Rapid switching kVp dual energy CT: Value of reconstructed dual energy CT images and organ dose assessment in multiphasic liver CT exams

Abstract

Purpose: Clinical applications of dual energy computed tomography (DECT) have been widely reported; however, the importance of the different image reconstructions and radiation organ dose remains a relevant area of investigation, particularly considering the different commercially available DECT equipment. Therefore, the purpose of this study was to assess the image reliability and compare the information content between several image reconstructions in a rapid-switching DECT (rsDECT), and assess radiation organ dose between rsDECT and conventional single-energy computed tomography (SECT) exams.

Materials and methods: This Institutional Review Board-approved retrospective study included 98 consecutive patients who had a history of liver cancer and underwent multiphasic liver CT exams with rsDECT applied during the late arterial phase between June 2015 and December 2015. Virtual monochromatic 70 keV, material density images (MDI) iodine (-water) and virtual unenhanced (VUE) images were generated. Radiation dose analysis was performed in a subset of 44 patients who had also undergone a multiphasic SECT examination within 6 months of the rsDECT. Four board-certified abdominal radiologists reviewed 24-25 patients each, and a fifth radiologist re-evaluated all the scans to reach a consensus. The following imaging aspects were assessed by the radiologists: (a) attenuation measurements were made in the liver and spleen in VUE and true unenhanced (TUE) images; (b) subjective evaluation for lesion detection and conspicuity on MDI iodine (-water)/VUE images compared with the virtual monochromatic images/TUE images; and (c) overall image quality using a five-point Likert scale. The radiation dose analyses were evaluated in the subset of 44 patients regarding the following parameters: CTDIvol, dose length product, patient's effective diameter and organ dose using a Monte Carlo-based software, VirtualDose™ (Virtual Phantoms, Inc.) to 21 organs.

Results: On average, image noise on the TUE images was 49% higher within the liver (p < 0.0001) and 48% higher within the spleen (p < 0.0001). CT numbers for the spleen were significantly higher on VUE images (p < 0.0001). Twenty-eight lesions in 24/98 (24.5%) patients were not observed on the VUE images. The conspicuity of vascular anatomy was considered better on MDI iodine (-water) Images 26.5% of patients. Using the Likert scale, the rsDECT image quality was considered to be satisfactory. Considering the subset of 44 patients with recent SECT, the organ dose was, on average, 37.4% less with rsDECT. As the patient's effective diameter decreased, the differences in dose between the rsDECT and SECT increased, with the total average organ dose being less by 65.1% when rsDECT was used.

Conclusion: VUE images in the population had lower image noise than TUE images; however, a few small and hyperdense findings were not characterized on VUE images. Delineation of vascular anatomy was considered better in around a quarter of patients on MDI iodine (-water) images. Finally, radiation dose, particularly organ dose, was found to be lower with rsDECT, especially in smaller patients.

Keywords: DECT; Rapid-switching kV; Virtual unenhanced.

Figure 1

Representative image types generated from the rapid-switching dual energy computed tomography (rsDECT) scan. (a) Axial, virtual monochromatic 70 keV images. (b) Material Density Image water (iodine)-Virtual Unenhanced. (c) Material Density Image iodine (-water). (d) Material Suppressed Iodine-Virtual Unenhanced. (e) Iodine map overlay.

Figure 2

Noise measurements performed in (a) single-energy computed tomography true unenhanced (SECT-TUE) (liver noise = 9.7 HU; Spleen noise = 10.2 HU) and (b) in virtual unenhanced (VUE) (liver noise = 3.8 HU; Spleen noise = 6.8 HU). Note the small kidney stone lost on the right kidney in the VUE image. For both images, the same slice location and window were selected.

Figure 3

From left to right: true unenhanced (TUE) images, material suppressed iodine - virtual unenhanced (MSI-VUE) images and material density image (MDI) - water (-iodine) - VUE. TUE and VUE images - MSI and MDI - water (-iodine). (a) Circled in white, VUE images did not characterize the hepatic steatosis causing the focal fat sparing to be less apparent on both MSI and MDI water (-iodine) (b) Hypoattenuating liver lesions (arrow) readily observed on TUE, but not found within the VUE images. (c) Small hemorrhagic cyst (arrow) was not seen on the VUE images. The circled object is iodine contrast inside a bowel loop.