Rationale and Objectives
Computed tomographic angiography is the standard in routine follow-up after endovascular aneurysm repair, causing radiation exposure; thus, dose-saving strategies should be applied. The aim of this study was to evaluate the novel sinogram-affirmed iterative reconstruction (SAFIRE) algorithm in terms of clinical usability and potential reduction of radiation exposure.
Materials and Methods
Forty-six patients underwent computed tomographic angiographic follow-up after endovascular aneurysm repair. Data were acquired using a dual-source computed tomographic scanner running both x-ray tubes at the same voltage (120 kV). Raw data were reconstructed using projections of both tubes with filtered back projection (FBP) and of only one tube with FBP and SAFIRE, corresponding to synthetic acquisition with half the radiation dose of the clinical routine radiation dose. Image sets were objectively compared regarding signal-to-noise ratio and edge sharpness. Two radiologists independently assessed a set of subjective criteria, including diagnostic usability, depiction of contrasted vessels, and image noise.
Results
Half-dose (HD) SAFIRE images showed significantly higher signal-to-noise ratios compared to full-dose FBP images ( P < .001), while having equal edge sharpness ( P = .56). Most of the subjectively assessed parameters, such as diagnostic usability and depiction of contrasted vessels, were rated similar for HD SAFIRE and full-dose FBP images. Full-dose FBP images depicted fine anatomic structures more clearly ( P < .05), while HD SAFIRE data sets showed less noise ( P < .01). HD FBP images performed worse on all criteria ( P < .001). Interrater agreement was good (κ = 0.74–0.80).
Conclusions
Using the SAFIRE algorithm, the radiation dose of high-contrast abdominal computed tomographic angiography is reducible from routine clinical levels by up to 50% while maintaining good image quality and diagnostic accuracy.
Endovascular aneurysm repair (EVAR) of infrarenal abdominal aortic aneurysm has become an attractive alternative to open surgery. However, this method can result in a variety of complications, including endoleak, graft migration, graft wire fracture, graft thrombosis, graft stenosis, graft kinking, and graft infections . Consequently, long-term follow-up is mandatory to identify and treat complications. Among other imaging modalities, contrast-enhanced computed tomographic (CT) angiography (CTA) has become the gold standard because of its availability, cost-effectiveness, objectivity, and high sensitivity . Routine follow-up includes CT examinations recommended at 1, 3, 6, and 12 months after the procedure, and yearly thereafter, resulting in a substantial cumulative radiation dose burden with an attributable lifetime cancer risk . To keep patients’ radiation dose as low as reasonably achievable and to minimize cumulative radiation exposure, several dose-saving strategies have proven beneficial, such as lowering the x-ray tube voltage, modulating and adapting the tube current, increasing the iodine concentration of the contrast media , and, more recently, optimizing postprocessing algorithms.
With respect to the latter approach, efforts have been made to improve image quality computationally. Iterative reconstruction techniques have been promising candidates for many years but were limited by computing power and consequent unreasonably long reconstruction times . Compared to traditional filtered back projection (FBP) reconstruction, in which spatial resolution is directly correlated with image noise , iterative reconstruction is able to decouple spatial resolution and image noise to a certain extent. Although resolution and edge sharpness are maintained, image noise in low-contrast areas is reduced . Several studies have demonstrated the benefit of iterative reconstruction in different body regions using iterative reconstruction in image space . A new development in this field is the so-called sinogram-affirmed iterative reconstruction (SAFIRE) technique (Siemens Healthcare, Forchheim, Germany) for CT raw data, which uses a refined noise-modeling technique supported by the raw data (sinogram data), with the aims of reducing noise and maintaining image sharpness. SAFIRE estimates the local noise content in each image pixel by analyzing the raw data contributing to this pixel and removes it from the current image data set.
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Materials and methods
Patients
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Table 1
Patient and CT Angiographic Acquisition Characteristics ( n = 46)
Variable Value Men 45 Age (y) 69.2 ± 10.0 (46–89) CT angiographic number after EVAR 2 (1–8) Interval between EVAR and CT angiography (d) 113 (1–2274) Anterior-posterior cross-section ∗ (cm) 28.7 ± 3.3 Lateral cross-section (cm) ∗ 35.2 ± 2.9 Scan parameters Tube voltage (kV) 120/120 Effective tube current–time product (mAs) 212.2 ± 68.8 CTDI (mGy) 17.9 ± 5.6 Scan length (mm) 535.7 ± 113.6 DLP (mGy · cm) 1008.8 ± 492.9 Pitch 1.1 Collimation (mm) 2 × 64 × 0.6 ED (mSv) 15.1 ± 7.4
CT, computed tomographic; CTDI, computed tomographic dose index; DLP, dose-length product; ED, effective radiation dose; EVAR, endovascular aneurysm repair.
Data are expressed as mean ± standard deviation (range) or as median (range).
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CT Image Acquisition
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Image Reconstruction
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SAFIRE
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Quantitative and Qualitative Image Assessment
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Radiation Dose Estimates
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Statistical Analysis
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Results
Image Noise, CT Numbers, and Edge Sharpness
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Table 2
Mean CT Number, Mean Image Noise, and SNR
ROI FD FBP HD FBP HD SAFIRE CT Number (HU) Image Noise (HU) SNR CT Number (HU) Image Noise (HU) SNR CT Number (HU) Image Noise (HU) SNR Aorta 272.76 ± 40.48 19.29 ± 3.50 14.62 ± 3.39 271.73 ± 40.72 28.31 ± 5.41 9.95 ± 2.36 271.91 ± 40.91 15.34 ± 2.70 18.32 ± 4.33 Muscle 51.66 ± 9.66 18.88 ± 4.34 2.91 ± 0.96 50.15 ± 9.80 27.42 ± 6.79 1.95 ± 0.65 50.02 ± 9.62 15.35 ± 3.64 3.47 ± 1.17 Fat −104.26 ± 5.93 15.22 ± 2.52 −7.03 ± 1.18 −103.83 ± 6.17 20.95 ± 4.01 −5.12 ± 0.93 −103.54 ± 5.97 11.87 ± 2.67 −9.12 ± 1.95 Liver 69.27 ± 10.13 17.20 ± 2.97 4.14 ± 0.91 67.29 ± 10.05 25.96 ± 5.40 2.70 ± 0.69 67.27 ± 9.67 14.03 ± 3.19 5.00 ± 1.25
CT, computed tomographic; FBP, filtered back projection; FD, full dose; HD, half dose; HU, Hounsfield units; ROI, region of interest; SAFIRE, sinogram-affirmed iterative reconstruction; SNR, signal-to-noise ratio.
Data are expressed as mean ± standard deviation.
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Subjective Quality
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Table 3
Subjective Image Quality Assessment ( n = 46)
n FD FBP HD FBP HD SAFIRE Common hepatic artery 46 3.95 ± 0.23 2.77 ± 0.42 3.96 ± 0.20 Lumbar artery 45 3.56 ± 0.73 2.26 ± 0.85 3.57 ± 0.73 Spleen artery 46 3.95 ± 0.23 2.72 ± 0.54 3.98 ± 0.15 Superior mesenteric artery 46 3.93 ± 0.29 2.80 ± 0.42 3.93 ± 0.29 Renal parenchyma 46 3.84 ± 0.42 2.58 ± 0.60 3.80 ± 0.48 Renal cysts 25 3.64 ± 0.59 2.36 ± 0.74 3.64 ± 0.59 Gallbladder wall 39 3.21 ± 0.72 1.53 ± 0.55 2.68 ± 0.72 Diverticula 14 3.79 ± 0.49 2.50 ± 0.73 3.82 ± 0.47 Blooming of the EVAR 46 2.64 ± 0.52 1.54 ± 0.52 2.87 ± 0.56 Noise 46 3.04 ± 0.41 1.53 ± 0.52 3.78 ± 0.51 Diagnostic usability 46 3.84 ± 0.50 1.98 ± 0.74 3.82 ± 0.51
EVAR, endovascular aneurysm repair; FBP, filtered back projection; FD, full dose; HD, half dose; SAFIRE, sinogram-affirmed iterative reconstruction.
Data are expressed as mean ± standard deviation. Subjective image quality parameters were rated on a scale ranging from 1 to 4 (1 = not diagnostic; 2 = moderate image quality, slightly affecting diagnostic accuracy; 3 = good image quality, not affecting diagnostic accuracy; 4 = excellent image quality).
Table 4
Statistical Analysis of Subjective Image Quality
Friedman FD FBP vs HD SAFIRE HD FBP vs HD SAFIRE FD FBP vs HD FBP Common hepatic artery <.001 NS <.01 <.01 Lumbar artery <.001 NS <.01 <.01 Spleen artery <.001 NS <.01 <.01 Superior mesenteric artery <.001 NS <.01 <.01 Renal parenchyma <.001 NS <.01 <.01 Renal cysts <.001 NS <.01 <.01 Gallbladder wall <.001 <.05 <.01 <.01 Diverticula <.001 NS <.01 <.01 Blooming of the EVAR <.001 NS <.01 <.01 Noise <.001 <.01 <.01 <.01 Diagnostic usability <.001 NS <.01 <.01
EVAR, endovascular aneurysm repair; FBP, filtered back projection; FD, full dose; HD, half dose; SAFIRE, sinogram-affirmed iterative reconstruction.
All P values are Bonferroni corrected.
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Dose Estimations
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Discussion
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Conclusions
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