高级迭代重建算法降低盆腔CT剂量潜能的模体研究

目的 探讨应用自适应统计迭代重建(ASiR)和基于模型的迭代重建(MBIR)降低盆腔CT扫描剂量的可行性。方法 应用宝石能谱CT以不同的管电流对标准男性模体扫描,记录不同毫安值扫描条件下的容积CT剂量指数(CTDI_vol)和剂量长度乘积(DLP),测量滤过反投影重建(FBP)、50% ASiR及MBIR 3种重建模式下图像的平均CT值、噪声及对比噪声比。由3位高年资医师对图像给予主观评价。结果 50% ASiR及MBIR重建算法比FBP重建算法噪声明显减少,对比噪声比明显提高。能够符合诊断要求的最小mA值分别为250 mA (FBP)、180 mA(50% ASiR) 和100 mA (MBIR)。在同等图像质量情况下,50% ASiR及 MBIR重建模式分别可减少剂量28.0% 及59.9%。结论 高级迭代重建算法能够减少图像噪声,提高图像对比噪声比,降低盆腔CT剂量。     

Dose reduction with adaptive statistical iterative reconstruction for paediatric CT: phantom study and clinical experience on chest and abdomen CT

Objectives

To assess the benefit and limits of iterative reconstruction of paediatric chest and abdominal computed tomography (CT).

Methods

The study compared adaptive statistical iterative reconstruction (ASIR) with filtered back projection (FBP) on 64-channel MDCT. A phantom study was first performed using variable tube potential, tube current and ASIR settings. The assessed image quality indices were the signal-to-noise ratio (SNR), the noise power spectrum, low contrast detectability (LCD) and spatial resolution. A clinical retrospective study of 26 children (M:F?=?14/12, mean age: 4 years, range: 1–9 years) was secondarily performed allowing comparison of 18 chest and 14 abdominal CT pairs, one with a routine CT dose and FBP reconstruction, and the other with 30 % lower dose and 40 % ASIR reconstruction. Two radiologists independently compared the images for overall image quality, noise, sharpness and artefacts, and measured image noise.

Results

The phantom study demonstrated a significant increase in SNR without impairment of the LCD or spatial resolution, except for tube current values below 30–50 mA. On clinical images, no significant difference was observed between FBP and reduced dose ASIR images.

Conclusion

Iterative reconstruction allows at least 30 % dose reduction in paediatric chest and abdominal CT, without impairment of image quality.

Key points

? Iterative reconstruction helps lower radiation exposure levels in children undergoing CT. ? Adaptive statistical iterative reconstruction (ASIR) significantly increases SNR without impairing spatial resolution. ? For abdomen and chest CT, ASIR allows at least a 30 % dose reduction.     

Accuracy of lung nodule volumetry in low-dose CT with iterative reconstruction: an anthropomorphic thoracic phantom study

Objective:

The purpose of this study was to assess accuracy of lung nodule volumetry in low-dose CT with application of iterative reconstruction (IR) according to nodule size, nodule density and CT tube currents, using artificial lung nodules within an anthropomorphic thoracic phantom.

Methods:

Eight artificial nodules (four diameters: 5, 8, 10 and 12 mm; two CT densities: −630 HU that represents ground-glass nodule and +100 HU that represents solid nodule) were randomly placed inside a thoracic phantom. Scans were performed with tube current–time product to 10, 20, 30 and 50 mAs. Images were reconstructed with IR and filtered back projection (FBP). We compared volume estimates to a reference standard and calculated the absolute percentage error (APE).

Results:

The APE of all nodules was significantly lower when IR was used than with FBP (7.5 ± 4.7% compared with 9.0 ±6.9%; p < 0.001). The effect of IR was more pronounced for smaller nodules (p < 0.001). IR showed a significantly lower APE than FBP in ground-glass nodules (p < 0.0001), and the difference was more pronounced at the lowest tube current (11.8 ± 5.9% compared with 21.3 ± 6.1%; p < 0.0001). The effect of IR was most pronounced for ground-glass nodules in the lowest CT tube current.

Conclusion:

Lung nodule volumetry in low-dose CT by application of IR showed reliable accuracy in a phantom study. Lung nodule volumetry can be reliably applicable to all lung nodules including small, ground-glass nodules even in ultra-low-dose CT with application of IR.

Advances in knowledge:

IR significantly improved the accuracy of lung nodule volumetry compared with FBP particularly for ground-glass (−630 HU) nodules. Volumetry in low-dose CT can be utilized in patient with lung nodule work-up, and IR has benefit for small, ground-glass lung nodules in low-dose CT.The volumetric measurement of a lung nodule with CT imaging is more accurate and consistent in the detection of growth and determination of tumour doubling time than simple manual axial diameter measurements used in the New Response Evaluation Criteria in Solid Tumours (revised RECIST guideline v. 1.1).^1,2 The recent Dutch–Belgian randomized lung cancer screening trial (NELSON) nodule management protocol was based on volumetric nodule assessment. A test was considered to be positive if the solid component of a nodule measured >500 mm³, or if the solid component of a nodule was 50–500 mm³ when the volume doubling time was less than 400 days.³ Therefore, pulmonary nodule volumetry is used for nodule identification and diagnostic strategy guidance in the follow-up of lung cancer screening as well as for monitoring tumour response to therapy.The increase in the use of CT has raised concern about the increasing risk of cancer from medical radiation exposure.⁴ Thoracic CT has been widely used in variable disease entities and frequent follow-up CT examinations may be needed. Additionally, lung cancer screening using CT is becoming more common. Therefore, further reduction of the radiation exposure during chest CT examinations would be required, and radiation dose reduction is very important issue in lung cancer screening and in lung nodule work-up. For lowering the radiation dose, the use of iterative reconstruction (IR) algorithms has become available, due to advances in technology and increased computational power. IR provides imaging at lower radiation doses with similar noise levels compared with routine-dose conventional filtered back projection (FBP), allowing dose reduction without compromising on image quality and diagnostic value.^5–10 Among the several IR algorithms offered by different vendors, we used adaptive iterative dose reduction system using a three-dimensional processing algorithm (AIDR 3D; Toshiba Medical Systems, Otawara, Japan).The accuracy of volumetric measurements of lung nodules can be affected by many sources of variability, such as nodule characteristics, CT scan parameters and measurement technology.^11–15 Several studies have examined the accuracy of volumetric measurement of lung nodules in low-dose CT.^16–18 However, it is still not well known whether IR algorithm can be a source of variability in nodule volume measurement. Furthermore, the effect of lower dose CT on volumetric measurement in relation to nodule density and image reconstruction algorithm has not been investigated.The purpose of this study was to assess the accuracy of lung nodule volumetry in low-dose CT using IR according to different nodule sizes, nodule densities, CT tube currents and scan types using spherical synthetic pulmonary nodules inside an anthropomorphic thoracic phantom.     

Usefulness of model-based iterative reconstruction in semi-automatic volumetry for ground-glass nodules at ultra-low-dose CT: a phantom study

This study aimed to investigate the effects of parameter presets of the forward projected model-based iterative reconstruction solution (FIRST) on the accuracy of pulmonary nodule volume measurement. A torso phantom with simulated nodules [diameter: 5, 8, 10, and 12 mm; computed tomography (CT) density: ? 630 HU] was scanned with a multi-detector CT at tube currents of 10 mA (ultra-low-dose: UL-dose) and 270 mA (standard-dose: Std-dose). Images were reconstructed with filtered back projection [FBP; standard (Std-FBP), ultra-low-dose (UL-FBP)], FIRST Lung (UL-Lung), and FIRST Body (UL-Body), and analyzed with a semi-automatic software. The error in the volume measurement was determined. The errors with UL-Lung and UL-Body were smaller than that with UL-FBP. The smallest error was 5.8% ± 0.3 for the 12-mm nodule with UL-Body (middle lung). Our results indicated that FIRST Body would be superior to FIRST Lung in terms of accuracy of nodule measurement with UL-dose CT.     

低管电压联合迭代模型重建技术在肝脏CT增强扫描中的可行性研究

目的 探讨低管电压联合迭代模型重建(IMR)技术在肝脏CT增强扫描中的可行性。方法 60例患者按随机数字表法分为A组和B组,每组30例。扫描方案A组动脉期100 kV,门静脉期120 kV,B组动脉期120 kV,门静脉期100 kV。各组管电流均固定为250 mAs。A组动脉期和B组门静脉期采用IMR,A组门静脉期和B组动脉期采用滤波反投影(FBP)重建,得到4组图像,包括A1组(动脉期,100 kV,IMR),B1组(动脉期,120 kV,FBP),A2组(门静脉期,120 kV,FBP)以及B2组(门静脉期,100 kV,IMR)。分别比较A1组和B1组,A2组和B2组的图像质量客观评价指标 [图像噪声、图像信噪比(SNR)、对比噪声比(CNR)] 和主观评价指标(低对比分辨力、病灶边缘锐利度、图像失真及诊断信心度),并计算有效剂量。结果 有效剂量A1组较B1组、B2组较A2组明显下降(t=11.05、11.64, P<0.01)。低对比分辨力、病灶边缘锐利度A1优于B1组、 B2优于A2组(Z=6.391、3.200、6.559、3.409, P<0.01),图像失真和诊断信心差异无统计学意义(P>0.05)。图像噪声A1组低于B1组,B2组低于A2组(t=12.889、15.163, P<0.01),SNR和CNR A1组高于B1组,B2组高于A2组(t=15.458、1.325、15.308、3.136, P<0.01)。结论 与常规管电压FBP重建相比,低管电压联合IMR重建可显著降低肝脏增强CT的辐射剂量,并提高其图像质量。     

双低扫描联合正弦波迭代重组技术在肝脏增强 CT扫描的可行性研究

目的探讨联合低剂量碘对比剂、低管电压和正弦波迭代重组（SAFIRE）技术用于肝脏增强CT扫描的可行性。方法收集2016年6月1日-7月31日行双源CT肝脏增强扫描（包括动脉期和静脉期）的病人200例,男118例,女82例,年龄21～82岁,平均（57.1±12.2）岁。病人随机分为2组：A组（100例）采用管电压100kV和SAFIRE重组,对比剂剂量400mg/kg;B组（100例）采用管电压120kV和滤过反投影法重组,对比剂剂量500mg/kg。采用两独立样本t检验比较影像质量评分、噪声、对比噪声比（CNR）、对比剂剂量和辐射剂量。结果A、B组肝脏动脉期和门静脉期的影像质量评分,影像噪声,腹主动脉、门静脉和肝实质的CNR差异均无统计学意义（均P>0.05）。A、B组动脉期有效辐射剂量分别为（3.21±0.63）?mSv和（4.08±1.16）?mSv（P<0.05）,A组较B组低21.3%;静脉期有效辐射剂量分别为（3.12±0.36）mSv和（3.88±1.11）?mSv（P<0.05）,A组较B组低19.6%。结论与常规120kV肝脏增强CT扫描相比,低剂量碘对比剂、100kV低管电压和SAFIRE重组技术可以在保持肝脏影像质量不降低的基础上,降低有效辐射剂量。     

A new phantom for evaluation of low-contrast detectability of X-ray CT scanner

It is possible to produce Xyron (styrene grafted polyphenylene ether) having different electron densities but virtually the same effective atomic number. The difference between the CT numbers obtained with two Xyron types of different electron densities was shown both theoretically and experimentally to be constant, regardless of the effective photon energy. A phantom for evaluation of low-contrast detectability of X-ray CT scanners was produced from these two types of Xyron and was shown to be effective for both comparative studies and daily quality assurance.     

Impact of radiation dose and iterative reconstruction on pulmonary nodule measurements at chest CT: a phantom study

PURPOSE

We aimed to identify the impact of radiation dose and iterative reconstruction (IR) on measurement of pulmonary nodules by chest computed tomography (CT).

METHODS

CT scans were performed on a chest phantom containing various nodules (diameters of 3, 5, 8, 10, and 12 mm; +100, −630 and −800 HU for each diameter) at 80, 100, 120 kVp and 10, 20, 50, 100 mAs (a total of 12 radiation dose settings). Each CT was reconstructed using filtered back projection, iDose⁴, and iterative model reconstruction (IMR). Thereafter, two radiologists measured the diameter and attenuation of the nodules. Noise, contrast-to-noise ratio and signal-to-noise ratio of CT images were also obtained. Influence of radiation dose and reconstruction algorithm on measurement error and objective image quality metrics was analyzed using generalized estimating equations.

RESULTS

The 80 kVp, 10 mAs CT scan was not feasible for the measurement of 3 mm sized simulated ground-glass nodule (GGN); otherwise, diameter measurement error was not significantly influenced by radiation dose (P > 0.05). IR did not have a significant impact on diameter measurement error for simulated solid nodules (P > 0.05). However, for simulated GGNs, IMR was associated with significantly decreased relative diameter measurement error (P < 0.001). Attenuation measurement error was not significantly influenced by either radiation dose or reconstruction algorithm (P > 0.05). Objective image quality was significantly better with IMR (P < 0.05).

CONCLUSION

Nodule measurements were not affected by radiation dose except for 3 mm simulated GGN on 80 kVp, 10 mAs dose setting. However, for GGNs, IMR may help reduce diameter measurement error while improving image quality.Recently, the National Lung Screening Trial demonstrated that three annual low-dose computed tomography (CT) screenings (cumulative average effective dose, 4.5 mSv) resulted in a 20% relative mortality reduction of lung cancer in comparison with chest radiographs for individuals at high risk of lung cancer (1, 2). Despite the great advantage of low-dose CT screenings, one of the biggest considerations must be radiation exposure.Currently, there are several techniques known to reduce radiation exposure from chest CT (3, 4). Modification of the tube current is the simplest method of radiation dose reduction and has been a mainstay of chest CT imaging for radiation dose reduction (5). As lowering radiation dose is often accompanied by increased noise in CT images reconstructed with the conventional filtered back projection (FBP) algorithm (6), noise-reducing iterative reconstruction (IR) algorithms have also become available. At present, all major CT vendors have their own unique IR techniques (6). A novel IR algorithm, iterative model reconstruction (IMR) (Philips Healthcare), was developed recently and this knowledge-based IR incorporates system optics as well as data statistics and image statistics.For clinical application of these IR algorithms in conjunction with reduced radiation dose, the feasibility of lesion characterization and radiologic measurements are fundamental prerequisites. Nodule measurements should be performed equally well without significant variability between low-dose IR-applied CT images and standard-dose CT reconstructed with FBP. This is of great significance since the management of small incidentally-detected pulmonary nodules, especially ground-glass nodules (GGNs), differs based on nodule size and their changes over follow-up examinations (7). Manual measurements of nodule diameter on chest CT of various radiation doses from standard to ultra-low doses have been studied previously; however, the effect of IR algorithms were not considered in those investigations (8, 9). In addition, considering the fuzzy, ill-defined border of GGNs and the fact that GGNs are usually followed-up with low-dose CT, advantage of IR for the measurement of GGNs is clearly expected. Nevertheless, IR has not been highlighted for evaluating GGNs to date.In this study, we hypothesized that the accuracy of manual nodule measurement would be potentially affected by various radiation dose settings and reconstruction algorithms such as iDose⁴ (Philips Healthcare) and IMR. We suspected that the measurement of GGNs would be particularly influenced by the variables. Therefore, we performed modeling analysis using an anthropomorphic chest phantom with simulated nodules.     

Computer-assisted solid lung nodule 3D volumetry on CT: influence of scan mode and iterative reconstruction: a CT phantom study

Objective

To evaluate the effect of high-resolution scan mode and iterative reconstruction on lung nodule 3D volumetry.

Methods

Solid nodules with various sizes (5, 8, 10 and 12 mm) were placed inside a chest phantom. CT images were obtained with various tube currents, scan modes (conventional mode, high-resolution mode) and iterative reconstructions [0, 50 and 100 % blending of adaptive statistical iterative reconstruction (ASiR) and filtered back projection]. The nodule volumes were calculated using semiautomatic software and compared with the assumed volume from the nodules.

Results

The mean absolute and relative percentage error improved when using iterative reconstruction especially when using the conventional scan mode; however, this effect was not significant. Significant reduction in volume overestimation was observed when using high-resolution scan mode (P = 0.011).

Conclusion

The high-resolution mode significantly reduces the volume overestimation of 3D volumetry. Iterative reconstruction shows a reduction in volume overestimation and error margin especially with the conventional scan mode; however, this effect was not significant.     

Feasibility of optimized ultralow-dose pulsed fluoroscopy for upper gastrointestinal tract examinations: a phantom study with clinical correlation

Purpose

To establish an optimized ultralow-dose digital pulsed fluoroscopy (FP) protocol for upper gastrointestinal tract examinations and to investigate the radiation dose and image quality.

Materials and methods

An Alderson-Rando-Phantom with 60 thermoluminescent dosimeters was used for dose measurements to systematically evaluate the dose–area product (DAP) and organ doses of the optimized FP protocol with the following acquisition parameters: 86.7 kV; 77 mA; 0.9 mm³, automatic image noise and contrast adaption. Subjective image quality, depiction of contrast agent and image noise (5-point Likert scale; 5 = excellent) were assessed in 41 patients, who underwent contrast-enhanced FP with the aforementioned optimized protocol by two radiologists in consensus. A conventional digital radiograph (DR) acquisition protocol served as the reference standard for radiation dose and image quality analyses.

Results

Phantom measurements revealed a general dose reduction of approximately 96% per image for the FP protocol as compared to the DR standard. DAP could be reduced by 97%. Significant dose reductions were also found for organ doses, both in the direct and scattered radiation beam with negligible orbital (FP 5.6 × 10^?3 vs. DR 0.11; p = 0.02) and gonadal dose exposure (female FP 2.4 × 10^?3 vs. DR 0.05; male FP 8 × 10^?4 vs. DR 0.03; p ≤ 0.0004). FP provided diagnostic image quality in all patients, although reading scores were significantly lower for all evaluated parameters as compared to the DR standard (p < 0.05).

Conclusion

Ultralow-dose FP is feasible for clinical routine allowing a significant reduction of direct and scattered dose exposure while providing sufficient diagnostic image quality for reliable diagnosis.

     

低管电压联合迭代重建算法对胸部CT血管造影图像质量及辐射剂量的影响

目的 探讨低管电压联合迭代重建算法对胸部CT血管造影成像图像质量及辐射剂量的影响.方法 利用ATOM 701-D型成年男性仿真人胸部模型,配置碘水混合对比剂注入输液管后置入胸部模拟胸部血管.采用320排容积CT以80、100和120 kV分别联合自适应迭代降剂量技术(AIDR 3D)迭代重建4个等级(关闭、轻、标准、强)共12种方案按管电流等其他参数不变进行胸部扫描,记录辐射剂量.获得横断面输液管CT值及噪声水平、纵隔区软组织CT值、信噪比(SNR)和对比噪声比(CNR).比较不同管电压扫描及不同迭代算法水平重建对图像质量的影响.结果 容积CT剂量指数(CTDI_vol)、剂量长度乘积(DLP)及有效剂量(E)在120 kV时为12.50 mGy、453.50 mGy ·cm、6.35 mSv;在100 kV时为7.60 mGy、276.50 mGy ·cm、3.87 mSv;在80 kV时为3.80 mGy、138.70 mGy ·cm、1.94 mSv.随着管电压的降低,辐射剂量显著降低.不同管电压时,输液管CT值、噪声标准差(SD)、SNR、CNR差异均具有统计学意义(F=1 029.24、8.56、3.44、4.09,P<0.05).按不同重建算法,输液管SD、SNR、CNR差异均具有统计学意义(F=33.99、54.80、54.72,P<0.05),但对输液管CT值没有显著影响(P>0.05).管电压或重建算法对纵隔区软组织CT值的影响差异均无统计学意义(P>0.05).结论 80 kV管电压扫描联合标准水平的迭代重建算法方案可降低有效剂量,并获得较高质量的胸部血管图像.     

Karl迭代技术降低16层CT胸部扫描辐射剂量的水模和临床研究

目的：探讨Karl迭代重建技术在16层CT胸部检查中的应用价值。方法：水模研究：采用标准模体,管电压120 kV,管电流分别为150、136、123、109、95、83、68、55和41 mAs,使用滤波反投影（FBP）或Karl迭代技术进行图像重建,测量和比较不同条件下图像噪声和对比噪声比（CNR）。临床研究：将100例进行胸部 CT 体检者随机分为2组,对照组（A组）采用标准剂量（120 kV、150 mAs）进行扫描并采用FBP法进行重建;低剂量组采用120 kV、83 mAs进行图像采集,并分别采用Karl技术（B组）和FBP法（C组）进行图像重建。对三组中的图像噪声、信号噪声比（SNR）、对比噪声比、图像质量主观评分、容积CT剂量指数（CTDIvol）和有效剂量（ED）等指标进行比较。结果：在水模研究中,当管电流为95和83 mAs时,辐射剂量分别为标准剂量的60％和50％,且使用Karl重建技术时图像噪声、CNR与使用标准剂量、FBP 法重建的图像间差异无统计学意义（P＞0．05）。在临床研究中,低剂量组的有效剂量为（2．38±0．26）mSv,较标准剂量组的（4．87±0．37）mSv减少51．1％（P＜0．05）。B 组的图像噪声、CNR、SNR、主观评分与 A 组比较,差异均无统计学意义（P＞0．05）;而B组的SNR、CNR及主观评分均高于C组,两组间差异有统计学意义（P＜0．05）。结论：在胸部 CT 平扫中,使用Karl迭代重建技术能在降低辐射剂量后有效地保证图像质量,优于FBP技术,具有较好的临床应用价值。     

兔脑CTP低剂量扫描联合迭代重建的可行性研究

目的:评价联合迭代重建技术(iDose4)的低剂量兔脑CT灌注成像参数的准确性及图像质量.方法:将60只成年雄性新西兰白兔随机均分为3组进行CT灌注成像,扫描参数:常规剂量组(RD)为80 kV、100 mAs、FBP,低剂量组(LD)为80 kV、50 mAs、iDose4和层级3),超低剂量组(ULD)为80kV、25mAs、iDose4和层级5.测量各组兔脑的灌注参数值(包括脑血流量CBF、脑血容量CBV、平均通过时间MTT、达峰时间TTP)并计算有效辐射剂量.对3组灌注图像质量进行评估和比较,包括图像噪声、脑实质信噪比(SNR)和图像质量评分.结果:共有54只兔脑CT灌注扫描成功,其中RD组16只,LD组20只,ULD组18只.三组的有效剂量分别为:RD组3.47 mSv,LD组1.53 mSv,ULD组0.75 mSv;与RD组比较,LD组和ULD组的有效辐射剂量分别下降55.9％和78.3％.各项灌注参数值(CBF、CBV、MTT和TTP)在三组间差异均无统计学意义(P＞0.05).图像噪声和脑实质SNR在三组间差异无统计学意义(P＞0.05).图像质量主观评分:ULD组低于RD组,差异有统计学意义(P=0.006);LD组与RD组,ULD组与LD组间差异无统计学差异(P＞0.05).两位观察者对图像质量主观评价结果的一致性中等(Kappa=0.471,P＜0.01).结论:通过联合不同层级的迭代重建技术,低剂量和超低剂量兔脑CT灌注成像可获得准确的灌注参数值以及可接受的图像质量.     

Six iterative reconstruction algorithms in brain CT: a phantom study on image quality at different radiation dose levels

Objective:

To evaluate the image quality produced by six different iterative reconstruction (IR) algorithms in four CT systems in the setting of brain CT, using different radiation dose levels and iterative image optimisation levels.

Methods:

An image quality phantom, supplied with a bone mimicking annulus, was examined using four CT systems from different vendors and four radiation dose levels. Acquisitions were reconstructed using conventional filtered back-projection (FBP), three levels of statistical IR and, when available, a model-based IR algorithm. The evaluated image quality parameters were CT numbers, uniformity, noise, noise-power spectra, low-contrast resolution and spatial resolution.

Results:

Compared with FBP, noise reduction was achieved by all six IR algorithms at all radiation dose levels, with further improvement seen at higher IR levels. Noise-power spectra revealed changes in noise distribution relative to the FBP for most statistical IR algorithms, especially the two model-based IR algorithms. Compared with FBP, variable degrees of improvements were seen in both objective and subjective low-contrast resolutions for all IR algorithms. Spatial resolution was improved with both model-based IR algorithms and one of the statistical IR algorithms.

Conclusion:

The four statistical IR algorithms evaluated in the study all improved the general image quality compared with FBP, with improvement seen for most or all evaluated quality criteria. Further improvement was achieved with one of the model-based IR algorithms.

Advances in knowledge:

The six evaluated IR algorithms all improve the image quality in brain CT but show different strengths and weaknesses.Iterative reconstruction (IR) algorithms are one of the most recent advances in CT. Since the introduction of the first IR algorithm in 2008 [1], multiple clinical studies have shown the potential of such algorithms to improve the image quality and allow for the reduction of radiation dose while maintaining diagnostic acceptability [2–7].Although all IR algorithms perform iterative image optimisation at some point in the CT image generation process, there are considerable technical differences between the available IR solutions. Furthermore, some vendors even offer more than one type of IR algorithm in their product range. Although detailed mechanisms of the current algorithms remain undisclosed, they can be classified into two basic categories [8,9] (