Automatic exposure control (AEC) for dual energy computed tomography (DECT)

DECT means acquiring the same object at two different energies, respectively two different tube voltages U₁ and U₂. The rawdata q₁ and q₂ undergo a decomposition process of type p = p(q₁, q₂). The rawdata p are reconstructed to obtain monochromatic images of the attenuation mu, of the object density p or of a specific material distribution. Given p and a rawdata-based projection-wise patient dose estimation D (alpha) we determine the optimal tube current curves I₁(alpha) and I₂ (alpha), with alpha being the view angle, that minimizes image noise for a given patient dose level. AEC for DECT can perform online, I₁(alpha) and I₂ (alpha) can be determined during the scan. Simulation studies using semianthropomorphic phantom data were carried out. In particular functions p that generate mu-images and density images were evaluated. Image quality was compared to standard scans at U_o = 120 kV (clinical CT) and U_o = 45 kV (micro-CT) that were taken at the same dose level (Do = D₁ + D₂) and identical spatial resolution. Appropriate choice of p(q₁,q₂) allows to obtain mu images that are both artifact-free and show image noise levels comparable to the noise of the standard scan. Here, mu images at 25 keV (micro-CT) and 70 keV (clinical CT) turned out to be optimal. Non-optimal choice of the decomposition function will, however, significantly increase image noise. In particular mu images at 511 keV, as needed for PET/CT attenuation correction, exhibit more than twice as much image noise as the standard scan. With DECT-AEC, that guarantees best dose usage possible, monochromatic images are generated with only slightly increased noise levels at the same dose compared to a standard scan. The benefit of significantly decreased artifacts appears to allow using DECT-AEC-generated monochromatic images in daily ro- utine.