Scatter Radiation

Ch 22

What does Collimation improve and reduce?

Reduces patient dose

improves image contrast

What should collimation not exceed?

The beam should never exceed size of IR

What is scatter radiation from patient?

Emitted form patient in all directions

What is the source of exposure to the technologist and radiologist?

Scatter from patient

What are negative factors of scatter radiation?

• reduces radiographic contrast

• no useful information

What is used to reduce scatter radiation?

Collimation and grids are used to reduce scatter

3 factors that influence intensity of scatter → IR

1. kVp

2. field size

3. patients or part thickness

What is DIRECTLY proportional to Kilovoltage?

Compton Interaction

As energy increases, so does….?

Compton Scatter

Low kVp =

low scatter radiation

Low kVp = low scatter radiation; resulting in

high patient dose (because it is absorbing in the patient)

Field Size decrease =

Scatter decrease

Smaller field size =

Lower scatter

Lower scatter =

less optical density

Less optical density =

more technique needed

Optical Density -

overall blackening of an image

Part / Patient thickness

thicker body parts scatter more radiation than thinner parts

Four benefits of Compression Devices

1. improve spatial resolution by reducing patient thickness

2. brings object closer to image receptor

3. Reduces patient dose

4. improves contrast resolutions

Beam Restricting Devices

limits size of x-ray field only to anatomic structures of interest

3 types of beam-restricting devices

1. aperture diaphragm

2. Cones or Cylinders

3. Variable - aperture collimator

Aperture Diaphram

A lead or lead-lined metal plate attached to x-ray tube with an opening designed to cover just less than the size of image receptor used

Cones and Cylinders SHAPE

Usually circular shape

Cones and Cylinders CONs

difficult to align x-ray source, cone and image receptor

Where does OFF FOCUS RADIATION happen?

happens in the tube

What is OFF FOCUS RADIATION?

when x-rays hits any area other then target are on anode

What is the most common beam restricting device?

Light Localizing Variable Aperture Collimator

What does Light Localizing Variable Aperture Collimator help clean?

clean up OFF FOCUS Radiation

Collimation Filtration

Total filtration = inherent filtration + added filtration

What is the principal function of a GRID?

Principle function is to improve image contrast

How do GRIDs improve image contrast

Device that reduced the level of scatter radiation that will reach the image receptor

How much can a GRID absorb?

80 - 90 % of scatter

What is the GRID designed to do?

Designed to transmit only the x-rays whose direction is on a straight line from source to image receptor

What happens to the X-rays that exit the patient and strikes the grid strips?

they are absorbed and do not reach the IR

3 Important Dimensions (GRID)

1. grid ratio

2. grid frequency

3. grid material

Grid Ratio equation

(h) height of grid strip divided by (d) interspace width

which grid ratio is more effective?

High ratio grids are more effective in reducing scatter radiation than low ratio grids

What are grid ration ranges?

5:1 (85%) or 16:1 (97%) clean up

Grid ratio determines what?

amount of scatter radiation

What is the grid frequency?

Number of grid strips or grid lines per centimeter

What is the difference between HIGH fz and LOW fz grids?

• High frequency grids show less distinct grids lines than low frequency grids

• High frequency grids have thinner interspace strips and higher grid ratios = higher patient dose

Most grids frequency range

range of 25 to 45 lines per centimeter

What is the interspace in most grids made of?

either Aluminum or Plastic

Why aluminum?

absorbs low energy photons

What is the grid material made of?

lead strips

CIF

Contrast Improvement Factor

What is CIF?

Ratio of Contrast of an image made with the grid to the contrast of an image without a grid

CIF refers to?

Image Quality

Equation for CIF

K = Contrast with grid / Contrast without grid

CIF higher or lower for high ratio grids?

higher

Most grids improvement between?

1.5 and 2.5

Bucky Factor AKA

(B), Grid Factor

What is Bucky Factor?

Measured how much of an increase is technique is needed with and without a grid

What does Bucky Factor indicate?

indicates how large the patient dose will be with a particular grid

Higher grid ratio =

higher patient dose

higher grid ratio = higher patient dose =

higher bucky factor

Grid types

1. parallel grids

2. crossed grids

3. focused grids

4. moving grids

5. virtual grids

Grid Cutoff

Undesirable absorption of primary or useful x-rays by the grid

Example of Grid Cutoff

The grid is at an angle and the photons that should have gone through, don’t and get absorbed

Parallel Grid aka

Linear Grid

What kind is Parallel Grid? how dose it affect scatter

Simplest type of grid

Cleans up scatter in only one direction

How is Parallel Grid normally used?

Used primarily with short SID or large area image receptor

Crossed Grids aka

Cross Hatch grid

Compared to linear grid

more effective

What does Cross grid consist of?

Consist of two linear grids placed perpendicular to one another

Advantage of Cross Grid ?

Advantage - better with use of higher kVp

Disadvantage of Cross Grid?

grid cut off

What is Focused Grid designed to do?

minimize grid cut off

How are the lead strips for Focused Grid?

Angled to match the imaginary divergent x-ray beam

Focused Grid: high ratio to low ratio

High ratio grids have less positioning latitude than low ratio

When can Focused grid be used?

• Can only be used at SID specified on grid by the manufacturer (or grid cut off will occur)

• Most often are moving grids

Moving Grids AKA

Potter-bucky diaphragm, bucky diaphragm or bucky grid

What is Moving grid?

Mechanism that is moved at the time of an x-ray exposure

Advantage of Moving Grid

Removes grid lines by motion (blurring)

2 types of Moving Grid

Reciprocating and Oscillating

Reciprocating

• Most common type

• Motor driven back and forth several tines during an exposure

Oscillating

• Powered by an electromagnet

• Grid moves in a circular pattern

Virtual Grid: what does each x-ray have?

Each incident x-ray on a digital IR has measurable energy and frequency

Virtual Grid: TECHNIQUE

Technique to identify each xray and assign it to an energy or frequency bin

Results from Virtual Grid

digitally reconstructed radiograph produced with fewer scatter interactions

Virtual Grid helps to?

1. Reduce patient dose

2. Imrpve image contrast

3. Eliminate artifacts

Grid Problems

1. off level grid

2. off center grid

3. off focus grid

4. upside down grid

Off-level Grid

Occurs by having the tube improperly positioned

Off-Center Grid AKA

Lateral decentering

Off Center Grid positioning

Center of grid myst be positioned directly under tube so beam passes through the centermost interspace of grid

Any lateral shift =

Grid Cut off

Off Focus Grid needs

Proper SID - important so grid cut off does not occur

What is Off Focus Gris most important for?

Most important for high ratio grids

What do Low Ratio gids have?

Have more positioning latitude

Upside Down Grids

Will cause severe grid cutoff

Grid Selection: when using high kVp

high ratio grids should be used

As grid ratio increase =

so does scatter radiation

What is the ratio often employed

8:1

Grid Selection: Rule of Thumb

• 8:1 grids for technique below 90 kVp

• Above 90 kVp = use higher grid ratios

What is a major disadvantage to grids

Increased patient dose

Air-Gap Technique

Alternative method of decreasing scatter radiation in place of grid

Air Gap Technique: where is the IR placed?

10 to 15 cm from patient, so fewer scattered x-ray interactions with image receptor and enhance contrast

How is technique changed for Air Gap Tehcnique

mAs is increased about 10% for every cm

Disadvantage of Air Gap Technique

image magnification