Value of virtual monochromatic spectral image of dual-layer spectral detector CT with noise reduction algorithm for image quality improvement in obese simulated body phantom

Abstract

Background: Dual-layer spectral detector CT (SDCT) may provide several theoretical advantages over pre-existing DECT approaches in terms of adjustment-free sampling number and dose modulation, beam hardening correction, and production spectral images by post-processing. In addition, by adopting noise reduction algorithm, high contrast resolution was expected even in low keV level. We surmised that this improvement would be beneficial to obese people. Therefore, our aim of study is to compare image quality of virtual monochromatic spectral images (VMI) and polychromatic images reconstructed from SDCT with different body size and radiation dose using anthropomorphic liver phantom.

Methods: One small and one large size of body phantoms, each containing eight (four high- and four low-contrast) simulated focal liver lesions (FLLs) were scanned by SDCT (at 120 kVp) using different Dose Right Indexes (DRIs). VMI were reconstructed from spectral base images from 40 keV to 200 keV. Hybrid iterative reconstruction (iDose⁴) was used for polychromatic image reconstruction. Image noise and contrast to noise ratio (CNR) were compared. Five radiologists independently rated lesion conspicuity, diagnostic acceptability and subjective noise level in every image sets, and determined optimal keV level in VMI.

Results: Compare with conventional polychromatic images, VMI showed superior CNR at low keV level regardless of phantom size at every examined DRIs (Ps < 0.05). As body size increased, VMI had more gradual CNR decrease and noise increase than conventional polychromatic images. For low contrast FLLs in large phantom, lesion conspicuities at low radiation dose levels (DRI 16 and 19) were significantly increased in VMI (Ps < 0.05). Subjective image noise and diagnostic acceptabilities were significantly improved at VMI in both phantom size.

Conclusions: VMI of dual-layer spectral detector CT with noise reduction algorithm provides improved CNR, noise reduction, and better subjective image quality in imaging of obese simulated liver phantom compared with polychromatic images. This may hold promise for improving detection of liver lesions and improved imaging of obese patients.

Keywords: Computed tomography; Dual-energy; Liver; Obesity; Phantom; Spectral detector.

Fig. 1

The appearance of the customized phantom (a) and pork belly-wrapped phantom to mimic a large body size (b). CT images of each phantom size are displayed for (c) small and (d) large body sizes, respectively

Fig. 2

The eight simulated focal liver lesions (FLLs) in body phantom. a Four hypo-attenuating FLLs are noted. Two were high-contrast hypo-attenuating FLLs (white arrows) and other two are low-contrast hypo-attenuating FLLs (empty arrows). b Other four hyper-attenuating FLLs are presented. Likewise, two were high-contrast hyper-attenuating FLLs (white arrows) and other two were low-contrast hyper-attenuating FLLs (empty arrows). The conventional polychromatic images are reconstructed using hybrid iterative reconstruction algorithm (iDose⁴) in a level of 4 (a, b). VMI are reconstructed using spectral level 4 and presented in 60 keV (c, d). All images were applied DRI 19. FLL = focal liver lesion

Fig. 3

An axial virtual monochromatic image that show ROIs manually drawn on anterior abdominal wall, bilateral paraspinal muscle, liver parenchyma and a hypo-attenuating FLL. Other ROIs drawn in other axial image (not shown) to measure the attenuation of hyper-attenuating FLLs

Fig. 4

The graphs show the mean noise in the (a) polychromatic images and VMI in small phantom. When the same radiation dose level (same DRI), noise of VMI were lower than polychromatic images (all P < 0.05) except low keV range. The graph (b) and (c) presented the CNR of (b) hyper- or (c) hypo-attenuating FLL in polychromatic images and VMI of the small phantom. The CNR value gradually decreases as keV increases and has a higher value in low keV ranges than that of the polychromatic images with equal DRI values. DRI dose right index, VMI virtual monochromatic spectral image, Poly polychromatic image

Fig. 5

Image noise of polychromatic image and VMI in small and large phantoms. The maximum noise gap increases as phantom size increases in all examined DRIs ([a] DRI 16, [b] DRI 19, [c] DRI 22, [d] DRI 25). DRI = dose right index, VMI virtual monochromatic spectral image, DRI dose right index

Fig. 6

CNR values of polychromatic image and VMI in small and large phantoms. VMI had more gradual CNR decrease than conventional polychromatic images in all examined DRIs ([a] DRI 16, [b] DRI 19, [c] DRI 22, [d] DRI 25). DRI dose right index, VMI virtual monochromatic spectral image, DRI dose right index

Fig. 7

Low-contrast hyper-attenuating FLL (arrow) in the left lateral segment of the liver with different body phantom sizes. On polychromatic iDose⁴ image, as the phantom size is bigger ([a] small and [b] large), the low-contrast FLL is less visible. When adjusting the image for a low keV level (56 keV) VMI on large size phantom (c), the FLL visibility is markedly improved compared polychromatic iDose⁴ images. FLL focal liver lesion, DRI dose right index, VMI virtual monochromatic spectral image