Quality Resource Guide l Clinical Considerations for Cone Beam Imaging in Dentistry 3rd Edition 4 www.metdental.com Exposure settings such as kilovoltage (kV) and/ or milliamperage (mA) may be “fixed” for certain patient types (adult vs. child) or manually adjusted. Selection should be based on the relative size of the patient and in compliance with manufacturer’s recommendations. These choices affect both image quality and patient radiation dose (Scarfe et al., 2018). The primary acquisition settings include: a) Field of view. The volume of tissue of the patient’s head irradiated during exposure is referred to as the field of view (FOV). These dimensions are most often fixed for different regions ( e.g. one jaw, both jaws, dental quadrant) or may be customized. The FOV should be correspond to the region of interest (ROI). This provides marked reduction in patient radiation exposure ranging from 25% to 66% depending on the machine, degree of collimation and location. (Ludlow and Ivanovic, 2008 da Silva Moura et al ., 2019) b) Acquisition time. The total number of basis images comprising the projection data of a single scan is usually fixed but may be variable on some units. This is determined by adjustment of the acquisition or scan time. While increasing scan time provides more basis images to produce less noisy images, this is achieved at a proportionately higher patient dose. c) Arc Trajectory. Most CBCT systems are now multimodal, based on the panoramic platform, and use a fixed limited arc of rotation of less than 3600. This reduces scan time and minimizes the opportunity for patient motion during the scan, however data must be extrapolated to provide a full volumetric dataset. Machines that use a complete circular (3600) rotation may offer a limited arc trajectory scan that, for some tasks ( e.g. periapical bone loss), produce images at lower radiation exposure with comparable diagnostic accuracy (Lennon et al ., 2011; da Silva Moura et al ., 2019 ) 2) Image Visualization Protocol: Several units now offer post-acquisition options to apply to the volumetric dataset prior to display, improving image quality. All techniques can be applied without increasing patient radiation exposure. a) Reconstruction algorithm. The Feldkamp, Davis and Kress (FDK) algorithm is the most widely used for 3-D reconstruction. It has a relatively fast processing time. Iterative reconstruction (IR) algorithms are now provided as an option in some units. These provide enhanced processing to images with reduced noise and artifacts, greater contrast and spatial resolution but have longer processing time. They are particularly useful in units with limited trajectory arcs. b) Spatial Resolution. The acquired voxel dimensions of a CBCT unit reflect the pixel size and matrix dimensions of the image sensor. For larger scans, addition of the data in adjacent pixels is often performed to reduce file size and improve contrast resolution using a process of pixel binning. Some CBCT units provide options where spatial resolution can be improved by post-processing. Resolution settings should be selected based on the diagnostic task with implant and orthodontic applications using a low resolution (0.25 to 0.4mm voxel size), TMJ assessments and tooth impactions using a standard resolution (0.125 to 0.25mm voxel size) and endodontic diagnosis using a high resolution (< 0.125 mm voxel size). c) Artifact Reduction. Two algorithms can be applied to the volumetric dataset prior to interpretation that may potentially improve image quality – noise and metallic artifact reduction. Both should be used with caution as they may add considerable time to the reconstruction phase and introduce other undesirable effects into the image. Selection of scanning and image visualization protocols should be based on the requirements of the imaging task – a concept referred to as task specific imaging. For example, a TMJ scan to determine the degree of translation of the condyle with jaw opening should be performed at the standard resolution, shortest scan time and reduced FOV. This provides optimal imaging at a nominal dose. Prepare patient. Whether the patient is standing, lying or seated, the jaws must be firmly stabilized during the entire scan. This reduces the potential for motion during the scan, a significant source of reduced image quality. (Bontempi et al., 2008) This can be accomplished using equipment such as chin rests and/or head holders and providing adequate instructions to the patient prior to exposure to remain still during the procedure and to keep the teeth closed either together or on a bite block. Protect patient. The patient should be draped in a lead torso apron to reduce scatter radiation. Use of a thyroid collar should also be considered when it does not interfere with the area to be imaged as this substantially reduces patient radiation by shielding exposure to the hyoid, esophagus and cervical spine. (Qu et al., 2012) Exposure. CBCT scan time is often comparable to that of panoramic radiography. However, unlike panoramic imaging, CBCT also incorporates correction of the collected images. The digital detector may require periodic correction, referred to as detector calibration, to prevent untoward artifacts affecting image quality. View the image. To assist in interpretation of the 3-D imaging volume, image display should be software-assisted, dynamic and performed as an interactive process – the value of the voxels must be adjusted (brightness, contrast), the volumetric dataset reoriented and the data reformatted for display using task specific protocol formatting. Relative Radiation Exposure Patient radiation dose is markedly influenced by the type and model of CBCT device, patient size (child vs. adult), region of interest (mandible vs. maxilla), exposure settings (kV, mA,) scan parameters (size of FOV, number of basis images, rotational arc, voxel size and resolution) and use of protective shielding. CBCT effective dose range from 11–252 μSv for small FOV, 28–652 μSv for medium FOV, and 52–1,073 μSv for large FOV comparable to approximately 1 to 70 times