In this study, the impact of varying CM densities at different tube potentials on the overall image quality and diagnostic CE for thoracic CT was investigated. Our results show that increasing CM densities leads to significantly higher diagnostic CE and image quality (CNR). While a reduction in tube potential was shown to increase CE and CNR, these differences were not statistically significant, regardless of employed scan technique or patient size.
The clear benefits of low-kV protocols have been a favoured topic, with resulting radiation and CM dose savings between 40 and 60% reported, when reducing tube potential from 120 to 80 kVp [23,24,25,26,27]. Further dose savings and improved image quality have been reported, when combining low kV and optimised injection protocols which would reduce CM volume or concentration [14, 28, 36, 38]. The latter is demonstrated in the present phantom-based study. Reducing the tube voltage from 120 to 70 kVp for reference diagnostic CE of 200 HU resulted in increases in CNR of up to 80% and 96% using dose modulation and constant CTDIvol, respectively, in the medium phantom size. When lowering the tube potential, there is a debate regarding which approach is optimal for lowering the CM dose. Fleischmann et al.  have reported moderate iodine concentrations (300 mg I/mL) to be superior to higher CM concentrations (400 mg I/mL) for a constant iodine delivery rate when using 70 kVp to achieve sufficient CE over 300 HU . The “double-low” approach has been favoured by several earlier studies [14, 28, 38,39,40] because of resultant lower effective doses and iodine doses with comparable image quality. According to the literature, a CE ≥ 200 HU in the thoracic region is clinically acceptable [3, 13]. For a routine chest CT, 60–70 mL of 350–370 mg I/mL CM has been suggested to be acceptable to achieve a CE of 150–200 HU .
The results of our study, despite investigating the impact of CM density on enhancement instead of CM concentration used in other studies, confirm the same tendency as reported in the study by Sun et al. . They suggested greater potential for lowering the iodine load even in obese patients by 27%, scanning with 100 kVp instead of 120 kVp. Our investigation has reported a 50% reduction of CM density (medium phantom size) for the same diagnostic CE (200 HU) and CNR, by reducing tube potential from 120 to 70 kVp, using dose modulation as shown in Figs. 2 and 3. Our findings are also in agreement with the 51% CM density reduction reported by Thor et al. . However, larger patient sizes as demonstrated in our study (see Fig. 3) may be a limitation with regards to the required tube output needed to achieve comparable CNR levels when using higher kVp values. Van Hamersvelt et al.  have shown a similar 40–60% iodine reduction, without loss of image quality using dual source and dual energy CT. In our study, a single source and single energy CT protocol was employed.
The CNR is primarily affected by CM signal and image noise, broadly becoming the most appropriate measure for investigating iodine-enhanced vessels and structures [27, 40]. When increasing contrast enhancement, by lowering photon energy towards the k-shell electron binding energy of iodine, more noise is accepted . As shown in Fig. 3, the CNR increased from 22.6 to 40.8, approximately 80%, for the medium-sized phantom, when tube potential was reduced from 120 to 70 kVp (see Fig. 3). CNR (22.6 to 40.8) increased by 55.4% when using a fixed CTDIvol, inherently improving the image quality (see Fig. 3). However, these differences in CNR were not statistically significant, when using both scan techniques (p < 0.094). Patient size nevertheless has a great impact on image noise as photon penetration decreases in larger patients and a higher x-ray beam energy is required to achieve the same noise level [36, 39]. In our study, the image quality remained diagnostically acceptable independent of phantom size.
The CNR values resulting in a diagnostic CE ≥ 200 HU were above 23 and 30, with and without dose modulation, for the medium-sized phantom (Fig. 3). When compared to constant CTDIvol, dose modulation continuously reduces the tube current to patient/phantom attenuation profile, while maintaining a given noise index. This may cause an effective dose reduction of 53% according to Kok et al.  which supports our observation with only a slight change in CNR using dose modulation compared to constant CTDIvol (Fig. 3).
There are several limitations in our study. This was a phantom study, thus, no anatomical noise or artefacts caused by breathing and pulsation were present in the images. Furthermore, for the fixed parameter settings, the CTDIvol was lower for the 70 and 80 kVp levels for the large phantom due to technical limitations. Still, the systematic evaluation of different CM concentrations in the different phantom sizes for different scan techniques and dose levels would not be possible to obtain in a clinical setting due to patient radiation dose issues. Thus, performing a phantom study as the first step of systematic evaluation prior to a clinical study is needed to fully assess different scan techniques and available parameter settings. Each ROI placed inside the plastic straws and the chest wall was separated by air, influencing the calculation of objective image quality. No assessment of subjective image quality was conducted in this study. Therefore, to fully assess and validate the findings in this study, clinical studies including both objective and subjective image quality evaluation are needed to confirm our findings in routine clinical care. However, our results show that increased CE at lower tube voltages can be employed in clinical practice.
In conclusion, this study demonstrated that the combination of lower CM densities (specific HU) combined with lower tube potentials (e.g., 70 kVp) resulted in improved CE enhancement (~ 90% higher), and maintained image quality (80% higher CNR) in chest CT when compared to acquisitions at 120 kVp. Using double-low method in thoracic CT examinations, CM density can be reduced by approximately 50% while maintaining CNR. Our findings were independent of scan technique and phantom size. To fully assess the potential of reduced CM densities for lower kVp in chest CT, clinical validation of the results from this study are needed.