What Is Radiography Image Quality.
The radiographic image quality refers to the ability of the field with which the anatomical structures that are being examined is rendered on the film. The radiographic image is not easy to define. The number of factors affect radiographic film quality.
The radiographic image quality is depends up on the following characteristics.
A) Resolution.
B) Contrast.
C) Noise.
D) Geometry factor.
A) Resolution:- The resolution is the ability to image two separate objects and visually distinguish one from the other. There are three types of resolution.
a) Spatial resolution..
b) Contrast resolution.
c) Temporal resolution.
a) Spatial resolution:- The spatial resolution is the ability to image small object that have high subject contrast. Such as bone soft tissue interface, a breast microcalsification. The function of spatial resolution is
– Point spread function.
– Line spread function.
– Edge spread function.
– Modulation transfer function.
b) Contrast resolution:-The contrast resolution is the ability to distinguish anatomical structures of similar subject contrast. Eg liver spleen.
c) Temporal resolution:- The Temporal resolution is the ability of the imaging system to localize the object in time from frame to frame and follow it’s movement. It is high for fluoroscopy.
B) Contrast:- The radiographic contrast viewed on an illumination is called radiographic contrast The contrast means the difference in density between two area of film. The contrast may be
a) Subject contrast.
b) Radiographic contrast.
c) Subjective contrast.
a) Subject contrast:- The subject Contrast refers, the difference in x-ray intensity of transmitted through different parts of body is called subject contrast. The thinner and the thicker body parts attention radiation capacity is different. The subject Contrast is proportional to the relative number of transmitted. The subject contrast depends on the following components.
– Patient thickness.
– Tissue density.
– Effective atomic number of tissue.
– Kilovoltage (Kvp).
– Contrast media.
– Scattered radiation.
b) Radiographic Contrast:- The various degree of brightness when light is transmitted through film is called radiographic film, which provides anatomical details in film. The radiographic contrast depends on the following parameters.
– Subject contrast
– Scattered radiation
– Intensifying screens.
– Film fog.
– Development.
– Film density.
– Types of film.
c) Subjective contrast:- The subjunctive contrast depends upon following parameters.
– Subject contrast.
– Radiographic contrast.
– The observer.
– Viewing boxes.
– Ambient light.
C) Noise:- The noise refers that is borrowed from electrical engineering. The radiographic noise is inherent in the image system. The low noise is a better radiographic image because it improves contrast resolution. There are four radiographic noise components are present. Films graininess, Structure mottle , Quantum mottle, Scatter radiation.
Film graininess refers to distribution in size and space of silver halides graininess in emulsion. The structure mottle is similar to film graininess but the structure mottle refers to the phosphor of the radiographic intensifying screen. The film graininess and structure mottle are inherent in in the screen film image receptor.
They are not under control the radiographer / technologist.
Quantum mottle refers to the random nature by which x-ray inherent with the image receptor. If the image is produced with few a x-ray, the quantum mottle will be higher then if the image formed from a large number of x-rays. The use of very fast intensifying screen results in increased quantum mottle. The quantum mottle is control under the radiographer / Technologist.
D) Geometric factors:-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 .
a) Magnification.
b) Distortion.
c) Focal spot blur.
a) 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.
b) 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 :-
1) Object thickness.
2) Object position.
3) Object shape.
1) 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.
2) 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.
3) 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.
C) 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.