Geometrical Factors

Question 1

Focal Spot Blur

Answer 1

From anode side to cathode side.
Effective focal spot size is larger on the cathode side and therefore the geometric unsharpness of a radiograph is greatest on the cathode side

Question 2

What happens to the radiographic density with motion?

Answer 2

Less dense, less dark. Causes blurry images

Question 3

The Ideal image (sharpest) would have

Answer 3

small focal spot size, long SID, smallest OID possible and no movement

Question 4

Size Distortion - Magnification (Film Screen)
OID increases ______ magnification
SID increases ________ magnification

Answer 4

Image is larger than the actual object size
OID increases - more magnification. We want the object as close as possible to the object size.

SID increases - reduce magnification. SID decreases (get closer) magnification increases - object larger

Question 5

Film screen radiography. We can never make an image size _______ than the object size. Always some degree of _______

Answer 5

can never make an image size smaller

always degree form of magnification

Question 6

Shape Distortion (True Distortion) 
Two types:

Answer 6

“misrepresentation” of an object’s true shape
Unequal magnification of different parts of the same object - affects all imaging.
Elongation and foreshortening

Question 7

Foreshortening

Answer 7

When the object is angled but CR remains perpendicular to the image receptor.
Image appears shorter or more squashed together. Caused by unequal magnification

Question 8

Elongation

Answer 8

object is parallel to the IR, CR is angled
CR is perpendicular to the object, IR is angled
Image appears stretched out. Can be used to an advantage - ie. - scaphoid

Question 9

Measuring Detail (film screen)

Answer 9

Measure the recorded detail with a lp/mm test tool
The greater the amount of line pairs that are visible in a given space, the better the recorded detail.
More line pairs/mm we can see, the higher the spatial frequency, the greater the spatial resolution.
*In digital it’s pixels - size of pixels

Question 10

Anode Heel Effect

Answer 10

The x-rays that constitute the useful beam emitted toward the anode side must traverse a greater thickness of target material than the x-rays emitted toward the cathode direction. The intensity of x-rays that are emitted through the “heel” of the target is reduced because they have a longer path through the target and therefore increased absorption. This is the heel effect

Question 11

When does the Anode Heel Effect become more important

Answer 11

when we are using large field size. Intensity change is larger with a greater field size. Anode Heel effect becomes more pronounced with larger field size.

Question 12

SID Decrease - more or less Anode Heel Effect

Answer 12

SID decrease requires the collimation to be opened up because of smaller field size so we can have a greater Anode Heel effect

Question 13

Distortion depends on 3 things

Answer 13

Object thickness - thick objects result in unequal magnification and thus greater distortion than thin objects
Object position - if the object plane and the image plane are not parallel, distortion occurs
Object shape

Question 14

Increase SID, ______ Distortion

Answer 14

DECREASE. Using more photons that are closer to 90 degrees (perpendicular). Diverging beams cause unsharpness, distortion. Objects on periphery become distorted because of diverging beams.

Question 15

Increase OID ______ Distortion

Answer 15

INCREASE. Greater OID - more magnification/distortion.

Question 16

To minimize shape distortion, indicate the most ideal relationship between the structures of interest and the film plane

Answer 16

Structure of interest should be as close to IR as possible and long axis parallel to IR. CR is perpendicular.

Question 17

What causes more distortion - angling the tube or angling the object?

Answer 17

Angling the tube causes more distortion - the further x-rays have to travel the more distortion you will see.

Question 18

Give two examples in which shape distortion is used to best advantage and describe how this is accomplished

Answer 18

Clavicle - angle CR tube to remove superimposition, clavicle is removed from ribs. Clavicle appears higher, curvature is removed. Elongated - the ends appear a lot longer.
Scaphoid - angle the tube or the IR to elongate the scaphoid
Coyle Method - angle the tube, radial head is elongated - stretched out.

Question 19

Anode Heel effect and SID

Answer 19

Increase SID, less Anode Heel effect.

Decrease SID - more Anode Heel effect (opening up field size with collimation so more peripheral photons)

Question 20

Tool that measures intensity

Answer 20

DOSIMETER