6 - Projection Geometry Flashcards
(24 cards)
The larger the focal spot area, what happens to the sharpness?
The greater the unsharpness
3 ways to maximize image sharpness:
- Use as small an effective focal spot as practical
- Increase the distance between the focal spot and the object by using a long, open-ended cylinder
- Minimize the distance between the object and the image receptor
What two things help maximize value from a radiograph?
- Clear understanding of normal anatomy
2. Mentally reconstruct a 3D view of anatomic structures of interest
define sharpness:
How well a boundary between 2 areas of differing radiodensity is revealed
Define spatial resolution
How well a radiograph is able to reveal small objects that are close together
T or F, Sharpness and Spatial Resolution are two distinct things yet are interdependent
True, They depend on each other
What is desirable to optimize imaging?
Have both high sharpness and resolution
T or F, As the object-to-image receptor distance is reduced, the zone of unsharpness increases
False, zone of unsharpness decreases resulting in enhanced image clarity
What is image size distortion
The increase in size of the image on the radiograph compared with the actual size of the object
What two things help to minimize image magnification or image size distortion?
- Increasing the focal spot-to-image receptor distance
- Decreasing the object-to-image receptor distance
*In other words the image size distortion results from the relative distances of the focal spot-to-image receptor and object-to-image receptor.
2 advantages to using the long,open-ended cylinder
- Reduces magnification of images on a periapical view
- Improves image sharpness by increasing the distance between the focal spot and object.
Image shape distortion is the result of what?
Unequal magnification of different parts of the same object.
***This situation arises when not all the parts of an object are at the same focal spot-to-object distance. This may be due to the physical shape of the object.
What 2 guidelines should be followed to minimize shape distortion?
- Position the image receptor parallel to the long axis of the object.
- Orient the central ray perpendicular to the object and image receptor.
What is foreshortening?
- It causes the radiographic image to be shorter than the object.
- When an image is distorted because of the unequal distances of the various parts of the object from the image receptor.
- *This occurs when the central ray is perpendicular to the image receptor, but the object is not parallel to image receptor.
What is elongation?
- Object appearing longer on the image receptor than its actual length
- *Occurs when the x-ray beam is orientated at right angles to the object but not to the image receptor.
What is the preferred method technique for making intraoral radiographs?
Paralleling technique
*The bisecting-angle technique has been used less frequently and the paralleling technique use has drastically increased
To achieve parallel orientation, the practitioner often must position the image receptor how?
Toward the middle of the oral cavity, away from the teeth.
*Note: this does result in some image magnification and loss of sharpness. To overcome this, the paralleling technique uses a relatively long open-ended aiming cylinder cone to increase the focal spot-to-object distance. The cone directs only most central and parallel rays of beam to teeth and image receptor.
What is object localization?
Deriving 3D information from 2D radiographic image
What two methods are used for object localization?
- Two images (views) projected at right angles to each other
- Tube shift technique - Identifies spatial position of object. (SLOB rule)
* *Also known as the buccal-object rule or Clark’s rule
The right-angle (or cross section) technique in object localization is best for what part of the mouth?
Mandible
**Maxillary occlusal view is not usually good because the features in the anterior part of the skull frequently obscures the area of interest
What is the SLOB rule
S-Same
L-Lingual
O-Opposite
B-Buccal
T or F, Position of object can be determined w/ respect to reference structures with use of the right-angle technique?
False, the Tube shift technique
Tube shift technique - If object does not move with respect to reference object when tube head is moved, object is where?
In the middle
What effect accounts for why normal structures such as the lamina dura, the border of the maxillary sinuses and nasal fossa, and abnormal structures, including the corticated walls of cysts and benign tumors, are well demonstrated on plain images?
Eggshell effect
