Comparison of Z-axis automatic tube current modulation technique with fixed tube current CT scanning of abdomen and pelvis

PURPOSE: To compare image quality, diagnostic acceptability, and radiation exposure associated with 16-section multi-detector row computed tomographic (CT) examinations of abdomen and pelvis performed with z-axis modulation technique of automatic tube current modulation and with manual selection of fixed tube current. MATERIALS AND METHODS: Sixty-two consecutive subjects (mean age, 60 years; age range, 19-84 years; male-to-female ratio, 35:27) underwent follow-up CT of abdomen and pelvis with use of a 16-section multi-detector row scanner and z-axis modulation technique (10.5-12.0-HU noise index, 10-380 mA). Scanning parameters included 140 kVp, 0.5-1.0-second gantry rotation time, 0.938:1 beam pitch, and 5-mm reconstructed section thickness. For each subject, images obtained with z-axis modulation were compared with previous images obtained with fixed tube current (200-300 mA) and with other parameters identical. Images were compared for noise and diagnostic acceptability by two subspecialty radiologists using a five-point scale (1, unacceptable; 3, acceptable; 5, excellent) at five levels: upper liver at diaphragm, porta hepatis, right kidney hilum, iliac crest, and upper margin of acetabulum. Tube current and gantry rotation time used for acquisitions at these levels were recorded. Data were analyzed with parametric and nonparametric statistical tests. RESULTS: Although no significant differences were found (P =.34), images acquired with z-axis modulation at the levels of the upper liver (diaphragm) and acetabulum had a higher noise and lower diagnostic quality, compared with images acquired with fixed tube current. Compared with fixed tube current, z-axis modulation resulted in tube current-time product reduction in 54 (87%) of 62 examinations (mean reduction, 71.2 mAs) and increase in eight (13%) (mean increase, 17.0 mAs). CONCLUSION: Compared with manually selected fixed tube current, z-axis automatic tube current modulation resulted in reduced tube current-time product and similar image noise and diagnostic acceptability at CT of abdomen and pelvis.     

Metallic prosthesis: technique to avoid increase in CT radiation dose with automatic tube current modulation in a phantom and patients

The institutional review board approved this Health Insurance Portability and Accountability Act-compliant study protocol, with waiver of informed consent. The purpose of the study was to retrospectively evaluate the combined automatic tube current modulation technique in patients with orthopedic metallic prostheses. Five hundred abdominal-pelvic computed tomographic (CT) studies performed with combined modulation technique were reviewed to identify nine patients with metallic prostheses (mean age, 66 years; range, 35-86 years; male-female ratio, 5:4). On the basis of age and transverse abdominal images, these patients were matched with nine others with no metallic prostheses (mean age, 56 years; range, 36-72 years; male-female ratio, 4:5) who were control patients. Images were graded for extent and severity of streak artifacts (grade 1, streak artifact present but not substantially compromising evaluation of adjacent structures; grade 2, streak artifact present and slightly compromising evaluation of adjacent structures; and grade 3, streak artifact present and severely compromising evaluation of adjacent structures). Student t test was performed for statistical analysis. There was no difference in mean effective tube current-time product between study and control patients (P > .49). With automatic tube current modulation, an increase in CT dose caused by metallic prostheses can be successfully avoided.     

64层螺旋CT自动毫安技术对降低下肢动脉 血管成像辐射剂量的研究

目的 探讨64层螺旋CT自动毫安技术(ATCM)对降低下肢动脉血管成像辐射剂量的可行性。方法 通过预实验改变噪声指数(NI)调节管电流,得出NI值为10 HU时图像质量与辐射剂量匹配较佳。搜集52例疑有下肢动脉闭塞糖尿病及高血压病患者病例,随机分为实验组和对照组行自肾下腹主动脉至足底的CT容积扫描,每组病例26例。对照组采用常规固定300 mA(FM组),实验组采用NI为10 HU的自动毫安(ATCM组)技术。分别选择肾下腹主动脉、髋关节水平股动脉、 NFDB1 动脉及踝关节胫前动脉平面作为观察平面,对两种毫安技术的图像质量、图像噪声和辐射剂量均采用 t 检验方法进行统计学分析。结果 FM组的辐射剂量CTDI_vol为10.81 mGy,平均DLP为(1280.4±124.7)mGy·cm,ATCM组的平均CTDI_vol为(5.14±1.23)mGy,平均DLP为(582.86±150.41)mGy·cm,比FM组降低约54.4%,两组差异有统计学意义(CTDI_vol、DLP的 t 值分别为13.13、17.36, P 值均<0.05)。FM组的4个选取平面图像噪声分别为13.8±4.04、14.53±2.79、5.69±1.38、(4.75±1.38) HU。ATCM组分别为10.76±2.11、12.13±2.96、10.47±2.96、(6.17±1.53) HU,两组差异有统计学意义( t 值分别为2.911、3.038、7.287及3.186, P 值均<0.05),ATCM组肾下腹主动脉、髋关节水平股动脉层面的图像噪声较FM组的低,而 NFDB1 动脉、踝关节胫前动脉层面的图像噪声较FM组的高,ATCM组图像噪声变异度小,图像更加均匀。FM组图像质量评分平均为2.654±0.562,ATCM组图像评分平均为2.462±0.582,两组图像质量评分差异无统计学意义( t =1.21, P ＞0.05)。结论 使用噪声指数为10的下肢动脉ATCM技术,在保证图像质量的情况下可以降低辐射剂量约54.4%。     

自动管电流调制技术结合迭代重建算法在成人盆腔低剂量CT中的初步应用

目的 探讨成人盆腔低剂量CT成像中应用自动管电流调制技术（ATCM）结合适应性统计迭代重建（ASIR）算法降低辐射剂量的可行性。方法 将45例先后行常规剂量和低剂量盆腔CT检查患者的图像纳入研究,共获得3组图像：A组为常规剂量图像,采用机器默认的滤波反投影重建（FBP）;B组为低剂量图像,采用ASIR,并分别按不同ASIR权重值分为B₁组（ASIR 0）和 B₂组（ASIR 50%）。由两位放射专家对3组图像质量进行评价。结果 B组的容积CT剂量指数（CTDI_vol）、剂量长度乘积（DLP）和有效剂量（E）均显著低于A组（t=12.30、12.10、12.15,P<0.05）。与A、B₂组相比,B₁组的影像噪声（SD）值增加（t=9.14、8.95,P<0.05）,而主观图像质量评分、信噪比（SNR）和对比噪声比（CNR）显著降低（t=8.72、10.62、11.83和8.40、9.55、11.42,P<0.05）。A组与B₂组差异无统计学意义。结论 应用自动管电流调制技术和ASIR 50%重建进行成人盆腔CT扫描可以明显降低辐射剂量而不降低图像质量。     

Comparison of angular and combined automatic tube current modulation techniques with constant tube current CT of the abdomen and pelvis

OBJECTIVE: The objective of our study was to compare image quality and radiation dose associated with abdominopelvic CT using combined modulation, angular modulation, and constant tube current. CONCLUSION: Compared with using a constant tube current to scan the abdomen and pelvis, the use of a combined modulation technique results in a substantial reduction (42-44%) in radiation dose with acceptable image noise and diagnostic acceptability.     

基于胸围的管电流调制技术在冠状动脉CT造影应用的可行性探讨

目的：建立以胸围为参考指标调节冠状动脉CT造影（CTA）成像管电流的函数模型,并探讨其个体化管电流调节模型的临床应用的可行性。方法：连续选取68例以自动管电流调节模式扫描的胸部CT病例,建立胸围与管电流之间的函数模型;再应用其建立的胸围-管电流函数模型对另外的连续64例病例进行冠状动脉CTA成像,对图像质量进行评分,并记录噪声值、管电流、辐射剂量等指标。结果：胸围与管电流之间的函数模型以POW函数拟合度最好（R²=0.691,P<0.05）。应用胸围-管电流函数模型冠状动脉CTA成像的平均图像等级评分为（3.38±0.72）分,噪声值为（31.02±3.97）HU,管电流为（390.63±89.30）mA,CTDI_vol为（34.83±10.72）mGy,DLP为（751.67±175.16）mGy·cm。 结论：以胸围为参考指标调节冠状动脉CTA成像管电流方法,能够较好地实现个体化辐射剂量控制,临床应用具有可行性。     

Z轴自动管电流调制技术在头颈部CT扫描血管成像中对甲状腺剂量的降低

目的 评价在进行头颈部CT扫描血管成像时,Z轴自动管电流调制技术(ATCM)对减少甲状腺的辐射剂量的作用及对图像噪声的影响。方法 回顾性地分析140例头颈部CT增强血管成像的病例,其中用固定管电流技术和 Z 轴自动管电流调节技术各70例,观察其成像质量,记录其客观噪声水平(由CT图像衰减值的标准差进行评估),并比较其单次扫描的加权CT剂量指数CTDI_w,管电流mA及剂量长度乘积DLP。结果 在扫描范围、扫描参数(管电压、螺距、准直器厚度等)、造影剂注射速率和注射部位完全相同的情况下,固定管电流技术和 Z 轴自动管电流调节技术的图像质量相同,甲状腺图像噪声分别为10.14和13.64 HU。单次扫描的加权CT剂量指数CTDI_w(mGy)分别为(43.22±1.42)和(35.99±1.31) mGy。剂量长度乘积分别为(1514.45±5.56)和(1121.39±5.51)mGy·cm, 剂量长度乘积降低约25.95%。结论 Z 轴自动管电流调节技术能有效降低总曝光量和累计剂量长度乘积,可以有效地降低患者的辐射剂量,特别是像甲状腺和眼晶体等射线敏感组织器官的辐射剂量降低,减少其辐射危害,但是图像噪声略有增加。     

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.     

Effective dose and image quality evaluations of an automatic CT tube current modulation system with an anthropomorphic phantom

The purpose of this study was to evaluate the consequences of different choices of acquisition parameters on the actual image noise and on the patient dose with an automatic tube current modulation system. The CT investigated was a GE Lightspeed 16-slice and an anthropomorphic phantom was used to simulate the patient. Several acquisitions were made varying noise index (NI), kilovoltage and pitch values. Tube current values were compared for the different acquisitions. Patient dose was evaluated in terms of volumetric computed tomography dose index (CTDI_vol) and also as effective dose. The noise actually present in the images was analyzed by a region of interest analysis considering representatively phantom sections in the regions of the shoulders, of the lungs and of the abdomen. The obtained results generally evidenced a good agreement between the noise index and the measured noise for the abdomen sections, whereas for the shoulders and the lungs sections the measured noise was respectively greater and lower of the NI. Varying the kV the automatic current modulation system provided images with a substantially constancy of the actual noise and of the patient dose. An increase of the pitch generally decreased the patient dose, whereas the noise was slightly greater for the lowest pitch and almost constant for the other pitch values. This study outlines some important relationships between an automatic tube current modulation system and other CT acquisition parameters, providing useful informations for the choice requested by radiologists in the task of optimization of the CT acquisition protocols. Unless there are other considerations in place, pixel pitches below 1.375 should be avoided, and kVp settings can be changed with no real impact on dose or image noise.     

螺旋CT自动管电流技术在胸部检查中的辐射防护价值

目的探讨螺旋CT自动管电流技术在胸部检查中辐射防护价值。方法实验组和对照组各30例，分别采用CT自动管电流（噪声指数=10．00）和固定管电流（200mA）技术进行胸部检查，对所获图像噪声特性、细节显示程度及CT设备本身剂量信息显示结果进行比较。结果实验组和对照组主动脉弓上层面、气管隆突和横膈上层面肺野内CT值差异不具显著性（P=0．136，0．289，0．176），主动脉弓上层面图像背景标准差差异无统计学意义（P=0．121），但气管隆突和横膈上层面图像背景标准差差异有统计学意义（P=0．001，0．000），实验组大于对照组。实验组和对照组肺内细小血管、细支气管、叶间裂等细微结构显示程度以及病变形态特征显示无差别。两组图像主动脉弓上层面均可见线状假影，但实验组有假影的层面多，图像噪声颗粒比对照组明显。实验组CTDIvol和剂量长度乘积（DLP）约为对照组的1／3。结论正确使用CT自动管电流技术可以在确保图像质量同时降低受检者所受辐射剂量。     

Automatic tube current modulation for whole-body polytrauma CT with immobilization devices: is there an increase in radiation dose and degradation of image quality?

The objective of this study was the assessment of the image quality and radiation dose in polytrauma CT using immobilization devices. An anthropomorphic whole body and a liver phantom were scanned on a 128-slice CT scanner with four different protocols using automatic tube current modulation (120 kVp, 150 ref. mAs; 120 kV, 200 ref. mAs; 140 kVp, 150 ref. mAs; and 140 kVp, 200 ref. mAs) and four different setups (no immobilization device (setup A), vacuum mattress 1 (setup B), vacuum mattress 2 (setup C), and spineboard (setup D)). Qualitative and quantitative image quality parameters and radiation dose were assessed. Image noise increased on average by 6.6, 11.2, and 9.4 %, and CNR decreased by 11.2, 13.9, and 6.5 for setups B, C, and D, respectively, compared with setup A. The CTDI_vol increased up to 6 % using immobilization devices. Severe streak artifacts, provoked by the inflation valve of the mattresses were detected at the level of the head and shoulder. Applying immobilization devices for whole-body CT with automatic tube current modulation increases the radiation dose and decreases the quantitative image quality slightly. Severe artifacts, induced by the inflation valve of the mattress, can influence the diagnostic accuracy at the level of the head and shoulder.     

Automatic exposure control in CT: the effect of patient size,anatomical region and prescribed modulation strength on tube current and image quality

Objectives

To study the effect of patient size, body region and modulation strength on tube current and image quality on CT examinations that use automatic tube current modulation (ATCM).

Methods

Ten physical anthropomorphic phantoms that simulate an individual as neonate, 1-, 5-, 10-year-old and adult at various body habitus were employed. CT acquisition of head, neck, thorax and abdomen/pelvis was performed with ATCM activated at weak, average and strong modulation strength. The mean modulated mAs (mAs_mod) values were recorded. Image noise was measured at selected anatomical sites.

Results

The mAs_mod recorded for neonate compared to 10-year-old increased by 30 %, 14 %, 6 % and 53 % for head, neck, thorax and abdomen/pelvis, respectively, (P?mod was lower than the preselected mAs with the exception of the 10-year-old phantom. In paediatric and adult phantoms, the mAs_mod ranged from 44 and 53 for weak to 117 and 93 for strong modulation strength, respectively. At the same exposure parameters image noise increased with body size (P?Conclusions The ATCM system studied here may affect dose differently for different patient habitus. Dose may decrease for overweight adults but increase for children older than 5 years old. Care should be taken when implementing ATCM protocols to ensure that image quality is maintained.

Key Points

? ATCM efficiency is related to the size of the patient’s body. ? ATCM should be activated without caution in overweight adult individuals. ? ATCM may increase radiation dose in children older than 5 years old. ? ATCM efficiency depends on the protocol selected for a specific anatomical region. ? Modulation strength may be appropriately tuned to enhance ATCM efficiency.     

Optimization of Z-axis automatic exposure control for multidetector row CT evaluation of neck and comparison with fixed tube current technique for image quality and radiation dose

BACKGROUND AND PURPOSE: Z-axis automatic exposure control (AEC) technique automatically adjusts tube current based on size of body region scanned. The purpose of the current study was to compare diagnostic acceptability, noise, and radiation exposure of multidetector row CT (MDCT) of neck performed with z-axis AEC and with fixed current. MATERIALS AND METHODS: Two study groups of 26 patients each underwent MDCT of neck using z-axis AEC with 8 noise index (NI), 150-440 mA, and 10 NI, 75-440 mA, respectively. A control group consisting of another 26 patients underwent MDCT of neck with fixed-current technique (300 mA). Objective noise and mean tube current-time products (mA . s) were recorded. Two radiologists evaluated images for diagnostic acceptability and subjective noise on a 5-point scale. RESULTS: All CT examinations of study and control groups were diagnostically acceptable, though objective noise was significantly more with z-axis AEC (shoulder: NI 8, 20.6 +/- 6.2 HU; NI 10, 22.2 +/- 4.6 HU) than with fixed current (16.2 +/- 6 HU) (P = .01). There was no significant difference between AEC and fixed current in diagnostic acceptability and subjective noise (P = .22-.42). AEC resulted in significant radiation dose reduction (NI 8, 186.3 +/- 20.5 mA . s; NI 10, 158.1 +/- 21.2 mA x s), compared with fixed current (235 +/- 21.8 mA x s). CONCLUSION: Z-axis AEC resulted in similar subjective noise and diagnostic acceptability, with radiation dose reduction of 21% for NI of 8 and 33% for NI of 10, respectively, for MDCT evaluation of neck, compared with those of fixed current technique.     

Detection of urinary tract stones at low-radiation-dose CT with z-axis automatic tube current modulation: phantom and clinical studies

PURPOSE: To evaluate depiction of urinary tract calculi at computed tomography (CT) with a z-axis modulation technique at various noise indexes to reduce radiation dose and preserve image quality. MATERIALS AND METHODS: Sixteen radiopaque kidney stones (2.5-19.2 mm in diameter) were embedded in the collecting systems of two bovine kidneys immersed in a water bath. A kidney phantom was made by placing the kidneys in an elliptical Plexiglas phantom (32 x 20 x 20 cm) filled with physiologic saline. The phantom was scanned at 16-detector row CT with a fixed tube current (300 mA) and z-axis modulation at noise indexes of 14, 20, 25, 35, and 50; remaining imaging parameters were held constant. Two abdominal radiologists reviewed images from most to least noisy. Images were evaluated for presence of stones and size, site, and attenuation value of each stone. Readers also graded conspicuity and margins of each stone on a five-point scale. In addition, follow-up studies of 22 patients (mean age, 46 years; range, 26-57 years; male-female ratio, 14:8) with kidney and ureteral stones who underwent CT with z-axis modulation (noise index, 14 and 20) were evaluated in the same manner. Statistical analysis was performed with the Student t test, Wilcoxon signed rank test, and kappa test of interobserver agreement. Institutional review board approval was obtained, and informed consent was not needed. RESULTS: In the phantom study, all 16 stones were identified on images obtained with a fixed tube current and z-axis modulation at noise indexes of 14, 20, and 25 (with a reduction in radiation dose of up to 77% compared with that of fixed tube current scanning). Three stones (<5 mm) were not visualized with z-axis modulation at noise indexes of 35 and 50. No significant difference was shown for conspicuity of kidney stones in 22 patients who underwent CT with z-axis modulation (with a 43%-66% reduction in radiation dose) when compared with results of previous fixed tube current studies (P > .05). CONCLUSION: Kidney stones (< or =2.5 mm) can be adequately depicted with the z-axis modulation technique, with a 56%-77% reduction in radiation dose. In patients with urinary tract stones, the technique results in a 43%-66% reduction in radiation dose at noise indexes of 14 and 20 without compromising stone depiction.     

Noise-based tube current reduction method with iterative reconstruction for reduction of radiation exposure in coronary CT angiography

Purpose

To investigate the potential of noise-based tube current reduction method with iterative reconstruction to reduce radiation exposure while achieving consistent image quality in coronary CT angiography (CCTA).

Materials and methods

294 patients underwent CCTA on a 64-detector row CT equipped with iterative reconstruction. 102 patients with fixed tube current were assigned to Group 1, which was used to establish noise-based tube current modulation formulas, where tube current was modulated by the noise of test bolus image. 192 patients with noise-based tube current were randomly assigned to Group 2 and Group 3. Filtered back projection was applied for Group 2 and iterative reconstruction for Group 3. Qualitative image quality was assessed with a 5 point score. Image noise, signal intensity, volume CT dose index, and dose-length product were measured.

Results

The noise-based tube current modulation formulas were established through regression analysis using image noise measurements in Group 1. Image noise was precisely maintained at the target value of 35.00 HU with small interquartile ranges for Group 2 (34.17–35.08 HU) and Group 3 (34.34–35.03 HU), while it was from 28.41 to 36.49 HU for Group 1. All images in the three groups were acceptable for diagnosis. A relative 14% and 41% reduction in effective dose for Group 2 and Group 3 were observed compared with Group 1.

Conclusion

Adequate image quality could be maintained at a desired and consistent noise level with overall 14% dose reduction using noise-based tube current reduction method. The use of iterative reconstruction further achieved approximately 40% reduction in effective dose.     

自动管电流调制技术结合迭代重建算法在低剂量CT肺动脉成像中的应用

目的 探讨自动管电流调制技术结合idose4迭代重建算法在低剂量CT肺动脉成像（CTPA）中的应用价值。方法 连续收集行CTPA的受检者80例,采用随机数字表法将其分为对照组和实验组,每组40例。两组管电压均采用80 kV。对照组固定管电流为180 mAs,采用滤波反投影法重建,获得A组图像;实验组采用自动管电流调制技术,分别采用滤波反投影法和idose4迭代重建算法重建,获得B组和C组图像。统计分析A、B和C 3组的图像质量主观评分、肺动脉平均CT值、图像噪声值、肺动脉的信噪比（SNR）和对比噪声比（CNR）。比较实验组和对照组的有效剂量（E）。结果 实验组、对照组的E分别为（1.2±0.2）和（1.9±0.3）mSv,实验组的ED较对照组明显降低了36.8%（t=-3.998,P<0.05）。3组图像质量主观评分均≥3,满足临床诊断要求,且差异无统计学意义（P>0.05）。3组SNR、CNR和噪声值组间比较,差异有统计学意义（F=10.4 11、7.630、13.021,P<0.05）;而肺动脉平均CT值差异无统计学意义（P>0.05）。结论 自动管电流调制技术结合idose4迭代重建算法在低剂量CTPA检查中,在获得良好图像质量的同时,可进一步降低辐射剂量。     

64层螺旋CT自动管电流调节技术在婴幼儿胸部低剂量CT扫描中应用的可行性研究

目的 探讨应用64层螺旋CT(MSCT),采用适宜噪声指数的Z轴自动管电流调节技术获得稳定的婴幼儿胸部CT扫描图像,并降低射线剂量的可行性研究.方法 选取100例根据临床需要行64层MSCT胸部扫描的3岁以下婴幼儿患儿,按年龄和体重用随机数字表法分为2组,每组各50例.对照组(中位年龄1.7岁)采用固定管电流扫描技术:1岁以下120 mA,≥1岁者150 mA;研究组(中位年龄1.4岁)采用z轴自动管电流调节扫描技术:1岁以下预设噪声指数为8,≥1岁为9,毫安设置范围为10～250 mA,其余扫描参数条件2组一致.记录2组CT剂最指数(CTDIvol)值.2名医师独立对图像质量采用5分制进行评估,＞13分被认为符合临床诊断要求.采用Kappa方法评价不同观察者评定结果间的一致性.结果 对照组的平均客观噪声和CTDIvol分别为(4.78 4±0.58)和(6.68±0.62)mGy,研究组的平均客观噪声和CTDIvol分别是(7.84±0.66)和(2.34±0.71)mGy;研究组和对照组的平均管电流分为(41.6 4±11.6)和(99.2±7.7)mAs.研究组的平均CTDIvol较对照组下降了 65%.研究组和对照组平均图像质量评分分别为(3.46±0.40)和(4.65±0.46)分,所用图像均获得了满意的临床诊断要求,2名医师的评估结果获得了中等一致性(Kappa值分别为0.474和0.536).结论 64层MSCT自动管电流调节技术应用于婴幼儿胸部扫描时可以在降低射线剂量的基础卜获得稳定的图像质量,噪声指数设定在8或9较为适宜.     

Dose reduction in CT by on-line tube current control: principles and validation on phantoms and cadavers

We investigated approaches to reducing the dose in CT without impairing image quality. Dose can be reduced for non-circular object cross-sections without a significant increase in noise if X-ray tube current is reduced at angular tube positions where the X-ray attenuation by the patients is small. We investigated different schemes of current modulation during tube rotation by simulation and phantom measurements. Both pre-programmed sinusoidal modulation functions and attenuation-based on-line control of the tube current were evaluated. All relevant scan parameters were varied, including constraints such as the maximum modulation amplitude. A circular, an elliptical and two oval water phantoms were used. Results were validated on six cadavers. Dose reduction of 10–45 % was obtained both in simulations and in measurements for the different non-circular phantom geometries and current modulation algorithms without an increase in pixel noise values. On-line attenuation-based control yielded higher reductions than modulation by a sinusoidal curve. The maximal dose reduction predicted by simulations could not be achieved due to limits in the modulation amplitude. In cadaver studies, a reduction of typically 20–40 % was achieved for the body and about 10 % for the head. Variations of our technique are possible; a slight increase in nominal tube current for high-attenuation projections combined with attenuation-based current modulation still yields significant dose reduction, but also a reduction in the structured noise that may obscure diagnostic details. We conclude that a significant reduction in dose can be achieved by tube current modulation without compromising image quality. Attenuation-based on-line control and a modulation amplitude of at least 90 % should be employed. Received: 1 April 1998; Revision received: 9 July 1998; Accepted: 10 July 1998     

Multislice helical CT of the heart with retrospective ECG gating: reduction of radiation exposure by ECG-controlled tube current modulation

Our objective was to evaluate image quality and radiation exposure of retrospectively ECG-gated multislice helical CT (MSCT) investigations of the heart with ECG-controlled tube current modulation. One hundred patients underwent MSCT scanning (Somatom VolumeZoom, Siemens, Forchheim, Germany) for detection of coronary artery calcifications. A continuous helical data set of the heart was acquired in 50 patients (group 1) using the standard protocol with constant tube current, and in 50 patients (group 2) using an alternative protocol with reduced radiation exposure during the systolic phase. The standard deviations (SD) of predefined regions of interest (ROIs) were determined as a measure of image noise and were tested for significant differences. There was no significant difference between group 1 and group 2 with respect to image noise. Radiation exposure with and without tube current modulation was 1.0 and 1.9 mSv (p<0.0001), respectively, for males and 1.4 and 2.5 mSv (p<0.0001), respectively, for females; thus, there was a mean dose reduction of 48% for males and 45% for females, respectively. The ECG-controlled tube current modulation allows significant dose reduction when performing retrospectively ECG-gated MSCT of the heart. Electronic Publication     

自动管电流联合迭代算法在颈部CT扫描中的应用研究

目的 探讨自动管电流技术联合迭代算法在颈部CT扫描中的应用价值。方法 连续收集行颈部CT扫描（平扫+增强）的患者80例,按随机数表法分为实验组和对照组,各40例。实验组平扫、增强均采用自动管电流技术+迭代算法重建,对照组平扫、增强均采用固定管电流200 mAs+滤波反投影法重建。对两组平扫、增强图像均进行主观评分、客观评价[甲状腺、斜方肌、胸锁乳突肌、颌下腺、翼外肌、脑组织的噪声值（SD）]。比较两组的有效剂量（E）。结果 实验组平扫、增强的E均低于对照组,差异有统计学意义（t=-2.451、-2.451,P<0.05）。实验组平扫、增强图像主观评分均高于对照组,差异有统计学意义（Z=-1.969、-2.056,P<0.05）;实验组平扫、增强图像甲状腺、斜方肌、胸锁乳突肌、颌下腺的SD均低于对照组,差异有统计学意义（t=2.400、2.516、2.120、2.411、4.134、4.674、2.711、2.892,P<0.05）。两组平扫、增强图像翼外肌、脑组织的SD差异均无统计学意义（P>0.05）。结论 自动管电流技术联合迭代算法在颈部CT扫描中,在提高图像质量的同时,可有效降低辐射剂量。