Grids Part 1 (RADS 230)

Scatter radiation is a result of

The Compton Effect

As the x-ray beam passes through the patient it is:

transmitted (Remnant), Absorbed (photoelectric), Scattered

Scatter leads to

noise on image decreasing contrast resolution and reducing the signal to noise ratio (SNR)

Does scatter increase or decrease contrast?

decrease, inverse relationship

Does scatter have any good benefits?

No, but we can not get rid of it all we can do is try to keep it from hitting the IR

SNR

signal to noise ratio, we want high signal (radiation) to noise (scatter, always 1)

ex. 1:1 vs 10:1 (better)

Scatter radiation

A form of secondary radiation that occurs when an x-ray beam has been deflected from its path by interaction with matter

-exposure to patient and others nearby

Less scatter means

decreased exposure to IR, increased contrast, and less dose to patient

What causes scatter?

- increase in kvp

- decreased collimation (larger field size)

- thicker part/patient

Increase in kVp (scatter)

absorption goes down meaning photoelectric interactions go down because of this compton interactions increases meaning more scatter occurs because compton causes scatter

More matter

more scatter (decreasing collimation, thicker part/patient)

Compression paddle (scatter)

you can improve spatial resolution and contrast resolution of the image by choosing the proper technique factors and using this device to reduce scatter radiation to the image receptor also lowering patient dose

-facilitates the size of the pt as usually pt thickness is not controlled by the tech

Example of when compression is used in radiography

Mammography

Beam restriction/ Collimation (scatter radiation)

reduces the quantity of x-ray photons in the primary beam which reduces scatter before it reaches the patient

-increased collimation (less matter) = decreased pt exposure & scatter radiation = increased radiographic contrast

With digital imaging contrast is mostly restored with...

computer post processing (automatic recalling), but pt dose becomes a risk (dose creep)

secondary and scatter radiation are produced as the beam...

passes through the pt.

How to decrease production of scatter?

-collimate

-proper kVp

-compression

-smaller pt.

all deal with the the primary beam, primary way to reduce radiation dose

Grids recieve the

remnant beam after exiting the pt, they do not control the production of scatter they only intercept it stopping it from reaching the IR.

Grids were invented by

Gustav Bucky in 1913

Most effective method for limiting the amount of scatter radiation reaching the IR is?

Grids

Grids

absorb scatter radiation exiting the patient, increasing radiographic contrast improving the image quality

-b/w patient and IR

Grids are designed to allow only...

x-rays whose direction is on a straight path from the source to reach the IR, x-ray photons that have deviated and are not straight (scattered photons) will be blocked

Most common interspace material for grids is...

aluminum

Grids are made up of...

lead strips that absorbs the scattered x-rays with a radiolucent interspace material placed between the lead strips

Primary radiation travels in

straight lines meaning it will pass through the spaces between the lead strips on grids

Grids can absorb (or clean up) up to _____ of scatter radiation

90%

scattered photons travel

at various angles so most of them would be absorbed by the lead strips

Grid construction is described by:

grid ratio and grid frequency

Grid ratio

the height of the lead strips to the distance between them

Grid Ration Formula

Grid ratio = height/Distance = ____ : 1

Most common grid ratios

5:1, 6:1, 8:1, 10:1 & 12:1, 16:1

The higher the grid ratio

the more effective it is in cleaning up scatter radiation

5:1 grid may clean up _____ of scatter vs 16:1 which can clean up as much as _____ of scatter

85% vs 97% (mostly 90%)

High ration grids (12:1, 16:1) are most often used in...

high kVp radiography

What is the major disadvantage of grids?

it increases patient dose becuase it requires higher exposure in order for a sufficient number of cray photons to get through the grids

High ratio grids are made by:

- reducing the width of the interspace (make lead strips closer together)

- or increasing the height of lead strips

- or both

The higher the grid ratio, the xrays passing through the grid will need to be...

staighter, this is why they are harder to use even though they have the better cleanup

grid frequency

the number of lead lines per unit (in.. cm., both)

What is the range in cm & in for grid frequency?

25-80 lines/cm, 63-200 lines/in

High grid fequency =

more lead lines, better scatter cleanup

Lower grid frequency =

less lead lines

As grid frequency increases there's more lead to absorb scatter, so what happens to the dose, and why?

dose is higher because a higher exposure technique is needed to reach the IR

Grid performance is described by

contrast improvement factor and bucky factor

Grid Performance

our ability to put a number on how much the grid improved the image, kind of grading the performance of it

Who is contrast improvement factor measured by?

physicist, not the rad tech

Contrast Improvemnet Factor (K)

the ration of radiographic contrast with a grid to the radiographic contrast without a grid

Contrast Improvemnt Factor Formula

K = rad. contrast with grid (higher #) / rad. contrast without grid

Most K factors are ____

( or what happens to contrast when grids are used?)

1.5 - 2.5, or contrast is doubled

- 1 means no improvement

Is K lower or higher for high grid ratios?

higher as high ratio grids give increased contrast because they remove more sctter

Although K is a complex function of:

- the x-ray emission spectrum

- patient thickness

- type of tissue irradiated

measured at 100 kVp

Bucky Factor

the amount of remnant radiation penetrating the grid compared to the amount of remnant radiation that approached the grid but didn't pass through it (incident)

The amount of x-rays penetrating though any grid is less than the original amount directed at the gird. Therefore, what is required of the exposure?

It must increase to compensate for the loss of x-ray photons from the use of a grid

Bucky factor measures

how much exposure factors must increase with grid use

contrast improvemnt measures

improvement in image quality with grid use

What is the relationship between kVp and Bucky factor?

direct, the higher the kVp, the higher the bucky factor

Grid conversion factor

mAs with grid/mAs without grid

5:1

6:1

8:1

10:1

12:1 & 16:1

kVp bucky factor

8

12

20

23

25

add or subract this from original kVp when adding or taking away a grid

5:1

6:1

8:1

10:1 & 12:1

16:1

mAs bucky factor

2

3

4

5

6

multiply or divide this from the original mAs when adding or taking away a grid

Frequently used grid ratios with general-purpose imaging systems

10:1 & 12:1, 16:1

Virtual grids

Scatter reduction by digitally reconstructing a radiograph, pt is nearly half og what it would be with a grid

-will eventually replace physical grids

Air gap technique is most effective with..

lower kVp exams