Beam Intensity and Penetrability Inverse square pt2

Maintaining IR Exposure

As SID changes, technical factors must be adjusted to maintain appropriate IR exposure. mAs is used to adjust for changes in SID. The change in mAs is directly proportional to the square of the distance change: mAs₂ / mAs₁ = (SID₂ / SID₁)².

Filtration Definition

A filter is anything placed in the primary x-ray beam that attenuates photons. The thicker the filter and the higher the atomic number, the greater the reduction in photon number. Measured in millimeters of aluminum equivalence (mm Al/eq).

Purpose of Filtration

Removes low-energy photons from the beam, increasing the average energy (beam hardening). Aluminum is the standard for measuring filtration effectiveness.

Compensating Filter Definition

Devices placed in the primary beam to compensate for variation in part thickness or tissue density. They vary beam intensity so that the image receptor receives uniform exposure.

Types of Filtration

1. Inherent Filtration – Oil around tube, glass window, and envelope (~0.5 mm Al/eq).

2. Added Filtration – Metal sheets and collimator mirror.

Total filtration = Inherent + Added (minimum 2.5 mm Al/eq).

Filtration Measurement

Expressed in Aluminum Equivalence (Al/eq): the thickness of aluminum that provides the same attenuation of the x-ray beam as the tested material.

Beam Quality Definition

Refers to the penetrating ability of the x-ray beam. Beam quality depends primarily on photon energy, determined by kVp and filtration.

Polyenergetic Beam

The x-ray beam is polyenergetic (contains photons of various energies) ranging from minimum to peak value (kVp). Average energy ≈ 1/3 to 1/2 of peak kVp. Example: 120 kVp ≈ 40–50 keV average.

Why the Beam Is Polyenergetic

1. Fluctuating kilovoltage (voltage ripple).

2. Characteristic vs. Bremsstrahlung interactions.

3. Multiple electron interactions with target atoms.

4. Off-focus radiation (created outside the target area).

Monoenergetic Beam

All photons have the same energy (e.g., gamma rays).

Beam Quality Units of Measure

1. Average keV – mean photon energy.

2. Average Frequency – higher frequency = greater penetrability.

3. Average Wavelength – shorter wavelength = higher penetrability.

4. Linear Energy Transfer (LET) – lower LET = greater penetrability.

5. Half-Value Layer (HVL) – higher HVL = more penetrability.

Half-Value Layer Definition

Thickness of material required to reduce beam intensity to one-half of its original value. Expressed in mm Al/eq.

HVL vs Filtration

HVL describes beam penetrability;

filtration refers to the physical quantity of filtering material. HVL measures quality, not quantity.

Factors Increasing HVL

1. Higher kVp.

2. Increased filtration.

3. Higher atomic number target material.

4. Reduced voltage ripple.

Common Misconceptions

Avoid confusing HVL with (1) filtration (physical quantity) or (2) radioactive half-life (time-based decay).

Emission Spectrum

Graph showing beam characteristics. Curve farther right on x-axis = higher quality (energy). Taller curve on y-axis = higher intensity (quantity).

Average kEV

Average of individuals energies of all photons in a beam

Average frequency

high frequency = higher penetrability

Average wavelength

short wavelength = higher penetrability

Linear energy transfer (LET)

the amount of energy a beam loses as it pass through matter. Low LET = more penetrability

Half value layer

higher half value layer = more penetrability typically measured in aluminum equivalency al/eq

increase in mAs does what to quality and quantity

Quality: none

Quantity: increased

Increase in kVp does what to quality and quantity

Quality: increased

Quantity: increased

Increase in Distance does what to quality and quantity 

Quality: none

Quantity: Reduced

Increased in filtration does what to quality and quantity

Quality : increased

Quantity: reduced