Optimized control of X-ray exposure and image noise using a particular multislice CT scanner

Patient dose reduction in computed tomography (CT) always results in a trade off between radiation exposure and image quality. There are few reports that estimate the relationship between image quality and X-ray exposure in CT examinations as one optimal index. The purpose of this study was to determine the optimal parameter settings enabling a low radiation exposure without compromising image quality using a particular 4-row multislice CT (MSCT) scanner (Aquilion VZ 4-slice CT scanner, Toshiba Medical Systems Corporation, Otawara, Tochigi, Japan). Normalized dose divided by image noise for helical pitches (nDNR: normalized dose to noise ratio) were calculated in consideration of beam collimation and tube current-time product. Optimal tube current-time product was calculated using the nDNR for the helical pitches based on user-defined standards of quality of the CT image. As a result, the nDNR proved to be well-supported to decrease the patient exposure in various exposure conditions of MSCT scans; however, the dose and image noise did not show a linear relation to the helical pitch. In conclusion, nDNR can be applied to patient dose reduction while keeping an acceptable image quality using a particular 4-row MSCT scanner.     

Effective doses in subjects undergoing computed tomography cardiac imaging with the 256-multislice CT scanner

BACKGROUND: The 256-multislice CT (256MSCT) obtains volumetric data with 128-mm coverage in a single rotation. This coverage allows satisfactory visualization of the whole heart, allowing the 256MSCT to visualize the cardiac chambers and coronary arteries by cine scan without ECG gating. These characteristics provide a solution to the problems of MSCT. Although a wider beam width provides more efficient imaging over a wider coverage area, patient doses with the 256MSCT are of considerable concern. OBJECTIVE: We assessed potential radiation exposure with the 256MSCT in a cardiac CT protocol and compared the results to those with 16- and 64MSCT (collimated 64x0.5mm using 256MSCT). METHODS: Organ or tissue doses were measured in an anthropomorphic phantom under a coronary artery imaging protocol with the 256MSCT in cine scan mode without ECG gating, and with the 16- and 64MSCT in helical scan mode with ECG gating. RESULTS: Average effective doses were 22.8mSv for the 16MSCT, 27.8mSv for the 64MSCT and 14.1mSv for the 256MSCT. The 16- and 64MSCT doses were thus approximately 1.6- and 2.0-fold higher than those of the 256MSCT. CONCLUSIONS: Use of the 256MSCT in cardiac volumetric cine imaging offers lower radiation exposure than 16- and 64MSCT, and suggests the potential of this equipment in single-beat cardiac imaging without ECG gating. This effective dose is acceptable for routine cardiac imaging.     

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     

Dose and pitch relationship for a particular multislice CT scanner.

OBJECTIVE: With single-slice helical CT, an increased pitch can decrease the radiation dose to the patient if all other parameters are constant. The purpose of this study was to determine whether the same relationship holds for a particular multislice helical CT system (Somatom Plus 4 VZ multislice helical CT scanner, version A11A) in our department. CONCLUSION: The measured radiation dose to the phantom was identical for all pitch selections on the multislice helical CT system we tested. This unexpected result was because of an automatic proportionate increase in the tube current when the pitch selection was increased. Radiologists and physicists should exercise caution when extrapolating dose reduction strategies from single-slice to multislice helical CT systems, and they must acquire a detailed understanding of the multislice helical CT scanner of their chosen manufacturer.     

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     

Radiation cost of helical high-resolution chest CT

OBJECTIVE: In our department, most high-resolution CT (HRCT) scans of the lungs are performed in conjunction with a standard helical examination to assess the entire chest. This requires scanning the patient twice. The goal of this study was to determine if the radiation dose could be decreased by performing a single combination helical scan of the chest from which both 5-mm standard and 1.25-mm HRCT images could be obtained. CONCLUSION: Because the total measured radiation dose is 32% greater from a single combination helical HRCT scan of the chest versus separate standard helical plus axial HRCT scans, helical HRCT is not a clinically advisable technique.     

Techniques and applications of automatic tube current modulation for CT

Introduction of slip-ring technology with subsequent development of single- and multi-detector row helical computed tomographic (CT) scanners have expanded the applications of CT, leading to a substantial increase in the number of CT examinations being performed. Owing to concerns about the resultant increase in associated radiation dose, many technical innovations have been introduced. One such innovation is automatic tube current modulation. The purpose of automatic tube current modulation is to maintain constant image quality regardless of patient attenuation characteristics, thus allowing radiation dose to patients to be reduced. This review discusses the principles, clinical use, and limitations of different automatic tube current modulation techniques.     

Radiation dose saving through the use of cone-beam CT in hearing-impaired patients

Purpose

Bionic ear implants provide a solution for deafness. Patients treated with these hearing devices are often children who require close follow-up with frequent functional and radiological examinations; in particular, multislice computed tomography (MSCT). Dental volumetric cone-beam CT (CBCT) has been reported as a reliable technique for acquiring images of the temporal bone while delivering low radiation doses and containing costs. The aim of this study was to assess, in terms of radiation dose and image quality, the possibility of using CBCT as an alternative to MSCT in patients with bionic ear implants.

Materials and methods

One hundred patients (mean age 26 years, range 7–43) with Vibrant SoundBridge implants on the round window underwent follow-up: 85 with CBCT and 15 with MSCT. We measured the average tissue-absorbed doses during both MSCT and CBCT scans. Each scan was focused on the temporal bone with the smallest field of view and a low-dose protocol. In order to estimate image quality, we obtained data about slice thickness, high- and low-contrast resolution, uniformity and noise by using an AAPM CT performance phantom.

Results

Although the CBCT images were qualitatively inferior to those of MSCT, they were sufficiently diagnostic to allow evaluation of the position of the implants. The effective dose of MSCT was almost three times higher than that of CBCT.

Conclusions

Owing to low radiation dose and sufficient image quality, CBCT could be considered an adequate technique for postoperative imaging and follow-up of patients with bionic ear implants.     

多层螺旋CT低剂量扫描在眼眶部外伤检查中的应用

目的 探讨眼眶部外伤检查时MSCT扫描的最适低剂量.方法 选择30例眶壁骨折患者的横轴面骨折层面影像,通过图像空间添加噪声软件模拟出30、70、100、140、170、200 mA 6种低剂量影像,根据影像质量及骨折等显示情况进行评价,找出满足诊断需求的最适球管电流量后进行临床应用.同时记录容积CT剂量指数(CTDIvol)、剂量长度乘积(DLP)、有效剂量(ED).影像质量按影像层次、噪声、解剖结构及能否满足诊断要求的显示情况而采用好、较好、一般、差、很差的5级制评价.等级资料使用秩和检验进行统计分析,计数资料采用X2检验.结果 在使用常规剂量300 mA扫描条件时30例眼眶外伤患者图像中显示眶骨骨折30例、眶内气肿19例、眼肌损伤12例、眶内异物1例.在低剂最模拟图像中,不同剂量的图像质量差异有统计学意义(X2=102.009,P=0.000).当剂量降低至70 mA时上述所有临床征象仍可清晰显示和准确诊断,但图像质量评价为好1例、较好8例、一般21例,无差及很差.当模拟剂量管电流降低至100 mA时,图像质量评价为好9例、较好17例、一般4例,无差及很差;且同常规剂量差异无统计学意义(P>0.05).临床实际应用100 mA管电流检查20例眼眶外伤患者,所得图像质量为好5例、较好13例、一般2例,无差及很差.CTDIvol、DLP、ED分别为20.84 mGy、125.04 mGy·cm、0.29 mSv.与常规剂量(300 mA)CTDIvol、DLP、ED分别为62.53 mGy、375.18 mGy·cm、0.86 mSv,比较管电流100 mA进行扫描时,ED下降了70%.结论 眼眶部外伤进行MSCT扫描时,管电流为100 mA所得图像即可满足临床诊断要求,又可明显降低对患者的辐射剂量.     

Current status of coronary multislice CT angiography

Multislice computed tomography (MSCT) is an emerging technique which has an enormous potential to improve the current practice of coronary artery imaging. This article reviews the current status of coronary MSCT angiography (MSCTA) with emphasis on the imaging techniques and clinical utilities of 16-slice CTA. Results and experiences gained from coronary MSCTA in the past few years have taught us that accurate diagnosis of coronary artery disease relies on good technical studies and can be achieved by optimizing image parameters including image timing and image reconstruction ECG-trigger delay. Current clinical applications of coronary MSCTA include: quantitative assessment of coronary artery stenosis, characterization of coronary atherosclerotic plaques, and follow-up of coronary artery stent and bypass graft. Furthermore, MSCT has brought an increasing awareness to the amount of radiation used in CT. This has prompted CT researchers and manufacturers to improve various techniques and develop new strategies to reduce radiation dose. It is anticipated that MSCT will become a sensitive and accurate tool for detecting coronary artery disease and monitoring outcomes after treatment for coronary artery disease.     

Combined magnetic resonance urography and targeted helical CT in patients with renal colic: a new approach to reduce delivered dose

PURPOSE: To determine whether magnetic resonance urography (MRU), obtained before helical computed tomography (CT) in patients with acute renal colic, can help delimit the obstructed area to be subsequently examined by a targeted CT scan, thus reducing the dose of radiation. MATERIALS AND METHODS: Patients (51) with symptoms of acute renal colic underwent MRU and a total urinary tract helical CT. CT images from the 5 cm below the level of ureteral obstruction as demonstrated by MRU were selected out. Combined interpretation of MRU and selected CT images constituted protocol A. Protocol B consisted of the entire unenhanced helical CT of the urinary tract. The two protocols were compared regarding the following points: 1) sensitivity in diagnosing the presence of obstructing urinary stones, and 2) the delivered radiation dose. RESULTS: Protocol A and protocol B had, respectively, 98% and 100% sensitivity in demonstrating ureteral stone as a cause of renal colic. Estimated average dose calculated from phantom study was 0.52 mSv for protocol A and 2.83 mSv for protocol B. Therefore, the effective radiation dose was 5.4 times lower in protocol A compared to protocol B. CONCLUSION: Combined MRU and short helical CT has a high sensitivity in detecting ureteral calculi with a reduced radiation dose.     

Radiation exposure at chest CT: a statement of the Fleischner Society

The introduction of helical single-detector row computed tomography (CT) and, more recently, multi-detector row CT has greatly increased the clinical indications for CT. Correspondingly, CT examinations now account for greater than one-half of the radiation dose due to medical procedures in the population of North America. The level of CT radiation dose, especially in the pediatric population, is of concern to radiologists, medical physicists, government regulators, and the media. This review addresses this problem with particular reference to radiation dose in chest CT. Specifically it outlines the topics of measurement units used to quantify radiation exposure, factors affecting CT scanner dose efficiency, scanner settings that determine the administered radiation dose, and radiation dose reduction in chest CT. A table of reference dose values is provided. Given the wide variation documented in chest CT radiation exposure, the authors suggest that reference standards be promoted to minimize excessive CT radiation exposure. In addition, further research into the complex relationship between radiation exposure, image quality, and diagnostic accuracy should be encouraged in order to establish the minimum radiation dose necessary to provide adequate diagnostic information for standard clinical questions.     

低剂量容积数据高分辨率CT重组在肺部疾病中的应用价值

目的:评价低剂量容积数据高分辨率CT重组图像在肺部疾病中的应用价值。方法:对50例肺部疾病患者进行低剂量多层螺旋CT(MSCT)、高分辨率CT(HRCT)扫描及容积数据HRCT重组,比较所得到的低剂量容积数据HRCT重组图像和HRCT扫描图像质量并进行分级评分。结果:50例肺部疾病患者的低剂量容积数据HRCT重组图像和HRCT扫描图像质量相同,两者差异无统计学意义(P=0.188)。所见征象评分P值均>0.05,肺实变、网状影、蜂窝影、肺气肿、肺大泡得分趋向一致。低剂量MSCT扫描降低80%辐射量,缩短扫描时间。结论:低剂量容积数据HRCT重组图像评价肺部疾病的价值与HRCT扫描图像相同。低剂量容积数据HRCT重组大大降低辐射量,明显缩短检查时间,复检患者尤其适用,可以替代HRCT扫描。     

Evaluation and comparison of dose profiles and patient doses among X-ray CT scanners

The increased clinical use of multi-detector-row CT (MDCT) may result in increased radiation doses for patients. The objective of this study was to compare radiation dose profiles between MDCT with 16 data acquisition systems, MDCT with 4 data acquisition systems, and single-detector-row CT (SDCT), and to compare effective doses among them to measure specific organ doses. When comparing radiation dose profiles on the Z-axis, doses outside the scanning range increased, especially when a 20 mm X-ray beam width was used. Effective doses also were higher with MDCT (low helical pitch) than with SDCT (helical pitch 1.0:1), but were not higher with MDCT (high helical pitch) than with SDCT (helical pitch 1.0:1). When a 20 mm X-ray beam width or high helical pitch was used, scanning time was shortened, but doses outside the scanning range were increased. When the standard deviation (SD) is the same, there is not much difference between SDCT and MDCT in terms of effective doses if the helical pitch is 1.0:1. It can be expected that X-ray over-wrap under low helical pitch and incorrect parameter setting cause increased radiation dose to the patient. Therefore, high helical pitch should be used if it is possible to disregard the influence of image noise. It is important to be cautious in regard to patient radiation dose when MDCT is used, and it is necessary to perform high-quality examinations at as low a dose as possible. Optimization of the scan parameters is an important issue for the future.     

Low-dose, volumetric helical CT: image quality, radiation dose, and usefulness for evaluation of bronchiectasis

RATIONALE AND OBJECTIVES: The aim of our study was to assess the image quality, radiation dose, and clinical applicability of low-dose, volumetric helical CT in the evaluation of bronchiectasis. METHODS: Volumetric helical CT scans (120 kVp, 3-mm collimation, pitch of 2, and reconstruction interval of 2 mm) were obtained through the thorax at four levels of tube current: 150, 100, 70, and 40 mA. There were a total of 12 patients who underwent CT scan either for suspected bronchiectasis or for lung cancer screening, with three patients allocated to each current level. Five radiologists assessed and compared image quality of the helical CT scans obtained at the various exposure levels. Radiation doses of helical CT performed with four different current settings and of high-resolution CT (120 kVp, 170 mA, 1-mm collimation, and 10-mm intervals) were measured. The diagnostic usefulness of the 40-mA helical CT images was compared with that of high-resolution CT by two observers in 52 patients with known or suspected airway diseases. RESULTS: With helical CT, there was no significant difference in image quality among the four different levels of current (P > 0.05). Radiation doses associated with the 40-, 70-, 100-, and 150-mA helical techniques were 3.21 mGy (range, 3.02-3.57), 4.81 mGy (range, 3.89-5.93), 6.46 mGy (range, 6.01-7.31), and 10.4 mGy (range, 8.93-12.1), respectively, whereas that of high-resolution CT was 2.17 mGy (range, 1.90-2.67). Of 52 patients, 44.5 and 47 patients (the mean of positive interpretations by the five observers) were diagnosed with bronchiectasis at high-resolution and low-dose helical CT, respectively. Of 928 segments, bronchiectasis was seen in 152.5 segments on high-resolution CT and in 193.5 segments on helical CT. The interobserver agreement (k-values) was acceptable for both techniques. CONCLUSIONS: With acceptable images and similar radiation dose, low-dose volumetric helical CT at 40 mA may offer more information than does high-resolution CT in the evaluation of bronchiectasis.     

胰腺癌的螺旋CT诊断与术前评估

胰腺癌临床表现隐慝，临床诊断困难。随着ＣＴ、多层螺旋CT的运用，多期扫描，血管重建等功能的实现，使CT成为胰腺癌诊断及评价的重要手段之一，本文着重介绍螺旋CT在胰腺癌的诊断及鉴别诊断，术前分期及其手术切除的可能性评价的重要作用，对胰腺癌的发病机制、临床特征和病理及CT扫描技术等方面也作了简介。     

16层螺旋CT检查输尿管结石的应用价值

 目的 探讨16层螺旋CT在输尿管结石病例中的诊断价值.方法 应用东芝Aquilion 16层螺旋CT对142例临床拟诊输尿管病变病例进行检查分析,采用标准和低剂量扫描两组对比分析,对确诊输尿管结石病例减少造影剂用量行增强扫描.结果 两组扫描方法对输尿管结石诊断准确性无差异,低剂量组辐射剂量仅为标准组的37.12%.增强扫描适当减少造影剂用量,也能满足临床制定治疗方案前评估肾功能的要求.结论 16层螺旋CT对诊断输尿管结石有很高的准确性,适当降低扫描条件不影响结石检出率,适当减少造影剂用量不影响临床评价肾功能,但却减少了辐射剂量和患者费用,在临床中应该得到大力推广.     

Comparison of doses for pulmonary embolism detection with helical CT and pulmonary angiography

The objective of this study was to compare the radiation exposure delivered by helical CT and pulmonary angiography (PA) for the detection of pulmonary embolism (PE), with an anthropomorphic phantom. A preliminary survey defined a representative standard procedure for helical CT and PA (n=148) by choosing the exposure settings most frequently used. Then, radiation doses were measured with thermoluminescent dosimeters TLD 100 (Lif) introduced into the depth of an anthropomorphic phantom. Average doses were approximately five times smaller with helical CT than with PA (6.4±1.5 and 28±7.6 mGy, respectively). The most important doses were abreast the pulmonary apex for CT, and abreast the pulmonary arteries for PA. Compared with PA, helical CT dose distribution was relatively uniform (10–13 mGy). Finally, concerning abdomen and pelvis, doses were more important for PA than for CT scan (0.06–2.86 and 0.2–11.5 mGy, respectively). For the diagnostics of PE, radiation exposure is five times smaller with helical CT than with pulmonary angiography.     

Impact of new technologies on dose reduction in CT

The introduction of slip ring technology enables helical CT scanning in the late 1980's and has rejuvenated CT's role in diagnostic imaging. Helical CT scanning has made possible whole body scanning in a single breath hold and computed tomography angiography (CTA) which has replaced invasive catheter based angiography in many cases because of its easy of operation and lesser risk to patients. However, a series of recent articles and accidents have heightened the concern of radiation risk from CT scanning. Undoubtedly, the radiation dose from CT studies, in particular, CCTA studies, are among the highest dose studies in diagnostic imaging. Nevertheless, CT has remained the workhorse of diagnostic imaging in emergent and non-emergent situations because of their ubiquitous presence in medical facilities from large academic to small regional hospitals and their round the clock accessibility due to their ease of use for both staff and patients as compared to MR scanners. The legitimate concern of radiation dose has sparked discussions on the risk vs benefit of CT scanning. It is recognized that newer CT applications, like CCTA and perfusion, will be severely curtailed unless radiation dose is reduced. This paper discusses the various hardware and software techniques developed to reduce radiation dose to patients in CT scanning. The current average effective dose of a CT study is ∼10 mSv, with the implementation of dose reduction techniques discussed herein; it is realistic to expect that the average effective dose may be decreased by 2-3 fold.     

扫描方式对64层螺旋CT头颅CTA图像质量和辐射剂量影响的研究

目的：探讨64层螺旋CT扫描方式对头颅CTA图像质量和辐射剂量的影响。方法：采用GE Lightspeed VCT对100例需要行头颅CTA检查的患者进行对比研究,50例行螺旋扫描,另50例行轴层扫描。两组数据经过VR、MIP、去骨VR和多平面重组等方法后处理,然后由3组医师通过5分评价法对CTA图像质量进行主观评价,取其均值纳入统计学分析;记录每组扫描的容积剂量指数（CTDIvol）和剂量长度乘积（DLP）。对两组数据进行两独立样本t检验。结果：轴向扫描图像质量评分是（4．66±0．42）分、螺旋扫描CTA的成像质量评分（4．67±0．38）分;两组数据差异无统计学意义（P〉0．05）。轴向扫描、螺旋扫描的CTDIvol分别是45．71和54．18mGy;DLP分别是731．43和954．68mGy·cm。两组间差异有统计学意义（P〈0．05）,轴层扫描方式辐射剂量更低。结论：64层CT头颅CTA轴扫可以显著降低辐射剂量而不降低图像质量,在临床运用中值得关注。