Polyenergetic

Having different levels of energy.

The x-ray beam is polyenergetic.

The kVP set by the radiographer will determine the maximum energy level of the photons created and they range will vary anywhere from zero (0) to maximum.

If the kVP set is 90kVp, then no photon created will exceed 90 keV (because 90 was the maximum selected on the control panel).

Heterogenous

Produced from more than one source.

The x-ray beam is heterongenous.

The x-ray photons from the x-ray tube can be produced from one of two target interactions:

1.  Bremsstrahlung Interactions

2. Characteristic Interactions

Primary Beam

the radiation (x-ray photons) that leave the x-ray tube.

Remnant Beam

the radiation that leaves the patient's body and strikes the image receptor

** the remnant beam is the radiation that actually forms the x-ray image

Filtration

The process of eliminating the undesirable low energy x-ray photons by inserting absorbing materials into the primary beam.

Expressed in terms of the thickness of Aluminum Equivalency (Al/Eq)

Filtration allows the radiographer to "shape" the beam and is sometimes called "hardening" the beam due to the removal of the low energy photons (called soft photons).

Filtering the primary beam wiill cause the average energy of the beam to increase.

***The primary reason for filtration is for PATIENT PROTECTION because the removal of the low energy photons will reduce (decrease) patient skin dose.

Filter

Any material that is designed to selectively absorb photons from the x-ray beam.

Filters are typically added between the source and the patient.

Aluminum is the most common material filters are made of.  Other materials used can be copper and plexiglass.

Types of Filters

There are two types of Filters:

1.  Compensating

2.  Compound

What is the primary reason for filtration?

For patient protection!!

Filtration allows the removal of low energy photons that only increase patient dose without enhancing the radiographic image.

Why are weak (low-energy) x-rays easily absorbed?

Because:

1. They have a long wavelength

2. They have a low frequency

3. They have a low keV value

**an ev is an electron volt.  Electron volts are the energy one electron will have when accelerated by an electrical potential of one volt**

Types of Filtration

1. Inherent Filtration

2. Added Filtration

**Total Filtration is the sum of the inherent filtration + added filtration**

Inherent Filtration

Filtration that is a result of the composition of the x-ray tube and the tube housing.

Examples:

1. Glass Window (provides most of the inherent filtration)

2. The Dielectric Insulating Oil

3. The thickness of the glass envelope

***The filtering effect of Inherent Filtration is 0.5 to 1.0mm Al/Eq***

Added Filtration

Any filtration that occurs outside the tube housing and before the image receptor.

Examples:

1. Collimator

2. Silver on the mirror

3. Vaporized tungstun on the glass envelope

4. Any external filters (compound/compensating)

**The collimators (and silver on mirror) have approx. a 0.75 to 1.0mm Al/Eq***

Total Filtration

The sum of the inherent filtration + the added filtration.

The federal standard for x-ray tubes that are capable of delievering 70kVp or more must have Total Filtration of:

2.5 mm Al/Eq

**This does not include any compound or compensating filters that are added**

Effects of Increasing the Amount of Filtration of the Beam

1. The beam quality will increase (the beam will have  more higher energy photons).

2. More weak photons will be absorbed

3. There will be fewer weak photons in the beam

4. Beam quantity will decrease. (There will be fewer #s of photons in the beam

5. Reduced radiographic density.  (Because beam quantity decreased, there are fewer photons that strike the IR which results in the decrease in density.

6. The average energy of the beam will increase.

7. There will be a reduction (decrease) in contrast.

Higher kVp leads to a decrease in radiographic contrast -- creating more scatter, more grays, and the decrease in contrast. Because the higher kVp creates more scatter in the image which will therefore create more grays (low contrast/long scale).  Because filtration increases the average energy of the beam, the photons that are attenuated by the patient will have a higher energy level

8. The beam will become "harder."

9. There will be a decrease in patient skin dose.  This is the primary reason for filtration.

Polychromatic

possesing different wavelengths

**The x-ray beam in polychromatic**

Compensating Filters

Filters that are usually designed to solve a problem involving unqeual subject densities and are used for body parts with unequal thickiness throughout their length.

Types

1. Wedge Filter

2. Trough Filter

Wedge Filter

A types of compensating filter that is thicker on one side and thinner on the other.

**The thickest portion of the filter should be placed over the thinnest section of the patient's anatomy.**

Trough Filter

A type of compensating filter that is thick on both ends and thinner in the middle.

**Used for exams like bi-lateral kidneys or even a PA chest**

Compound Filters

A type of filter that uses two or more materials that complement on another in their absorbing abilities.

Most are constructed so that each layer of material will absorb the characteristic photons that were created by the previous layer.  Because of this, compund filters are sometimes called K-Edge Filters.

The filters are designed so that the material with the highest atomic number (z number) will be placed closer to the tube while the material with the lowest atomic number is placed closest to the patient.

The Thoraeus Filter is an example of a compound filter.

Thoreaus Filter

A special type of compund filter that is used in Radiation Therapy.

Composed of Tin (Sn), Copper (Cu), and Aluminum (Al)

**The tin portion of the filter has the highest atomic number and is therefore placed closer to the tube while the aluminum portion is placed closest to the patient.**

Half Value Layer

(HVL)

the amount of absorbing material that will reduce the intensity (quantity) of the primary beam to 1/2 (half) of it's original value.

HVL is a method of determining beam quality and filtration.

Is affected by both kVp and filtration.

The HVL range for diagnostic equipment ranges from 1.0 to 5.0mm Al/Eq

Tenth-Value Layer

(TVL)

the amount of absorbing material placed in the beam that will reduce the intensity of the beam to 1/10th its original value.

TVL is a methof for determinging beam quality and filtration.

1 TVL = 3.3 HVLs

For example:

if the HVL is = to 5.0mm Al/Eq, what is the TVL?

Multiply the TVL by the HVL:

(3.3)(5mm) = 16.5 Al/Eq

Factors that Affect HVL and TVL

1. KvP

2. Filtration

KVP:

the higher the energy of the beam, the greater its value layer; therefore, it takes more absorbing material to reduce the strength of a 100keV beam vs. a 50keV beam.  So, if kVp is set higher, the keV will increase as well.

Filtration:

As the amount of total filtration is increased, the average energy of the beam is increased; therefore, more absorbing material is needed to reduce the intensity of the beam.

Filtration and Effect on Output and Technical Factors

Filtration is useful because it removes low energy photons that don't add any value to the radiographic image.  However, a portion of the useful beam will also be absrobed.  Because of this, radiographic density will be decreased.

When filtration is increased, the exposure factors must also be increased to maintain the same image receptor exposure.

**A point of diminishing returns is reached beyond 3.0mm Al/Eq.  This means that the reduction in skin dose does not warrant the tube-loading increase from the increased exposure factors.

Spectrum

a continuous sequence or range

The x-ray emission spectrum is:

A plot of the relative number of photons emitted (Quantity) vs. the energy of each photon (Quality)

Beam Quantity

refers to the number of photons that has been emitted from the x-ray tube

Quantity is also known as:

1. mAs

2. # of photons (*)

3. Amplitude

**The quantity will be located along the Y axis (up and down)**

Beam Quality

Refers to the energy level of the photons emitted (average energy)

Quality is also known as:

1. keV

2. Average energy

3. Position

4. Energy

**Located along the x axis (across)

**the kVp level selected will determine the maximum keV possible for any photon; photons will never exceed 90keV if 90kVp is selected by the radiographer

Incident Electrons

the electrons formed by thermionic emission from the filament in the cathode that will strike to anode (target)

When the incident electrons strike the anode, they convert their kinetic energy to the atoms of the target material.  This interaction produces the x-ray photons.

The greater the mass or speed of the incident electrons, the greater the quality (energy) and quantity (#) of photons produced.

Changing kVp will change the speeds of the electrons slightly

Incident Electrons are also known as:

1. Incoming Electrons

2. Thermions

3. i