geometry of image formation

Question 1

image sharpness

Answer 1

how well defined is a boundary between 2 areas of differing radiodensities (i.e. DEJ)

Question 2

image resolution

Answer 2

how well an image is able to reveal very small objects that are very close together

Question 3

what are the 4 desired characteristics of image formation?

Answer 3

minimum magnification
minimum distortion
minimum superimposition
MAXIMUM detail

Question 4

magnification depends on what 3 distances?

Answer 4

1. object-film distance (OFD)
2. source-object distance (SOD)
3. source-film distance (SFD)

Question 5

We want OFD to be ________ for clear and sharp images.

Answer 5

SHORT

Question 6

we want SFD to be ________ for defined images

Answer 6

LONG

Question 7

if you increase OFD, the magnification _______

Answer 7

increases

Question 8

constant OFD + decreased SFD ____________ magnification

Answer 8

increases

Question 9

constant OFD + increased SFD ___________ magnification

Answer 9

decreases

Question 10

constant SOD + increased OFD (film moves away from object) __________ magnification

Answer 10

increases

Question 11

constant SFD + increased OFD (object moves toward source) ____________ magnification

Answer 11

increases

Question 12

decreased SFD __________ magnification

Answer 12

increases

Question 13

4 names for source of radiation

Answer 13

source
target
focal spot
anode

Question 14

distortion

Answer 14

variation seen in image from true shape or proportions of object

Question 15

distortion in an image is due to what?

Answer 15

disproportionate magnification

Question 16

distortion depends on ___________; magnification depends on __________

Answer 16

angulation; distance

Question 17

minimum distortion occurs when object and sensor are ________ and central ray is ______________ to both __________ and _______

Answer 17

parallel
perpendicular
object
film

Question 18

what happens when central ray is directed at right angle to object?

Answer 18

elongation

Question 19

what happens when central ray is directed at right angle to sensor?

Answer 19

foreshortening

Question 20

parallel technique

- image shifts __________ if vertical angle is too great and shifts _____________ if vertical angle is too small

Answer 20

occlusally

apically

Question 21

bisecting angle technique
– image shifts __________/_________ if vertical angle is too great and shifts _____________ if vertical angle is too small

Answer 21

occlusally/incisally

apically

Question 22

paralleling technique (3 characteristics)

Answer 22

• placement of sensor parallel to teeth is easy
• if mistake is made no foreshortening or magnification
• long cone is mandatory due to need for increased SFD

Question 23

bisecting angle technique (3 characteristics)

Answer 23

• direct invisible central ray of invisible x-ray beam at right angle to imaginary bisector
• if mistake is made it will result in foreshortening or elongation
• long cone is not necessary

Question 24

radiographic definition

Answer 24

recorded sharpness of boundaries of structures

Question 25

umbra

Answer 25

innermost and darkest part of a shadow where light source is completely blocked by occluding body

Question 26

the _________ the umbra, the sharper the image

Answer 26

smaller

Question 27

penumbra

Answer 27

space in periphery of main target of radiation therapy

Question 28

size of source is _________ related to definition | size of source is _________ related to unsharpness

Answer 28

inversely | directly

Question 29

all factors that ________ magnification will decrease definition

Answer 29

increase

Question 30

movement of source of radiation =

Answer 30

geometric unsharpness

Question 31

movement of patient or image receptor =

Answer 31

unsharpness

Question 32

zero degree VA

Answer 32

central ray is parallel to occlusal plane

Question 33

increased/positive VA: source of radiation is __________ to point of entry of central ray (PID is pointing _________) and the image shifts _________ (_______/_________ edges may be projected off receptor)

Answer 33

superior downwards down incisal/occlusal

Question 34

are all maxillary radiographs positive?

Answer 34

yes

Question 35

decreased/negative VA: image shifts ____ (_______ may be projected off receptor)

Answer 35

up | apices

Question 36

for mandibular teeth a the source of radiation for negative VA is _________ to point of entry of central ray. PID is pointing _________

Answer 36

inferior | upwards

Question 37

are all mandibular radiographs negative?

Answer 37

yes

Question 38

increasing VA in mandibular teeth causes image to shift ______ where what edges may be projected off the receptor?

Answer 38

up | occlusal/incisal

Question 39

decreasing VA in mandibular teeth causes image to shift _____ where ______ may be projected off the receptor

Answer 39

down | apices

Question 40

VA for maxillary periapicals is ____________ than bitewings

Answer 40

greater

Question 41

VA for maxillary anterior periapicals is __________ than VA for posterior periapicals

Answer 41

greater

Question 42

Horizontal angulation | - 0 degrees: central ray is _________ to sagittal plane and directed _____________

Answer 42

parallel | anteroposteriorly

Question 43

HA | - 90 degrees: central ray is __________ to sagittal plane

Answer 43

perpendicular

Question 44

HA | - 180 degrees: central ray is __________ to sagittal plane and directed _____________

Answer 44

parallel | posteroanteriorly

Question 45

improper HA causes _________

Answer 45

interproximal overlapping

Question 46

increasing HA causes images to shift ____________ | decreasing HA causes images to shift ___________

Answer 46

anteriorly | posteriorly

Question 47

source of radiation moves ____________

Answer 47

posteriorly

Question 48

in periapical radiography, HA varies from ____ degrees (incisor PA) to _____ degrees (molar PA)

Answer 48

0-90