This study shows the effect of different reconstruction techniques at four decreasing radiation dose levels. While FBP resulted in an overestimation of emphysema on CT at reduced dose, both HIR and MIR resulted in an underestimation of the amount of emphysema compared to reconstruction of the images with FBP at routine dose. Furthermore, we showed that by using different thresholds or percentages in HIR and MIR, it was possible to derive results comparable to FBP at routine dose.
There are two commonly used measures to quantify emphysema on CT based on densitometry, namely the density at the 15th percentile of the attenuation curve and the percentage of voxels below -950 HU. Previous studies indicated that those thresholds show the strongest correlation with microscopic and macroscopic emphysema findings in studies using FBP [8, 9]. The 15th percentile and the -950 HU thresholds are widely used; however, different thresholds have been applied in the literature [8, 9, 20, 21]. Several other studies have investigated the effect of dose and image reconstruction on pulmonary emphysema quantification. Schilham et al. [22] compared a clinical routine dose CT acquisition with a low dose acquisition in 25 patients. A post-processing filter was used to reduce the amount of noise in the low dose images and three different thresholds (-950, -930 and -910 HU) were used to quantify emphysema. The application of the filter resulted in a reduction of the effect of noise on the emphysema percentage. A different study by Mets et al. [23] in 75 patients who underwent a routine dose CT acquisition reconstructed with FBP and HIR reported an underestimation of the amount of emphysema with HIR when the cut-off was not adjusted. In a study by Nishio et al. [24], the application of IR at reduced dose improved the agreement in emphysema quantification with routine dose FBP. Three studies comparing a routine dose acquisition with a low-dose acquisition in the same patient all reported an overestimation with low-dose FBP while IR resulted in an underestimation [25,26,27]. Messerli et al. [27] reduced the radiation dose to chest x-ray equivalent dose levels of 0.14 mSv; at this dose level, HIR resulted in a similar emphysema measurement as FBP at routine dose (1.7 mSv). Similar results were found in the study by Nishio et al. [28]. Therefore, by carefully selecting the dose reduction level, emphysema overestimation can be compensated for by using IR, since the latter results in reduced emphysema with CT quantification. To our best knowledge, only the study by Choo et al. [15] investigated the effects of both HIR and MIR. No dose reduction was used and they reported that MIR resulted in a larger underestimation than HIR compared to FBP, which is comparable to the results of the current study.
The effect of reconstruction technique and radiation dose can be explained by the density histogram. IR algorithms result in a different density distribution, which subsequently affects emphysema quantification. Due to the noise reduction with IR, the extremes of the attenuation distribution are affected [23], leading to a smaller peak in the density histogram. Dose reduction, on the other hand, results in increased image noise, leading to a broadening of the density histogram [29].
In the current study, FBP at routine dose was used as the reference standard. However, ideally a pathological reference standard should be applied or a realistic phantom to determine what is closest to the truth and if thresholds should be adapted. It is important to be aware that differences in emphysema quantification can occur and to keep the radiation dose and reconstruction algorithm constant in longitudinal follow-up studies.
Although this within-patient study systematically assessed the effect of dose and reconstruction on emphysema quantification, there are several limitations. First, the patients included in this study had a low amount of emphysema. Second, the sample size was relatively low; however, due to the within-patients study design, the statistical power of the study was increased. Although we showed that adapting the commonly used thresholds can prevent underestimation of emphysema with IR, the sample size was too small to give a clear recommendation about the optimal threshold. Third, one software package and IR algorithms from only one vendor were studied and results may differ for other packages and other vendors. Fourth, only an inspiratory chest CT was acquired; therefore, air-trapping could not be studied. Fifth, the effect of slice thickness and reconstruction kernel were not investigated in the current article. Gierada et al. [30] investigated the effect of reconstruction kernel and slice thickness and reported that patients with 10–30% emphysema are most sensitive for the effect of kernel and slice thickness, while lower emphysema percentages (such as in the current study) resulted in more stable measurements.
In conclusion, as compared to FBP at routine dose, both HIR and MIR result in an underestimation of CT emphysema at routine dose and reduced dose while FBP results in an overestimation at reduced dose. This can potentially be solved by using adapted thresholds.