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.
Highlights
Since it was first approved by the U.S Food and Drug Administration (FDA) twenty years ago, cone beam computed tomography (CBCT) has found widespread applications in dentistry, including dental implant planning and pre-operative and post-operative assessment of surgical procedures [1,2,3,4]
The results demonstrated that the virtual monoenergetic images generated using the proposed dual energy CBCT (DE-CBCT) were effective in reducing the metal artifacts caused by the presence of metal objects in the anthropomorphic head phantom, consistent with prior reports using other dual-energy CT/CBCT techniques
The results from this study show that, for the intended dental imaging applications, the required x-ray flux can be delivered by a fixed anode carbon nanotube (CNT) x-ray source and the DE-CBCT scanning time is comparable to that of the current single-energy CBCT at the same imaging dose
Summary
Since it was first approved by the U.S Food and Drug Administration (FDA) twenty years ago, cone beam computed tomography (CBCT) has found widespread applications in dentistry, including dental implant planning and pre-operative and post-operative assessment of surgical procedures [1,2,3,4]. The presence of strong metal artifacts has been a major limitation in using CBCT for dental imaging [11]. The strong x-ray attenuation from common metal features, such as dental restorations and implants, results in beam hardening and photon starvation. This leads to artifacts in the form of streaks and halos in the reconstructed 3D images [1, 11]. These artifacts degrade the image quality, compromise the diagnostic accuracy, and make dental CBCT ineffective for postoperative evaluation of the osseointegration of implants [12]. The results of these algorithms have demonstrated limited utility [13]
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