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      Development and validation of a low dose simulator for computed tomography

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          Abstract

          Purpose

          To develop and validate software for facilitating observer studies on the effect of radiation exposure on the diagnostic value of computed tomography (CT).

          Methods

          A low dose simulator was developed which adds noise to the raw CT data. For validation two phantoms were used: a cylindrical test object and an anthropomorphic phantom. Images of both were acquired at different dose levels by changing the tube current of the acquisition (500 mA to 20 mA in five steps). Additionally, low dose simulations were performed from 500 mA downwards to 20 mA in the same steps. Noise was measured within the cylindrical test object and in the anthropomorphic phantom. Finally, noise power spectra (NPS) were measured in water.

          Results

          The low dose simulator yielded similar image quality compared with actual low dose acquisitions. Mean difference in noise over all comparisons between actual and simulated images was 5.7 ± 4.6% for the cylindrical test object and 3.3 ± 2.6% for the anthropomorphic phantom. NPS measurements showed that the general shape and intensity are similar.

          Conclusion

          The developed low dose simulator creates images that accurately represent the image quality of acquisitions at lower dose levels and is suitable for application in clinical studies.

          Related collections

          Most cited references 13

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          Application of the noise power spectrum in modern diagnostic MDCT: part I. Measurement of noise power spectra and noise equivalent quanta.

          Dose reduction efforts in diagnostic CT have brought the tradeoff of dose versus image quality to the forefront. The need for meaningful characterization of image noise beyond that offered by pixel standard deviation is becoming increasingly important. This work aims to study the implementation of the noise power spectrum (NPS) and noise equivalent quanta (NEQ) on modern, multislice diagnostic CT scanners. The details of NPS and NEQ measurement are outlined and special attention is paid to issues unique to multislice CT. Aliasing, filter design and effects of acquisition geometry are investigated. While it was found that both metrics can be implemented in modern CT, it was discovered that NEQ cannot be aptly applied with certain non-traditional reconstruction filters or in helical mode. NPS and NEQ under a variety of conditions are examined. Extensions of NPS and NEQ to uses in protocol standardization are also discussed.
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            CT colonography at different radiation dose levels: feasibility of dose reduction.

            To investigate the sensitivity and specificity of polyp detection and the image quality of computed tomographic (CT) colonography at different radiation dose levels and to study effective doses reported in literature on CT colonography. CT colonography and colonoscopy were performed with 100 mAs in 50 consecutive patients at high risk for colorectal cancer; 50- and 30-mAs CT colonographic examinations were simulated with controlled addition of noise to raw transmission measurements. One radiologist randomly evaluated all original and simulated images for the presence of polyps and scored image quality. Differences in image quality were assessed with the Wilcoxon rank test. Scan protocols from the literature and recent (unpublished) updates were collected. In nine of 10 patients with polyps 5 mm in diameter or larger (sensitivity, 90%) and in seven of 17 patients with polyps smaller than 5 mm, polyps were correctly identified with CT colonography at all dose levels. Specificity for patients without polyps 5 mm or larger was 53%-60% at all dose levels and for patients without any polyps was 26% (at 100 and 50 mAs) and 48% (at 30 mAs). Image quality decreased significantly as the dose level decreased. The median effective doses (supine and prone positions) calculated from protocols reported in the literature and updates were 7.8 and 8.8 mSv, respectively. Although image quality decreases significantly at 30 mAs (3.6 mSv), polyp detection remains unimpaired. The median dose for CT colonography at institutions that perform CT colonographic research is currently 8.8 mSv.
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              Computer-simulated radiation dose reduction for abdominal multidetector CT of pediatric patients.

              Limiting CT radiation dose is especially critical when imaging children. The purpose of our study was to modify and test an accurate and safe tool for evaluating systematic dose reduction for abdominal multidetector CT (MDCT) in pediatric patients. After validating the computer-simulation technique with a water phantom, we subjected the original digital scanning data for 26 contrast-enhanced abdominal MDCT scans (120 mA) obtained in infants and children (age range, 1 month-9 years; mean age, 3.1 years) to simulated tube current reduction (100, 80, 60, and 40 mA) by adding noise. this procedure created four additional examinations per child that were identical to the originals except for image noise. The 130 examinations were scored randomly, independently, and without prior knowledge of the children's diagnoses by three radiologists for depiction of high-visibility structures, such as adrenal glands and fat in the intrahepatic falciform ligament, and low-visibility structures, such as the extrahepatic hepatic artery, small intrahepatic vessels, and common bile duct. Aligned rank and Wilcoxon's signed rank tests were used for statistical analyses. Simulated tube current reduction significantly affected the detection of low-visibility structures (p 0.5). The results of this computer simulation suggest that accurate abdominal MDCT can be performed in pediatric patients using substantially reduced radiation, depending on the indication for imaging. (In our case, the reduction was between 33% and 67%, depending on whether a high-visibility or low-visibility structure was being assessed.) This simulation technology can be applied to MDCT of other organ systems for systematic evaluation of radiation dose reduction.
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                Author and article information

                Contributors
                +31-71-5263687 , +31-71-5248256 , R.M.S.Joemai@lumc.nl
                Journal
                Eur Radiol
                European Radiology
                Springer-Verlag (Berlin/Heidelberg )
                0938-7994
                1432-1084
                30 September 2009
                30 September 2009
                April 2010
                : 20
                : 4
                : 958-966
                Affiliations
                [1 ]Radiology Department, Leiden University Medical Centre, Albinusdreef 2, 2333 ZA Leiden, The Netherlands
                [2 ]Radiology Department, Leiden University Medical Centre, PO Box 9600, 2300 RC Leiden, The Netherlands
                Article
                1617
                10.1007/s00330-009-1617-x
                2835638
                19789877
                © The Author(s) 2009
                Categories
                Physics
                Custom metadata
                © European Society of Radiology 2010

                Radiology & Imaging

                x-ray, computer simulation, physics, radiation, noise, computed tomography

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