Image Receptor Exposure and Brightness (RADS 230)

IRE

Image Receptor Exposure

-kVp & mAs

When you have an underexposed IRE you should...

increase the exposure factors (kVp and mAs)

When you have an overexposed IRE you should...

decrease exposure factors

Controlling Factor of the IRE

mA and time (mAs)

mAs and IRE relationship

directly proportional (twins), whatever you do to one the same happens to the other

-if mAs doubles, the amount of radiation exiting the tube doubles

mAs only affects

quantity or amount of radiation making it the QUANTITATIVE FACTOR

-nothing to do with penetration

Time of exposure

actual duration of x-ray production, in seconds

Time of exposure and IRE relationship

directly proportional, quantity of radiation produced & time exployed

mAs

the amount of electron charge transferred from cathode to anode during the exposure (mA x time)

As mAs is increased

quantity of radiation is increased and IRE is increased

-and vice versa

What % change in mAs in necessary to see a visible change on the radiographic image?

30% (50% for clinic because it's easier to divide by 2)

Example of applying the 50% rule for change in mAs.

45 mAs, too underexposed, what is the new mAs?

45/2 =22.5

22.5 + 45 = ~70 mAs

Primary Influencing Factor

kVp (but is more powerful than mAs)

kVp

quality factor of the beam controlling the penetrating ability of the beam & speed of electrons

kVp affects

quality and quantity

kVp and penetration relationship

direct

increasing kVp, increases penetration

kVp and absorption relationship

inverse

increasing kVp, decreases absorption (going faster so it will penetrate rather than this)

Subject contrast

the difference in intensity between different areas of the image carried by the remnant beam, created by the xray absorption characteristics of different body tissues

-the ability to look at a radiograph and see different between neighboring structures

kVp and subject contrast relationship

inverse

increasing kVp, decreases subject contrast

-ex. abdomen (lots of greys) vs finger (black and white)

kVp and IRE relationship

direct but not directly proportional, increase in kVp increases IRE but by a lot

kVp must be used to ___________ the part, and mAs to _______________

penetrate (bony tissue); produce proper IRE (soft tissue delineation)

15 % rule

a 15% change in kVp is equivalent to double or half the mAs

can mAs compensate for inadequate kvp?

No because we won't be able to penetrate the bone with mAs

To maintain exposure to image receptor

-Increase mA and proportionally decrease time

-Increase time and proportionally decrease mA

-Increase kVp 15% and decrease mAs by half

-Decrease kVp 15% and increase mAs by two times (not done often because we want low mAs)

increase kVp by 15%

decrease mAs by half

Decrease kVp by 15%

increase mAs by 2 times (double mAs)

-not done often

For patient safety you always want...

high kVp and low mAs

To increase exposure to image receptor

increase mA, exposure time, or kVp (and vice versa to decrease)

Digital image receptors have...

wide dynamic range (many exposure factors can provide an image)

Image brightness is not dependent on...

mAs and kVp

What else influences IRE?

Patient Factors

Distance

Beam Limiting Devices (collimator)

Grids

Filters

Anode Heel Effect

Source-to-Image Receptor Distance (SID)

distance measured between the focal spot (target) of an x-ray tube to the image receptor (IR)

object-to-image receptor distance (OID)

The distance between the subject/ object and the image receptor (IR)

source-to-object distance (SOD)

distance from the source (focal spot/target) to object/subject

SID is a

secondary influencing factor for IRE

-as this is changed a noticeable difference in IRE is produced

As distance increases... (SID)

the total area influenced by the beam increases, decreasing IRE

SID and IRE Relationship

Inverse square law, as SID increases, IRE decreases as a result of the square of the distances (and vice versa)

inverse square law

numerically illustrates what happens to the intensity of radiation as distance changes

-used for exposure to pt., tech, air or the entrance skin exposure

Because intensity of radiation changes with SID changes we must use the...

mAs distance formula (direct) to maintain IRE regardless of SID changes

mAs distance formula

mAs new/mAs old = D new^2/D old^2

-direct square law, direct relationship

-used for changes to the control panel

Object-image distance (OID) vs IRE Relationship

inverse relationship

-increase in OID causes a decrease in IRE because the remnant beam will scatter before reaching the IR

A double/ half in SID (by 2) changes IRE by a factor of

4, this is the greatest influence on IRE

ex. Increasing the SID by 2 times, decreases IRE by a factor of 4

Beam Limitation Devices vs IRE relationship

inverse relationship

-increase collimation (reduce beam), you decrease exposure

Grids

limit the amount of scatter radiation reaching the IR

Grids vs IRE relationship

inverse relationship

-using a grid, removes scatter/photons, which will reduce the amount of IRE

Filters vs IRE relationships

inverse

-increase filters will absorb the low or soft energy photons, which will reduce the IRE slightly

When adding a grid (kVp)

you add to the original kVp

When taking away a grid (kVp)

you subtract from kVp

Grid Sizes (in order)

5:1

6:1

8:1

10:1 & 12:1

16:1

kVp addition or subtraction units for Grids (in order)

8

12

20

23

25

If the field size is increased from the the beam limitation device (decrease collimation)

the amount of tissue irradiated is increased, increasing the amount of secondary and scatter radiation, increasing IRE

- MORE MATTER, MORE SCATTER

do we ever square mAs or kVp?

No, we square distance

Uneven changes in SID and want to quantify the rate at which exposure is changing

use the mAs maintenance formula and plug in 10 for mAs old and solve for the mAs new