TOPIC: PRINCIPLE OF PROJECTION
🌟 IMPORTANT POINTS TO REMEMBER
🔹Image sharpness: It is the degree with which how well a boundary between two differing radiodensities is delineated.
🔹Image resolution: It is the degree with which how well a radiograph records separate objects that are close together.
🔹Anode: It is having 20° inclination which makes effective focal spot of 1 × 1 mm of actual 1 × 3 mm size.
🔹Small effective focal spot should be used as practically possible.
🔹Dimensional distortion: Image shape distortion is the unequal magnification of different parts of the same object, also called dimensional distortion.
🔹Foreshortening: If the film is not placed parallel to the object and the central ray is perpendicular to the film only, the resultant image will be recorded shorter than its actual size; this type of dimensional distortion is called foreshortening of the image.
🔹Elongation: If the film is not placed parallel to the object and the central ray is perpendicular to the object only, the resultant image will be recorded as elongated than its actual size; this type of distortion is called elongation of the image.
🔹Overlapping: If film and object are placed parallel to each other but the central ray is not perpendicular to either of them, the geometric distortion will result in overlapping of adjacent structures.
📌 IDEAL POSITIONING REQUIREMENT
💡The tooth under investigation and the film packet should be in contact or as close as possible.
💡The tooth and the film packet should be parallel to one another.
💡The film packet should be positioned with its long axis vertically for the incisor and canine and horizontally for premolar and molar, sufficiently beyond the apices to record the apical tissue.
💡The X-ray tube head should be positioned such that the central beam is perpendicular to the film and object in both vertical and horizontal planes.
💡The positioning should be reproducible.
📌 SIX RULES FOR IMAGE FORMATION
💡Rule 1: The source of radiation (effective focal spot) should be as small as possible.
💡Rule 2: The distance between the object and the film (object–film distance) should be as small as possible.
💡Rule 3: The object and the film should be parallel to each other.
💡Rule 4: The source distance should be as long as possible.
▪️ Rule 4a: Focal spot-to-object distance
▪️ Rule 4b: Focal spot to film distance (target film distance)
💡Rule 5: The central ray must strike the tooth, object and film at right angles.
💡Rule 6: There should be no movement of the tube, film or patient during exposure.