Radiography in the Digital Age Chapter 21

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).

72 (180 cm) Signup and view all the answers

It is generally desirable to use the ---------- feasible SID.

maximum Signup and view all the answers

At 72" (180 cm) distance, magnification of the heart is only about ----- %, which is statistically negligible.

3 Signup and view all the answers

Dedicated chest units, however, can have a locked-in SID of ------ inches because chest techniques tend to use very low mAs values.

96 Signup and view all the answers

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 -----------.

diminishes Signup and view all the answers

The longer the SID, the ---------- the recorded detail in the image.

sharper Signup and view all the answers

In reality, it is the --------- distance that is directly responsible for these sharpness effects, but changing the SID always changes this too.

SOD Signup and view all the answers

All other factors equal, increasing the SID ---------- magnification of the image size.

reduces Signup and view all the answers

Since for most radiographic applications, magnification is undesirable, the goal is generally to use the ----------- feasible SID.

maximum Signup and view all the answers

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.

shorter Signup and view all the answers

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.

one-half (1/2) Signup and view all the answers

There is a direct relationship between SID and shape distortion.

False Signup and view all the answers

Just like forces such as gravity, magnetism, electrical force and all kinds of radiation, x-rays also follow the ----------- law as they spread out.

inverse square Signup and view all the answers

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.

quantum mottle Signup and view all the answers

For digital imaging systems, it does not take a very great loss of ----------- for mottle to become apparent in the image.

exposure Signup and view all the answers

Any change greater than ------ % in the SID should be compensated for by adjusting the mAs.

10 Signup and view all the answers

Distance changes are normally compensated for by using ---------, although the kVp can be adapted to make the correction by applying the 15% rule.

mAs Signup and view all the answers

From a 40" technique, since the 80" distance would require two doublings in technique, 60 inches SID will require --------- doubling of the mAs.

2x Signup and view all the answers

From a 40" technique, a 30" distance would require ----------- the mAs.

1/2 Signup and view all the answers

The relationship between a 40-inch (100 cm) technique and a 72-inch (180 cm) technique is a factor of ---------.

3x Signup and view all the answers

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.

one-third (1/3) Signup and view all the answers

SID is a factor impacting subject contrast in the image.

False Signup and view all the answers

Decreasing the SID ---------- penumbra.

increases Signup and view all the answers

Image sharpness is directly proportional to which radiographic distance?

SOD Signup and view all the answers

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.