Selection of technical factors - 507

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Which of the following could be used to improve recorded detail?

1. Slower screen/film combination
2. Smaller focal-spot size

Other imaging factors such as milliampere-seconds and kilovoltage selection and correct photocell selection influence the visibility of recorded detail by affecting

density and contrast

Which of the following is (are) associated with subject contrast?

1. Patient thickness
2. Tissue density
3. Kilovoltage

Radiographic contrast is the sum of

film emulsion contrast and subject contrast

Subject contrast

Several factors influence subject contrast, each as a result of

beam attenuation differences in the irradiated tissues

increased

As kilovoltage increases

higher energy photons are produced, beam attenuation is decreased, and subject contrast decreases

Using a short (25-30 inches) SID with a large size (14 x 17 inches) image receptor is likely to

increase the anode heel effect

anode angles of 10° or less

cause the anode heel effect to be much more apparent

The x-ray beam needs to diverge more to cover a

large-size image receptor

decreases

The x-ray beam has no problem diverging toward the cathode end of the beam, but as it tries to diverge toward the anode end of the beam, it is eventually stopped by the

anode (x-ray photons are absorbed by the anode).

This causes a decrease in beam intensity at the anode end of the beam and is characteristic of the

anode heel effect

To change nongrid exposures to grid exposures, or to adjust exposure when changing from one grid ratio to another, you must remember the factor for each grid ratio:

No grid = 1 x the original mAs
5:1 grid = 2 x the original mAs
6:1 grid = 3 x the original mAs
8:1 grid = 4 x the original mAs
12:1 grid = 5 x the original mAs
16:1 grid = 6 x the original mAs

To adjust exposure factors, you simply compare the old with the new:

(old mAs) = (old grid factor)
```````````` ```````````````````
(new mAs) = (new grid factor)

Which of the following is most likely to produce a radiograph with a long scale of contrast?

Increased photon energy

kilovoltage

Kilovoltage regulates the

penetrability of x-ray photons; it regulates their wavelength—the amount of energy with which they are associated.

The higher the related energy of an x-ray beam, the greater its

penetrability (kilovoltage and photon energy are directly related; kilovoltage and wavelength are inversely related).

As kilovoltage (photon energy) is increased

the number of grays increases, thereby producing a longer scale of contrast

as screen speed increases, so does

contrast (resulting in a shorter scale of contrast).

An increase in mAs is frequently accompanied by an appropriate decrease in

kilovoltage, which would also shorten the contrast scale.

unrelated

Grid ratio is defined as

the height of the lead strips compared to (divided by) the width of the interspace material

grid ratio.

Although the stated focal spot size is measured directly under the actual focal spot, focal spot size really varies along the length of the x-ray beam. At which portion of the x-ray beam is the effective focal spot the largest?

At the cathode end

X-ray tube targets are constructed according to the

line focus principle—the focal spot is angled (usually 12° to 17°) to the vertical

As the actual focal spot is projected downward, it is foreshortened; thus, the effective focal spot is always smaller than the

actual focal spot

actual size

As it is projected toward the anode end, it gets smaller because of the

anode "heel" effect.

How are mAs and radiographic density related in the process of image formation?

mAs and radiographic density are directly proportional.

Radiographic density is described as the

overall degree of blackening of a radiograph or a part of it.

The mAs regulates

the number of x-ray photons produced at the target, and thus regulates radiographic density

proportional

The speed of an intensifying screen is influenced by which of the following factors?

1. Phosphor layer thickness
2.. Phosphor type used

Rare earth phosphors have a much higher conversion efficiency (and therefore speed) and have all but replaced the older

calcium tungstate screens.

speed.

Antihalation backing

a component of single-emulsion film that prevents crossover of fluorescent light within an image receptor.

In radiography of a large abdomen, which of the following is (are) effective way(s) to minimize the amount of scattered radiation reaching the IR?

1. Use of close collimation
2. Use of compression devices

One way to minimize scattered radiation reaching the IR is to use

optimal kilovoltage

Close collimation is also important because

the smaller the volume of irradiated material, the less scattered radiation is produced

Low-ratio grids allow a greater percentage of

scattered radiation to reach the IR.

Use of a high-ratio grid will clean up a greater amount of

scattered radiation before it reaches the IR.

grid

mAs by 30%

mAs is required.

mAs regulates

the number of x-ray photons produced at the target.

An increase or decrease in mAs of at least

30% is necessary to produce a perceptible effect.

Increasing the kVp by 15% will have about the same effect as

doubling the mAs.

density.

Which of the following influences geometric unsharpness?

1. OID
2. Source-object distance
3. SID

magnification

magnification

increases

magnification

Greater latitude is available to the radiographer in which of the following circumstances?

1. Using high-kVp technical factors
2. Using a slow film-screen combination
3. Using a low-ratio grid

error

error

high-ratio grids

Decreasing field size from 14 x 17 into 8 x 10 inches will?

decrease radiographic density and decrease the amount of scattered radiation generated within the part.

Limiting the size of the radiographic field serves to limit the amount of ?

scattered radiation produced within the anatomic part.

As the amount of scattered radiation generated within the part decreases, so does the?

resultant density within the radiographic image

beam restriction is a very effective means of reducing the quantity of non-information-carrying?

scattered radiation (fog) produced, resulting in a shorter scale of contrast with fewer radiographic densities.

Slow-speed screens are used

to image fine anatomic details.

The slower the screen speed, the smaller the quantity of?

fluorescent light emitted during x-ray exposure.

slow-speed screens require more?

x-ray exposure to provide adequate radiographic density and cannot be used when exposure reduction or fast exposure time is essential.

because they are associated with less diffusion of fluorescent light, they produce better?

recorded detail and are used to image structures requiring excellent recorded detail.

Pediatric radiography is likely to require?

fast screens to reduce exposure time and dose.

Shape distortion is influenced by the relationship between the

1. x-ray tube and the part to be imaged.
2. part to be imaged and the image recorder.
3. image recorder and the x-ray tube.

Shape distortion is caused by ?

misalignment of the x-ray tube, the part to be radiographed, and the image recorder/film.

Only one of the three need be misaligned for

distortion to occur.

Which of the following devices is used to overcome severe variation in patient anatomy or tissue density, providing more uniform radiographic density?

Compensating filter

A compensating filter is used when ?

the part to be radiographed is of uneven thickness or density (in the chest, mediastinum vs lungs)

is constructed in such a way that it will absorb much of the primary radiation that would expose the low-tissue density area, while allowing the primary radiation to pass unaffected to the high-tissue density area.

A collimator is used to

decrease the production of scattered radiation by limiting the volume of tissue irradiated.

The grid functions to

trap scattered radiation before it reaches the IR, thus reducing scattered radiation fog.

If a lateral projection of the chest is being performed on an asthenic patient and the outer photocells are selected, what is likely to be the outcome?

Decreased density

caliper

A technique chart identifies?

the standardized factors that should be used with that particular x-ray unit, for various examinations/positions, of anatomic parts of different sizes.

To be used effectively, these technique charts require?

that the anatomic part in question be measured correctly with a caliper.

Exposure latitude will be reduced in which of the following situations?

1. Use of low kilovoltage technical factors
2. Use of high-speed film

In the low-kilovoltage ranges, a difference of just a few kilovolts makes a very noticeable radiographic difference; that is, there is little

exposure latitude.

The faster the speed of the film emulsion:

the less the latitude/margin of error.

What is the best way to reduce magnification distortion?

Decrease the OID.

Shape distortion relates to?

the alignment of the x-ray tube, the part to be radiographed, and the image recorder

OID and the SID.

Magnification can be reduced by either?

increasing the SID or decreasing the OID.

an increase in SID must be accompanied by an increase in ?

mAs to maintain density.

is therefore preferable, in the interest of exposure, to reduce

OID whenever possible.

Artifacts can be a result of?

exposure, handling and storage, or processing.

Exposure artifacts include

motion, double exposure, poor screen-film contact—the effects of these are seen as a result of the exposure

Handling and storage artifacts include

static electricity discharge, crinkle marks, scratches, and light or radiation fog—all these occur as a result of improper usage or storage.

Processing artifacts occur

while the film is in the automatic processor and include pi lines, guide shoe marks, and chemical fog.

The effect described as differential absorption is

2. a result of attenuating characteristics of tissue.
3. minimized by the use of high kVp.

Differential absorption refers to

the x-ray absorption characteristics of neighboring anatomic structures

t may be enhanced with high-contrast technical factors, especially using

low kilovoltage levels.

At low kilovoltage levels

the photoelectric effect predominates.

Diagnostic x-ray photons interact with tissue in a number of ways, but mostly they are involved in the production of

Compton scatter or in the photoelectric effect

Compton scatter occurs

when a relatively high-energy (kV) photon uses some of its energy to eject an outer-shell electron. In doing so, the photon is deviated in direction and becomes a scattered photon.

Compton scatter causes

objectionable scattered radiation fog in large structures such as the abdomen and poses a radiation hazard to personnel during procedures such as fluoroscopy.

In the photoelectric effect

a relatively low-energy x-ray photon uses all its energy to eject an inner-shell electron, leaving a "hole" in the K shell. An L-shell electron then drops down to fill the K vacancy, and in so doing emits a characteristic ray whose energy is equal to the difference between the binding energies of the K and L shells.

The photoelectric effect occurs with

high-atomic-number absorbers such as bone and positive contrast media, and is responsible for the production of radiographic contrast

photoelectric effect

is helpful for the production of the radiographic image, but it contributes to the dose received by the patient (because it involves complete absorption of the incident photon).

Factor(s) that can be used to regulate radiographic density is (are)

1. milliamperage.
2. exposure time.
3. kilovoltage.

Factors that regulate the number of x-ray photons produced at the target can be used to control

radiographic density, namely milliamperage and exposure time (mAs)

mAs

one-half

Although kilovoltage is used primarily to regulate radiographic contrast, it may also be used to regulate

radiographic density in variable-kVp techniques, according to the 15% rule.

Congestive heart failure and pleural effusion involve abnormal amounts of fluid in the chest and thus require

an increase in exposure factors.

An increase in kVp will have which of the following effects?

1. More scattered radiation will be produced.
2. The exposure rate will increase.

An increase in kilovoltage (photon energy) will result in

a greater number (ie, exposure rate) of scattered photons (Compton interaction)

These scattered photons

carry no useful information and contribute to radiation fog, thus decreasing radiographic contrast.

ratio

Grid frequency

refers to the number of lead strips per inch.

are terms denoting the distance range with which a focused grid may be used.

24-inch OID

Magnification radiography may be used to?

delineate a suspected hairline fracture or to enlarge tiny, contrast-filled blood vessels.

mammography

To magnify an object to twice its actual size:

the part must be placed midway between the focal spot and the IR.

Focal spot blur is greatest

toward the cathode end of the x-ray beam.

Focal spot blur, or geometric blur, is caused by

photons emerging from a large focal spot.

The actual focal spot is always larger than the?

effective (or projected) focal spot, as illustrated by the line focus principle.

In addition, the effective focal spot size varies along the longitudinal tube axis, being greatest in size at the ?

cathode end of the beam and smallest at the anode end of the beam

Because the projected focal spot is greatest at the cathode end of the x-ray tube, geometric blur is also greatest?

at the corresponding part (cathode end) of the radiograph.

Which of the following is (are) classified as rare earth phosphors?

1. Lanthanum oxybromide

Rare earth phosphors have a greater

conversion efficiency than do other phosphors.

Lanthanum oxybromide is

a blue-emitting rare earth phosphor,

a green-emitting rare earth phosphor.

Cesium iodide

the phosphor used on the input screen of image intensifiers; it is not a rare earth phosphor

How is the mAs adjusted in an AEC system as the film-screen speed combination is decreased?

The mAs remains unchanged as film-screen speed decreases.

As the speed of the film-screen system decreases:

an increase in mAs is usually required to maintain radiographic density

when an automatic exposure control (phototimer or ionization chamber) is used, the system is programmed for:

the use of a particular film-screen speed

If a slower-speed screen cassette-image receptor is placed in the Bucky tray, the AEC has no way of recognizing it as different and will:

time the exposure for the system that it is programmed for

For example, if the system is programmed for a 400-speed film-screen combination, and if a 200-speed screen cassette-image receptor was placed in the Bucky tray, the resulting radiograph would

have half the required radiographic density.

SID affects recorded detail in which of the following ways?

Recorded detail is directly related to SID.

recorded detail.

Because the part is being exposed by more perpendicular (less divergent) rays, less

magnification and blur are produced.

Although the best recorded detail is obtained using a

long SID, the necessary increase in exposure factors and resulting increased patient exposure becomes a problem.

An optimal 40-inch SID is used for most radiography, with the major exception being

chest examinations.

more density

More scattered radiation is generated within a part as:

the kilovoltage is increased, the size of the field is increased, and the thickness and density of tissue increases.

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