Iterative reconstruction improves the reliability of disease severity differentiation and reduces the occurrence of false-positive findings in coronaries with heavy calcification or stenting through a more effective suppression of beam-hardening and blooming artifacts compared with FBP at constant x-ray tube settings.39,43,44 Several additional artifacts, such as scatter and motion, can also be reduced using iterative reconstruction.47
Besides dose-reduced protocols for chest CT, the use of the iterative reconstruction technique allows for an improved depiction of anatomic and pathologic imaging findings. In a study comprising 24 patients, substantial better visualization of small normal lung structures (eg, interlobular septa, centrilobular arteries, small bronchi and bronchioles) as well as pathologic patterns (eg, reticular pattern, small nodules, changes in lung attenuation) was achieved by using ASIR, particularly in combination with a specific high-detail reconstruction kernel.48 In addition, the sensitivity for detection of pulmonary nodules using a computer-aided detection software can be significantly increased with increasing levels of ASIR compared with FBP. However, the number of false-positive findings per scan increased with higher influence of iterative reconstruction.49 Within the same context, Willemink et al50 showed no significant impact on volume measurements of solid pulmonary nodules using the iDose algorithm. Therefore, the clinical value of improved image quality through iterative reconstruction techniques remains to be seen. However, regarding the potential dose reduction, lung-screening trials using dose-reduced CT protocols appear to be a promising application of iterative reconstructions in chest imaging.51,52
Despite advances in CT technology, the use of cCTA in obese patients is still challenging because of noise-impaired image quality.53–57 Image noise in relation to obesity can be explained by quantum noise and Compton scattering. Quantum noise is typically inversely proportional to the square root of the delivered dose.15,58 As soft-tissue absorption of low-energy x-ray photons in obese patients is increased, a reduced number of photons reach the detector; this decreases the SNR.15,55,59 Compton scattering, in simple terms, is the alteration of an x-ray photon’s trajectory following interaction with a substrate electron, with a comparable decrease in the frequency of the photon. This angular displacement causes the photon to proceed toward a different coordinate on the detector, leading to erroneous data. As body volume increases, x-ray scattering will increase, resulting in a deterioration in CT image quality.14,15 In addition, the degree of arterial enhancement can be further impaired in CTA of obese patients because of higher cardiac output, higher central blood volume, and beam-hardening artifacts.55,59,60
The superiority of iterative reconstruction techniques with regard to image quality and the potential for radiation dose reduction has been verified already by several reports.16–19,48,61,62 However, fewer studies have compared the CT image quality of iterative reconstruction with that of FBP in obese patients.19,63 In a study by Wang and colleagues with 109 consecutive patients using IRIS and FBP reconstruction, the study population was divided into 3 groups according to body mass index (BMI): normal patients were defined as those having a BMI of 18.50 to 24.99 kg/m2, overweight patients as those having a BMI of 25.00 to 29.99 kg/m2, and obese as those with a BMI≥30.00 kg/m2. IRIS demonstrated an incremental improvement in subjective image quality in all BMI groups, compared with FBP. In obese and overweight patients, the number of assessable coronary segments was increased with IRIS. Image noise, SNR, and CNR were also significantly improved in all BMI groups when using IRIS, compared with FBP. When IRIS and FBP were compared for cCTA with coronary catheterization as the referenced standard, diagnostic accuracy, specificity, and positive predictive value for detection of significant stenosis were significantly improved with IRIS (over FBP) in obese and overweight patients.
Moreover, iterative reconstruction has provided a different degree of dose reduction dependent on the level of obesity. Several studies using iterative reconstruction reported radiation dose reductions relative to obesity compared with traditional FBP.11,19 In a clinical study, a total of 33 obese patients (average BMI 43) who underwent pulmonary CTA were compared with 33 individuals controls (average BMI 22). Compared with FBP, the application of the iDose algorithm revealed a significant decrease in noise and increase in the CNR and also provided a subjective assessment of image quality.64 Regarding radiation exposure, it has been shown that the potential dose reduction percentage increases as BMI decreases. In a study cohort of 12 patients with iterative reconstruction and FBP, although patients with a BMI<20 exhibited CT dose index (CTDI) and dose-length product reductions of 65%, patients with a BMI≥25 had reductions of 29% to 35%.11 In a more recent and larger study consisting of 162 patients, iterative reconstruction in patient groups with BMI <25 and ≥25 demonstrated effective radiation doses of 40% and 51%, respectively, compared with routine-dose CT using FBP.19 Wang et al63 also reported similar results in a study involving obese patients using SAFIRE. Effective radiation dose at 100 kV cCTA with SAFIRE was 4.41±0.83 mSv and significantly lower with comparable image quality, compared with 8.83±1.74 mSv at 120 kV cCTA with FBP. An example of a cCTA of an obese patient is provided in Figure 6.
With advances in CT technology, the following strategies have been applied to minimize radiation dose during a cardiothoracic CT scan: automated tube current modulation,65–69 noise reduction filter,70,71 low tube voltage,72–79 reduction of z-axis scan length,73,80,81 ECG pulsing,82 heart rate adaptive pitch,83 sequential ECG triggering,78,84 high-pitch acquisition,85 and reduction in the duration of padding.86–89
The FBP reconstruction algorithm has fundamental limitations for radiation dose reduction because of the resultant image noise. Several recent studies involving iterative reconstruction consistently support that iterative reconstruction algorithms permit significant radiation dose reductions while maintaining superior image quality compared with FBP reconstruction.5,11,16,18,19,24,90–96
Most importantly, iterative reconstruction can be applied along with other image acquisition techniques, such as prospective ECG triggering or traditional retrospective ECG gating, providing additional dose reduction to existing dose-reduction strategies.5,43 Therefore, iterative reconstruction can provide a low-dose CT protocol for pediatric patients and obese patients and in the case of multiple follow-up or multiphasic studies.4,12
A disadvantage of earlier-generation iterative reconstruction has been the prohibitively long computational processing time compared with FBP. In recent times, advanced iterative reconstruction techniques have reduced processing times as a result of the use of computationally faster algorithms.4 At the time of writing this paper, the mean duration for data reconstruction of SAFIRE is longer than that of FBP, with reconstruction speeds of approximately 20 frames/s in SAFIRE and 40 frames/s in FBP.5,95 However, its clinical utility is not affected. As mentioned above, Veo is even more time-consuming because of a different iterative approach, which currently limits its implementation into clinical routine. Reported reconstruction times for data sets obtained over a scan length of 140 to 400 mm are 20 to 60 minutes, which equals roughly 0.2 to 0.5 frames/s.101 However, as computational power continues to evolve, the processing time of iterative reconstruction is expected to decrease even further.
Another disadvantage of the current iterative reconstruction technologies is that, in cases of highly iterative reconstruction, artificial-looking, noise-free images are produced with significantly smoothed borders, which can decrease diagnostic confidence. This appearance due to highly iterative reconstruction can usually be controlled by the amount (ie, strength) of iterative reconstruction,4 depending on the available algorithm. For instance, Gervaise et al20 reported no artificial-looking images with the use of AIDR—an algorithm that does not allow for the choice between different strength levels by the user. However, as more powerful iterative reconstruction algorithms emerge, noise will be reduced more efficiently and with better preservation of an image’s natural appearance.4,12,16
Iterative reconstruction can allow radiation dose reduction while maintaining superior image quality. Iterative reconstruction also provides improvement of certain artifacts (eg, blooming) in comparison with the FBP algorithm. Given what we have seen, the future prospects of iterative reconstruction are bright, with the expectation of it becoming an even more robust and easy-to-apply method with the emergence of more effective iterative reconstruction algorithms. Currently, these reconstruction techniques demonstrate powerful noise reduction and thereby permit further reduction in x-ray tube current/voltage for lower radiation doses, reducing patient risk and improving diagnostic outcomes.
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