Comparison of low- and ultralow-dose computed tomography protocols for quantitative lung and airway assessment

Abstract

Purpose: Quantitative computed tomography (CT) measures are increasingly being developed and used to characterize lung disease. With recent advances in CT technologies, we sought to evaluate the quantitative accuracy of lung imaging at low- and ultralow-radiation doses with the use of iterative reconstruction (IR), tube current modulation (TCM), and spectral shaping.

Methods: We investigated the effect of five independent CT protocols reconstructed with IR on quantitative airway measures and global lung measures using an in vivo large animal model as a human subject surrogate. A control protocol was chosen (NIH-SPIROMICS + TCM) and five independent protocols investigating TCM, low- and ultralow-radiation dose, and spectral shaping. For all scans, quantitative global parenchymal measurements (mean, median and standard deviation of the parenchymal HU, along with measures of emphysema) and global airway measurements (number of segmented airways and pi10) were generated. In addition, selected individual airway measurements (minor and major inner diameter, wall thickness, inner and outer area, inner and outer perimeter, wall area fraction, and inner equivalent circle diameter) were evaluated. Comparisons were made between control and target protocols using difference and repeatability measures.

Results: Estimated CT volume dose index (CTDIvol) across all protocols ranged from 7.32 mGy to 0.32 mGy. Low- and ultralow-dose protocols required more manual editing and resolved fewer airway branches; yet, comparable pi10 whole lung measures were observed across all protocols. Similar trends in acquired parenchymal and airway measurements were observed across all protocols, with increased measurement differences using the ultralow-dose protocols. However, for small airways (1.9 ± 0.2 mm) and medium airways (5.7 ± 0.4 mm), the measurement differences across all protocols were comparable to the control protocol repeatability across breath holds. Diameters, wall thickness, wall area fraction, and equivalent diameter had smaller measurement differences than area and perimeter measurements.

Conclusions: In conclusion, the use of IR with low- and ultralow-dose CT protocols with CT volume dose indices down to 0.32 mGy maintains selected quantitative parenchymal and airway measurements relevant to pulmonary disease characterization.

Keywords: airway measurements; chronic obstructive pulmonary disease; low-dose computed tomography; lung disease assessment; quantitative CT protocols.

Figure 1

CT Data Acquisition Summary. For each inspiratory lung volume, three CT datasets were acquired in a single enforced breath hold — one control CT scan followed immediately by two target CT scans. Each breath hold was preceded by 2 min of recruitment and followed by 5 min of normal ventilation. This process of CT data collection was repeated for all protocols. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

Airway tree segmentations. The left most airway tree illustrates the eight airways isolated for analysis. Blue (BX) denotes the medium‐sized airways, while red (RX) denotes the small airways. The remaining airway trees represent the segmented airway trees from all acquired protocols, denoted in the lower right corner of each image, with a corresponding CT cross section in the upper left corner.

Figure 3

Specific airway results for small airways. Cumulative change in average difference magnitudes for small airways showing normal variability across breath holds and within the control protocol. Results are presented with differences compared to control (black) and showing repeatability (white) and summed across measurement to assess by protocol (left) and summed across protocol to assess by measurement (left).

Figure 4

Specific airway results for medium airways. Cumulative change in average difference magnitudes for medium airways showing normal variability across breath holds and within the control protocol. Results are presented with differences compared to control (black) and showing repeatability (white) and summed across measurement to assess by protocol (left) and summed across protocol to assess by measurement (left).