Exposures II: Chapter 26 - Image Receptor Exposure

Refers to the fact that a radiographic image is visible only because there is sufficient exposure and contrast present.

What is visibility of detail?

the number of x-ray photons that strike the image receptor.

What is the definition of IR exposure?

Contrast is the difference between adjacent densities or shades of gray

What is the definition of contrast?

Looking at the exposure indicator (EI). 

• This will indicate if the IR was properly exposed, under exposed (quantum mottle), or over exposed (low contrast).

How is IR exposure assessed?

The overexposed image is the better choice because it has too much information recorded. 

• Post processing algorithms may be used to see the recorded information. 

• An underexposed image is simply missing information.

If a choice must be made between an overexposed or underexposed image, which is preferable and why?

Exposure doubles

What happens to exposure when mAs is doubled or kVp in increased by 15%?

It is a direct relationship. 

• As mAs is increased, exposure is increased.

What type of relationship does mAs have with exposure?

Doubling or halving

When adjusting mAs to change exposure, what increments should be used?

When kVp is increased → the electrons inside the tube have more energy (faster) → They produce more target interactions → therefore more x-ray photons. 

• This means more exposure to the image receptor.

Explain how kVp is an influencing factor on IR exposure.

15% to 25%

How much should you actually adjust kVp?

The 15% rule takes into account scatter adding exposure to the IR for an average kVp setting and an average part thickness

• When kVp is very high or very thick part is imaged = ↑ scatter = reduction of more than 15% to halve exposure

• Low kVp or very thin parts = ↓ scatter = increase kVp more than 15% to double exposure

Explain why the 15% kVp rule should actually be a variable 15 - 25% rule.

Increase kVp more than 15% to double exposure

• Low kVp or very thin parts = ↓ scatter

How would you adjust kVp when kVp is very low and a very thin part is imaged?

Reduction of more than 15% to halve exposure.

• When kVp is very high or very thick part is imaged = ↑ scatter

How would you adjust kVp when kVp is very high and a very thick part is imaged?

Because the reasons to adjust kVp are for:

1. Penetration power

2. Adjusting contrast

When only a change in exposure is needed, why is it not a good idea to adjust kVp?

Generator phase affects voltage ripple. 

• The greater the ripple, the greater the number of low energy x-ray photons produced, and the lesser the IR exposure. 

• A 1-faced 2p unit has 100% voltage ripple and produces less IR exposure than a high frequency unit which has very little voltage ripple.

How does generator phase affect exposure?

Higher energy

What level of energy do less ripple and more pulses have in a generator phase?

Less ripples = ↑ pulses = ↑ energy x-ray photons = ↑ IR exposure

The least amount of ripples in a generator phase has what effect on IR exposure?

Half the mAs

How do you convert low frequency to high frequency?

Double the mAs

How do you convert high frequency to low frequency?

When a machine has dual filaments, the actual mA for the large and small filament is slightly adjusted by the machine.

• If the machine is in need of recalibration, then a difference in exposure may be noticed when using the same mAs with different focal spot sizes. 

• Blooming of the focal spot may occur when high mA settings are used. This will result in a reduction of exposure and a replacement of the tube will be necessary.

Explain how an improperly functioning x-ray machine may result in focal spot size being an influencing factor on exposure

The machine needs to be recalibrated

How do you solve a focal spot size issue when there is a difference in exposure when using the same mAs with different focal spot sizes?

Focusing cup (a concave negative metal) is not repelling electrons in a nice close stream therefore, not all electrons are striking the target.

What is the cause of a focal spot size issue when a high mAs setting is used but there is a reduction in IR exposure?

Replacement of the tube

How do you solve a focal spot size issue when a high mAs setting is used but there is a reduction in IR exposure?

The intensity of the beam is the greatest on the cathode side. These differences are greatest at the peripheral portions of the beam and is most noticeable when the collimator is open wide.

• The intensity of the beam and thus exposure of the IR will be greatest towards the cathode.

Explain how the anode heel effect is an influencing factor on exposure.

X-ray photons are being absorbed by the target in anode and don’t ever exit the target therefore they don’t exit the tube = less x-ray photons on anode side

Why are there more x-ray photons on cathode side based on anode heel effect?

45%

How many more x-ray photons are on the cathode side compared to the anode side?

• Thicker end of the part should be placed towards the cathode side

• Thinner end towards the anode side

How can the anode heel effect be used to our advantage when x-raying a part with varying thicknesses?

1. Smaller effect to focal spot = ↑ detail

2. More heat loading capacity of the tube (can set high kVp and mAs without heating the target)

What are the small target angles advantages?

1. More pronounced anode heeling effect (more x-ray photons being absorbed on the anode side)

What are the small target angles disadvantages?

As SID increases, the beam diverges. The angled photons miss the IR = fewer photons striking the receptor = less exposure.

Explain how SID affects exposure.

↑ OID = ↓ scatter strikes the IR (scatter comes in funny angles therefore doesn’t strike the IR when OID is increased) = ↓ Exposure

As OID increases, what happens to exposure

↑ Filtration (removes low energy x-ray photons) = ↑ x-ray photons absorbed = ↓ exposure

As filtration increases, what happens to exposure?

↑ Collimation = ↓ field size = ↓ exposure

• because fewer photons are present in the beam, creating fewer scattered photons, and thus less exposure to the IR.

• More lead on the path of the beam = all x-ray photons are absorbed = less interaction with patient = less scatter striking the IR = less IR exposure

As collimation increases, what happens to exposure?

↑ thickness (more absorption of the beam by patient) = ↓ exposure

As the thickness of the part increases, what happens to exposure?

↑ effective atomic number = ↑ absorption of beam = ↑ PE absorption = ↓ exposure

As the effective atomic number of the part increases, what happens to exposure?

↑ tissue density = ↑absorption of the beam = ↓ exposure

As tissue density increases, what happens to exposure?

Additive conditions (increase in atomic # + tissue density) = ↑ absorption = ↓ exposure

• increase kVp by 5%-15%

What effect do additive conditions have on exposure?

Destructive condition (decrease atomic # + tissue density) = ↓ absorption = ↑ exposure

• decrease mAs by 25%-50%

What effect do destructive conditions have on exposure?

Radiopaque (contains Barium and Iodine) = ↑ absorption = ↓ exposure

What effect does contrast material such as radiopaque have on exposure?

Radioluscent (containing air) = ↓ absorption = ↑ exposure

What effect does contrast material such as radioluscent have on exposure?

↑ Grid ratio = strips are closer together = ↑ absorption of scatter = ↓ exposure

As grid ratio increases, what happens to exposure?

↑ grid frequency = strips are thinner = ↓ absorption = ↑ exposure

What does a high grid frequency have an effect on IR exposure?

↓ grid frequency = strips are thicker = ↑ absorption = ↓ exposure

What does a low grid frequency have an effect on IR exposure?

Grid cutoff = ↑ absorption = ↓ exposure

What does a grid cut off have an effect on IR exposure?

Through LUT

How is brightness adjusted?

CR = less sensitive to x-ray photons = more mAs can be used

How should you change the mAs when using CR, since it is less sensitive to x-ray photons?

DR = more sensitive to x-ray photons = less mAs can be used

How should you change the mAs when using DR, since it is more sensitive to x-ray photons?

Post-processing that controls brightness

What does window level control?

Histogram analysis error

What error is made when the image is too dark/bright and EI is incorrect?

30%

What is the percentage image brightness difference between 2 images in order for the eye to detect there’s a change in brightness?

mAs

What is the controlling factor of IR exposure?

kVp

What controls energy of electrons?

Compton scatter

What interaction predominates when kVp is increased?

1. ↑ Part thickness/tissues

2. ↑ Energy of x-ray photons

What affects scatter?

↑ kVp = ↑ compton scatter

When kVp is increased, what happens to scatter?