Chapter 9: X-ray Emission

What are the two main properties we care about in an X-ray beam?

How it was produced and how it behaves in its interactions with matter

What are the two components of X-ray beam characteristics?

Quantity and quality

What is filtration?

The use of a material to absorb X-ray photons from the X-ray beam

What are the types of filtration?

Inherent, added, and compensating

What makes up inherent filtration?

Components built into the X-ray tube such as glass envelope and insulating oil

What makes up added filtration?

Additional components added externally such as the collimator box

What is compensating filtration used for?

To make a uniform exposure out of a non-uniform body part

Does compensating filtration reduce patient dose?

No, it only equalizes image density

What is the total filtration of most diagnostic X-ray tubes?

About 2.5 mm aluminum equivalent

Where is added filtration placed?

Between the tube window and the top of the collimator

What is total filtration?

The combination of inherent and added filtration

What do compensating filters adjust for?

Variations in patient thickness or density to create uniform exposure

Where are compensating filters attached?

To the bottom of the collimator or placed on the anatomic part of interest

What is beam quantity?

The number of photons in the X-ray beam; radiation dose

What factor primarily controls beam quantity?

mAs

What happens when mAs is increased?

Quantity and patient dose increase

What happens when mAs is decreased?

Quantity and patient dose decrease

How does kVp affect beam quantity?

Increases exponentially with kVp

What is the 15% rule?

A 15% increase in kVp is equivalent to doubling the mAs

How does distance affect beam quantity?

It decreases with the square of the distance (inverse square law)

What is the inverse square law formula?

I₁/I₂ = (D₂² / D₁²)

How does filtration affect beam quantity?

Decreases beam quantity depending on material and thickness

What are the four factors that affect beam quantity?

mAs, kVp, distance, and filtration

How does increasing mAs affect quantity?

Increases quantity (direct relationship)

How does increasing kVp affect quantity?

Increases quantity exponentially

How does increasing distance affect quantity?

Decreases quantity (inverse square law)

How does increasing filtration affect quantity?

Decreases quantity

What is beam quality?

The energy level or penetrating ability of the X-ray beam

What controls beam quality?

kVp and filtration

What is a high-quality (hard) beam?

An X-ray beam with high energy and greater penetration

What is a low-quality (soft) beam?

An X-ray beam with low energy and lower penetration

How does filtration affect beam quality?

Removes low-energy photons and increases the average energy of the beam

What happens to quantity when filtration is increased?

Quantity decreases because photons are removed

What happens to quality when filtration is increased?

Quality increases

What is the half-value layer (HVL)?

The thickness of a material that reduces beam intensity to half its original value

What is the normal HVL for diagnostic beams?

3 to 5 mm aluminum

What does HVL measure?

Both the energy and photon quantity characteristics of an X-ray beam

What is attenuation?

The reduction of X-ray intensity resulting from absorption and scattering

Do distance and mAs affect beam quality?

No, only kVp and filtration affect quality

What is the primary beam?

X-rays exiting the collimator and exposing the patient

What is the remnant beam?

X-rays that exit the patient and expose the image receptor

Why do larger patients require higher technique?

Because more photons are absorbed and fewer reach the IR

What is X-ray intensity also called?

X-ray exposure or quantity, measured in roentgens or grays

What unit is now typically used for exposure measurement?

Gray (Gy)

What happens to inherent filtration as the tube ages?

It increases due to tungsten deposits on the tube window

How does kVp relate to HVL?

Higher kVp increases HVL due to higher beam energy

What is the relationship between kVp and beam quality?

Direct—higher kVp increases beam quality

What is the relationship between kVp and beam quantity?

Exponential—higher kVp increases beam quantity

What happens to the emission spectrum when kVp increases?

Shift to the right and increase in amplitude (more quantity and quality)

What happens when mA increases?

Amplitude of the emission spectrum increases (quantity increases)

What happens when generator type improves (single → high frequency)?

Both quantity and quality of X-rays increase; emission spectrum shifts right

How does filtration affect the beam?

Increases quality but decreases quantity

What factors increase beam quantity?

Increased kVp, increased mAs, decreased distance, decreased filtration

What factors increase beam quality?

Increased kVp and increased filtration

What is the primary purpose of filtration?

Remove low-energy photons to reduce patient dose and increase beam quality

What is the definition of the compensating filter?

A device that adjusts exposure for non-uniform anatomy to produce uniform image density

What happens when kVp increases by 15%?

Exposure doubles; mAs can be cut in half to maintain exposure

What happens to beam quantity when distance doubles?

It decreases to one-fourth due to inverse square law

How does kVp control the kinetic energy of electrons?

Higher kVp increases electron kinetic energy and photon energy

What happens to X-ray beam quality and quantity when filtration is added?

Quality increases, quantity decreases

What is meant by "beam penetrability"?

The ability of X-rays to pass through matter

What is a "hard" beam?

A beam with higher energy and greater penetrability

What is a "soft" beam?

A beam with lower energy and less penetration

How does tube age affect filtration and quality?

Inherent filtration increases, slightly improving beam quality but decreasing quantity