CT automated exposure control using a generalized detectability index.

PURPOSE: Identifying an appropriate tube current setting can be challenging when using iterative reconstruction due to the varying relationship between spatial resolution, contrast, noise, and dose across different algorithms. This study developed and investigated the application of a generalized detectability index (d'gen) to determine the noise parameter to input to existing AEC systems to provide consistent image quality across different reconstruction approaches.

METHODS: This study proposes a task-based automated exposure control (AEC) method using a generalized detectability index (d'gen). The proposed method leverages existing AEC methods that are based on a prescribed noise level. The generalized d'gen metric is calculated using look-up tables of task-based modulation transfer function (MTF) and noise power spectrum (NPS). To generate the look up tables, the American College of Radiology CT accreditation phantom was scanned on a multi-detector CT scanner (Revolution CT, GE Healthcare) at 120 kV and tube current varied manually from 20 mAs to 240 mAs. Images were reconstructed using a reference reconstruction algorithm and four levels of an in-house iterative reconstruction algorithm with different regularization strengths (IR1 - IR4). The task-based MTF and NPS were estimated from the measured images to create look up tables of scaling factors that convert between d'gen and noise standard deviation. The performance of the proposed d'gen-AEC method in providing a desired image quality level over a range of iterative reconstruction algorithms was evaluated using the ACR phantom with elliptical shell and using a human reader evaluation on anthropomorphic phantom images.

RESULTS: The study of the ACR phantom with elliptical shell demonstrated reasonable agreement between the d'gen predicted by the look up table and d' measured in the images, with a mean absolute error of 15% across all dose levels and maximum error of 45% at the lowest dose level with the elliptical shell. For the anthropomorphic phantom study, the mean reader scores for images resulting from the d'gen-AEC method were 3.3 (reference image), 3.5 (IR1), 3.6 (IR2), 3.5 (IR3) and 2.2 (IR4). When using the d'gen -AEC method, the observers' image quality scores for the reference reconstruction were statistical equivalent to the scores for IR1, IR2, and IR3 iterative reconstructions (p-value > 0.35). The d'gen-AEC method achieved this equivalent image quality at lower dose for the IR scans compared to the reference scans.

CONCLUSIONS: A novel AEC method, based on a generalized detectability index, was investigated. The proposed method can be used with some existing AEC systems to derive the tube current profile for iterative reconstruction algorithms. The results provide preliminary evidence that the proposed d'gen-AEC can produce similar image quality across different iterative reconstruction approaches at different dose levels. This article is protected by copyright. All rights reserved.