Radiation exposure has a greater potential risk in a pe-diatric CT than an adult CT because children havegreater radio-sensitivity and a longer life expectancy (1).

Thus, particular emphasis has been posed on the reduc-tion of radiation dose for pediatric CT (2) by means ofvarious strategies as follow (3-9). Adaptation of thetube current corresponding to the patient age (3) or bodyweight (4) is central to dose-saving for the CT protocol.Moreover, the use of low tube potential and tube cur-rent modulation has recently been recommended forfurther optimization of CT dose (5-9).

To optimize a CT protocol, a radiologist should bal-ance dose reduction and image quality because adjust-

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Pediatric 16-slice CT Protocols: Radiation Dose and Image Quality1

Dong Hyun Yang, M.D., Hyun Woo Goo, M.D.

1Department of Radiology and Research Institute of Radiology, AsanMedical Center, University of Ulsan College of Medicine, Korea. Received June 30, 2008 ; Accepted September 1, 2008Address reprint requests to : Hyun Woo Goo, M.D., Department ofRadiology and Research Institute of Radiology, Asan Medical Center,University of Ulsan College of Medicine, 388-1 Poongnap-2 dong,Songpa-gu, Seoul 138-736, KoreaTel. 82-2-3010-4388 Fax. 82-2-476-4719 E-mail: hwgoo@amc.seoul.kr

Purpose: To assess radiation dose and image quality of our pediatric 16-slice CT protocolsand to compare them with published standards.Materials and Methods: For 540 weight-based pediatric 16-slice CT examinations in sixanatomic regions, CTDIvol, DLP, effective dose, and image noise were determined.Two radiologists evaluated the visual quality of CT images by consensus. We analyzedthe relationship of CTDIvol and image noise with body diameter. Our results werecompared with published data.Results: The average CTDIvol (mGy), DLP (mGy·cm), effective dose (mSv), and im-age noise (HU) were as follows: 4.1/125.5/1.6/16.2 for chest CT, 3.3/54.2/1.2/13.7 forheart CT, 5.8/256.6/3.8/13.0 for abdomen-pelvis CT, 6.8/318.7/5.9/12.0 for dynamic ab-domen CT, 3.5/86.2/0.35/7.9 for neck CT and 25.4/368.0/1.6/3.7 for brain CT, respec-tively. All CT images were diagnostic upon visual analysis. The CTDIvol and imagenoise were proportional to body diameter. Our dose parameters were comparable tothe first quartile of the cited German survey, whereas image noise in our study wassimilar to published data.Conclusion: Our pediatric CT dose is at the lower end of published standards and ourimage noise can be used as a target noise for each protocol in developing better pedi-atric multi-slice CT protocols.

Index words : Child Tomography, X-Ray Computed Quality assurance, health care Radiation dosage

ing each parameter represents a trade-off. Thus, oneshould be familiar with the parameters of radiation doseand image quality and their relationships. Although re-cent dose surveys have described current dose levels inpediatric CT imaging, no evaluations of image qualityparameters, (e.g. image noise) have been made (10, 11).For pediatric CT imaging, only a small number of phan-tom studies have simultaneously considered radiationdose and image quality (2, 12, 13). Since 2000, we havedeveloped our own pediatric low-dose CT protocol towhich a combination of body weight-based tube currentadaptation, low tube potential, and tube current modu-lation were applied (6, 13). We carried out the presentstudy to assess radiation dose and image quality of ourpediatric multi-slice CT protocols and to compare themwith published standards.

Materials and Methods

Our Institutional Review Board approved this retro-spective study and informed consent was waived.

Study group

During the six months from July 2006 to December2006, 1142 pediatric CT examinations were performedusing eight CT scanners following the low-dose protocolat 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 0.375 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-sliceCT (n = 1, Somatom Sensation 16; Siemens; gantry rota-tion time of 0.5 s), and a dual source CT (n = 1, SomatomDefinition; Siemens). Among them, 64 CT examinations,

including upper abdomen CT images, high-resolutionchest CT images, unenhanced body CT images and elec-trocardiography (ECG)-synchronized heart CT imageswere excluded because the number of examinations wastoo small to be analyzed. Thus, the six examination pro-tocols included in our study included chest, non-ECG-synchronized heart, abdomen-pelvis, dynamic abdomen,neck, and brain CT images (Table 1). Further, 15 exami-nations in which the dose report (CTDIvol and DLP) wasnot available were excluded from the study. Finally, a to-tal 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 exam-ination included: malignancies (n = 280), congenitalanomalies (n = 124), infections (n = 59), inflammatorydiseases other than infections (n = 34), benign tumors (n= 17) and others (n = 26).

CT imaging

In our CT protocol, tube current and tube potentialwere adjusted for six body-weight groups (Table 1). AllCT examinations other than brain CT imaging were per-formed with a spiral scan, followed by the application oftube current modulation (CARE Dose 4D; Siemens,Forchheim, Germany). For brain CT imaging, axialscans were used with a gantry rotation time of 1.0 sec-ond to avoid image blurring and artifacts related to mul-ti-slice spiral scans. A gantry rotation time of 0.375 sec-onds was chosen for all spiral scans to minimize motionartifacts. Beam collimation, reconstruction section thick-ness, reconstruction kernel, and scan range are summa-rized in Table 2. In all spiral scans, a beam pitch of 1.0was used. In addition, iodinated contrast agent (Iomeron300 or 400; Bracco Imaging SpA, Milan, Italy) was ad-ministered intravenously (1.2-1.5 mL/kg), followed bya saline flush using a dual power injector (OptivantageDH, Tyco Health/Mallinckrodt, St Louis, U.S.A.). For

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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)05-09.9 kg 080/50 (2) 00080/65 (30) 0080/80 (6) 00080/90 (15) 0100/50 (1) 100/140 (8)10-19.9 kg 0080/65 (75) 00080/90 (11) 0080/105 (67) 080/120 (7) 0100/60 (7) 100/190 (5)20-39.9 kg 0080/90 (65) 080/120 (9) 0080/130 (48) 00100/85 (16) 0100/70 (7) 0120/160 (13)40-59.9 kg 0100/65 (47) 0100/85 (0) 00100/95 (32) 100/100 (3) 0100/80 (7) 120/170 (5)60-69.9 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.

heart and dynamic abdomen CT images in which vascu-lar opacification is of paramount importance, iodinatedcontrast agent containing 400 mg I/mL (Iomeron 400)was used, whereas iodinated contrast agent containing300 mg I/mL (Iomeron 300) was used for all other CT ex-aminations. An injection rate ranging from 0.3 to 3.0mL/sec was determined according to the gauge (24-18)of the peripheral venous catheter. Further, a scan delaytime was determined using a bolus tracking method inchest CT, heart CT, and the arterial phase of dynamicabdomen CT. A fixed delay time was employed in ab-domen-pelvis CT (50 - 65 sec), the portal venous phaseof dynamic abdomen CT (50 - 65 sec) and neck CT (40- 60 sec). Conforming to the “As Low As ReasonablyAchievable (ALARA)” principle, an unenhanced CT wasnot performed other than for a brain CT (unenhancedscan only, n = 22; unenhanced and enhanced scans, n= 14).

Non-ECG-synchronized heart CT imaging was per-formed to assess the anatomy of the heart and thoracicgreat vessels in patients with congenital heart disease.Dynamic CT imaging of the abdomen, which consists ofarterial and portal venous phases, was performed to as-sess hepatic vessels for liver transplantation or enhance-ment patterns of focal hepatic lesions. In the two exami-nation protocols requiring multiplanar and three-dimen-sional reformations, a thinner collimation with a rela-tively greater radiation dose was used, compared tochest and abdomen-pelvis CT images requiring only axi-al images (Tables 1, 2).

Radiation dose parameters

A volume CT dose index (CTDIvol; mGy), dose lengthproduct (DLP; mGy·cm), and effective dose (mSv)were described in our study as the three important radi-ation dose parameters. CTDIvol and DLP were automati-cally generated and saved as a dose report after each CTexamination. The dosimetric measurement of these pa-rameters was not performed. The CTDIvol is a phantom-based dose parameter and is based on the unit volume.In a Sensation 16 CT, the phantom size used to calculateCTDIvol is predetermined by a specific scan mode (e.g.,16-cm phantom for head mode, 32-cm phantom forbody mode) irrespective of patient size. In pediatric CTimaging, even in body scan mode, 16-cm phantom-based dose parameters have been commonly used be-cause these values provide more realistic estimates of ra-diation dose for children than 32-cm phantom-baseddose parameters (10, 11). Moreover, the effective dosein pediatric CT images has been estimated from 16-cmphantom-based values (10, 11). In a Sensation 16 CTscanner, the radiation dose based on a 16-cm phantom isknown to be approximately two times higher than thevalue based on a 32-cm phantom (15); consequently, weconverted 32-cm phantom-based dose parameters forthe CT protocols (chest, heart, abdomen-pelvis, and dy-namic abdomen) performed in body scan mode to 16-cm phantom-based ones by multiplying the 32-cm doseparameters by a factor of two.

In order to determine the effective dose, we applied aformula which had been used in a German pediatric CT

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Table 2. CT Imaging Parameters and Scan Ranges in CT Examination Protocols

Chest Heart Abdomen-pelvis Dynamic Abdomen * Neck Brain

Beam Collimation (mm) 1.5×16 0.75 or 1.5×16 1.5×16 0.75 or 1.5×16 0.75×16 1.5×12Reconstructed Section

Thickness (mm) 3-5 2-4 3-5 3-5 2-3 4.5Reconstruction Kernel B30f B20f or B30f B30f B30f H30f H40sScan 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 basehilum 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 notobtained.† 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 adjustedto 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 pur-pose 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.

dose survey (10):

where E (mSv) is the effective dose, Pf refers to thephantom factor, Cf (mSv/mGy·cm) is the effectivedose normalized to the DLP (mGy·cm), sC is the scan-ner correction factor in head mode (sChead) or bodymode (sCbody), aC is the patient age correction factor,and x is a factor required for scanner correction in chil-dren (Appendix). The age correction factor was appliedto five age groups (newborn [up to 1 month], up to 1year, 2-5 years, 6-10 years, and 11-15 years).Consistent with the aforementioned German survey,

the 11-15 year age group, a 32-cm phantom-based val-ue of DLP (DLP32 cm) was used to estimate the effectivedose of body CT images, while a 16-cm phantom-basedvalue (DLP32 cm×2 in our study) was used in other agegroups. The commonly used software ImPACT CTdosimetry spreadsheet (16) was not employed for effec-tive dose calculation in our study because a pediatricmathematical phantom was not available in the soft-ware program.

Image quality

Image noise (HU), the standard deviation of the CTdensity, was measured in two organs and in backgroundair for each protocol by placing a rectangular region ofinterest (ROI) (Table 3). The ROI measurement was per-

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E = ×Cf×sC×aC×xsChead

sCbody

DLPPf

A B

Fig. 1. Locations of the rectangular regions of interest for imagenoise measurement of the chest or heart CT images. Three setsof CT images with different window/width levels (HU) weremerged (950/220 for the thoracic aorta, 300/50 for the paraspinalmuscle, and 1500/-700 for the background air). The imagenoise 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 thoracicaorta, 9.6 for the paraspinal muscle, and 7.1 for the backgroundair.

C

formed at three predefined levels (Table 3) by a board-certified radiologist with three-years of experience usingour PACS workstation. Following the measurement, thethree measured values were averaged (Fig. 1). Becausethe target organs are diverse in size in the pediatric CT,the ROI was drawn as large as possible within a ho-mogenous area of each organ (for organs, ROI up to 100mm2; for background air, ROI fixed at 100 mm2). Tominimize the potential influence of window settings,image noise was measured under a fixed window set-ting (width/level) for each organ and for background air:vessel (950/220), liver and muscle (300/50), backgroundair (1500/-700) and brain (100/30) (Fig. 1). The measure-ment of image noise was performed at the portal venousphase for dynamic abdomen CT imaging and the unen-hanced scan for brain CT imaging.

The visual quality of the CT images was evaluated bytwo board-certified radiologists (11-years of experience

and three-years of experience, respectively) for imagenoise, beam hardening or motion artifacts, and enhance-ment of target organs. The overall image quality wasthen graded by consensus using a four-point scale (1non-diagnostic images, 2 suboptimal but acceptablequality images, 3 good quality images, 4 excellent im-ages) (Fig. 2). The CT images showing grade 2 or morewere regarded as diagnostic.

Body diameter

Body diameters were measured on all CT examina-tions at a predetermined level for each anatomic region.The anterior-to-posterior and transverse diameters weremeasured on a PACS workstation by an experienced ra-diologist. The mean value represented the body diame-ter of a patient for that CT protocol. The measurementlevels for each CT protocol included: the four cardiacchambers for chest and heart CT images, celiac axis for

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A B

C

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 portalvein in a 3-year-old girl (CTDIvol 4.46 mGy, body diameter 15cm, image noise 10.7 HU in the liver). (B) Visual grade 3 of a CTimage in a 4-year-old boy (CTDIvol 4.46 mGy, body diameter 17cm, image noise 13.2 HU in the liver). (C) Visual grade 2 of a CTimage in a 3-year-old boy (CTDIvol 4.48 mGy, body diameter 17cm, image noise 12.6 HU in the liver).

abdomen-pelvis and dynamic abdomen CT images,mandibular angle for neck CT, and the center of thethalamus for the brain CT images.

Data analysis and comparison with previously published

data

Radiation dose and image noise were summarized foreach protocol and for each body-weight group. All para-meters were expressed as the mean ± standard devia-

tion. We correlated the measured body diameter withCTDIvol and image noise using a linear regression analy-sis. The statistical analyses were performed using a sta-tistical software package (MedCalc 7.4; MedCalcSoftware, Mariakerke, Belgium). A p-value of 0.05 wasconsidered to indicate a statistically significant differ-ence.

Our radiation dose parameters were compared withtwo recent German and UK CT dose surveys for each

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Table 5A. Summary of Radiation Dose Parameters, Image Noise, and Visual Grades for Chest CT images and Heart CT images as a func-tion of Body-weight Group

CT ProtocolCTDIvol DLP Effective Dose Image Noise (HU)

Visual Grade(mGy) (mGy·cm) (mSv) Organ 1 Organ 2 Air

Chest (kV/mAs)< 5 kg (80/40) 1.9 ± 0.4 30.7 ± 3.1 1.0 ± 0.2 12.3 ± 1.8 09.9 ± 3.0 6.7 ± 1.1 2.3 ± 0.5005-9.9 kg (80/50) 1.9 ± 0.1 39.0 ± 4.2 1.0 ± 0.1 16.3 ± 1.1 11.7 ± 1.3 8.0 ± 0.6 2.0 ± 0.010-19.9 kg (80/65) 2.7 ± 0.2 64.8 ± 6.3 1.2 ± 0.2 15.9 ± 2.5 13.0 ± 1.9 7.4 ± 0.9 2.8 ± 0.520-39.9 kg (80/90) 3.9 ± 0.3 116.2 ± 15.3 1.4 ± 0.3 16.8 ± 2.5 14.8 ± 2.7 7.2 ± 1.2 3.0 ± 0.540-59.9 kg (100/65) 5.7 ± 0.2 202.1 ± 17.9 2.0 ± 0.4 16.3 ± 2.1 14.7 ± 2.1 7.4 ± 1.2 2.7 ± 0.560-69.9 kg (100/100) 8.8 ± 0.2 326.8 ± 29.7 3.2 ± 0.5 16.1 ± 1.3 14.7 ± 1.0 7.1 ± 0.4 2.7 ± 0.5

Heart (kV/mAs)< 5 kg (80/50) 2.5 ± 0.1 28.1 ± 3.4 0.9 ± 0.2 13.6 ± 1.7 10.6 ± 2.3 7.2 ± 1.0 2.8 ± 0.4005-9.9 kg (80/65) 3.0 ± 0.4 37.2 ± 8.5 1.0 ± 0.2 12.6 ± 2.2 10.4 ± 2.1 6.5 ± 1.2 2.9 ± 0.610-19.9 kg (80/90) 4.5 ± 0.1 72.9 ± 7.5 1.5 ± 0.2 15.2 ± 2.4 12.4 ± 1.4 6.8 ± 1.3 3.2 ± 0.620-39.9 kg (80/120) 5.4 ± 0.8 160.2 ± 80.1 2.1 ± 0.9 16.3 ± 1.8 12.7 ± 1.7 6.3 ± 0.8 3.2 ± 0.4

Note ─ Data are expressed as the mean values ± standard deviation.

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(37.1 ± 22.0) (37.5 ± 22.1)

Abdomen-pelvis / Liver Paraspinal muscle Liver dome / Hilum/ Inferior angleDynamic Abdomen (95.1 ± 8.9) (78.0 ± 29.0)

Neck Internal jugular vein Sternocleidomastoid muscle Hyoid bone/ Vocal cord/ Thyroid (20.1 ± 7.6) (20.1 ± 7.6) gland

Brain Gray matter Cerebrospinal fluid Cerebellum/ Thalamus/ Centrum (51.8 ± 11.2) (16.5 ± 10.6) semiovale

Note ─ The numbers in parentheses indicate the size of the region of interest (mm2, mean ± standard deviation).

Table 4. Summary of Radiation Dose Parameters, Image Noise, and Visual Grades in Our CT Protocols

CT ProtocolCTDIvol DLP Effective Dose Image Noise (HU)

Visual Grade(mGy) (mGy·cm) (mSv) Organ 1 Organ 2 Air

Chest (n=202) 04.1 ± 1.6 125.5 ± 72.10 1.6 ± 0.6 16.2 ± 2.4 14.0 ± 2.4 7.3 ± 1.1 2.8 ± 0.5Heart (n=75) 03.3 ± 1.1 054.2 ± 50.00 1.2 ± 0.5 13.7 ± 2.4 11.1 ± 2.2 6.7 ± 1.1 2.9 ± 0.5Abdomen-pelvis (n=160) 05.8 ± 1.9 256.6 ± 131.8 3.8 ± 1.1 13.0 ± 1.8 12.1 ±1.7 6.7 ± 0.9 2.6 ± 0.6Dynamic Abdomen (n=41)* 06.8 ± 1.9 318.7 ± 150.8 5.9 ± 1.8 12.0 ± 1.6 10.9 ± 1.6 6.5 ± 1.1 2.9 ± 0.5Neck (n=26) 03.5 ± 0.8 86.2 ± 35.6 0.3 ± 0.1 07.9 ± 1.9 07.7 ± 1.6 4.1 ± 1.1 2.7 ± 0.5Brain (n=36)† 25.4 ± 6.8 368.0 ± 129.8 1.6 ± 0.3 03.7 ± 0.5 03.7 ± 0.7 2.4 ± 0.5 2.7 ± 0.4

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 en-hanced CT scans.

age group (10, 11). In addition to these surveys, theCTDIvol of our CT protocol was compared with previ-ously published CT protocols (4, 6, 13). We calculatedthe CTDIvol of the previous protocols based on theirimaging parameters and CT models with the ImPACTprogram (16). In these pediatric CT protocols, imagingparameters were specific to either body-weight or agegroup. A known conversion table was used to convertbody weight to patient age (13). Moreover, the imagenoises for the aorta, liver, muscle, and brain in our studywere compared with corresponding data available in theliterature (17-23).

Results

The radiation dose parameters, image noise, and visu-al image quality grades for each protocol and body-weight group are summarized in Tables 4 and 5. The ra-diation dose parameters revealed a tendency to be lessin lighter-weight groups (Table 5) and in younger-agegroups (Fig. 3), except for the effective dose for brainCT. Image noise showed a slight tendency to be less inlighter-weight groups, except for the neck CT. All CTimages were of diagnostic quality and had mean scores

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Table 5B. Summary of Radiation Dose Parameters, Image Noise, and Visual Grades for Abdomen-pelvis CT images and DynamicAbdomen CT images as a function of Body-weight Group

CT ProtocolCTDIvol DLP Effective Dose Image Noise (HU)

Visual Grade(mGy) (mGy·cm) (mSv) Organ 1 Organ 2 Air

Abdomen-pelvis (kV/mAs)< 5 kg (80/65) 02.6 ± 1.0 071.0 ± 29.7 2.3 ± 1.5 11.2 ± 2.2 11.6 ± 3.6 6.5 ± 1.3 2.5 ± 0.5005-9.9 kg (80/80) 03.4 ± 0.2 101.3 ± 7.20 3.1 ± 0.5 11.3 ± 0.6 10.8 ± 1.2 6.2 ± 0.8 2.2 ± 0.410-19.9 kg (80/105) 04.5 ± 0.1 161.7 ± 13.3 3.5 ± 0.8 12.4 ± 1.4 11.3 ± 1.4 6.5 ± 0.9 2.7 ± 0.520-39.9 kg (80/130) 05.6 ± 0.4 252.4 ± 27.7 3.6 ± 0.9 13.7 ± 2.1 12.8 ± 1.6 6.8 ± 1.0 2.5 ± 0.640-59.9 kg (100/95) 08.4 ± 0.2 440.0 ± 23.2 4.6 ± 1.1 13.2 ± 1.7 12.6 ± 1.5 7.0 ± 0.9 2.5 ± 0.560-69.9 kg (100/130) 12.1 ± 0.8 654.8 ± 49.8 6.7 ± 0.9 13.9 ± 1.1 14.0 ± 1.7 7.1 ± 0.4 2.2 ± 0.7

Dynamic Abdomen (kV/mAs) *005-9.9 kg (80/90) 4.8 ± 0.1 172.0 ± 25.6 5.5 ± 1.5 12.0 ± 1.7 10.0 ± 1.3 6.5 ± 1.2 2.8 ± 0.410-19.9 kg (80/120) 6.4 ± 4.0 0287.4 ± 165.2 5.6 ± 1.7 11.3 ± 1.4 11.1 ± 1.3 6.1 ± 0.6 2.9 ± 0.320-39.9 kg (100/85) 8.5 ± 0.1 426.3 ± 59.0 6.3 ± 2.1 12.2 ± 1.5 11.1 ± 1.3 6.5 ± 1.2 2.9 ± 0.640-59.9 kg (100/100) 9.3 ± 1.1 552.0 ± 59.0 6.4 ± 1.8 13.6 ± 1.6 13.7 ± 2.3 6.9 ± 0.8 2.7 ± 0.5

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 func-tion of Body-weight Group

CT ProtocolCTDIvol DLP Effective Dose Image Noise (HU)

Visual Grade(mGy) (mGy·cm) (mSv) Organ 1 Organ 2 Air

Neck (kV/mAs)< 5 kg (100/40) 1.8 17.0 0.2 12.8 4.7 6.5 3005-9.9 kg (100/50) 2.3 39.0 0.3 7.3 7.4 4.1 210-19.9 kg (100/60) 2.8 ± 0.0 58.9 ± 9.1 0.2 ± 0.0 6.9 ± 1.1 7.8 ± 1.2 3.9 ± 0.5 2.6 ± 0.520-39.9 kg (100/70) 3.4 ± 0.3 078.4 ± 14.3 0.2 ± 0.0 8.2 ± 1.8 8.2 ± 1.5 4.3 ± 1.0 2.7 ± 0.540-59.9 kg (100/80) 4.1 ± 0.4 111.3 ± 14.7 0.3 ± 0.0 8.4 ± 1.8 8.0 ± 1.9 4.2 ± 1.5 2.7 ± 0.560-69.9 kg (100/100) 4.7 ± 0.2 148.3 ± 3.20 0.4 ± 0.0 7.3 ± 0.6 6.5 ± 1.2 3.4 ± 0.5 3.0 ± 0.0

Brain* (kV/mAs)< 5 kg (100/120) 15.2 ± 0.4 176.1 ± 32.7 1.3 ± 0.3 3.1 ± 0.4 2.8 ± 0.7 2.2 ± 0.1 2.3 ± 0.4005-9.9 kg (100/140) 17.5 ± 1.2 234.9 ± 23.8 1.7 ± 0.3 3.5 ± 0.6 3.5 ± 0.8 2.3 ± 0.3 2.6 ± 0.510-19.9 kg (100/190) 24.3 ± 0.4 337.6 ± 21.4 1.8 ± 0.6 3.8 ± 0.5 3.9 ± 0.7 2.6 ± 0.7 2.6 ± 0.520-39.9 kg (120/160) 30.9 ± 0.6 450.6 ± 21.0 1.5 ± 0.2 3.9 ± 0.3 3.9 ± 0.4 2.4 ± 0.6 2.8 ± 0.440-59.9 kg (120/170) 32.5 ± 0.4 485.6 ± 69.1 1.4 ± 0.2 4.1 ± 0.6 3.9 ± 0.8 2.3 ± 0.3 3.0 ± 0.060-69.9 kg (120/200) 36.3 691.0 1.9 4.1 3.5 2.0 3

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 andenhanced CT scans.

of visual grade as follows: 2.8 in chest CT images, 2.9 inheart CT images, 2.6 in abdomen-pelvis CT images, 2.9in dynamic abdomen CT images, 2.7 in neck CT images,and 2.7 in brain CT images (Table 6).

The CTDIvol of our CT protocols were highly correlat-ed with the measured body diameter (r = 0.88 in chestCT, r = 0.88 in heart CT, r = 0.87 in the abdomen-pelvis CT images, r = 0.91 in dynamic abdomen CT im-ages, r = 0.83 in neck CT images, and r = 0.90 in brainCT images; p <0.001 for all protocols). The scatter plots

between the CTDIvol and the measured body diameterappeared stepwise rather than linear due to the employ-ment of stepwise CT imaging parameters based on eachbody-weight group (Fig. 4). On the other hand, imagenoise showed variable degrees of association with themeasured body diameter in all CT protocols except forneck CT images (r = 0.26, p < 0.001 in chest CT images,r = 0.49, p < 0.001 in heart CT images, r = 0.46, p <0.001 in abdomen-pelvis CT images, r = 0.40, p = 0.01in dynamic abdomen CT images; r = 0.78, p < 0.001 in

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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) 0 51 141 10Heart (n=75) 0 13 053 09Abdomen-pelvis (n=160) 0 69 089 02Dynamic Abdomen (n=41) 0 08 031 02Neck (n=26) 0 08 018 00Brain (n=36) 0 10 026 00

Note ─ Data are expressed as the number of examinations.

A B

C

Fig. 3. Graphs showing the mean effective dose estimates ineach age group for (A) chest and heart CT images, (B) abdomen-pelvis and dynamic abdomen CT images, and (C) neck andbrain CT images.

brain CT; and r = - 0.07, p = 0.71 in neck CT) (Fig. 5).Our CT dose parameters were substantially lower

than the reference doses, which comprised the thirdquartile of both the cited German and UK dose surveys(10, 11), and were comparable to the first quartile of theGerman survey (Fig. 6) (Table 7). Our mean effectivedoses were 46%, 61%, and 62% of the German refer-ence dose for chest, abdomen-pelvis, and brain CT im-

ages, respectively. The CTDIvol of our chest and ab-domen-pelvis CT images were also comparable to thoseby Greess et al. (6) and Huda et al. (13) (Fig. 6). Imagenoise levels of CT protocols were similar to that of previ-ously published data (17-23) (Table 8).

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A BFig. 4. Scatter plots showing the correlation between CTDIvol and measured body diameter in (A) chest CT images and (B) ab-domen-pelvis CT images. The CTDIvol shows a statistically significant linear correlation between body diameter for the chest CTimages (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 ratherthan linear because of weight-group-based adjustment of the CTDIvol.

A BFig. 5. Scatter plots show a correlation between image noise and measured body diameter in the (A) chest CT images and (B) ab-domen-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).

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Table 7. Summary of Radiation Dose for the current Study, the German Survey, and the UK Survey

CTDIvol (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

ChestNewborn 1.9 3.6 (2.0-4.2)0 029 46 (25-47)0 1.0 1.9 (1.0-2.1)Up to 1 year 2.7 5.4 (2.6-6.9)0 11 (5.1-12) 057 77 (36-93)0 159 (68-204)0 1.3 2.2 (1.2-2.6) 6.3 (2.6-7.9)2-5 years 2.8 6.1 (3.0-8.4)0 11 (7.6-13) 070 116 (60-137)0 198 (119-228) 1.3 2.6 (1.4-3.2) 3.6 (2.1-4.1)6-10 years 4.2 08.5 (3.7-11.9)0 14 (9.5-17) 124 194 (98-257)0 303 (174-368) 1.7 3.0 (1.4-3.9) 3.9 (2.3-4.8)11-15 years 5.5 12.4 (8.0-16.0)0 195 360 (224-488) 1.9 3.4 (2.1-4.4)

Abdomen-pelvisNewborn NA 4.2 (3.1-4.6)0 NA 71 (49-81)0 NA 3.6 (2.3-4.6)Up to 1 year 4.0 6.2 (4.8-6.8)0 129 148 (87-164)0 3.6 4.8 (2.4-4.9)2-5 years 4.6 7.9 (4.2-8.3)0 164 219 (103-261) 3.6 5.4 (2.2-5.6)6-10 years 5.7 10.7 (6.0-13.7)0 249 342 (220-477) 3.9 5.8 (3.7-7.2)11-15 years 7.8 16.6 (13.0-20.2) 404 656 (408-804) 4.2 6.7 (4.4-8.0)

BrainNewborn 16.8 21.9 (16.1-26.1) 198 227 (162-275) 2.0 2.3 (1.6-2.7)Up to 1 year 18.0 26.2 (16.9-33.6) 25 (16-28) 238 302 (179-393) 230 (160-270) 1.7 2.2 (1.3-2.8) 2.5 (1.8-3.0)2-5 years 27.1 35.7 (21.6-49.0) 34 (24-43) 390 452 (271-611) 383 (280-465) 1.6 1.9 (1.2-2.6) 1.5 (1.1-1.9)6-10 years 30.6 43.7 (29.3-58.0) 44 (32-51) 442 582 (394-711) 508 (402-619) 1.5 2.0 (1.4-2.5) 1.6 (1.3-2.0)11-15 years 31.8 53.2 (44.4-64.5) 490 764 (588-920) 1.4 2.2 (1.7-2.7)

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

C

Fig. 6. The graphs show the CTDIvol (16 cm-phantom-based val-ue) of our protocols and previously published CT protocols forthe (A) chest CT images, (B) abdomen-pelvis CT images, and (C)brain CT images. The amount and pattern of CTDIvol in ourstudy are comparable to those of previous protocols with the ex-ception of the protocol by Donnelly et al. (6) (dashed line). Inthis protocol, the CTDIvol is higher in the younger age group (2-5 years) than the older age group (6-15 years) due to an abruptchange in pitch between the two.

Discussion

In the present study, we described radiation dose pa-rameters and image noise in our weight-based pediatricmulti-slice CT protocol. Our study group of 540 clinicalCT examinations encompassed nearly the entire rangeof children’s body-weight groups and anatomical re-gions. To the best of our knowledge, a study compre-hensively describing radiation dose parameters, imagenoise, and visual image quality for pediatric brain, neck,and body CT protocols has not been performed. Itshould be noted that our CT protocols utilized all avail-able dose reduction strategies, including body-weight-based tube current adaptation, low tube potential, andtube current modulation.

We found that our CTDIvol approximates the Germanfirst quartile (10) and recently proposed pediatric CTprotocols (6, 13) in both its values and pattern among thedifferent age groups (Fig. 6). As expected, radiation doseparameters showed a tendency to be less in lighter-weight and younger-age groups. The only exception wasthe effective dose for brain CT images, in which the val-ue showed a tendency to be less in older age groups (Fig.3C). Notably, other researchers previously demonstrat-ed that the effective dose from pediatric brain CT de-creases exponentially with increasing body length (24).For heart and dynamic abdomen CT protocols, the ef-fective dose was less in the 11-15-year age group thanin the 6-10-year age group (Fig. 3A, B). This might bedue to a change in imaging parameters or suboptimalparameters. Similarly, CTDIvol was higher in the 2-5-

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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 1.5-10 120 37.5-75* 12.5 16.2 / 13.7 Aorta 120 12.5-25* 39.5 (Chest / Heart)

Sigal- Adults 5 B40f 120 60* 12.0Cinqualbre 080 180* 20.0et al. 080 135* 24.0

Liver Nyman et al. Adults 5 B30f 120 115 13.9 13.0 / 12.0Wilting et al. Adults 5 Soft 120-140 75-250* 11.6 (Abdomen-pelvis /

Dynamic abdomen)Muscle Mulkens et al. Adults 5 B40s 130 78 13.9 12.1 / 10.9 (Abdomen) (Abdomen-pelvis /

Dynamic abdomen)130 68 14.8

Muscle Namasivayam Adults 2.5 Standard 120 251* 5.9 7.7(Neck) et al. 120 198* 7

120 168* 6.9Brain Mullins et al. Adults 5 NA 140 227* 3.6 3.7(Gray 140 120* 4.9Matter)

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).

AppendixVariables for Effective Dose Calculation in the Sensation 16 CT [12]

ScanPhantom Effective Dose

Scanner Correction Region

Factor Normalized to DLP Age Correction Factor (aC)(Pf) (Cf) (mGy·cm)

Factor (sC)

16-cm 32-cm Male FemaleHead Body (sCbody)

NewbornUp to 2-5 6-10 11-15

(sChead) 16-cm 32-cm 1 year years years years

Chest 0.77 0.45 0.0068 0.0088 0.90 0.90 1.00 3.90 (1.5) 2.70 (1) 2.03 (1) 1.53 (0.5) 1.11 (0)Abdomen 0.77 0.45 0.0072 0.0104 0.90 0.90 1.00 4.47 (1.5) 2.94 (1) 2.12 (1) 1.55 (0.5) 1.10 (0)Head / Neck 0.76 0.0022 0.0024 0.90 0.90 3.53 (1.5) 2.63 (1) 1.51 (1) 1.25 (0.5) 1.05 (0)

The number in parentheses is the value of x.

year age group than in the 6-15-year age groups inbody CT protocols by Donnelly et al. (4) (Fig. 6) proba-bly due to a change in pitch between the two groups.

Overscanning or overranging is defined as extra-expo-sure along the z-axis that extends beyond the plannedscan region for data interpolation. The value is scanner-specific and is proportional to beam collimation, recon-structed slice width, and pitch (25, 26). However, over-scanning is not related to the planned scan length. Thus,the contribution of overscanning to the total CT doseshould be greater when the planned scan length is short,as in pediatric CT imaging. In a study using pediatric an-thropomorphic phantoms and a Sensation 16 CT scan-ner (pitch = 1.0) (25), the overscanning contribution tothe 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 considera-tion in dose estimation of pediatric CT imaging. In ourstudy, the overscanning length was taken into accountin the calculation of the effective dose because the DLPis a product of CTDIvol and total scan length (plannedlength + overscanning length). Fortunately, this over-scanning, which is a growing dose concern in pediatricmulti-slice CT imaging, is expected to be excluded fromthe total CT dose by a new collimation technique.

The diagnostic level of image noise in clinical CT ex-aminations is difficult to determine because the levelmust differ among the different diagnostic tasks or radi-ologists (27). In addition, multiple factors, including ra-diation dose, image reconstruction kernel, and methodof contrast enhancement, influence the image noise.Since image noise is anticipated to be greater as a resultof the lower tube potential used in our study, the meanimage noise values in our CT protocols are similar tothose of previous studies (17-23) (Table 8). This unex-pected finding in image noise may be attributed to thedifferences in the section thickness (equal or thicker inour study), reconstruction kernel (smoother in ourstudy), and the application of tube current modulation.Nevertheless, the aforementioned result may partly sup-port the speculation that our pediatric multi-slice CTprotocols may provide acceptable image noise.Moreover, the results of visual image quality assessmentmay also support the clinical acceptability of our imagenoise level. Comparative studies between image noiseand subjective image quality in adults proposed cut-offvalues for acceptable image noise (10 HU for the liver,20 HU for the thoracic aorta, and 6 HU for the brain)

(18, 28-30). Because there are only a few studies re-garding image quality in pediatric CT imaging studies,further work is needed to establish the diagnostic levelof image quality for specific pediatric CT protocols.From this viewpoint, our results of image noise for vari-ous pediatric CT protocols seem to be helpful for futurestudies.

In our study, image noise was slightly less in childrenwith smaller body diameters than in those with largerbody diameter in all CT protocols except for neck CTimages (Fig. 5). Previous studies have found that it is un-reasonable to require the same level of image noise, in-dependent of body diameter (10, 20). There are severalreasons why CT images in smaller children requireslightly lower image noise than that of larger children.Smaller patients generally have a small amount of bodyfat, which is an inherent contrast agent. Also, for a con-stant noise level, subjectively scored image quality wasworse in smaller patients (20). Therefore, the pattern ofcorrelation between image noise and body diametersuggests that our CT protocols adapt well to patient size.Based on X-ray physics, radiation dose is reduced byhalf for every 4-cm decrease in body diameter (the so-called “factor of 2 per 4-cm” rule) for the same level ofimage noise (9, 20). To obtain slightly less image noise insmaller patients, dose reduction should be slightly lessthan 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 bodyCT images (10, 31). Interestingly, the slopes of the corre-lation between CTDIvol and body diameter in our bodyCT seemed to be a good fit with the ‘factor of 2 per 8-cm’ rule (Fig. 4). For example, a chest CT demonstrateda factor of 1.96 per 8-cm between CTDIvol and body di-ameter (5.54 mGy and 2.82 mGy in 24-cm and 16-cmbody diameters, respectively), whereas abdomen-pelvisCT images demonstrated a factor of 1.91 per 8-cm be-tween CTDIvol and body diameter (8.58 mGy and 4.50mGy in 24-cm and 16-cm body diameters, respectively).As mentioned above, the stepwise pattern of the correla-tion was a result of CT imaging parameters determinedin an interrupted manner by body-weight groups ratherthan contiguously by an algebraic equation. Currently,such a group-based CT protocol (either body weight orage) is the most commonly used protocol for pediatricCT imaging (32). However, cross-sectional dimensionsand attenuation of the scanned body region provide amore homogeneous CT image quality than body weightdoes (33). Tube current modulation is usually regarded

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as insufficient for that purpose in pediatric CT imaging.Dose reduction by tube current modulation was report-ed to be lower in newborns due to a symmetric cross-section of the body (7). Nevertheless, tube current mod-ulation should be always used in pediatric CT since it isposes no risk of harm.

The benefit of low tube potential in pediatric CT im-ages is two-fold (7, 9, 11); First, lowering the tube poten-tial is a reasonable option for dose saving in children (9,12, 22). Second, low tube potential increases the signal-to-noise ratio or contrast-to-noise ratio of tissue contain-ing a relatively high concentration of iodine (9, 18).However, low tube potential may provide unacceptablehigh image noise in the scanned body regions that arelarge size or high attenuation. Therefore, we did not use80 kVp for the brain CT because this study requires avery low level of image noise and because the head isencircled by the skull. In abdomen CT imaging, a lowkVp was employed with high tube current to compen-sate for increased image noise in our study. Other inves-tigators reported that the contrast-to-noise ratio of hy-pervascular liver lesion can be substantially increasedand the CT dose reduced by using an 80-kVp, high tubecurrent CT technique (34). In addition, the low tube po-tential should be carefully used in combination withtube current modulation because the use of low tube po-tential in larger patients may deactivate the tube currentmodulation by reaching its maximal capacity of the X-ray tube (8).

Several limitations exist in our study. First, our resultsshould be appropriately modified to other CT modelsbecause they were determined from one particular CTmodel. However, it is probable that our results could beused as a reference by other pediatric radiologists be-cause we provided both the radiation dose parametersand image noise. Therefore, our results can be modifiedto a different CT model by cross-referencing these para-meters. A report concerning radiation dose in variousCT models may be helpful for such a modification (35).Secondly, validation of radiation dose with dosimetricmeasurement was not performed in our study. In aSensation 16 CT, the difference between the measuredCTDI and the value displayed on the console was re-ported to be within a range of 10% in our beam collima-tion and tube potentials used in our study (15), and weconsidered this range to be acceptable in the clinical set-ting. Further validation studies of dose estimation meth-ods are necessary. Finally, some body weight groupshad a small number of examinations, particularly in the

less than 5 kg group, which represents an unequal sam-ple effort (Table 1). We believe that a paucity of CT ex-aminations in this small baby group has been a commonproblem in other studies dealing with pediatric CT im-ages(10, 17). For instance, newborn CT images account-ed for only 3% of all pediatric CT examinations in theGerman survey (10).

In conclusion, various dose-saving strategies reduceour pediatric multi-slice CT dose to the lower end ofpublished standards. Moreover, the image noise mea-sured in our study can be used as a target noise level foreach protocol in developing better pediatric multi-sliceCT protocols.

Acknowledgments

The authors thank Joo Kim, RT, and other CT techni-cians for their support and participation in performingpediatric CT at our institution.

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mization of imaging techniques. J Korean Radiol Soc 2005;52:1-515. Theocharopoulos N, Perisinakis K, Damilakis J, Karampekios S,

Gourtsoyiannis N. Dosimetric characteristics of a 16-slice comput-ed tomography scanner. Eur Radiol 2006;16:2575-2585

16. ImPACT CT Patient Dosimetry Calculator (version 0.99x20/01/06). London : ImPACT. 2006. Available from URL : http://www.impactscan.org/ctdosimetry.

17. Rogalla P, Stover B, Scheer I, Juran R, Gaedicke G, Hamm B. Low-dose spiral CT: applicability to paediatric chest imaging. PediatrRadiol 1999;29:565-569

18. Sigal-Cinqualbre AB, Hennequin R, Abada HT, Chen X, Paul JF.Low-kilovoltage multi-detector row chest CT in adults: feasibilityand effect on image quality and iodine dose. Radiology 2004;231:169-174

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21. Mulkens TH, Bellinck P, Baeyaert M, Ghysen D, Van Dijck X,Mussen E, et al. Use of an automatic exposure control mechanismfor dose optimization in multi-detector row CT examinations: clini-cal evaluation. Radiology 2005;237:213-223

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대한영상의학회지 2008;59:333-347

16-절편 소아 CT 프로토콜: 방사선량과 영상의 질1

1울산대학교 의과대학 서울아산병원 영상의학과교실, 영상의학연구소

양 동 현·구 현 우

목적: 이 연구의 목적은 16-절편 CT를 이용한 소아 CT 프로토콜의 방사선량과 영상의 질을 평가하고 이를 이전

연구와 비교하는 것이다.

대상과 방법: 환자의 몸무게를 기준으로 방사선량을 조절하여 검사한 16-절편 소아 CT 540예에서 CTDIvol,

DLP, 유효선량, 그리고 영상잡음을 평가하였다. 두 명의 영상의학 전문의가 합의로 시각적인 영상의 질을 평가

하였다. CTDIvol과 영상소음의 체직경에 대한 상관관계를 분석하였다. 우리의 연구결과를 출간된 자료들과 비교

하였다.

결과: 각 신체부위의 CT에서 평균 CTDIvol (mGy), DLP (mGy·cm), 유효선량 (mSv), 그리고 영상소음 (HU)은

다음과 같다: 4.1/125.5/1.6/16.2 흉부 CT, 3.3/54.2/1.2/13.7 심장 CT, 5.8/256.6/3.8/13.0 복부-골반 CT,

6.8/318.7/5.9/12.0 역동적 복부 CT, 3.5/86.2/0.3/7.9 경부 CT, 그리고 25.4/368.0/1.6/3.7 두부 CT. 모든 CT 검

사는 진단 가능한 영상의 질을 보였다. CTDIvol과 영상소음은 체직경에 각각 양의 상관 관계를 보였다. 방사선량

지표들은 최근 독일의 전국적 방사선량 조사의 일사분위수와 비슷했다. 영상소음은 출간된 자료들과 비슷했다.

결론: 저자들의 소아 CT 방사선량은 출간된 자료들의 낮은 극단에 속한다. 본 연구의 영상소음은 앞으로 좀더 개

선된 소아 다중 절편 CT 프로토콜을 확립할 때 목표치로 사용될 수 있을 것이다.