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Questions and Answers
List three image qualities which are directly affected by the source-to-image distance (SID).
Exposure level, Magnification, Sharpness of recorder detail.
The source-to-image distance (SID) has come to be known by many as the fourth of the '---------- factor of radiography,' along with mA, time and kVp.
prime
A red plus-sign or 'X' is engraved in the anode end of the x-ray tube housing to mark the exact location of the ---------- ----------.
focal spot
When a tape measure is not available on a mobile procedure, one way to estimate the SID is to use the human 'wingspan' from fingertip to fingertip, which for a 6-feet (2 m) tall person measures about ------ inches (cm).
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It is generally desirable to use the ---------- feasible SID.
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At 72" (180 cm) distance, magnification of the heart is only about ----- %, which is statistically negligible.
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Dedicated chest units, however, can have a locked-in SID of ------ inches because chest techniques tend to use very low mAs values.
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When the x-ray tube is moved farther away from the patient and image receptor, the amount of penumbra produced at the edges of the image -----------.
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The longer the SID, the ---------- the recorded detail in the image.
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In reality, it is the --------- distance that is directly responsible for these sharpness effects, but changing the SID always changes this too.
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All other factors equal, increasing the SID ---------- magnification of the image size.
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Since for most radiographic applications, magnification is undesirable, the goal is generally to use the ----------- feasible SID.
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In cases where there is good separation between the overlying anatomy and the anatomy of interest, intentional use of a ----------- SID can both blur and magnify the obscuring anatomy.
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When a 30-inch (76-cm) SID is used, technique should be compensated by cutting the mAs to approximately ----------, as compared to a 40-inch technique.
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There is a direct relationship between SID and shape distortion.
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Just like forces such as gravity, magnetism, electrical force and all kinds of radiation, x-rays also follow the ----------- law as they spread out.
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With digital image processing, this loss of exposure represents a loss of original data that can be fed into the computer, and can quickly lead to visible image noise in the form of ----------- ---------- in the image.
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For digital imaging systems, it does not take a very great loss of ----------- for mottle to become apparent in the image.
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Any change greater than ------ % in the SID should be compensated for by adjusting the mAs.
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Distance changes are normally compensated for by using ---------, although the kVp can be adapted to make the correction by applying the 15% rule.
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From a 40" technique, since the 80" distance would require two doublings in technique, 60 inches SID will require --------- doubling of the mAs.
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From a 40" technique, a 30" distance would require ----------- the mAs.
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The relationship between a 40-inch (100 cm) technique and a 72-inch (180 cm) technique is a factor of ---------.
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When a 72-inch (180 cm) chest technique must be adapted for a supine trauma patient on a stretcher- with the x-ray tube 40 inches above the gurney, use -------- the mAs from a 72-inch technique.
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SID is a factor impacting subject contrast in the image.
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Decreasing the SID ---------- penumbra.
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Image sharpness is directly proportional to which radiographic distance?
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Study Notes
Source-to-Image Distance (SID) Impact on Radiography
- SID directly affects image quality through exposure level, magnification, and sharpness of recorded detail.
- Known as the "fourth prime factor" in radiography alongside mA, time, and kVp.
- A red plus sign or "X" on the x-ray tube housing indicates the location of the focal spot.
Estimating SID
- In the absence of a tape measure, the "wingspan" of a 6-foot tall person (approximately 72 inches or 180 cm) can be used to estimate SID.
- Maximum feasible SID is generally preferred for improved image quality.
Magnification and Sharpness
- At 72 inches (180 cm), heart magnification is about 3%, which is statistically negligible.
- Dedicated chest units often use a fixed SID of 96 inches for low mAs techniques.
- Moving the x-ray tube further from the patient decreases penumbra at the image edges, enhancing detail.
Relationship Between SID and Image Characteristics
- Longer SID results in sharper recorded detail.
- SOD (Source-to-Object Distance) is the primary determinant of sharpness, and changes in SID affect it as a result.
- Increasing SID reduces image size magnification, while maximum SID usage is the objective due to undesirable magnification.
Shorter SID Usage
- In situations requiring separation of overlying anatomy, using a shorter SID can intentionally blur and magnify obscuring anatomy.
Technique Adjustments for SID Changes
- Adjusting mAs is vital when changing SID; a 30-inch (76 cm) SID requires halving the mAs compared to a 40-inch SID.
- No direct relationship exists between SID and shape distortion.
- X-rays adhere to the inverse square law as they disperse, similar to other forces.
Digital Imaging Considerations
- Loss of exposure in digital systems can lead to image noise manifested as quantum mottle.
- Mottle is noticeable even with minor exposure losses.
- Adjustments to mAs should be made for any over 10% change in SID.
Doubling mAs for Distance Variations
- Transitioning from a 40-inch technique to an 80-inch distance necessitates doubling the mAs; similarly, a 60-inch SID requires 2x mAs.
- Conversely, transitioning to a 30-inch distance from a 40-inch technique warrants halving the mAs.
Comparison of Techniques
- The relationship between 40-inch (100 cm) and 72-inch (180 cm) techniques is a factor of 3x, indicating the required increase in mAs.
- When adapting a 72-inch technique for a supine trauma patient, use one-third of the original mAs.
SID and Subject Contrast
- SID is not a determining factor affecting subject contrast in images.
- Decreasing SID increases penumbra, which can degrade image sharpness.
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Description
This quiz focuses on Chapter 21 of 'Radiography in the Digital Age', highlighting the key aspects of source-to-image receptor distance (SID). It covers the impact of SID on image quality, including exposure level, magnification, and sharpness. Perfect for students looking to reinforce their understanding of this critical topic.
