Concepts of Panoramic Radiography Theory of Panoramic Imaging

1. Concepts of Panoramic RadiographyTheory of Panoramic Imaging DHY 202 Clinical Radiology I Dental Hygiene Department William Rainey Harper College

2. Panoramic Imaging : General Principles • Employs scanography (slit beam) & tomography

3. Panoramic Imaging : General Principles • Tomography: allows radiographing in one plane of an object while blurring or eliminating images from structures in other planes. • “Tomo” is Greek for section • View sections or radiographic slices

4. Panoramic Imaging : General Principles • Tomography • used extensively in medicine • basis for CT (computed tomography) & MRI (magnetic resonance imaging)

5. Panoramic Imaging : General Principles • Tomogram • Client remains stationary while xray source & film move in opposite directions in a fixed relationship through one or a series of rotation points. Rotation points can be inside or outside of the focal trough

6. Panoramic Imaging : General Principles • Focal trough in tomogram • Or “plane of acceptable detail,” or “image layer,” is the plane that is not blurred on the radiograph

7. Panoramic Imaging : General Principles • Focal trough in pantogram • Width & thickness governed by many factors • Objects lying within the focal spot are shown clearly; objects outside are blurred

8. Panoramic Imaging : General Principles • A panoramic radiograph or pantomogram is produced using curved-surface tomography. Curved surface Flat surface

9. Panoramic Imaging : General Principles • Rotational panoramic radiography is accomplished by rotating a narrow beam of radiation in the horizontal plane around an invisible pivot point/axis positioned intraorally. • Film & tube travel in opposite directions around the client

10. Panoramic Imaging : General Principles • Client remains stationary as xray tube and film cassette-holder (which are connected) both rotate around the client

11. Panoramic Imaging : General Principles • A vertical, narrow beam is used compared with the larger, circular or rectangular beam used in conventional intraoral radiography

12. Panoramic Imaging : General Principles • The pivot point/axis is called therotation center • The center of rotation changes as the film and tubehead rotate which allows the image layer to conform to the elliptical shape of the dental arches

13. Panoramic Imaging: Projection in the Vertical Plane • Vertical dimension unaffected by horizontal rotation • Vertical angulation same as conventional intraoral projection • Slight negative angulation; passes beneath occipital area (-4 to -7 degrees)

14. Panoramic Imaging:Projection in the Horizontal Plane • Horizontal image affected by horizontal rotation of the beam • Xrays appear to diverge from intraoral source but really originate outside of the client • Apparent intraoral source is called the center of rotation

15. Panoramic Imaging:Projection in the Horizontal Plane True intraoral source & focus of projection Effective focus of projection Rotating beam projected on stationary film Uniform magnification

16. Panoramic Imaging:Projection in the Horizontal Plane Rotating beam and moving film has proportions restored • Discrepancy in horizontal versus vertical magnification eliminated by using a moving film to equalize the magnification in the horizontal dimension with the vertical • Film moves in direction opposite to the horizontal rotation of the beam

17. Panoramic Imaging: Principles of Image Layer Formation • Film placed on circular drum or a moving flat cassette • Horizontal magnification is reduced to match vertical magnification by adjusting speed of film in respect to projection of beam

18. Panoramic Imaging: Principles of Image Layer Formation • Vertical and horizontal dimensions match only when the object lies within a particular plane called the central plane or sharply depicted plane of the image layer

19. Panoramic Imaging: Principles of Image Layer Formation • The image layer is called the focal trough

20. Panoramic Imaging: Image Layer/Focal Trough Defined • A three-dimensional zone in which structures are reasonably well-defined • A zone in an object defined as containing those object points depicted with sufficient detail to be distinguished • Determines where dental arches must be positioned to achieve clearest image

21. Panoramic Imaging: Principles of Image Layer Formation • Objects outside this sharply depicted plane will appear distorted, fuzzy, or may not be visible

22. Panoramic Imaging: Width of the Image Layer • Determined by • Distance from center of rotation to central plane of image layer • Width of long, narrow slit beam (the narrower the beam , the wider the image layer)

23. Panoramic Imaging: Position of the Image Layer • Changes in film speed alter the position of the image layer • Increased film speed = image farther away from rotation center • Decreased film speed = image closer to rotation center • This is how the image layer is shaped to center the jaws (anteriors narrower)

24. Panoramic Imaging: Movement Pattern of the Xray Beam • Movement pattern of the xray beam chosen to obtain a favorable projection of the jaws • Depending on the manufacturer, number and location of rotational centers differ

25. Panoramic Imaging: Movement Pattern of the Xray Beam • Continuously moving rotation center are most popular

26. Panoramic Imaging: Image Layer Analysis • Objects closest to film will be narrowed • Objects closest or toward the source will be widened • Buccal objects projected lower • Lingual objects projected higher • Objects in the center of the layer are magnified 20-30%

27. Panoramic Imaging: Image Layer Analysis • Less definition than than intraoral film • More horizontal than vertical magnification • All objects, even those outside the focal trough are projected onto the film, but most are not seen • Objects with the greatest density are displayed in two places: intended image and ghost image