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Basic Concepts of Other Imaging Modalities Dent 5101 Body-section Radiography • A special radiographic technique that blurs out the shadows of superimposed structures • Object of interest less blurred • Does not improve the sharpness Tube and Film Move in Opposite Direction • Tube and film move in opposite direction, and rotate about a fulcrum • The level of the fulcrum is the focal plain Blurring • Determined by: – Distance of the tube travel – Distance from the focal plain – Distance from the film – Orientation of tube travel Types of Tomographic Motions Linear Circular Elliptical Figure 8 Hypocycloidal Trispiral Panoramic Radiography Panoramic Radiography • Obtained by rotating a narrow beam of radiation in the horizontal plane • The film is rotated in the opposite direction while the object (jaws) is stationary Focal Trough • A 3-dimensional curved zone or image layer in which structures are reasonably well defined. Types of Panoramic Machines • Panorex – Two centers of rotation. Interruption of exposure in the midline • Orthopantomogram – Three centers of rotation. Continuous image Panorex Image Orthopantpmograph Image Intensification Early Fluoroscopy • Early fluoroscopy done by direct observation • Screen was poorly illuminated - image perception inadequate Image Intensification • Image intensifier improved viewing of fluoroscopy Intensifier Tube • Four parts: – Input phosphor and photocathode – Electrostatic focusing lens – Accelerating anode – Output phosphor Intensifier Tube (Cont.) • Input phosphor: cesium iodide (CsI) or zinccadmium-sulfide. • Photocathode: A photo-emissive metal. • Electrostatic focusing lens: series of negatively charged electrodes—focuses the electron beam. • Output phosphor: Provides thousand-fold more light photons. Intensifier Tube • Used in: – Sialography – Arthrography Digital Imaging Digital Imaging • Conventional film-intensifying screen radiograph - analog image. • Digital radiograph—film-less. • Conventional films can be digitized, with a likelihood of loss of information. Photostimulable Phosphor (PSP) • Storage phosphor • Indirect Digital Radiography • Similar to the intensifying screen phosphors • Difference - PSP traps a significant number of electrons in its phosphor, which is later read by a laser beam Photostimulable Phosphor (PSP) Charge-coupled Devices • An amorphous silicon wafer containing an array of pixels (picture elements) • Each pixel acts as a capacitor storing charge • On radiation exposure, electric charge is deposited in the pixels proportional to the intensity of the beam • The variation in charge deposition can be digitally converted to an image Pixels • Each square (pixel) has its own color (numerical value) Pixel Array CMOS • Complimentary Metal Oxide Semiconductor • Principle similar to CCD • Simpler circuit design CCD, CMOS Resolution • Ability to identify two different objects that are close to each other • D-speed films: >10 line pair/mm • Digital images:7-10 line pair/mm • Eyes: 4-6 line pair/mm Digital Subtraction • Shows changes in an area over time • Requires two films exposed with similar geometry Digital Subtraction • Two radiographs are obtained • Identical position • One superimposed over another • Differences in two images identified digitally • Allows identifying changes in hard tissue that occurred between the two examinations Radiation Dose Full Mouth Series Receptor Collimation D E Digital D E Digital Round Round Round Rectangular Rectangular Rectangular Background equivalent 8 4 2 4 2 1 days days days days days day Image Processing/Reconstruction • To improve diagnostic accuracy • May improve the diagnosis of one disease, while obscure another • Fraud Digital Radiography: Advantages • • • • • Instant images Consistent quality High signal/noise ratio Image Manipulation Lower radiation dose Disadvantages • • • • • Lower resolution Quality depends on monitor and printer Print quality often not optimal Higher initial cost Unwanted manipulation of images