Projection Geometry

Question 1

What are three problems that will always be present in radiographs

Answer 1

unsharpness, magnification, distortion

Question 2

Why are there always projection problems

Answer 2

1. the focal spot is an area, never a point
2. The X-rays diverge from their source
3. Images are 2-d representation of 3 d structures

Question 3

What is sharpness

Answer 3

the degree of how well the image can display the boundary between two differing radio densities

Question 4

the unsharpness at the boundary of an object

Answer 4

penumbra

Question 5

Spatial resolution

Answer 5

the degree of how well the image can display two objects that are close together

Question 6

How is spatial resolution measured

Answer 6

line pairs/ mm

lp/mm

Question 7

_______ spatial resolution is desirable

Answer 7

higher

Question 8

Spatial resolution and sharpness are both influenced by

Answer 8

1. focal spot size
2. receptor composition
3. movement

Question 9

____ focal spot size has higher sharpness and spatial composition

Answer 9

small

Question 10

Order the following receptors based on there sharpness and spatial resolution
PSP, Film, CCD, CMOS

Answer 10

Film> CCD> CMOS > PSP

Question 11

Movement of the tube head is

Answer 11

source movement

Question 12

movement of the patient is

Answer 12

object movement

Question 13

patient movement of sensor/film in mouth is

Answer 13

receptor movement

Question 14

Large focal spot will cause sharpness to

Answer 14

decrease

Question 15

long source to object distance will cause sharpness to

Answer 15

improve

Question 16

long object to receptor distance will cause sharpness to

Answer 16

decrease

Question 17

What angulation is the dental x-ray set at

Answer 17

20 degrees

Question 18

What is the size of the focal spot in dental X-rays

Answer 18

1mmx3mm

Question 19

Distortion can cause

Answer 19

foreshortening or elongation

Question 20

foreshortening is due to

Answer 20

the tooth not parallel to the receptor, beam perpendicular to the receptor

Question 21

elongation is due to

Answer 21

the receptor is not parallel to the tooth and it is not perpendicular to the beam, but is perpendicular to the beam

Question 22

this technique increased sharpness, and decrease distortions

Answer 22

parallel technique

Question 23

In parallel technique the tooth is parallel to the

Answer 23

receptor

Question 24

This technique is used due to its accuracy in length

Answer 24

bisecting angle

Question 25

A downfall of the bisecting technique is

Answer 25

the distortion of other anatomic features

Question 26

two ways to improve image distortion

Answer 26

1. object parallel to the receptor

2. beam is perpendicular to the long axis of the tooth

Question 27

What are methods to localize objects in radiographs

Answer 27

SLOB and cone beam computed tomography, image at right angle to the