Cone Beam CT PDF

CONE BEAM CT Mohd Hafizi Mahmud CT Physics and Instrumentation MRD530  Limitations of 2D Dental Imaging  Inability to observe cross-section changes  Geometric distortion  Overlapping of the disease process with neighboring, dense anatomical structures -...

CONE BEAM CT Mohd Hafizi Mahmud CT Physics and Instrumentation MRD530  Limitations of 2D Dental Imaging  Inability to observe cross-section changes  Geometric distortion  Overlapping of the disease process with neighboring, dense anatomical structures - Intra-oral anatomical noise OPG  Role of CBCT in Dental and Maxillofacial Imaging  CBCT involves 3D acquisition providing volumetric and multi-planar display leading to accurate representation: Lesional extent Involvement of adjacent anatomic structures Lesional borders Internal lesional details Presence and degree of root resorption, particularly on the buccal or lingual/palatal aspects of the lesion or tooth, respectively  Principle of CBCT  CBCT uses rotating gantry to which an X-ray source and detector are fixed.  A divergent pyramidal- or cone-shaped source of ionizing radiation is directed through the middle of the area of interest onto an area X-ray detector on the opposite side.  The X-ray source and detector rotate around a fixed fulcrum within the region of interest (ROI).  During the exposure sequence hundreds of sequential planar projection images are acquired of the field of view (FOV) in an arc generally of at least 180°.  In this single rotation, CBCT provides precise, essentially immediate and accurate three- dimensional (3D) radiographic image volumes.  As CBCT exposure incorporates the entire FOV, only one rotational sequence of the gantry is necessary to acquire enough data for image reconstruction.  Principle of CBCT - Cone Beam Acquisition  With 2D X-ray area detectors and cone-beam geometry, a 3D volume must be reconstructed from 2D projection data –referred to as cone beam reconstruction.  Most common cone beam reconstruction - FDK (Feldkamp-Davis-Kress) algorithm, employs a convolution-back projection method.  Cone beam acquisition uses a beam geometry providing multiple transmission images that are integrated directly forming volumetric information  Development of Maxillofacial CBCT Systems  CBCT : Hamamatsu Photonics K.K (Hamamatsu City, Japan), Varian Medical Systems (Salt Lake City Utah, USA), and Samsung (Seoul, South Korea).  X-ray generators used in maxillofacial CBCT machines are far simpler than those in MDCT with an operating voltage between 80 kVp and 120 kVp.  Focal spot size is no different than MDCT (0.5–0.8 mm nominally), unlike MDCT the anode is stationary in most systems CBCT Prototype (2003) - DentoCAT CBCT Models  CBCT Image Production *Most CBCT models use rectangular collimation, resulting in a pyramid- shaped beam  Projection Geometry  Image acquisition is performed through a single partial (≥180°) or full (360°) rotational scan of an X-ray source and a reciprocating 2D flat detector array.  The axis of rotation of this configuration is centered at a certain region of interest (ROI) within the patient’s head.  Throughout the rotation, divergent pyramidal or cone-shaped beam of X-rays is directed towards the detector on the opposite side, with the field of view (FOV) being determined by the physical collimation applied from a slightly different horizontal angle.  These image projections constitute the raw data and are individually referred to as basis, frame, projection, or raw images.  Because X-ray beam projections incorporate the entire FOV, only one rotational sequence (or, at the very least, half a rotation) of the gantry is necessary. Cone beam CBCT vs Fan beam MDCT IMAGE DETECTOR  Image intensifier (II) - The attenuated X-ray beam is converted into electrons. These electrons are then amplified before being reconverted into photons, which are recorded using a charge-coupled device (CCD). - Prone to geometric distortion - Currently, few CBCT units incorporate II detectors  Flat panel detectors (FPD)  1. Indirect FPD system  Use scintillator which converts X-ray radiation into visible light and a photon detector which converts light into an electrical signal, which can then be digitized.  2. Direct FPD system  The most recent technology  Comprise an amorphous selenium (a-Se), cadmium telluride (CdTe), or cadmium zinc telluride (CdZnTe) photoconductor, which converts X-ray photons into an electrical charge, directly connected to a TFT or CMOS  Images from direct detector systems are inherently less unsharp than those from indirect detector systems IMAGE RECONSTRUCTION  Each projection image consists of a pixel matrix with a 12- to 16-bit value (proportionate to the detected X-ray intensity) assigned to each pixel.  This data is then reconstructed into a 3D volumetric dataset composed of cubical volume elements (voxels) by a sequence of software algorithms.  The most widely used reconstruction algorithm in CBCT is the Feldkamp (FDK) algorithm, which is a modified filtered back- projection (FBP) method CBCT VS MDCT Similarities and differences between CBCT & MDCT Similarities and differences between CBCT & MDCT CBCT Configurations Field of View (FOV)  Volume Acquisitions  CBCT units can be assigned into four broad categories based on the vertical and horizontal dimensions of the FOV :  Large (Maxillofacial)  Covers most of the craniofacial skeleton, at least from below the hard tissue of the chin to the nasion. Usually greater than 15 cm in any dimension.  Dentoalveolar (both jaws)  Usually 8 cm or more in diameter and height.  Single jaw/dual TMJ  Can cover a single full jaw (excl. ramus for mandibular scans) or both temporomandibular joints. Wide in diameter (≥ 10 cm, or ≥ 14 cm if used for TMJs) but small in height (4–6 cm).  Small (localized)  As small as 3 cm in any dimension, covering localized regions such as 2– 4 teeth and surrounding alveolus or a single temporomandibular joint.  Projection Images  The number of projections acquired during a CBCT scan is determined by the frame rate (number of projections acquired per second), the extent of the rotation arc (180°–360°), and the speed of the rotation.  Exposure time (s)  In CBCT, the exposure time is proportionate to the number of acquired projections.  CBCT using pulsed exposure have a lower exposure time for a given number of projections than those with continuous exposure.  Frame Rate  Higher frame rates allow for shorter scan time, which can lead to images with less artifacts and better image quality.  High frame rates require detectors with pixels sensitive enough to reach an adequate signal-to- noise ratio (SNR) during a short time frame.  In CBCT, the number of projections typically ranges between 150 and 1000.  While the use of few projections results in aliasing artifacts due to under-sampling.  An increased number of projections provides more information to reconstruct the image, resulting in a decreased image noise.  An increased number of projections would therefore allow the image to be reconstructed at a smaller voxel size while keeping noise at a reasonable level, effectively increasing spatial resolution. Exposure Panel Multi-planar reformation (MPR) Casto et al (2014) End of slide Thank you