VETN 205B: Exposure Variables

What is intensity or beam quantity?

Number of beam every seconds AKA energy/sec

What is thermionic emission?

When the cathode heats via filaments

What is mA?

Milliamperage - the amount of heat needed to heat the filaments

• Measures the number of electrons

The higher the mA…

The higher the number of electrons

The longer the time…

The more x-rays created

What affects intensity of x-rays?

Time and mA (heat)

What is mAs?

mA x time (secs) = mAs

OR milliamperage in seconds

The higher the mA…

The lower the time

The lower the mA…

The higher the time

What kind of relationship does mA and time have?

Inverse

What are some advantages to mAs?

You can decrease motion artifacts (blur) and personnel exposure by decreasing time

You can penetrate through thick parts with higher mA

The shorter the wavelength…

The higher the penetrating power

What is kVp?

Kilovoltage peak - the penetrating power of photons

• The electrical difference between anode and cathode

The larger the difference…

The higher the kinetic energy and the faster electrons travel

How does kV reach its peak?

Superimposed waves - multiple waves at the same time

The higher the kVp…

The more penetrating photons and the more film exposure

AKA the more “black” the film is

What is Santes’ rule?

(2 x thickness (cm)) + 40 = kVp (from table top)

What is involved in beam quantity?

Time and mA (mAs)

What is involved in beam quality?

kVp

What is the relationship between mAs and kVp?

Inverse relationship - that way they have around the same exposure

The higher the kVp… (in terms of mAs)

The lower the mAs

What is SID?

Source to image distance OR FFD (focal film distance)

What is the inverse square law?

Intensity will increase as distance decreases (gets closer)

The closer (less) the SID…

The higher the intensity

If distance is cut by 1/2…

The intensity is increased by 4x

What are some factors affecting quality?

1. Radiographic density - # of electrons

2. Radiographic contrast - kVp (penetration)

3. Geometric factors

What are geometric factors?

• Focal spot size

• Motion - blurring

• SID

• Object to film distance (OFD)

• Distortion

The smaller the focal spot…

The clearer the image

What creates penumbra?

Big focal spot - creates a halo around object (overexposed)

What is OFD and what does changes to it cause?

Object to film distance and can cause penumbra

• The larger the OFD, the worse the quality

• The smaller the OFD, the clearer the quality

What are some distortion factors?

1. Magnification

2. Elongation

3. Foreshortening

What is magnification?

The image is bigger than what its supposed to be

• If we change OFD and make it larger (meaning object is farther away from film)

What is elongation?

When an object looks longer than it should be

How does elongation come to be?

When the source of the x rays isn’t different (not perpendicular to the table)

• If the position of source is changed

What is foreshortening?

When the object looks shorter than what it should be

How does foreshortening come to be?

If object is not parallel with the source (X-ray tube)

• If position of object is changed

What is density?

Degree of “blackness” on the film AKA the amount of x-rays reaching the film and exposing it

What are some exposure variables when it comes to density?

• mAs - # of photons

• kVp - penetration/heat it takes for filaments

What are some subject variables?

Thickness and tissue density of subject

Ex: Thick part of abdomen vs. thin part of abdomen

Ex: Muscle (fat) vs. bone

When it comes to density, the higher the thickness…

The more absorption it needs

When it comes to density, the higher the tissue density…

The lower the film density or the more blacker (?) / higher kVp

What are the 5 basic densities?

1. Air

2. Fat

3. Water

4. Bone

5. Metal

The higher the density means…

More x-rays are absorbed and reach the film

What is contrast?

Film density difference between adjacent structures OR the different shades of gray

What is high contrast?

There are more contrasting colors (opposite colors) which means more black and whites

• Also called short scale

• Low kVp

What is low contrast?

There are less contrasting colors which means more shades of gray

• AKA long scale

• High kVp

What contrast should be used for bone?

High contrast - only black and white (so low kVp)

What contrast should be used for soft tissue?

Low contrast or high kVp - so more shades of gray and you can see the differences between tissues

How do you get high contrast?

Low kVp, high mAs

How do you get low contrast?

High kVp, low mAs

What exposure factor affects density?

MAs - X-ray quantities

What is the exposure factor that affects contrast?

KVp - penetration

Subject thickness

What is the relationship between kVp and scatter?

The higher the kVp, the more scatter

What is scatter?

Unwanted film exposure

What is scatter caused by?

Low contrast OR high kVp

How do we minimize scatter?

1. Beam limiting

2. Filters

3. Grids

4. Air gap

What do grids do?

Absorb scatter via lead strips

How do grids absorb scatter?

They do it in alternating layers of lead strips and spacers

What are the functions of lead strips?

To absorb unwanted x-rays from scatter

What is the function of spacers in grids?

Lets wanted x-rays inside

Where should grids be located?

Between the subject and film - more photons and less distance (OFD)

When it comes to grids, what should be increased?

MAs should be increased - meaning more photons - bc some photons are blocked by the grid

What are some type of grids?

1. Parallel

2. Focused

3. Linear

4. Crossed

What makes grid lines?

Lead strips over film

What gets rid of grid lines?

Potter-Bucky diaphragms - moves grid which blurs lines

What is a disadvantage of Bucky diaphragms?

They make noises/vibrate which can scare or startle the patient

What creates geometric unsharpness?

1. Focal spot size - big focal spot size makes penumbra (which is a halo)

2. Motion - creates bluriness

What are types of geometric distortion?

1. Magnification - SID is closer (less)

2. Elongation - beam (source) is not perpendicular to receptor

3. Foreshortening - subject (object) is not parallel to receptor