Medical Image acquisition unit 1 Exam

Transmission,

goes through the body

Absorption

, high subject density, whites on image bones, hardware, good for image quality, photoelectric effect

Scatter

Unwanted noise, useless to image

Compton scatter

Contributes most to scatter effects image quality, increases patient dose, increases tech dose, decrease image quality

Compton scatter

Incident x-ray photon interacts with outer shell electron, the photon scatters and has less energy, recoil electron

What does the effect scatter have on image?

decreases contrast, low contrast image, has fog or noise on image, more shades of grey

3 factors that impact scatter production?

1. Patient thickness

2. kVp

3. field size/beam size

Part thickness

Increasing the size of patient or the body part

increases scatter

• increases tissue density

• - increases scatter More matter more scatter

Field size

Increasing the field size exposed

• increased scatter

kVp

• Increasing kVp= decrease in photoelectric and Compton interactions

• there are more Compton interactions than photoelectric interactions.

• Scatter photons has higher remaining energy when higher kVp is used

• Increase kVp=Increased scatter

• Transmission occurs but more scatter

Mass effect on scatter?

No affect

kVp

Lower contrast images do not have differences in shades more grey

Scatter control

Beam restricting devices:

affects scatter production

decrease the x-ray beam field size and the amount of tissue irradiated, reducing the amount of scatter radiation produced in the patient

Radiographic grids-

Affects scatter clean up after production

Used to improve radiographic image quality by absorbing scatter radiation that exists the patient, reducing the amount of scatter reaching the image receptor

Beam restriction

aslo know as Collimation

Beam restriction two main purposes

1. Limit patient exposure

2. reduce scatter production installed outside of the x-ray tube housing changes the shape and size of the primary beam

increase collimation= decrease field size

decrease collimation= increase field size

LOCATED BELOW THE TUBE HOUSING, change the size to what we want

Increase collimation

decrease field size

decrease collimation

Increase field size

Increasing beam restriction

• decrease patient dose

• decrease the scatter produced in the patient

• decreases scatter reaching the IR

• increased radiographic contrast

Increase collimation

Increase intensity of scatter radiation

Significant increase collimation

• Quantum noise increases as the number of photons reaching the IR decreases

• mAs settings must be increased to provide sufficient photon quantity

• Not adjusting KVP because it would impact image contrast

Types of beam restriction devices

1. aperture diaphragm

2. collimator

3. cylinder

4. cones

5. lead mask/shield

6. Automatic Collimators (PBL)

Aperture Diaphragm

A flat piece of lead (diaphragm) that has a hole (aperture) in it and is placed directly below the x-ray tube window

• very easy to use

• can be cut out of lead

DOWNFALLS- field size cannot be adjusted

because of the proximity to the radiation source their is a large area of unsharpness that surrounds the image, because the device is so close to the tube

Cones and Cylinders

• an aperture diaphragm with an extended flange attached to it

• slide onto the tube directly below the window

• The flange can be made to telescope to increase its total length

• typically fit to a specific equipment easy to use

Cones and Cylinders

• Limit the unsharpness surrounding the image

• come in limited sizes

• May not be interchangeable among the tube housings

• Cylinders are more useful than cones

• LIMITED IN SIZE AND SHAPE THAT CAN BE PRODUCED

• NOT INTERCHANGABLE TO CERTAIN TUBE HOUSINGS

• CYLINDERS ARE MORE USEFUL than cones

Cones and Cylinders

Produce a circular projected field

Collimators Located?

Located immediately below the tube window

Collimators

2 sets of lead shutters

First lead shutter

entrance shutters

• located directly below tube

• limit the x-ray beam just as much as a aperture diaphragm would

Second lead shutter

Adjustable shutters

located 3-7 inches below the tube

• consists of both longitudinal and lagitunal leads or blades, adjustable

• limits off focus radiation

off focus radiation

• adds noise to the image

• x-rays not aligned with regular beam

• x-rays created anywhere outside of focal spot

Collimators equipped with

• A white light source and a mirror to project a light field onto the patient which indicated the exposure area

• An x-ray field measurement guide in case of light failure

• A plastic template with crosshairs to indicate centering

Collimation should match what?

field guide

light bulb and mirror could?

come out of alignment due to rough handling/ off centering. Can put plastic template to show where crosshairs should be

Automatic collimators

Positive beam limiting (PBL) devices

PBL

Automatically limits the size and the shape of the primary beam to the size and shape of the image receptor

Override mechanism

overrides the positive beam limitation

Lead blocker/ Masks

Similar to an aperture diaphragm

• limited to that shape and size

• cut out the shape of body part

• can be placed behind someone for thicker body parts/preventing scatter from reaching the image receptor

Lead blocker

can make it for the patient/ cut out/ or a piece of rubber to be placed in the patients back.

what does a Lead blocker do?

CLEANS UP SCATTER

why is the Lead blocker is placed behind the thick body part

cleans up scatter

Scatter produced where?

in the patient

radiographic grids do what with scatter?

clean up scatter

Who invented the first grid

Gustav bucky

the first grid was a

Cross hatch grid, most effective means of limiting scatter reaching image receptor

Hollis potter grids

• LINEAR STRIPS ONLY

• thinner strips

• Potter Bucky diaphragm- moved grid during exposure to blur grid lines

• moved during exposure to blur grid lines

• thinner stripes less you would see grid lines

lead does what?

absorbs radiation

Radiographic grids are?

◦Very thin lead strips absorb scatter from the patient before it can reach the IR

◦Placed between patient and IR

◦Typical grid use:

◦Part >10 cm 

◦kVp above 60

Radiographic grids are placed?

between patient and image receptor

Grids do what

improve contrast

Grid setback?

adjust technique, increase mAs,

Grids are used?

10 cm or greater

Grid kVp that is required to use?

kVp above 60

Grid size to use for a chest x-ray?

30 cm or greater

Grids why we increase mass?

reduce the transmitted photons and still let some scatter photons pass through. They are not perfect thats why we increase mass

what happens if you do not use a grid?

scattered photons reach the IR and add unwanted exposure

Can grids be removed?

yes most remove in and out of tables.

Grids size?

mostly 14 by 17 inches,

Thickness of a grid?

a quarter of an inch thick

Grids range in sizes!

8 ×10 or 17×19 size

The patients body is the what?

SOURCE OF SCATTER!!!

Grids are used to?

clean up and not put any unnessarsary information on image

when using film it is

important to use grids! shows more detail less grey!

What are the two most common material in grids?

aluminum and plastic!

they are the spacing

Strips are made of?

lead

The interspace material can be made of what?

aluminum or plastic

Why does lead absorb radiation

high atomic number

easy to shape less expensive

Aluminum or plastic

radiolucent, easy to use and durable

Front and back of grid are covered by?

aluminum to protect them

what are the grid dementions?

• height (H)

• thickness lead strip (T)

• distance between strips (D)

What is the T

the thickness of the lead stip how thick or wide

what is D

measures the amount of space between one stip of lead to the other, represents the thickness between the interspaces material

What is H

the height or how tall

What is the height of the grid or the distance between the lead strips?

Grid ratio

What is grid ratio related to?

efficeincy of the grid

Grid construction is described by?

grid ratio

grid frequency

grid frequency

number of grid lines between a certain distance, inch or centimeter

Grid ratio

compares height of the distance to the grid directly related to the efficiency of the grid

Grid ratio also known as

GR

Grid ratio

Height of lead strips (H)

Distance between strips (D)

◦Ranges:  5:1 to 16:1

◦As the ratio increases, so does the efficiency of scatter cleanup

◦

Ex.  The lead strips of a grid are 2mm high and distance between is 0.2mm.   What is the grid ratio?

            2mm (H)        =   10 

             0.2 mm (W)

             This grid has a 10:1 grid ratio.

Grid ratio is determined by

many factors of the patient

Grid ratio lowest?

5:1 less clean up

Grid ratio higher?

16:1 more clean up reduces scatter more

we can determine grid radio by what?

the equation!

H/W=GR

height divided by width equal to the grid ratio

The more lead in grid the more what?

more clean up will happen

how is the grid ratio changed?

Changing the height of strips or the distance between them changes the grid ratio  -  the leeway for scatter changes.

Increasing the GR does what?

Increase scatter cleanup, imporve image quality.contrast

As we increase ratio it will?

decrease the angle of the openings and be absorbed

decrease ratio

more opening to be less absrobed

What describes the number of lead lines per unit length?

Grid frequency

Grid frequency is?

Number of lead lines per unit length (cm/mm/ inches)

◦Ranges: 60 – 110 lines/ inch (25-45 lines/cm)

◦Most common: 85 -103 LPI

Grid frequency varies in ranges of?

60-110 lines/inch (25-45 lines/cm)

Most common lines per inch for grid frequency?

85-103

Grid frequency math

get the thickness of one line strip, then space between and divide by 1 inch which gets the line pairs or lead strips in one inch of the grid.

Thicker strips

Greater frequency

Relationship between grid ratio, frequency, and about of lead content in grid?

increase grid ratio to same frequency the lead content will increase with more scatter production.

However Relationship between grid ratio, frequency, and about of lead content in grid?

If grid frequency is increased with same grid ratio, their will be less lead content because the width of the inner space or thickness have been decreased. DECREASES LEAD CONTENT AND SCATTER PRODUCTION.

thicker strips

more scatter clean up

Linear grid pattern?

lead lines wun in only one direction

• allows angulation of entral ray along length of lead strips because only absorbs scatter in one direction

What is the most popular grid pattern?

linear

Crossed grid pattern is?

lead lines run at right angles to one another