B - Beam restriction, Collimation and Grids

Question 1

What is scatter radiation?

Answer 1

Secondary radiation that is produced within our patient via a Compton scatter interaction

Question 2

Why don’t we want scatter radiation?

Answer 2

Increases patient dose
Increases scatter fog
Reduced image contrast

Question 3

How can we reduce scatter radiation?

Answer 3

Beam Restricting Devices (Cones, Collimation): These limit field size top reduce scatter and primary radiation
Grids: Absorb scatter before it reaches the image detector

Question 4

What are the 3 main types of beam restricting device?

Answer 4

Aperture Diaphragm
Cones/Cylinders
Collimators

Question 5

What is an Aperture Diaphragm?

Answer 5

Consist of a sheet of lead with a hole in the centre that determines the size and shape of the beam and attaches directly to the x-ray tube

Question 6

What are cones/cylinders?

Answer 6

Circular metal tubes that attach to the attach to the x-ray tube, limiting the x-ray beam to a predetermined size and shape. Cones can be either flared or straight

Question 7

What are Collimators?

Answer 7

Consist of two sets of adjustable lead shutters inside the x-ray housing unit. The particular device uses a light to show the technologist the field that is to be irradiated and allows you to adjust specifically to the size of the area of interest.

Question 8

What are the advantages of light beam collimator?

Answer 8

Widely variable primary beam size

Light within collimator shows beam size/shape

Thin shadow lines indicate the central ray (CR)

Less scatter:
Decreased patient dose
Improved image quality
Less scatter fog = more contrast = better visualisation of anatomic structures

Question 9

What are the disadvantages of light beam collimator?

Answer 9

Sometimes you need to x-ray a large body part (e.g. Lumbar spine, chest) and collimation not large enough

Question 10

What are Grids?

Answer 10

Extremely effective at reducing the amount of scatter that reaches the image detector
Improve image contrast if used correctly
Placed between patient and the image detector
Consist of strips of lead with an interspaces of aluminium or organic spacer
The strips can be oriented either straight or angled
Generally used where anatomy is thicker than 10cm

Question 11

What is grid ratio?

Answer 11

The working ability of a grid is described by the grid ratio, which is the ratio of the height of the lead strips to the distance between two strips (the interspaces).
The higher the grid ratio, the better the image contrast but at a cost of increased patient dose
Common grid ratios include 4:1, 6:1,8:1, 10:1 or 12:1
Grid ratio of 8:1 is generally used for 70-90 kVp technique and 12:1 is used for >90 kVp technique

Question 12

What are the three important dimensions on a grid?

Answer 12

Width of the grid strip (T)

Width of the interspace material (D)

Height of the grid (h)

Question 13

How does a grid work?

Answer 13

Primary/non-scattered beam x-rays striking the interspace material are allowed to pass to the film

Secondary radiation (scatter) that strikes the interspace material will be absorbed

High quality grids will attenuate 80% to 90% of the scatter radiation

Question 14

What are the different types of grids?

Answer 14

Focused Grid (most common): Strips are slightly angled
Parallel Grid: Strips are straight. Mostly used for short field or long distances
Moving Grid/Potter-Bucky Grid: Eliminates fine grid lines that may appear when using a focused or parallel grid

Question 15

What are the disadvantages of grids?

Answer 15

↑ patient dose (even more with moving grids)
Expensive
Fragile
Prone to misalignment
Cost and upkeep and all the problems you can imagine with mechanical devices that have moving parts if using Bucky

Question 16

What is the air gap technique?

Answer 16

Radiography technique used to reduce the quantity of scatter reaching the image detector
Increase the distance between scatter point (in your patient) and image detector
Referred to an increase of OID (object to image distance)
Air gap of 10-15cm is most effective

Question 17

What does SID mean?

Answer 17

Source to Image Distance. This is the distance between x-ray source and image receptor.

Question 18

What does SOD mean?

Answer 18

Source to Object Distance. This is the distance between the x-ray source and the object/patient.

Question 19

What does OID mean?

Answer 19

Object to Image Distance. This is the distance between the object/patient and the image receptor.

Question 20

How does the air gap technique work?

Answer 20

Reduces the low energy scatter photons that reach the image receptor
Causes the energy of photons to decrease – Especially in first 10cm due to beam divergence
Primary radiation is not affected or reduced

Question 21

When do we use air gap technique?

Answer 21

Should only use it when an air gap is unavoidable i.e. lateral c-spine, and when scatter must be reduced
Can use it instead of an anti-scatter grid for some projections
Using a gap 0f 10cm can replace a 10:1 grid, and doing this in a DR system can result in dose reduction of approximately 70%
Projections: Lateral hip, lateral c-spine, chest, pelvis

Question 22

How do you perform a lateral cervical spine x-ray?

Answer 22

Unavoidable air gap
Using a grid despite having this inherent air gap can increase dose compared with using just the air gap
Using a grid and filter combination can allow better visualisation of C7-T1 junction
The decision to use either an air gap alone or a grid and air gap combination for scatter reduction should be made on an equipment and patient basis
SID is 180cm for this projection, increase in OID is compensated for by increasing SOD distance

Question 23

How do you perform a Lateral Hip (Horizontal Beam) x-ray?

Answer 23

Unavoidable air gap
Due to the thickness of the proximal anatomy, an anti-scatter grid may also be necessary for optimal scatter reduction
Recent research found that the optimum dose and image quality scenario for this projection involves using both an anti-scatter grid and a 45 cm air gap

Question 24

How do you perform a chest x-ray with air gap?

Answer 24

Technique still used with older equipment (CR cassettes)
Involves using high kVp technique (125kVp)
Improves image contrast
Using air gap reduces patient dose compared to grid use
Intensity of primary beam remains relatively unchanged but scatter reduced significantly

Question 25

What are the advantages of the air gap technique?

Answer 25

Reduction of surface skin entrance dose
Comparable to using a grid if one is not available
Reduces amount of scattered radiation that reaches image receptor
Improves contrast resolution of image

Question 26

What are the disadvantages of the air gap technique?

Answer 26

Exposure factors must be increased in order to compensate for less photons reaching ID due to increased distance (Must increase exposures so they are comparable to using an 8:1 grid)
Increased patient dose if you do not compensate for increased OID by also increasing SOD (overall SID)
100cm for most projections, 180cm for lat cervical spine, lateral hip and PA chest
Magnification of anatomy
May need a larger detector if magnification factor high
Equipment limitations may prevent additional increase of SID, resulting in reduction of spatial resolution and image sharpness

Question 27

What is the importance of magnification?

Answer 27

PA Chest imaging, we do not want to magnify the heart!
Heart should be no larger than third of the width of the chest
If it is larger this can indicate ventricular failure and the cardiac pathology/disease