Evaluation of skin exposure during cerebral CT perfusion studies on a phantom

Objectives

To evaluate the skin dose during cerebral CT perfusion on a phantom, and estimate the weighted CT dose index (CTDIw) to maximum skin dose conversion factors for four types of CT scanners.

Study design

We evaluated the relationship between surface dose during cerebral CT perfusion and distance from the scan center in the x–y plane using a 64-multidetector row CT scanner. Skin doses were also assessed with 4 different 64-multidetector CT scanners.

Results

The surface doses decreased with the distance from the scan center in the x–y plane. The surface doses at the points 6 cm and 10 cm from the scan center in the x–y plane were different from the dose at the point 8 cm by about 15%. CTDIw and skin doses differed among the CT scanners (CTDIw, 143–590 mGy; averaged temporal skin dose, 126–590 mGy). For all the four types of CT scanner, the doses increased in the following order: occipital point < frontal point < temporal points. The ratios of the maximum skin dose (averaged temporal skin dose) to CTDIw differed among the CT scanners (64–100%).

Conclusions

The maximum skin dose during cerebral CT perfusion and the dose to CTDIw ratios differs among CT scanners. The CTDIw is useful for estimation of the maximum skin dose during cerebral CT perfusion using a proper conversion factor specific to each type of CT scanner.     

Relationships between physical dose quantities and patient dose in CT

Patient dose in CT is usually expressed in terms of organ dose and effective dose. The latter is used as a measure of the stochastic risk. Determination of these doses by measurements or calculations can be time-consuming. We investigated the efficacy of physical dose quantities to describe the organ dose and effective dose. For various CT examinations of the head, neck and trunk, organ doses and effective doses were determined using conversion factors. Dose free-in-air on the axis of rotation (Dair) and weighted computed tomography dose index (CTDIw) were compared with the absorbed doses of organs which are located totally within the body region examined. Dose-length product (DLP) was compared with the effective dose. The ratio of the organ dose to CTDIw was 1.37 (0.87-1.79) mSv mGy-1. DLP showed a significant correlation with the effective dose (p < 0.005). The average ratio of effective dose to DLP was 0.28 x 10(-2) mSv (mGy cm)-1 for CT of the head, 0.62 x 10(-2) mSv (mGy cm)-1 for CT of the neck and 1.90 x 10(-2) mSv (mGy cm)-1 for CT of the trunk. CTDIw and DLP can be used for estimating the organ dose and effective dose associated with CT examinations of the head, neck and trunk.     

Spatial distribution of dose in computed tomography with special reference to thin-slice techniques

The spatial dose distribution in a cylindrical polystyrene phantom with a diameter of 200 mm was measured for seven computed tomography (CT) scanners. The measurements were performed in the head mode and mainly for narrow slices in the range 1.5 to 4 mm. Both radial and axial dose profiles were measured and the dose distribution for multiple-scan procedures was calculated. The ratio between the surface and centre doses for a single scan varied between the extremes of 1.8 and 4.3 and was generally higher for narrow than for wide slices. With multiple nominally contiguous scans the difference in absorbed dose between surface and centre locations in the object decreased, on account of scattered radiation. The CT dose index for centre locations varied considerably between the tested scanners, with a range from 5.6 to 27.2 mGy per nominal 100 mAs. For a simulated multiple-scan procedure, comparable to a CT examination of the orbits, the multiple-scan average dose varied between 4.3 and 16.4 mGy per nominal 100 mAs.     

Radiation dose to the lens of eye and thyroid gland in paranasal sinus multislice CT

CT has become an established examination in the evaluation of the paranasal sinuses. Until recently this was achieved by the direct coronal technique on conventional and single slice helical scanners. With the advent of multislice technology, thin slice axial CT with excellent coronal and sagittal reconstructions is now the norm. We describe a study designed to evaluate the radiation dose to the lens of the eye and thyroid gland in the axial and coronal planes on a Siemens Volume Zoom quad slice scanner at 140 kV and effective mAs of 100 using 1 mm collimation. Thermoluminescent dosimeters were placed on the eyelid and thyroid gland of 29 patients scanned axially in the supine position and a further 28 patients scanned coronally in the prone position with gantry tilt. The results show mean doses of 35.1 mGy (lens) and 2.9 mGy (thyroid gland) in the coronal plane compared with 24.5 mGy (lens) and 1.4 mGy (thyroid gland) in the axial plane. Results obtained from a head phantom and from using the ImPACT CT dose calculator were comparable. The kV and mAs were then reduced to 120 and 40, respectively, and the axial study repeated using the head phantom and predicted doses using the ImPACT CT dose calculator. The low dose scanning technique revealed a lens dose of 9.2 mGy and thyroid dose of 0.4 mGy. The eye dose on a multislice scanner is still substantially less than the threshold dose of 0.5-2 Gy for detectable lens opacities. These results indicate that, in addition to the established perceived advantages of multislice axial sinus CT, i.e. patient comfort, no artefact from dental amalgam and reproducible true coronal images, should be included a decreased radiation dose to both the eye lens and thyroid gland compared with direct coronal scanning.     

新生儿头颅多层螺旋CT低剂量扫描的临床应用

目的：评价新生儿头颅多层螺旋CT低剂量扫描的临床价值。方法：选取头颅CT检查的新生儿80例,随机等分成2组,分别使用120kV、90mAs及120kV、260mAs各扫描40例。其余扫描参数为：准直1．5mm,层厚6mm,重建间隔6mm,床速11．7mm／r,扫描时间0．75s。分别比较2种扫描剂量产生的总mAs、CT权重剂量指数（CTDIw）及剂量长度乘积（DLP）,并作t检验。由3名医师采用盲法评价CT图像。按正常图像、图像有少许伪影、图像有严重伪影的等级对每帧图像进行质量评价,并进行统计学处理。结果：90mAs、260mAs组扫描的CTDIw分别为17．28mGy、49．85mGy,DPL分别为245mGy·cm、711mGy·cm。经t检验,90mAs组的CTDIw、DLP明显低于260mAs组（P〈0．01）。满足诊断需要的低剂量扫描图像所占比例（98．6％）与常规剂量扫描（99．9％）相比无显著差异（P〉0．05）。结论：新生儿头颅多层螺旋CT低剂量扫描的辐射剂量为常规剂量扫描的35％,而且图像不影响诊断,低剂量扫描适用于新生儿头颅CT检查。     

小儿头部多层螺旋CT检查的放射剂量评价

目的　评价小儿头部低剂量与常规剂量多层螺旋CT检查的放射剂量 ,为小儿头部多层螺旋CT检查提供扫描剂量参数。资料与方法　 (1)按年龄把 12 0例 0～ 6岁小儿分成 2组 ,患儿 <6个月 ,12 0kVp、90mAs扫描 30例 ;6个月～ 6岁 ,12 0kVp ,15 0mAs扫描 30例 ;常规扫描剂量为 12 0kVp、2 6 0mAs,依照上述年龄段各扫描 30例。其余扫描参数为 :准直 1.5mm ,层厚 6mm ,重建间隔 6mm ,床速 11.7mm/r,扫描时间 0 .75s。分别比较 2种扫描剂量产生的有效mAs、CT权重剂量指数 (weightedCTdoseindex ,CTDIw)及剂量长度乘积 (dose lengthproduct,DLP) ,并作 χ2检验。 (2 )由 3名医师盲法评价CT图像。按正常图像、图像有少许伪影、图像有严重伪影的等级对每帧图像进行质量评判 ,并进行统计学处理。结果　 (1)小儿各年龄段低剂量 (90mAs、15 0mAs)扫描的CTDIw为 17.2 8mGy、2 8.8mGy ,分别是常规剂量 (2 6 0mAs)扫描的 34.6 %、5 7.8% ;前者的DLP分别为 2 37mGy·cm、4 2 3mGy·cm ,明显低于后者的 6 83mGy·cm、731mGy·cm(P <0 .0 1)。 (2 ) 98%以上小儿头部低剂量CT图像可满足临床影像诊断需要 ,与常规剂量小儿头部图像相比无显著差异 (P >0 .0 5 )。结论　小儿头部低剂量多层螺旋CT扫描的辐射剂量为常规剂量扫描的 35     

Image quality vs radiation dose of four cone beam computed tomography scanners

OBJECTIVES: To evaluate image quality by examining segmentation accuracy and assess radiation dose for cone beam CT (CBCT) scanners. METHODS: A skull phantom, scanned by a laser scanner, and a contrast phantom were used to evaluate segmentation accuracy. The contrast phantom consisted of a polymethyl methacrylate (PMMA) cylinder with cylindrical inserts of air, bone and PMMA. The phantoms were scanned on the (1) Accuitomo 3D, (2) MercuRay, (3) NewTom 3G, (4) i-CAT and (5) Sensation 16. The structures were segmented with an optimal threshold. Thicknesses of the bone of the mandible and the diameter of the cylinders in the contrast phantom were measured across lines at corresponding places in the CT image vs a ground truth. The accuracy was in the 95th percentile of the difference between corresponding measurements. The correlation between accuracy in skull and contrast phantom was calculated. The radiation dose was assessed by DPI(100,c) (dose profile integral (100,c)) at the central hole of a CT dose index (CTDI) phantom. RESULTS: The results for the DPI(100,c) were 107 mGy mm for (1), 1569 mGy mm for (2), 446 mGy mm for (3), 249 mGy mm for (4) and 1090 mGy mm for (5). The segmentations in the contrast phantom were submillimeter accurate in all scanners. The segmentation accuracy of the mandible was 2.9 mm for (1), 4.2 mm for (2), 3.4 mm for (3), 1.0 mm for (4) and 1.2 mm for (5). The correlation between measurements in the contrast and skull phantom was below 0.37 mm. CONCLUSIONS: The best radiation dose vs image quality was found for the i-CAT.     

Survey of computed tomography technique and radiation dose in Sudanese hospitals

The purpose of this study was to survey technique and radiation absorbed dose in CT examinations of adult in Sudan and to compare the results with the reference dose levels. Questionnaire forms were completed in nine hospitals and a sample of 445 CT examinations in patients. Information on patient, procedure, scanner, and technique for common CT examinations were collected. For each facility, the radiation absorbed dose was measured on CT dose phantom measuring 16 cm (head) and 32 cm (body) in diameter and was used to calculate the normalized CT air kerma index. Volume CT air kerma index (CVOL), CT air kerma-length product (PKL,CT) values were calculated using the measured normalized CT air kerma index and questionnaire information. The effective dose, E estimates was determined by using PKL,CT measurements and appropriate normalized coefficients. Assuming the sample to offer a fair representative picture of CT practice patterns in Sudan, the mean CVOL and PKL,CT values were comparable or below the reference doses: 65 mGy and 758 mGy cm, respectively at head CT; 11.5 mGy and 327 mGy cm, respectively at chest CT; 11.6 mGy and 437 mGy cm, respectively at abdominal CT; and 11.0 mGy and 264 mGy cm, respectively at pelvis CT. Estimated effective doses were 1.6, 4.6, 6.6 and 4.0 mSv, respectively. The study offered a first national dose survey and provided a mean for quality control and optimization of CT practice within the country.     

CT头颈部检查所致婴幼儿眼晶状体吸收剂量估算方法的模体研究

目的 探讨CT不同扫描方案检查所致婴幼儿眼晶状体吸收剂量估算方法,并寻求快速估算眼晶状体吸收剂量的实用方法。方法 通过设置7种临床标准扫描方案,对1岁年龄组仿真模体进行扫描,利用布放在模体不同位置的热释光探测器（TLD）测量剂量,最后测量结果分别用组织因子转换和个人剂量当量转换两种方法来估算眼晶状体吸收剂量,同时将眼晶状体吸收剂量与CT剂量指数（CTDI）建立线性回归方程。结果 7种临床标准儿童扫描方案CT检查所致的婴幼儿眼晶状体吸收剂量分别为（9.96±0.69）mGy（头部轴向）、（7.01±0.42）mGy（头部螺旋）、（12.60±0.97）mGy（副鼻窦）、（12.97±0.42）mGy（内耳高分辨）、（0.63±0.03）mGy（颈部软组织）、（8.89±0.44）mGy（颈部颈椎）和（0.34±0.01）mGy（胸部常规）,不同组之间剂量差异有统计学意义（F=846.826,P<0.05）。不同扫描部位,CTDI值与眼晶状体吸收剂量之间均存在线性关系（r=0.986~0.999,P<0.05）。结论 采用儿童CT扫描条件,婴幼儿眼晶状体吸收剂量单次剂量范围一般不会超过阈剂量。另外,通过读取CTDI值,利用线性关系,可快速估算眼晶状体吸收剂量。     

Absorbed radiation dose of the female breast during diagnostic multidetector chest CT and dose reduction with a tungsten-antimony composite breast shield: preliminary results

AIM: To determine the absorbed radiation dose to the female breast during chest computed tomography (CT), and whether a custom-designed breast shield can reduce that dose. MATERIALS AND METHODS: Bilateral breast phantoms were combined with an anthropomorphic torso phantom. Each breast phantom contained 20 thermoluminescent dosimeter (TLD) cavities. Eight cavities were used per phantom. Absorbed radiation was measured using TLD 100 s. Three-stacked TLDs comprised a set. Three sets of three TLDs were positioned at eight designated locations and three depths (surface; 1 cm; 4 cm). One set of three TLDs was positioned at eight additional designations, 1cm deep. Each breast was divided anatomically into quadrants. In total, 32 TLD sets/96 TLDs were deployed. The breast-torso phantom was consecutively imaged using a 16-detector array CT machine. Subsequently, 32 new TLD sets were similarly placed, the phantom re-imaged in a likewise manner, but with the application of a tungsten-antimony composite breast shield. TLD readings were averaged and calculated. RESULTS: Average absorbed radiation doses for unshielded right and left breast phantoms ranged from 13.83-19.36 mGy, and 14-20.47 mGy, respectively. The absorbed dose in the shielded right and left breast was reduced to 6.64-8.12 mGy, and 6.7-8.03 mGy, respectively. Average absorbed radiation doses based on the depth for the unshielded breasts ranged from 15.4-18.3 mGy. Shielding reduced this dose to 7-7.9 mGy. Unshielded absorbed radiation doses based on anatomic quadrants ranged from 17.5-18.9 mGy. Shielding reduced this dose to 7-7.5 mGy. CONCLUSIONS: The average absorbed radiation dose to the unshielded female breast phantom is approximately 14-20 mGy. An externally applied shield can reduce this absorbed dose by 56-61%.     

Female breast surface radiation exposure during FDG PET/CT examinations

BACKGROUND: The combined positron emission tomography (PET) and computed tomography (CT) scanners have been developed in which CT data can be used for both anatomical landmarks and attenuation correction of PET images. However, this modality potentially introduces more radiation burden to patients compared to conventional PET scanning as a result of the added radiation exposure received from CT examination. The purpose of our study was to determine the breast radiation doses of combined PET/CT examination. MATERIAL AND METHODS: Patients' superficial breast doses were calculated using thermoluminescence dosimeters (TLDs) placed onto the surface of the breasts. TLDs were positioned before FDG injection and removed after 24 h. We also determined the average superficial and glandular breast radiation doses from the anthropomorphic dosimetric phantom imaged using similar CT protocol (low dose) to the patients' study. RESULTS: The mean superficial breast dose of the breast skin measured from the PET/CT studies was 14.42+/-2.41 mGy. The average superficial and glandular breast doses of the anthropomorphic phantom measured from the low-dose CT was 9.50 mGy and 5.94 mGy, respectively. CONCLUSION: This study showed that radiation exposure to the breasts during PET/CT was higher than the recommended doses. Therefore, combined PET/CT scanning must be used for essential indications, particularly in women of reproductive age and preferentially a low-dose CT protocol should be implemented to avoid overexposure in such patients.     

Thyroid dose from common head and neck CT examinations in children: is there an excess risk for thyroid cancer induction?

This study was conducted to estimate thyroid dose and the associated risk for thyroid cancer induction from common head and neck computed tomography (CT) examinations during childhood. The Monte Carlo N-particle transport code was employed to simulate the routine CT scanning of the brain, paranasal sinuses, inner ear and neck performed on sequential and/or spiral modes. The mean thyroid dose was calculated using mathematical phantoms representing a newborn infant and children of 1year, 5 years, 10 years and 15 years old. To verify Monte Carlo results, dose measurements were carried out on physical anthropomorphic phantoms using thermoluminescent dosemeters (TLDs). The scattered dose to thyroid from head CT examinations varied from 0.6 mGy to 8.7 mGy depending upon the scanned region, the pediatric patient’s age and the acquisition mode used. Primary irradiation of the thyroid gland during CT of the neck resulted in an absorbed dose range of 15.2–52.0 mGy. The mean difference between Monte Carlo calculations and TLD measurements was 11.8%. Thyroid exposure to scattered radiation from head CT scanning is associated with a low but not negligible risk of cancer induction of 4–65 per million patients. Neck CT can result in an increased risk for development of thyroid malignancies up to 390 per million patients.     

Estimation of exposure dose on MDCT examination--the measurement of organ dose and effective dose by anthropomorphic phantom

In order to evaluate the exposure dose in CT examinations, we measured the tissue and organ doses by test site in 4-row, 16-row, and 64-row multi detector CT by using an anthropomorphic phantom and fluorescent glass dosimeters. Furthermore, we calculated the effective dose by using the tissue weighting factor recommended by the ICRP in 2007. The effective dose in the head and neck examinations was 1.4-3.1 mSv, whereas the maximum skin dose was 278.9 mGy in head perfusion CT. The effective dose in examinations of the body trunk was 10.1-35.2 mSv. In addition, the organ dose and skin dose in the scanning range was similar to the CTDI(vol) in head and neck examinations, while it was higher than the CTDI(vol) in examinations of the body trunk. The exposure dose of patients undergoing CT is high in comparison to other radiological examinations. As a result, due to consecutive examinations, an absorbed dose of more than 100 mGy is possible. A future problem therefore remains how to lower the overall exposure dose with the introduction of new radiographic diagnostic modalities, such as phase scan or coronary CT angiography.     

Assessment of computed tomography radiation doses for paediatric head and chest examinations using paediatric phantoms of three different ages

IntroductionWith the rapid development of computed tomography (CT) scanners, the assessment of the radiation dose received by the patient has become a heavily researched topic and may result in a reduction in radiation exposure risk. In this study, radiation doses were measured using three paediatric phantoms for head and chest CT examinations in Najran, Saudi Arabia.MethodsThirteen scanners were included in the study to estimate the CT radiation doses using three phantoms representing three age groups (1-, 5-, and 10-year-old patients).ResultsThe volume CT dose index (CTDI_vol) estimated for each phantom ranged from 6.56 to 41.12 mGy and 0.292 to 11.10 mGy for the head and chest examinations, respectively. The estimation of lifetime attributable risk (LAR) indicated that the cancer risk could reach approximately 0.02–0.16% per 500 children undergoing head and chest CT examinations.ConclusionThe comparison with the published data of the European Commission (EC) and countries reported in this study revealed that the mean CTDI_vol for the head examinations was within the recommended dose reference levels (DRLs). Meanwhile, chest results exceeded the international DRLs for the one-year-old phantoms, suggesting that optimisation work is required at a number of sites.Implications for practiceThe variation among CT doses reported in this study showed that substantial standardisation is needed.     

Radiation dose evaluation in 64-slice CT examinations with adult and paediatric anthropomorphic phantoms

The objective of this study was to evaluate the organ dose and effective dose to patients undergoing routine adult and paediatric CT examinations with 64-slice CT scanners and to compare the doses with those from 4-, 8- and 16-multislice CT scanners. Patient doses were measured with small (<7 mm wide) silicon photodiode dosemeters (34 in total), which were implanted at various tissue and organ positions within adult and 6-year-old child anthropomorphic phantoms. Output signals from photodiode dosemeters were read on a personal computer, from which organ and effective doses were computed. For the adult phantom, organ doses (for organs within the scan range) and effective doses were 8–35 mGy and 7–18 mSv, respectively, for chest CT, and 12–33 mGy and 10–21 mSv, respectively, for abdominopelvic CT. For the paediatric phantom, organ and effective doses were 4–17 mGy and 3–7 mSv, respectively, for chest CT, and 5–14 mGy and 3–9 mSv, respectively, for abdominopelvic CT. Doses to organs at the boundaries of the scan length were higher for 64-slice CT scanners using large beam widths and/or a large pitch because of the larger extent of over-ranging. The CT dose index (CTDI_vol), dose–length product (DLP) and the effective dose values using 64-slice CT for the adult and paediatric phantoms were the same as those obtained using 4-, 8- and 16-slice CT. Conversion factors of DLP to the effective dose by International Commission on Radiological Protection 103 were 0.024 mSv⋅mGy⁻¹⋅cm⁻¹ and 0.019 mSv⋅mGy⁻¹⋅cm⁻¹ for adult chest and abdominopelvic CT scans, respectively.X-ray CT scanners have made remarkable advances over the past few years, contributing to the improvement of diagnostic image quality and the reduction of examination time. CT scanners with 64 slices, the clinical use of which started quite recently in many medical facilities, has enabled a large number of thin slices to be acquired in a single rotation. 64-slice CT technology accelerated the practical use of three-dimensional body imaging techniques such as coronary CT angiography and CT colonography with an increasing number of CT examinations. The increase in CT examination frequency not only for adults but also for children and the higher doses in CT examinations compared with other X-ray diagnostic procedures have raised concerns about patient doses and safety. An understanding of patient doses requires the evaluation of organ and effective doses for patients undergoing CT examinations, although these dose values in 64-slice CT scans have seldom been reported.One common method for estimating organ and effective doses is dose calculation from the CT dose index (CTDI) or dose–length product (DLP), which are both used as readily available indicators of radiation dose in CT examinations. Organ and effective doses can be estimated from the CTDI or DLP, and conversion factors derived from Monte Carlo simulation of photon interactions within a simplified mathematical model of the human body [1]. Another method is based on measurement using thermoluminescence dosemeters (TLDs) implanted in various organ positions within an anthropomorphic phantom [2–6]. Although TLD dosimetry is considered to be the standard method for measuring absorbed doses in a phantom, the dose measurement is laborious and time consuming. Hence, we devised an in-phantom dosimetry system using silicon photodiode dosemeters implanted in various organ positions, where absorbed dose at each position could be read electronically. In the present study, we evaluated organ and effective doses with 64-slice CT scan protocols used clinically for adult and paediatric patients undergoing chest and abdominopelvic CT examinations. We compared the doses with published dose values for 4-, 8- and 16-slice CT, and indicated the conversion factor of DLP to the effective dose in each examination of the chest and abdomen–pelvis for 64-slice CT scanners.     

中国人仿真胸部体模检测多层螺旋CT扫描组织器官剂量的研究

目的 利用中国人仿真胸部模型来测量不同噪声指数下胸部各组织器官的吸收剂量,计算有效剂量(ED)并对MSCT胸部扫描进行剂量评估.方法 对CDP-1C型中国人仿真胸部体模在CT体层解剖和X线衰减两方面进行等效性论证;通过在体模内布放热释光剂量计(TLD)来测量不同噪声水平下各组织器官的吸收剂量,并记录相应的剂量长度乘积(DLP);将两者分别换算为ED后选择单因素t检验方法进行对比研究,分析自动管电流调制(ATCM)技术时不同噪声指数胸部CT扫描的剂量水平.结果 中国人仿真胸部体模与成人CT胸部图像的结构相似.体模主要器官平均CT值为肺-788.04 HU、心脏45.64 HU、肝脏65.84 HU、脊柱254.32 HU,与成人偏差程度分别为肺0.10%、心脏3.04%、肝脏4.49%、脊柱4.36%.肝脏的平均CT值差异有统计学意义(t=-8.705,P＜0.05);肺、心脏和脊柱平均CT值与人体差异无统计学意义(t值分别为-0.752、-1.219、-1.138,P＞0.05).当噪声指数从8.5逐渐增至22.5时,DLP从393.57 mGy·cm递减至78.75 mGy·cm,各器官吸收剂量呈下降趋势(以肺为例,平均吸收剂量从22.38 mGy递减至3.66 mGy).应用DLP所计算的ED较器官吸收剂量计算的ED偏低(以噪声指数为8.5为例,两种方法的ED分别为6.69和8.77 mSv).结论 应用中国人仿真体模来进行CT剂量评估更为准确;基于ATCM技术的胸部CT扫描噪声指数设定至少应大于8.5.

Abstract：

Objective Using the Chinese anthropomorphic chest phantom to measure the absorbed dose of various tissues and organs under different noise index, and to assess the radiation dose of MSCT chest scanning with the effective dose(ED). Methods The equivalence of the Chinese anthropomorphic chest phantom(CDP-1C) and the adult chest on CT sectional anatomy and X-ray attenuation was demonstrated. The absorbed doses of various tissues and organs under different noise index were measured by laying thermoluminescent dosimeters(TLD) inside the phantom, and the corresponding dose-length products(DLP) were recorded. Both of them were later converted into ED and comparison was conducted to analyze the dose levels of chest CT scanning with automatic tube current modulation (ATCM) under different noise index. Student t-test was applied using SPSS 12.0 statistical software. Results The Phantom was similar to the human body on CT sectional anatomy. The average CT value of phantom are -788.04 HU in lung,45.64 HU in heart,65.84 HU in liver,254.32 HU in spine and the deviations are 0.10%,3.04%, 4.49% and 4.36% respectively compared to humans. The difference of average CT value of liver was statistically significant(t=-8.705,P＜0.05),while the differences of average CT values of lung, heart and spine were not significant(t value were -0.752,-1.219,-1.138,respectively and P＞0.05).As the noise index increased from 8.5 to 22.5, the DLP decreased from 393.57 mGy·cm to 78.75 mGy·cm and the organs dose declined. For example, the average absorbed dose decreased from 22.38 mGy to 3.66 mGy in lung. Compared to ED calculating by absorbed dose, the ED calculating by DLP was lower. The ED values of the two methods were 6.69 mSv and 8.77 mSv when the noise index was set at 8.5. Conclusions Application of the Chinese anthropomorphic chest phantom to carry out CT dose assessment is more accurate. The noise index should be set more than 8.5 during the chest CT scanning based on ATCM technique.     

Dosisoptimierung in der thorakalen Computertomographie

Radiation exposure in computed tomography (CT) by far exceeds radiation dose in chest radiography. Dose requirements in CT of the chest, however, are much smaller than for the abdomen because of low x-ray absorption in the lungs. This article describes scanner parameters that influence patient exposure and image quality. Suitable compromises will be explained that allow for dose reduction in chest CT without jeopardising image quality. Dose reduction in chest CT should be performed depending on the clinical indication and requires active reduction of mAs settings. For helical CT, a pitch of 1.5 to 2 should be employed. Dedicated low-dose techniques for screening of bronchogenic carcinoma are described. Dose reduction decreases image quality but the detrimental effects can be reduced by applying proper parameters for scanning and image reconstruction. Thus, images of the mediastinum should be reconstructed with a smoothing filter, while a higher resolution filter should be used for the lungs. In multislice CT, reconstruction of thicker axial or multiplanar sections retains spatial resolution but keeps image noise and thus dose requirements low. The effective, weighted CT dose index (CTDIw,eff) can serve as a rough estimate of the effective patient dose (E) in helical or multislice CT of the whole chest: for most scanners and age groups the conversion factors are 0.5 mSv/mGy for females and 0.4 mSv/mGy for males.     

宁夏儿童头颅、胸部CT辐射剂量状况分析

目的 评估宁夏地区儿童头颅、胸部CT检查的辐射剂量水平,为不同年龄段儿童的CT辐射剂量优化提供基础。方法 采用分层整群抽样的方法,实地采集宁夏地区不同市、县、区不同规模医院1~2周内儿童（≤15岁）头颅、胸部CT的扫描参数、容积CT剂量指数（CTDI_vol）及剂量长度乘积（DLP）,计算患者有效剂量（E）值;并将CTDI_vol、DLP的第75百分位数（P₇₅）与其他国家推荐的DRL值进行比较;所有儿童分4个年龄组：<1岁、1~5岁、6~10岁、11~15岁。结果 走访调查39家医院,调查CT设备47台,采集头颅断层扫描1 134例,胸部平扫636例。头颅CTDI_vol、DLP的P₇₅分别为：<1岁：44.2 mGy、456.2 mGy·cm;1~5岁：57.2 mGy、659.6 mGy·cm;6~10岁：61.1 mGy、668.7 mGy·cm;11~15岁：63.6 mGy、849.3 mGy·cm。胸部CTDI_vol、DLP的P₇₅分别为：<1岁：5.0 mGy、89.2 mGy·cm;1~5岁：5.9 mGy、124.8 mGy·cm;6~10岁：6.0 mGy、167.9 mGy·cm;11~15岁：7.1 mGy、235.0 mGy·cm。结论 宁夏地区儿童胸部CT的辐射剂量与其他报道相近,但头颅CT的辐射剂量相对偏高,且各年龄段均存在偏高现象,尤以婴儿患者较著;应加强宁夏地区儿童头颅CT的辐射剂量优化与监管,增强儿科医生、放射科医生的剂量控制意识,提高对辐射相关风险的认识。     

Quality criteria implementation for brain and lumbar spine CT examinations

A study was undertaken to implement the quality criteria proposed by the European Commission for brain general and lumbar spine (disc herniation) CT examinations. The proposed criteria were evaluated for samples including 93 brain and 86 lumbar spine CT examinations, with special emphasis on the diagnostic and radiation dose requirements. The extent to which the image criteria had been achieved was evaluated after two independent observers had each read the images twice. Dose measurements were conducted in parallel to estimate the proposed dose quantities needed to obtain the images. For brain examinations, we found that a group of image criteria were largely met, and met uniformly in all sites. One criterion (1.2.5) was frequently fulfilled but had intermediate values for two sites; the remaining criteria were fulfilled to different extents, although for criteria 1.2.1 and 1.2.2, scores were lower than 50% and 70%, respectively. The mean percentage image quality score had values between 57% and 78%, with variation coefficients in the range 30-68%. Mean values of the dose quantities were in the ranges 44-74 mGy for weighted CT dose index (CTDIw), 497-1018 mGy cm for dose-length product (DLP) and 1.1-2.2 mSv for effective dose (E). CTDIw and DLP were not correlated because of significant variations in the scanned length, whereas DLP and E were strongly correlated. A weak relationship between image quality score and DLP was found for the sample as a whole. For lumbar spine examinations, none of the critical reproduction image criteria was systematically achieved. One group of criteria (1.2.7, 1.2.8 and 1.2.9) was fulfilled to a large extent in many departments, but fulfilment of the remainder varied widely. The mean score fluctuated in the range 39-88%, with three groups of differences: low (39-51%), intermediate (67-71%) and high (85-88%). Mean values of the CTDIw varied between sites in the range 27-48 mGy. Mean DLP values varied between 188 mGy cm and 333 mGy cm, and the mean effective dose ranged between 3 mSv and 5 mSv. There were significant differences in effective dose between men and women. By sites, there was no relationship between DLP and mean score, with the highest image score associated with intermediate dose values. The percentage disagreement among the observers about a given criterion ranged between 2% and 22% for brain, and between 3% and 46% for lumbar spine.     

Absorbed dose resulting from a specially designed computed tomography technique for examination of the urinary bladder

The absorbed spatial dose distribution resulting from a specially designed CT protocol for examination of the urinary bladder has been investigated with TLD rods in a body phantom containing tissue equivalent material. The CT examination consisted of scout view and both pre- and postcontrast scan series with 5 mm slice thickness and 5 mm unscanned intervals between successive scans. Cross-sectional dose distribution for one scan in the plane of the ovaries was measured as well as the dose profile for one scan along a line through the ovary parallel to the axis of rotation. Based on these measurements, the dose resulting from the whole CT examination was calculated, both with contiguous and noncontiguous scans. The ovarian dose was calculated for different positions of the ovaries in relation to the scanned area. The absorbed dose varied between 8.3 mGy and 9.7 mGy with the actual technique used. When contiguous scans with the same thickness were taken, the ovarian dose increased with a factor from 1.7 to 1.9. The dose resulting from the CT protocol of the urinary bladder was of the same magnitude as absorbed dose resulting from urography. When the diagnostic gain from a precise definition of tumor extent was taken into account, the dose resulting from the CT protocol was judged acceptable.