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. 2022 Feb 3;17(2):e0262713.
doi: 10.1371/journal.pone.0262713. eCollection 2022.

Feasibility of dual-energy CBCT by spectral filtration of a dual-focus CNT x-ray source

Boyuan Li  1 , Derrek Spronk  2 , Yueting Luo  2 , Connor Puett  3 , Christina R Inscoe  1 , Donald A Tyndall  4 , Yueh Z Lee  5 , Jianping Lu  1 , Otto Zhou  1
Affiliations

Feasibility of dual-energy CBCT by spectral filtration of a dual-focus CNT x-ray source

Boyuan Li et al. PLoS One. .

Abstract

Cone beam computed tomography (CBCT) is now widely used in dentistry and growing areas of medical imaging. The presence of strong metal artifacts is however a major concern of using CBCT especially in dentistry due to the presence of highly attenuating dental restorations, fixed appliances, and implants. Virtual monoenergetic images (VMIs) synthesized from dual energy CT (DECT) datasets are known to reduce metal artifacts. Although several techniques exist for DECT imaging, they in general come with significantly increased equipment cost and not available in dental clinics. The objectives of this study were to investigate the feasibility of developing a low-cost dual energy CBCT (DE-CBCT) by retrofitting a regular CBCT scanner with a carbon nanotube (CNT) x-ray source with dual focal spots and corresponding low-energy (LE) and high-energy (HE) spectral filters. A testbed with a CNT field emission x-ray source (NuRay Technology, Chang Zhou, China), a flat panel detector (Teledyne, Waterloo, Canada), and a rotating object stage was used for this feasibility study. Two distinct polychromatic x-ray spectra with the mean photon energies of 66.7keV and 86.3keV were produced at a fixed 120kVp x-ray tube voltage by using Al+Au and Al+Sn foils as the respective LE and HE filters attached to the exist window of the x-ray source. The HE filter attenuated the x-ray photons more than the LE filter. The calculated post-object air kerma rate of the HE beam was 31.7% of the LE beam. An anthropomorphic head phantom (RANDO, Nuclear Associates, Hicksville, NY) with metal beads was imaged using the testbed and the images were reconstructed using an iterative volumetric CT reconstruction algorithm. The VMIs were synthesized using an image-domain basis materials decomposition method with energy ranging from 30 to 150keV. The results were compared to the reconstructed images from a single energy clinical dental CBCT scanner (CS9300, Carestream Dental, Atlanta, GA). A significant reduction of the metal artifacts was observed in the VMI images synthesized at high energies compared to those from the same object imaged by the clinical dental CBCT scanner. The ability of the CNT x-ray source to generate the output needed to compensate the reduction of photon flux due to attenuation from the spectral filters and to maintain the CT imaging time was evaluated. The results demonstrated the feasibility of DE-CBCT imaging using the proposed approach. Metal artifact reduction was achieved in VMIs synthesized. The x-ray output needed for the proposed DE-CBCT can be generated by a fixed-anode CNT x-ray source.

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Conflict of interest statement

Otto Zhou has equity ownership and serves on the board of directors of Xintek, Inc., to which the technologies used or evaluated in this project have been licensed. This does not alter our adherence to PLOS ONE policies on sharing data and materials. Jianping Lu has equity ownership in Xintek, Inc. All activities have been approved by institutional COI committees.

Figures

Fig 1
Fig 1. Drawing of proposed system and clinical scanner used for comparison.
A drawing illustrating the proposed DE-CBCT using a CNT x-ray source with two focal spots and a common flat-panel detector (left). In the feasibility study the source and detector were stationary, and the object was rotated. For comparison the same head phantom was imaged using a clinical CBCT scanner (Carestream CS9300) (Right). A Rando adult skull and tissue-equivalent head phantom with a 1/4” and a 1/8” diameter stainless steel metal bead was imaged.
Fig 2
Fig 2. The simulated x-ray energy spectra.
The simulated x-ray energy spectra from an x-ray source with W anode, 120kVp tube voltage before (left) and after (right) the object. Yellow: with the intrinsic filter (0.4mm Fe); Blue: with the low-energy filter (7mmAl + 0.05mm Au); and Red: with the high-energy filter (11mm Al + 0.63mm Sn). A multiplication factor was applied so that the low-energy and high-energy spectra combined produces approximately the same air kerma (per mAs) as the original spectrum.
Fig 3
Fig 3. Reconstructed CBCT images of the Rando phantom.
a) the clinical CBCT scanner without the external metal bead; b) the clinical scanner after attaching the external metal bead; c) the testbed using the LE beam; and d) the testbed using the HE beam. Because of the slight differences in the phantom positioning, the images from the clinical scanner and the testbed are not at same plane.
Fig 4
Fig 4. Zoomed in VMIs focusing on the region with the metal artifact.
From left to right: 40, 60, 80 KeV (1st row); and 100, 120 and 140keV (2nd row). The images were set at the same window level (750HU:1500HU).
Fig 5
Fig 5. ROI and quantitative analysis.
a) A CT image of the phantom showing the region-of-interest (ROI) used for the calculation. b), c), and d): The metal artifact index (MAI), Hounsfield Unit (HU) values as a function of the monoenergetic energy. The square and circle correspond to the values from the input low and high energy images respectively. In the MAI plot, the blue line is from the metal artifact region 1, and the red is from the metal artifact region 2 indicated in a).
Fig 6
Fig 6. Region with the metal artifacts from the small metal bead.
From left to right: Single-energy clinical CBCT, LE, HE, and VMI at 120KeV.
See this image and copyright information in PMC

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Grants and funding

The work was supported by the U.S. Department of Defense (DOD) under Grant DM180025 and the David Godschalk Distinguished Professor fund at the University of North Carolina at Chapel Hill. There was no additional external fund received for this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Otto Zhou has equity ownership and serves on the board of directors of Xintek, Inc., to which the technologies used or evaluated in this project have been licensed. This does not alter our adherence to PLOS ONE policies on sharing data and materials. Jianping Lu has equity ownership in Xintek, Inc. All activities have been approved by institutional COI committees. We thank Dr. Enrique Platin for helpful discussions.