Rationale and Objectives
This study aimed to investigate the feasibility of reducing radiation exposure and contrast medium (CM) dose in follow-up computed tomography angiography (CTA) after thoracic endovascular aortic repair (TEVAR) using low tube voltage and knowledge-based iterative model reconstruction (IMR).
Materials and Methods
Thirty-six patients that required follow-up CTA after TEVAR were included in this intra-individual study. The conventional protocol with standard tube voltage of 120 kVp and CM volume of 70 mL was applied in the first follow-up CTA of all the patients (control group A). The ultra-low CM dose protocol with low tube voltage of 80 kVp and weight-adapted CM volume of 0.4 mL/kg was utilized in the second follow-up CTA (study group B). Set A.FBP (group A filtered back-projection) contained images for group A that were reconstructed through FBP method. Three sets (B.FBP, B.HIR, and B.IMR) for group B were reconstructed using three methods, FBP, hybrid iterative reconstruction (HIR), and IMR, respectively. Objective measurements including aortic attenuations, image noise, contrast-to-noise ratios (CNRs), and figure of merit of CNR (FOM CNR ), and subjective rating scores of the four image sets were compared.
Results
Compared to the images in set A.FBP, the images in set B.IMR had better quality in terms of equivalent attenuation values, equivalent subjective scores, lower noise, higher or equivalent CNRs, and higher FOM CNR . The quality of images in sets B.FBP and B.HIR was unacceptable. The radiation exposure and CM dose in group B were 1.94 mGy and 28 ± 5 mL, respectively, representing reductions of 77.6% ( P < .001) and 60% ( P < .001) as compared to those in group A.
Conclusions
In follow-up examinations after TEVAR, CTA with ultra-low radiation exposure and CM dose is feasible using low tube voltage and IMR for nonobese patients.
Introduction
Owing to the lower risk of mortality and morbidity compared to traditional open surgery, thoracic endovascular aortic repair (TEVAR) has been shown to be a less invasive alternative in the treatment of type B aortic dissections . Careful surveillance for patients who have undergone TEVAR is crucial to the long-term success of the procedure. Currently, computed tomography angiography (CTA) is the preferred follow-up method after TEVAR, which facilitates detection of endoleaks and other procedure-related complications . However, repeated CTA scans in the follow-up period could result in large contrast medium (CM) dosage, fast CM injection rate, and high X-ray radiation exposure, which potentially induce serious complications. For example, CM dose has been shown to be positively correlated to the occurrence of contrast-induced nephropathy , fast CM injection rate may lead to failure of intravenous access , and radiation exposure in computed tomography (CT) scans is a major risk factor of radiation-induced malignancies . Hence, minimal volume and injection rate of CM and radiation dose is preferred in follow-up CTA after TEVAR.
Advancements in low tube voltage have enabled reductions in CM dose and radiation exposure for CTA examinations . Unfortunately, an important limitation is that the noise and susceptibility to beam hardening artifacts of images increase as the tube voltage decreases. To overcome the problem, iterative reconstruction (IR) techniques have been introduced into clinical practice to counterbalance the image noise and improve overall image quality . The knowledge-based iterative model reconstruction (IMR, Philips Medical Systems, Best, Netherlands) is the latest development in the field of IR techniques . It effectively controls the noise reduction level through enforcing the image smoothness in an optimization process where reconstruction is incorporated as a part and constraints are imposed on a penalty-based cost function that considers characteristics of a given CT system . Analyses of the joint application of low tube voltage and IMR on aortic CTA are still rare. The only study in this area is a pilot study that investigated the feasibility of applying low tube voltage and IMR to reduce CM volume to a fixed level of 60 mL (24 gI) in follow-up CTA for abdominal aortic aneurysm . To our knowledge, the impact of combining low tube voltage with IMR has not been investigated for patients undergoing follow-up CTA after TEVAR.
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Materials and Methods
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Study Population
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CT Examinations
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CT Image Reconstruction
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Image Analysis
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CM Volume and Radiation Dose Evaluation
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Statistical Analysis
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Results
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Quantitative Image Analysis
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TABLE 1
CT Attenuation (HU) Results in All Evaluated Structures
Item Ascending Aorta Descending Aorta Celiac Trunk Renal Artery Iliac Artery Set A.FBP 372 ± 33 357 ± 33 351 ± 33 352 ± 32 354 ± 38 Set B.FBP 343 ± 51 345 ± 45 348 ± 48 347 ± 43 343 ± 40 Set B.HIR 343 ± 52 343 ± 45 344 ± 42 343 ± 41 343 ± 39 Set B.IMR 343 ± 51 344 ± 46 344 ± 41 343 ± 39 345 ± 39P value .045 .59 .90 .78 .13 A.FBP vs B.FBP 0.12 1.00 1.00 1.00 0.28 A.FBP vs B.HIR 0.12 1.00 1.00 1.00 0.25 A.FBP vs B.IMR 0.12 1.00 1.00 1.00 0.43 B.FBP vs B.HIR 1.00 1.00 1.00 1.00 1.00 B.FBP vs B.IMR 1.00 1.00 1.00 1.00 1.00 B.HIR vs B.IMR 1.00 1.00 1.00 1.00 1.00
A.FBP, group A filtered back-projection; B.FBP, group B filtered back-projection; B.HIR, group B hybrid iterative reconstruction; B.IMR, group B iterative model reconstruction; CT, computed tomography; HU, Hounsfield unit.
Data are presented as mean ± standard deviation.
TABLE 2
Contrast-to-Noise Ratios in All Evaluated Structures
Item Ascending Aorta Descending Aorta Celiac Trunk Renal Artery Iliac Artery Set A.FBP 16.3 ± 3.4 15.5 ± 3.5 15.2 ± 3.5 15.3 ± 3.5 15.8 ± 3.5 Set B.FBP 3.6 ± 1.4 3.6 ± 1.3 3.6 ± 1.2 3.6 ± 1.3 3.6 ± 1.2 Set B.HIR 5.7 ± 1.9 5.7 ± 1.7 5.7 ± 1.6 5.7 ± 1.6 5.7 ± 1.6 Set B.IMR 18.2 ± 4.9 18.3 ± 4.5 18.3 ± 4.2 18.3 ± 4.1 18.4 ± 4.0P value <.001 <.001 <.001 <.001 <.001 A.FBP vs B.FBP <0.001 <0.001 <0.001 <0.001 <0.001 A.FBP vs B.HIR <0.001 <0.001 <0.001 <0.001 <0.001 A.FBP vs B.IMR 0.13 0.003 <0.001 <0.001 0.005 B.FBP vs B.HIR 0.06 0.04 0.03 0.03 0.02 B.FBP vs B.IMR <0.001 <0.001 <0.001 <0.001 <0.001 B.HIR vs B.IMR <0.001 <0.001 <0.001 <0.001 <0.001
A.FBP, group A filtered back-projection; B.FBP, group B filtered back-projection; B.HIR, group B hybrid iterative reconstruction; B.IMR, group B iterative model reconstruction.
Data are presented as mean ± standard deviation.
![Figure 2, Box and whisker graph plots show median, interquartile range, and extreme cases of figure of merit of contrast-to-noise ratio (FOM CNR ). FOM CNR increased by 3.6 times for low-tube-voltage scan (80 kVp with ultra-low contrast medium [CM] dose) compared to that for the standard-tube-voltage protocol (120 kVp with standard dose of CM). The values of the median, first quartile, and third quartile are 20.3, 14.7, and 30.7 for the standard-tube-voltage protocol, and 93.2, 58.5 and 136.9 for the low-tube-voltage protocol ( P < .001). FBP, filtered back-projection; IMR, iterative model reconstruction.](https://storage.googleapis.com/dl.dentistrykey.com/clinical/LowTubeVoltageandIterativeModelReconstructioninFollowupCTAngiographyAfterThoracicEndovascularAorticRepair/1_1s20S1076633217304750.jpg)
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Qualitative Image Analysis
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TABLE 3
Qualitative Assessments of Image Quality
Item Image Noise Vessel Sharpness Overall Image Quality Set A.FBP 4.8 ± 0.3 4.8 ± 0.3 4.9 ± 0.3 Set B.FBP 1.5 ± 0.6 1.7 ± 0.6 1.5 ± 0.5 Set B.HIR 2.5 ± 0.6 2.9 ± 0.4 2.5 ± 0.5 Set B.IMR 4.6 ± 0.5 4.7 ± 0.4 4.8 ± 0.4 Kappa value 0.76 0.75 0.80P value <.001 <.001 <.001 A.FBP vs B.FBP <0.001 <0.001 <0.001 A.FBP vs B.HIR <0.001 <0.001 <0.001 A.FBP vs B.IMR 1.00 1.00 1.00 B.FBP vs B.HIR 0.002 0.16 0.19 B.FBP vs B.IMR <0.001 <0.001 <0.001 B.HIR vs B.IMR <0.001 <0.001 <0.001
A.FBP, group A filtered back-projection; B.FBP, group B filtered back-projection; B.HIR, group B hybrid iterative reconstruction; B.IMR, group B iterative model reconstruction.
Data are presented as means ±standard deviation.
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CM Volume and Injection Rate
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TABLE 4
Radiation Exposures, and Contrast Medium (CM) Doses and Injection Rates
Item Control Group A Study Group B_P_ value CM volume (mL) 70 28 ± 5 <.001 Iodine dose (g) 24.5 9.8 ± 1.8 <.001 Injection rate (mL/s) 5.0 2.4 ± 0.4 <.001 Radiation exposure CTDI vol (mGy) 9.7 ± 3.1 1.94 <.001 DLP (mGy.cm) 621.4 ± 174.2 148.9 ± 77.5 <.001 ED (mSv) 11.6 ± 3.2 2.8 ± 1.4 <.001
CTDI vol , CT volume dose index; DLP, dose-length product; ED, effective dose.
Data are presented as mean ± standard deviation.
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Radiation Dose
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Discussion
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Conclusion
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