Filtration & Prime Factors

Filtration

Increased Filtration

Filters out the lower energy x-ray photons

Decrease Filtration

Does NOT add useful image information

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Half-Value-Layer (HVL)

HVL too low

The amount of filtration needed to reduce the beam to 1/2 of its original intensity.

Too many low-energy photons in the beam

kVp

mAs

Distance

Milliampere (mA)

Source to image-receptor distance (SID)

Exposure time (s)

Energy level of the photons

Beam penetrability

how many photons will penetrate anatomy

Quality

Measured in eV (Electron Volts)

Represented numerically by HVL

Measured in mm al/eq

Quantity

Measured in coulomb/kilogram or air kerma

Coulomb/Kilogram

Number of electrons liberated per kilogram of Air (output intensity of x-ray equipment)

Air Kerma

Quantity of radiation delivered to a given point (energy deposited)

Energy Deposited

Output intensity of x-ray equipment

The measure of tube current

Determines beam quantity or intensity

Changing the mA station changes the filament used

Changes the number of electrons released though thermionic emission

A RATE (how many electrons flow from cathode to anode per second)

1 Ampere (A) = 1 Coulomb (C)/sec

1 mA = 6.3 x10^15 electrons/sec

1C =6.3 x 10^18 electrons/sec

Tube Current

Measured in

The flow of electrons from cathode to anode

DIRECT relationship to intensity

Doubling mA

Doubles the number of electrons emitted

Doubles the number of photons

Doubles patient dose (IF no change is made to exposure time)

Increase Patient dose

mAs Reciprocity

As long as the total mAs is the same, any combination of mA and time can be used to produce the same quantity of radiation.

Increasing kVp

Increases the speed and energy of electrons crossing from cathode to anode.

Space Charge Compensator

Corrects for the increase in electron speed to maintain a constant number of electrons/second

15% Rule

Increases kVp 15% (+10kVp) and reduce mAs 50% (1/2)

Decreases kVp 15% (-10kVp) and doubles mAs 50% (x2)

Influences beam quantity

More interactions with higher kVp (more photons produced)

Influences scatter/secondary radiation production

Fewer interactions occur in the body at higher kVp

Inverse Square Law

image

Doubling the distance, reduces the beam intensity by a factor of 4

Applies to all EM Radiation

Direct Square Law (DSL)

image

Used to maintain the same overall IR exposure

Also known as the "mAs Distance Compensation Formula" or "Exposure Maintenence Formula"

Anode Heel Effect

Smaller & Steeper angle = Smaller Effective Focal Spot

Better Resolution/Detail Sharpness

The x-ray on the cathode side is more intense than that on the Anode side

The physicist will add filtration

More Penetration (Travel farther in matter)

Should not be used to control IR exposure

No practical value

Doubling kVp = 4x X-ray photon #

Doesn't account for changes in penetration and scatter production

Fewer interactions occur in the body

A higher % of interactions are Compton scatter (not photoelectric)

Lowers contrast

X-Ray Machine Indicators

Agfa

Kodak (Carestream)

Fuji

Low E = Low LogM 2.05

High E = High LogM 2.35

Low E = High S# 400

High E = Low S# 100

Low E = Low EI 1800

High E = High EI 2200

Uses EI

Low = Low & High = High

Low = Low & High = High

Uses LogM

Uses S#

High = Low & Low = High

increasing Distance

Doubling distance

Primary controller of x-ray intensity

Reduces beam intensity by a factor of 4

Reduces Entrance Skin Exposure (ESE)

Improves Image Quality

Patient Dose

Longer SID counteracts Anode Heel Effect

Smaller anode angle

Increases the impact of the anode heel effect

Image contrast is ALWAYS controlled through computer post-processing