Iliac crest upper pole (venous) Note Beam collimation is expressed as the effective detector-row width multiplied by the number of data channels. Beam pitch (table feed per gantry revolution / beam collimation) was 1.0 for a chest, heart, abdomen-pelvis, dynamic abdomen, and neck CT. * For a dynamic abdomen CT, enhanced scanning was performed in the arterial and portal venous phases: unenhanced scans were not obtained. When a chest CT and abdomen-pelvis CT were performed at the same time (n = 90), the lower scan range of the chest CT was adjusted to minimally overlap the upper range of the abdomen-pelvis CT. A pediatric radiologist (G. H. W.) checked all parameters, including the scan range of all heart CT examinations, depending on the purpose of the examination and the category of cardiac defects. ** B30f = medium smooth kernel in body mode, B20f = smooth kernel in body mode, H30f = medium smooth kernel in head mode, H40s = medium kernel in head mode. 335

4 dose survey (10): E = DLP P f C f sc ac sc head sc body where E (msv) is the effective dose, Pf refers to the phantom factor, Cf (msv/mgy cm) is the effective dose normalized to the DLP (mgy cm), sc is the scanner correction factor in head mode (sc head ) or body mode (sc body ), ac is the patient age correction factor, and x is a factor required for scanner correction in children (Appendix). The age correction factor was applied to five age groups (newborn [up to 1 month], up to 1 year, 2 5 years, 6 10 years, and years). Consistent with the aforementioned German survey, x the year age group, a 32-cm phantom-based value of DLP (DLP 32 cm ) was used to estimate the effective dose of body CT images, while a 16-cm phantom-based value (DLP 32 cm 2 in our study) was used in other age groups. The commonly used software ImPACT CT dosimetry spreadsheet (16) was not employed for effective dose calculation in our study because a pediatric mathematical phantom was not available in the software program. Image quality Image noise (HU), the standard deviation of the CT density, was measured in two organs and in background air for each protocol by placing a rectangular region of interest (ROI) (Table 3). The ROI measurement was per- A B Fig. 1. Locations of the rectangular regions of interest for image noise measurement of the chest or heart CT images. Three sets of CT images with different window/width levels (HU) were merged (950/220 for the thoracic aorta, 300/50 for the paraspinal muscle, and 1500/ 700 for the background air). The image noise was measured at the predefined three levels (i.e., carina (A), aortic valve (B), and cardiac apex (C). In this examination, the mean measured image noise (HU) was 17.6 for the thoracic aorta, 9.6 for the paraspinal muscle, and 7.1 for the background air. C 336

5 formed at three predefined levels (Table 3) by a boardcertified radiologist with three-years of experience using our PACS workstation. Following the measurement, the three measured values were averaged (Fig. 1). Because the target organs are diverse in size in the pediatric CT, the ROI was drawn as large as possible within a homogenous area of each organ (for organs, ROI up to 100 mm 2 ; for background air, ROI fixed at 100 mm 2 ). To minimize the potential influence of window settings, image noise was measured under a fixed window setting (width/level) for each organ and for background air: vessel (950/220), liver and muscle (300/50), background air (1500/ 700) and brain (100/30) (Fig. 1). The measurement of image noise was performed at the portal venous phase for dynamic abdomen CT imaging and the unenhanced scan for brain CT imaging. The visual quality of the CT images was evaluated by two board-certified radiologists (11-years of experience and three-years of experience, respectively) for image noise, beam hardening or motion artifacts, and enhancement of target organs. The overall image quality was then graded by consensus using a four-point scale (1 non-diagnostic images, 2 suboptimal but acceptable quality images, 3 good quality images, 4 excellent images) (Fig. 2). The CT images showing grade 2 or more were regarded as diagnostic. Body diameter Body diameters were measured on all CT examinations at a predetermined level for each anatomic region. The anterior-to-posterior and transverse diameters were measured on a PACS workstation by an experienced radiologist. The mean value represented the body diameter of a patient for that CT protocol. The measurement levels for each CT protocol included: the four cardiac chambers for chest and heart CT images, celiac axis for A B Fig. 2. Visual grading of CT images for the abdomen-pelvis CT. (A) Visual grade 4 of a CT image at the level of the right portal vein in a 3-year-old girl (CTDI vol 4.46 mgy, body diameter 15 cm, image noise 10.7 HU in the liver). (B) Visual grade 3 of a CT image in a 4-year-old boy (CTDI vol 4.46 mgy, body diameter 17 cm, image noise 13.2 HU in the liver). (C) Visual grade 2 of a CT image in a 3-year-old boy (CTDI vol 4.48 mgy, body diameter 17 cm, image noise 12.6 HU in the liver). C 337

6 abdomen-pelvis and dynamic abdomen CT images, mandibular angle for neck CT, and the center of the thalamus for the brain CT images. Data analysis and comparison with previously published data Radiation dose and image noise were summarized for each protocol and for each body-weight group. All parameters were expressed as the mean standard deviation. We correlated the measured body diameter with CTDI vol and image noise using a linear regression analysis. The statistical analyses were performed using a statistical software package (MedCalc 7.4; MedCalc Software, Mariakerke, Belgium). A p-value of 0.05 was considered to indicate a statistically significant difference. Our radiation dose parameters were compared with two recent German and UK CT dose surveys for each Table 3. Region of Interest Location for Each Scan Region CT Protocol Organ 1 Organ 2 Level Chest / Heart Thoracic aorta Paraspinal muscle Carina/ Aortic valve/ Cardiac apex ( ) ( ) Abdomen-pelvis / Liver Paraspinal muscle Liver dome / Hilum/ Inferior angle Dynamic Abdomen ( ) ( ) Neck Internal jugular vein Sternocleidomastoid muscle Hyoid bone/ Vocal cord/ Thyroid ( ) ( ) gland Brain Gray matter Cerebrospinal fluid Cerebellum/ Thalamus/ Centrum ( ) ( ) semiovale Note The numbers in parentheses indicate the size of the region of interest (mm 2, mean standard deviation). Table 4. Summary of Radiation Dose Parameters, Image Noise, and Visual Grades in Our CT Protocols CT Protocol CTDI vol DLP Effective Dose Image Noise (HU) (mgy) (mgy cm) (msv) Organ 1 Organ 2 Air Visual Grade Chest (n=202) Heart (n=75) Abdomen-pelvis (n=160) Dynamic Abdomen (n=41)* Neck (n=26) Brain (n=36) Note Data are mean values standard deviation. * DLP and effective dose for dynamic abdomen CT are the sum of those of the arterial and portal venous phases. DLP and effective dose for brain CT are those of the unenhanced CT scan. The radiation doses were equal for the unenhanced and enhanced CT scans. Table 5A. Summary of Radiation Dose Parameters, Image Noise, and Visual Grades for Chest CT images and Heart CT images as a function of Body-weight Group CT Protocol CTDI vol DLP Effective Dose Image Noise (HU) (mgy) (mgy cm) (msv) Organ 1 Organ 2 Air Visual Grade Chest (kv/mas) < 5 kg (80/40) kg (80/50) kg (80/65) kg (80/90) kg (100/65) kg (100/100) Heart (kv/mas) < 5 kg (80/50) kg (80/65) kg (80/90) kg (80/120) Note Data are expressed as the mean values standard deviation. 338

7 age group (10, 11). In addition to these surveys, the CTDI vol of our CT protocol was compared with previously published CT protocols (4, 6, 13). We calculated the CTDI vol of the previous protocols based on their imaging parameters and CT models with the ImPACT program (16). In these pediatric CT protocols, imaging parameters were specific to either body-weight or age group. A known conversion table was used to convert body weight to patient age (13). Moreover, the image noises for the aorta, liver, muscle, and brain in our study were compared with corresponding data available in the literature (17 23). Results The radiation dose parameters, image noise, and visual image quality grades for each protocol and bodyweight group are summarized in Tables 4 and 5. The radiation dose parameters revealed a tendency to be less in lighter-weight groups (Table 5) and in younger-age groups (Fig. 3), except for the effective dose for brain CT. Image noise showed a slight tendency to be less in lighter-weight groups, except for the neck CT. All CT images were of diagnostic quality and had mean scores Table 5B. Summary of Radiation Dose Parameters, Image Noise, and Visual Grades for Abdomen-pelvis CT images and Dynamic Abdomen CT images as a function of Body-weight Group CT Protocol CTDI vol DLP Effective Dose Image Noise (HU) (mgy) (mgy cm) (msv) Organ 1 Organ 2 Air Visual Grade Abdomen-pelvis (kv/mas) < 5 kg (80/65) kg (80/80) kg (80/105) kg (80/130) kg (100/95) kg (100/130) Dynamic Abdomen (kv/mas) * kg (80/90) kg (80/120) kg (100/85) kg (100/100) Note Data are mean values standard deviation. * DLP and effective dose of the dynamic abdomen CTs represent the sum of the arterial and portal venous phase CTs. Table 5C. Summary of Radiation Dose Parameters, Image Noise, and Visual Grades for Neck CT images and Brain CT images as a function of Body-weight Group CT Protocol CTDI vol DLP Effective Dose Image Noise (HU) (mgy) (mgy cm) (msv) Organ 1 Organ 2 Air Visual Grade Neck (kv/mas) < 5 kg (100/40) kg (100/50) kg (100/60) kg (100/70) kg (100/80) kg (100/100) Brain* (kv/mas) < 5 kg (100/120) kg (100/140) kg (100/190) kg (120/160) kg (120/170) kg (120/200) Note Data are expressed as the mean values standard deviation. * DLP and effective dose for brain CT images are those for the unenhanced CT scan. The radiation doses were equal for unenhanced and enhanced CT scans. 339

8 of visual grade as follows: 2.8 in chest CT images, 2.9 in heart CT images, 2.6 in abdomen-pelvis CT images, 2.9 in dynamic abdomen CT images, 2.7 in neck CT images, and 2.7 in brain CT images (Table 6). The CTDI vol of our CT protocols were highly correlated with the measured body diameter (r = 0.88 in chest CT, r = 0.88 in heart CT, r = 0.87 in the abdomenpelvis CT images, r = 0.91 in dynamic abdomen CT images, r = 0.83 in neck CT images, and r = 0.90 in brain CT images; p <0.001 for all protocols). The scatter plots between the CTDI vol and the measured body diameter appeared stepwise rather than linear due to the employment of stepwise CT imaging parameters based on each body-weight group (Fig. 4). On the other hand, image noise showed variable degrees of association with the measured body diameter in all CT protocols except for neck CT images (r = 0.26, p < in chest CT images, r = 0.49, p < in heart CT images, r = 0.46, p < in abdomen-pelvis CT images, r = 0.40, p = 0.01 in dynamic abdomen CT images; r = 0.78, p < in Table 6. Summary of Visual Image Quality Analyses CT Protocol Grade 1 Grade 2 Grade 3 Grade 4 (Non-diagnostic) (Suboptimal but Acceptable) (Good) (Excellent) Chest (n=202) Heart (n=75) Abdomen-pelvis (n=160) Dynamic Abdomen (n=41) Neck (n=26) Brain (n=36) Note Data are expressed as the number of examinations. A B Fig. 3. Graphs showing the mean effective dose estimates in each age group for (A) chest and heart CT images, (B) abdomenpelvis and dynamic abdomen CT images, and (C) neck and brain CT images. C 340

9 J Korean Radiol Soc 2008;59: brain CT; and r = , p = 0.71 in neck CT) (Fig. 5). Our CT dose parameters were substantially lower than the reference doses, which comprised the third quartile of both the cited German and UK dose surveys (10, 11), and were comparable to the first quartile of the German survey (Fig. 6) (Table 7). Our mean effective doses were 46%, 61%, and 62% of the German reference dose for chest, abdomen-pelvis, and brain CT im- ages, respectively. The CTDIvol of our chest and abdomen-pelvis CT images were also comparable to those by Greess et al. (6) and Huda et al. (13) (Fig. 6). Image noise levels of CT protocols were similar to that of previously published data (17-23) (Table 8). A B Fig. 4. Scatter plots showing the correlation between CTDIvol and measured body diameter in (A) chest CT images and (B) abdomen-pelvis CT images. The CTDIvol shows a statistically significant linear correlation between body diameter for the chest CT images (r = 0.88, p<.001) and abdomen-pelvis CT images (r = 0.87, p<.001). The best-fit lines are shown as a solid line (chest CT, intercept = -2.62, slope = 0.34; abdomen-pelvis CT, intercept = -3.66, slope = 0.51). The scatter plots appear stepwise rather than linear because of weight-group-based adjustment of the CTDIvol. A B Fig. 5. Scatter plots show a correlation between image noise and measured body diameter in the (A) chest CT images and (B) abdomen-pelvis CT images. Image noise shows a positive correlation with body diameter for both the chest CT images (r = 0.26, p<0.01) and the abdomen-pelvis CT images (r = 0.46, p<0.01). For each scatter plot, the best-fit lines are shown as a solid line (chest CT, intercept = 13.24, slope = 0.15; abdomen-pelvis CT, intercept = 8.32, slope = 0.25). 341

10 Table 7. Summary of Radiation Dose for the current Study, the German Survey, and the UK Survey CTDI vol (mgy) DLP (mgy cm) Effective Dose (msv) CT protocol Our German UK Our German UK Our German UK Study Survey Survey Study Survey Survey Study Survey Survey Chest Newborn ( ) (25 47) ( ) Up to 1 year ( )0 11 (5.1 12) (36 93)0 159 (68 204) ( ) 6.3 ( ) 2 5 years ( )0 11 (7.6 13) (60 137)0 198 ( ) ( ) 3.6 ( ) 6 10 years ( )0 14 (9.5 17) (98 257)0 303 ( ) ( ) 3.9 ( ) years ( ) ( ) ( ) Abdomen-pelvis Newborn NA 4.2 ( )0 NA 71 (49 81)0 NA 3.6 ( ) Up to 1 year ( ) (87 164) ( ) 2 5 years ( ) ( ) ( ) 6 10 years ( ) ( ) ( ) years ( ) ( ) ( ) Brain Newborn ( ) ( ) ( ) Up to 1 year ( ) 25 (16 28) ( ) 230 ( ) ( ) 2.5 ( ) 2 5 years ( ) 34 (24 43) ( ) 383 ( ) ( ) 1.5 ( ) 6 10 years ( ) 44 (32 51) ( ) 508 ( ) ( ) 1.6 ( ) years ( ) ( ) ( ) Note Data are expressed as mean values. The numbers in parentheses indicate the first-third quartile in each age group. NA = not available. A B Fig. 6. The graphs show the CTDI vol (16 cm-phantom-based value) of our protocols and previously published CT protocols for the (A) chest CT images, (B) abdomen-pelvis CT images, and (C) brain CT images. The amount and pattern of CTDI vol in our study are comparable to those of previous protocols with the exception of the protocol by Donnelly et al. (6) (dashed line). In this protocol, the CTDI vol is higher in the younger age group (2 5 years) than the older age group (6 15 years) due to an abrupt change in pitch between the two. C 342

11 Discussion In the present study, we described radiation dose parameters and image noise in our weight-based pediatric multi-slice CT protocol. Our study group of 540 clinical CT examinations encompassed nearly the entire range of children s body-weight groups and anatomical regions. To the best of our knowledge, a study comprehensively describing radiation dose parameters, image noise, and visual image quality for pediatric brain, neck, and body CT protocols has not been performed. It should be noted that our CT protocols utilized all available dose reduction strategies, including body-weightbased tube current adaptation, low tube potential, and tube current modulation. We found that our CTDI vol approximates the German first quartile (10) and recently proposed pediatric CT protocols (6, 13) in both its values and pattern among the different age groups (Fig. 6). As expected, radiation dose parameters showed a tendency to be less in lighterweight and younger-age groups. The only exception was the effective dose for brain CT images, in which the value showed a tendency to be less in older age groups (Fig. 3C). Notably, other researchers previously demonstrated that the effective dose from pediatric brain CT decreases exponentially with increasing body length (24). For heart and dynamic abdomen CT protocols, the effective dose was less in the year age group than in the 6 10-year age group (Fig. 3A, B). This might be due to a change in imaging parameters or suboptimal parameters. Similarly, CTDI vol was higher in the 2 5- Table 8. Image Parameters and Image Noise of Previously Published CT Protocols (24 30) Organ Author Age Group Slice Reconstruction Tube Tube Current Image Our Study Thickness Kernel Potential (kv) (Effective Noise mas) (HU) (Image Noise) Thoracic Rogalla et al. Children * / 13.7 Aorta * 39.5 (Chest / Heart) Sigal- Adults 5 B40f * 12.0 Cinqualbre * 20.0 et al * 24.0 Liver Nyman et al. Adults 5 B30f / 12.0 Wilting et al. Adults 5 Soft * 11.6 (Abdomen-pelvis / Dynamic abdomen) Muscle Mulkens et al. Adults 5 B40s / 10.9 (Abdomen) (Abdomen-pelvis / Dynamic abdomen) Muscle Namasivayam Adults 2.5 Standard * (Neck) et al * * 6.9 Brain Mullins et al. Adults 5 NA * (Gray * 4.9 Matter) Note * The values of effective tube current-time product (effective mas) were calculated using given values of mas and pitch in the literature (effective mas = mas/pitch). Appendix Variables for Effective Dose Calculation in the Sensation 16 CT [12] Scan Region Phantom Effective Dose Scanner Correction Factor Normalized to DLP Age Correction Factor (ac) Factor (sc) (Pf) (Cf) (mgy cm) 16-cm 32-cm Male Female Head Body (sc body ) Up to Newborn (sc head ) 16-cm 32-cm 1 year years years years Chest (1.5) 2.70 (1) 2.03 (1) 1.53 (0.5) 1.11 (0) Abdomen (1.5) 2.94 (1) 2.12 (1) 1.55 (0.5) 1.10 (0) Head / Neck (1.5) 2.63 (1) 1.51 (1) 1.25 (0.5) 1.05 (0) The number in parentheses is the value of x. 343

12 year age group than in the 6 15-year age groups in body CT protocols by Donnelly et al. (4) (Fig. 6) probably due to a change in pitch between the two groups. Overscanning or overranging is defined as extra-exposure along the z-axis that extends beyond the planned scan region for data interpolation. The value is scannerspecific and is proportional to beam collimation, reconstructed slice width, and pitch (25, 26). However, overscanning is not related to the planned scan length. Thus, the contribution of overscanning to the total CT dose should be greater when the planned scan length is short, as in pediatric CT imaging. In a study using pediatric anthropomorphic phantoms and a Sensation 16 CT scanner (pitch = 1.0) (25), the overscanning contribution to the total dose was up to 43% for the head-neck CT, 70% for the chest CT, and 36% for the abdomen-pelvis CT, which were approximately twice the adult values (26). Therefore, overscanning should be taken into consideration in dose estimation of pediatric CT imaging. In our study, the overscanning length was taken into account in the calculation of the effective dose because the DLP is a product of CTDI vol and total scan length (planned length + overscanning length). Fortunately, this overscanning, which is a growing dose concern in pediatric multi-slice CT imaging, is expected to be excluded from the total CT dose by a new collimation technique. The diagnostic level of image noise in clinical CT examinations is difficult to determine because the level must differ among the different diagnostic tasks or radiologists (27). In addition, multiple factors, including radiation dose, image reconstruction kernel, and method of contrast enhancement, influence the image noise. Since image noise is anticipated to be greater as a result of the lower tube potential used in our study, the mean image noise values in our CT protocols are similar to those of previous studies (17 23) (Table 8). This unexpected finding in image noise may be attributed to the differences in the section thickness (equal or thicker in our study), reconstruction kernel (smoother in our study), and the application of tube current modulation. Nevertheless, the aforementioned result may partly support the speculation that our pediatric multi-slice CT protocols may provide acceptable image noise. Moreover, the results of visual image quality assessment may also support the clinical acceptability of our image noise level. Comparative studies between image noise and subjective image quality in adults proposed cut-off values for acceptable image noise (10 HU for the liver, 20 HU for the thoracic aorta, and 6 HU for the brain) 344 (18, 28 30). Because there are only a few studies regarding image quality in pediatric CT imaging studies, further work is needed to establish the diagnostic level of image quality for specific pediatric CT protocols. From this viewpoint, our results of image noise for various pediatric CT protocols seem to be helpful for future studies. In our study, image noise was slightly less in children with smaller body diameters than in those with larger body diameter in all CT protocols except for neck CT images (Fig. 5). Previous studies have found that it is unreasonable to require the same level of image noise, independent of body diameter (10, 20). There are several reasons why CT images in smaller children require slightly lower image noise than that of larger children. Smaller patients generally have a small amount of body fat, which is an inherent contrast agent. Also, for a constant noise level, subjectively scored image quality was worse in smaller patients (20). Therefore, the pattern of correlation between image noise and body diameter suggests that our CT protocols adapt well to patient size. Based on X-ray physics, radiation dose is reduced by half for every 4-cm decrease in body diameter (the socalled factor of 2 per 4-cm rule) for the same level of image noise (9, 20). To obtain slightly less image noise in smaller patients, dose reduction should be slightly less than the amount determined by the factor of 2 per 4- cm rule. Some researchers proposed a factor of 2 per 8- cm rule as an appropriate dose reduction rate in body CT images (10, 31). Interestingly, the slopes of the correlation between CTDI vol and body diameter in our body CT seemed to be a good fit with the factor of 2 per 8- cm rule (Fig. 4). For example, a chest CT demonstrated a factor of 1.96 per 8-cm between CTDI vol and body diameter (5.54 mgy and 2.82 mgy in 24-cm and 16-cm body diameters, respectively), whereas abdomen-pelvis CT images demonstrated a factor of 1.91 per 8-cm between CTDI vol and body diameter (8.58 mgy and 4.50 mgy in 24-cm and 16-cm body diameters, respectively). As mentioned above, the stepwise pattern of the correlation was a result of CT imaging parameters determined in an interrupted manner by body-weight groups rather than contiguously by an algebraic equation. Currently, such a group-based CT protocol (either body weight or age) is the most commonly used protocol for pediatric CT imaging (32). However, cross-sectional dimensions and attenuation of the scanned body region provide a more homogeneous CT image quality than body weight does (33). Tube current modulation is usually regarded

13 as insufficient for that purpose in pediatric CT imaging. Dose reduction by tube current modulation was reported to be lower in newborns due to a symmetric crosssection of the body (7). Nevertheless, tube current modulation should be always used in pediatric CT since it is poses no risk of harm. The benefit of low tube potential in pediatric CT images is two-fold (7, 9, 11); First, lowering the tube potential is a reasonable option for dose saving in children (9, 12, 22). Second, low tube potential increases the signalto-noise ratio or contrast-to-noise ratio of tissue containing a relatively high concentration of iodine (9, 18). However, low tube potential may provide unacceptable high image noise in the scanned body regions that are large size or high attenuation. Therefore, we did not use 80 kvp for the brain CT because this study requires a very low level of image noise and because the head is encircled by the skull. In abdomen CT imaging, a low kvp was employed with high tube current to compensate for increased image noise in our study. Other investigators reported that the contrast-to-noise ratio of hypervascular liver lesion can be substantially increased and the CT dose reduced by using an 80-kVp, high tube current CT technique (34). In addition, the low tube potential should be carefully used in combination with tube current modulation because the use of low tube potential in larger patients may deactivate the tube current modulation by reaching its maximal capacity of the X- ray tube (8). Several limitations exist in our study. First, our results should be appropriately modified to other CT models because they were determined from one particular CT model. However, it is probable that our results could be used as a reference by other pediatric radiologists because we provided both the radiation dose parameters and image noise. Therefore, our results can be modified to a different CT model by cross-referencing these parameters. A report concerning radiation dose in various CT models may be helpful for such a modification (35). Secondly, validation of radiation dose with dosimetric measurement was not performed in our study. In a Sensation 16 CT, the difference between the measured CTDI and the value displayed on the console was reported to be within a range of 10% in our beam collimation and tube potentials used in our study (15), and we considered this range to be acceptable in the clinical setting. Further validation studies of dose estimation methods are necessary. Finally, some body weight groups had a small number of examinations, particularly in the 345 less than 5 kg group, which represents an unequal sample effort (Table 1). We believe that a paucity of CT examinations in this small baby group has been a common problem in other studies dealing with pediatric CT images(10, 17). For instance, newborn CT images accounted for only 3% of all pediatric CT examinations in the German survey (10). In conclusion, various dose-saving strategies reduce our pediatric multi-slice CT dose to the lower end of published standards. Moreover, the image noise measured in our study can be used as a target noise level for each protocol in developing better pediatric multi-slice CT protocols. Acknowledgments The authors thank Joo Kim, RT, and other CT technicians for their support and participation in performing pediatric CT at our institution. References 1. Brenner DJ, Hall EJ. Computed tomography-an increasing source of radiation exposure. N Engl J Med 2007;357: Slovis TL. Introduction to seminar in radiation dose reduction. Pediatr Radiol 2002;32: Cody DD, Moxley DM, Krugh KT, O Daniel JC, Wagner LK, Eftekhari F. Strategies for formulating appropriate MDCT techniques when imaging the chest, abdomen, and pelvis in pediatric patients. 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