Exposure Chapter 7 Image Quality: The Geometric Factors

Magnification Formula Alternative

% Magnification = Image Width - Object Width / Object Width x100

Penumbra

(also called image blur) is defined as the area of image unsharpness seen at the edges of the fine sturctures within the anatomical part and the outermost edges of the anatomical part being radiographed.

X-ray tubes used in angiographic procedures, magnification radiography, and mammography can have a FSS as small as:

.1 mm.

Typical focal spot sizes for standard radiographic equipment range from:

.5-1.2 mm.

For example, to calculate the geometric unsharpness produced with a .6-mm focal spot, a 5-in. OID, and a 40-in. SID, you can apply the formula as follows (pg. 216):

.6 mm x 5 in. Geometric unsharpness = _________________ 35 in. = 3 mm ________ 35 = .086 mm

Factors affecting penumbra are:

1. OID 2. SID

Because of the angle of the anode disk, penumbra is more pronounced on the ___ side of the beam than it is on the __ side of the beam.

1. cathode 2. anode

Factors influencing recorded detail:

1. focal spot size 2. SID 3. OID 4. patient motion 5. x-ray tube motion 6. image receptor motion

Factors controlling recorded detail:

1. penumbra (geometric unsharpness) 2. motion unsharpness

Involuntary Motion

Cardiac movement, respiration, peristalsis, spasm and tremor

Divergence can potentially do what to a small fracture?

Close it

As SID increases, magnification ________________

Decreases

All radiographic images contain some sort of __.

Distortion

When a CR is not centered on the midpoint of the part of interest, this occurs.

Divergence

Shape distortion exists as either _________ or ______.

Elongation or Foreshortening

The Geomteric unsharpness formula is written as:

FSS x OID Geometric unsharpness = _______________ SOD

Voluntary Motion

General body movements such as movement of the limbs, or flexion, extension, rotation, and lateral bending of the torso or neck.

Magnification Factor Proportion

Image Width/Object Width = SID/SOD

SID and magnification have an ___________

Inverse Relationship

Classifications of Patient Movement

Involuntary and Voluntary motion

Magnification Factor Formula

MF= SID/SOD

Radiographic distortion forms are _______ and _____.

Magnification (size distortion) and shape distortion

This is always written as a %

Magnification Factor

Motion Unsharpness

Most significant form of image unsharpness, appears as uniform blurring of the entire anatomical part or a specific area of the part.

Causes of motion unsharpness are ___ and ___.

Movement of the patient, x-ray tube, and cassette.

The most important factor influencing magnification is ______________

OID

Since increased OID increases magnification, what is also increased?

Penumbra

Shape Distortion Examples

Pg. 231

Table 7-2

Pg. 235

Penumbra caused by OID can be __________ by increasing SID

Reduced

Involuntary motion can often be avoided by:

Shortest practical exposure time

When elongation and foreshortening are used properly for advantageous reasons, we can do what to anatomical parts?

Stretch out or open up

Geometric unsharpness (penumbra)

The loss of recorded detail as a result of how the anatomical information is projected through space toward the image receptor.

Radiographic distortion

The radiographic misrepresentation of the anatomical part of interest.

Controlled patient movement

Used to enhance visualization of anatomical parts

When performing a chest radiograph, using a longer SID is more advantageous because:

a large focal spot can be used (allowing a shorter exposure time), and the penumbra produced is similar to the penumbra produced using a small focal spot.

Penumbra is most pronounced when using:

a large focal spot size (FSS), a long OID, and a short SID.

Blooming

a phenomenon caused by a high mA and low kV setting, the electron beam tends to enlarge as it travels from cathode to anode.

Smaller anode angles and smaller Actual focal spot sizes produce:

a smaller Projected focal spot.

The diameter of the electron beam represents the size of the ___ focal spot.

actual.

The Geometric unsharpness formula is used to determine the ___ in a radiographic image.

amount of penumbra.

Angiographic procedures

an interventional procedure where contrast is injected into the arteries or veins to produce a picture of the blood vessels in the body.

Recorded detail is also called:

definition, resolution, or sharpness.

There is a ___ relationship between FSS or OID and penumbra.

direct.

Suboptimal photographic factors

excessive density or insufficient contrast of the part being radiographed.

Geometric factors

factors that affect either recorded detail or distortion.

The focusing cup is least efficient when a ___ and ___ settings are used.

high mA, low kV.

Since long exposure times can cause motion unsharpness, a ___ and ___ are usually used.

higher mA setting and lower exposure time.

As SID ___, penumbra ___.

increases, decreases.

As FSS or OID ___, penumbra ___.

increases.

The radiographer can minimize the effect of OID on penumbra by:

increasing the SID.

___ the kV and ___ the mA can reduce the effect of blooming.

increasing, decreasing.

There is a ___ relationship between SID and penumbra.

inverse.

Umbra

is the "true" anatomical image.

Magnification radiography or Macroradiography

is used to exaggerate small structures to improve their visualization.

When using a higher mA setting the larger filament is engaged because:

it protects the anode disk from damage. The larger filament size distributes the electron beam over a larger surface area on the anode disk, creating a less concentrated electron beam.

The ___ filament is automatically engaged for mA settings beyond 300mA.

larger.

Tabletop radiographic images exhibit ___than Bucky radiographic images.

less penumbra.

More of the perpendicular x-rays (central portion of the beam) is used at ___ SIDs.

longer.

More mAs is obtained by increasing ___ and ___.

mA, exposure time.

Penumbra existing around the edges of the fine structures is a main concern especially in:

mammography and skeletal radiography.

Macroradiography is commonly used in ___ as a supplument to further evaluate suspect legions detected on the routine images.

mammography.

X-ray tubes with a small FSS are called:

microfocus or fractional focal spot x-ray tubes.

Penumbra negatively affects recorded detail, and should be ___ when possible.

minimized.

At longer SIDs, the beam spreads out over a larger area than shorter SIDs, therefore:

more collimation is needed to limit the beam to the size of the film.

When large anatomical parts are radiographed ___ is needed.

more mAs.

The hazy, "ghostlike" edge of the "true" image.

penumbra.

The high mA setting:

produces a large electron cloud where the electrons, each having a negative charge, repel eachother outward.

If the electron beam spreads out, the actual focal spot increases, and the ___ is negatively affected.

recorded detail.

The low kV setting:

reduces the speed at which the electrons travel from cathode to anode.

The Line-focus principle

refers to the angle of the anode disk, coupled with the Actual focal spot size which determines the size of the Projected focal spot.

Recorded detail

refers to the fine anatomical structures seen in the recorded image.

Distortion

refers to the inaccurate radiographic representation of either the size or shape of the anatomical part being radiographed.

Projected focal spot

represents multiple points on the anode disk, some of the photons carry the edges of the part at a wider angle than other photons in the beam. This produces penumbra.

Penumbra is less pronounced with a ___ than with a ___.

shorter OID, longer OID.

Penumbra is more pronounced with a ___ than with a ___.

shorter SID, longer SID.

A ___ Projected focal spot produces better recorded detail than does a ___ Projected focal spot.

small, large.

Smaller Actual focal spot size are used on:

smaller anatomical parts and patients who are able to remain immobile during the exposure.

The ___ filament is automatically engaged for mA settings up to 300mA.

smaller.

(SOD) is figured out by:

subtracting OID from SID.

Unavoidable OID exists between the ___ and ___ when a Bucky mechanism is used.

tabletop, casette.

The formula used to reduce blooming is___.

the 15% rule.

The Anode heel effect is more pronounced when:

the anode angle is very small.

If the smaller filament were used with a high mA setting instead:

the electron beam would be too narrow and intense on a small area of the anode disk. This intense focused beam can cause surface melting of the anode disk.

The distance from the focal spot to the ___ is an important factor affecting recorded detail. (SID)

the image receptor

The Projected or Effective focal spot is determined by:

the line-focus principle.

The size of the Actual focal spot is determined by:

the size of the anatomical part being radiographed, and the cooperation of the patient.

The FSS is determined by:

the size of the filament used.

The Actual focal spot

the spot where the electrons are focused.

A very small anode angle would be most beneficial because it allows for:

the use of a large Actual focal spot, but also obtain a small Projected focal spot.

The Projected or Effective focal spot

the useful portion of the x-ray beam.

The radiographic factors, density and contrast, affect the ___ of the recorded detail.

visibility.