Physics Test (second year)

Which technical factor is responsible for the proper quantity of exposure to the IR?

mAs

Which exposure factor is directly proportional to patient dose?

mAs

What type of dose is better for the patient?

higher kVp, low mAs

Since Compton x-rays are responsible for occupational dose, which technical factor contributes most to student/tech dose?

kVp

Which technical factor is responsible for penetration?

kVp

Which technical factor affects Compton/PE ratio?

kVp

How does kVp affect Quantity at the anode?

An increase in kVp directly and exponentially increases the quantity of x-rays produced at the anode

How does kVp affect the Quantity in the beam?

Increasing kVp directly increases the number of photons in the beam

How does kVp affect the Quality of the beam?

Increasing kVp directly increases the energy of the beam which increases the quality of the beam

How does kVp affect the electrons and the x-ray beam?

Increasing it increases the speed at which electrons are pulled across to the anode

How does kVp affect the penetration?

Increasing kVp increases the penetration

How does kVp affect the quantity at the image receptor?

A 15% increase in kVp doubles the quantity at the IR

How does kVp affect the percentage of scatter?

Increasing kVp increases the percentage of scatter being produced

How does kVp affect PE's?

Increasing kVp decreases the likelihood of photoelectric interactions to occur

How does kVp affect the Compton to PE ratio?

Increasing kVp significantly increases the Compton to PE ratio meaning as the energy of the x ray beam increases the likelihood of Compton interactions becomes much greater compared to photoelectric interactions

How can kVp be used to change exposure (both patient and IR)?

To decrease patient dose while maintaining IR exposure: increase the kVp by 15% and half the mAs
To increase IR exposure: increase kVp by 15% and leave mAs the same

Why it shouldn't kVp be used as a first choice to control IR exposure?

Because kVp mainly controls the QUALITY of the beam rather than the quantity

How can we use kVp if we need a lower dose technique?

15% rule: increase kVp by 15% and half the mAs

A 15% increase in kVp equals a __________________% increase in mAs.

100%

A 15% decrease in kVp equals a __________________% decrease in mAs

50%

A 15% increase in kVp does what to IR exposure if no change is made to mAs?

Doubles the IR exposure

A 15% increase in kVp does what to IR exposure if you compensate with mAs?

Remains the same

A 15% increase in kVp does what to Patient exposure if no change is made to mAs?

Increases patient exposure

A 15% increase in kVp does what to Patient exposure if you compensate with mAs?

Decreases patient dose

A 15% decrease in kVp does what to IR exposure if no change is made to mAs?

Decreases the IR exposure

A 15% decrease in kVp does what to IR exposure if you compensate with mAs?

Remains the same

A 15% decrease in kVp does what to Patient exposure if no change is made to mAs?

Decreases the patient dose

A 15% decrease in kVp does what to Patient exposure if you compensate with mAs?

Remains the same

Using the 5% rule- a 5% change in kVp is equivalent to how much of a change in mAs?

A 5% increase in kVp increases the IR exposure by 30%

Increases in kVp do what to PE's? Comptons?

PE's: decrease
Comptons: increase

Decreases in kVp do what to PE's? Comptons?

PE's: increase
Comptons: decrease

How does scatter and PE affect contrast?

Scatter: decreases the contrast
PE's: increase the contrast

What "image quality" factors does kVp affect? (Scatter, visibility functions and IR exposure)

- scatter
- IR exposure

Why do we (usually) not want to use kVp to adjust exposure for patient size and most pathologies?

Increases in kVp increases scatter

What other equipment may be needed where high kVp techniques are used?

Grids (above 70 kVp use a grid)

How can kVp be used to control motion?

increased kVp results in lower mAs

Why shouldn't kVp be your first choice to control motion (what might be introduced if mAs gets too low?)?

Because if our mAs is too low we can introduce quantum mottle

What 2 types of noise artifacts will using kVp to control motion increase the likelihood of being present on the IR?

- fog
- quantum mottle

If kVp has to be used to affect a change in IR exposure, what is the minimum amount that should be considered?

5% increase

How do you manipulate MAs to control motion?

increase mA and decrease time (reciprocity)

How much of an increase in mAs is required when using a 15% reduction in kVp if you wish to maintain exposure?

double your mAs (100% increase)

Does mAs affect PE ratio?

no

How much of a change in mAs is required to affect a minimum change in IR exposure?

30% increase is the minimum to see any difference

If an image needs to be repeated for inadequate exposure, how much of a change in mAs is required?

By a factor of 2

How do changes in mAs affect quantity/exposure at the anode?

as mAs increases, the quantity increases directly

How do changes in mAs affect quantity/exposure at the IR?

as mAs increases, the quantity/exposure increases directly

How do changes in mAs affect patient exposure?

as mAs increases, patient dose increases

What do we want to use mAs to control? Why not kVp?

Motion, we don't want to use kVp because we can introduce quantum mottle

What is reciprocity?

A give and take relationship. We give more mA to reduce time.
(used when exposure needs to remain the same but contrast needs to be adjusted)

What type of noise artifact is present in the lack of mAs?

Quantum mottle: speckled appearance on the image, mA is too low

Can insufficient mAs effect sharpness of recorded detail (think WAY underexposed)?

Yes

Can too high mAs exposures affect SRD?

Yes, high mA can lead to blooming which increases the actual FSS and decreases SRD

How does mAs affect heat loads?

mAs controls the heating of the filament

How do high mA techniques affect tube life?

Long prep times lead to high hear and blooming which shortens tube life

How does heating of the filament affect the imaging characteristics of the tube?

Over time tanning occurs which increases the inherent filtration of the tube

What type of exposures are best for tube safety?

high kVp, low mAs

What does mAs control in the tube?

mA: thermionic emissions on the filament
mAs: the number of x-rays created on the anode, the number of x-rays emitted from the tube

What type of timers allow for the shortest exposure time?

electronic timers

What type of generators can use a synchronous timer?

single phase machines

Types of tissue from easiest to hardest to penetrate:

air/lung > fat > muscle > bone

Types of tissue that create more scatter?

adipose tissue (fat)

How do you change your technique for a decrease in thickness (subtractive/destructive pathologies)?

30-50% decrease of mAs

How do you change your technique for a increase in thickness (additive/constructive pathologies)?

30-50% increase in mAs

Why do we usually use mAs for a change in tissue thickness?

Because increases in kVp increases scatter

When x-raying casting material what technique do you change?

kVp because you need to penetrate through the casting material

What type of tissue creates more PE interactions?

bone

What are three types of contrast media?

iodine, barium, and air

What does contrast media do to density on the IR?

increases the density on the IR

Which type of contrast media requires the highest kVp?

barium

What is the purpose of a grid? What does it do?

The purpose of a grid is to improve the quality of the image. It cleans scatter up after its created

For what types of tissues is it most important to use a grid?

adipose tissue, thick tissue (10cm or thicker)

What flaw was inherent in the design of grids?

They produce grid strips in the image

What is the grid made of?

alternating strips of lead and spacer material (aluminum) in a protective case

Who fixed the flaw in grids?

Hollis E. Potter

How was the flaw in grids fixed?

The moving bucky (also called a Potter-Bucky) was created where the grid moves while the exposure is being made. They move just before the exposure is made and continues moving after. The grid lines disappear at little increase of patient radiation dose.

What are the two types of moving grids and how do they work?

Reciprocating grid: a grid motor driven back and forth several times during x-ray exposure. The total distance drive is approximately 2cm.
Oscillating grid: positioned within a frame with a 2- to 3- cm tolerance on all sides between the frame and the grid. There is a magnet that pull the grid to one side and releases it at the beginning of the exposure and the grid oscillates in a circular fashion around the grid frame for 20 to 30 seconds before resting.

What does grid use do the image?

- improve image contrast
- clean up scatter after its created
- reduce the level of scatter reaching the IR
- affects the VRD, might slighly change SRD
- decreases exposure to IR
- makes the dark areas darker and the light areas lighter

What does a grid do to patient dose?

Increases patient dose since we are losing numbers reaching the IR we need to compensate by increasing the mAs

When should a grid be used?

If part thickness is over 10cm, and if kvp is over 70.

What is grid ratio?

The ratio of the height of the lead strips to the distance between them GR=h/D
It is the most effective measure of grid effectiveness

What do you do when you go from no grid to a grid technique?

You will multiply your mAs by the bucky factor of the grid you are using

What do you do when you go from one grid ratio to the other?

Divide the mAs by the bucky factor of the grid you are using to get to a no grid technique then multiply that technique by the bucky factor of the grid you need to use.

What grid ratios are best for pediatric patients or when you want to keep the dose low?

Low ratio grids are best since they don't require as much of a mAs increase. (5:1)

What type of grids clean up more of the scatter?

High ratio grids, but more mAs is required

What is grid radius?

-distance at which the primary x-ray beam is parallel to the focused lead strips of the grid.
-the distance from the surface of a focused grid to the point above the grid where all the lead strips would converge
-seen in focused grids

What is grid frequency?

number of grid strips per cm
- grids with high frquency show less distinct grid lines than grids with low frequency
- the higher the grid frequency the thinner the interspace must be and the higher the GR will be

What is grid efficiency?

How well a grid cleans up scatter
A grid 3 important dimensions:
1. height of the strip (h): taller strips=harder for scatter to make it through(think long hallway)
2. thickness of the strip (T): thicker strip=more lead, more x-rays absorbed
3. width of the interspace between (D): thinner interspace material=harder for scatter to get through(think thinner hallway)
- higher grid ratios are more effective

What is a parallel grid?

All the strips are parallel to each other, NOT the x-ray beam
- grid cut off most common with parallel grids
- infinite grid radius
- can angle CR but only longitudinally

What is a cross-hatched grid?

Usually 2 parallel grids together
- problems: little positioning latitude, and allows for no CR angling

What is a focused grid?

The lead strips coincide with the beam divergence
- designed to minimize grid cut off
- specific SIDs must be used
- "canting": leaning of the strips

What is a focused cross hatched grid?

a type of X-ray grid in medical imaging where the lead strips are angled towards the center of the image field (like a focused grid) and are arranged in a criss-cross pattern, with one set of strips running perpendicular to the other, maximizing the absorption of scattered radiation from all directions

What grid has the least positioning latitude?

focused cross hatched

What grid has the greatest positioning latitude?

parallel grids

What grid has the best clean up and highest dose?

cross hatch

Types of grid cut-off:

Off-level: "tilted", grid surface not perpendicular to the CR or CR is angled across the grid, cut off across the IR, caused by improperly positioned x-ray tube
Off-center: usually focused grid, occurs in focused grids with improper lateral centering, more a result of improper positioning of the tube than the grid
Off-focus: major prob w/ focused grids, SID too low or too far, cut off toward periphery, proper distance more important w/ high ratio grids
Upside down grid: "inverted", focused grid, severe grid cut-off in the periphery

What type of focus do parallel grids have?

infinite focus

Where is cut off most likely to be present with parallel grids even when used properly?

In the periphery

With parallel grids, does cut off decrease with changes in SID?

Cut off is worse at short SID's

What would cause grid lines over most of the grid (but maybe darker on one side?)

off center/ off focus

What causes the Moire'/ aliasing effect?

Occurs because of similarities between the CR scanning and grid frequency. Overcome with high grid frequency.