Variability in CT lung-nodule volumetry: Effects of dose reduction and reconstruction methods

Abstract

Purpose: Measuring the size of nodules on chest CT is important for lung cancer staging and measuring therapy response. 3D volumetry has been proposed as a more robust alternative to 1D and 2D sizing methods. There have also been substantial advances in methods to reduce radiation dose in CT. The purpose of this work was to investigate the effect of dose reduction and reconstruction methods on variability in 3D lung-nodule volumetry.

Methods: Reduced-dose CT scans were simulated by applying a noise-addition tool to the raw (sinogram) data from clinically indicated patient scans acquired on a multidetector-row CT scanner (Definition Flash, Siemens Healthcare). Scans were simulated at 25%, 10%, and 3% of the dose of their clinical protocol (CTDIvol of 20.9 mGy), corresponding to CTDIvol values of 5.2, 2.1, and 0.6 mGy. Simulated reduced-dose data were reconstructed with both conventional filtered backprojection (B45 kernel) and iterative reconstruction methods (SAFIRE: I44 strength 3 and I50 strength 3). Three lab technologist readers contoured "measurable" nodules in 33 patients under each of the different acquisition/reconstruction conditions in a blinded study design. Of the 33 measurable nodules, 17 were used to estimate repeatability with their clinical reference protocol, as well as interdose and inter-reconstruction-method reproducibilities. The authors compared the resulting distributions of proportional differences across dose and reconstruction methods by analyzing their means, standard deviations (SDs), and t-test and F-test results.

Results: The clinical-dose repeatability experiment yielded a mean proportional difference of 1.1% and SD of 5.5%. The interdose reproducibility experiments gave mean differences ranging from -5.6% to -1.7% and SDs ranging from 6.3% to 9.9%. The inter-reconstruction-method reproducibility experiments gave mean differences of 2.0% (I44 strength 3) and -0.3% (I50 strength 3), and SDs were identical at 7.3%. For the subset of repeatability cases, inter-reconstruction-method mean/SD pairs were (1.4%, 6.3%) and (-0.7%, 7.2%) for I44 strength 3 and I50 strength 3, respectively. Analysis of representative nodules confirmed that reader variability appeared unaffected by dose or reconstruction method.

Conclusions: Lung-nodule volumetry was extremely robust to the radiation-dose level, down to the minimum scanner-supported dose settings. In addition, volumetry was robust to the reconstruction methods used in this study, which included both conventional filtered backprojection and iterative methods.

FIG. 1.

Torso phantom scanned at 7 quality reference mAs (left) and simulated 7 quality reference mAs derived from a 300 reference mAs scan (right) showed good qualitative agreement. Mean and standard deviations were calculated for each region of interest (“lung,” “mediastinum,” and “outside”). Standard deviations are shown.

FIG. 2.

Scanned and simulated means and standard deviations of the HU values in three regions of interest agreed well across a range of quality reference mAs settings from 7 (minimum scanner-supported tube-current setting) to 200 mAs.

FIG. 3.

Longest in-plane diameters for the full set of 33 nodules acquired under the reference protocol, averaged across readers.

FIG. 4.

Interdose reproducibility for all cases (N = 33) at three reduced-dose levels showed good agreement.

FIG. 5.

Clinical-dose repeatability (top row) and interdose reproducibility (rows 2–4) for the subset of 17 cases in the repeatability experiment. Interdose reproducibility showed good agreement with the baseline repeatability at clinical dose.

FIG. 6.

Axial slices through an 11 mm nodule at clinical dose (measurements 1 and 2) and 10% dose. The reader’s contours were consistent, with proportional differences of 3.6% and −2.2% for clinical 2 and 10% dose, respectively. Images were reconstructed every 1 mm but this figure only shows every 2 mm for convenience.

FIG. 7.

Axial slices through a 10 mm nodule at clinical dose (measurements 1 and 2) and 10% dose. Reader 1’s contours showed some disagreement due to the complexity of the spiculated nodule boundary. Proportional differences were 16.3% (Clinical 2) and −27.0% (10% dose). Again, images were reconstructed every 1 mm but this figure only shows every 2 mm for convenience.

FIG. 8.

Inter-reconstruction-method reproducibility for all nodules and all readers (N = 99) showed good agreement at clinical dose.

FIG. 9.

Repeatability under the reference protocol (top row) and inter-reconstruction-method reproducibility (rows 2 and 3) for the subset of 17 cases in the repeatability experiment. Inter-reconstruction-method reproducibility showed good agreement with the baseline repeatability at clinical dose.

FIG. 10.

Axial slices through a 9 mm nodule (2 mm increment) at clinical dose. Reader 3’s contours agreed well across reconstruction methods, despite the nodule complexity. Proportional differences were −7.0% and −2.2% for I44 strength 3 and I50 strength 3 methods, respectively.

FIG. 11.

Reader 2’s contours showed some disagreement for this 16 mm, noncircular nodule. Proportional differences were 19.0% for I44 strength 3 and −17.3% for I50 strength 3.