Iterative Reconstruction Technique For Radiation Dose Reduction In Chest CT: Phantom And Clinical Study

【Background】Since its introduction, CT has become the major imaging modality for investigatingchest disorders. Compared with chest radiography, CT has greater diagnostic accuracy in arange of clinical situations. This improvement in image quality, however, is associated witha100fold increase in radiation dose compared with conventional radiography. It wasestimated that the number of CT examinations has increased23times, from3million in1980to more than68.7million in2007in the united states. It has estimated that the use ofCT may be responsible for approximately2%of all incident cancer cases in the unitedstates[1]. Therefore, the optimation of CT protocols and reduction in radiation dose havebecome an important focus for research.Providing diagnostic image quality at the lowest radiation dose is an important focus ofCT research in this world. In the NLST[2], a20%decrease in mortality from lung cancerwas observed in the low dose CT group as compared with the radiography group. The rateof positive results was higher with low-dose CT screening than with radiographicscreening. It may be difficult to evaluate GGO nodules by low-dose thin-section helical CT.Li et al[3]reported that32%of lung cancers with solid nodules were missed on CT lungcancer screens performed with low-dose CT. At MDCT, GGO nodules with a CT numberof-650HU or less were difficult to detect at the lower dose settings (21and45mAs)[4].A number of CT dose reduction techniques and researches have been developed withthe goal of preserving image quality while reducing radiation dose. Because the primaryeffect of reduced radiation dose is increased image noise. Most commercial CT systemscurrently use Filtered back projection(FBP) as the standard image reconstruction algorithm.FBP, while fast and fairly robust at routine radiation dose, is prone to image noise andartifacts that result in non-diagnostic images at extremely low doses. As a result, thepotential to reduce the dose while maintaining image quality using FBP is limited. Iterativereconstruction(IR) is now a hot topic to enable the reductions of radiation dose and imagenoise. Numerous IR techniques are available commercially, such as IRIS (iterativereconstruction in image space; Siemens Healthcare, Forchheim, Germany), ASIR(adaptive statistical iterative reconstruction; GE Healthcare, Milwaukee, WI), and iDose(Philips Healthcare) and so on. These techniques can significantly reduce noise andimprove image quality, have a potential to decrease the radiation dose[5-10]. The objective of this work was to investigate the potential value of iterativereconstruction when applying to the low dose chest CT both in a phantom and in patients.Part OneEffect of Iterative reconstruction technique on radiation dose and image qualityfor low dose chest CT: phantom study1.1Impact of low-dose HRCT and iterative reconstruction algorithm(iDose) onthe CT number: a pulmonary phantom study【Objective】 To evaluate the impact of low-dose HRCT and iterative reconstructionalgorithm(iDose) on the CT number using a pulmonary phantom.【Materials andmethods】 HRCT scan were performed on chest phantom with5different tube currents(200mAs、100mAs、50mAs、25mAs、10mAs), scanned with Philips Brilliance128scanner.All data were created using3reconstructions, i.e. filtered back projection (FBP) andmoderate-and high-level iterative reconstructions (iDose-4, iDose-7). Effective dose(ED)was calculated on the basis of dose length product and volumetric CT dose index.Hounsfield unit (HU) and standard deviation (SD) of CT values in different regions of thephantom were recorded.【Results】 No significant differences were found on the CTnumbers obtained using different tube currents or different reconstructions(p>0.05). Andthe SD was increased with the current decrease. Raw data-based iterative reconstructionreduces SD values compared to FBP.【Conclusion】 Low dose HRCT can measure theCT number accurately, And the noise was increased with the current decrease. Rawdata-based iterative reconstruction reduces image noise compared to FBP, The larger theiDose level is, the larger the noise reduction is.1.2Detection of Ground-glass opacities using hybrid iterative reconstruction (iDose)and low-dose HRCT: a phantom study【Objective】 The purpose is to investigate the ability to detect GGO in a chest phantomusing a low-dose HRCT and hybrid IR named iDose.【Materials and methods】 HRCT scan were performed on chest phantom with5different tube currents (200mAs、100mAs、50mAs、25mAs、10mAs), scanned with Philips Brilliance128scanner. All data werecreated using3reconstructions, i.e. filtered back projection (FBP) and moderate-andhigh-level iterative reconstructions (iDose-4, iDose-7). Effective dose(ED) was calculatedon the basis of dose length product and volumetric CT dose index. Standard deviation (SD)of CT values in different regions of the phantom were recorded. Visually assessment ofidentification and image quality of GGNs by two chest radiologists.【Results】Kappavalue of overall image quality were substantial or almost perfect(k>0.4). The identificationof GGN was possible at currents of>10mAs. iDose had better image quality at a lowerradiation dose than images acquired with a conventional FBP reconstruction algorithm.The image quality was significantly lower at50mAs-FBP than at25mAs-iDose（p<0.001).【Conclusion】 GGO nodules of5mm in diameter can be detected with128-slice spiralCT using10mAs at experimental study，The use of hybrid IR techniques enabled improvethe image quality.1.3Impact of radiation dose and iterative reconstruction on the Volumetricmeasurement accuracy of ground-glass opacity nodules【Objective】 To evaluate the impact of the radiation dose and iterative reconstruction onthe volumetric measurement accuracy of ground-glass opacity nodules(GGN) detection.【Material and Methods】 CT scans were performed on chest phantom containing9artificial GGN with5different tube currents (200mAs、100mAs、50mAs、25mAs、10mAs),All data were reconstructed with slice thickness of1.0mm and bone algorithm forreconstruction. The images were reconstructed with regular filtered back projection (FBP)and an iterative reconstruction technique (iDose). Nodule volumes were measured usingLung Analysis software. Effective dose(ED) was calculated. The standard reference usednodule volume measured at200mAs. The relative percent error (RPE) between the CTmeasured volume and the reference volume of GGN were calculated.【Results】 Whentube current was10mAs, volume of GGNs can’t measured using the software. The RPEbetween the CT measured volume and the reference volume of GGN at100mAs,50mAs,25mAs were （-0.96±1.28）、(-2.24±2.86)、(-11.97±5.48) respectively. There werestatistically significant differences between the RPE measured at25mAs and other groups(p<0.05). There was a significant negative correlation between RPE and the tubecurrent(r=-0.739，p=0.000). The RPE of groups reconstructed with iterative reconstruction were （-0.095±0.39）、(-0.93±0.97)、(-2.18±3.07)、（-3.12±3.85）, There were statisticallysignificant differences between the RPE measured at10mAs and other groups (p<0.05).【Conclusion】 The different radiation and reconstruction techniques could have effectson the accuracy of volumetric measurement. Compared with regular FBP, iterativereconstructions enable significant reduction of image noise without loss of diagnosticinformation, thus having the potential to decrease radiation dose and reduce the error ofmeasurement on nodule volume.1.4Effect of kernel and iterative reconstruction on image quality of ground-glassopacities on low-dose CT: A phantom study【Objective】 To evaluate the impact of the kernel and iterative reconstruction on imagequality of ground-glass opacity nodules(GGN) on low-dose chest CT.【Material andMethods】 CT scans were performed on chest phantom containing9artificial GGN with4different tube currents (100mAs、50mAs、30mAs、25mAs), The images werereconstructed with regular filtered back projection (FBP) standard kernel,FBP withsmooth kernel and an iterative reconstruction technique (iDose). After scanning, Wemeasured image noise, CT number Effective dose(ED) and assessed image quality.【Results】 No significant differences were found on the CT numbers obtained usingdifferent tube currents or different reconstructions and kernels(p>0.05). There was astatistically significant difference between FBP with standard kernel and FBP with smoothkernel and iDose-4/6with respect to image noise (p<0.05), Regarding to25mAs,30mAs,50mAs, There were no statistically significant differences between FBPwith smooth kernel and iDose-4(p>0.05). Image noise on50mAs images reconstructedwith iDose-4remained constant compared to100mAs+FBP images. Image noise on25/30mAs images reconstructed with iDose-6remained constant compared to100mAs+FBP images. Image quality was better with iDose than FBP with standard orsmooth kernel.【Conclusion】 From our findings, FBP with smooth kernel can alsoreduce the image noise, but make the lesion more smooth. iDose had better image qualityat a lower radiation dose than images acquired with a conventional FBP reconstructionalgorithm. Part TwoUltra low dose non-enhanced chest CT similar to chest X-ray examination: Potentialvalue of iterative reconstruction【Objective】 The purpose of this study was to assess the diagnostic image quality ofultra-low-dose chest computed tomography (ULD-CT) obtained with a radiation dosecomparable to chest radiography and reconstructed with filtered back projection (FBP) anditerative reconstruction (iDose) in comparison with routine low-dose diagnostic CT(LDD-CT).【Material and Methods】 Unenhanced chest CT images of42patientsacquired with ULD-CT were compared with images obtained with routine LDD-CT in thesame examination. The image noise, CT numbers and effective dose (ED) with eachprotocol were assessed. Normal lung structures and Lesion conspicuity of chest lesions onall CT data sets were assessed on a three-point scale.【Results】 The radiation doseof ULD-CT was0.215±0.018mSv compared with2.6±0.7mSv for LDD-CT(P<0.0001). The CT number of ULD-CT was significantly higher using FBP than thatusing iDose-L4and iDose-L6(p<0.05). Image quality of ULD-CT and LDD-CT increasedsignificantly when using iDose-L4or iDose-L6compared with FBP (P<0.001). ULD-CTreconstructed with iDose enabled to detect the lesions as seen on LDD-CT. However,image quality of ULD-CT reconstructed with iDose was clearly inferior forcharacterisation of ground glass opacities.【Conclusion】 Iterative reconstruction allowsdetection the lesions with ULD-CT with radiation exposure in the range of a posterior toanterior (PA) and lateral chest X-ray.Part ThreeThe effect of iterative reconstruction on image quality of contrast-enhanced chest CTwith low tube voltage settings【Objective】To evaluate the image quality of an iterative reconstruction algorithm(iDose)in contrast-enhanced chest CT with low tube voltage settings in comparison withstandard-dose filtered back projection(FBP) CT in patients with normal body massindex(BMI).【Material and Methods】 Eighty patients with normal body massindex(BMI) were referred for a contrast-enhanced chest CT, The patients were randomlyassigned into groups120-KVp and80-KVp. Standard convolution FBP was used to reconstruct120-KVp (group A) and80-KVp(group C) image sets, and iterativereconstructions(iDose) was used to reconstruct80-KVp (group B) image sets. The meanimage noise, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and effective dose(ED) with each protocol were assessed. Image quality were graded (scale:1-3) andcompared among3groups.【Results】Radiation dose was71.35%less for the low doseprotocol. Noise was significantly lower in low dose images reconstructed with iDose(Group B)compared to images reconstructed with FBP（Group C). Group B had the highestSNR and CNR. There was no difference in subjective image quality scores between GroupB and A(p>0.05).【Conclusion】 In contrast-enhanced chest CT examinations, Imagesreconstructed with iterative reconstruction had better image quality at a lower radiationdose than images acquired with a conventional FBP reconstruction algorithm.