Innovations in CT Dose Reduction Strategy: Application of the Adaptive Statistical Iterative Reconstruction Algorithm
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1 Medical Physics and Informatics Pictorial Essay Silva et al. T Dose Reduction Medical Physics and Informatics Pictorial Essay Downloaded from by on 12/18/17 from IP address opyright RRS. For personal use only; all rights reserved lvin. Silva 1 Holly J. Lawder my Hara Jennifer Kujak William Pavlicek Silva, Lawder HJ, Hara, Kujak J, Pavlicek W Keywords: adaptive statistical iterative reconstruction (SIR), filtered back projection, iterative reconstruction algorithm, low-dose T, radiation reduction DOI: /JR Received pril 23, 2009; accepted after revision June 26, Supported by a grant from GE Healthcare. 1 ll authors: Department of Radiology, Mayo linic rizona, E Shea lvd., Scottsdale, Z ddress correspondence to.. Silva (silva.alvin@mayo.edu). JR 2010; 194: X/10/ merican Roentgen Ray Society Innovations in T Dose Reduction Strategy: pplication of the daptive Statistical Iterative Reconstruction lgorithm OJETIVE. The purpose of this article is to discuss the application of a new T reconstruction algorithm, adaptive statistical iterative reconstruction (SIR), to reduce radiation dose at body T and to provide imaging examples in comparison with low-dose and standard-dose filtered back projection T. ONLUSION. The SIR reconstruction algorithm is a promising technique for providing diagnostic quality T images at significantly reduced radiation doses. s the number of T examinations performed in the United States has exploded into more than 60 million studies annually [1, 2], concerns regarding the effects of medical radiation exposure have also increased [3]. lthough controversial, recent data suggest that medical radiation exposure may significantly increase the risk of adverse radiation effects [4 7]. The lifetime cancer risk based on current T usage has been estimated to be as high as 2.0% [8]. umulative risks may be even higher for certain adult patient populations, particularly for those requiring multiple or multiphase studies, as well as in obese patients requiring increased radiation dose for adequate tissue penetration. Unfortunately, inherent in lowering the T radiation dose is increased image noise because the current reconstruction method, filtered back projection (FP), is unable to consistently generate diagnostic quality images with reduced x-ray tube currents (m) [9]. However, a new method for noise reduction based on iterative reconstruction algorithms has recently been developed that is able to correct image data using a system of models to improve image noise. This new algorithm, adaptive statistical iterative reconstruction (SIR), has the potential to preserve and enhance the diagnostic capability of T studies performed at reduced doses. This article will describe the technical aspects of the SIR algorithm and illustrate applications for improving image quality and reducing radiation dose in obese patients and current low-dose protocol examinations. In addition, its potential use for reducing radiation but preserving image quality for routine, standard-dose T examinations, as well as potential disadvantages, will be discussed. Technical: SIR Iterative reconstruction techniques attempt to accurately rebuild images by concentrating on noise reduction. One type of iterative reconstruction technique, SIR, uses information obtained from the FP algorithm as an initial building block for image reconstruction. The SIR model then uses matrix algebra to transform the measured value of each pixel (y) to a new estimate of the pixel value (y ). This pixel value is then compared with the ideal value that the noise model predicts. The process is repeated in successive iterative steps until the final estimated (X ) and ideal pixel values ultimately converge. X = arg min{l (x, y) + alphag(x)} Where alphag(x) is a regularization term and L is a cost function that penalizes poor statistics. Using this method, SIR is able to selectively identify and then subtract noise from an image. Thus, SIR reconstructs images with lower image noise compared with FP techniques (Fig. 1). Imaging in Obese Patients: Photon-Starved T Examinations Imaging of obese patients (defined as a MI of 25 or more in our patient population) is often complicated by scatter and relatively increased image noise at routine radiation JR:194, January
2 Silva et al. Downloaded from by on 12/18/17 from IP address opyright RRS. For personal use only; all rights reserved doses. Even if the dose is increased for adequate tissue penetration, some examinations are suboptimal because the required energy flux is greater than the maximal output of the current generation of x-ray tubes, with resultant relative photon-starved images (Fig. 2). This is potentially problematic because image noise can obscure normal anatomy (Fig. 3) or mimic disease (Fig. 4). T performed on obese patients with SIR allows image noise reduction and improved image quality, not otherwise possible with current-generation T scanners reconstructed with FP. Imaging Dedicated Low-Dose Protocols: Intended Low-Dose T With specific clinical indications whereby high-contrast lesions are surrounded by lowcontrast structures, dedicated low-dose protocols have been routinely used in an effort to minimize radiation dose. For example, several studies have suggested that low-dose T may be adequate for renal calculus evaluation [10, 11], T colonography [12, 13], and pulmonary T angiography [14]. lthough satisfactory for the dedicated evaluation of the intended anatomy, these protocols were implemented with the realization that image quality would be significantly reduced outside the area of interest (Fig. 5). In our experience, application of the SIR algorithm to these low-dose studies significantly reduces image noise and improves overall image quality when compared with low-dose standard FP (Figs. 6 and 7). SIR not only reduces noise for standard low-dose T protocols, it also allows even more aggressive dose reduction with ultra-low-dose techniques. For example, T colonography is typically performed at 50 ms. With SIR, we have been able to reduce the dose in half to 25 ms without a loss in image quality (Fig. 8). Routine Imaging at a Low Dose With SIR lthough desirable for all patients, dose reduction is of particularly high priority for young patients, patients who are pregnant, and patients requiring multiple follow-up or multiphase studies. Even though dose reductions have been achieved for the specific indications described, these techniques have not been widely implemented for routine T because of poor image quality outside the area of interest. However, low-dose imaging with SIR reconstructions appears to have potential in displaying similar or better overall image quality compared with routine-dose contrast-enhanced (Fig. 9) and unenhanced examinations (Fig. 10) that use conventional reconstruction techniques. preliminary phantom and patient study indicated that dose reductions of up to 65% without a significant loss in image quality were possible using SIR [15]. This new reconstruction algorithm may thus have significant impact in imaging young patients requiring T (Fig. 11), patients requiring serial T follow-up (Fig. 12), and pregnant patients in whom imaging is deemed medically necessary but who are unable or unwilling to undergo MRI. In addition, SIR may allow even greater dose reductions in nonobese patients, while maintaining acceptable image quality (Fig. 13). Potential Disadvantages of SIR disadvantage of traditional iterative reconstruction algorithms has been the lengthy computation time requirements. However, SIR is a modified and computationally faster technique that uses a single, less-complex corrective model to remove image noise while reducing processing time. To date, SIR reconstructions are slightly longer relative to FP reconstructions, with processing times approximately 30% higher for a standard abdominopelvic T (65 seconds vs 50 seconds for FP). However, as this method continues to evolve, the expectation is that the processing time for SIR will continue to decrease. nother potential disadvantage is a noisefree appearance that can present as an artifactual oversmoothing of the image. Whether this has any diagnostic significance is still uncertain. However, the amount of oversmoothing can be controlled by the amount of SIR processing applied to the image (Fig. 14). onclusion SIR can allow significant radiation dose reduction without sacrificing image quality at T compared with FP alone. This has important implications for routine and screening T, both low dose and standard dose, as well as imaging obese patients. References 1. IMV Medical Information Division. IMV 2006 T Market Summary Report. Des Plaines, IL: IMV Medical Information Division, Mettler F Jr, Thomadsen R, hargavan M, et al. Medical radiation exposure in the U.S. in 2006: preliminary results. Health Phys 2008; 95: Lee I, Haims H, Monico EP, rink J, Forman HP. Diagnostic T scans: assessment of patient, physician, and radiologist awareness of radiation dose and possible risks. Radiology 2004; 231: Hall EJ, renner DJ. ancer risks from diagnostic radiology. r J Radiol 2008; 81: renner DJ, Doll R, Goodhead DT, et al. ancer risk attributable to low doses of ionizing radiation: assessing what we really know. Proc Natl cad Sci 2003; 100: renner DJ, Ellison D. Estimated radiation risks potentially associated with full-body T screening. Radiology 2004; 232: National Institute of Environmental Health Sciences Website. List of cancer-causing agents grows. ccessed March 28, renner DJ, Hall EJ. omputed tomography: an increasing source of radiation exposure. N Engl J Med 2007; 357: Kalra MK, Maher MM, Sahani DV, et al. Lowdose T of the abdomen: evaluation of image improvement with use of noise reduction filters pilot study. Radiology 2003; 228: Rogalla P, Kluner, Taupitz M. Ultra-low-dose T to search for stones in kidneys and collecting system [in German]. ktuelle Urol 2004; 35: White WM, Zite N, Gash J, Waters J, Thompson W, Klein F. Low-dose computed tomography for the evaluation of flank pain in the pregnant population. J Endourol 2007; 21: ohnen M, Vogt, eck, et al. Feasibility of MDT colonography in ultra-low-dose technique in the detection of colorectal lesions: comparison with high-resolution video colonoscopy. JR 2004; 183: van Gelder RE, Venema HW, Florie J, et al. T colonography: feasibility of substantial dose reduction comparison of medium to very low doses in identical patients. Radiology 2004; 232: Heyer M, Mohr PS, Lemburg SP, Peters S, Nicolas V. Image quality and radiation exposure at pulmonary T angiography with 100- or 120-kVp protocol: prospective randomized study. Radiology 2007; 245: Hara K, Paden RG, Silva, Kujak JL, Lawder HJ, Pavlicek W. Iterative reconstruction technique for reducing body radiation dose at T: feasibility study. JR 2009; 193: JR:194, January 2010
3 T Dose Reduction Downloaded from by on 12/18/17 from IP address opyright RRS. For personal use only; all rights reserved Fig. 1 omparison of image noise and contrast in merican ollege of Radiology Phantom (Data Spectrum orp.) using standard-dose and 50% reduced-dose T without and with adaptive statistical iterative reconstruction (SIR) algorithm., T images of phantom at 650 ms with standard filtered back projection (FP) algorithm (), 325 ms with standard FP (), and 325 ms with SIR (). Note expected increase in noise between standard- and low-dose study without SIR (arrows, and ) but similar noise measurements between standard- and low-dose images with SIR (arrows, and ). ontrast-to-noise ratios are relatively decreased on low-dose FP image (circle, ) but are similar between standard- and low-dose images with SIR (circles, and ). Fig. 2 omparison of image noise in 56-year-old obese man at standard-dose T. and, Standard filtered back projection () and adaptive statistical iterative reconstruction (SIR) () coronal multiplanar reformations from T angiography at 2-mm slice thickness show improved visualization of accessory left renal artery (arrow) on SIR image. JR:194, January
4 Silva et al. Fig. 3 omparison of image quality and noise in 78-year-old woman with body mass index of 35. and, ontrast-enhanced T images obtained at 120 kvp, 2-mm slice thickness, and T dose index of 26 using standard filtered back projection () and adaptive statistical iterative reconstruction () algorithms show diminished noise within renal parenchyma, renal cyst (arrow), and perinephric fat. Downloaded from by on 12/18/17 from IP address opyright RRS. For personal use only; all rights reserved Fig. 4 omparison of image noise and quality in 64-year-old man with abdominal aortic aneurysm and body mass index of 46. and, ontrast-enhanced abdominal T images obtained at 120 kvp, 2-mm slice thickness, and T dose index of 27 using standard filtered back projection () and adaptive statistical iterative reconstruction () algorithms show diminished noise within liver, spleen, and aortic lumen (arrows). Fig. 5 omparison of image noise and quality in 64-year-old man with medullary sponge kidney. and, Low-dose renal stone protocol coronal multiplanar reconstruction T images obtained at 120 kvp, slice thickness 0.8 mm, and T dose index of 9.7 using standard filtered back projection () and adaptive statistical iterative reconstruction (SIR) () algorithms show presence of multiple renal calcifications (arrows) and diminished noise within solid abdominal organs on SIR image. 194 JR:194, January 2010
5 T Dose Reduction Downloaded from by on 12/18/17 from IP address opyright RRS. For personal use only; all rights reserved Fig year-old woman with hepatic cyst. and, Low-dose renal stone protocol coronal multiplanar reconstruction T images obtained at 120 kvp, 0.6-mm slice thickness, and T dose index of 14 using standard filtered back projection () and adaptive statistical iterative reconstruction () algorithms. Note that diminished noise within solid abdominal organs improves visualization of incidentally discovered hepatic cyst (arrows) adjacent to gallbladder (G). Fig. 7 Low-dose pulmonary T angiogram in 43-year-old woman. and, xial T images obtained at 100 kvp, 0.6-mm slice thickness, and T dose index of 12 using standard filtered back projection () and adaptive statistical iterative reconstruction () algorithms show diminished noise within pulmonary arterial trunk and main pulmonary arteries. JR:194, January
6 Silva et al. Downloaded from by on 12/18/17 from IP address opyright RRS. For personal use only; all rights reserved D Fig. 8 Low-dose T colonography in 61-year-old man. D, Three-dimensional volume-rendered images obtained at 50 ms and T dose index (TDI) of 4.2 using filtered back projection (FP) () and adaptive statistical iterative reconstruction (SIR) () algorithms and at 25 ms and TDI of 2.1 using FP () and SIR (D) algorithms. Note speckled snowstorm appearance of noise on FP images ( and ) is improved compared with their respective dose SIR images ( and D). In addition, image noise is relatively improved on 25 ms SIR image (D) compared with 50 ms FP image (). 196 JR:194, January 2010
7 T Dose Reduction Downloaded from by on 12/18/17 from IP address opyright RRS. For personal use only; all rights reserved Fig. 9 omparison of low-dose renal stone protocol T without and with adaptive statistical iterative reconstruction (SIR) versus routine-dose T in 64-year-old man with multiple bilateral renal calculi., Low-dose coronal multiplanar reconstruction (MPR) images obtained at 120 kvp, 5-mm slice thickness, T dose index (TDI) of 7.34 with filtered back projection (FP) () and SIR () algorithms and routine-dose coronal MPR image () obtained at 120 kvp, 5-mm slice thickness, and TDI of Note universal decrease in noise on low-dose SIR image () compared with low-dose FP image (), with improved conspicuity of small renal calculi (arrows). lso note that image quality on low-dose SIR image () is comparable to routine-dose FP image () despite significant reduction in radiation dose (TDI 7.34 vs 24.67, respectively). Fig. 10 Low-dose T enterography in 36-year-old man with rohn s disease and thickening of terminal ileum. D, oronal () and axial () low-dose images with adaptive statistical iterative reconstruction (SIR) at 3.75-mm slice thickness and T dose index (TDI) of 5.12 and coronal () and axial (D) routine-dose images at 3-mm slice thickness and TDI of Note overall comparable image quality despite approximate 70% dose reduction for SIR reconstructions ( and ). D JR:194, January
8 Silva et al. Downloaded from by on 12/18/17 from IP address opyright RRS. For personal use only; all rights reserved Fig. 11 omparison of image noise and quality in 78-year-old woman with hepatic cyst., ontrast-enhanced low-dose abdominal T obtained at 120 kvp, 2.5-mm slice thickness, and T dose index (TDI) of 6.84 using filtered back projection () and adaptive statistical iterative reconstruction (SIR) () algorithms and routine-dose image () obtained at 140 kvp, 3-mm slice thickness, and TDI of Note reduced noise and improved image quality when SIR is applied () compared with low-dose without SIR (). lso note similar image quality between low dose with SIR () versus routine dose (). Fig. 12 bdominal contrast-enhanced T in 73-year-old man with hepatocellular carcinoma and incidental left renal (short arrow) and splenic (long arrow) cysts. and, Low-dose image with adaptive statistical iterative reconstruction (SIR) algorithm obtained at 120 kvp, 5-mm slice thickness, T dose index (TDI) of () and routine-dose image obtained at 140 kvp, 5-mm slice thickness, and TDI of (). Overall lesion conspicuity between two images is comparable despite 33% reduction in radiation dose with SIR. Fig. 13 Unenhanced abdominal T in 81-year-old man with multiple hepatic metastases (short arrow) and simple hepatic cysts (long arrow). and, Low-dose image with adaptive statistical iterative reconstruction (SIR) obtained at 120 kvp, 3.75-mm slice thickness, and T dose index (TDI) of () and routine-dose image obtained at 140 kvp, 3-mm slice thickness, and TDI of (). Overall image quality on this unenhanced study is comparable between two images despite 34% reduction in radiation dose with SIR. 198 JR:194, January 2010
9 Downloaded from by on 12/18/17 from IP address opyright RRS. For personal use only; all rights reserved T Dose Reduction Fig. 14 Schematic in 41-year-old woman shows differing amounts of adaptive statistical iterative reconstruction (SIR) processing combined with filtered back projection (FP) processing. Single image of liver is shown using only FP technique (0%). Same image is shown with 40%, 60%, 80%, and 100% SIR processing. Note smoothed appearance of 100% SIR image versus FP image. JR:194, January
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