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Radiographic Quality
Visibility and Sharpness
By Prof. Stelmark
A primary responsibility of the radiographer is to evaluate radiographic images to
determine whether sufficient information exists for a diagnosis. Evaluating
radiographic quality requires the radiographer to assess the image for both its
visibility of recorded detail (photographic properties) and its sharpness of recorded
detail (geometric properties). Radiographic quality is the combination of both the
visibility and the sharpness of recorded detail.
Brightness
Photographic Properties (Visibility)
Photographic properties (visibility factors) of recorded detail are determined by
the extent to which the structural components of the anatomic area of interest
can be seen on the recorded image.
Visibility of the recorded detail is achieved by the proper balance of
radiographic density and radiographic contrast.
Radiographic Density
Radiographic density is the amount of overall blackness produced on the image
after processing. A radiograph must have sufficient density to visualize the
anatomic structures of interest. A radiograph that is too light has insufficient
density to visualize the structures of the anatomic part . Conversely, a radiograph
that is too dark has excessive density, and the anatomic part cannot be well
visualized.
The ability to determine when a radiograph is unacceptable as a result of either
insufficient or excessive density requires knowledge of the radiographic factors
and clinical experience.
If a radiograph is deemed unacceptable, the radiographer must determine what
factors contributed to the density error. Knowledge about the factors that affect the
density on a radiograph is critical to developing effective problem-solving skills.
Factors that directly affect density are identified as controlling factors, whereas
factors that indirectly affect density are identified as influencing factors
Controlling Factors
Milliamperage
Exposure time
Influencing Factors
Kilovoltage
Distance
Grids
Film-screen speed
Collimation
Anatomic part
Anode heel effect
Reciprocity law
Generator output
Filtration
Film processing
Exposure Factors and Digital Imaging
The relationship among the exposure factors of mAs, kVp, and SID and their
effect on the intensity of radiation reaching the image receptor holds true for digital
imaging.
It has been stated that exposure errors ±50% can be adequately adjusted during
digital image processing. Exposure errors beyond ±50% can be adjusted, but the
quality of the image may be sacrificed and the patient overexposed.
It is important for radiographers to select exposure factors that produce optimal
quality images regardless of whether film-screen or digital image receptors are
used. Selecting appropriate exposure factors ensures production of a quality
image that provides the maximum amount of information needed for diagnosis
with the least amount of exposure to the patient.
Excessive Radiation Exposure and Digital Imaging
Although the computer can adjust for exposure errors in digital imaging,
routinely using more radiation exposure than needed will increase the
radiation dose to the patient unnecessarily.
Radiographic Contrast
Radiographic contrast is a photographic factor that also affects the visibility of
recorded detail. Contrast is the degree of difference between adjacent
densities. The ability to distinguish between densities enables differences in
anatomic tissues to be visualized. When the absorption characteristics of an
object differ, the image presents with varying densities. In tissues where the
absorption characteristics differ, recorded detail is best visualized when
contrast is optimized for the area of interest.
Radiographic contrast is the combined result of two categories: film (image
receptor) contrast and subject contrast. Film, or image receptor, contrast is a
result of the inherent properties manufactured into the type of film and how it is
radiographed (direct exposure or with intensifying screens), along with the
processing conditions. Subject contrast is a result of the absorption
characteristics of the anatomic tissue radiographed and the level of kilovoltage
used.
Subject Contrast
Film (Image Receptor) Contrast
Kilovoltage
Film type
Tissue composition
Direct exposure or intensifying screens
Contrast medium
Processing conditions
CONTROLLING FACTOR
Kilovoltage is considered the controlling factor for radiographic contrast (Box 4-6).
The quality or penetrating power of the x-ray beam has the most direct effect on
controlling the desired level of contrast.
Altering the penetrating power of the x-ray beam affects its absorption and
transmission through the anatomic tissue being radiographed. High kilovoltage
increases the penetrating power of the x-ray beam and results in less absorption,
more transmission, and fewer density differences in the anatomic tissues.
Controlling Factor
Kilovoltage
Influencing Factors
Grids
Collimation
Object-to-image receptor distance
Anatomic part
Contrast media
Processing
DIGITAL IMAGING
Kilovoltage remains an important exposure factor for providing adequate
penetration of the anatomic part. However, altering contrast to best visualize
the area of interest occurs during digital image processing and is less
dependent on the actual kVp selected. In addition, contrast can be further
manipulated once the digital image is displayed on a computer monitor.
Influencing factors such as grids, contrast media, and the composition of the
anatomic part will still affect the subject contrast created on the digital
image. It is important to note that image receptors used in digital imaging
are more sensitive to scatter radiation than film-screen receptors.
Scatter Radiation and Digital Imaging
Digital imaging receptors are more sensitive to scatter radiation than filmscreen receptors. Efforts must be routinely made to limit the amount of
scatter radiation reaching the digital image receptor.
Analog System
Radiographic Variables
Density
Contrast
↑ mAs
↑
0
↓ mAs
↓
0
↑ kVp
↑
↓
↓ kVp
↓
↑
↑ Grid ratio
↓
↑
↓ Grid ratio
↑
↓
↑ Film-screen speed
↑
0
↓ Film-screen speed
↓
0
↑ Collimation
↓
↑
↓ Collimation
↑
↓
Digital System
Radiographic Variables
Density
Contrast
↑ mAs
0
0
↓ mAs
0
0
↑ kVp
0
↓
↓ kVp
0
↑
↑ Grid ratio
0
↑
↓ Grid ratio
0
↓
↑ Film-screen speed
0
0
↓ Film-screen speed
0
0
↑ Collimation
0
↑
↓ Collimation
0
↓
Geometric Properties (Sharpness)
The geometric properties of a film-screen image refer to the sharpness of
structural lines recorded in the radiographic image. A radiographic image
cannot be an exact reconstruction of the anatomic structure. Some information
is always lost during the process of image formation. It is the radiographer's
responsibility to minimize the amount of information lost by accurately
manipulating the factors that affect the sharpness of the recorded image.
Optimal geometric quality is achieved by maximizing the amount of recorded
detail and minimizing the amount of image distortion
Recorded Detail
Recorded detail refers to the distinctness or sharpness of the structural lines
that make up the recorded image. The ability of a radiographic image to
demonstrate sharp lines will determine the quality of the recorded detail. The
imaging process makes it impossible to produce a radiographic image without
some degree of unsharpness. A radiographic image that has a greater amount
of recorded detail will minimize the amount of unsharpness of the anatomic
structural lines. The amount of recorded detail is controlled by minimizing
geometric unsharpness and receptor unsharpness and by eliminating motion
unsharpness.
GEOMETRIC UNSHARPNESS
The amount of geometric unsharpness is a result of the relationship among the
size of the focal spot, SID, and OID
IMAGE RECEPTOR UNSHARPNESS
The type of device used to record the image also affects the amount of
unsharpness recorded in the image. In conventional radiography, various
intensifying screen-film combinations have created a complex system of image
receptors. Variations in the construction and composition of the intensifying
screen combined with different types of radiographic film affect not only the
photographic properties of the image but also its geometric properties.
MOTION UNSHARPNESS
Motion unsharpness has the most detrimental effect on the recorded detail of the
radiographic image. Motion of the tube, part, or image receptor causes a
profound decrease in recorded detail. Motion must not just be decreased; it must
be eliminated.
Distortion
Distortion results from the radiographic misrepresentation of either the size
(magnification) or shape of the anatomic part. When the image is distorted,
recorded detail is also reduced.
SIZE DISTORTION (MAGNIFICATION)
The term size distortion/magnification refers to an increase in the object's image
size compared with its true, or actual, size. Radiographic images of objects are
always magnified in terms of the true object size. The distances used (SID and
OID) play an important role in minimizing the amount of size distortion of the
radiographic image.
SHAPE DISTORTION
In addition to size distortion, objects that are being imaged can also be
misrepresented radiographically by distortion of their shape. Shape distortion
can appear in two different ways radiographically: elongation or
foreshortening. Elongation refers to images of objects that appear longer than
the true objects. Foreshortening refers to images that appear shorter than the
true objects
Radiographic
Variables
Density
Contrast
Recorded Detail
Distortion
↑mAs
↑
0
0
0
↓mAs
↓
0
0
0
↑kVp
↑
↓
0
0
↓kVp
↓
↑
0
0
↑SID
↓
0
↑
↓
↓SID
↑
0
↓
↑
↑OID†
↓
↑
↓
↑
↓OID
↑
↓
↑
↓
↑Grid ratio
↓
↑
0
0
↓Grid ratio
↑
↓
0
0
↑Film-screen speed
↑
0
↓
0
↓Film-screen speed
↓
0
↑
0
↑Collimation
↓
↑
0
0
↓Collimation
↑
↓
0
0
↑Focal spot size
0
0
↓
0
↓Focal spot size
0
0
↑
0
↑Central ray angle
↓
0
↓
↑