Latest CT Technologies in Lung Cancer Screening

Protocols and Radiation Dose Reduction

Marleen Vonder; Monique D. Dorrius; Rozemarijn Vliegenthart

Disclosures

Transl Lung Cancer Res. 2021;10(2):1154-1164. 

In This Article

Radiation Dose Reduction

In the past 5 years, multiple studies have evaluated the impact of lowering the radiation further from LDCT to ultra-low dose CT (ULDCT) with submilliSievert dose level. Common applied dose reduction techniques are reducing the tube output by decreasing the tube current and/or tube voltage or by using beam filtration with a tin-filter. Reduction of the output would lead to decreased image quality and therefore these techniques are often applied in a protocol with iterative reconstruction.

Decreasing tube current and iterative reconstruction

In the dose reduction studies, the reference LDCT protocols, CT systems and iterative reconstruction algorithms differ considerably.[39–44] In the study of Lim et al. a high-pitch scan mode with iterative reconstruction at 120 kVp with fixed tube current of 30 mAs resulted in 34% dose reduction (2.1 vs. 3.1 mGy) with similar image quality and reduced cardiac motion artifacts compared to reference low-dose chest CT at 120 kVp with FBP and low pitch.[44] Nevertheless, a fixed tube current of 30 mAs was used in 30 healthy volunteers and reconstructions were made at 3.0/3.0 slice thickness and increment, making the study set-up less suitable to determine the impact on lung nodule evaluation.

In a study of Ye et al. the authors specifically investigated the sensitivity for nodule detection of an ULDCT protocol compared to LDCT protocol in 188 individuals.[39] The ULDCT was performed at 120 kVp with 10 mA while the LDCT was executed at 120 kVp with 50 mA. In both protocols IR 50% [adaptive statistical iterative reconstruction V (ASiR-V)] was applied, and images were reconstructed at slice thickness and increment of 2.5 mm. The authors found that nodule type (solid, sub-solid) and nodule size (diameter) had influence on the sensitivity. For example, the sensitivity for nodule detection was overall 90.4% while this could be increased to 98.2% if only nodules with diameter of ≥6 mm were considered. Although the dose could be reduced from 0.93 to 0.096 mSv based on ULDCT, images were reconstructed at relatively thick slice thickness. Reducing the slice thickness to the preferred 1.0 mm may drastically impede image quality and sensitivity for nodule detection. Nevertheless, in a study of Zhang et al. ULDCT protocols with IR [iterative model reconstruction (IMR)] and with thin slices were used (1.0 mm). This study showed that the use of ULDCT with IMR resulted in similar nodule detectability as the reference LDCT with FBP and with hybrid IR, while diagnostic image quality for lung and mediastinum was preserved. However, the ULDCT protocol in this study was with 0.67 mSv considerably higher than the former study, while obese patients were excluded.[42]

Tin-filter and Iterative Reconstruction

Several CT systems from one vendor are equipped with a tin filter than can be applied to block the low-energy part of the X-ray spectrum. This part of the spectrum would otherwise have been absorbed by the patient's body and would not have contributed to the image quality but would have increased the total radiation dose.

In a phantom study from Martini et al. the sensitivity of a low-dose tin filter protocol combined with IR [advanced modeled iterative reconstruction (ADMIRE) level 5] for solid and sub-solid nodule detection was determined.[45] Sensitivity was 100% and 93.8% for solid and sub-solid nodules ranging in size from 2–10 mm at dose of 0.10 mGy. With a slightly higher dose (0.30 mGy), by increasing the tube current from 30 to 80 mAs, also a sensitivity of 100% for sub-solid nodules could be reached. In addition, in a phantom study of Eberhard et al. the impact of low dose tin filter protocol on volume measurements of solid nodules was determined. This study showed similar results for standard dose CT and the tin filter protocol for nodule volume ranging from 34–524 mm3 (diameter 4–10).[46] However, absolute percentage error increased for images reconstructed with higher levels of IR (ADMIRE 3 and 5) compared to FBP. This may affect calculation of volume doubling times and consequently impact the clinical management. Nonetheless, volume measurement of solid nodules may still be reliably performed by using a Sn100kVp protocol with FBP at CTDIvol of 0.10 mGy.

In a patient study of Messerli et al. an almost similar CT protocol was applied as in the phantom study of Martini et al., namely Sn100kVp with IR (ADMIRE level 3) at 70 mAs.[47] The ultra low-dose Sn protocol was applied in 202 patients and sensitivity and image quality were compared to the full dose protocol with 110 kVp and IR (ADMIRE level 3). This patient study showed slightly lower sensitivity of 97.3% and 92.6% for all nodules types and sub-solid nodules of >5 mm, respectively, compared to the phantom study. However, the majority of false negative nodules on ultra low-dose scans had diameter of ≤2.0 mm on the full dose scan, which is well below the lowest size threshold in nodule management protocols. Although the image quality was significantly lower in the ultralow dose protocol this seemed not to affect the detection of relevant lung nodules. Therefore, the radiation dose could be reduced from 3.9 to 0.3 mGy by applying tin-filter protocol with IR. Nonetheless, the impact of the ultralow dose protocol on diameter or volume measurement error was not evaluated in this patient study.

To conclude, the radiation dose may be further decreased beyond the currently recommended guidelines, by using iterative reconstruction and a low tube current or tin-filter. However, the majority of studies investigated only the impact on nodule detection based on certain diameter size with cut-off values varying between studies. Also, only a few studies specified sensitivity by nodule type and many studies excluded obese patients. Therefore, one should be cautious in reducing the radiation to ultralow dose settings.

processing....