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Chapter 19 & 20 Image Quality & Techniques. There are three geometric factors that affect radiographic quality. Magnification Distortion Focal Spot Blur We have explored these factors in the laboratory. Magnification.
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Chapter 19 & 20 Image Quality & Techniques • There are three geometric factors that affect radiographic quality. • Magnification • Distortion • Focal Spot Blur • We have explored these factors in the laboratory.
Magnification • All objects on the radiograph are larger that their actual size. This is called magnification. • The magnification factor is the image size divided by the object size. • At 40” (100 cm) factor is 1.1 • At 72” (180 cm) factor is 1.05
Magnification • Usually we do not know the size of the object so we must determine the magnification factor another way. • Image size S0D • MF = ----------------= ------------ • Object Size SID
Minimizing Magnification • Large SID: Use the less divergent beam. • Chest X-rays are done at 72” SID to minimize magnification. • Lateral C-spine done at 72” • Small OID: Get patient as close to the film as possible. • Basic principle for positioning.
Distortion • Distortion is the misrepresentation of the true size and shape of the object being radiographed. • The amount of distortion depends upon the thickness, position and shape.
Thickness • Thick objects are more distorted than thin objects because of the greater change in Object Image Distance.
Thickness • The position of the object relative to the central axis will cause greater distortion with thick and/or irregular shaped objects.
Object Position • If the object plane and image plane are parallel the image will not be distorted. • If the object plane and image plane are not parallel, distortion will occur.
Spatial Distortion • When multiple objects at different OID’s occur, we get spatial distortion due to unequal magnification. • Two arrows appear as one. • When shifted laterally more distortion occurs
Object Shape Distortion • When the object plane is not parallel to the image plane as when inclined, shape distortion occurs. • This will result in foreshortening.
Focal Spot Blur • Focal spot blur is caused by the effective size of the focal spot, which is larger at the cathode side. • Focal spot blur is the most important factor in determining spatial resolution.
Focal Spot Blur • Focal spot blur is impacted by the Object to Image Distance.
Focal Spot Blur Heel Effect • There is more to the heel affect than just the attenuation of the beam by the anode. • The focal spot blur is smaller at the anode side and larger at the cathode side.
Did you see a problem ? • If the tube is mounted correctly for the AP Full spine, Chest and A-P Thoracic Spine, the patient must stand on his head for the lateral thoracic spine!!!! • For erect radiography, the use of the anode heel affect is limited.
Object Factors that Affect Quality • Subject Contrast • Patient or part thickness • Tissue mass density • Affective atomic number • Object shape • kVp
Radiographic Contrast • Radiographic Contrast is how the film looks. • It is the combination of receptor contrast and subject contrast. • Contrast occurs between structures of different densities.
Thickness • The thicker the body part or body section, the greater attenuation of the beam. Contrast is relative to the number of x-rays leaving the body. • Remember that x-rays are merely shadows of the anatomy based upon attentation of the beam.
Thickness • Radiographs of thin patients will have more contrast than those of large patients. • Thicker object absorb more rays and will appear lighter than thin objects.
Tissue Mass Density • Different sections of the body have may equal thickness yet different mass density. • Two wrist may be the same thickness but the one that is swollen will have greater mass density due to water in the tissues.
Effective Atomic Number • While Compton interactions with tissues are not impacted by the relative atomic number of tissues, the photoelectric effect vary with the cube of the atomic number. • When the effective atomic number of adjacent tissues is very different, subject contrast is very high.
Object shape • The object shape not only influences the geometry but also through its contribution to subject contrast. • A vessel on end has high contrast while other have lower contrast.
Varying tissue densities • Bone absorbs most x-rays leaving a white shadow. • Water absorbs less x-rays leaving a light gray shadow • Fat absorbs fewer x-rays leaving a dark gray shadow. • Air absorbs little x-rays and is black on the film.
Varying densities in the Body • Air, oil, water and metal (natural) absorb different degrees of the x-rays and produce contrast. • Heavy metals are used as contrast media to enhance contrast in the body in medical radiology. Principle ones are Barium and Iodine.
kVp • We have little control over the previous factors controlling subject contrast. • BUT!!!!! • The absolute magnitude of subject contrast is greatly influenced by the kVp of operation.
kVp • kVp also influences film contrast but not to the extend it controls subject contrast. • Low kVp = high contrast = short scale • High kVp = low contrast = broad scale
Motion Blur • If any element of the x-ray moves during exposure, we get motion blur. • Patient motion is the most common cause of motion blur. • Motion blur is more common in erect radiography.
Ways to Control Motion Blur • Use the shortest possible exposure time • Restrict patient motion by instructions or restraining device. • Use a large SID • Use a small OID
Tools to Improve Quality • Patient Positioning • Get the patient close to the film reduce OID. • Center the beam to the area of interest. • Get the area of interest parallel to the beam or film. • Restrain motion and communicate with the patient. • Use short exposure times.
Tools to Improve Quality • Image receptors • Use the correct film & screen combination for the examination. Extremity for wrist. Regular for spines. • Intensifying screens reduce patient exposure at least 20 times. • As the speed of the image receptor increases, radiographic noise and contrast resolution decreases.
Tools to Improve Quality • Image receptors • Low contrast imaging procedures have wider latitude and a larger margin of error in producing acceptable radiographs. • Use the highest speed system that will provide adequate contrast and density over the entire spectrum of examinations.
Tools to Improve Quality • Technique selection • We must select the optimum technical factors. • We must get the exposure time as low as possible so high frequency machine are important. • Contrast controlled by the kVp used • Density controlled by the mAs used.
Tools to Improve Quality • kVp has a greater influence than mAs. • Technique selection is a balancing act. Balancing contrast, density and exposure.
Chapter 20 Radiographic Technique • Several factors influence the selection of technical factors. The primary factors that impact exposure and image quality are: • SID • mAs • kVp
Patient Factors • The anatomic thickness and body composition greatly impact the technical factors. • The technique chart is based upon the Sthenic Body Type.
Patient Factors • Sthenic is strong & active • Hyposthenic is thin but healthy • Hyperstenic is obese • Astenic is small, frail sometime emaciated, and often elderly
Patient Thickness • The thickness of the patient should not be guessed. • It should be measured with calipers. • Patient thickness is measured in cm.
Body composition • The type of tissue in the area of exposure will impact the technical factors. • The tissue types in the chest are different from the abdomen. • Disease processes will also impact the exposure factors. Obtaining a good clinical history is important. History must be communicated to the radiographer.
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Image Quality Factors • Image quality factors include • OD • Contrast • Image Detail • Image Distortion • OD is the optical density or radiographic density. OD is controlled by the mAs and SID.
Optical Density • Numerically low OD is a low number like 0.25. • Dark is a high number like 2.20 to 4.0 • Light is underexposed • Dark is over exposed • If density is the only factor that needs to be changed, change the mAs.
Optical Density • A 30% change in mAs is needed to make a perceptible change in optical density. • Usually when a change in optical density is needed, the mAs is either doubled or halved. • kVp must be changed by 4% to produce the same change in optical density. • Changing kVp will also impact penetration and contrast.
30 - 50 Rule • If the film is under exposed, double the mAs. • If the film is over exposed, cut the mAs in half. • If the film is slightly underexposed, increase the mAs 30%. • If the film is slightly overexposed, reduce the mAs 30%.
30% Density Change • The lower image was the first image taken. It was dark but normally would be acceptable. • The top image was the mAs reduced 30%. The air fluid levels in the sinus is easier to see.
15% Rule • The OD can be changed with kVp but it will also impact exposure and contrast also. • Increase of 15% in kVp is equal to cutting the mAs in half. • Decrease of 15% in kVp is equal to doubling the mAs. • If the film is underexposed, increase kVp 15%. • If the film is overexposed, decrease kVp 15%.
Contrast • The function of contrast is to make the anatomy more visible. • Contrast is the difference in density of adjacent structures. • The relative penetrability of the x-ray through different tissues determines the image contrast.
Contrast • Contrast can be measured as the Gray Scale of Contrast. It is the range of optical density from white to black on the image. • Contrast is controlled by kVp.
Adjusting Contrast with 15% Rule • An 15% increase in kVp and a reduction of mAs by 50% will produce the same OD but lower contrast. • Used to reduce exposure or reduce exposure time/ • An 15% decrease in kVp and doubling the mAs will produce the same OD but higher contrast.
Image Detail • The sharpness of image detail refers to the ability to see structural lines or borders of tissue in the image. • The visibility of image detail is best measured by the contrast resolution. • The geometric factors of focal spot selection, SID and OID will impact sharpness.
Image Detail • Visibility of image detail is impacted by factors such as image fog. • Scatter radiation reduces the ability to visualize lines of detail. • Light fog or processing can impact the visibility of structures. • Collimation, screen combination and the use of a grid are other factors that impact image detail.