Cone-beam Computed Tomography

September 14, 2024
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TOPIC : CONE-BEAM COMPUTED TOMOGRAPHY

🌟 IMPORTANT POINTS TO REMEMBER

🔹History: CBCT was first developed and described for use in angiography. In 1998, Mozzo et al. reported the first CBCT unit developed specifically for dental and maxillofacial use.

🔹Principle Of CBCT: CBCT is based on fixed X-ray source detector with a rotating gantry. A divergent pyramidal or cone-shaped source of ionizing radiation is directed through the middle of the ROI onto an area X-ray detector on the opposite side.

🔹Difference Between CBCT and CT: First, CBCT uses a low-energy fixed anode tube, similar to that used in dental panoramic X-ray machines. Second, CBCT machines rotate around the patient only once, capturing the data using a cone-shaped X-ray beam.

🔹Detector Used: Detectors used in CBCT are flat-panel detector and IIT/CCD.

🔹IIT/CCD: It utilizes a phosphor screen to convert X-rays into visible light. The light is then projected onto a photon cathode that converts the light into electrons.

🔹FPDs: They are used in more recent -generation CBCT. In FPD, detection of X-rays occurs in sensor elements which are produced in a thin film of hydrogenated amorphous silicon. The sensor elements in the scintillator layer are composed of terbium-activated gadolinium oxysulphide or thallium-doped caesium iodide.

🔹Image Formation: Patient is exposed to continuous cone-shaped beam. Alternately, the X-ray beam may be pulsed to coincide with the detector sampling, which means that actual exposure time is markedly less than scanning time. Image data are recorded in a single rotation varying between 200 and 360°.

🔹Reconstruction Stage: It consists of relating the collected images to each other to create a volumetric data set.

🔹View Taken: CBCT data display four primary views: (i) the axial view, (ii) the panoramic view, (iii) the 3D reconstructed view and (iv) the cross-sectional views.

🔹FBP: The most commonly used algorithm for tomographic reconstruction is FBP.

🔹Raysum Or Ray Casting (Raysum): The detector is composed of single cells (measurement units), each of which records the incoming intensity of rays passing the object.

🔹Artefacts In CBCT: They are beam hardening, noise, scatter, patient-related artefacts, partial volume averaging, undersampling, cone-beam effect and ring artefacts.

🔹Application Of CBCT: Implant imaging, orthodontic and orthognathic surgeries, endodontics and periodontics.

📌 APPLICATION OF CBCT :

🔸Implant Imaging :

  • Location-related implant success rate:
  • Functional implant zone 1: Traumatic zone.
  • Functional implant zone 2: Sinus zone.
  • Functional implant zone 3: Interforaminal zone (the mandibular alveolar ridge between mental foramen and first premolar on each side).
  • Functional implant zone 4: Alveolar process of the mandible posterior to mental foramen (from the second premolar to retromolar pad).

🔸Orthodontic and Orthognathic Surgeries : 

  • Simulation of virtual positioning of the jaw, osteotomy, distraction osteogenesis 
  • 3D reconstruction of jawbones

🔸Endodontics : 

  • Precise imaging modality to measure the length and width of the root canal, prevent iatrogenic exposure of the apex and improve prognosis of root canal therapy.
  • Diagnose vertically fractured roots and horizontal root fracture, and measure the depth of dentin fracture, using high-resolution images

🔸Periodontics :

  • CBCT can accurately evaluate intrabony defects and assess dehiscence, fenestration defects and periodontal cysts.

📣 Advantages and Disadvantages of CBCT :

 🔺Advantages :

  • Rapid scan time
  • Beam limitation
  • Image accuracy
  • Reduced patient radiation dose
  • Interactive display modes applicable to maxillofacial imaging
  • Multiplanar reformation

🔻Disadvantages :

  • Extinction artefacts
  • Beam hardening artefacts
  • Partial volume effect and edge
  • Aliasing artefacts
  • Ring artefacts
  • Motion artefacts (misalignment artefacts)

📌 MULTIPLE CHOICE QUESTIONS (MCQs)

💡 According to Tolstunov functional implant zones, ‘sinus zone’ is

  1. Functional implant zone 1
  2. Functional implant zone 2
  3. Functional implant zone 3
  4. Functional implant zone 4

Answer : 2

💡 Traumatic zone consists of alveolar ridge of premaxilla and

  1. Eight anterior teeth: four incisors, two canines and two first premolars
  2. Six anterior teeth: four incisors and two canines
  3. Four anterior teeth, i.e. four incisors
  4. Ten anterior teeth: four incisors, two canines, two first premolars and two second premolars

Answer : 1

💡Dental implants in the mandibular premolar region are dictated by the size and the location of the mental foramen. Its location varies from the mandibular canine to the first molar. The most accurate method for detecting the mental foramen is

  1. MRI
  2. Conventional radiograph
  3. CBCT
  4. None of the above

Answer : 3

💡 The image detector used in CBCT is

  1. CCD
  2. FPD
  3. Phosphor detector
  4. None of the above

Answer : 2

💡Following are the disadvantages of CBCT except

  1. Inability to detect soft tissue
  2. Less radiation dose
  3. Beam hardening artefacts
  4. Cost of equipment

Answer: 2

💡 Which of the following is not a beam-related artefact in CBCT?

  1. Partial volume averaging
  2. Undersampling
  3. Cone-beam effect
  4. Ring artefacts

Answer : 4

💡CBCT delivers radiation dose compared to conventional radiographs as

  1. 5–16 times more
  2. 5–16 times less
  3. 48–68 times more
  4. 48–68 times less

Answer: 1

💡Aliasing artefacts are due to

  1. Divergence of cone beam, diverging towards the periphery of the reconstructed volume
  2. Defect or uncalibrated detector elements
  3. Restoration, implants, etc.
  4. None of the above

Answer: 1

💡All of the following are advantages of FPD over CCD except

  1. The flat-panel detector arrangement is less complicated.
  2. Flat-panel detectors require a slightly greater radiation exposure but are inexpensive.
  3. Flat-panel detectors cause reduced peripheral distortion and high resolution.
  4. All of the above.

Answer : 4

Oral Medicine and Radiology