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Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for patients with bronchial carcinoma and intrapulmonary metastases
Clinical Radiology xxx (2016) 1e8
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Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for patients with bronchial carcinoma and intrapulmonary metastases €fer a, *, L. Lu € demann b, G. Bo € ning a, J. Kahn a, S. Fuchs a, M.-L. Scha B. Hamm a, F. Streitparth a €tsmedizin Berlin, Campus Virchow Klinikum, Augustenburger Platz 1, Department of Radiology, Charit e - Universita 13353 Berlin, Germany b €tsklinikum Essen, Hufelandstr. 55, 45147 Essen, Germany Department of Radiotherapy, Universita a
art icl e i nformat ion Article history: Received 4 September 2015 Received in revised form 7 January 2016 Accepted 14 January 2016
AIM: To compare the radiation dose and image quality of 64-row chest computed tomography (CT) in patients with bronchial carcinoma or intrapulmonary metastases using full-dose CT reconstructed with filtered back projection (FBP) at baseline and reduced dose with 40% adaptive statistical iterative reconstruction (ASIR) at follow-up. MATERIALS AND METHODS: The chest CT images of patients who underwent FBP and ASIR studies were reviewed. Doseelength products (DLP), effective dose, and size-specific dose estimates (SSDEs) were obtained. Image quality was analysed quantitatively by signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) measurement. In addition, image quality was assessed by two blinded radiologists evaluating images for noise, contrast, artefacts, visibility of small structures, and diagnostic acceptability using a five-point scale. RESULTS: The ASIR studies showed 36% reduction in effective dose compared with the FBP studies. The qualitative and quantitative image quality was good to excellent in both protocols, without significant differences. There were also no significant differences for SNR except for the SNR of lung surrounding the tumour (FBP: 3517, ASIR: 3922). DISCUSSION: A protocol with 40% ASIR can provide approximately 36% dose reduction in chest CT of patients with bronchial carcinoma or intrapulmonary metastases while maintaining excellent image quality. Ó 2016 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction The 2014 World Cancer Report of the International Agency for Research on Cancer, the specialised cancer €fer, Institut fu € r Radiologie, * Guarantor and correspondent: M.-L. Scha € r Strahlenheilkunde, Charite Berlin, Campus Virchow, Klinik fu Augustenburger Platz 1, 13353 Berlin, Germany. Tel.: þ49 176 20 78 73 18. E-mail address: [email protected] (M.-L. Sch€ afer).
agency of the World Health Organisation WHO, concluded that, by 2025, approximately 20 million people could develop cancer each year, about 40% more than at present. With 1.8 million new cases, lung cancer was the most prevalent cancer in 2012.1 To monitor patients with bronchial carcinoma during therapy, computed tomography (CT) represents the method of choice.2 The number of CT procedures has increased rapidly since it became commercially available.3 CT is fast and readily available and provides high-
http://dx.doi.org/10.1016/j.crad.2016.01.013 0009-9260/Ó 2016 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
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resolution cross-sectional images.4 Although controversial, recent data suggest that medical radiation exposure may significantly increase the risk of adverse radiation effects.4e6 The lifetime cancer risk based on current CT usage has been estimated to be as high as 2%.7 To counteract these developments, radiologists must carefully check whether alternative imaging methods, such as ultrasound or magnetic resonance imaging (MRI), can be used. In the large number of cases, where CT cannot be replaced, the major challenge is to achieve adequate image quality and maintain the high diagnostic benefit of CT with minimum radiation exposure. Several technical developments that help the radiologist reduce radiation exposure of CT, such as automatic exposure control have been successfully implemented.8,9 Other recent CT dose reduction techniques include optimising imaging parameters such as reducing tube peak voltage.10 Other important strategies to reduce radiation doses include the use of shorter scan lengths11 or avoiding unnecessary multiphase CT studies.12 The acquisition of CT images with lower tube voltage or tube current may increase image noise, which is one of the most important factors determining image quality and hence diagnostic accuracy.13 A technique to overcome the problem of image noise and artefacts resulting from the use of reduced voltage and current is the adaptive statistical iterative reconstruction (ASIR) method (GE Healthcare, Milwaukee, WI, USA). This method attempts to accurately rebuild images by concentrating on noise reduction.14 Several studies have investigated the use of ASIR12e15; however, there are differing reports on the application of ASIR to chest CT16,17 and, to the authors’ knowledge, no studies have been performed to assess the potential benefits of ASIR in chest CT examinations of patients with bronchial carcinoma or pulmonary metastases. It was hypothesised that a low-dose protocol, when used in conjunction with the ASIR technique, will significantly reduce the radiation exposure of patients with bronchial carcinoma or pulmonary metastases undergoing chest CT without compromising the image quality and diagnostic accuracy compared to a routine dose protocol reconstructed with FBP. Therefore, the purpose of this study was to retrospectively compare radiation dose and image quality of chest CT images of patients with bronchial carcinoma or pulmonary metastases performed using a routine dose protocol with FBP and a low-dose protocol with ASIR using the same CT system.
Materials and methods
The interval between the two CT examinations was 14 months at maximum. A total of 1957 patients were referred to the authors’ institution for body CT regarding diagnostic evaluation or possible treatment. Following application of various exclusion criteria (see Fig 1), 79 consecutive patients (36 women, 43 men; mean age: 6411 years) were retrospectively enrolled in this study. Among others, patients with differences in scan lengths of >40 mm in the FBP and ASIR CT examinations were excluded. Patients with implanted pacemakers or port systems were also excluded from the study to avoid bias in noise measurements. For the additional evaluation of SSDE, eight patients were excluded due to missing data, such as missing scout. All bronchial carcinoma or pulmonary metastases were histologically proven, the diagnoses are summarised in Table 1. This study was approved by the human research committee of the institutional review board.
CT examination All CT examinations were performed using a Lightspeed VCT 64-row CT system (GE Healthcare). The chest CT protocol was used in helical mode: 0.4 s gantry rotation time, 120 kVp, 0.984:1 beam pitch, 40 mm table feed per gantry rotation, a z-axis tube current modulation was used (100/ 500 mA min/max tube current), with a noise index of 25 and a 640.625 mm collimation. Before using ASIR in clinical practice, different levels of ASIR were tested using a torso phantom imaged on the same CT system. In these phantom experiments, the image quality was assessed by two independent radiologists. The blend of 40/60% ASIR/FBP was chosen for the chest CT examinations after trying preliminary reconstruction with various ASIR percentages from 10% to 100%. The 40% blending of ASIR was the best blending to reduce noise sufficiently without excessive smoothing artefact resulting from higher percentages of ASIR blends. The contrast medium injection protocol and patient preparation were the same for the two CT examinations analysed for intra-individual comparison. Chest CT examinations were performed with a concentration of 300 mg iodine/ml of the intravenous contrast agent, iobitridol (Xenetix, Laboratoires Guerbet, Roissy, France). For contrast enhancement, 1e1.5 ml/kg of contrast medium was injected through an upper extremity peripheral intravenous line, followed by a saline chaser of 0.5 ml/kg. The injection speed was 2.5 ml/s (delay time 80 seconds). The imaging parameters of the routine study protocol and the ASIR study protocol were completely identical, except for the reduced tube current.
Patients and study design Image analysis A retrospective review of the database was undertaken for patients with bronchial carcinoma or pulmonary metastases who had undergone contrast-enhanced low-dose chest CT with ASIR from September 2012 to March 2013 and a previous chest CT study with a routine dose protocol on the same CT system using an otherwise identical protocol.
The chest CT images of all studies with bronchial carcinoma or pulmonary metastases acquired with the low-dose protocol and 40% ASIR (the ASIR study) were retrospectively reviewed by two independent blinded radiologists, who also reviewed the corresponding chest CT images previously
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
€fer et al. / Clinical Radiology xxx (2016) 1e8 M.-L. Scha
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Figure 1 Exclusion criteria for the study population.
taken with the routine dose protocol and FBP (the routine study) in the same set of patients. Qualitative scoring and quantitative measurements were obtained for each study using our institution’s picture archiving and communication
Table 1 Overview of diagnoses in the study patients (79)*. Diagnosis
n
Non-small cell lung cancer (NSCLC) Small cell lung cancer (SCLC) Pulmonary metastases Pancreas carcinoma Mamma carcinoma Neuroendocrine tumour Gastric carcinoma Oesophagus carcinoma Oropharynx carcinoma Renal cell carcinoma Chordoma Leiomyosarcoma of the uterus Alveolar soft part sarcoma (ASPS) Malignant peripheral nerve sheath tumour (MPNST) Osteosarcoma Squamous cell carcinoma of the glandula submandibularis Ovary carcinoma
39 12 28 7 3 3 2 2 2 2 1 1 1 1 1 1 1
*All diagnoses were proven histologically.
system (PACS) workstation (Centricity Radiology RA1000; GE Medical Systems).
Qualitative analysis Two radiologists with 6 and 10 years of experience in reading chest CT performed qualitative analysis independently on a high-resolution diagnostic monitor comparing CT images from both the routine and the ASIR studies in a lung window with a level setting of 1500/500 HU. The images were randomised and made anonymous so that the reader was unaware of patient data and imaging parameters. Image quality was graded on a five-point Likert scale as follows: 5 ¼ excellent image quality; 4 ¼ good quality; 3 ¼ fair quality: 2 ¼ poor quality but adequate for evaluation; 1 ¼ unacceptable image. The two radiologists independently recorded their subjective impression of diagnostic validity, qualitative noise, visualisation of small structures, visualisation of tumour, and presence of artefacts.
Quantitative measurements Objective image noise was measured by obtaining the standard deviation (SD) of the mean radiodensity value
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
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(HU) within a homogeneous region of interest (ROI). For the statistical measurements all values were normalised. The contrast-to-noise ratio (CNR) was calculated using the following equation: . CNR ¼ ðROITissue1 ROITissue2 Þ SDbackground
using Pearson’s chi-square test, Fisher’s exact test, or a t-test for unpaired samples. The Wilcoxon signed rank test was used to assess subjective image quality scores of the routine (FBP) and the ASIR studies. A p-value of <0.05 was considered to be statistically significant.
where ROITissue1 and ROITissue2 are the mean HU values in the corresponding tissues, and SDbackground is the standard deviation in the surrounding background. Another quantitative measure was the signal-to-noise ratio (SNR), which was defined as specific organ attenuation divided by standard deviation of noise (i.e. air)18:
Results
ROI SNR ¼ SD This analysis was performed by retrospectively comparing CT images from both the routine and the ASIR studies in a mediastinal window with a level setting of 400/ 40 HU. The ROIs were positioned to encompass a homogeneous portion without surrounding structures. A single circular ROI was drawn in the centre of the trachea at the level above the bifurcation and in the prethoracic air inside the display field of view (FOV) to measure noise with SD. ROIs were positioned in the tumour area of the lung as well as in the surrounding lung tissue (e.g., for detection of lymphangitis carcinomatosa) and in the healthy pulmonary tissue. Additional ROIs were placed in the pulmonary artery main stem, in the subscapularis muscle and in the liver. Next, different CNRs for tumour/surrounding lung, tumour/healthy lung, tumour/pulmonary artery, tumour/ subscapularis muscle, and tumour/liver were calculated. The subscapularis muscle was chosen because it is typically at the widest part of the chest, where noise caused by the shoulders and clavicles is the greatest.19
Dose analysis Doseelength product (DLP; measure of ionising radiation exposure during the entire acquisition of images) values were recorded on the dose page for each of the studies, and the effective dose was calculated to estimate radiation dose by multiplying DLP values by age-specific conversion factors.20 In addition, a novel dose estimation method, called sizespecific dose estimate (SSDE) was used (Fig 2). SSDE was calculated for 71 of the 79 examinations by using the method described in the American Association of Physicists in Medicine task group report 204, with the scout CT images used to measure the transverse and anteroposterior diameters of each patient.21 Transverse and anteroposterior diameters were summed to determine the conversion factor needed for SSDE determination.22
Statistical analysis The data were analysed using SPSS 20.0 (SPSS, Chicago, IL, USA). Differences such as age, radiation dose, and objective image noise were tested for statistical significance
All examinations were successfully performed in all patients. There was no significant difference in acquisition time between FBP and ASIR analysis.
Qualitative analysis Overall diagnostic validity was excellent for both ASIR and FBP studies. With regard to the five features that were assessed qualitatively, diagnostic validity, qualitative noise, visualisation of small structures, visualisation of tumour, and presence of artefacts, there were no statistically significant differences between the FBP and ASIR images. Both pulmonary metastasis (see Fig 3) and bronchial carcinoma, e.g., at the hilum (Fig 4), were reliably detected. Qualitative analysis yielded the following results: all CT examinations were estimated to be of excellent image quality without artefacts compromising diagnosis (Level 5) in terms of diagnostic validity, visualised tumour, and image artefacts (Table 2). Regarding qualitative noise and visualisation of small structures, no statistically significant differences resulted, with qualitative noise estimated at 4.60.5 and 4.40.4 for the FBP and ASIR protocol. For visualisation of small structures the results were 4.60.5 and 4.70.5, respectively. There were no significant differences between the two groups (Table 2).
Quantitative measurements Comparing the CNRs on FBP and ASIR images, no statistically significant difference was found between the two protocols for tumour/surrounding lung (p¼0.587), tumour/ healthy lung (p¼0.201), tumour/pulmonary artery (p¼0.183), tumour/subscapularis muscle (p¼0.359), or tumour/liver (p¼0.314) (Table 3). Comparison of the SNR of FBP and ASIR images revealed no significant differences, except for the tumour/surrounding lung (p¼0.022). All other SNR values (tumour, p¼0.196; healthy lung, p¼0.631; pulmonary artery, p¼0.848; subscapularis muscle, p¼0.566; liver, p¼0.572) were not significantly different between the two groups (Table 3).
Radiation dose The average effective dose delivered with the ASIR protocol was 4.12.5 mSv, whereas that of the standard FBP protocol was 6.43.7 mSv, corresponding to an average reduction of 36%. The difference between values obtained with ASIR and FBP was statistically significant (p<0.0001, Wilcoxon test), as was that between the DLP values (275170 versus 430240 mGy∙cm; p<0.0001). DLP was
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
€fer et al. / Clinical Radiology xxx (2016) 1e8 M.-L. Scha
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Figure 2 Analysis of the imaging parameters and dose parameters with an Isocenter graphic and tube current modulation by use of DoseWatch (General Electric).
36% lower for the ASIR protocol compared to the CT examinations without ASIR (Table 4). Mean patient SSDE for FBP examinations was 9.33 mGy and mean patient SSDE for ASIR examinations was 6.22.1 mGy. On average, the mean patient dose based on SSDE was 34% lower for the ASIR protocol than for the FBP protocol and was statistically significant (p<0.0001, Wilcoxon test; Table 4).
Discussion In recent decades, CT has become widespread in all developed countries due to its speed, ease of use, and availability.23 For monitoring of the patients with bronchial
carcinoma during therapy, CT represents the method of choice.2 Its use, however, is not free from risks, and it is hoped that the dialogue among radiologists, emergency room staff, and other physicians will be ongoing to slow the increase in CT usage and dose, without compromising patient care.4 For image reconstruction in CT, FBP has been used since the 1970s. FBP allows very rapid image reconstruction; however, this algorithm suffers from drawbacks due to approximation of the real focal spot and detector sizes, and the data are corrupted by quantum and electronic noise during the acquisition, thus propagating image noise.24 Dose reduction strategies have been implemented; the most important are reducing scan time and extent and
Figure 3 A 71-year-old female patient with pulmonary metastases of uterine leiomyosarcoma (arrows). Note the excellent image quality for both FBP (a) and ASIR (b) images, with a significant dose reduction in ASIR images of 47%. €fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
€fer et al. / Clinical Radiology xxx (2016) 1e8 M.-L. Scha
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Figure 4 A 68-year-old male patient with non-small-cell lung cancer in the right lung perihilar (arrow). Note the comparable visualised tumour for both FBP (a) and ASIR (b) images, with a significant dose reduction in ASIR images of 43%.
modulating X-ray tube parameters. Additional technologies such as prepatient collimation, improved filters, and automatic tube current modulation25 have been developed to reduce radiation exposure without decreasing image quality. Most recently, types of iterative reconstructions from various vendors have been introduced, all indicating their potential for lowering the radiation dose of CT studies.26e30 ASIR is an example of a new reconstruction algorithm designed to reduce noise levels. Lower radiation doses can then be used without sacrificing image quality. In this study, the use of ASIR to reduce radiation exposure of patients being evaluated for bronchial carcinoma or pulmonary metastases was examined. In the present study, image noise was lower in examinations performed with the ASIR protocol, both on subjective and objective measurements. These observations are comparable to the study of Romagnoli et al.31 Reduction of the mean effective dose achieved with the use of the ASIR protocol used in combination with modulation of tube parameters (pitch increase, tube-intensity modulation, and voltage reduction in normal-to-underweight patients) was 36% on average, in line with the study of Pontana et al.,27,28 and was similar to previous reported clinical studies on chest and abdominal CT (range, 25e44%).32,33 The variability of the reported data on dose reduction may be explained by the heterogeneity of patients and by the great variability of CT protocols used in published studies. Table 2 Qualitative analysis of images with FBP- and ASIR-protocol.
Diagnostic validity Qualitative noise Visualisation of small structures Visualised tumour Artefacts
FBP-protocol
ASIR-protocol
5 4.6 0.5 4.7 0.5 5 5
5 4.4 0.4 4.6 0.5 5 5
Comparing FBP and ASIR examinations acquired under otherwise similar conditions, no statistically significant difference was found in the objective evaluation of image quality, as assessed by the measurements of image noise, SNR, and CNR, except for the SNR of the lung tissue around tumours (p¼0.022). This exception may be due to strong scatter, especially in the surrounding lung tissue and may possibly result from the presence of lymphangiosis carcinomatosa. The overall qualitative image scores did not differ between the FBP group and the ASIR group, confirming the hypothesis that iterative reconstruction could provide comparable image quality despite the 36% dose reduction applied to the follow-up examination. The implementation of iterative reconstruction in routine clinical practice requires significant hardware efforts to avoid excessive image reconstruction times. Despite methodological differences, the present results can be compared to those Table 3 Contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR); standard deviation in parentheses.
CNR Tumour/surrounding lung Tumour/healthy lung Tumour/pulmonary artery Tumour/subscapularis muscle Tumour/liver SNR Tumour Surrounding tumour lung Healthy lung Pulmonary artery Subscapularis muscle Liver FBP, filtered back reconstruction.
FBP
ASIR
Statistical significance (paired t-test)
118.5 (39.2) 126.9 (39.8) 34.6 (36.9) 21.1 (34.3) 25.5 (34.7)
120.8 (40.0) 132.2 (38.7) 29.7 (31.5) 18.0 (28.8) 22.1 (29.7)
0.587 0.201 0.183 0.359 0.314
153.9 (39.9) 35.4 (17.1) 27.1 (12.3) 188.5 (34.2) 174.5 (31.1) 179.3 (31.5)
159.9 (42.1) 39.1 (21.7) 27.6 (11.5) 189.3 (45.2) 176.8 (42.0) 181.6 (43.2)
0.196 0.022 0.631 0.848 0.566 0.572
projection;
ASIR,
adaptive
statistical
iterative
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
€fer et al. / Clinical Radiology xxx (2016) 1e8 M.-L. Scha Table 4 Radiation dose with FBP- and ASIR-protocol. Significance FBP protocol ASIR protocol Dose reduction (%) Effective dose, mSv 6.3 3.7 Doseelength 430 240 product, mGy∙cm Size-specific dose 9.3 3 estimate, mGy FBP, filtered back reconstruction.
projection;
4.1 2.5 275 170
36 36
<0.0001 <0.0001
6.2 2.1
34
<0.0001
ASIR,
adaptive
statistical
iterative
of Prakash et al.29 Similar to the present study, they investigated the images for small anatomical details in patients with diffuse lung disease, abnormal findings, image quality, image noise, and artefacts. They reported the possibility of reducing patient dose by 27.6% by using ASIR, while image noise was simultaneously reduced by 24.1%. This dose reduction is lower than in the present study (36% dose reduction with simultaneous reduction of image noise when using ASIR); this might be a result of different noise indices (12.5e18.75 [noise indexes per body weight] in the study of Prakash et al. versus noise index of 25 in the present study). The study of Neroladaki et al.34 assessed the diagnostic image quality of ultra-low-dose chest CT with 40% and 80% ASIR and with model-based iterative reconstruction (MBIR, Veo) in comparison with standard dose diagnostic CT (SDDCT) or with low-dose diagnostic CT (LDD-CT). Using this technique, they achieved a dose reduction of 98.6% compared with SDD-CT and 94% compared with LDD-CT, but the implementation of MBIR results in much longer image reconstruction time, requiring an hour or more.34 The study of Smith et al.35 achieved a dose reduction of approximately 45% in paediatric patients with an FBP and ASIR protocol, whereas the present study detected lower dose reduction with 34% in SSDE measurements. In contrast to the present study, they compared a reduced-dose CT protocol with MBIR technology, VEO (GE Healthcare) and a standard-dose CT protocol (30% ASIR) with FBP in paediatric body CT. A few limitations of the present study should be highlighted. A major limitation of the present study, as well as of most studies published so far,14,19,27,29,31,34 is the retrospective design. Patients were not scanned at both standard and reduced tube currents in 1 day, as it would be impractical to prospectively scan each patient twice (once at standard dose and once at reduced dose). The investigated patients, the CT system, and the imaging protocol parameters were identical except for the use of ASIR. An additional limitation was blinding of the radiologists; it was difficult because the visual appearance of ASIR images was different from the appearance of the images the reviewers were used to in their clinical practice, i.e., FBP images; however, the radiologists were blinded to the identity of the ASIR and FBP images, as well as to the clinical and pathological information. Image quality evaluation was based on the subjective impression of two readers. A further limitation might be the heterogeneous histology in the present patient population with bronchial
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carcinoma and pulmonary metastases with different primary tumours. Such heterogeneity might give rise to bias. Finally, ASIR was not compared with other noisereducing algorithms, such as iterative reconstruction in image space (IRIS; Siemens Medical Solutions), MBIR (GE Healthcare), sinogram-affirmed iterative reconstruction (SAFIRE; Siemens Medical Solutions), or adaptive iterative dose reduction (AIDR; Toshiba Medical Systems). In this initial study, the potential correlations between stage of tumour and nodule size and between intrapulmonary masses and other lung diseases (e.g., nodes in sarcoidosis) were not analysed; these could be interesting issues in prospective studies. In conclusion, the results of the present study show that ASIR at the 40% setting can be used to significantly decrease radiation exposure during CT of the chest in patients with bronchial carcinoma or with pulmonary metastases without sacrificing quantitative image quality. This is especially important in patients with frequent CT examinations, thus reducing the accumulative radiation exposure.
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27. Pontana F, Pagniez J, Flohr T, et al. Chest computed tomography using iterative reconstruction vs filtered back projection (Part 1): evaluation of image noise reduction in 32 patients. Eur Radiol 2011;21:627e35. 28. Pontana F, Duhamel A, Pagniez J, et al. Chest computed tomography using iterative reconstruction vs filtered back projection (Part 2): image quality of low-dose CT examinations in 80 patients. Eur Radiol 2011;21:636e43. 29. Prakash P, Kalra MK, Ackman JB, et al. Diffuse lung disease: CT of the chest with adaptive statistical iterative reconstruction technique. Radiology 2010;256(1):261e9. 30. Winklehner A, Goetti R, Baumueller S, et al. Automated attenuationbased tube potential selection for thoracoabdominal computed tomography angiography: improved dose effectiveness. Invest Radiol 2011;46:767e73. 31. Romagnoli A, Funel V, Meschini A, et al. Optimisation of low-dose CT with adaptive statistical iterative reconstruction in total body examination. Radiol ed 2012;117:1333e46. 32. Baumueller S, Alkadhi H, Stolzmann P, et al. Computed tomography of the lung in the high-pitch mode. Is breath holding still required? Invest Radiol 2011;46:240e5. 33. Leipsic J, Nguyen G, Brawn J, et al. A prospective evaluation of dose reduction and image quality in chest CT using adaptive statistical iterative reconstruction. AJR Am J Roentgenol 2010;195:1095e9. 34. Neroladaki A, Botsikas D, Boudabbous S, et al. Computed tomography of the chest with model-based iterative reconstruction using a radiation exposure similar to chest X-ray examination: preliminary observations. Eur Radiol 2013;23:360e6. 35. Smith EA, Dillman JR, Goodsitt MM, et al. Model-based Iterative Reconstruction: effect on patient radiation dose and image quality in pediatric body CT. Radiology 2014;2:526e34.
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
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Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for patients with bronchial carcinoma and intrapulmonary metastases €fer a, *, L. Lu € demann b, G. Bo € ning a, J. Kahn a, S. Fuchs a, M.-L. Scha B. Hamm a, F. Streitparth a €tsmedizin Berlin, Campus Virchow Klinikum, Augustenburger Platz 1, Department of Radiology, Charit e - Universita 13353 Berlin, Germany b €tsklinikum Essen, Hufelandstr. 55, 45147 Essen, Germany Department of Radiotherapy, Universita a
art icl e i nformat ion Article history: Received 4 September 2015 Received in revised form 7 January 2016 Accepted 14 January 2016
AIM: To compare the radiation dose and image quality of 64-row chest computed tomography (CT) in patients with bronchial carcinoma or intrapulmonary metastases using full-dose CT reconstructed with filtered back projection (FBP) at baseline and reduced dose with 40% adaptive statistical iterative reconstruction (ASIR) at follow-up. MATERIALS AND METHODS: The chest CT images of patients who underwent FBP and ASIR studies were reviewed. Doseelength products (DLP), effective dose, and size-specific dose estimates (SSDEs) were obtained. Image quality was analysed quantitatively by signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) measurement. In addition, image quality was assessed by two blinded radiologists evaluating images for noise, contrast, artefacts, visibility of small structures, and diagnostic acceptability using a five-point scale. RESULTS: The ASIR studies showed 36% reduction in effective dose compared with the FBP studies. The qualitative and quantitative image quality was good to excellent in both protocols, without significant differences. There were also no significant differences for SNR except for the SNR of lung surrounding the tumour (FBP: 3517, ASIR: 3922). DISCUSSION: A protocol with 40% ASIR can provide approximately 36% dose reduction in chest CT of patients with bronchial carcinoma or intrapulmonary metastases while maintaining excellent image quality. Ó 2016 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction The 2014 World Cancer Report of the International Agency for Research on Cancer, the specialised cancer €fer, Institut fu € r Radiologie, * Guarantor and correspondent: M.-L. Scha € r Strahlenheilkunde, Charite Berlin, Campus Virchow, Klinik fu Augustenburger Platz 1, 13353 Berlin, Germany. Tel.: þ49 176 20 78 73 18. E-mail address: [email protected] (M.-L. Sch€ afer).
agency of the World Health Organisation WHO, concluded that, by 2025, approximately 20 million people could develop cancer each year, about 40% more than at present. With 1.8 million new cases, lung cancer was the most prevalent cancer in 2012.1 To monitor patients with bronchial carcinoma during therapy, computed tomography (CT) represents the method of choice.2 The number of CT procedures has increased rapidly since it became commercially available.3 CT is fast and readily available and provides high-
http://dx.doi.org/10.1016/j.crad.2016.01.013 0009-9260/Ó 2016 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
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€fer et al. / Clinical Radiology xxx (2016) 1e8 M.-L. Scha
resolution cross-sectional images.4 Although controversial, recent data suggest that medical radiation exposure may significantly increase the risk of adverse radiation effects.4e6 The lifetime cancer risk based on current CT usage has been estimated to be as high as 2%.7 To counteract these developments, radiologists must carefully check whether alternative imaging methods, such as ultrasound or magnetic resonance imaging (MRI), can be used. In the large number of cases, where CT cannot be replaced, the major challenge is to achieve adequate image quality and maintain the high diagnostic benefit of CT with minimum radiation exposure. Several technical developments that help the radiologist reduce radiation exposure of CT, such as automatic exposure control have been successfully implemented.8,9 Other recent CT dose reduction techniques include optimising imaging parameters such as reducing tube peak voltage.10 Other important strategies to reduce radiation doses include the use of shorter scan lengths11 or avoiding unnecessary multiphase CT studies.12 The acquisition of CT images with lower tube voltage or tube current may increase image noise, which is one of the most important factors determining image quality and hence diagnostic accuracy.13 A technique to overcome the problem of image noise and artefacts resulting from the use of reduced voltage and current is the adaptive statistical iterative reconstruction (ASIR) method (GE Healthcare, Milwaukee, WI, USA). This method attempts to accurately rebuild images by concentrating on noise reduction.14 Several studies have investigated the use of ASIR12e15; however, there are differing reports on the application of ASIR to chest CT16,17 and, to the authors’ knowledge, no studies have been performed to assess the potential benefits of ASIR in chest CT examinations of patients with bronchial carcinoma or pulmonary metastases. It was hypothesised that a low-dose protocol, when used in conjunction with the ASIR technique, will significantly reduce the radiation exposure of patients with bronchial carcinoma or pulmonary metastases undergoing chest CT without compromising the image quality and diagnostic accuracy compared to a routine dose protocol reconstructed with FBP. Therefore, the purpose of this study was to retrospectively compare radiation dose and image quality of chest CT images of patients with bronchial carcinoma or pulmonary metastases performed using a routine dose protocol with FBP and a low-dose protocol with ASIR using the same CT system.
Materials and methods
The interval between the two CT examinations was 14 months at maximum. A total of 1957 patients were referred to the authors’ institution for body CT regarding diagnostic evaluation or possible treatment. Following application of various exclusion criteria (see Fig 1), 79 consecutive patients (36 women, 43 men; mean age: 6411 years) were retrospectively enrolled in this study. Among others, patients with differences in scan lengths of >40 mm in the FBP and ASIR CT examinations were excluded. Patients with implanted pacemakers or port systems were also excluded from the study to avoid bias in noise measurements. For the additional evaluation of SSDE, eight patients were excluded due to missing data, such as missing scout. All bronchial carcinoma or pulmonary metastases were histologically proven, the diagnoses are summarised in Table 1. This study was approved by the human research committee of the institutional review board.
CT examination All CT examinations were performed using a Lightspeed VCT 64-row CT system (GE Healthcare). The chest CT protocol was used in helical mode: 0.4 s gantry rotation time, 120 kVp, 0.984:1 beam pitch, 40 mm table feed per gantry rotation, a z-axis tube current modulation was used (100/ 500 mA min/max tube current), with a noise index of 25 and a 640.625 mm collimation. Before using ASIR in clinical practice, different levels of ASIR were tested using a torso phantom imaged on the same CT system. In these phantom experiments, the image quality was assessed by two independent radiologists. The blend of 40/60% ASIR/FBP was chosen for the chest CT examinations after trying preliminary reconstruction with various ASIR percentages from 10% to 100%. The 40% blending of ASIR was the best blending to reduce noise sufficiently without excessive smoothing artefact resulting from higher percentages of ASIR blends. The contrast medium injection protocol and patient preparation were the same for the two CT examinations analysed for intra-individual comparison. Chest CT examinations were performed with a concentration of 300 mg iodine/ml of the intravenous contrast agent, iobitridol (Xenetix, Laboratoires Guerbet, Roissy, France). For contrast enhancement, 1e1.5 ml/kg of contrast medium was injected through an upper extremity peripheral intravenous line, followed by a saline chaser of 0.5 ml/kg. The injection speed was 2.5 ml/s (delay time 80 seconds). The imaging parameters of the routine study protocol and the ASIR study protocol were completely identical, except for the reduced tube current.
Patients and study design Image analysis A retrospective review of the database was undertaken for patients with bronchial carcinoma or pulmonary metastases who had undergone contrast-enhanced low-dose chest CT with ASIR from September 2012 to March 2013 and a previous chest CT study with a routine dose protocol on the same CT system using an otherwise identical protocol.
The chest CT images of all studies with bronchial carcinoma or pulmonary metastases acquired with the low-dose protocol and 40% ASIR (the ASIR study) were retrospectively reviewed by two independent blinded radiologists, who also reviewed the corresponding chest CT images previously
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
€fer et al. / Clinical Radiology xxx (2016) 1e8 M.-L. Scha
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Figure 1 Exclusion criteria for the study population.
taken with the routine dose protocol and FBP (the routine study) in the same set of patients. Qualitative scoring and quantitative measurements were obtained for each study using our institution’s picture archiving and communication
Table 1 Overview of diagnoses in the study patients (79)*. Diagnosis
n
Non-small cell lung cancer (NSCLC) Small cell lung cancer (SCLC) Pulmonary metastases Pancreas carcinoma Mamma carcinoma Neuroendocrine tumour Gastric carcinoma Oesophagus carcinoma Oropharynx carcinoma Renal cell carcinoma Chordoma Leiomyosarcoma of the uterus Alveolar soft part sarcoma (ASPS) Malignant peripheral nerve sheath tumour (MPNST) Osteosarcoma Squamous cell carcinoma of the glandula submandibularis Ovary carcinoma
39 12 28 7 3 3 2 2 2 2 1 1 1 1 1 1 1
*All diagnoses were proven histologically.
system (PACS) workstation (Centricity Radiology RA1000; GE Medical Systems).
Qualitative analysis Two radiologists with 6 and 10 years of experience in reading chest CT performed qualitative analysis independently on a high-resolution diagnostic monitor comparing CT images from both the routine and the ASIR studies in a lung window with a level setting of 1500/500 HU. The images were randomised and made anonymous so that the reader was unaware of patient data and imaging parameters. Image quality was graded on a five-point Likert scale as follows: 5 ¼ excellent image quality; 4 ¼ good quality; 3 ¼ fair quality: 2 ¼ poor quality but adequate for evaluation; 1 ¼ unacceptable image. The two radiologists independently recorded their subjective impression of diagnostic validity, qualitative noise, visualisation of small structures, visualisation of tumour, and presence of artefacts.
Quantitative measurements Objective image noise was measured by obtaining the standard deviation (SD) of the mean radiodensity value
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
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(HU) within a homogeneous region of interest (ROI). For the statistical measurements all values were normalised. The contrast-to-noise ratio (CNR) was calculated using the following equation: . CNR ¼ ðROITissue1 ROITissue2 Þ SDbackground
using Pearson’s chi-square test, Fisher’s exact test, or a t-test for unpaired samples. The Wilcoxon signed rank test was used to assess subjective image quality scores of the routine (FBP) and the ASIR studies. A p-value of <0.05 was considered to be statistically significant.
where ROITissue1 and ROITissue2 are the mean HU values in the corresponding tissues, and SDbackground is the standard deviation in the surrounding background. Another quantitative measure was the signal-to-noise ratio (SNR), which was defined as specific organ attenuation divided by standard deviation of noise (i.e. air)18:
Results
ROI SNR ¼ SD This analysis was performed by retrospectively comparing CT images from both the routine and the ASIR studies in a mediastinal window with a level setting of 400/ 40 HU. The ROIs were positioned to encompass a homogeneous portion without surrounding structures. A single circular ROI was drawn in the centre of the trachea at the level above the bifurcation and in the prethoracic air inside the display field of view (FOV) to measure noise with SD. ROIs were positioned in the tumour area of the lung as well as in the surrounding lung tissue (e.g., for detection of lymphangitis carcinomatosa) and in the healthy pulmonary tissue. Additional ROIs were placed in the pulmonary artery main stem, in the subscapularis muscle and in the liver. Next, different CNRs for tumour/surrounding lung, tumour/healthy lung, tumour/pulmonary artery, tumour/ subscapularis muscle, and tumour/liver were calculated. The subscapularis muscle was chosen because it is typically at the widest part of the chest, where noise caused by the shoulders and clavicles is the greatest.19
Dose analysis Doseelength product (DLP; measure of ionising radiation exposure during the entire acquisition of images) values were recorded on the dose page for each of the studies, and the effective dose was calculated to estimate radiation dose by multiplying DLP values by age-specific conversion factors.20 In addition, a novel dose estimation method, called sizespecific dose estimate (SSDE) was used (Fig 2). SSDE was calculated for 71 of the 79 examinations by using the method described in the American Association of Physicists in Medicine task group report 204, with the scout CT images used to measure the transverse and anteroposterior diameters of each patient.21 Transverse and anteroposterior diameters were summed to determine the conversion factor needed for SSDE determination.22
Statistical analysis The data were analysed using SPSS 20.0 (SPSS, Chicago, IL, USA). Differences such as age, radiation dose, and objective image noise were tested for statistical significance
All examinations were successfully performed in all patients. There was no significant difference in acquisition time between FBP and ASIR analysis.
Qualitative analysis Overall diagnostic validity was excellent for both ASIR and FBP studies. With regard to the five features that were assessed qualitatively, diagnostic validity, qualitative noise, visualisation of small structures, visualisation of tumour, and presence of artefacts, there were no statistically significant differences between the FBP and ASIR images. Both pulmonary metastasis (see Fig 3) and bronchial carcinoma, e.g., at the hilum (Fig 4), were reliably detected. Qualitative analysis yielded the following results: all CT examinations were estimated to be of excellent image quality without artefacts compromising diagnosis (Level 5) in terms of diagnostic validity, visualised tumour, and image artefacts (Table 2). Regarding qualitative noise and visualisation of small structures, no statistically significant differences resulted, with qualitative noise estimated at 4.60.5 and 4.40.4 for the FBP and ASIR protocol. For visualisation of small structures the results were 4.60.5 and 4.70.5, respectively. There were no significant differences between the two groups (Table 2).
Quantitative measurements Comparing the CNRs on FBP and ASIR images, no statistically significant difference was found between the two protocols for tumour/surrounding lung (p¼0.587), tumour/ healthy lung (p¼0.201), tumour/pulmonary artery (p¼0.183), tumour/subscapularis muscle (p¼0.359), or tumour/liver (p¼0.314) (Table 3). Comparison of the SNR of FBP and ASIR images revealed no significant differences, except for the tumour/surrounding lung (p¼0.022). All other SNR values (tumour, p¼0.196; healthy lung, p¼0.631; pulmonary artery, p¼0.848; subscapularis muscle, p¼0.566; liver, p¼0.572) were not significantly different between the two groups (Table 3).
Radiation dose The average effective dose delivered with the ASIR protocol was 4.12.5 mSv, whereas that of the standard FBP protocol was 6.43.7 mSv, corresponding to an average reduction of 36%. The difference between values obtained with ASIR and FBP was statistically significant (p<0.0001, Wilcoxon test), as was that between the DLP values (275170 versus 430240 mGy∙cm; p<0.0001). DLP was
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
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Figure 2 Analysis of the imaging parameters and dose parameters with an Isocenter graphic and tube current modulation by use of DoseWatch (General Electric).
36% lower for the ASIR protocol compared to the CT examinations without ASIR (Table 4). Mean patient SSDE for FBP examinations was 9.33 mGy and mean patient SSDE for ASIR examinations was 6.22.1 mGy. On average, the mean patient dose based on SSDE was 34% lower for the ASIR protocol than for the FBP protocol and was statistically significant (p<0.0001, Wilcoxon test; Table 4).
Discussion In recent decades, CT has become widespread in all developed countries due to its speed, ease of use, and availability.23 For monitoring of the patients with bronchial
carcinoma during therapy, CT represents the method of choice.2 Its use, however, is not free from risks, and it is hoped that the dialogue among radiologists, emergency room staff, and other physicians will be ongoing to slow the increase in CT usage and dose, without compromising patient care.4 For image reconstruction in CT, FBP has been used since the 1970s. FBP allows very rapid image reconstruction; however, this algorithm suffers from drawbacks due to approximation of the real focal spot and detector sizes, and the data are corrupted by quantum and electronic noise during the acquisition, thus propagating image noise.24 Dose reduction strategies have been implemented; the most important are reducing scan time and extent and
Figure 3 A 71-year-old female patient with pulmonary metastases of uterine leiomyosarcoma (arrows). Note the excellent image quality for both FBP (a) and ASIR (b) images, with a significant dose reduction in ASIR images of 47%. €fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
€fer et al. / Clinical Radiology xxx (2016) 1e8 M.-L. Scha
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Figure 4 A 68-year-old male patient with non-small-cell lung cancer in the right lung perihilar (arrow). Note the comparable visualised tumour for both FBP (a) and ASIR (b) images, with a significant dose reduction in ASIR images of 43%.
modulating X-ray tube parameters. Additional technologies such as prepatient collimation, improved filters, and automatic tube current modulation25 have been developed to reduce radiation exposure without decreasing image quality. Most recently, types of iterative reconstructions from various vendors have been introduced, all indicating their potential for lowering the radiation dose of CT studies.26e30 ASIR is an example of a new reconstruction algorithm designed to reduce noise levels. Lower radiation doses can then be used without sacrificing image quality. In this study, the use of ASIR to reduce radiation exposure of patients being evaluated for bronchial carcinoma or pulmonary metastases was examined. In the present study, image noise was lower in examinations performed with the ASIR protocol, both on subjective and objective measurements. These observations are comparable to the study of Romagnoli et al.31 Reduction of the mean effective dose achieved with the use of the ASIR protocol used in combination with modulation of tube parameters (pitch increase, tube-intensity modulation, and voltage reduction in normal-to-underweight patients) was 36% on average, in line with the study of Pontana et al.,27,28 and was similar to previous reported clinical studies on chest and abdominal CT (range, 25e44%).32,33 The variability of the reported data on dose reduction may be explained by the heterogeneity of patients and by the great variability of CT protocols used in published studies. Table 2 Qualitative analysis of images with FBP- and ASIR-protocol.
Diagnostic validity Qualitative noise Visualisation of small structures Visualised tumour Artefacts
FBP-protocol
ASIR-protocol
5 4.6 0.5 4.7 0.5 5 5
5 4.4 0.4 4.6 0.5 5 5
Comparing FBP and ASIR examinations acquired under otherwise similar conditions, no statistically significant difference was found in the objective evaluation of image quality, as assessed by the measurements of image noise, SNR, and CNR, except for the SNR of the lung tissue around tumours (p¼0.022). This exception may be due to strong scatter, especially in the surrounding lung tissue and may possibly result from the presence of lymphangiosis carcinomatosa. The overall qualitative image scores did not differ between the FBP group and the ASIR group, confirming the hypothesis that iterative reconstruction could provide comparable image quality despite the 36% dose reduction applied to the follow-up examination. The implementation of iterative reconstruction in routine clinical practice requires significant hardware efforts to avoid excessive image reconstruction times. Despite methodological differences, the present results can be compared to those Table 3 Contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR); standard deviation in parentheses.
CNR Tumour/surrounding lung Tumour/healthy lung Tumour/pulmonary artery Tumour/subscapularis muscle Tumour/liver SNR Tumour Surrounding tumour lung Healthy lung Pulmonary artery Subscapularis muscle Liver FBP, filtered back reconstruction.
FBP
ASIR
Statistical significance (paired t-test)
118.5 (39.2) 126.9 (39.8) 34.6 (36.9) 21.1 (34.3) 25.5 (34.7)
120.8 (40.0) 132.2 (38.7) 29.7 (31.5) 18.0 (28.8) 22.1 (29.7)
0.587 0.201 0.183 0.359 0.314
153.9 (39.9) 35.4 (17.1) 27.1 (12.3) 188.5 (34.2) 174.5 (31.1) 179.3 (31.5)
159.9 (42.1) 39.1 (21.7) 27.6 (11.5) 189.3 (45.2) 176.8 (42.0) 181.6 (43.2)
0.196 0.022 0.631 0.848 0.566 0.572
projection;
ASIR,
adaptive
statistical
iterative
€fer M-L, et al., Radiation dose reduction in CT with adaptive statistical iterative reconstruction (ASIR) for Please cite this article in press as: Scha patients with bronchial carcinoma and intrapulmonary metastases, Clinical Radiology (2016), http://dx.doi.org/10.1016/j.crad.2016.01.013
€fer et al. / Clinical Radiology xxx (2016) 1e8 M.-L. Scha Table 4 Radiation dose with FBP- and ASIR-protocol. Significance FBP protocol ASIR protocol Dose reduction (%) Effective dose, mSv 6.3 3.7 Doseelength 430 240 product, mGy∙cm Size-specific dose 9.3 3 estimate, mGy FBP, filtered back reconstruction.
projection;
4.1 2.5 275 170
36 36
<0.0001 <0.0001
6.2 2.1
34
<0.0001
ASIR,
adaptive
statistical
iterative
of Prakash et al.29 Similar to the present study, they investigated the images for small anatomical details in patients with diffuse lung disease, abnormal findings, image quality, image noise, and artefacts. They reported the possibility of reducing patient dose by 27.6% by using ASIR, while image noise was simultaneously reduced by 24.1%. This dose reduction is lower than in the present study (36% dose reduction with simultaneous reduction of image noise when using ASIR); this might be a result of different noise indices (12.5e18.75 [noise indexes per body weight] in the study of Prakash et al. versus noise index of 25 in the present study). The study of Neroladaki et al.34 assessed the diagnostic image quality of ultra-low-dose chest CT with 40% and 80% ASIR and with model-based iterative reconstruction (MBIR, Veo) in comparison with standard dose diagnostic CT (SDDCT) or with low-dose diagnostic CT (LDD-CT). Using this technique, they achieved a dose reduction of 98.6% compared with SDD-CT and 94% compared with LDD-CT, but the implementation of MBIR results in much longer image reconstruction time, requiring an hour or more.34 The study of Smith et al.35 achieved a dose reduction of approximately 45% in paediatric patients with an FBP and ASIR protocol, whereas the present study detected lower dose reduction with 34% in SSDE measurements. In contrast to the present study, they compared a reduced-dose CT protocol with MBIR technology, VEO (GE Healthcare) and a standard-dose CT protocol (30% ASIR) with FBP in paediatric body CT. A few limitations of the present study should be highlighted. A major limitation of the present study, as well as of most studies published so far,14,19,27,29,31,34 is the retrospective design. Patients were not scanned at both standard and reduced tube currents in 1 day, as it would be impractical to prospectively scan each patient twice (once at standard dose and once at reduced dose). The investigated patients, the CT system, and the imaging protocol parameters were identical except for the use of ASIR. An additional limitation was blinding of the radiologists; it was difficult because the visual appearance of ASIR images was different from the appearance of the images the reviewers were used to in their clinical practice, i.e., FBP images; however, the radiologists were blinded to the identity of the ASIR and FBP images, as well as to the clinical and pathological information. Image quality evaluation was based on the subjective impression of two readers. A further limitation might be the heterogeneous histology in the present patient population with bronchial
7
carcinoma and pulmonary metastases with different primary tumours. Such heterogeneity might give rise to bias. Finally, ASIR was not compared with other noisereducing algorithms, such as iterative reconstruction in image space (IRIS; Siemens Medical Solutions), MBIR (GE Healthcare), sinogram-affirmed iterative reconstruction (SAFIRE; Siemens Medical Solutions), or adaptive iterative dose reduction (AIDR; Toshiba Medical Systems). In this initial study, the potential correlations between stage of tumour and nodule size and between intrapulmonary masses and other lung diseases (e.g., nodes in sarcoidosis) were not analysed; these could be interesting issues in prospective studies. In conclusion, the results of the present study show that ASIR at the 40% setting can be used to significantly decrease radiation exposure during CT of the chest in patients with bronchial carcinoma or with pulmonary metastases without sacrificing quantitative image quality. This is especially important in patients with frequent CT examinations, thus reducing the accumulative radiation exposure.
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