Efficient use of automatic exposure control systems in computed tomography requires correct patient positioning

Area of Science:

• Medical Imaging
• Radiological Physics

Background:

• Computed tomography (CT) image quality and patient radiation dose are critical parameters.
• Automatic exposure control (AEC) with tube current modulation (TCM) aims to maintain consistent image quality by adjusting tube current based on patient anatomy.

Purpose of the Study:

• To investigate the influence of patient centering on tube current-time product (mAs) and image noise in CT scanners.
• To assess the variability in AEC performance across different CT scanner models.

Main Methods:

• An oval acrylic phantom was scanned in various off-center positions across three CT scanners.
• Tube current-time product (mAs) and image noise were measured at different phantom locations.
• Pearson correlation was used to analyze the relationship between position, mAs, and noise.

Main Results:

• AEC-driven mAs varied significantly with the y-position of the phantom across all scanners, with correlations up to ±0.98.
• mAs deviations from the iso-center reached up to +70% and -34% in the y-direction.
• Image noise showed correlations with phantom position, particularly in the y-direction for two scanners.

Conclusions:

• Patient centering critically affects AEC performance in CT, leading to substantial variations in tube current modulation.
• Differences in scanner design and scout radiograph positioning influence the direction and magnitude of mAs changes.
• Off-center positioning can compromise image quality and diminish the intended radiation dose reduction benefits of AEC.