Digital Imaging (390) Test #2

What is the preferred way to measure/ estimate SID?

1. Detent 2. Built in tape measure

What is the backup way to measure/estimate SID?

1. Wingspan - 72" 2. Half span - 40"

If you decrease penumbra, what happens to sharpness?

Increase

If you increase SID, what happens to penumbra and sharpness?

Decrease Penumbra & Increase Sharpness

What is directly responsible for the effects of sharpness?

SOD

What three things does SID effect?

1. Sharpness 2. Magnification 3. Exposure Level

Increased SID does what to magnification?

Decrease

Increased SID does what to Umbra?

Decrease

Increased SID does what to visualized anatomy?

Increase

SID does not affect what?

Shape Distortion

With a shorter SID the shape will become more what?

Obviously distorted

Increased SID does what to intensity?

Decrease

X-rays will diverge as they are emitted in ________ __________ and _________ in intensity as they travel.

straight lines; lesson

SID changes only alter the _______ of the x-ray beam?

Intensity

What are not affected by SID changes?

1. Energy Levels 2. Penetrability

X-rays ________ as they are emitted from the focal spot, they __________ in intensity as they travel from the source.

1. Diverge 2. lesson

A radiographer must compensate for the __________ for significant change in _________ due to loss of ________ at the IR.

mAs; SID; exposure

If there is no compensation for changes in increased SID, what is going to be on the image?

Quantum Mottle

If we don't compensate for the changes in a decreased SID, what is the result to our patient exposure?

Hot Image (unnecessary increase of patient exposure)

What is considered a "significant change" in SID?

Any change greater than 15% should be compensated by mAs.

What law is this Intensity of radiation at given distance from point source is inversely proportional to the square of the distance?

Inverse Square Law

What law is this Used to compensate mAs in order to maintain the original exposure level when changes in the SID are made?

Direct Square Law

Adjusting the __________ restores the original intensity without changing the _________.

mAs; quality

If you increase SID what happens to sharpness and penumbra?

Increase sharpness (visualized anatomy) & Decrease penumbra (magnification exposure intensity)

What image qualities are not directly affected by SID?

1. Subject Contrast 2. Shape Distortion

OID has an impact on everything except what?

Shape Distortion

Secondary x-rays that are traveling in a different direction than the original x-ray beam are known as what?

Scatter Radiation

Scatter radiation impairs image quality by placing exposures on the IR that are?

Unrelated to patient anatomy

Scatter Radiation destroys what 2 things?

1. Subject Contrast 2. Gray Scale (in remanent beam)

Increased OID does what to scatter radiation reaching the IR?

Decrease

Increased OID does what to subject contrast?

Increase

T/F: A larger OID allows for the intensity of scatter radiation to spread out over a larger area.

True

What happens to geometry of the primary beam with changes in OID?

It does not change

What factor affects the penetrability of the primary beam?

kVp

What is the best method to improve image quality?

Air Gap Technique

What are the 2 methods to improve image quality using the air gap technique?

1. Restrict primary beam (collimation) 2. Grid use when appropriate

If you have improved contrast what happens to sharpness?

Reduced

Total exposure at the IR =

Primary Radiation + Scatter Radiation

Reduction in scatter = what in total exposure?

Reduction

What is the main goal when using OID?

Minimization

If you have increased OID what happens to scatter and total exposure at the IR?

Decrease (for both)

If you have decreased OID what happens to scatter and total exposure at the IR?

Increase (for both)

If you increase OID what happens to sharpness?

Decrease

If there is an increase in OID what happens to penumbra?

1. It spreads 2. Increase blur on object edge

If you increase OID what happens to magnification?

Increase

If there is true magnification what happens to the umbra?

Increase

T/F: OID does not affect shape distortion?

True

What is shape distortion affected by?

1. Tube 2. IR 3. Part

What is the magnification formula?

SID/SOD

If you are only given OID and SID how do you find SOD?

OID-SID = SOD

Relative sharpness is proportional and controlled by what formula?

SOD/OID

If there is any change in SID, SOD, and OID what will happen?

Offset by proportionate change in one of the other distances.

What three things rely on ratio distances?

penumbra, magnification, & sharpness

If the resulting number of the ratio is more than 1.0 the image is what?

Sharper

If the resulting number of the ratio is less then 1.0 the image got what?

blurrier

Ideal alignment happens when?

anatomical part and IR are parallel with a perpendicular CR

The extent of distortion depends on what 2 factors?

1. Degree of off centering/ off angling 2. Size and shape of actual object

Off centering and off angling both result in what?

1. Angles 2. Diverging peripheral x-ray beam

Off centering had identical types of effects to what?

angling the beam or part

What is the ideal way to minimize distortion clincally?

1. Part parallel to IR 2. CR perpendicular to part and IR

What are the 2 types of distortion?

1. Elongation 2. Foreshortening

What is image length measures longer than that of the real object (ALWAYS bc of misaligned tube or IR)?

Elongation

What is Image length measures shorter than that of the real object (ALWAYS bc of the misaligned part)?

Foreshortening

What objects in general will have their projected image distorted under more circumstances, and to a greater degree?

Spherical & Cubical

If the object is thicker what happens to the distortion?

Greater distortion (Causing elongation)

Even if the structure does not have an axis (sphere) it will still create what?

an angle (elongation)

Spherical and cubical structures will:

1. Distort under more circumstances 2. Distort to a greater degree than other objects

T/F: If you off center an object, it causes more severe distortion effects at short SID's than long SID's.

True

What are never the cause of distortion?

Distances

What are the 4 geometric objectives of radiographic positioning?

1. Increase Sharpness (anatomy closer to IR)

2. Reduce Magnification (anatomy closer to IR)

3. Minimize Shape Distortion (optimal alignment of anatomy of x-ray & IR)

4. Increase Visibility (Decrease superimposition of contrasting anatomical structures which create noise)

Why must at least 2 projections be taken as a general rule for diagnosis?

Accurate size, shape location of foreign bodies, displacement of fractures & pathological process

Does patient motion blur the recorded image whether it be voluntary or involuntary?

Yes

What are the 3 ways to control the effects of motion?

1. Patient cooperation (good tech communication)

2. Immobilization of examined part

3. Short exposure times

T/F: Movement of the object extends the spread of penumbra at the margins of the image

True

If you have increased motion what happens to contrast?

Decrease

Does motion cause shape distortion?

NO

What is defined as new image created by interaction between anatomy present & motion itself. It is basically a new image that does not represent the real object at all.

False Image

What does the angle of the anode bevel affect?

1. Effective focal spot

2. X-ray intensity distribution (anode heel affect)

What reduces the effective focal spot size for sharper images while maintaining heat dispersion.

Line-focus principle

What happens to image sharpness as the effective focal spot gets smaller?

Sharpness increases

What is crucial to image sharpness in radiography?

Size of the effective focal spot

What controls the effective focal spot size?

Width of the electron beam (determined by filament size)

What is the effective focal spot size at a 45 degree anode angle?

Same as the electron beam width

What happens when the anode is beveled at a lesser angle?

1. surface becomes steeper 2. effective focal spot gets smaller

Standard anode bevel angles in diagnostic x-ray tubes range from:

12-17 degrees

What is the focal spot size for large focal spot setting?

1-2mm

What is the focal spot size for small focal spot setting?

0.5-1 mm

When is large focal spot preffered?

1. Higher techniques 2. more heat dispersion 3. less need for sharpness

What end of the field has the sharpest image due to the line-focus principle?

Anode end

Which end of the image receptor has the largest projected focal spot?

Cathode end

What is the physical area on the anode struck by electrons?

Actual focal spot

What is the projected area of x-ray origin seen on the IR?

Effective focal spot

What is the actual focal spot used for?

Heat Dispersion

Why can't we just use a 0.5 mm actual focal spot?

Too much heat -> risk of anode damage/melting

What's the goal of the line-focus principle?

Max sharpness + good heat dispersion

What causes the anode heel effect?

1. X-rays on anode side pass through more material = reduced intensity

How does the anode heel effect intensity?

Cathode side = more intensity

Anode Side = less intensity

What increases the anode heel effect?

1. steeper anode angles

2. Larger focal spots

3. Short SID

4. Larger imaging plates

When using anode heel effect, where should the thicker body part be placed?

Toward the cathode (Fat Cat)

Why does the collimator increase heel effect at shorter SID?

Wider beam exposes more heel-affected regions