IMAGE QUALITY
Image Quality refers to the exactness of representation of the patient’s anatomy on the image.
Three Major Factors a radiographer should understand are the following:
1. Film Factors
2. Geometric Factors
3. Subject Factors
Radiographic Quality
Refers to the fidelity with which the anatomic structure being examined is imaged in the radiograph. A radiograph that faithfully reproduces structure and tissues is identified as a high- quality radiograph. A radiologist needs a good- quality radiograph to prevent reexamination and missed diagnoses.
Factors Affecting Quality of the Radiograph
1. Spatial Resolution
2. Contrast Resolution
3. Noise
4. Artifacts
Resolution
Is the ability to image two separate objects and visually distinguish one form the other.
Spatial Resolution
Refers to the ability to image small objects that have high subject contrast, such as bone- soft tissue interface, a breast micro calcification, or a calcified lung nodule. This improves as screen blur decreases, motion blur decreases, and geometric blur decreases.
Contrast Resolution
Is the ability to distinguish anatomic structure of similar subject contrast such as liver- spleen and gray matter- white matter.
Detail of Recorded Detail
Terms sometimes used in spatial and contrast resolution.
Visibility of Detail
Refers to the ability to visualize recorded detail when image contrast and optical density are optimized.
Noise
Refers to the random fluctuation in he optical density of the image
Two Kinds of Noise
1. Audio Noise
2. Video Noise
Four Components of the Radiographic Noise
1. Film Graininess- refers to the distribution in size and space of the silver Halide grains in the emulsion.
2. Structure Mottle- is similar to film graininess but refers to the phosphor of the radiographic intensifying screen. This structure mottle and film graininess is inherent in the image receptor and are under the control of the radiologic technologist.
3. Quantum Mottle- this is somewhat under the control of radiologic technologist, this refers to the random nature in which x-rays interact with the image receptor.
Speed
Refers to the sensitivity of the screen- film combination. This influences the spatial and contrast resolution and radiographic noise.
Radiographic Quality Rules
1. Fast IR have high noise and low spatial resolution and contrast resolution.
2. High spatial resolution and contrast resolution require low noise and slow IR.
3. Low noise accompanies slow IR with high spatial resolution and contrast resolution.
Film Factors
Studies to Test the Effectiveness of the Film
1. Sensitometry Test
This is done first; the film under investigation is exposed- flashed- through the sensitometer. When processed, the film will have areas of increasing OD corresponding to optical wedge steps. The sensitometer is fabricated so that the relative intensity of light under each step can be determined.
2. Densitometry Test
This is done first; a light sensing device is positioned on the opposite side of the film. The film is positioned between the pinhole and the light sensor and the amount of light transmitted through each step of the radiographic image is measured.
Characteristic Curve
Relationship between the OD and radiation exposure. This is also called the “H & D Curve”
Two Parts of H & D Curve
1. Toes- contains underdeveloped images
2. Shoulder- shows overdeveloped images
Optical Density
The degree of blackness of a radiograph. This can be expressed in OD= log10 (Io/ It). Most unexposed and processed film has an OD in the range of 1.1 to 0.3. These OD’s are due to base and fog density. The base density is inherent in the base of the film. It is due to composition of the base and the tint added to the base. Base density has a value of 0.1 or 0.05 OD. Fog density results from inadvertent exposure of the film during storage, handling and undesirable chemical contamination. Fog density in the radiograph should not be exceeding to 0.2 mR or 0.05 mR.
The useful range of OD is approximately 0.25 to 2.5.
Reciprocity Law
States that the OD on a radiograph is proportional only to the total energy imparted to the radiographic film.
Contrast
Refers to the OD variations when the radiograph is viewed in the view box.
Two Factors the Produce Contrast
1. Image receptor contrast- is inherent in the screen- film combination and influenced somewhat by processing of the film.
2. Subject contrast- determined by the size, shape, and x-ray attenuating characteristics of the anatomy being examined and the energy of eh x-ray beam.
Average Gradient
A method use to specify image receptor contrast, it is the slope of a straight line drawn between the two points on the characteristic curve at OD’s 0.25 and 2.0 above base and fog density
Toe Gradient- is probably more important than the average gradient.
Shoulder Gradient- is more important in mammography.
Latitude
Refers to the range of technique factor to produce an acceptable image. This is inversely proportional to the contrast.
Film Processing
Is required for optimal image receptor contrast because the degree of development has a pronounced effect on the level of fog density and on the ODs resulting from a given exposure at a given IR speed.
Factor Affecting the Finished Radiograph
1. The concentration of processing chemicals.
2. The degree of chemistry agitation during development.
3. The development time.
4. The development temperature.
Geometric Factor
Magnification
The increase of size of the image in the radiograph. This is expressed in Magnification Factor (MF).
MF= IMAGE SIZE/ SUBJECT SIZE and MF= SID/ OID
Magnification Radiography- an examination where magnification is required.
Radiographs taken at 100 cm SID, the MF is approximately 1.1 and for radiographs taken at 180 cm SID, the MF is approximately 1.05.
Two Factors Affecting Magnification
1. SID
2. OID
How to Minimize Magnification
1. Use large SID.
2. Use small OID.
Distortion
Refers to the unequal magnification of different portions of the same object.
Factors Contributing Distortion
1. Object Thickness
Thick objects are more distorted than thin objects. With a thick object, the OID changes measurably across the object.
2. Object Position
If the object plane and the image parallel, the image is not distorted.
Foreshortening- occurs when the image of an inclined object is smaller than the object itself.
Elongation- the severe foreshortening of an inclined object.
3. Object Shape
Focal- spot blur
This occurs because the focal- spot is not a point. This is a phenomenon where radiographers have a little control. This is the most important factor in determining the spatial resolution and is undesirable.
Focal- spot blur= (effective focal spot) OID/ SOD
Heel Effect
This is the disadvantage of the line- focus principle where the radiation intensity on the cathode side is greater than on the anode side.
Subject Factors
This is concerned with the patient.
The following are the Subject contrast:
1. Subject Contrast- a contrast of a radiograph viewed in an illuminator, it is expressed in Radiographic contrast= film contrast x subject contrast
2. Patient thickness- the thicker the body section, the more x-rays it attenuates. The degree of subject contrast is directly proportional to the relative number of x-rays leaving those sections of the body.
3. Tissue Mass density- it is an important factor affecting the subject contrast
4. Effective Atomic Number- this affects the subject contrast, when the effective atomic number of adjacent tissues is very much different, the subject contrast is very high.
5. Object Shape- it influence the radiograph quality not only through its geometry but also through its contribution to subject contrast.
Absorption blur- a characteristic of the subject to affect the subject contrast. It reduces both spatial and contrast resolution.
6. kVp- this is under the control of the rad.tech. and this also influences the film contrast but not to the extent that it controls subject contrast.
Motion blur
The loss of radiographic quality due to the movement of patient or x-ray tube during the exposure.
Procedures for Reducing the Motion Blur
1. Use the shortest possible exposure time.
2. Restrict patient motion by instruction or restraining device.
3. Use a large SID.
4. Use a small OID.
Tools for Radiographic Quality
Patient Positioning
Proper patient positioning requires that the anatomic structure under investigation be placed as close to the image receptor as is practical and that the axis of this structure lie in a plane parallel to the plane of the image receptor. The various techniques applied to radiographic positioning are designed to produce radiographs with minimal image distortion and maximum resolution.
Image Receptors
Principles to be considered when planning a particular examination:
1. Use of IS decreases patient dose by a factor of at least 20.
2. As the speed of the IR increases, radiographic noise increases and spatial resolution is decreased.
3. Low- contrast imaging procedures have wider latitude, margin of error, in producing acceptable image.
Selection of Technique Factor
The rad.tech. is responsible for this in applying an optimal factors to produce a good quality radiograph and low patient dose. As short as possible, use a shorter time of exposure to decrease patient dose. The primary control of radiographic contrast is the kVp and the primary control of the OD is the mAs.
To produce a good quality radiograph, the radiographer must be aware of the effects of every technique factor to prevent from over exposure of radiation.
