What Are Geometric Factors In Radiography.
The sharpness of the shadow image on the wall is a function of a number of geometric factors. The radiographic image quality depends up on the geomagnetic factors. The geometric factors also apply to the production of high quality radiographs. Three principle geometric factors affect radiographic quality are:-
1) Magnification.
2) Distortion.
3) Focal spot blur.
1) Magnification:- The radiographic image are largest the the object is called magnification. The all radiographic images are magnified. The magnification is the ratio between the image size and object size.
M = SID /SOD
Where M is magnification, SID is the source to image distance and the SOD is the source to object distance. If the object is closer to the source distance then the magnification is large. If the object moves away from the source distance then the magnification is decrease.
It is depends up on the geometric conditions. The magnification is minimize by following condition. In chest radiographic PA views are taken at 180 cm source to image distance so the magnification is unity. In mammography imaging system are designed for 50 to 70 cm source to image distance. It is short but this distance is necessary at low kVp and low radiation intensity.
2) Distortion:- The unequal magnification of different positions of the same object is called distortion. The distortion may missed the patient diagnosis. The distortion may caused by following methods :-
a) Object thickness.
b) Object position.
c) Object shape.
a) Object thickness:- The thicker objects are more distortion as compared to thin object. When the thicker object , the object to image distance changes measurably across the object. The change in object to image distance across the thicker structure, the image of the object is more distortion as compared to thinner structure.
b) Object position:- If the image plane and object plane are not parallel the distortion may be occurs. If the image plane and the object plane is parallel the images are not distortion. Distortion is possible in every radiographic examination if the patient is not properly positioned.
c) Object shape:- Shape distortion is sometimes referred to as “true distortion”. It’s defined as the misrepresentation of the shape (length or width) of an object. This is caused by beam/part alignment.
3) Focal spot blur:- A blurred region on the radiograph over which radiologic technologist have little control results because the Effective focal spot has size. The phenomenon is called focal- spot blur. It is greater on the cathode side of the image. Focal spot blur is the most important factor for determining spatial resolution. The geomagnetic factors magnification the focal spot blur. The source, object and image is produced greater magnification and increased focal spot blur. The focal spot blur should minimize for using small focal spots and position the patient so that the anatomical part under examination is close to the image receptor.