Metallic materials in the exomass impair cone beam CT voxel values

Introduction

Cone beam CT (CBCT) is an imaging technique that allows for three-dimensional visualization of structures of interest in dentomaxillofacial diagnosis. Compared to multidetector CT (MDCT), CBCT offers some advantages such as higher spatial resolution, lower radiation dose and cost.¹ The scientific literature has demonstrated the great importance of CBCT for dental implant planning, with consistent and accurate evaluation of alveolar bone height, thickness and volume,^2–4 as well as for endodontic,⁵ periodontal,⁶ orthodontic⁷ and oral and maxillofacial surgical treatments.^{8, 9}

The CBCT image is a matrix composed of small volumetric units known as voxel. Each voxel represents the linear attenuation coefficient of inside structure by exhibiting a specific level of grey with a related numerical value (voxel value). More attenuating materials have more hyperdense image with higher voxel values. However, the CBCT voxel values are not accurate due to inherent characteristics of this image modality such as geometric, energetic and computational factors.^10–12 The variability of the voxel values in CBCT contributes to generate artefacts in the reconstructed images,^13–15 which may reduce diagnostic accuracy.^16–18

It is well-known that an important source of image artefact is the presence of metal objects in the field of view (FOV).^{16, 17} When the FOV encompasses just a portion of an object which is entirely located in the path of the X-rays (between the focal area and the image detector), this is known as local tomography.^{14, 19} Moreover, when this phenomenon occurs, any structure that lies outside of the FOV, this constitutes the so-called exomass (Figure 1). In this case, described in the scientific literature as a truncation effect, CBCT image reconstruction attempts to disregard the X-ray attenuation from the exomass to avoid negative interference in the reconstructed image.²⁰

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Currently, the use of small FOV sizes has increased because of the significant concern of minimizing radiation doses, the need for higher spatial resolution in many diagnostic purposes and the greater accessibility.^{21, 22} However, smaller FOV sizes result in an indirect increase of the exomass, which has shown to negatively affect the voxel values.²³ Considering the wide use of metallic materials in dentistry associated with the increased indication of small FOV sizes, the objective of this study was to assess the influence of artefacts arising from representative metallic materials in the exomass on CBCT voxel values.

Methods and materials

Custom-made exomass phantom

A thin layer of wax was fixed on the inside bottom of a polypropylene cylindrical container (diameter, 167 mm; height, 65 mm). An aqueous solution of dipotassium phosphate (K₂HPO₄) was prepared at a concentration of 1000 mg ml⁻¹ to fill 16 polypropylene tubes of 0.2 ml (Figure 2a). K₂HPO₄ is a salt with high water solubility and density very close to that of calcium hydroxyapatite (2.44 and 3.16 g cm⁻³, respectively). This concentration was selected to simulate alveolar bone density.²⁴ The 16 tubes were placed vertically in two concentric circles using a transparent guide that was placed on the wax. The inner and outer circles contained, respectively, 4 and 12 tubes (Figure 2b). To induce the formation of artefacts arising from metallic materials in the exomass, cylindrical metallic materials (diameter, 5 mm; height, 15 mm) of three compositions—titanium implant (KOPP, Curitiba, Brazil), cobalt–chromium alloy (Talmax, Curitiba, Brazil) and silver amalgam (SSWhite, Rio de Janeiro, Brazil; Figure 2a)—could be vertically positioned at the vertices of an imaginary isosceles triangle and at 15 mm from the outer circle of the tubes (Figure 2b). The physical density of the cylinders was calculated based on the Archimedes’ principle using an analytical balance discovery (OHAUS Corporation, Parsippany, NJ).²⁵ Finally, the container was filled with water to simulate the absorption and scattering of the X-rays in soft tissues.

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CBCT scans

The phantom was placed in the centre of a 5 × 5 cm FOV (diameter × height) such that it could cover all the 16 tubes. Tomographic scans were obtained using three CBCT units: CS9300 (Carestream, Rochester, NY) at 90 kVp, 3.2 mA, exposure time of 20 s, voxel size of 0.09 mm; Picasso Trio (Vatech, Seoul, Republic of Korea) at 90 kVp, 3.0 mA, exposure time of 24 s and voxel size of 0.20 mm; and NewTom Giano (QR, Verona, Italy) at 90 kVp, 3.0 mA, exposure time of 9 s and voxel size of 0.10 mm. All scans resulted in 16 bits images. 10 repeated CBCT scans were obtained of the phantom under the following protocols: control (no metallic material in the exomass); titanium implant (one, two or three titanium implants in the exomass); cobalt–chromium alloy (one, two or three cylinders of cobalt–chromium alloy in the exomass); amalgam (one, two or three cylinders of amalgam in the exomass). When only one metallic material was present in the exomass, it was located in the right region of the FOV. When two materials were present, they were located on both sides of the FOV. When three materials were present, the third one was located in the anterior region of the FOV (Figure 2b). To avoid additional adverse voxel value variability, the three CBCT units were warmed-up (two initial scans) before starting the experiments, and a time interval of 10 min was respected between each acquisition.²⁶Figures 3–6 show representative images obtained from each protocol.

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Results

Intraobserver agreement was 0.99 for both mean voxel values and voxel value variability.

Discussion

The FOV size in CBCT has demonstrated to be one of the most important factors influencing the radiation dose delivered to the patient. Under similar exposure settings, smaller FOV sizes lead to lower doses of X-rays and better theoretical image quality by collimating the primary beam and reducing scattering.^{21, 22} In 2012, the European project SEDENTEXCT developed evidence-based guidelines on use of CBCT in dentistry and recommended the use of the smallest FOV available on the equipment and consistent with the individual diagnostic task.⁹ However, the use of local tomography by reducing the FOV size in the axial plane indirectly increases the exomass impact and the occurrence of truncation effect^{19–21,27,28} and inconsistencies in the reconstructed volume.¹⁴ To the best of the authors’ knowledge, it seems that the effect of artefacts arising from metal objects in the exomass on CBCT voxel values has not been studied yet. Hence, considering that such inconsistencies may cause general voxel value variability rather than a definite pattern,¹⁴ the current methodology included different CBCT units capable of producing limited scan volumes to better understand this occurrence.

In the present study, metallic materials in the exomass produced noticeable hypodense and hyperdense streaks (Figures 4–6), which are image artefacts resulting from beam hardening and scattering, as well as data extinction.^22,27–29 For instance, mean voxel value reduced (image darkening) in the presence of any metallic material in the exomass in the CS9300 and Picasso Trio, and in the presence of two cylinders of cobalt–chromium or amalgam in the NewTom Giano. This indicates that hypodense artefacts were more predominant in the image and, in a clinical situation, could possibly affect the diagnosis of hypodense conditions, such as fracture lines.

Conversely, in the NewTom Giano, the presence of one or three cylinders of cobalt–chromium or amalgam induced greater formation of hyperdense artefacts. This difference in results observed between CBCT units is probably a consequence of some exposure parameters, such as milliampere-second (mAs), which, although adjusted to be as similar as possible, was not exactly the same. The CS9300, Picasso Trio and NewTom Giano operated at 64, 72 and 27 mAs, respectively, and at 90 kVp. Additionally, since Newtom Giano is equipped with an automatic exposure control system (SafeBeam™ technology), the milliamperage could have been regulated to keep the detector signal constant. CBCT units operating at higher mAs produce more photons of X-rays, which, at the same energy level, may increase the impact of beam hardening-related hypodense artefacts over the whole image. Despite this hypothesis, Oliveira et al did not find a significant influence of milliamperage on voxel value variability.²⁴

In the NewTom Giano, the presence of two cylinders of any material induced a reduction of mean voxel value, which could be explained by the arrangement of the cylinders in the exomass. As detailed in the methodology section, even this being a purely in vitro study, the cylinders were placed at specific distances at the vertices of an imaginary isosceles triangle to simulate one anterior and two posterior areas of a human mandible. In all CBCT scans, when only two cylinders were present, they were aligned in both sides of the “posterior” region, which induced photon starvation and the formation of an impacting artefact of data extinction. This substantial reduction of mean voxel value showed to be enough to overcome the high voxel values of the hyperdense artefacts. Furthermore, the CS9300, Picasso Trio and NewTom Giano made use of different number of projections (frames), which were 480, 720 and 360, respectively. A reduced data sampling may lead to misregistration, sharper edges and noisier images, because of aliasing, where fine striations appear.^{30, 31} However, when comparing results of different CBCT units, besides discussing quantitative parameters, possible confidential image reconstruction features should be considered.

It is well-known in the scientific literature that CBCT voxel values are inhomogeneous and do not present a standardized scale.¹¹ Thus, the imaging phantom used in the present study was composed of tubes with a radiopaque solution under the same concentration such that the standard deviation of their voxel values could indicate voxel value variability.

The presence of metallic materials in the exomass increased voxel value variability in the three CBCT units is in agreement with previous studies that evaluated the influence on image quality of metallic materials within the FOV.^{22, 27,29} Oliveira et al showed a reduction of voxel value variability in the presence of a thin and uniform layer of water as the exomass, and they suggest that such a homogeneous exomass should have acted as a filter of the X-ray beam.²⁴ In the study of Nardi et al metallic materials (e.g. orthodontic appliances, osteosynthesis devices, dental implant, prosthetic pin, prosthetic metal structure, endodontically treated tooth and/or prosthetic crown) did not interfere with the image quality.³² However, these results were subjectively obtained by means of visual assessment.

The presence of only one cylinder of any material in the exomass presented reduced voxel value variability when compared with the presence of two or three cylinders, which may possibly indicate that reduced image deterioration can be expected of patients with fewer metallic objects. The physical properties of an object directly impact the presence/magnitude of artefacts in CBCT. In the present study, amalgam and titanium promoted the highest and lowest voxel value variability, respectively. Considering the similarities in shape and size of the cylinders, this difference should be related to additional physical properties. The atomic number of the titanium is 22 and the density of the cylinder was 4.301 kg m^–3. The cylinder of amalgam had a density of 10.153 kg m^–3, and the atomic number of the components is mercury-80, silver-47, tin-50 and copper-29. High atomic number and physical density are expected to increase artefact formation when interacting with the X-ray beam.^{33, 34}

Some scientific studies have shown that reduced CBCT voxel value variability can be expected from larger FOV sizes.^{22, 27,32,35} However, under similar energetic parameters, it is not justified to indicate larger FOV sizes just to theoretically improve image quality, considering that the patient will be receiving higher dose of X-ray. Instead, priority should be given to understand and, possibly, reduce the effects caused by the exomass. An important limitation of most studies assessing different CBCT units is the inability of standardizing the exposure factors to have all machines operating under the same conditions.

Conclusion

Artefacts arising from metallic materials in the exomass have a significant negative influence on CBCT voxel values. Further studies and technological approaches should consider the impact of exomass on image quality.