Dual-layer dual-energy computed tomography for the assessment of hypovascular hepatic metastases: impact of closing k-edge on image quality and lesion detectability

Objectives To evaluate the image quality of virtual-monoenergetic-imaging (VMI) from dual-layer dual-energy CT (DLCT) for the assessment of hypovascular liver metastases and its effect on lesion detectability. Methods Eighty-one patients with hypovascular-liver-metastases undergoing portal-venous-phase abdominal DLCT were included. Polyenergetic-images (PEI) and VMI at 40–200 keV (VMI 40–200 , 10-keV interval) were reconstructed. Image noise, tumor-to-liver contrast, and contrast-to-noise ratio (CNR) of hepatic parenchyma and metastatic nodules ( n = 288) were measured to determine the optimal monoenergetic levels. Two radiologists independently and subjectively assessed the image quality (image contrast, image noise, and diagnostic confidence) of PEI and optimal VMI on 5-point scales to determine the best energy. For 38 patients having up to 10 metastases each with diameters < 25 mm (153 lesions), we compared blindly assessed lesion detectability and conspicuity between PEI and VMI at the best energy. Results Image noise of VMI 40–200 was consistently lower than that of PEI ( p < 0.01). Tumor-to-liver contrast and CNR increased as the energy decreased with CNR at VMI 40–70 being higher than that observed on PEI ( p < 0.01). The highest subjective score for diagnostic confidence was assigned at VMI 40 followed by VMI 50–70 , all of which were significantly better than that of PEI ( p < 0.01, kappa = 0.75). Lesion detectability at VMI 40 was significantly superior to PEI, especially for lesions with diameters of < 10 mm ( p < 0.01, kappa ≥ 0.6). Conclusions VMI 40–70 provided a better subjective and objective image quality for the evaluation of hypovascular liver metastases, and the lesion detectability was improved with use of VMI 40 compared with conventional PEI. Key Points • DLCT-VMI at 40–70 keV provides a superior subjective and objective image quality compared with conventional PEI for the assessment of hypovascular hepatic metastases during portal venous phase. • Tumor-to-liver contrast and CNR of hypovascular hepatic metastases was maximized at 40 keV without a relevant increase in the image noise. • VMI at 40 keV yields a superior lesion detectability, especially for small (< 1 cm) metastatic nodules compared with conventional PEI.