Cone Beam CT Flashcards
What is CBCT (Cone beam Computed tomography?
- Form of cross-sectional imaging suitable for assessing radiodense structures
- Used in secondary and primary healthcare (private)
What are some non-DMFR uses of CBCT?
- Temporal bone imaging
- paranasal sinus imaging
- Radiotherapy planning
What are the 3 basic principles of CBCT?
- Ionising radiation
- Conical/pyramidal x-ray beam and sqaure digital detector that rotates around head
- Captures many 2-dimensional images e..g 200 which are made into cylindrical 3D image
What are the unit types and what is the pt positioning of CBCT?
- Can have single or multi-purpose units
- Pt position specific to unit but usally standing or sitting
- Head positioning same as panoramic, horizontal frankfort plane, vertical midsagittal plane
What are some benefits of CBCT over plain radiography?
- No superimposition
- Ability to view subject from any angle
- No magnification/distortion
- Allows for volumetric (3D) reconstruction
What are some negatives of CBCT over plain radiography?
- Increased radiation dose to pt
- Lower spatial resolution (not as sharp)
- Susceptible to artefacts
- Equipment more expensive
- Images more complicated to manipulate and interpret
- Requires additional training to justify, operate and interpret
What are the main CBCT benefits over CT?
- Lower radiation dose
- Potential for sharper images due to higher resolution
- Cheaper initial cost, running and maintenance
- Smaller footprint
What are the main benefits of CT over CBCT?
- Differentiate soft tissues better
- Cleaner images due to better signal to noise ratio
- Larger field of view is poss
What are some common uses of CBCT in dentistry?
- Clarifying relationship between impacted mandibular third molar and IANC prior to intervention
- Implant placement to measure alveolar bone dimension
- Visualising complex root canal morphology to aid endo txt
- Investigate external root resorption next to impacted teeth
- Assess large cystic jaw lesions and involvement of important anatomical structure
What orthogonal plane is this?
- Axial
What orthogonal plane is this?
- Sagittal
What orthogonal plane is this?
- Coronal
What are the used of 3D volume reconstruction?
- Help clinician tp picture the extent/shape of disease
- Can be informative teaching aid for pt
What are some drawbacks of 3D volume reconstruction?
- It is a modified reconstruction of data and can create misleading images (poor at showing chin bone)
There are some imaging factors/variables that are set before the CBCT scan begins. Give some examples and what principle should be used when deciding
Examples
- Field of view
- Voxel size
- Acquisition time
ALARP principle as each will alter info obtained and pt dose
What is the field of View? What does increased size mean for the pt
- Size of the captured vol of data
- Increase size = increased radiation dose + increased number of tissue irradiated + increased scatter + more to report
What is the voxel size? What does decreased voxel size give for pt?
- The image resolution (aka 3D pixels)
- Decreased voxel size indirectly leads to increased radiation dose
- Decreased voxel size leads to increased scan time
What are the range of options for voxel size of CBCT?
- 0.4mm^3 - 0.085mm^3
In an endodontic case what imaging factor choice do you want?
- FOV small as poss e.g. 5x5cm unless large apical pathology
- Smaller voxel size to see fine detail of root canal and apex
In an implant planning case what factor imaging choice is correct?
- FOV depends on number/pos of implants
- Large voxel size as only needing to see the bone surrounding
What is the radiating dose of panoramic / intraoral and what is the rough guide for CBCT?
Panoramic = 3-24uSv
Intraoral = 4uSv
CBCT = approx 2-3X dose of panoramic
What are artefacts ? What are the two main types?
- Visualised structures on scan not present in object investigated
- Movement artefacts
- Streak artefacts
When does movement artefact occur? What can it lead to? How is it reduced?
- Occurs if pt not completely still during full exposure (affects whole scan)
- Lead to general blurriness or extra contours
- Reduced by using fixation aids like a chin rest or head strap
What are streak artefacts caused by? What are the main issues with them?
- Caused by high-attenuation objects e.g. amalgam
- Can prevent caries assessment adjacent to restorations
- Can prevent assessment of perforations/missed canals in RCTed teeth
What are some contra-indications for CBCT?
- If plain radiographs are sufficient
- Pathology requiring soft tissue visualisation i.e. Malignancy or infection spreading in soft tissue
- If high risk of debilitating artefacts
- Pt factors e.g. pt unable to stay still like Parkinsons disease or Pt unable to fit in machine like obsese/bodybuilder
What justification do you require for CBCT?
- Preceded by clinical exam
- If plain radiography unable to provide sufficient info
- FGDP guidance to assist
