Collimation-Field Size

Question 1

What is collimation also known as?

Answer 1

Field size limitation

Question 2

What is collimation designed to do?

Answer 2

Reduce patient dose and preserve/increase subject contrast

Question 3

What is a general rule of thumb for field size?

Answer 3

The field size must never be larger than the anatomy of interest or the size of the IR

Question 4

How are collimation and field size related?

Answer 4

They are inversely related, as collimation increases field size decreases and vice versa

Question 5

What is the affect of collimation/field size on patient dose?

Answer 5

As collimation increases and field size decreases, the patient dose decrease

Question 6

What is the affect of collimation/field size on scatter?

Answer 6

As collimation increases and field size decreases, scatter decreases

Question 7

What is the affect of collimation/field size on radiographic contrast?

Answer 7

As collimation increases and field size decreases, radiographic contrast increase

Question 8

What is the affect of collimation/field size on exposure to the IR?

Answer 8

As collimation increases and field size decreases, exposure to the IR decreases

Question 9

What is cropping?

Answer 9

Referes to the removing or elimination of infortmation by electronically changing the field size

Question 10

Can cropping be used as a replacement for pre-exposure collimation?

Answer 10

No, you should isolate the part via collimation and then shoot

Question 11

Who created the first type of collimation device?

Answer 11

William Rollins

Question 12

What were the first type of collimation devices called?

Answer 12

Diaphragms

Question 13

What are diaphragms?

Answer 13

Devices with cut openings that were meant to change the beam shape

Question 14

How do diaphragms work to limit the field size?

Answer 14

They limit the beam to the size of the cutout and cuts off peripheral areas of the beam

Question 15

How does collimation impact contrast?

Answer 15

Closed collimation produces better contrast, since there is less scatter

Question 16

What are the other types of collimation devices?

Answer 16

Lead cones and cylinders

Question 17

What is important when using lead cones and filters?

Answer 17

Positioning of the patient, since they need to remain within the field while in use

Question 18

What is the collimator box?

Answer 18

Part of the x-ray unit that allows technologists to adjust the length and width of the field size

Question 19

What do collimator boxes often work with?

Answer 19

Positive beam limitation

Question 20

What is PBL?

Answer 20

When the size of the cassette is detected when its placed in a bucky and the collimation automatically adjusts to the CR size

Question 21

How should collimation be set if the box is not aligned perfectly with the beam?

Answer 21

1/2 inch of light beyond the anatomy should be kept

Question 22

What does over collimation result in?

Answer 22

Clipped anatomy, which results in a repeat image and double the dose

Question 23

What is a double aperture collimator?

Answer 23

Creates a sharp border at the edge of the field by utilizing two shutters

Question 24

What do double aperture collimators provide?

Answer 24

Better reduction of off-focus/extrafocal radiation

Question 25

What is off-focus radiation?

Answer 25

Photons produced outside the focal spot

Question 26

How much of the beam is made up of off-focus radiation?

Answer 26

As much as 25%

Question 27

What part of the double aperture collimator cathces the off-focus rays?

Answer 27

The secondary shutters

Question 28

What affect does SID have on field size?

Answer 28

The closer the tube moves to the IR the smaller the collimated field is and the further the tube is the bigger it is