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 Table of Contents  
REVIEW ARTICLE
Year : 2014  |  Volume : 26  |  Issue : 3  |  Page : 293-297

Image deteriorating factors in cone beam computed tomography, their classification, and measures to reduce them: A pictorial essay


Department of Oral Medicine and Radiology, Mahatma Gandhi Vidyamandir's KBH Dental College and Hospital, Nashik, Maharashtra, India

Date of Submission22-Jul-2014
Date of Acceptance29-Oct-2014
Date of Web Publication19-Nov-2014

Correspondence Address:
Ajay Ramesh Bhoosreddy
Professor and Head, Department of Oral Medicine and Radiology, Mahatma Gandhi Vidyamandir's KBH Dental College and Hospital, Nashik, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-1363.145009

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   Abstract 

Cone beam computed tomography (CBCT) is now an established technology in dentistry, used for three-dimensional imaging of the teeth and jaws. But the current CBCT technology has various limitations causing deterioration of images. Every dentist must be familiar with these limitations while interpreting CBCT images. This article aims to present a pictorial essay describing the various CBCT faults and artifacts, which can help to better understand the factors causing image deterioration in CBCT. To further simplify, we are also presenting a classification of these artifacts and listing the measures to reduce them.

Keywords: Cone beam computed tomography artifacts, cone beam imaging, image deteriorating factors, image deterioration


How to cite this article:
Bhoosreddy AR, Sakhavalkar PU. Image deteriorating factors in cone beam computed tomography, their classification, and measures to reduce them: A pictorial essay . J Indian Acad Oral Med Radiol 2014;26:293-7

How to cite this URL:
Bhoosreddy AR, Sakhavalkar PU. Image deteriorating factors in cone beam computed tomography, their classification, and measures to reduce them: A pictorial essay . J Indian Acad Oral Med Radiol [serial online] 2014 [cited 2020 Jan 19];26:293-7. Available from: http://www.jiaomr.in/text.asp?2014/26/3/293/145009


   Introduction Top


Cone beam computed tomography (CBCT) is now an established technology in dentistry, used for three-dimensional imaging of the teeth and jaws. With innovation in computers and developments in scanning technology, CBCT has become one of the important diagnostic modalities to the practicing dentists and researchers in the rapidly changing field of digital dentistry. It is becoming widely available and has applications in all the fields of dentistry. It has also become a standard of care in the field of implant dentistry.

The usefulness of CBCT imaging can no longer be questioned. CBCT is a useful task-specific imaging modality and an important technology in comprehensive evaluation. Patients have benefited since the advent of CBCT by receiving better diagnostics, enhanced treatment planning, and ultimately, safer and more foreseeable surgeries. This also facilitates better patient education, understanding, and treatment acceptance.

CBCT may have created a paradigm shift in the science of imaging with several expanding clinical applications, but current CBCT technology has some limitations related to cone beam projection geometry, detector sensitivity, and contrast resolution. Faulty factors like lower resolution, reduced usability, [1] and subsequent image degradation can sometimes lead to inaccurate diagnosis or misdiagnosis.

This article aims to present a pictorial essay describing the various CBCT faults and artifacts, which can help to better understand the factors causing image deterioration in CBCT. To further simplify, we are also presenting a classification of these artifacts and listing the measures to reduce them.


   Faults and Artifacts Top


It is important to understand the basic concept of a fault and an artifact. Fault is an imperfection, a mistake or error, where flaws will hinder interpretation of the radiograph, whereas an artifact is any distortion or error in the image that is unrelated to the (tissues/organs of the) subject being studied. Artifacts can be classified according to their cause. [1] It is important for a radiologist to be able to recognize and diagnose the faults and artifacts in the image and to understand the cause, thus preventing their occurrence in subsequent images.


   Classification Top


[Figure 1] shows the classification of various image deteriorating factors in CBCT. They are classified as follows:
Figure 1: Classifi cation of image deteriorating factors in cone beam CT

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1. Artifacts:

1a. Beam-related artifacts

1a (i). Beam hardening artifact

1a (ii). Cone-shaped beam-related faults

1a (iii). Scatter

1a (iv). Exponential edge gradient effect (EEGE)

1a (v). Photon deprivation

1a (vi). Full mouth restoration (metallic) artifact

1b. Patient-related artifacts

1b (i). Unsharpness

1b (ii). Double image

1c. Scanner-related artifacts

1d. Foreign objects

2. Image noise

3. Poor soft tissue contrast


   1. Artifacts Top


1a. Beam-related artifacts

The beam-related artifacts include:

1a (i). Beam hardening artifact

The most prominent artifacts seen in CBCT images are beam hardening artifacts. [2],[3],[4] These are made by heavy metal restorations because of their high density. Beam hardening artifact is seen because the mean energy of beam increases as the lower energy photons are absorbed more in comparison to higher energy photons. [1] This shows effects in the distortion of metallic structures as a result of disturbance in the reconstruction process. This phenomenon produces two types of artifacts:

  1. Cupping artifacts: They are seen as a distortion of metallic structures as a result of differential absorption [Figure 2]a. [5]
  2. Streaks and dark bands: They can be seen between two dense objects [5] [Figure 2]a. They can significantly deteriorate the image quality and are more prominently seen in the axial planes and 3D reconstruction images [Figure 2]b.
Figure 2: (a) Image showing: 1. cupping effect, 2. streaks, and 3. and 4. dark bands seen as a result of beam hardening. (b) Beam hardening effect seen on 3D image

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1a (ii). Cone-shaped beam-related faults

The cone beam projection geometry and the image reconstruction method produce three types of artifacts:

  1. Partial volume averaging: It occurs when the selected voxel resolution of the scan is greater than the spatial or contrast resolution of the object to be imaged. Partial volume averaging artifacts occur in regions where surfaces are rapidly changing in the z direction (e.g. in the temporal bone).
  2. Under sampling: This is a type of aliasing artifact. It is seen when very few basis projections are provided for the reconstruction [Figure 3].
  3. Cone beam effect: This type of artifact is seen in the peripheral portions of the scan and is seen because of the divergence of X-rays in those areas. The outcome of cone beam effect is image distortion, streaks, and peripheral noise.
Figure 3: Under sampling due to insufficient basic projections

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1a (iii). Scatter

Scattering is caused by those photons that are diffracted from their original path after interaction with matter. [3] Most of the scattered radiation is produced omnidirectionally and is recorded by pixels on the cone beam area detector, which does not reflect the actual attenuation of the object within a specific path of the X-ray beam. [1] The resultant outcome increases the image noise and reduces contrast. Reconstruction error is proportional to the amount of scatter.

1a (iv). Exponential edge gradient effect

This effect is caused because of the sharp edges of the metallic crown borders producing high contrast, as it reduces the computed density value. [6] As sharp edges of high contrast may commonly occur in the oral cavity, e.g. at metallic crown borders, this artifact also has to be considered in dental CBCT. [3] The EEGE is known to cause streaks tangent to long straight edges in the projection direction. [4]

1a (v). Photon deprivation

This is a result of severe beam hardening, generally seen next to titanium implants or other heavy metal restorations. Due to the high density of metallic restorations, sufficient photons do not reach the detector and a complete void exists in the image, which is known as photon starvation. [7] In particular, photon deprivation effects can present as apparent "pseudo" fracture on axial images [Figure 4].
Figure 4: Image showing: 1. photon deprivation, 2. streaks, and 3. pseudofracture

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1a (vi). Full mouth restoration (metallic) artifact

This artifact is seen in patients with full mouth metallic restorations or long bridges. This is the combination of streaks, dark bands, and photon deprivation to the extent that the image loses all of its diagnostic quality [Figure 5] and [Figure 6]. Patients with long metallic bridges, cast partial dentures, and full mouth metallic restorations may not be indicated for fine details in CBCT.
Figure 5: Image degradation due to full mouth metallic restorations. Image also shows: 1. pseudofracture and 2. ring artifact

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Figure 6: Metallic artifact in cross-sectional view

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1b. Patient-related artifacts

Patient motion can cause faulty registration of data, which appears as unsharpness or double image in the reconstructed image. If an object moves during the scanning process, the reconstruction process does not account for that move [Figure 7]. Positive correlation is present with the amount of artifacts, reduced quality images, and the presence of restorations.
Figure 7: Faulty registration of data due to patient movement

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1c. Scanner-related artifacts

These artifacts typically present as circular or concentric rings centered on the location of the axis of rotation. [1] This is due to the malfunction in the detector following faulty calibration or imperfections in scanner detection, which causes a consistent and repetitive reading at each angular position of the detector [Figure 5], [Figure 8], and [Figure 9].
Figure 8: Ring artifact

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Figure 9: Image deterioration due to heavy metal jewellery. Image shows the following: 1. metal earrings, 2. streaks, 3. bands, 4. mouth block, and 5. ring artifact

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1d. Foreign objects

The bite block may cast a shadow as per its shape and size, which may be confused as a foreign object. However, metal foreign objects such as nose rings, earrings, clips, etc. can cast a shadow as a result of beam hardening [Figure 9].


   2. Image Noise Top


This is an important image deteriorating factor. It is the result of inconsistent attenuation values in the projection images [Figure 10]. [3] Most of the scattered radiation is produced in all directions and is recorded as pixels by the detector. This is different from the actual attenuation of the object within a specific path of the X-ray beam. [1] Because of the use of an area detector, much of this nonlinear attenuation is recorded and contributes to image degradation seen as a noise [Figure 10].
Figure 10: Noise

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   3. Poor Soft Tissue Contrast Top


This drawback is the result of all other faults, mainly scatter and noise.


   Methods to Reduce these Drawbacks Top


With the advancement in computer technology, image reconstruction, and scanning, these artifacts occur less frequently in comparison with earlier machines. Measures to minimize drawbacks and to enhance the image quality are summarized in [Table 1]. In a study by Bechara et al., [8] the metal artifact reducing (MAR) algorithm reduced the effects of the beam hardening and scattering caused by a metallic structure.
Table 1: Measures to minimize drawbacks and to enhance the image quality

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   Conclusion Top


In conclusion, CBCT imaging is becoming an essential imaging modality in day-to-day clinical practice. For the effective use of this technology, it is necessary to not only know the advantages but also the limitations. Though newer technology with high definition and artifact reducing software has reduced these limitations to negligible levels, every practitioner must be competent to know what is normal and what is not. However, as this advancement in technology continues, we can hope for more accurate and better images leading to a better diagnosis.

 
   References Top

1.Scarfe WC, Farman AG. What is cone-beam CT and how does it work? Dent Clin North Am 2008;52:707-30.  Back to cited text no. 1
    
2.Schulze RK, Berndt D, d'Hoedt B. On cone-beam computed tomography artefacts induced by titanium implants. Clin Oral Implants Res 2010;21:100-7.  Back to cited text no. 2
    
3.Schulze R, Heil U, Gross D, Bruellmann DD, Dranischnikow E, Schwanecke U, et al. Artefacts in CBCT: A review. Dentomaxillofac Radiol 2011;40:265-73.  Back to cited text no. 3
    
4.De Man B, Nuyts J, Dupont P, Marchal G, Suetens P. Metal streak artifacts in x-ray computed tomography: A simulation study. IEEE Trans Nucl Sci 1999;46:691-6.  Back to cited text no. 4
    
5.Scarfe WC, Farman AG. Cone-beam computed tomography. In: White SC, Pharoah MJ, editors. Oral Radiology: Principles and Interpretations. 6 th ed. St. Louis, Missouri: Mosby Elsevier; 2009. p. 225-43.  Back to cited text no. 5
    
6.Joseph PM, Spital RD. The exponential edge-gradient effect in x-ray computed tomography. Phys Med Biol 1981;26:473-87.  Back to cited text no. 6
    
7.Scarfe WC, Farman AG. Interpreting CBCT images for implant assessment: Part 1--Pitfalls in image interpretation. Australas Dent Pract 2010;20:106-14.  Back to cited text no. 7
    
8.Bechara BB, Moore WS, McMahan CA, Noujeim M. Metal artefact reduction with cone beam CT: An in vitro study. Dentomaxillofac Radiol 2012;41:248-53.  Back to cited text no. 8
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
 
 
    Tables

  [Table 1]



 

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