Minimum size and positioning of imaging field for CBCT-scans of impacted lower third molars: a retrospective study

Anne-Mari Ilo,Janna Waltimo-Sirén,Marja Ekholm,Elmira Pakbaznejad Esmaeili

doi:10.1186/s12903-021-02029-6

Abstract

BackgroundCone-beam Computed Tomography (CBCT) is widely used for preoperative 3D imaging of lower third molars. Hence, for this imaging indication, the present study aimed to define the minimum field-of-view (FOV) size and its optimum placement, to decrease radiation exposure, and highlight the need of computer-assisted FOV centering technique for dental CBCT devices. To facilitate proper placement of image field, lower second molar was chosen as reference.MethodsThe retrospective study included 50 CBCT-scans of 46 patients with mean age of 34 years. Based on the lower second molar, a three-dimensional coordinate was formed and the location of mandibular canal (MC) and the dimensions and locations of the lower third molars, and possible associated pathological findings were assessed. Accordingly, the FOV size and position for third-molar imaging were optimized, while ensuring encompassment of all relevant structures.ResultsThe minimum cylindrical volume, covering lower third molars and MC, was 32.1 (diameter) × 31.6 (height) mm, placed in relation to the second molar crown, top 2.2 mm above cusp tips, anterior edge 6.7 mm in the front of the most distal point of the crown, and lingual edge 7.9 mm on the medial side of the lingual wall.ConclusionsThe optimized FOV for lower third molars was smaller than common standard small FOVs. We recommend using FOV volume 3.5∅ × 3.5 cm for third molars without associated pathology. Accurate FOV protocols are essential for development of new CBCT-devices with computer-assisted and indication-specific FOV placement.

Highlights

Cone-beam Computed Tomography (CBCT) is widely used for preoperative 3D imaging of lower third molars
Rapid development of CBCT-technology and access to CBCT-devices have increased the associated risk from radiation exposure received by patient because radiation doses are generally higher in CBCT than in conventional radiography [7, 10,11,12,13]
The 50 CBCT-scans were of 46 patients, 32 females and 14 males, aged 19 to 67 years with a mean age of 34

Summary

Introduction

Cone-beam Computed Tomography (CBCT) is widely used for preoperative 3D imaging of lower third molars. For this imaging indication, the present study aimed to define the minimum field-of-view (FOV) size and its optimum placement, to decrease radiation exposure, and highlight the need of computer-assisted FOV centering technique for dental CBCT devices. A Finnish nationwide survey reported that of the monthly 1,345 CBCT-scans 25% were due to mandibular third molars, making the assessment of the relationship between the lower third molar and MC the third most common clinical indication for CBCT-imaging [9]. CBCT guidelines in the North America conclude that the selected field-ofview (FOV) is an important consideration to reduce to dose to the patient [16]. The general guideline is to keep FOV as small as possible, encompassing only the region of interest [7, 8, 11]

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