DOM 8220 CT CBCT Principles WEBSITE PDF

Computed Tomography and Cone Beam CT Learning Objectives Computed tomography – Understand the concept of creation of 3D images – Understand the physical principles of CT – Know the process of image creation, including windowing of the gray scale Computed Tomograp...

Computed Tomography and Cone Beam CT Learning Objectives Computed tomography – Understand the concept of creation of 3D images – Understand the physical principles of CT – Know the process of image creation, including windowing of the gray scale Computed Tomography (CT): Concept Planar (2D) imaging uses a stationary x-ray source and sensor Projects all structures in the beam path on a flat plane on the receptor Superimposition of structures makes visualization of tissues at different layers impossible Example: Posterior-anterior skull radiograph Computed Tomography (CT): Concept CT imaging: X-ray source and sensor rotate around the body while making hundreds of exposures, each from a different angle Creates hundreds of different patterns of absorption, transmission, and scatter as the beam circles the structures Computer records each exposure and creates a 3D array of data Computed Tomography (CT): Concept By exposing structures from all angles, they can be separated from each other in the image Superimposition is eliminated! The fundamental principle behind computed tomography is to acquire multiple views of an object over a range of angular orientations Eliminates superimposition and distinguishes different tissues Computed Tomography (CT): Concept Planar (2D) image collapses all structures on a plane CT (3D) separates the structures, depicting each structure separately  Computed Tomography (CT): Concept Planar 2D PA Skull CT 3D sections through the orbits and the maxillary and sphenoid sinuses Computed Tomography (CT): Concept X-ray beam is confined to a narrow fan-shape This results in the production of numerous sections (slices) corresponding to the fan beam The section images are viewed individually but can be formatted as a single 3D view Tomography: Section (tomo) image (graphy) Recording of exposures in each section is performed by a computer “Computed tomography” Computed Tomography (CT): Concept Computed Tomography (CT) Godfrey Hounsfield Engineer at EMI Inventor of Computed Tomography His proposed research was very costly Where would EMI get the money to fund it?? Computed Tomography (CT) EMI had another branch --- It was also a music recording company In the 1960s, EMI had a rock ‘n roll group under contract that was somewhat profitable … finletes invented  on he Computerized Axial Tomography  Computed Tomography (CT): Physical Principles X-ray beam rotates around the patient Frigga Over 1000 radiographic projections (basis images), each from a different angle Exposure field limited to a thin section of the body (tomography) imaginingsection Projection data captured by detector array Computed Tomography (CT): Physical Principles Fan beam Detector array  Helical CT iaii.am Multidetector computed tomography: MDCT Computed Tomography (CT): Physical Principles Computer now has data from hundreds of basis images of the tissue section Different angles of projections result in different patterns of x-ray absorption in tissue Computed Tomography (CT): Physical Principles Computer uses data to divide tissue section into thousands of blocks called voxels (volume elements) Each voxel is assigned a CT number (Hounsfield Number) proportional to degree of x-ray absorption by tissues in the voxel Computed Tomography (CT): Physical Principles Over 4,000 CT numbers: – Air: -1,000 – Water: 0 now – Bone: +1,000 these – Metal: +3,000 Computed Tomography (CT): Physical Principles Each voxel represented in the image by a square called a pixel (picture element) CT image consists of thousands of small square pixels Pixel is assigned the CT number generated for the voxel it represents Computed Tomography (CT): Physical Principles CT: Image Creation Image creation: – Gray shade assigned to each pixel based on its CT number – Lower CT numbers get darker shades (CT number for air is -1,000 and appears black) blackestShadeofgrey – Higher CT numbers get lighter shades (CT number for metal is +3,000 and appears clear) – Same as intraoral digital radiology! CT: Image Creation Image creation: – 256 gray shades are applied to image – Don’t want to stretch 256 gray levels over 4,000 CT numbers – Range of CT numbers per gray level would be so large that subtle alterations (disease) would not be detectable: very poor contrast CT: Image Creation Image creation: Gray scale applied over limited parts of CT range: windowing c important concept – To visualize soft tissues, center the gray scale (128) on a pixel with a low CT number – Range of gray limited to soft tissues with low CT numbers – Can see subtle differences in soft tissues; all hard tissues are uniformly light – Soft tissue window CT Images: Soft Tissue Window Cysts in parotid glands CT: Image Creation Image creation: – To visualize hard tissues, center the gray scale (128) on a pixel with a high CT number – Range of gray limited to hard tissues with high CT numbers – Can see subtle differences in bone; all soft tissues are generally dark – Bone window  CT Images: Bone Window Zygomaticomaxillary fracture  Iii CT: Image Creation c 404mg Learning Objectives Cone beam computed tomography – Know the similarities and differences between CT and CBCT – Understand the steps of acquisition, reconstruction, and volume presentation – Recognize artifacts on CBCT images – Know the indications and contraindications for CBCT – Recognize important anatomic landmarks  Cone Beam Computed Tomography Modification of conventional computed tomography Developed in 1980s for angiography Some similarities to CT Some important differences doctorspiller.com Cone Beam Computed Tomography: What Does It Do? Provides views of the cranium and maxillofacial complex in three planes of space Eliminates superimposition of structures Generates images with minimal distortion and magnification beaterty get  CBCT: How Does It Work? Acquisition: Capturing the image – X-ray generation and detection Reconstruction: Building the image – Voxels and pixels – Gray scale and multiplanar formatting Volume presentation: What you see on the screen – Image manipulation  CBCT: Acquisition A conical x-ray beam and detector rotate around the patient’s head Exposes approximately 400 basis images, each from a different angle Complete series of basis images constitutes the projection data Projection data captured by flat panel detector (in most units) unlike et  CBCT: Acquisition Imaging Sciences International i-CAT Carestream CS8100  CBCT: Acquisition Conical beam of x-rays rotates around head at specific kVp and mA settings Number of basis images depends on:  Frame rate (exposures/second)  Rotation path (360o or 180o)  Rotation speed doctorspiller.com Generally produces one basis image approximately at each degree of rotation  CBCT: Acquisition Difference between CT and CBCT: Beam size Narrow tan beam IEEE Conventional CT: Cone Beam CT: Fan beam (multiple in MDCT) Conical beam  CBCT: Acquisition Field of View Éenth filed Field of view (FOV) can vary from a small section of the jaws to the entire head Some units have only smaller FOVs Some units have a large FOV that can be collimated to a smaller size www.i-cat.com  CBCT: Acquisition Field of View FOV should be selected for task Small FOV: portions of one jaw; 8 x 8 cm or smaller Medium FOV: entire jaw or both jaws; 10 x 6 cm to 16 x 10 cm Large FOV: most or all of cranio- facial skeleton; larger than 16 x 10 cm www.i-cat.com  CBCT: Reconstruction Voxels The projection data is divided into thousands of 3-D voxels (volume elements) CBCT voxels are isotropic: same size on all sides, so no distortion flinin Ysgeona Voxel array Voxel jmorita.com CBCT: Reconstruction Voxels Voxel size is determined mostly by the receptor matrix Voxels are usually small (no larger than 0.4 mm on each side), smaller than CT voxels Smaller voxels make the image relatively sharp www.theendoblog.com CBCT: Reconstruction Can't Voxels dowith CT Data set can be reconstructed with voxel sizes different than the acquired sizes CBCT: Reconstruction Voxels and Spatial Resolution Spatial resolution: ability of an imaging system to distinguish closely spaced objects Measured in line pairs/mm Voxel size is only one factor in determining resolution Voxel size is NOT the same as resolution CBCT: Reconstruction Voxels Resolution also depends on: – Nominal pixel size of detector – Beam projection geometry – Detector motion blur – Patient motion – Scattered radiation from patient CBCT: Reconstruction Pixels Each voxel is represented by a 2-D pixel (picture element) CBCT image consists of thousands of small square pixels Pixel  CBCT: Reconstruction Gray Scale Each pixel is assigned a gray level proportional to amount of x-ray absorption by tissue in the voxel Lots of hard tissue = light shade Lots of soft tissue = dark shade Bit depth: number of gray shades available (usually 8-bit, or 256 shades) Need sufficient number of gray shades to produce acceptable contrast for diagnosis The one limitation Of CBCTis Softissue resolution is bad  CBCT: Reconstruction Gray Scale WARNING! “HU” tools included in many CBCT units may make you think that Hounsfield units are being created, as in a CT scan. no hounsfieldunits But they are not! CBCT has non-standardized gray shades  CBCT: Reconstruction Exposure settings, FOV, voxel and pixel size, resolution … How do we decide which exposure parameters are best for our patients? Signal vs. noise obscured image Stuff too wanna see Sadhguru.com  CBCT: Reconstruction Orthodontics and implant treatment planning for multiple sites: Large FOV – greater radiation dose but desired anatomy is visualized Larger pixel size – spatial resolution is decreased but this is not usually a problem with pixels of 0.4 mm or smaller  CBCT: Primary Reconstruction Large FOV for orthodontics: 23 x 16 cm, voxel size 0.4 mm  CBCT: Primary Reconstruction Visualization of abnormalities or TMJs: Medium FOV – produces an acceptable balance between radiation dose and resolution Medium FOV for TMJ: 10 x 8 cm, voxel size 0.3 mm  CBCT: Primary Reconstruction Endodontic applications: Small FOV – limited exposure dose to patient Small pixel size – improves spatial resolution if exposure is sufficient to overcome noise needto have a larger exposuredose to overcome mise  CBCT: Primary Reconstruction Small FOV for endodontics: 5 x 5 cm, voxel size 0.120 mm LargerFOV May not be abletoviewfracturesaswell CBCT: Reconstruction Multiplanar Reformatting CBCT data set appears on screen through multiplanar reformatting (MPR) in three orthogonal images: – Axial, sagittal, and coronal Can scroll through the images to view all planes in entire thickness – Axial (top to bottom) – Sagittal (side to side) – Coronal (anterior to posterior)  CBCT: Secondary Reconstruction Visible Image i Axial, sagittal, and coronal views together on the monitor  CBCT: Volume Presentation Since CBCT voxels are isotropic, they can be viewed in various non-orthogonal configurations: – Curved planar view aligns long axis of imaging plane with dental arch – Produces panoramic-like format – Also produces serial transplanar (cross- sectional) images at right angles to the arch for implant site assessment – MUCH BETTER than MDCT! CBCT: Volume Presentation 3-D Surface Rendering Provides shading to represent voxels at outer surface of tissue Permits more accurate distinction of buccal-lingual relation of structures by adding the surface Supplements 2-D formats for depicting bony abnormalities at surface William C. Scarfe Lingual displacement of roots  Artifacts “Who are you going to believe – me or your own eyes?” Groucho Marx Artifacts Artifact: any feature not naturally present in the subject being examined but formed by artificial means due to an extrinsic agent CBCT is prone to many types of artifacts  Artifacts Scattered radiation – X-rays that are deflected after hitting tissue – Since the sensor is generally large, these photons are recorded where they shouldn’t be This is why CBCT fails – Grainy appearance (quantum to depict soft tissue: noise) computer can’t track – Streaks and bands scattered photons to their origin in soft tissue Artifacts Streaks and bands due to scattered radiation Artifacts Partial volume averaging – Occurs when voxel is larger than the size of the object being examined – If a voxel contains soft tissue and a thin layer of bone, pixel has a gray value representing the weighted average of the tissues – Can produce a step defect appearance Artifacts Partial volume averaging in delicate bony walls of an expansile lesion S T.EE i ieii Walls of lesions appear discontinuous but are intact – just very thin! Artifacts Beam hardening – When x-ray beam passes through tissue its mean energy increases Dark areas adjacent to radiopaque material Radiopaque and radiolucent streaks Cupping of metal restorations Dark areas: beam hardening artifacts can simulate dental caries  Artifacts Beam hardening: radiolucent line adjacent to metal post, simulating root fracture 11th may Artifacts Procedure-related artifacts result from errors in technique or faulty equipment – Ring artifact – Patient motion  Artifacts Ring artifact – Caused by detector that is miscalibrated – Can result from imperfections in sensor detection  Artifacts Ring artifact Due to hiscalibrationoftheXray Source Bhoosreddy AR, Sakhavalkar PU. J Indian Acad Oral Med Radiol 2014;26:293-7  Artifacts Mostcommonartifact Patient motion – Patient may move if head is not supported and stabilized – Double edges appear – Motion unsharpness is probably the most obvious (and avoidable) artifact  Artifacts Doubleedges Signify Sagittal Patient motion double Double edge Doubleedge