Chapter 24 – Alignment and Motion

What does misalignment in radiography refer to

It refers to both off-centering and off-angling of the beam, part, or IR.

What is the general imaging effect of misalignment

It causes shape distortion.

Where should the central ray (CR) pass when imaging a joint

It should pass parallel through the joint space.

How should the CR be aligned to the IR for proper imaging

CR should be perpendicular to the IR.

What are the two primary causes of misalignment

Off-centering and off-angling.

What happens when anatomy is off-centered in relation to the CR

It is placed into increasingly diverging beams, leading to distortion.

How much off-centering is typical in radiography

Usually limited to a few inches.

Which is more extreme: off-centering or off-angling

Off-angling can be much more extreme.

How can off-centering and off-angling cause similar problems

Both can cause distortion depending on alignment and angulation issues.

Why is beam angulation sometimes required in radiography

To de-superimpose obstructing anatomy, especially in trauma or limited ROM cases.

What is the effect of object thickness on distortion

The thicker the object, the greater the distortion.

Which objects are more distorted: spherical/cubical or flat/tubular/wedge-shaped

Spherical or cubical objects are more distorted.

Give an example of spherical or cubical structures that distort. Cranium, femoral condyles, vertebral bodies.

Give examples of flat anatomical structures with less distortion. Sternum and scapula.

Give an example of a wedge-shaped anatomical structure. The sacrum.

What type of distortion occurs when flat or tubular objects are tilted relative to the IR

Foreshortening distortion.

What distortion occurs when the CR is angled perpendicular to a tilted object

Elongation distortion.

What does Ceiszynski's Law of Isometry state

Angle the CR one-half of the angle formed between the object and IR to reduce distortion.

What happens when a coin is parallel to the IR and CR is perpendicular to the coin

A normal, undistorted image appears.

What happens when a coin is tilted 45° but CR is perpendicular to IR

The coin appears shorter (foreshortened).

What happens when a coin is tilted 45° and CR is angled perpendicular to the coin

The coin appears stretched (elongated).

What happens when a coin is tilted 45° and CR is angled 22.5°

The coin appears regular (no major distortion).

What happens when a coin is parallel to IR but CR angled 30°

The coin appears slightly stretched.

What type of distortion occurs in diverging rays of an off-centered beam

Elongation distortion.

At 40" SID, how many degrees of divergence occur per inch from the CR

About 1.7 degrees, approximated to 2° per inch.

At 72" SID, how many degrees of divergence occur per inch

About 0.9 degrees, approximated to 1° per inch.

Why is beam divergence greater at a shorter SID

Because rays spread out more over a shorter distance.

How does reducing SID affect distortion

It places the object in more divergent beams, increasing distortion.

Why must SID be adjusted when angling the CR

Angling increases the SID, which reduces exposure if not compensated.

What is the compensation rule for tube angulation

For every 5° of angulation, reduce SID by 1 inch (2.5 cm).

What three image qualities are directly affected by positioning

Sharpness of detail, magnification, and shape distortion.

How does positioning affect visibility

By reducing superimposition and improving recognition of structures.

What four objectives does every projection aim for

Increase sharpness, reduce magnification, minimize distortion, increase visibility.

How do you reduce penumbra

Increase your SID.

How do you reduce magnification

Reduce OID and/or increase SID.

What factor alignment minimizes distortion

Beam, part, and IR alignment.

What does desuperimposing anatomy do

It increases visibility of the area of interest.

Why are multiple projections often required

To evaluate displacement of fractures, locate foreign bodies, and assess extent of pathology.

What is the effect of motion on radiographic images

It blurs sharpness of recorded detail.

What are the two general types of motion

Voluntary and involuntary motion.

What are three ways to control motion

Patient cooperation, immobilization, and short exposure times.

What is the number one enemy of image sharpness

Motion.

Besides the patient, what equipment issues can cause motion blur

Drifting of an unlocked tube or IR vibration from the Potter-Bucky mechanism.

What immobilization methods can be used

Communication, clamps, compression bands, sponges, and sandbags.

What is the best strategy for uncooperative patients when motion cannot be eliminated

Use very short exposure times (e.g., 0.03 sec).

How does severe motion affect image contrast

It causes high contrast (black and white appearance).

What is unique about motion's effect on subject contrast

It is the only factor that affects subject contrast without scatter radiation.

What digital processing feature enhances edges

Edge enhancement improves local contrast and visibility.

Can digital processing fix motion blur

No, motion cannot be corrected post-image digitally.

Does motion affect geometric distortion

No, it does not cause true geometric distortion.

What false effect can severe motion introduce

False images, which are a form of noise.

How is a false image defined

An image altered in shape by the effects of movement.

Review: What is the effect of object thickness on distortion

Thicker objects produce greater distortion.

Review: What does increasing SID do to beam divergence

It reduces beam divergence, improving sharpness.

Review: What is the rule of thumb for beam divergence at 40" SID

Approximately 2 degrees per inch.

Review: What is the rule of thumb for beam divergence at 72" SID

Approximately 1 degree per inch.

Review: What exposure time can nearly freeze motion in radiography

About 0.03 seconds.

Review: What does proper beam, part, and IR alignment minimize

It minimizes shape distortion.