Filtered back projection vs. iterative reconstruction for CBCT: effects on image noise and processing time

Abstract

Objectives: To assess the effect of standard filtered back projection (FBP) and iterative reconstruction (IR) methods on CBCT image noise and processing time (PT), acquired with various acquisition parameters with and without metal artefact reduction (MAR).

Methods: CBCT scans using the Midmark EIOS unit of a human mandible embedded in soft tissue equivalent material with and without the presence of an implant at mandibular first molar region were acquired at various acquisition settings (milliamperages [4mA-14mA], FOV [5 × 5, 6 × 8, 9 × 10 cm], and resolutions [low, standard, high] and reconstructed using standard FBP and IR, and with and without MAR. The processing time was recorded for each reconstruction. ImageJ was used to analyze specific axial images. Radial transaxial fiducial lines were created relative to the implant site. Standard deviations of the gray density values (image noise) were calculated at fixed distances on the fiducial lines on the buccal and lingual aspects at specific axial levels, and mean values for FBP and IR were compared using paired t-tests. Significance was defined as p < 0.05.

Results: The overall mean for image noise (± SD) for FBP was 198.65 ± 55.58 and 99.84 ± 16.28 for IR. IR significantly decreased image noise compared to FBP at all acquisition parameters (p < 0.05). Noise reduction among different scanning protocols ranged between 29.7% (5 × 5 cm FOV) and 58.1% (5mA). IR increased processing time by an average of 35.1 s.

Conclusions: IR significantly reduces CBCT image noise compared to standard FBP without substantially increasing processing time.

Keywords: Computer-Assisted Image Processing; Dental Implants; MeSH terms: Cone-Beam Computed Tomography; Radiographic Image Enhancement.

Figure 1.

Lateral (a) and superior (b) views of the mandible with the right first molar extracted are shown embedded in ballistic gelatin. Right lateral view of the mandible (c) with the titanium implant inserted in the lower right molar region.

Figure 2.

Matrix of six representative images acquired at 10mA, HQ resolution, and reconstructed with MAR at the level of the axial reference plane images showing the location of fiducial lines. Each row represents one of two different reconstruction algorithms: filtered back projection (FBP) or iterative reconstruction (IR). (a) FBP/9 × 10 cm; (b) IR/9 × 10 cm; (c) FBP/6 × 8 cm; (d) IR/6 × 8 cm; (e) FBP/5 × 5 cm; (f) IR/5 × 5 cm.

Figure 3.

Histogram comparing the effect of reconstruction algorithm (FBP or IR) on mean image noise (mean standard deviation of gray values) ± standard error for each condition, including the presence of an implant, FOV, resolution, application of MAR, and increases in milliamperage (mA). All comparisons of FBP vs IR were significantly different (p < 0.001).