Intro to CBCT Flashcards
CBCT machine components
X ray source
Head Stabilizing Apparatus
Detector
Control Panel
What are the principles of CBCT imaging
The X ray beam is cone shaped
X ray source and detector rotate 180-360 degrees around the patient’s head, with the rotation center as a FIXED POINT in the center of the ROI
Hundreds of 2D basis projection images are taken as the source and detector rotate
Projection data is used to construct a 3D volume using algorithms like filtered back projection
What is filtered back projection
Attenuation profiles of each basis projection image are layered on top of each other during reconstruction to increase contrast
What is attenuation?
The number of X rays reduced when passing through an object, dependent on density of said object
Strengths of CBCT wrt OPG
3D so can determine buccolingual info and get an accurate representation of spatial r/s
No superimposition, distortion or magnification (unlike OPGs) which make measurements inaccurate
Limitations of CBCT wrt OPG and CT scan
Higher radiation doses than OPG (on average 15x the dose but can be 4-47x based on settings and area captured)
Lack of soft tissue contrast wrt conventional CT scan, cannot interpret pathologies in salivary gland, muscle, BV etc
Doesn’t use Hounsfield Units (unlike CT scans), so characterisation of tissue and objects is less reliable
Artefacts like metal streak artefacts and beam hardening which are not found in OPGs
Patient motion causes appearance of double lines/structures, but can be mitigated by using head straps or asking px to close eyes
What are metal streak artefacts?
Dense objects such as metal crowns, titanium and zirconium oxide implants absorb X rays as they pass through.
Metal artefacts appear as white streaks/scatter due to complete absorption of X rays
Tends to be seen at occlusal plane
What is beam hardening?
Dense objects such as metal crowns, titanium and zirconium oxide implants absorb X rays as they pass through.
Beam hardening appears as dark bands due to absorption of low energy (soft) X rays, leaving only high energy (hard) and highly penetrative X rays
Tends to be seen at occlusal plane
What is the LNT model
Linear non-threshold model
Risk of cancer is present at ANY radiation dose, there is no “safe” threshold! Higher dose = higher risk
ALARA
We don’t have dosage readings at low levels so use data from historical or epidemiological events, extrapolating a linear graph down to point of origin
Indications for CBCT
Implant planning (alveolar bone, anatomic danger zones, pathology)
Assessment of impacted teeth and their relationships with vital structures (e.g. IDN) and planning of surgical approach
Evaluation of intraosseous pathology like cysts or tumours (LESION characteristics, size/extent, proximity to vital structures)
Craniofacial evaluation (cleft lip/palate/orthognathic surgery px to determine defect width, volume of bone graft needed, adjacent impacted/supernumerary teeth)
Evaluation of trauma (esp dental alveolar fracture)
Endodontic evaluation (complex root morphology, unfilled canals, canal number, course of roots)
Evaluation of paranasal sinuses (chronic sinusitis - endodontic or odontogenic?)
TMJ evaluation (flattened/round? sclerosis? osteophyte? degenerative joint disease)
Inappropriate usages of CBCT
Wrong FOV
Replacing 2D imaging
Caries diagnosis
Soft tissue pathology
FOV for CBCT
I.e. the scan volume
Small = just teeth
Medium = similar to DPT
Large = jaws, max sinus, orbit and top of skull
Determined by collimation of the X ray beam
Voxel size for CBCT
Isotropic volume element, CBCT can be sliced in any plane as required
Ranges from 0.075mm to 0.4mm
Determines spatial resolution
Smaller voxels = higher resolution but higher dosage
Use largest voxel size for the task
Multiplanar reformation for CBCT
Sectioning of the 3D data to produce a 2D image
Orthogonal: axial (horizontal), sagittal (like latceph), coronal (facing forward)
Para: prefix used when you want to change orthogonal view from 90 degrees for better evaluation of structures like condyles
Non-orthogonal: change focal trough according to ROI
> Oblique: like DPT
> Serial cross sectional: successive slices perpendicular to teeth, allowing for bucco-lingual imaging of teeth and alveolar bone width, and for nerve tracing
> curved planar/panoramic reconstruction
> surface volume render: visualization of surface details and pathology
> maximum intensity projection: only shows bones and metal and soft tissue calcifications
