Validation of a Monte Carlo tool for patient-specific dose simulations in multi-slice computed tomography

Estimating the dose delivered to the patient in X-ray computed tomography (CT) examinations is not a trivial task. Monte Carlo (MC) methods appear to be the method of choice to assess the 3D dose distribution. The purpose of this work was to extend an existing MC-based tool to account for arbitrary scanners and scan protocols such as multi-slice CT (MSCT) scanners and to validate the tool in homogeneous and heterogeneous phantoms. The tool was validated by measurements on MSCT scanners for different scan protocols under known conditions. Quantitative CT Dose Index (CTDI) measurements were performed in cylindrical CTDI phantoms and in anthropomorphic thorax phantoms of various sizes; dose profiles were measured with thermoluminescent dosimeters (TLD) in the CTDI phantoms and compared with the computed dose profiles. The in-plane dose distributions were simulated and compared with TLD measurements in an Alderson-Rando phantom. The calculated dose values were generally within 10% of measurements for all phantoms and all investigated conditions. Three-dimensional dose distributions can be accurately calculated with the MC tool for arbitrary scanners and protocols including tube current modulation schemes. The use of the tool has meanwhile also been extended to further scanners and to flat-detector CT.     

Estimation and comparison of effective dose (E) in standard chest CT by organ dose measurements and dose-length-product methods and assessment of the influence of CT tube potential (energy dependency) on effective dose in a dual-source CT

Purpose

To determine effective dose (E) during standard chest CT using an organ dose-based and a dose-length-product-based (DLP) approach for four different scan protocols including high-pitch and dual-energy in a dual-source CT scanner of the second generation.

Materials and methods

Organ doses were measured with thermo luminescence dosimeters (TLD) in an anthropomorphic male adult phantom. Further, DLP-based dose estimates were performed by using the standard 0.014 mSv/mGycm conversion coefficient k. Examinations were performed on a dual-source CT system (Somatom Definition Flash, Siemens). Four scan protocols were investigated: (1) single-source 120 kV, (2) single-source 100 kV, (3) high-pitch 120 kV, and (4) dual-energy with 100/Sn140 kV with equivalent CTDIvol and no automated tube current modulation. E was then determined following recommendations of ICRP publication 103 and 60 and specific k values were derived.

Results

DLP-based estimates differed by 4.5–16.56% and 5.2–15.8% relatively to ICRP 60 and 103, respectively. The derived k factors calculated from TLD measurements were 0.0148, 0.015, 0.0166, and 0.0148 for protocol 1, 2, 3 and 4, respectively. Effective dose estimations by ICRP 103 and 60 for single-energy and dual-energy protocols show a difference of less than 0.04 mSv.

Conclusion

Estimates of E based on DLP work equally well for single-energy, high-pitch and dual-energy CT examinations. The tube potential definitely affects effective dose in a substantial way. Effective dose estimations by ICRP 103 and 60 for both single-energy and dual-energy examinations differ not more than 0.04 mSv.     

多层CT螺距与辐射剂量的关系探讨

目的 探讨多层螺旋CT的扫描参数螺距的变化对辐射剂量的影响.方法 对于LightSpeed 16层螺旋CT和Sensation 16层和64层螺旋CT,将长杆电离室分别置于16 mm剂量体模的5个孔中,选用不同的螺距进行扫描并测量辐射剂量.结果 3种机型轴扫和螺旋扫描的CTDIvol分别为：（1）对于lightSpeed 16层螺旋CT,在轴扫及螺距分别为0.562、0.938,1.75时,分别等于28.9、51.4、30.8,16.5;（2）对于Sensation16层螺旋CT,在轴扫及螺距分别为0.5,1、1.5时,分别等于41.2、40.3、41.5,43.2;（3）对于Sensation64层螺旋CT,在轴扫及螺距分别为0.5,1.1.5时,分别等于41.2、40.3、41.5,43.2.结论 对于LightSpeed 16层螺旋CT,随着螺距的增加,辐射剂量降低,为保持恒定的图像质量,必须增大mAs作为补偿;Sensation 16层和64层螺旋CT,因采用了mAs自动调整技术,在不同的螺距下辐射剂量相同,图像质量稳定.日常操作中当螺距改变时,应根据不同机型来确定调整管电流的方法.     

Radiation dose associated with common computed tomography examination

Objective

To survey computed tomography (CT) radiation dose associated with non-contrast spiral Multislice CT examination in our institute.

A comparison of the CT-dosimetry software packages based on stylized and boundary representation phantoms

IntroductionWith the rapid development of computed tomography (CT) equipment, the assessment of effective and organ dose using suitable tools becomes an important issue and will provide health professionals with useful information regarding the radiation risks and the development of standard imaging protocols. Different clinical centres and/or institutions may use several software packages, each with different methods and algorithms for CT dose evaluation. Consequently, radiation doses calculated with these computer software packages might be different for the same patient and representative scanner models.MethodsThe effective and organ doses calculated by VirtualDose, CT-expo, and ImPACT software were compared for both males and females using kidney, chest, head, pelvis, abdomen, and whole-body CT protocols. The calculation of radiation dose in these software depends on the use of stylized and boundary representation (BREP) phantoms.ResultsIn general, the results showed that there was a discrepancy between the effective dose values calculated by the three packages. The effective dose in all examinations varied by factors ranging from 1.1 to 1.5 for male and from 1.1 to 1.3 for female. For the female phantom, the VirtualDose shows the highest effective doses in kidney and abdomen examinations while CT-expo gives the highest doses for head and pelvis examinations. For the male phantom, the VirtualDose shows the highest effective doses were for chest examinations.ConclusionVirtualDose approach gives the most accurate estimation, however, further work using a size-based method are necessary to improve the assessment of the effective and equivalent organ dose in CT examinations using these packages.Implications for practiceThe re-evaluation dosimetry software in comparison with patient size would allow for a more accurate estimation of dose and support the optimization process.     

Hi-ART螺旋断层放疗机MV螺旋CT剂量指数的测量

目的 探讨Hi-ART螺旋断层放疗机MV扇形束CT图像获取过程中患者接受的剂量。方法 用PTWTM30009CT电离室分别在T40017头部和T40016躯干模体中,选择扫描层厚2、4及6mm和改变扫描范围等参数,分别测量加权CT剂量指数,计算相应的剂量长度乘积,并与XVIkV锥形束CT、ACQSim模拟定位CT的结果进行比较。结果 Hi-ART螺旋断层治疗机的CT剂量指数与层厚成反比,剂量长度乘积与扫描范围成正比。临床应用条件下Hi-ART的CT剂量指数在头颈部比XVIkV锥形束CT大,但躯干较小。结论 CT剂量指数能反映患者成像过程中接受的剂量,可以作为治疗保证与控制的指标。图像引导过程中应该合理选择层厚,减少扫描范围,最大限度减少患者接受剂量。     

Radimetrics系统在CT辐射剂量评估上的初步应用

目的通过Radimetrics系统对CT辐射剂量的数据分析,评估扫描方案能否达到辐射防护的要求。 方法把Toshiba Aquilion 64排和Siemens Emotion 16排CT的3980例患者数据自动传输到Radimetrics工作站,并生成患者靶组织及敏感组织辐射剂量报表,采用SPSS17.0统计学软件统计出头颅、胸部、腹部、盆腔CT检查的辐射剂量分布情况并行t检验。 结果（1）头颅、胸部、腹部、盆腔CT平扫的辐射剂量的中位值分别为2.31、4.82、3.65、4.48 mSv;增强扫描的辐射剂量的中位值分别为4.58、9.58、10.51、8.71 mSv。（2）多数患者辐射剂量在ICRP103报告书范围内,仅有80例超出范围;（3）与扫描部位越邻近的组织,受到的辐射剂量越大;（4）动脉期、静脉期的辐射剂量低于平扫;（5）64排CT辐射剂量小于16排CT。 结论Radimetrics系统可通过对CT辐射剂量分析来有效评估扫描方案的合理性。     

单层与多层螺旋CT所致儿童受检者辐射剂量研究

目的 研究和评价儿童受检者在单层与多层螺旋CT扫描中所受到的辐射剂量。方法 测试21台CT机的头部和体部剂量指数,并结合0～1岁组、5岁组、10岁组儿童和成年人的头部和胸部常规扫描条件,计算CTDI_w、CTDI_vol、DLP值,再由DLP与有效剂量转换系数计算头部和胸部常规扫描所致各年龄组儿童和成年人的有效剂量。 结果 单位mAs的头部CTDI大于体部CTDI;在头部常规扫描中,0～1岁组、5岁组、10岁组儿童受到的有效剂量分别为2.2、1.3、1.1 mSv;在胸部常规扫描中,0～1岁组、5岁组、10岁组儿童受到的有效剂量分别为5.3、3.1、3.4 mSv;每单位mAs所致儿童有效剂量平均比成人高1.8倍;多层CT的儿童头部CTDI_vol、DLP、有效剂量值均大于单层与双层CT,多层与双层CT的儿童胸部CTDI_vol、DLP、有效剂量值均小于单层CT。 结论 与成年人相比,儿童在CT检查中可能受到更大辐射危害,应严格遵循儿童CT检查适应证,并合理选择CT扫描参数,尽可能降低儿童受到的辐射剂量。     

Strategies for formulating appropriate MDCT techniques when imaging the chest, abdomen, and pelvis in pediatric patients

OBJECTIVE: Our aim was to formulate appropriate MDCT chest and abdominopelvic CT scan protocols for pediatric patients. MATERIALS AND METHODS: Surface radiation dose measurements from a set of anthropomorphic phantoms (nominal 1 year old, 5 year old, and 10 year old) and an adult phantom were compared with standard CT dose index measurements. Image-noise values on axial 5-mm-thick anthropomorphic phantom images were obtained as a measure of image quality. RESULTS: Peripheral CT dose index values obtained with the standard 16-cm acrylic phantom were within approximately 10% of the CT surface dose measurements for the pediatric anthropomorphic phantoms for both chest and abdominopelvic scan protocols. The noise value for the adult phantom image acquired using a typical clinical CT technique was identified, and targeting this level of noise for pediatric CT examinations resulted in a decrease in dose of 60-90%. Initially, 80 kVp was selected for use with very small children; however, beam-hardening artifacts were severe enough to cause us to abandon this option. Current pediatric protocols at M. D. Anderson Cancer Center rely on 100- and 120-kVp settings. The display field-of-view parameter can be used as a surrogate for patient size to develop clinical pediatric CT protocol charts. CONCLUSION: CT dose index measurements obtained using the 16-cm standard acrylic phantom are sufficiently accurate for estimating chest and abdominopelvic CT entrance exposures for pediatric patients of the same approximate size as the anthropomorphic phantoms used in this study. Image-noise measurements can be used to adjust chest and abdominopelvic CT techniques for pediatric populations, resulting in a decrease in measured entrance dose by 60-90%.     

PET-CT检查致前列腺癌患者辐射剂量研究

目的 评估¹⁸F-Choline、¹¹C-Choline和⁶⁸Ga-PSMA PET-CT检查致前列腺癌患者的有效剂量和器官剂量。方法 回顾性研究2017年5月至2018年6月广西医科大学附属肿瘤医院接受PET-CT检查的150例前列腺癌患者,按照注射正电子放射性药物类型分为3组,每组50例。CT定位扫描电压和电流分别为120 kV和35 mA,全身CT扫描电和电流分别为120 kV和（135.6±9.4） mA。PET部分的剂量利用基于医学内照射剂量（MIRD）计算方法的OLINDA/EXM （version 1.1）软件行计算。利用有效剂量转换因子和ImPACT （version 1.0.4） CT剂量计算器计算CT部分剂量,CT剂量指数（CTDI）利用标准体模测量和ImPACT CT计算,组织权重因子取自国际放射防护委员会（ICRP）103号报告,PET和CT剂量之和为患者总有效剂量。结果 注射¹⁸F-Choline、¹¹C-Choline和⁶⁸Ga-PSMA的活度分别为（279.2±13.2）、（350.2±39.9）和（186.8±19.4） MBq,有效剂量分别为（5.0±0.2）、（1.6±0.2）和（3.0±0.3） mSv,差异有统计学意义（F=837.0,P<0.001）。CT有效剂量为（11.4±0.2） mSv。3组总有效剂量分别为（16.4±0.3）、（13.0±0.3）和（14.4±0.4） mSv。PET检查3组器官当量剂量平均值比较,差异有统计学意义（F=381.2~1 637.7,P<0.001）。¹⁸F-Choline和⁶⁸Ga-PSMA PET-CT检查器官当量剂量最高为肾脏,而¹¹C-Choline PET-CT检查最高为甲状腺。结论 PET-CT检查致前列腺癌患者的有效剂量为13.0~16.4 mSv,其中绝大部分的剂量来自CT扫描。¹¹C-Choline PET-CT检查致患者的辐射剂量最低,有望成为潜在的前列腺癌PET显像药物。     

胸部能谱成像模式与常规CT扫描的辐射剂量及图像质量的仿真体模研究

目的 通过仿真胸部体模研究胸部能谱CT不同方案成像模式与常规胸部CT扫描的辐射剂量、图像质量、对比噪声比及主观评分对比,获得最佳能谱扫描参数。方法 对仿真胸部体模分别进行常规胸部CT扫描以及3种不同扫描方式的能谱CT成像。3种能谱模式为宝石能谱CT （GSI）Assist模式（方案A）、管电流平均值时的GSI模式（方案B）及管电流最大值时的GSI模式（方案C）。所有扫描方式分别在噪声指数（NI）=9和11时,螺距0.984∶1,依次扫描。记录辐射剂量,同时测量感兴趣区（ROIs）5个不同层面水平的脂肪和肌肉组织的图像噪声值（SD）以评价图像质量。所有扫描序列由两位资深放射医师对肺窗肺纹理及分支5分制主观评分。结果 NI=9和11时,常规CT平扫与方案A、B、C的有效剂量（E）值分别为（8.0、8.5、6.2、10.4）和（5.3、5.1、4.3、6.2）mSv。NI=9时,常规胸部CT平扫与方案A、C的SD值差异有统计学意义（F=4.496,P<0.05）;NI=11时,方案A、B、C与常规胸部CT平扫SD值差异有统计学意义（F=8.425,P<0.05）;常规胸部CT扫描中,NI分别为9和11时SD值差异有统计学意义（t=－2.570,P<0.05）;在相同能谱扫描模式中,NI不同,SD值差异均无统计学意义（P>0.05）。NI相同扫描模式不同及扫描模式相同NI不同时,CNR及主观评分差异均无统计学意义（P>0.05）。结论 合理的能谱扫描模式与常规扫描的辐射剂量没有明显差异,但能谱扫描模式可以获得较高的图像质量。此外,选择合适的噪声指数在图像质量相仿的同时可以明显降低辐射剂量。综合辐射剂量及图像质量,能谱CT智能模式可以达到辐射剂量及图像质量的双向平衡。     

基于中国参考人的典型CT扫描患者剂量模拟与分析

目的 基于中国参考人体模,对典型CT患者剂量进行模拟与分析。开发一款在线评估CT扫描所致受检者器官吸收剂量和有效剂量的软件,为快速评估CT扫描辐射剂量提供工具。方法 采用蒙特卡罗方法,对GE LightSpeed 16型号CT进行建模,基于中国参考人体模计算中国成年男性、女性和1岁儿童受CT扫描照射后各组织器官的剂量,建立单层轴扫器官剂量数据库。开发基于web的CT剂量评估软件,根据用户输入的CT扫描参数,从器官剂量数据库中读取数据并快速计算器官吸收剂量和有效剂量。结果 开发了一款新的CT扫描剂量评估软件,可用于评估中国成年男女和1岁儿童在不同型号CT检查中,不同管电压、管电流、准直器宽度等CT参数下的器官剂量。CT扫描剂量评估软件的计算结果与两个文献数据进行对比,对于完全包含在扫描范围内的器官剂量差异较小,相对差异分别在15%和25%以内。结论 CT扫描剂量评估软件为简单快速评估CT扫描辐射剂量提供了有力的工具。     

Radiation dose to the lens in sequential and spiral CT of the facial bones and sinuses

A tendency for over-utilization of the radiological examinations of facial bones and sinuses has been observed. The radiosensitive lens is exposed to radiation during such examination. Comparison of routine roentgenography and spiral axial and coronal CT in sequential and spiral acquisition modes with different mAs values was undertaken using Alderson Rando Phantom. The spiral-CT technique with pitch 1.5 performed in the coronal plane gave the lowest radiation dose to the lens as compared with other CT methods and only seven times higher than routine sinus roentgenography. Received: 27 March 2000 Revised: 13 July 2000 Accepted: 13 July 2000     

CT扫描所致受检者器官剂量的体模实验研究

目的 了解不同部位X射线CT扫描所致受检者器官或组织的吸收剂量及其分布。方法 实测体模中重要组织器官的CT值,并转换成线性吸收系数与人体正常值进行比较;在体模中 布放光致辐射发光玻璃剂量计,分别模拟测量头部、胸部、腹部和盆腔CT扫描所致受检者主要器官或组织的吸收剂量。结果 实验用仿真人体模具有良好的组织等效性。头部扫描吸收剂量最大的器官是大脑,胸部扫描吸收剂量较大的器官是甲状腺、乳腺、肺和食道,腹部扫描吸收剂量较大的器官是肝、胃、结肠和肺,单次盆腔扫描体所致骨表面和结肠的吸收剂量可达50 mGy以上。结论 X射线CT扫描所致受检者的器官剂量及其分布随扫描部位的不同而异。盆腔扫描时结肠、红骨髓、性腺和膀胱等主要器官的吸收剂量较大,应引起注意。     

基于体型特异性剂量估算值推算冠状动脉CT血管成像中患者器官剂量和个体有效剂量的可行性研究

目的 探讨将体型特异性剂量估算值（SSDE）用于估算冠状动脉CT血管成像（CTA）中患者器官剂量和个体有效剂量的可行性。方法 回顾性连续纳入冠状动脉CTA患者421例,均于第3代双源Force型CT采用前瞻性心电门控触发轴扫协议检查。通过Radimetrics计算患者水当量直径以计算每位患者的SSDE;使用Monte Carlo模拟估算患者扫描范围内器官的吸收剂量包括心脏、肺、肝和乳腺。使用国际放射防护委员会（ICRP）103报告的器官敏感加权系数,将患者主要敏感器官的剂量加权求和计算个体有效剂量。使用线性相关分析验证SSDE与器官剂量及个体有效剂量的相关性,并推导基于SSDE估算器官剂量和个体有效剂量的转换系数。使用平均差值比评价该估算方法的准确性。结果 容积CT剂量指数（CTDI_vol）为（16.8±8.7） mGy,SSDE为（20.8±8.8） mGy,个体有效剂量为（4.4±2.9） mSv。基于SSDE估算器官剂量的线性拟合公式为：心脏Y=1.2X-6.4（R²=0.91,P<0.05,平均误差0.1%）;乳腺Y=1.4X-7.4（R²=0.91,P<0.05,平均误差7.9%）;肺脏Y=0.89X-4.6（R²=0.86,P<0.05,平均误差8.3%）;肝脏Y=0.36X-1.8（R²=0.64,P<0.05,平均误差-17.9%）。基于SSDE估算个体有效剂量的线性拟合公式为：男Y=0.21X-1.2（R²=0.92,P<0.05,平均误差0.2%）;女Y=0.39X-2.2（R²=0.93,P<0.05,平均误差1.7%）。结论 在冠状动脉CTA检查中通过SSDE和相应的转换系数可估算被照射器官吸收剂量和个体有效剂量,将有助于在临床工作中实现患者辐射剂量及风险的个性化评估和精准管理。     

16层螺旋CT低剂量扫描在女性骨盆中的应用

目的 评价16层螺旋CT低剂量扫描在女性骨盆中的应用。方法 30例志愿者用16层螺旋CT行女性骨盆扫描，分别采用低剂量扫描（CareDose）和传统方法扫描，将两组女性骨盆图像按各影像颗粒均匀性、解剖结构细节、界面清晰度和有无伪影等评定图像质量。同样对两组的CT检查辐射剂量进行对照研究。结果 两组扫描方法对女性骨盆图像质量的显示差异无统计学意义；剂量扫描与传统扫描相比，辐射剂量大大降低（P〈0．01）。结论 16层螺旋CT低剂量扫描在女性骨盆图像质量不下降的同时，可以较传统CT扫描减少受检者的辐射剂量。     

核设施流出物照射人体器官的当量剂量和有效剂量的计算

目的研究在核设施流出物照射条件下ＩＣＲＰ６０号出版物建议的新辐射量（当量剂量和有效剂量）算法和有关参数,建立在核设施防护领域应用新辐射量的剂量学资料。方法采用ＡＤＡＭ和ＥＶＡ拟人计算模型和组织空气比的方法计算器官当量剂量和有效剂量。结果给出了核设施正常运行和事故时可能释放的近２００种核素,在空气浸没、水中浸没和地表沉积物３种照射方式下,器官当量剂量率因子和有效剂量率因子,考虑了男性１８种和女性２０种器官;有效剂量与有效剂量当量相差在２０％以内。结论新旧辐射量相差不大,本文结果可用于核设施流出物外照射人员的剂量估计。     

铋屏蔽对头颈部多层螺旋CT中眼晶状体辐射剂量的降低作用

目的 探讨扫描平面内铋屏蔽在头颈部多层螺旋CT(MSCT)扫描中对影像质量的影响和眼晶状体辐射剂量的降低作用.方法 分别使用颅脑、颞骨和鼻窦临床扫描条件,在无屏蔽、1层、2层和3层铋屏蔽覆盖眼部区域时,对标准水模和离体头颅标本进行扫描,用热释光剂量片测量头颅标本每次扫描时的眼晶状体器官剂量.在屏蔽材料和被扫描体间放置5、10、15和20 mm厚的海绵时,使用鼻窦扫描条件采集影像,并测量眼晶状体的剂量.测量水模影像中与屏蔽物为2、4、6和8 cm距离处的CT值,主观评价头颅标本影像中伪影对解剖结构的影响.结果 颅脑、颞骨和鼻窦CT临床扫描中眼晶状体的器官剂量分别为24.31、27.60和20.01 mGy.使用铋屏蔽时,均使得眼晶状体剂量有显著下降,但下降幅度随着铋屏蔽物的增加而降低.在各种厚度的屏蔽物时,屏蔽物间隙越大,眼晶状体剂量的降低程度越小,测量兴趣区CT值的增加程度也显著降低.颅脑和颞骨CT扫描分别使用2层和3层铋屏蔽,在不影响诊断的前提下,可有效降低眼晶状体剂量分别为47.1%和59.1%;鼻窦CT扫描时,1层屏蔽无间隙、2层屏蔽1.5 cm间隙不影响诊断,可降低眼晶状体剂量分别为31.5%和34.5%.结论 扫描平面内铋屏蔽材料的合理应用,可有效降低头颈部CT扫描中眼晶状体的辐射剂量.     

Estimating cancer induction risk from abdominopelvic scanning with 6- and 16-slice computed tomography

Purpose: The biological effects of ionizing radiation (BEIR VII) report estimates that the risk of getting cancer from radiation is increased by about a third from current regulation risk levels. The propose of this study was to estimate cancer induction risk from abdominopelvic computed tomography (CT) scanning of adult patients using 6- and 16-slice CT scanners.

Materials and methods: A cross-sectional study on 200 patients with abdominopelvic CT scan in 6- and 16-slice scanners was conducted. The dose-length product (DLP) and volume CT Dose Index (CTDI_vol) values from the scanners as well as the effective dose values from the ImPACT CT patient dosimetry calculator with the biological effects of ionizing radiation (BEIR VII) method were used to estimate the cancer induction risk.

Results: The mean (and standard deviation) values of CTDI_vol and DLP were 6.9 (±1.07) mGy and 306.44 (±?60.57) mGy.cm for 6-slice, and 5.19 (±0.91) mGy and 219.7 (±49.31) mGy.cm for 16-slice scanner, respectively. The range of effective dose in the 6-slice scanner was 2.61–8.15 mSv and, in the 16-slice scanner, it was 1.47–4.72 mSv. The mean and standard deviation values of total cancer induction risk in abdominopelvic examinations were 0.136?±?0.059% for men and 0.135?±?0.063% for women in the 6-slice CT scanner. The values were 0.126?±?0.051% for men and 0.127?±?0.056% for women in the 16-slice scanner.

Conclusions: The cancer induction risk of abdominopelvic scanning was noticeable. Therefore, radiation dose should be minimized by optimizing the protocols and applying appropriate methods.     

Radiation dose in 320-slice multidetector cardiac CT: A single center experience of evolving dose minimization

BackgroundMinimization of radiation exposure remains an important subject that occurs in parallel with advances in scanner technology.ObjectiveWe report our experience of evolving radiation dose and its determinants after the introduction of 320-multidetector row cardiac CT within a single tertiary cardiology referral service.MethodsFour cohorts of consecutive patients (total 525 scans), who underwent cardiac CT at defined time points as early as 2008, are described. These include a cohort just after scanner installation, after 2 upgrades of the operating system, and after introduction of an adaptive iterative image reconstruction algorithm. The proportions of nondiagnostic coronary artery segments and studies with nondiagnostic segments were compared between cohorts.ResultsSignificant reductions were observed in median radiation doses in all cohorts compared with the initial cohort (P < .001). Median dose-length product fell from 944 mGy · cm (interquartile range [IQR], 567.3–1426.5 mGy · cm) to 156 mGy · cm (IQR, 99.2–265.0 mGy · cm). Although the proportion of prospectively triggered scans has increased, reductions in radiation dose have occurred independently of distribution of scan formats. In multiple regression that combined all groups, determinants of dose-length product were tube output, the number of cardiac cycles scanned, tube voltage, scan length, scan format, body mass index, phase width, and heart rate (adjusted R² = 0.85, P < .001). The proportion of nondiagnostic coronary artery segments was slightly increased in group 4 (2.9%; P < .01).ConclusionWhile maintaining diagnostic quality in 320-multidetector row cardiac CT, the radiation dose has decreased substantially because of a combination of dose-reduction protocols and technical improvements. Continued minimization of radiation dose will increase the potential for cardiac CT to expand as a cardiac imaging modality.