Radiology Lecture 2: X-Ray Filtration & Collimation Terms

Removes low-energy x-rays

Filtration

Collimation

Limits the size of the x-ray beam

- Reduces the radiation dose

- Reduces scattered radiation

Inherent filtration

X-rays produced at target go in all directions; the process where most are absorbed by the tube or tube head

Other x-rays exit the tube head through the diaphragm at the window to become the _________

Primary beam

Primary beam photons have __________

Wide range of energy (some very strong, some weak)

Added filtration is used to _______________ from the primary beam before they hit the patient

Absorb the low energy photons

What is filtration measured in?

mm Al

What is the typical range of inherent filtration?

Usually 0.5 to 2.0 mm Al

What is added filtration?

Aluminum plate at opening of tubehead

What must added filtration be with kVp lower than 70?

Must be 1.5 mm Al

What must added filtration be with kVp at 70 or above?

Must be 2.5 mm Al

Added filtration is the _____________ at __________ of tubehead

Aluminum plate; base

By removing low energy photons, there is a _________ proportion of high energy photons

Greater

Filtration ___________ the average energy of the x-ray photons in the primary beam, but ________ number of photons

Increases; decreases

Energy (quality) of primary beam photons increased with:

Increased kVp

Increased added filtration

Number (quantity) of primary beam photons increased with:

-Increased mA

-Increased exposure time

-Increased kVp

-Decreased added filtration

Collimator

Opening in a plate on outer surface of the added filtration

What does the collimator determine?

Size and shape

What is collimation a combination of?

Collimator and the position indicating device at the end

What does the position indicating device (PID) do?

Limits size of beam and reduces area exposed

- Reducing absorbed dose

- Reducing scattered photons

What is the best PID shape?

Rectangular; reduces radiation dose by 65%

What is the best PID length?

Long; 16" reduces radiation dose by 35%

What PID is used in UDM radio clinic?

12" SFD rectangular

What is the inverse square law?

Intensity of radiation varies inversely with the square of the distance between source and absorbing material

T/F: Intensity is increased with long distance

False

- Intensity is increased with short distance

- Decreased with long distance

Length of PID affects ___________

Intensity of radiation

Changes in PID length require ______________

Changes in exposure time

Shorter PIDs will produce ____________ on the sensor than long PIDs, with the difference proportional to the ________ of the difference in length.

- Image will be ________

More intense radiation; square

- Darker

Longer PIDs will produce ____________ on the sensor than short PIDs, with the difference proportional to the ________ of the difference in length.

- Image will be ________

Less intense radiation; square

- Lighter

To compensate for inverse square law,

- Short PID requires ________ exposure time

- Long PID requires ________ exposure time

Short; long

- Note: Difference in time varies with the square of the difference in distance

New exp time / Old exp time = (New PID length)² / (Old PID length)²

Example of changing exposure time to compensate for inverse square law:

- Original PID is 8", original exposure time is 0.2 sec

- New PID is 16", what exposure time should be used?

- X sec/0.2 sec = (16")^2/(8")^2= 4

- X sec= 0.2 sec * 4 = 0.8 sec

- Exposure time with 16" must be 0.8 sec to maintain the same darkness of the image as 12 impulses with an 8"

What makes the image darker?

Overexposure

- Too high kVp, mA, and/or exposure time

- Too short PID

What makes the image lighter?

Underexposure

- Too low kVp, mA, and/or exposure time

- Too long PID

What are three possible interactions?

1. Transmission

2. Absorption

3. Scatter

Label A, B, and C

A= scatter

B= absorption

C= transmission

What color images does transmission, absorption, and scatter make?

Transmission= dark

Absorption= light areas

Scatter= bounce off in diff. directions

What is the incidence of transmission, absorption, and scatter?

Transmission = 9%

Absorption = 27%

Scatter = 64%

What are the two types of scatter?

coherent scatter and compton scatter

What is Coherent (Rayleigh, elastic) scatter?

- Emits photon with same energy

- 7%

What are Compton scatter?

- Emits photon with lower energy

- 57%

What is the most common interaction of x-rays with matter?

Compton scattering

Fog

Undesirable darkening of the image

- Caused by scatter

What is transmission more common in? How does it appear?

Soft tissues are radiolucent

What is absorption more common in? How does it appear?

Hard matter are radiopaque

Absorption occurs through the _________

Photoelectric effect

Where is scatter more likely to occur with?

High energy x-rays

During a dental radiographic exposure, which is the most common interaction of x-ray photons and the patient?

A. Transmission

B. Absorption

C. Coherent scatter

D. Compton scatter

D. Compton scatter

Dark images have ________ radiographic density

High

Light images have ________ radiographic density

Low

What is radiographic density?

The degree of darkness on the radiograph

Radiographic density is increased with....

- High energy x-rays

- Soft tissues

- Thin tissue sections

- High vKp, mA, and exposure time

- Short PID

= Darker image

Greater x-ray exposure leads to ________

Increased radiographic density

Thinner parts allow __________, which causes images to be ________

- More transmission

- Dark (increased radiographic density

Thicker parts of a stepwedge ________________; less _______________ causes ____________

- Absorb more radiation

- Transmission of x-rays to film/sensor

- Light appearance

What is radiographic contrast?

Difference in density (darkness) between different parts of the image

What are the two components of radiographic contrast?

- Subject contrast

- Viewing conditions

What does subject contrast result from?

Differential absorption of x-rays through different tissues in the patient's body

= Different degrees of darkness

How does a high kVp affect subject contrast?

- High kVp→ gradual decrease in transmission→ gradual decrease in darkness→ less difference= low subject contrast

How does a low kVp affect subject contrast?

- Low kVp→ drastic transmission→ greater difference in darkness= high subject contrast

High kVp x-rays produce many shades of gray between black and clear (low contrast). This is called a ______________

Long gray scale or long-scale contrast

Low kVp x-rays produce few shades of gray (high contrast). This is called _________

Short gray scale or short-scale contrast

What is contrast controlled by?

Degree of exposure (how dark is the image?)

- Underexposure = too light

- Overexposure = too dark

Latitude

Range of exposures in which contrast and density are acceptable

Post-acquisition enhancement

Change brightness and contrast in digital images to correct for small errors in exposure time

T/F: Post-acquisition enhancement will fix a radiograph with severe errors in exposure time

False; it will NOT

What is fog?

Degree of unwanted darkness on radiograph

What causes fog?

Scattered radiation

How is scattered radiation/fog reduced?

By using long, rectangular PIDs

What does viewing x-rays in a darker room do?

Allows detection of subtle differences in darkness

What are the rules for viewing radiographs?

Don't look at them on a tablet PC in the bright clinic if you are trying to detect caries, periapical disease, or periodontal defects

- You will probably miss something

Radiographs should be examined in a darkened, quiet room