Cone Beam CT in Dental Imaging

Cone Beam CT (CBCT)

• CBCT uses a rotating gantry with a stationary X-ray source and detector.
• A cone-shaped beam of ionizing radiation passes through the area of interest.
• The X-ray source and detector rotate around a fixed point within the region of interest (ROI).
• Hundreds of sequential planar projection images are acquired in an arc of at least 180°.
• A single rotation provides essentially immediate and accurate 3D radiographic image volumes.
• The entire field of view (FOV) is incorporated in the exposure sequence, so only one gantry rotation is needed for reconstruction.

Limitations of 2D Dental Imaging

• Inability to observe cross-sectional changes.
• Geometric distortion.
• Overlapping of the disease process with neighboring dense anatomical structures, producing anatomical noise.

Role of CBCT in Dental and Maxillofacial Imaging

• CBCT provides volumetric and multi-planar display, allowing for accurate representation of lesions.
• CBCT can depict lesional extent, involvement of adjacent structures, lesional borders, and internal lesional details.
• It's helpful in evaluating the presence and degree of root resorption, especially on the buccal, lingual, or palatal aspects of the affected tooth or lesion.

Principle of CBCT - Cone Beam Acquisition

• 2D X-ray area detectors and cone beam geometry are used to reconstruct a 3D volume from 2D projection data.
• The most common reconstruction algorithm is FDK (Feldkamp-Davis-Kress).
• It employs a convolution-back projection method.
• CBCT generates multiple transmission images, which are integrated to create volumetric information.

Different Types of CBCT Gantries

• CBCT gantries can accommodate seated, standing, and supine patient positions.

Development of Maxillofacial CBCT Systems

• CBCT systems are manufactured by Hamamatsu, Varian, and Samsung.
• X-ray generators in maxillofacial CBCT machines are simpler than those in MDCT.
• Focal spot size is similar to MDCT (nominally 0.5-0.8 mm).
• The anode is stationary in most CBCT systems.

CBCT Prototype (2003) - DentoCAT

• Early CBCT prototype used a rotating gantry with an X-ray source and a detector array.
• The X-ray source and detector rotated around a fixed fulcrum within the region of interest.

CBCT Models

• Numerous CBCT models exist from various manufacturers (e.g., Acteon, Asahi Roentgen, etc.).

CBCT Image Production

• Projections are acquired during a 180° to 360° rotation of the X-ray tube and detector.
• The raw data from the projections is reconstructed into a 3D volumetric dataset.
• The reconstruction process usually includes reformatting into orthogonal slices for viewing.

Projection Geometry

• CBCT imaging acquisition is performed using a single partial (≥180°) or full (360°) rotational scan.
• The X-ray beam diverges and cone-shaped as it passes through the region of interest (ROI).
• These projections are individually referred to as basis, frame, projection, or raw images.

Cone beam CBCT vs Fan beam MDCT

• CBCT uses a cone-shaped beam, whereas MDCT uses a fan-shaped beam.
• CBCT uses a single rotation for reconstruction, while MDCT uses multiple axial slices.

Image Detector

• Attenuated X-ray beams are converted to electrons, then amplified and reconverted into photons.
• CCD or complementary metal–oxide–semiconductor (CMOS) detectors are used.
• Flat panel detectors (FPDs) are the most common.
• Indirect FPD systems employ a scintillator, which converts X-rays to light, and a photodetector that converts that light into an electric signal.
• Direct FPD systems employ a photoconductor (e.g., amorphous selenium, CdTe), that directly converts X-rays into an electric charge, which is then digitized.

Image Reconstruction

• Each projection image is a pixel matrix where each pixel has a 12- to 16-bit value (proportional to the detected X-ray intensity).
• Raw data is converted into a 3D volumetric dataset composed of voxels.
• The Feldkamp-Davis-Kress (FDK) algorithm, a modified filtered back-projection (FBP) method, is commonly used for reconstruction.

CBCT vs MDCT

• Both technologies use X-ray beams.
• CBCT usually has a less full coverage.
• CBCT can be a stationary unit, while MDCT is usually a mobile unit with a larger field of view.
• A wide variety of image processing may be used in CBCT.

Volume Acquisitions

• Large (Maxillofacial): Covers most of the craniofacial skeleton.
• Dentoalveolar (both jaws): Covers both jaws.
• Single jaw/dual TMJ : Covers a single full jaw or both temporomandibular joints.
• Small (localized): Covers localized regions (e.g., 2-4 teeth or a single TMJ).

Projection Images

• The number of projections reflects the frame rate (projections/second), rotation extent (180°-360°), and rotation speed.

• Exposure time is proportional to the number of acquired projections.

• Continuous exposure systems require longer exposure times compared to pulsed systems.

• Higher frame rates lead to shorter scan times, less artifacts, and better image quality, requiring detectors with more sensitive pixels.

Exposure Panel

• Exposure settings vary based on the patient's size, type of scan, and equipment type.
• Control panels may exhibit different settings for different protocols.

Multi-planar reformation (MPR)

• MPRs allow for examination in multiple planes (axial, coronal, and sagittal) from the dataset, providing 2D images of multiple sections from the 3-dimensional data.