Rad Science Ch.10 pt 2

What happens to the number of photons reaching the IR when kvp/energy increases?

More photons reach the IR because higher energy photons penetrate the patient more easily.

When kvp increases, what happens to the total number of interactions (PEE & Comptons) in the patient?

Interaction with patient atoms decreases because more photons pass straight through

As XR energy increases, what happens to Compton Scatter probability?

It decreases

Compton probability is inversely proportional to what?

Energy (1/E)
Higher energy —> less Compton Scatter

As XR energy increases, what happens to PEE?

It decreases a lot

PEE probability is inversely proportional to what?

Energy ³ (1/E³)
So small increases in energy cause BIG DECREASES in PEE

Which interaction decreases faster when kvp increases?

PEE

When energy increases, which interaction becomes more dominant compared to the other?

Compton Scatter b/c PEE drops much faster

At low energy / low kvp, which interaction dominates?

PEE

At high energy/ high kvp, which interaction dominates?

Compton Scatter

PEE probability is proportional to what?

Atomic number³ (Z³)

What does Z³ relationship mean?

Small increases in atomic number cause big increases in PEE

Does Compton Scatter depend on atomic number?

No

Why does bone produce more PEE than soft tissue?

Bone has a higher atomic numberWh

What is differential absorption?

The difference in XR absorption b/w tissues that create image contrast

Which interaction produces useful image contrast?

PEEWh

What does Compton scatter do to the image?

Creates radiographic fog and lowers contrast

Why can’t the IR tell if a photon came from scatter or straight transmission?

The IR only detects the photon, not its path.

In radiography, differential absorption mainly reflects the amount of what interaction?

PEE

About how many photons that hit the patient reach the IR?

Less than 1%

What happens to differential absorption when kvp decreases?

It increases

Why does low kvp increase differential absorption?

Because PEE interactions increase

What is the downside of using very low kvp?

Higher patient dose

Why do we use higher kvp for thicker body parts?

To increase penetration

What device us used to reduce scatter when using higher kvp?

Grid

What happens to photon interactions as mass density increases?

More interactions occur

What happens to photons reaching the IR when mass density increases?

Fewer photons reach the IR

What is attenuation?

Reduction of XR photons due to absorption or scatter

What two processes cause attentuation?

Absorption (PEE) and Scatter (Compton)Wh

How does patient thickness affect attenuation?

Attenuation increases exponentially as thickness increases

As energy/kvp increases, what will happen to PEE?

PEE decreases dramatically

How much of X-RAY beam makes it to the IR?

Less than 1% of the photons that strike the patient reach the IR

As energy/kvp decreases what will happen to Compton scatter?

Compton scatter increases

What is the formula for the probability of PEE as energy changes?

PEE ∝ 1 / E³

What is the formula for the probability of Compton interactions as energy changes?

Compton ∝ 1 / E

What happens to Compton interactions as the atomic number increases?

Nothing significant, Compton does not depend on atomic number.

What happens to Compton interactions as the atomic number decreases?

Nothing significant, Compton does not depend on atomic number

When energy/kVp increases what happens to Compton interactions compared to PEE?

The percent of Compton increases relative to PEE

(BOTH decrease but PEE decreases much faster, so the percent Compton increases relative to PEE)

When using 40 kVp for a diagnostic exam, most photons undergo what interaction?

PEE

(40 KVP is low energy, so photons have low energy and not very penetrating)

What happens to total interactions (PEE + Compton) as energy/kVp decreases? Why?

Increase, because fewer photons reach the IR (less penetration, more patient interactions)

What happens to total interactions (PEE + Compton) as energy/kVp increases? Why?

Decrease, because more photons reach IR (more penetration, less interaction with patient)

Differential absorption compares _______ to ______

amount of PEE to photons reaching IR (Compton + direct transmission)

As PEE increases differential absorption _______

Increases

What happens to PEE interactions as atomic number increases?

Increase significantly (Z³ relationship)

What happens to differential absorption as atomic number increases?

Increases

As energy increases what happens to Compton interactions compared to PEE?

Compton increases relative to PEE (PEE decreases faster)

What two things cause attenuation?

Absorption and scatter

Even small changes in energy/kVp or atomic number will change the likelihood of ______ interactions

PEE

Why are there fewer PEE and Compton interactions as kVp/energy increases what happens?

Because more photons reach the IR by direct transmission

If PEE is high/increased a lot, your image will look____?

White/light (radiopaque)

What happens to differential absorption as kVp/energy decreases?

Increases

What does Bushong say the sweet spot for Kevin is when imaging bone?

40 keV

What happens to differential absorption as atomic number decreases?

Decreases

As mass density increases, what happens to the number of Compton interactions?

Increases

As mass density increases what happens, what happens to the number of PEE interactions?

Increases