Effect of Radiation Dose Reduction and Reconstruction Algorithm on Image Noise, Contrast, Resolution, and Detectability of Subtle Hypoattenuating Liver Lesions at Multidetector CT: Filtered Back Projection versus a Commercial Model-based Iterative Reconstruction Algorithm

Abstract

Purpose To determine the effect of radiation dose and iterative reconstruction (IR) on noise, contrast, resolution, and observer-based detectability of subtle hypoattenuating liver lesions and to estimate the dose reduction potential of the IR algorithm in question. Materials and Methods This prospective, single-center, HIPAA-compliant study was approved by the institutional review board. A dual-source computed tomography (CT) system was used to reconstruct CT projection data from 21 patients into six radiation dose levels (12.5%, 25%, 37.5%, 50%, 75%, and 100%) on the basis of two CT acquisitions. A series of virtual liver lesions (five per patient, 105 total, lesion-to-liver prereconstruction contrast of -15 HU, 12-mm diameter) were inserted into the raw CT projection data and images were reconstructed with filtered back projection (FBP) (B31f kernel) and sinogram-affirmed IR (SAFIRE) (I31f-5 kernel). Image noise (pixel standard deviation), lesion contrast (after reconstruction), lesion boundary sharpness (average normalized gradient at lesion boundary), and contrast-to-noise ratio (CNR) were compared. Next, a two-alternative forced choice perception experiment was performed (16 readers [six radiologists, 10 medical physicists]). A linear mixed-effects statistical model was used to compare detection accuracy between FBP and SAFIRE and to estimate the radiation dose reduction potential of SAFIRE. Results Compared with FBP, SAFIRE reduced noise by a mean of 53% ± 5, lesion contrast by 12% ± 4, and lesion sharpness by 13% ± 10 but increased CNR by 89% ± 19. Detection accuracy was 2% higher on average with SAFIRE than with FBP (P = .03), which translated into an estimated radiation dose reduction potential (±95% confidence interval) of 16% ± 13. Conclusion SAFIRE increases detectability at a given radiation dose (approximately 2% increase in detection accuracy) and allows for imaging at reduced radiation dose (16% ± 13), while maintaining low-contrast detectability of subtle hypoattenuating focal liver lesions. This estimated dose reduction is somewhat smaller than that suggested by past studies. ^© RSNA, 2017 Online supplemental material is available for this article.

Figure 1

Graphical user interface of LesionTool software package. (a) Tool allows user to create lesion models either by random generation or by fitting to segmented real lesions. (b) Tool can also be used to insert lesion models into CT data or to export lesion models for projection-based lesion insertion, as was done for this study.

Figure 2

(a, b) Examples of lesion-present (a) and lesion-absent (b) ROI at same location. (c) Difference of these images, I_diff, shows how lesion itself is rendered by reconstruction algorithm. For each lesion in study, noise, contrast, and CNR were measured by using sub-ROIs (within lesion location), as shown. The resolution sub-ROI consisted of the band between the black and white lines, representing the boundary region of the lesion.

Figure 3

Graphical user interface of 2AFC experiment shows, A, image with lesion and, B, image without lesion. For this experiment, the reader was asked to choose which image is most likely to contain the lesion. Reader outcomes (ie, detection or nondetection) were recorded for each trial and the ensemble of outcomes across all combinations of radiation dose level (n = 6), reconstruction algorithm (n = 2), reader (n = 16), and lesion (n = 105) were input into a generalized linear mixed effects statistical model to examine how these factors affected detection accuracy.

Figure 4

Montages of ROI images used as basis of comparing FBP with SAFIRE. A, C, Montages of CT images show lesions within natural background. B, D, Montages show lesions themselves with background subtracted, which represents how lesion appears after being transferred through reconstruction algorithm. Images were obtained from 12.5% radiation dose level scans.

Figure 5

(a–d) Fan chart plots show noise (a), contrast (b), CNR (c), and average relative gradient magnitude (d) as a function of radiation dose. Dots represent individual lesions (blue = FBP, red = SAFIRE), shaded regions represent percentiles of data from 15% (darkest) to 85% (lightest), and solid lines represent medians. (e–h) Corresponding linear regression plots show how those factors compare between FBP and SAFIRE images. Data points (blue circles) are shown with their regression lines (red line) and diagonal (y=x, dotted line).

Figure 6

Results of 2AFC detection experiment show detection accuracy as a function of radiation dose for FBP and SAFIRE. Small markers represent individual readers, large circles are average across readers, and thick solid lines represent detection accuracy averaged across readers and lesions as function of dose, Ā_i(D), as predicted by statistical model. Equation for Ā_i (D) is shown. Dashed line represents radiation dose reduction potential of SAFIRE.