R e s i d e n t s ’ S e c t i o n • S t r u c t u r e d R ev i ew Padole et al. Iterative Reconstruction Techniques
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Residents’ Section Structured Review
Residents
inRadiology Atul Padole1 Ranish Deedar Ali Khawaja Mannudeep K. Kalra Sarabjeet Singh Padole A, Khawaja RD, Kalra MK, Singh S
CT Radiation Dose and Iterative Reconstruction Techniques Key Points 1. CT radiation dose optimization is one of the major concerns for the scientific community. 2. CT image quality is dependent on the selected image reconstruction algorithm. 3. Iterative reconstruction algorithms have reemerged with the potential of radiation dose optimization by lowering image noise. 4. Tube current is the most common parameter used to reduce radiation dose along with iterative reconstruction. 5. Tube potential (kV) is also used for dose optimization with iterative reconstruction in CT angiography protocols and small patients.
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Keywords: CT image quality, CT radiation dose optimization, iterative reconstruction techniques DOI:10.2214/AJR.14.13241 Received May 20, 2014; accepted after revision August 29, 2014.
T radiation dose optimization is an important concern to lower the associated population risks. Several efforts have been taken by the imaging community to reduce the dose to as low as reasonably achievable (ALARA). Scanner manufacturers have contributed by developing technologies to reduce doses while maintaining image quality, such as automatic exposure control, noise reduction filters, and iterative reconstruction algorithms. Image reconstruction algorithms play a vital role in maintaining or improving image quality in reduced-dose CT [1]. Reduced-dose CT can be achieved by modifying the scanning parameters (tube current, tube potential, pitch, and rotation time). Iterative reconstruction techniques can then be applied to these reduced-dose CT examinations to improve the image quality. In this article, we will review clinical applications of iterative reconstruction techniques for chest, abdominal, head and neck, and pediatric CT.
1
All authors: Department of Radiology, Massachusetts General Hospital, Harvard Medical School, 55 Fruit St, Founders-202, Boston, MA 02114. Address correspondence to S. Singh ([email protected]).
WEB This is a web exclusive article. AJR 2015; 204:W384–W392 0361–803X/15/2044–W384 © American Roentgen Ray Society
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Physical Basis of Iterative Reconstruction The first commercial CT scanners used filtered backprojection (FBP) technique because of its faster reconstruction and ease of implementation [1]. FBP has served the CT community for the past 40 years; however, its performance has been challenged because of the need for reducing radiation dose while improving resolution (both spatial and tem-
poral). To address some of these concerns, scanner manufacturers have introduced newer image reconstruction algorithms—namely, iterative reconstruction techniques. Although iterative reconstruction techniques were introduced clinically in 2009 as newer algorithms, this was actually the reemergence of existing technology because of advancement in the computational power of computers [1, 2]. Conventional FBP is associated with higher image noise and artifacts at reduced doses because it is based on some mathematic assumptions of the CT system. For example, FBP ignores key information about the x-ray photon statistics, such as the Poisson distribution of photons and system hardware details (focal spot size, active detector area, and image voxel shape) [1, 2]. Iterative reconstruction techniques, as the name suggests, iterate the image reconstruction several times to better estimate these mathematic assumptions and generate images with lower noise [1–3]. This iteration of the massive raw, or sinogram, CT data requires longer computational time and robust computers. Different scanner manufacturers have taken different algorithmic approaches to iterate different components of the image reconstruction algorithm. However, the common endpoint of all the iterative reconstruction algorithms is to produce lower image noise and higher resolution by maintaining edges and lower artifacts. This ability of iterative reconstruction
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Iterative Reconstruction Techniques techniques enables use of reduced-dose CT with lowering of scanning parameters, such as tube current or even tube potential [2–24]. Several studies with various iterative reconstruction techniques have shown the potential for lowering radiation dose on different scanners (Adaptive Statistical Iterative Reconstruction [ASIR] and Model-Based Iterative Reconstruction [MBIR or Veo], GE Healthcare; Iterative Reconstruction in Image Space [IRIS], Sinogram-Affirmed Iterative Reconstruction [SAFIRE], and Advanced Model-Based Iterative Reconstruction [ADMIRE], Siemens Healthcare; iDose and Iterative Model Reconstruction [IMR], Philips Healthcare; Adaptive Iterative Dose Reduction [AIDR], Toshiba America Medical Systems; and SafeCT, Medic Vision) [2–24]. Clinical Applications of Iterative Reconstruction Chest CT Chest CT dose optimization is important because of the direct exposure to some of the most radiation-sensitive tissues in the human body, including thyroid, breast, and lungs. High inherent contrast and low attenuation in lung parenchyma enable tolerance of image noise and substantial dose reduction compared with other body regions, such as the abdomen or head. Reduction in applied tube current is a common way of reducing dose with iterative reconstruction techniques. Singh et al. [3] reported that lung lesions can be adequately assessed with ASIR30 at 3 mGy (40 mAs), compared with FBP at 12 mGy (150 mAs). Noncalcified nodules can be seen well with MBIR at 0.3 mGy with 4 mAs (the lowest allowed mAs on the scanner) as noted in a recent prospective clinical study (n = 57 patients) [4] (Figs. 1 and 2). Conspicuity of ground-glass attenuation, ill-defined micronodules, and emphysematous lesions was improved on IRIS (settings of 3 and 5) at estimated volume CT dose index (CTDIvol) of 3–4 mGy (dose-length product [DLP], 126 mGy × cm) with 35% reduced noise compared with FBP [5]. Another iterative reconstruction from the same vendor, SAFIRE, has shown improved diagnostic confidence and lesion conspicuity at CTDIvol of 2.5 mGy when compared with routine chest CT examinations with FBP at 7 mGy [6]. Noise-reduction capabilities of iterative reconstruction also enable more frequent use of lower tube potential (measured as kilovolt-
age) for dose reduction compared with FBP. Chest CT angiography performed at 100 kV (n = 53) and 80 kV (n = 27) (median CTDIvol = 1.2 mGy) with SAFIRE (strength, 3) provided substantially lower noise compared with initial CT at 120 and 100 kV (CTDIvol = 2.6 mGy) with FBP [7]. Another study was performed on third-generation dual-source CT at 70 kVp, 100 kVp with tin (Sn) filtration at 100 Sn kVp, and 150 Sn kVp (CTDIvol 3.1, 0.3, and 0.15 mGy, respectively) and processed with ADMIRE (strength levels, 3 and 5) [8]. Authors have shown lower noise at 100 Sn kVp (0.3 mGy) and higher sensitivity for nodule detection with ADMIRE 5 when compared with 150 Sn kVp. For evaluation of coronary artery disease, Layritz et al. [9] reported that SAFIRE improves image quality for reduced dose (0.7 mSv) prospectively ECG-triggered coronary dual-source CT angiography compared with FBP. Details of the iterative reconstruction techniques are tabulated in Table 1. Abdominal CT Abdominal CT, on the other hand, is more challenging for dose optimization because of organs with low contrast enhancement, such as the liver. Iterative reconstruction techniques have also shown potential for lowering noise, thus making abdominal CT diagnostically acceptable at reduced doses. Tube current is once again the most commonly tweaked scanning parameter to lower the scanner output and optimize radiation dose for abdominal CT examinations. Singh et al. [2] showed lower image noise and improved diagnostic confidence for routine abdominal CT at 8 mGy with ASIR compared with a standard dose of 17 mGy with FBP [2]. Several other studies have shown similar results of image noise reduction with different iterative reconstruction methods, such as MBIR, IRIS, SAFIRE, iDose, IMR, and AIDR [10– 14] (Figs. 3 and 4). CT colonography (CTC) is routinely performed at a reduced dose compared with routine abdominal CT because of the high contrast between air in the colon lumen and soft-tissue wall. Several studies have further reduced the dose for CTC protocols with the use of iterative reconstruction [11]. Flicek et al. [11] scanned patients (n = 18) at a standard CTC dose of 50 mAs, or CTDIvol of 4.2 mGy, in the supine position and at a reduced dose of 25 mAs, or CTDIvol of 2.1 mGy, with 40% ASIR in the prone position. Authors have shown no significant differences in image quality between standard-dose and
reduced-dose ASIR images evaluated at a prominent fold within the rectosigmoid junction, at the splenic flexure of the colon, and at the ileocecal valve. Similar to chest CT with iterative reconstruction, use of lower tube potential in abdominal CT has also increased with use of iterative reconstruction techniques, particularly for arterial phase imaging and that of renal stone follow-up. Prior studies have explored the role of iterative reconstruction in enabling low tube potential for detection of hepatocellular carcinoma and suspected renal colic [12, 13]. Hur et al. [14] performed a retrospective study with liver donor CT protocol at 100 kV (n = 51, CTDIvol of 9 mGy) and compared the studies with an age- and size-matched standard group (n = 51, 10.4 mGy) at 120 kV. The authors found significantly higher scores for diagnostic confidence in detecting anatomic variants of hepatic vessels with MBIR at 100 kV compared with FBP at 120 kV. Head and Neck CT Iterative reconstruction can also allow radiation dose reduction for head and neck CT examinations [15, 16] (Fig. 5). Rapalino et al. [15] evaluated reduced-dose (49 mGy) unenhanced adult head CT reconstructed with different settings of ASIR (20–100% in increments of 20% ASIR) compared with 66 mGy FBP. The authors reported improved signal-to-noise ratio and contrast-to-noise ratio values in reduced-dose CT images with ASIR compared with FBP [15]. Ren et al. [16] evaluated image quality and lesion conspicuity of brain CT with ASIR at 200 mAs (38 mGy) and FBP at 300 mAs (57 mGy). The results of their study showed that ASIR allows greater than 30% dose reduction compared with FBP without compromising diagnostic confidence. Bodelle et al. [18] reported that intracranial hemorrhages were better seen on SAFIRE at reduced dose (1.7 mSv) compared with FBP without significant loss of diagnostic quality. Notohamiprodjo et al. [19] reported that at 0.9 mSv MBIR improves the image quality of head CT compared with ASIR and therefore allows substantial dose reduction. Pediatric CT Prior studies showed that iterative reconstruction techniques reduce radiation dose and improve image quality of pediatric CT [19–21] (Fig. 6). Singh et al. [17] combined iterative reconstruction with color-coded pe-
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Padole et al. TABLE 1: Tabulated Summary of Iterative Reconstruction Technique and Radiation Dose in Different Body Regions and in Children
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Protocol
Iterative Reconstruction
Lowest Acceptable CTDIvol (mGy)
Results
First Author
Year
Head CT
ASIR
49
Improved SNR and CNR with low-dose ASIR
Rapalino
2012
Head CT
ASIR
38
ASIR allows 30% dose reduction compared with FBP for brain CT without compromising diagnostic confidence
Ren
2012
Routine chest CT
ASIR
3
Lung lesions can be adequately assessed with ASIR at 75% reduced dose
Singh
2011
Routine chest CT
MBIR
0.3
Noncalcified lung nodules can be seen well with MBIR
Yamada
2012
Routine chest CT
IRIS
3–4
Conspicuity of ground-glass attenuation and emphysematous lesions improved on IRIS compared with FBP
Pontana
2011
Routine chest CT
SAFIRE
2.5
SAFIRE improves diagnostic confidence and lesion conspicuity compared with FBP
Kalra
2013
Chest CT angiography
SAFIRE
1.2
At 1.2 mGy, chest CT angiography has lower image noise than at 2.6 mGy
Pontana
2013
Routine abdomen CT
ASIR
8
CT colonography
ASIR
2.1
Liver CT
MBIR
9
Routine abdomen CT
MBIR
2.0–2.7
Routine abdomen CT
SAFIRE
2.5–6
ASIR lowers image noise and improves diagnostic Singh confidence of abdominal CT compared with FBP
2010
CT colonography can be performed with low-dose ASIR
2010
Flicek
At 8.7 mGy, diagnostic confidence for detecting Hur variants of hepatic vessels improved with MBIR
2014
Aggressive noise reduction leads to decreased diagnostic confidence with MBIR
Vardhanabhuti
2014
50–75% dose reduction can be achieved with SAFIRE compared with FBP
Kalra
2012
Pediatric chest and abdomen CT ASIR
3.7
ASIR improves image quality compared with FBP Singh
2012
Pediatric head CT
22
ASIR allowed 28% dose reduction for 3- to 12-year-old age group and 48% reduction for more than 12 years old compared with FBP
2014
ASIR
McKnight
Note—CTDIvol = volume CT dose index, SNR = signal-to-noise ratio, CNR = contrast-to-noise ratio, FBP = filtered back projection, ASIR = Adaptive Statistical Iterative Reconstruction (GE Healthcare), MBIR = Model-Based Iterative Reconstruction (GE Healthcare), IRIS = Iterative Reconstruction in Image Space (Siemens Healthcare), SAFIRE = Sinogram-Affirmed Iterative Reconstruction (Siemens Healthcare).
diatric protocols, using automatic tube current modulation, clinical indication, and weight-based fine tuning of scanning parameters. Authors have shown that lowtube-voltage (80–100 kVp) chest CT reconstructed with ASIR30 has 20% lower noise at 45% (3.7–7 mGy) reduced radiation doses compared with FBP. Pediatric head CT with ASIR at 22 mGy allowed 28% dose reduction for 3- to 12-year-old patients and 48% in reduction at 30 mGy for patients older than 12 years compared with FBP [20]. Smith et al. [21] reported that at 3.1 mGy MBIR maintained the diagnostic confidence of pediatric CT compared with ASIR at 6 mGy. Limitations Although several studies have documented the potential for dose reduction with iterative
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reconstruction, these studies have also found oversmoothing of images with higher strengths of iterative reconstruction [3–17]. Hence, it is vital that the appropriate radiation dose level as well as the strength of the iterative reconstruction technique is selected. This oversmoothing due to aggressive noise reduction is reported as distinctive image texture or appearance or “waxiness” or “pixillation” from a variety of iterative reconstruction techniques from different CT vendors. Different image texture is also associated with “stepwise” or “blocky” appearance of tissue margins, such as mediastinal structures [3]. In addition, loss of visibility of major fissures in the lung parenchyma due to this smoothing of images has been reported with iterative reconstruction [6]. Most iterative reconstruction techniques are vendor specific and only available for
state-of-the-art scanners. SafeCT offers an image-based vendor-neutral iterative reconstruction technique that can be applied to any CT system regardless of its age [24]. FBP is currently available on all scanners and is still commonly used in clinical practice. Some iterative reconstruction techniques, such as MBIR, have a long reconstruction time (30– 60 minutes for a single dataset). Thus, their clinical use is limited. These limitations can be overcome by use of hybrid iterative reconstruction techniques that blend with FBP. These hybrid iterative reconstruction techniques have less pixillation or oversmoothing problems compared with the pure iterative reconstruction techniques that do not blend with FBP. In addition, hybrid iterative reconstruction techniques reconstruct images almost in real time (< 1 minute for single dataset).
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Iterative Reconstruction Techniques How to Practically Apply Iterative Reconstruction? Most available iterative reconstruction techniques have different settings or strengths to reduce noise or improve image quality. The first step is to cross-check the dose levels of standard-dose CT at your institution and then compare with either the American College of Radiology Dose Index Registry or the published literature. Second, reduce the dose in small steps of 10–20% by first fine-tuning tube current and tube voltage for CT angiography. This lowering of dose with tube current or voltage can then be accompanied by selecting the milder settings of iterative reconstruction and gradually transitioning to higher settings. Conclusion Iterative reconstruction techniques have consistently shown improved image quality and reduced image noise for chest, abdominal, and head and neck CT as well as for pediatric CT. Iterative reconstruction techniques have the potential to enable CT radiation dose optimization by either lowering tube current or tube potential. CT dose reduction with iterative reconstruction techniques should be achieved in a gradual stepwise approach. Some iterative reconstruction techniques are also associated with limitations, which include texture changes and longer reconstruction time. References 1. Hsieh J. Computed tomography: principles, design, artifacts, and recent advances. Bellingham, WA: SPIE Press, 2003 2. Singh S, Kalra MK, Hsieh J, et al. Abdominal CT: comparison of adaptive statistical iterative and filtered back projection reconstruction techniques. Radiology 2010; 257:373–383 3. Singh S, Kalra MK, Gilman MD, et al. Adaptive statistical iterative reconstruction technique for radiation dose reduction in chest CT: a pilot study. Radiology 2011; 259:565–573 4. Yamada Y, Jinzaki M, Tanami Y, et al. Modelbased iterative reconstruction technique for ultralow-dose computed tomography of the lung: a pilot study. Invest Radiol 2012; 47:482–489 5. Pontana F, Pagniez J, Flohr T, et al. Chest com-
puted tomography using iterative reconstruction vs filtered back projection. Part 1. Evaluation of image noise reduction in 32 patients. Eur Radiol 2011; 21:627–635 6. Kalra MK, Woisetschläger M, Dahlström N, et al. Sinogram-affirmed iterative reconstruction of low-dose chest CT: effect on image quality and radiation dose. AJR 2013; 201:[web]W235–W244 7. Pontana F, Pagniez J, Duhamel A, et al. Reduceddose low-voltage chest CT angiography with sinogram-affirmed iterative reconstruction versus standard-dose filtered back projection. Radiology 2013; 267:609–618 8. Gordic S, Morsbach F, Schmidt B, et al. Ultralowdose chest computed tomography for pulmonary nodule detection: first performance evaluation of single energy scanning with spectral shaping. Invest Radiol 2014; 49:465–473 9. Layritz C, Schmid J, Achenbach S, et al. Accuracy of prospectively ECG-triggered very low-dose coronary dual-source CT angiography using iterative reconstruction for the detection of coronary artery stenosis: comparison with invasive catheterization. Eur Heart J Cardiovasc Imaging 2014; 15:1238–1245 10. Singh S, Kalra MK, Do S, et al. Comparison of hybrid and pure iterative reconstruction techniques with conventional filtered back projection: dose reduction potential in the abdomen. J Comput Assist Tomogr 2012; 36:347–353 11. Flicek KT, Hara AK, Silva AC, Wu Q, Peter MB, Johnson CD. Reducing the radiation dose for CT colonography using adaptive statistical iterative reconstruction: a pilot study. AJR 2010; 195:126–131 12. Gervaise A, Naulet P, Beuret F, et al. Low-dose CT with automatic tube current modulation, adaptive statistical iterative reconstruction, and low tube voltage for the diagnosis of renal colic: impact of body mass index. AJR 2014; 202:553–560 13. Yu MH, Lee JM, Yoon JH, et al. Low tube voltage intermediate tube current liver MDCT: sinogramaffirmed iterative reconstruction algorithm for detection of hypervascular hepatocellular carcinoma. AJR 2013; 201:23–32 14. Hur BY, Lee JM, Joo I, et al. Liver computed tomography with low tube voltage and model-based iterative reconstruction algorithm for hepatic vessel evaluation in living liver donor candidates. J Comput Assist Tomogr 2014; 38:367–375 15. Rapalino O, Kamalian S, Kamalian S, et al. Cra-
nial CT with adaptive statistical iterative reconstruction: improved image quality with concomitant radiation dose reduction. AJNR 2012; 33:609–615 16. Ren Q, Dewan SK, Li M, et al. Comparison of adaptive statistical iterative and filtered back projection reconstruction techniques in brain CT. Eur J Radiol 2012; 81:2597–2601 17. Singh S, Kalra MK, Shenoy-Bhangle AS, et al. Radiation dose reduction with hybrid iterative reconstruction for pediatric CT. Radiology 2012; 263:537–546 18. Bodelle B, Klein E, Naguib NN, et al. Acute intracranial hemorrhage in CT: benefits of sinogramaffirmed iterative reconstruction techniques. AJNR 2014; 35:445–449 19. Notohamiprodjo S, Deak Z, Meurer F, et al. Image quality of iterative reconstruction in cranial CT imaging: comparison of model-based iterative reconstruction (MBIR) and adaptive statistical iterative reconstruction (ASiR). Eur Radiol 2015; 25:140–146 20. McKnight CD, Watcharotone K, Ibrahim M, Christodoulou E, Baer AH, Parmar HA. Adaptive statistical iterative reconstruction: reducing dose while preserving image quality in the pediatric head CT examination. Pediatr Radiol 2014; 44:997–1003 21. Smith EA, Dillman JR, Goodsitt MM, Christodoulou EG, Keshavarzi N, Strouse PJ. Modelbased iterative reconstruction: effect on patient radiation dose and image quality in pediatric body CT. Radiology 2014; 270:526–534 22. Vardhanabhuti V, Riordan RD, Mitchell GR, Hyde C, Roobottom CA. Image comparative assessment using iterative reconstructions: clinical comparison of low-dose abdominal/pelvic computed tomography between adaptive statistical, model-based iterative reconstructions and traditional filtered back projection in 65 patients. Invest Radiol 2014; 49:209–216 23. Kalra MK, Woisetschläger M, Dahlström N, et al. Radiation dose reduction with sinogram affirmed iterative reconstruction technique for abdominal computed tomography. J Comput Assist Tomogr 2012; 36:339–346 24. Pourjabbar S, Singh S, Kulkarni N, et al. Dose reduction for chest CT: comparison of two iterative reconstruction techniques. Acta Radiol; [Epub 2014 Jun 9] (Figures start on next page)
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Padole et al.
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Fig. 1—73-year-old man with pulmonary nodule (arrows). A–D, Transverse chest CT images acquired at 7 mGy (baseline volume CT dose index) (A) and at 1.6 mGy (reduced dose) (B–D) and reconstructed with various iterative reconstruction techniques. At 1.6 mGy, pulmonary nodule was depicted optimally on iterative reconstruction images with SafeCT (Medic Vision) (B), Adaptive Statistical Iterative Reconstruction (ASIR, GE Healthcare) (C), and Model-Based Iterative Reconstruction (MBIR, GE Healthcare) (D) techniques.
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Iterative Reconstruction Techniques
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Fig. 2—85-year-old woman with lung nodule showing ground-glass opacity in anterior right upper lobe. A–D, Reduced-dose transverse chest CT images (volume CT dose index of 0.9 mGy [A and B] and 0.2 mGy [C and D]) reconstructed with filtered back projection (FBP) (A and C) and iterative model reconstruction (IMR) (B and D). At 0.9 mGy, there is little perceived difference between FBP and IMR images, but at 0.2 mGy, IMR image is better in terms of noise and definition of ground-glass opacity in anterior right upper lobe.
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Padole et al.
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Fig. 3—62-year-old woman with liver cysts (arrows). A–D, Transverse abdominal CT images acquired at baseline volume CT dose index of 8 mGy (A) and reduced dose at 1.2 mGy (B–D) and reconstructed with filtered backprojection (FBP) and various iterative reconstruction techniques. At 1.2 mGy, liver cysts were depicted optimally on iDose (Philips Healthcare) (C) and Iterative Model Reconstruction (IMR, Philips Healthcare) (D) images and suboptimally on FBP image (B).
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Fig. 4—Female cadaver with kidney stone (arrows) (age at death, 67 years). A–D, Postmortem transverse abdominal CT images acquired at volume CT dose index (CTDIvol) of 8.0 mGy (A and B) and 2.8 mGy (C and D) and reconstructed with filtered backprojection (FBP) (A and C) and Sinogram-Affirmed Iterative Reconstruction (SAFIRE, Siemens Healthcare). Confidence of detecting kidney stone is much higher on reduceddose SAFIRE images at 2.8 mGy CTDIvol compared with corresponding FBP images.
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Iterative Reconstruction Techniques
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Fig. 5—57 years male cadaver with no evidence of stroke. Patient expired from aspiration pneumonia and pulmonary edema. A–D, Postmortem transverse head CT images acquired at 58 mGy (A and B) and 15 mGy (C and D) and reconstructed with filtered backprojection (FBP) (A and C) and Model-Based Iterative Reconstruction (MBIR, GE Healthcare) (D). Left middle cerebral artery (arrows) was optimally seen with all 58-mGy images. On 15-mGy images, left middle cerebral artery was optimally seen with MBIR and suboptimally with FBP.
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Padole et al.
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Fig. 6—7-year-old girl with weight loss and diarrhea. A–D, Coronal abdominal CT images at 4.3 mGy (baseline volume CT dose index) (A) and 1.3 mGy (reduced dose) (B–D) and reconstructed with filtered backprojection (FBP) (B), Adaptive Statistical Iterative Reconstruction (ASIR, GE Healthcare) (C), and Model-Based Iterative Reconstruction (MBIR, GE Healthcare) (D). There is marked improvement in image quality with MBIR at 1.3 mGy compared with FBP and ASIR images.
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Residents’ Section Structured Review
Residents
inRadiology Atul Padole1 Ranish Deedar Ali Khawaja Mannudeep K. Kalra Sarabjeet Singh Padole A, Khawaja RD, Kalra MK, Singh S
CT Radiation Dose and Iterative Reconstruction Techniques Key Points 1. CT radiation dose optimization is one of the major concerns for the scientific community. 2. CT image quality is dependent on the selected image reconstruction algorithm. 3. Iterative reconstruction algorithms have reemerged with the potential of radiation dose optimization by lowering image noise. 4. Tube current is the most common parameter used to reduce radiation dose along with iterative reconstruction. 5. Tube potential (kV) is also used for dose optimization with iterative reconstruction in CT angiography protocols and small patients.
C
Keywords: CT image quality, CT radiation dose optimization, iterative reconstruction techniques DOI:10.2214/AJR.14.13241 Received May 20, 2014; accepted after revision August 29, 2014.
T radiation dose optimization is an important concern to lower the associated population risks. Several efforts have been taken by the imaging community to reduce the dose to as low as reasonably achievable (ALARA). Scanner manufacturers have contributed by developing technologies to reduce doses while maintaining image quality, such as automatic exposure control, noise reduction filters, and iterative reconstruction algorithms. Image reconstruction algorithms play a vital role in maintaining or improving image quality in reduced-dose CT [1]. Reduced-dose CT can be achieved by modifying the scanning parameters (tube current, tube potential, pitch, and rotation time). Iterative reconstruction techniques can then be applied to these reduced-dose CT examinations to improve the image quality. In this article, we will review clinical applications of iterative reconstruction techniques for chest, abdominal, head and neck, and pediatric CT.
1
All authors: Department of Radiology, Massachusetts General Hospital, Harvard Medical School, 55 Fruit St, Founders-202, Boston, MA 02114. Address correspondence to S. Singh ([email protected]).
WEB This is a web exclusive article. AJR 2015; 204:W384–W392 0361–803X/15/2044–W384 © American Roentgen Ray Society
W384
Physical Basis of Iterative Reconstruction The first commercial CT scanners used filtered backprojection (FBP) technique because of its faster reconstruction and ease of implementation [1]. FBP has served the CT community for the past 40 years; however, its performance has been challenged because of the need for reducing radiation dose while improving resolution (both spatial and tem-
poral). To address some of these concerns, scanner manufacturers have introduced newer image reconstruction algorithms—namely, iterative reconstruction techniques. Although iterative reconstruction techniques were introduced clinically in 2009 as newer algorithms, this was actually the reemergence of existing technology because of advancement in the computational power of computers [1, 2]. Conventional FBP is associated with higher image noise and artifacts at reduced doses because it is based on some mathematic assumptions of the CT system. For example, FBP ignores key information about the x-ray photon statistics, such as the Poisson distribution of photons and system hardware details (focal spot size, active detector area, and image voxel shape) [1, 2]. Iterative reconstruction techniques, as the name suggests, iterate the image reconstruction several times to better estimate these mathematic assumptions and generate images with lower noise [1–3]. This iteration of the massive raw, or sinogram, CT data requires longer computational time and robust computers. Different scanner manufacturers have taken different algorithmic approaches to iterate different components of the image reconstruction algorithm. However, the common endpoint of all the iterative reconstruction algorithms is to produce lower image noise and higher resolution by maintaining edges and lower artifacts. This ability of iterative reconstruction
AJR:204, April 2015
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Iterative Reconstruction Techniques techniques enables use of reduced-dose CT with lowering of scanning parameters, such as tube current or even tube potential [2–24]. Several studies with various iterative reconstruction techniques have shown the potential for lowering radiation dose on different scanners (Adaptive Statistical Iterative Reconstruction [ASIR] and Model-Based Iterative Reconstruction [MBIR or Veo], GE Healthcare; Iterative Reconstruction in Image Space [IRIS], Sinogram-Affirmed Iterative Reconstruction [SAFIRE], and Advanced Model-Based Iterative Reconstruction [ADMIRE], Siemens Healthcare; iDose and Iterative Model Reconstruction [IMR], Philips Healthcare; Adaptive Iterative Dose Reduction [AIDR], Toshiba America Medical Systems; and SafeCT, Medic Vision) [2–24]. Clinical Applications of Iterative Reconstruction Chest CT Chest CT dose optimization is important because of the direct exposure to some of the most radiation-sensitive tissues in the human body, including thyroid, breast, and lungs. High inherent contrast and low attenuation in lung parenchyma enable tolerance of image noise and substantial dose reduction compared with other body regions, such as the abdomen or head. Reduction in applied tube current is a common way of reducing dose with iterative reconstruction techniques. Singh et al. [3] reported that lung lesions can be adequately assessed with ASIR30 at 3 mGy (40 mAs), compared with FBP at 12 mGy (150 mAs). Noncalcified nodules can be seen well with MBIR at 0.3 mGy with 4 mAs (the lowest allowed mAs on the scanner) as noted in a recent prospective clinical study (n = 57 patients) [4] (Figs. 1 and 2). Conspicuity of ground-glass attenuation, ill-defined micronodules, and emphysematous lesions was improved on IRIS (settings of 3 and 5) at estimated volume CT dose index (CTDIvol) of 3–4 mGy (dose-length product [DLP], 126 mGy × cm) with 35% reduced noise compared with FBP [5]. Another iterative reconstruction from the same vendor, SAFIRE, has shown improved diagnostic confidence and lesion conspicuity at CTDIvol of 2.5 mGy when compared with routine chest CT examinations with FBP at 7 mGy [6]. Noise-reduction capabilities of iterative reconstruction also enable more frequent use of lower tube potential (measured as kilovolt-
age) for dose reduction compared with FBP. Chest CT angiography performed at 100 kV (n = 53) and 80 kV (n = 27) (median CTDIvol = 1.2 mGy) with SAFIRE (strength, 3) provided substantially lower noise compared with initial CT at 120 and 100 kV (CTDIvol = 2.6 mGy) with FBP [7]. Another study was performed on third-generation dual-source CT at 70 kVp, 100 kVp with tin (Sn) filtration at 100 Sn kVp, and 150 Sn kVp (CTDIvol 3.1, 0.3, and 0.15 mGy, respectively) and processed with ADMIRE (strength levels, 3 and 5) [8]. Authors have shown lower noise at 100 Sn kVp (0.3 mGy) and higher sensitivity for nodule detection with ADMIRE 5 when compared with 150 Sn kVp. For evaluation of coronary artery disease, Layritz et al. [9] reported that SAFIRE improves image quality for reduced dose (0.7 mSv) prospectively ECG-triggered coronary dual-source CT angiography compared with FBP. Details of the iterative reconstruction techniques are tabulated in Table 1. Abdominal CT Abdominal CT, on the other hand, is more challenging for dose optimization because of organs with low contrast enhancement, such as the liver. Iterative reconstruction techniques have also shown potential for lowering noise, thus making abdominal CT diagnostically acceptable at reduced doses. Tube current is once again the most commonly tweaked scanning parameter to lower the scanner output and optimize radiation dose for abdominal CT examinations. Singh et al. [2] showed lower image noise and improved diagnostic confidence for routine abdominal CT at 8 mGy with ASIR compared with a standard dose of 17 mGy with FBP [2]. Several other studies have shown similar results of image noise reduction with different iterative reconstruction methods, such as MBIR, IRIS, SAFIRE, iDose, IMR, and AIDR [10– 14] (Figs. 3 and 4). CT colonography (CTC) is routinely performed at a reduced dose compared with routine abdominal CT because of the high contrast between air in the colon lumen and soft-tissue wall. Several studies have further reduced the dose for CTC protocols with the use of iterative reconstruction [11]. Flicek et al. [11] scanned patients (n = 18) at a standard CTC dose of 50 mAs, or CTDIvol of 4.2 mGy, in the supine position and at a reduced dose of 25 mAs, or CTDIvol of 2.1 mGy, with 40% ASIR in the prone position. Authors have shown no significant differences in image quality between standard-dose and
reduced-dose ASIR images evaluated at a prominent fold within the rectosigmoid junction, at the splenic flexure of the colon, and at the ileocecal valve. Similar to chest CT with iterative reconstruction, use of lower tube potential in abdominal CT has also increased with use of iterative reconstruction techniques, particularly for arterial phase imaging and that of renal stone follow-up. Prior studies have explored the role of iterative reconstruction in enabling low tube potential for detection of hepatocellular carcinoma and suspected renal colic [12, 13]. Hur et al. [14] performed a retrospective study with liver donor CT protocol at 100 kV (n = 51, CTDIvol of 9 mGy) and compared the studies with an age- and size-matched standard group (n = 51, 10.4 mGy) at 120 kV. The authors found significantly higher scores for diagnostic confidence in detecting anatomic variants of hepatic vessels with MBIR at 100 kV compared with FBP at 120 kV. Head and Neck CT Iterative reconstruction can also allow radiation dose reduction for head and neck CT examinations [15, 16] (Fig. 5). Rapalino et al. [15] evaluated reduced-dose (49 mGy) unenhanced adult head CT reconstructed with different settings of ASIR (20–100% in increments of 20% ASIR) compared with 66 mGy FBP. The authors reported improved signal-to-noise ratio and contrast-to-noise ratio values in reduced-dose CT images with ASIR compared with FBP [15]. Ren et al. [16] evaluated image quality and lesion conspicuity of brain CT with ASIR at 200 mAs (38 mGy) and FBP at 300 mAs (57 mGy). The results of their study showed that ASIR allows greater than 30% dose reduction compared with FBP without compromising diagnostic confidence. Bodelle et al. [18] reported that intracranial hemorrhages were better seen on SAFIRE at reduced dose (1.7 mSv) compared with FBP without significant loss of diagnostic quality. Notohamiprodjo et al. [19] reported that at 0.9 mSv MBIR improves the image quality of head CT compared with ASIR and therefore allows substantial dose reduction. Pediatric CT Prior studies showed that iterative reconstruction techniques reduce radiation dose and improve image quality of pediatric CT [19–21] (Fig. 6). Singh et al. [17] combined iterative reconstruction with color-coded pe-
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Padole et al. TABLE 1: Tabulated Summary of Iterative Reconstruction Technique and Radiation Dose in Different Body Regions and in Children
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Protocol
Iterative Reconstruction
Lowest Acceptable CTDIvol (mGy)
Results
First Author
Year
Head CT
ASIR
49
Improved SNR and CNR with low-dose ASIR
Rapalino
2012
Head CT
ASIR
38
ASIR allows 30% dose reduction compared with FBP for brain CT without compromising diagnostic confidence
Ren
2012
Routine chest CT
ASIR
3
Lung lesions can be adequately assessed with ASIR at 75% reduced dose
Singh
2011
Routine chest CT
MBIR
0.3
Noncalcified lung nodules can be seen well with MBIR
Yamada
2012
Routine chest CT
IRIS
3–4
Conspicuity of ground-glass attenuation and emphysematous lesions improved on IRIS compared with FBP
Pontana
2011
Routine chest CT
SAFIRE
2.5
SAFIRE improves diagnostic confidence and lesion conspicuity compared with FBP
Kalra
2013
Chest CT angiography
SAFIRE
1.2
At 1.2 mGy, chest CT angiography has lower image noise than at 2.6 mGy
Pontana
2013
Routine abdomen CT
ASIR
8
CT colonography
ASIR
2.1
Liver CT
MBIR
9
Routine abdomen CT
MBIR
2.0–2.7
Routine abdomen CT
SAFIRE
2.5–6
ASIR lowers image noise and improves diagnostic Singh confidence of abdominal CT compared with FBP
2010
CT colonography can be performed with low-dose ASIR
2010
Flicek
At 8.7 mGy, diagnostic confidence for detecting Hur variants of hepatic vessels improved with MBIR
2014
Aggressive noise reduction leads to decreased diagnostic confidence with MBIR
Vardhanabhuti
2014
50–75% dose reduction can be achieved with SAFIRE compared with FBP
Kalra
2012
Pediatric chest and abdomen CT ASIR
3.7
ASIR improves image quality compared with FBP Singh
2012
Pediatric head CT
22
ASIR allowed 28% dose reduction for 3- to 12-year-old age group and 48% reduction for more than 12 years old compared with FBP
2014
ASIR
McKnight
Note—CTDIvol = volume CT dose index, SNR = signal-to-noise ratio, CNR = contrast-to-noise ratio, FBP = filtered back projection, ASIR = Adaptive Statistical Iterative Reconstruction (GE Healthcare), MBIR = Model-Based Iterative Reconstruction (GE Healthcare), IRIS = Iterative Reconstruction in Image Space (Siemens Healthcare), SAFIRE = Sinogram-Affirmed Iterative Reconstruction (Siemens Healthcare).
diatric protocols, using automatic tube current modulation, clinical indication, and weight-based fine tuning of scanning parameters. Authors have shown that lowtube-voltage (80–100 kVp) chest CT reconstructed with ASIR30 has 20% lower noise at 45% (3.7–7 mGy) reduced radiation doses compared with FBP. Pediatric head CT with ASIR at 22 mGy allowed 28% dose reduction for 3- to 12-year-old patients and 48% in reduction at 30 mGy for patients older than 12 years compared with FBP [20]. Smith et al. [21] reported that at 3.1 mGy MBIR maintained the diagnostic confidence of pediatric CT compared with ASIR at 6 mGy. Limitations Although several studies have documented the potential for dose reduction with iterative
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reconstruction, these studies have also found oversmoothing of images with higher strengths of iterative reconstruction [3–17]. Hence, it is vital that the appropriate radiation dose level as well as the strength of the iterative reconstruction technique is selected. This oversmoothing due to aggressive noise reduction is reported as distinctive image texture or appearance or “waxiness” or “pixillation” from a variety of iterative reconstruction techniques from different CT vendors. Different image texture is also associated with “stepwise” or “blocky” appearance of tissue margins, such as mediastinal structures [3]. In addition, loss of visibility of major fissures in the lung parenchyma due to this smoothing of images has been reported with iterative reconstruction [6]. Most iterative reconstruction techniques are vendor specific and only available for
state-of-the-art scanners. SafeCT offers an image-based vendor-neutral iterative reconstruction technique that can be applied to any CT system regardless of its age [24]. FBP is currently available on all scanners and is still commonly used in clinical practice. Some iterative reconstruction techniques, such as MBIR, have a long reconstruction time (30– 60 minutes for a single dataset). Thus, their clinical use is limited. These limitations can be overcome by use of hybrid iterative reconstruction techniques that blend with FBP. These hybrid iterative reconstruction techniques have less pixillation or oversmoothing problems compared with the pure iterative reconstruction techniques that do not blend with FBP. In addition, hybrid iterative reconstruction techniques reconstruct images almost in real time (< 1 minute for single dataset).
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Iterative Reconstruction Techniques How to Practically Apply Iterative Reconstruction? Most available iterative reconstruction techniques have different settings or strengths to reduce noise or improve image quality. The first step is to cross-check the dose levels of standard-dose CT at your institution and then compare with either the American College of Radiology Dose Index Registry or the published literature. Second, reduce the dose in small steps of 10–20% by first fine-tuning tube current and tube voltage for CT angiography. This lowering of dose with tube current or voltage can then be accompanied by selecting the milder settings of iterative reconstruction and gradually transitioning to higher settings. Conclusion Iterative reconstruction techniques have consistently shown improved image quality and reduced image noise for chest, abdominal, and head and neck CT as well as for pediatric CT. Iterative reconstruction techniques have the potential to enable CT radiation dose optimization by either lowering tube current or tube potential. CT dose reduction with iterative reconstruction techniques should be achieved in a gradual stepwise approach. Some iterative reconstruction techniques are also associated with limitations, which include texture changes and longer reconstruction time. References 1. Hsieh J. Computed tomography: principles, design, artifacts, and recent advances. Bellingham, WA: SPIE Press, 2003 2. Singh S, Kalra MK, Hsieh J, et al. Abdominal CT: comparison of adaptive statistical iterative and filtered back projection reconstruction techniques. Radiology 2010; 257:373–383 3. Singh S, Kalra MK, Gilman MD, et al. Adaptive statistical iterative reconstruction technique for radiation dose reduction in chest CT: a pilot study. Radiology 2011; 259:565–573 4. Yamada Y, Jinzaki M, Tanami Y, et al. Modelbased iterative reconstruction technique for ultralow-dose computed tomography of the lung: a pilot study. Invest Radiol 2012; 47:482–489 5. Pontana F, Pagniez J, Flohr T, et al. Chest com-
puted tomography using iterative reconstruction vs filtered back projection. Part 1. Evaluation of image noise reduction in 32 patients. Eur Radiol 2011; 21:627–635 6. Kalra MK, Woisetschläger M, Dahlström N, et al. Sinogram-affirmed iterative reconstruction of low-dose chest CT: effect on image quality and radiation dose. AJR 2013; 201:[web]W235–W244 7. Pontana F, Pagniez J, Duhamel A, et al. Reduceddose low-voltage chest CT angiography with sinogram-affirmed iterative reconstruction versus standard-dose filtered back projection. Radiology 2013; 267:609–618 8. Gordic S, Morsbach F, Schmidt B, et al. Ultralowdose chest computed tomography for pulmonary nodule detection: first performance evaluation of single energy scanning with spectral shaping. Invest Radiol 2014; 49:465–473 9. Layritz C, Schmid J, Achenbach S, et al. Accuracy of prospectively ECG-triggered very low-dose coronary dual-source CT angiography using iterative reconstruction for the detection of coronary artery stenosis: comparison with invasive catheterization. Eur Heart J Cardiovasc Imaging 2014; 15:1238–1245 10. Singh S, Kalra MK, Do S, et al. Comparison of hybrid and pure iterative reconstruction techniques with conventional filtered back projection: dose reduction potential in the abdomen. J Comput Assist Tomogr 2012; 36:347–353 11. Flicek KT, Hara AK, Silva AC, Wu Q, Peter MB, Johnson CD. Reducing the radiation dose for CT colonography using adaptive statistical iterative reconstruction: a pilot study. AJR 2010; 195:126–131 12. Gervaise A, Naulet P, Beuret F, et al. Low-dose CT with automatic tube current modulation, adaptive statistical iterative reconstruction, and low tube voltage for the diagnosis of renal colic: impact of body mass index. AJR 2014; 202:553–560 13. Yu MH, Lee JM, Yoon JH, et al. Low tube voltage intermediate tube current liver MDCT: sinogramaffirmed iterative reconstruction algorithm for detection of hypervascular hepatocellular carcinoma. AJR 2013; 201:23–32 14. Hur BY, Lee JM, Joo I, et al. Liver computed tomography with low tube voltage and model-based iterative reconstruction algorithm for hepatic vessel evaluation in living liver donor candidates. J Comput Assist Tomogr 2014; 38:367–375 15. Rapalino O, Kamalian S, Kamalian S, et al. Cra-
nial CT with adaptive statistical iterative reconstruction: improved image quality with concomitant radiation dose reduction. AJNR 2012; 33:609–615 16. Ren Q, Dewan SK, Li M, et al. Comparison of adaptive statistical iterative and filtered back projection reconstruction techniques in brain CT. Eur J Radiol 2012; 81:2597–2601 17. Singh S, Kalra MK, Shenoy-Bhangle AS, et al. Radiation dose reduction with hybrid iterative reconstruction for pediatric CT. Radiology 2012; 263:537–546 18. Bodelle B, Klein E, Naguib NN, et al. Acute intracranial hemorrhage in CT: benefits of sinogramaffirmed iterative reconstruction techniques. AJNR 2014; 35:445–449 19. Notohamiprodjo S, Deak Z, Meurer F, et al. Image quality of iterative reconstruction in cranial CT imaging: comparison of model-based iterative reconstruction (MBIR) and adaptive statistical iterative reconstruction (ASiR). Eur Radiol 2015; 25:140–146 20. McKnight CD, Watcharotone K, Ibrahim M, Christodoulou E, Baer AH, Parmar HA. Adaptive statistical iterative reconstruction: reducing dose while preserving image quality in the pediatric head CT examination. Pediatr Radiol 2014; 44:997–1003 21. Smith EA, Dillman JR, Goodsitt MM, Christodoulou EG, Keshavarzi N, Strouse PJ. Modelbased iterative reconstruction: effect on patient radiation dose and image quality in pediatric body CT. Radiology 2014; 270:526–534 22. Vardhanabhuti V, Riordan RD, Mitchell GR, Hyde C, Roobottom CA. Image comparative assessment using iterative reconstructions: clinical comparison of low-dose abdominal/pelvic computed tomography between adaptive statistical, model-based iterative reconstructions and traditional filtered back projection in 65 patients. Invest Radiol 2014; 49:209–216 23. Kalra MK, Woisetschläger M, Dahlström N, et al. Radiation dose reduction with sinogram affirmed iterative reconstruction technique for abdominal computed tomography. J Comput Assist Tomogr 2012; 36:339–346 24. Pourjabbar S, Singh S, Kulkarni N, et al. Dose reduction for chest CT: comparison of two iterative reconstruction techniques. Acta Radiol; [Epub 2014 Jun 9] (Figures start on next page)
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Fig. 1—73-year-old man with pulmonary nodule (arrows). A–D, Transverse chest CT images acquired at 7 mGy (baseline volume CT dose index) (A) and at 1.6 mGy (reduced dose) (B–D) and reconstructed with various iterative reconstruction techniques. At 1.6 mGy, pulmonary nodule was depicted optimally on iterative reconstruction images with SafeCT (Medic Vision) (B), Adaptive Statistical Iterative Reconstruction (ASIR, GE Healthcare) (C), and Model-Based Iterative Reconstruction (MBIR, GE Healthcare) (D) techniques.
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Iterative Reconstruction Techniques
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Fig. 2—85-year-old woman with lung nodule showing ground-glass opacity in anterior right upper lobe. A–D, Reduced-dose transverse chest CT images (volume CT dose index of 0.9 mGy [A and B] and 0.2 mGy [C and D]) reconstructed with filtered back projection (FBP) (A and C) and iterative model reconstruction (IMR) (B and D). At 0.9 mGy, there is little perceived difference between FBP and IMR images, but at 0.2 mGy, IMR image is better in terms of noise and definition of ground-glass opacity in anterior right upper lobe.
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Fig. 3—62-year-old woman with liver cysts (arrows). A–D, Transverse abdominal CT images acquired at baseline volume CT dose index of 8 mGy (A) and reduced dose at 1.2 mGy (B–D) and reconstructed with filtered backprojection (FBP) and various iterative reconstruction techniques. At 1.2 mGy, liver cysts were depicted optimally on iDose (Philips Healthcare) (C) and Iterative Model Reconstruction (IMR, Philips Healthcare) (D) images and suboptimally on FBP image (B).
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Fig. 4—Female cadaver with kidney stone (arrows) (age at death, 67 years). A–D, Postmortem transverse abdominal CT images acquired at volume CT dose index (CTDIvol) of 8.0 mGy (A and B) and 2.8 mGy (C and D) and reconstructed with filtered backprojection (FBP) (A and C) and Sinogram-Affirmed Iterative Reconstruction (SAFIRE, Siemens Healthcare). Confidence of detecting kidney stone is much higher on reduceddose SAFIRE images at 2.8 mGy CTDIvol compared with corresponding FBP images.
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Fig. 5—57 years male cadaver with no evidence of stroke. Patient expired from aspiration pneumonia and pulmonary edema. A–D, Postmortem transverse head CT images acquired at 58 mGy (A and B) and 15 mGy (C and D) and reconstructed with filtered backprojection (FBP) (A and C) and Model-Based Iterative Reconstruction (MBIR, GE Healthcare) (D). Left middle cerebral artery (arrows) was optimally seen with all 58-mGy images. On 15-mGy images, left middle cerebral artery was optimally seen with MBIR and suboptimally with FBP.
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Fig. 6—7-year-old girl with weight loss and diarrhea. A–D, Coronal abdominal CT images at 4.3 mGy (baseline volume CT dose index) (A) and 1.3 mGy (reduced dose) (B–D) and reconstructed with filtered backprojection (FBP) (B), Adaptive Statistical Iterative Reconstruction (ASIR, GE Healthcare) (C), and Model-Based Iterative Reconstruction (MBIR, GE Healthcare) (D). There is marked improvement in image quality with MBIR at 1.3 mGy compared with FBP and ASIR images.
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