Exam 1- Geometry of Image formation

What is image magnification?

X-ray beam is divergent

Some magnification always occurs

What is image magnification controlled by?

SFD (source–film distance)

SOD (source–object distance)

OFD (object–film distance)

What is the goal for OFD and SFD?

OFD as small as possible, SFD as large as possible

What is image distortion?

Image does not match true shape

Uneven magnification

When is the minimal image distortion?

Tooth and film are parallel

Central ray (CR) is 90° to both

What is elongation?

Image looks too long

CR is 90° to object but film is angled

What is elongation caused by?

Caused by too little vertical angulation

What is foreshortening?

Image looks too short

CR is 90° to film

What causes foreshortening?

Caused by too much vertical angulation

What is the Angle Bisector principal?

Film and tooth are not parallel

CR is directed perpendicular to the bisector

Based on rule of isometry

What is the parallel principal?

Tooth and film are parallel

CR is 90° to both

Least distortion

Long cone required

What is the right angle technique?

Another name for the parallel technique

CR is perpendicular to tooth and film

What is the long cone technique?

Uses long PID

Increases SFD

Reduces magnification

Improves image quality

How do you calculate percent magnification?

Magnification=(Image size - Object size/ Object size)×100

What is radiographic detail?

Sharpness of edges

What is radiographic contrast?

Difference between light and dark areas

What is resolution?

Ability to see fine details

What is radiographic definition(sharpness)?

How clear image edges appear

What is radiographic unsharpness?

Blurred edges

What are factors effecting definition?

Size of focal spot

SFD, SOD, OFD

Motion

What is point source?

ideal, sharp image

What is area source?

larger focal spot, less sharp (used to protect anode)

What is an umbra?

Darkest part of shadow

What is penumbra

Fuzzy edge of shadow

What is geometric unsharpness?

Due to beam divergence

Not very significant

What is motion unsharpness?

Patient movement

Very significant

What is a positive vertical angle?

Used for maxillary, bitewings, occlusals

Too much → image shifts down, crowns off film

Too little → image shifts up, apices off film

What is a negative vertical angle?

Used for mandibular molars

Too much → image shifts up

Too little → image shifts down

What is the horizontal angle?

Controls overlapping

Correct angle → open contacts

Incorrect angle → overlapping