FOV (Field of visualization) and its relation to treatment requirements in CBCT imaging.
Updated: Jul 30, 2020
CBCT imaging should always be conducted using protocols which are derived from specific indications of the case requiring investigation. It should provide the required clinical information to the clinician and have reduced radiation dose for the patient, along with being cost effective. The radiation dose is dictated by age of the patient and the anatomical region of interest (ROI). The field of visualization (FOV) needs to be tailored according to resolution requirements for the investigation and patient condition. These are the CBCT use recommendations after primary diagnosis by using intraoral radiography :
Cases in Endodontics
Intraoral or Periapical radiographs should be considered as the primary imaging modality of choice in the endodontic patient.
Small/Limited FOV CBCT should be considered as the imaging modality of choice for diagnosis in patients who present with contradictory or nonspecific clinical signs and symptoms associated with untreated or previously endodontically treated teeth.
Limited FOV CBCT is a focused scan covering only the region of interest, that is the concerned tooth under investigation and its supporting structures. It also minimizes the effective radiation dose to the patient.
Pre-operative:
A small or limited FOV CBCT (5x5 CM or 4x5 CM) should be considered for initial endodontic treatment because of its voxel properties. Voxel size is directly related to the spatial resolution of an image. Hence, the small FOV is selected due to increased resolution (highest possible resolution with smallest possible voxel size) to improve the diagnostic accuracy by visualization of small anatomical features like:
accessory canals, missed canals, calcified canals etc. e.g. missed MB2 canal in the maxillary first molars.
anatomical variations : e.g. presence of distolingual root in the mandibular first molars.
suspected complex morphology: e.g. presence of accessory mesial canals within the mesial root of mandibular molars. Intercommunication between the canal systems within the root in which they are present.
dental anomalies: e.g. fused teeth; dens evaginatus, dens invaginatus, C-shaped canal system, dilacerated teeth.
Coronal and cross sectional images at 125µm (limited FOV) showing a case of Dens invaginatus (Oehlers Type III) with 12 along with localized, expansile osteolytic lesion. The axial image shows location of the mesial canal orifice ir.t 12. The mesial canal is lined by enamel along the internal aspect till middle 3rd level.
Radix Entomolaris : Axial image at 180µm on the left side shows separate distolingual root (white marker) i.r.t 46 with history of endodontic treatment.
Cross sectional images for 46 show partially filled distobuccal root (image no.66), curved distolingual root (image no.67, red marker) and mesial root with 2 canals (image no.78). Periapical osteolytic lesion involving the inferior alveolar canal is noted.
Intraoperative:
It is recommended for a case which is currently in treatment without a prior CBCT investigation, and is suspected to have calcified/ missed canals or crown-root fracture. In such situation, a limited FOV CBCT should be considered for detection of such anatomical details or pathology which is not visualized on a periapical radiograph.
E.g. A voxel size of 0.2mm (200µm) or less is recommended to detect periapical pathosis.
When canals are identified but are difficult to negotiate due to calcification,a small FOV CBCT is useful to determine the extent of calcification and morphology of the canal so as to plan the further sequence of instrumentation for the treatment.
It can also help in cases with nonspecific pain by detection of root canal or furcal perforation; identification and location of fractured files/ instruments. The detection of these defects and choice of proper clinical therapy after proper assessment of clinical signs is important to determine a predictable treatment outcome and helps in the analysis of failures.
