Dosimetry and adequacy of CT-based attenuation correction for pediatric PET: phantom study

PURPOSE: To evaluate the dose from the computed tomographic (CT) portion of positron emission tomography (PET)/CT to determine minimum CT acquisition parameters that provide adequate attenuation correction. MATERIALS AND METHODS: Measurements were made with a PET/CT scanner or a PET scanner, five anthropomorphic phantoms (newborn to medium adult), and an ionization chamber. The CT dose was evaluated for acquisition parameters (10, 20, 40, 80, 160 mA; 80, 100, 120, 140 kVp; 0.5 and 0.8 second per rotation; 1.5:1 pitch). Thermoluminescent dosimetry was used to evaluate the germanium 68/gallium 68 rod sources. A phantom study was performed to evaluate CT image noise and the adequacy of PET attenuation correction as a function of CT acquisition parameters and patient size. RESULTS: The volumetric anthropomorphic CT dose index varied by two orders of magnitude for each phantom over the range of acquisition parameters (0.30 and 21.0 mGy for a 10-year-old with 80 kVp, 10 mAs, and 0.8 second and with 140 kVp, 160 mAs, and 0.8 second, respectively). The volumetric anthropomorphic CT dose index for newborn phantoms was twice that for adult phantoms acquired similarly. The rod source dose was 0.03 mGy (3-minute scan). Although CT noise varied substantially among acquisition parameters, its contribution to PET noise was minimal and yielded only a 2% variation in PET noise. In a pediatric phantom, PET images generated by using CT performed with 80 kVp and 5 mAs for attenuation correction were visually indistinguishable from those generated by using CT performed with 140 kVp and 128 mAs. With very-low-dose CT (80 kVp, 5 mAs) for the adult phantom, undercorrection of the PET data resulted. CONCLUSION: For pediatric patients, adequate attenuation correction can be obtained with very-low-dose CT (80 kVp, 5 mAs, 1.5:1 pitch), and such correction leads to a 100-fold dose reduction relative to diagnostic CT. For adults undergoing CT with 5 mAs and 1.5:1 pitch, the tube voltage needs to be increased to 120 kVp to prevent undercorrection.     

Effective dose determination using an anthropomorphic phantom and metal oxide semiconductor field effect transistor technology for clinical adult body multidetector array computed tomography protocols

PURPOSE: To determine the organ doses and total body effective dose (ED) delivered to an anthropomorphic phantom by multidetector array computed tomography (MDCT) when using standard clinical adult body imaging protocols. MATERIALS AND METHODS: Metal oxide semiconductor field effect transistor (MOSFET) technology was applied during the scanning of a female anthropomorphic phantom to determine 20 organ doses delivered during clinical body computed tomography (CT) imaging protocols. A 16-row MDCT scanner (LightSpeed, General Electric Healthcare, Milwaukee, Wis) was used. Effective dose was calculated as the sum of organ doses multiplied by a weighting factor determinant found in the International Commission on Radiological Protection Publication 60. Volume CT dose index and dose length product (DLP) values were recorded at the same time for the same scan. RESULTS: Effective dose (mSv) for body MDCT imaging protocols were as follows: standard chest CT, 6.80 +/- 0.6; pulmonary embolus CT, 13.7 +/- 0.4; gated coronary CT angiography, 20.6 +/- 0.4; standard abdomen and pelvic CT, 13.3 + 1.0; renal stone CT, 4.51 + 0.45. Effective dose calculated by direct organ measurements in the phantom was 14% to 37% greater than those determined by the DLP method. CONCLUSIONS: Effective dose calculated by the DLP method underestimates ED as compared with direct organ measurements for the same CT examination. Organ doses and total body ED are higher than previously reported for MDCT clinical body imaging protocols.     

降低儿童16层螺旋CT检查辐射剂量的研究

目的论证CT扫描参数kVp和mAs与剂量和图像噪声的关系,在不影响临床诊断的基础上,修正并验证一种基于成人扫描参数的安全可行的儿童16层螺旋CT检查的扫描参数。方法利用16层螺旋CT,采用标准CT剂量指数(CTDI)测试仪、100mm笔型电离室,分别测量16cm和32cm直径模体在2mm×5mm准直宽度时不同kVp和mAs的CTDI;采用20cm标准水模,测量单一感兴趣区域(ROI)标准偏差值SD代表噪声水平。以成人扫描参数的不同百分比修正为不同年龄段儿童CT扫描的参数供临床验证。结果随着kVp和mAs的增加,CTDI随之增加,并与mAs呈线性关系;16cm直径模体的表面CTDI要高于32cm模体58%;实际的加权CTDIw值高于CT扫描仪显示的CTDIw;mAs相同时,kVp越高,图像噪声SD值越低,在kVp固定时,随着mAs的增加,图像噪声SD随之减少,当mAs增加到一定程度后,图像噪声趋向平稳。结论在不影响临床诊断的图像噪声水平下,根据年龄和体型特点,儿童16层CT检查mAs可以比成人降低10%～85%。     

A comparative study of thoracic radiation doses from 64-slice cardiac CT

The goal of this study was to measure radiation doses for 64-slice cardiac CT angiography studies and to study the dose-savings features of these CT scanners. This was done using various phantoms. These radiation doses were compared with those from typical helical body CT scans, fluoroscopy cardiac catheterization studies and mammography examinations. Radiation measurements were made with a CT ionization detector and a solid state dosimeter. A GE 64-slice Lightspeed VCT and a Siemens Somatom Sensation 64 CT were used to scan a standard 32 cm acrylic phantom and an anthropomorphic phantom. Data were collected in axial and various gated cardiac helical modes. Organ doses and the effective doses were calculated from the measurements. In gated CT cardiac mode with the 32 cm acrylic phantom, the measured radiation doses per study were generally three to seven times greater than those from typical body helical CT examinations; the range depended upon selectable scan parameters. With the anatomical phantom, the surface doses in the anteroposterior (AP) plane were typically 20-60% higher than those measured using the 32 cm phantom. The lateral surface doses were -4% to +15%. These results can be attributed to the shorter AP dimension and the air in the lungs. The CT skin entrance radiation doses were 80-90% less than diagnostic cardiac catheterization studies, and organ doses were similar. Because 64-slice cardiac gated CT uses pitches equal to 0.20-0.27 and high mAs values, the patient radiation doses are appreciably higher than in routine body CT examinations. The female breast, which could receive a radiation dose 10-30 times that received from mammography screening, is an organ of particular concern.     

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.     

5岁儿童仿真模体千伏锥束CT模拟剂量测量

目的 利用热释光探测器（TLD）在CIRS 5岁仿真儿童模体内测量瓦里安千伏锥束CT（kV-CBCT）标准扫描参数下各重要器官剂量,并以此计算有效剂量。方法 挑选一致性在2%以内的TLD并退火。首先基于相同骨盆扫描模式分别用CT电离室和TLD测量CIRS骨盆仿真模体相同体积内的剂量和读数,二者比值即为TLD转换系数;将夹在组织等效插件中的TLD放入儿童模体器官内预留的插孔,在头部、胸部和骨盆3种标准扫描条件模式下,测量器官剂量,并计算有效剂量。结果 TLD转换系数是3.91 mGy/每读数;在头部、胸部和骨盆3种标准扫描条件下,得出全身有效剂量分别是0.63、6.85和19.3 mSv。结论 用CT电离室刻度过的TLD测量kV-CBCT给儿童仿真模体带来的辐射剂量的方法具有可行性。本研究中骨盆扫描条件的有效剂量高于胸部和头部,即该条件预期产生的辐射危害较大,诱发继发性癌症风险较高。     

Radiation dose and image quality in pediatric CT: effect of technical factors and phantom size and shape

PURPOSE: To evaluate effects of varying tube current and voltage on radiation dose, image noise, and image contrast with different phantom sizes and shapes. MATERIALS AND METHODS: Four round lucite phantoms with 8-32-cm diameters were scanned with multi-detector row computed tomography (CT) and 80-120 kVp. Radiation dose was based on CT dose index, image noise, and iodine contrast and measured with constant and variable tube currents that were age appropriate for each tube voltage. Radiation dose and image noise and contrast were compared in round and oval 24-cm phantoms. For various combinations of technical factors and phantom sizes and shapes, percentage differences were calculated for radiation dose and image noise and contrast. Associations between tube voltage and radiation dose, image noise, and image contrast in round and oval phantoms were determined by fitting second-degree polynomials to data. Differences in radiation dose and image noise and contrast, which were attributable to differences in tube voltage, were tested with paired t tests. RESULTS: With 165-mAs tube current, radiation doses with 140- and 80-kVp tube voltages were 103% ([41.9 mGy - 20.6 mGy]/20.6 mGy) and 58% ([10.2 mGy - 4.2 mGy]/10.1 mGy) higher in the 8-cm phantom than in the 32-cm phantom. When tube current was adapted for phantom size, radiation dose at 80 kVp in the 8-cm phantom was reduced by 82% ([10.1 mGy - 1.8 mGy]/10.1 mGy). In the 8-cm phantom, tube voltage was decreased from 120 to 80 kVp and tube current remained at 165 mAs, resulting in a 68% noise increase ([3.1 HU - 1.8 HU]/1.8 HU). With variable tube current, 80-kVp tube voltage in the 8-cm phantom led to a 138% noise increase ([7.3 HU - 3.1 HU]/3.1 HU). With reduced tube voltage, image contrast increased. In the 8-cm phantom, with a constant 165-mAs tube current and a decrease in tube voltage from 120 to 80 kVp, there was a 35% ([333 HU - 217 HU]/333 HU) increase in contrast. No difference was noted in radiation dose or noise between round and oval phantoms (P = .604 and P = .06, respectively), but a small statistically significant difference (1%) in contrast attenuation was demonstrated (P = .025). CONCLUSION: Reduced tube voltage for pediatric contrast material-enhanced CT reduces radiation dose and maintains image contrast. Image noise increases, but the effect is minimal in smaller phantoms. An additional reduction in tube current further reduces radiation dose.     

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.     

A PC program for estimating organ dose and effective dose values in computed tomography

Dose values in CT are specified by the manufacturers for all CT systems and operating conditions in phantoms. It is not trivial, however, to derive dose values in patients from this information. Therefore, we have developed a PC-based program which calculates organ dose and effective dose values for arbitrary scan parameters and anatomical ranges. Values for primary radiation are derived from measurements or manufacturer specifications; values for scattered radiation are derived from Monte Carlo calculations tabulated for standard anthropomorphic phantoms. Based on these values, organ doses can be computed by the program for arbitrary scan protocols in conventional and in spiral CT. Effective dose values are also provided, both with ICRP 26 and ICRP 60 tissue-weighting coefficients. Results for several standard CT protocols are presented in tabular form in this paper. In addition, potential for dose reduction is demonstrated, for example, in spiral CT and in quantitative CT. Providing realistic patient dose estimates for arbitrary CT protocols is relevant both for the physician and the patient, and it is particularly useful for educational and training purposes. The program, called WinDose, is now in use at the Erlangen University hospitals (Germany) as an information tool for radiologists and patients. Further extensions are planned. Received: 9 March 1998; Revision received: 4 June 1998; Accepted: 4 November 1998     

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.     

Estimation of fetal radiation dose from computed tomography scanning in late pregnancy: depth-dose data from routine examinations

RATIONALE AND OBJECTIVES: To provide depth-dose data for estimating fetal radiation dose from routine computed tomography (CT) examinations of the trunk. METHODS: Doses were measured during CT examinations of the thorax, upper abdomen, abdomen, and pelvis in two anthropomorphic phantoms simulating pregnant women in the second and third trimesters. Thermoluminescent dose meters were used for dose measurements. RESULTS: In CT examinations of the abdomen, doses of 30.0 to 43.6 mGy and of 29.1 to 42.0 mGy were measured at the measuring points in the phantom simulating pregnancy in the second and third trimesters, respectively. In CT examinations of the upper abdomen, pelvis, and thorax, both phantoms received lower doses of radiation. Knowledge of the normalized weighted dose index of the CT scanner and of the kVp and mAs settings of the protocol used for examination of the pregnant woman is needed to adjust the dose data found in the present study to modified protocols and different CT equipment. CONCLUSIONS: These dosimetric data may be used to guide the management of pregnant patients undergoing CT examinations of the trunk.     

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。     

胸部低剂量CT扫描管电流与噪声分布相关性研究

目的 分析胸部低剂量CT图像噪声分布特点,优化低剂量扫描参数.方法 利用图像空间添加噪声软件对中国人仿真胸部体模CT图像进行6种不同噪声指数的模拟,并分析预设噪声指数与模拟图像噪声值的差异.使用该软件对20例志愿者常规扫描图像进行噪声添加,模拟出10、30、50、80、100、120、150、180及240 mA 9组胸部低剂量图像,记录每幅图像模拟噪声值,并对不同剂量组模拟噪声值进行统计学分析.结果 图像噪声添加法计算的模拟噪声值与预设噪声指数差异不大.胸部低剂量扫描时,图像模拟噪声值随着管电流的降低而增加,当管电流在30～50 mA时,噪声降低显著(F=24.09 ～ 40.79,P＜0.05);80～240 mA时,噪声值降低幅度变缓;80mA组与120 mA组之间差异不大.结论 图像空间添加噪声软件可应用于胸部低剂量CT图像噪声的评价研究.胸部低剂量CT检查采用管电流80 mA,能够保证图像噪声较小,同时明显降低辐射剂量.     

Dose classification scheme for computed tomography of the paranasal sinuses

PURPOSE: The purpose of this study was to define objective and reproducible standards for the quality of CT images as a function of radiation doses and therapeutic validity. MATERIALS AND METHODS: CT images of the paranasal sinuses of 145 patients (77 female, 68 male; 5-83 years old; mean age, 39.9 years) were classified both subjectively (with a view toward their validity for the planning of functional endoscopic sinus surgery, FESS) and objectively by defining the pixel noise (the standard deviation, STD, of the CT number) in a homogeneous region of interest (ROI), centered on the M. masseter and on the frontal lobe. These measurements were then compared to measurements obtained from scan images of a water-filled Perspex phantom. RESULTS: The pixel noise measured in the phantom images was nearly identical to the respective values on the M. masseter on the patient images. The use of an edge-enhancing reconstruction algorithm and low-dose protocols, with a pixel noise amounting to 70-90 Hounsfield Units (HU), are indicated for children, chronic sinusitis, and septum deviation, while standard protocols, with a pixel noise of 50-70 HU, are recommended for the preoperative planning and postoperative control of FESS. The pixel noise for high-dose protocols is less than 50 HU; nonetheless, such protocols should generally be avoided. CONCLUSION: The pixel noise measured in a water-filled Perspex phantom is indicative of the clinical potential and image quality of paranasal sinus CT scans. Alternatively, the M. masseter can be chosen as an ROI to measure the pixel noise in order to obtain a rough estimate of the image quality or radiation dose class.     

Assessment of a theoretical formalism for dose estimation in CT: an anthropomorphic phantom study

Dose assessment in computed tomography (CT) is challenging due to the vast variety of CT scanners and imaging protocols in use. In the present study, the accurateness of a theoretical formalism implemented in the PC program CT-EXPO for dose calculation was evaluated by means of phantom measurements. Phantom measurements were performed with four 1-slice, four 4-slice and two 16-slice spiral CT scanners. Firstly, scanner-specific _nCTDI_w values were measured and compared with the corresponding standard values used for dose calculation. Secondly, effective doses were determined for three CT scans (head, chest and pelvis) performed at each of the ten installations from readings of thermoluminescent dosimeters distributed inside an anthropomorphic Alderson phantom and compared with the corresponding dose values computed with CT-EXPO. Differences between standard and individually measured _nCTDI_w values were less than 16%. Statistical analysis yielded a highly significant correlation (P<0.001) between calculated and measured effective doses. The systematic and random uncertainty of the dose values calculated using standard _nCTDI_w values was about –9 and ±11%, respectively. The phantom measurements and model calculations were carried out for a variety of CT scanners and representative scan protocols validate the reliability of the dosimetric formalism considered—at least for patients with a standard body size and a tube voltage of 120 kV selected for the majority of CT scans performed in our study.     

The development and use of a chest phantom for optimizing scanning techniques on a variety of low-dose helical computed tomography devices

PURPOSE: To develop a chest phantom for determining the optimal scan conditions for chest computed tomography (CT) screening. METHODS: The basic structure of the phantom is an arms-elevated-positioned anthropomorphic chest phantom. The internal structure includes simulated tumors (which are assumed to be the target lesions of chest CT screening examinations) placed at the levels of the lung apex, the bifurcation of the trachea, and the lung base of both simulated lungs. The image contrast of the simulated tumors is taken as the difference in CT value and is specified as 1 of 2 target values: Delta270 and Delta100. An opening for placement of a dosimeter is provided on the central axis of the phantom. Initial field tests were conducted focusing on the scan conditions for chest CT screening. RESULTS: Images similar to those obtained in chest CT screening examinations and clearly showing pathologic changes were obtained. The dose measurement at the center was 2.0 mGy. The diameters of the simulated tumors that could be detected were 6 mm for Delta270 and 10 mm for Delta100. CONCLUSION: The use of this phantom makes it possible to determine the optimal scan conditions for chest CT screening based on objective evaluation criteria.     

Coronary artery calcium: a multi-institutional, multimanufacturer international standard for quantification at cardiac CT

PURPOSE: To develop a consensus standard for quantification of coronary artery calcium (CAC). MATERIALS AND METHODS: A standard for CAC quantification was developed by a multi-institutional, multimanufacturer international consortium of cardiac radiologists, medical physicists, and industry representatives. This report specifically describes the standardization of scan acquisition and reconstruction parameters, the use of patient size-specific tube current values to achieve a prescribed image noise, and the use of the calcium mass score to eliminate scanner- and patient size-based variations. An anthropomorphic phantom containing calibration inserts and additional phantom rings were used to simulate small, medium-size, and large patients. The three phantoms were scanned by using the recommended protocols for various computed tomography (CT) systems to determine the calibration factors that relate measured CT numbers to calcium hydroxyapatite density and to determine the tube current values that yield comparable noise values. Calculation of the calcium mass score was standardized, and the variance in Agatston, volume, and mass scores was compared among CT systems. RESULTS: Use of the recommended scanning parameters resulted in similar noise for small, medium-size, and large phantoms with all multi-detector row CT scanners. Volume scores had greater interscanner variance than did Agatston and calcium mass scores. Use of a fixed calcium hydroxyapatite density threshold (100 mg/cm(3)), as compared with use of a fixed CT number threshold (130 HU), reduced interscanner variability in Agatston and calcium mass scores. With use of a density segmentation threshold, the calcium mass score had the smallest variance as a function of patient size. CONCLUSION: Standardized quantification of CAC yielded comparable image noise, spatial resolution, and mass scores among different patient sizes and different CT systems and facilitated reduced radiation dose for small and medium-size patients.     

Variation of patient dose in head CT.

CT dose varies with both equipment related and operator dependent factors. Thermoluminescence dosimetry (TLD) was employed in two phantoms to investigate the variation in absorbed dose for head CT scans, using a cylindrical head CT dose phantom. Dose profiles were plotted and the computed tomography dose index (CTDI) calculated for a single 10 mm thick slice on 14 CT scanners. An anthropomorphic head phantom was also scanned from the base-of-skull to the vertex using 10/10 mm slices. The absorbed dose measured at the centre of the scan series is reported (Dmid). The mean CTDIw for the 14 scanners was 60.0 mGy, while the mean Dmid was 45.8 mGy. Dmid better represents the absorbed dose in human tissues. The CTDIw and Dmid normalized to mAs varied by up to a factor of 2.2 for the different scanners. Equipment related factors contribute to such variations. However, variations due to operator dependent factors such as the choice of exposure factors, scanning protocol and positioning technique must also be considered. When such factors are taken into account the absorbed dose received by the patient can vary considerably, by as much as 16.2 for lens dose. Increased awareness of the factors influencing CT dose and the standardization of scanning protocols is recommended.     

Comparing Effective Doses During Image-Guided Core Needle Biopsies with Computed Tomography Versus C-Arm Cone Beam CT Using Adult and Pediatric Phantoms

Purpose

To compare the effective doses of needle biopsies based on dose measurements and simulations using adult and pediatric phantoms, between cone beam c-arm CT (CBCT) and CT.

Method

Effective doses were calculated and compared based on measurements and Monte Carlo simulations of CT- and CBCT-guided biopsy procedures of the lungs, liver, and kidney using pediatric and adult phantoms.

Results

The effective doses for pediatric and adult phantoms, using our standard protocols for upper, middle and lower lungs, liver, and kidney biopsies, were significantly lower under CBCT guidance than CT. The average effective dose for a 5-year old for these five biopsies was 0.36 ± 0.05 mSv with the standard CBCT exposure protocols and 2.13 ± 0.26 mSv with CT. The adult average effective dose for the five biopsies was 1.63 ± 0.22 mSv with the standard CBCT protocols and 8.22 ± 1.02 mSv using CT. The CT effective dose was higher than CBCT protocols for child and adult phantoms by 803 and 590 % for upper lung, 639 and 525 % for mid-lung, and 461 and 251 % for lower lung, respectively. Similarly, the effective dose was higher by 691 and 762 % for liver and 513 and 608 % for kidney biopsies.

Conclusions

Based on measurements and simulations with pediatric and adult phantoms, radiation effective doses during image-guided needle biopsies of the lung, liver, and kidney are significantly lower with CBCT than with CT.

     

Radiation exposure of patients undergoing whole-body dual-modality 18F-FDG PET/CT examinations.

We investigated radiation exposure of patients undergoing whole-body 18F-FDG PET/CT examinations at 4 hospitals equipped with different tomographs. METHODS: Patient doses were estimated by using established dose coefficients for 18F-FDG and from thermoluminescent measurements performed on an anthropomorphic whole-body phantom. RESULTS: The most relevant difference between the protocols examined was the incorporation of CT as part of the combined PET/CT examination: Separate low-dose CT scans were acquired at 2 hospitals for attenuation correction of emission data in addition to a contrast-enhanced CT scan for diagnostic evaluation, whereas, at the other sites, contrast-enhanced CT scans were used for both purposes. Nevertheless, the effective dose per PET/CT examination was similar, about 25 mSv. CONCLUSION: The dosimetric concepts presented in this study provide a valuable tool for the optimization of whole-body 18F-FDG PET/CT protocols. Further reduction of patient exposure can be achieved by modifications to the existing hardware and software of PET/CT systems.