Radiology Chapter 3
Terms
undefined, object
copy deck
- short scale contrast films
- high-contrast films, appear mainly in black and white, 65 kVp, may not reveal early pathologic changes
- object density
- teeth, bone, soft tissue- determined by the structure of the object being radiographed
- film density
- the degree of blackness on a film
- factors that influence detail
- size of the tube focal or target area, FFD, OFD, movement of patient, film or tube, type of intensifying screen(if used) and image contrast
- contrast
- the difference in the degrees of blackness on the film between adjacent areas
- paralleling technique
- the film is held parallel to the long axis of the tooth, results in an increased OFD in most areas of the mouth, to compensate, use a increased FFD
- size of tube focal area
- the smaller the focal area at the anode, the better the image detail will be, the heat produced limits how small the focal area can be
- how is the focal area in the tube tilted
- usually at an angle of 20 degrees to the cathode
- penumbra
- the unsharpness or blurring that surrounds the edge of a radiographic image
- maximum FFD
- the distance between the focal spot at the anode and the film in the patients mouth, the maximal distance allows the more parallel rays from the center of the x-ray beam to strike the object and the film
- an ideal radiograph can be made by
- establishing a maximum FFD, determining a minimal OFD and positioning the object and the film parallel to each other in their long axes and the central ray perpendicular to both
- image detail
- the visual quality of a radiograph that depends on definition or sharpness, measured in line pairs per millimeter
- most common FFDs
- 8, 12 and 16 inches
- long scale contrast films
- low contrast films, many gray tones, 90 to 100 kVp, early detection of bone loss and incipient decay
- minimal OFD
- the tooth and the film should be as close together as possible- the closer they are, the less enlarged the image is on the film
- inverse square law
- an expression of the relationship between the exposure time and FFD. " the intensity of radiation is inversely proportional to the square of the distance between a point source and the irradiated surface "
- object contrast is determined by
- thickness of the object, density of the object, chemical composition of the object, quality of the x-ray beam and scatter radiation
- umbra
- the sharp area of a radiographic image
- best FFD choice
- 16 inches
- bisecting-angel technique
- the film is held as close to the tooth as possible, the central x-ray is directed vertically perpendicular at an imaginary line that bisects the angle formed by the long axis of the tooth and the film packet
- FFD that may cause magnification
- 8 inches or less
- how to keep the penumbra as small as possible
- using a small focal spot, angulation of the target, an increased FFD and a small OFD