267 terms

Image Production & Criteria for Evaluation - 267

A foreign body in the image receptor
will be sharply imaged on the finished radiograph.
an artifact that has sharply delineated edges indicates that it is located
adjacent to the intensifying screens within the cassette.
An inverted focused grid will result in
an area of exposure down the middle of the image and grid cutoff everywhere else
As kVp is increased, the percentage of scattered radiation relative to primary radiation
increases, hence a grayer appearance
chemical fog usually produces
a very gray image
Examples of processing artifacts are
guide shoe marks, roller marks, and scratches
excessively high contrast caused by
too low kVp, too short a scale of contrast
factors are most likely to be the cause of quantum noise/mottle?
Fast film and screens with low mAs and high kVp
Focal spot blur would cause
a slight blur of anatomic details
Grid cutoff appears
as a gradually decreasing density
Grid cutoff is absorption of the primary beam by the grid and usually results in
loss of density and visibility of grid lines.
Grids are designed to
selectively absorb scattered radiation while absorbing as little of the primary radiation as possible
Images produced with higher ratio grids will possess
: fewer grays than those made with lower ratio grids
light fog generally results in
a very black finished radiograph
Overdevelopment makes a radiograph appear
very dark, and sometimes gray
Poor screen-film contact results in
very blurry areas of the finished radiograph.
Quantum noise, or mottle, is a grainy appearance on a finished radiograph that is seen especially in
fast-imaging systems
Scattered radiation fog gives the radiograph a
flat, gray appearance.
The farther the object is from the image receptor, the more
: blurred its edges will be as a result of magnification distortion.
The film exposed by a large static discharge ("tree static") frequently exhibits
black, branching artifacts
the most effective way to remove scattered photons from those exiting the patient?
a grid
The production of scattered radiation will also be limited if the field size is
as small as possible
too much background density is caused by
too much mAs
Use of optimal kilovoltage for each anatomic part is helpful in keeping scatter
to a minimum
describes how closely fine details may be associated and still be recognized as separate details before seeming to blend into each other and appear "as one."
The degree of resolution transferred to the IR is a function of:
the resolving power of each of the system components and can be expressed in line pairs per millimeter (lp/mm), line-spread function (LSP), or modulation transfer function (MTF).
Line pairs per millimeter can be measured using:
a resolution test pattern; a number of resolution test tools are available
LSP is measured using a
10-m x-ray beam
MTF measures
the amount of information lost between the object and the IR
line-focus principle
The effective focal spot is the foreshortened size of the actual focal spot as it is projected down toward the IR, that is, as it would be seen looking up into the emerging x-ray beam
Small crescent-shaped artifacts on processed x-ray film are usually the result of
acute bending of the film before or after exposure.
A radiograph made with a parallel grid demonstrates decreased density on its lateral edges. This is most likely due to
decreased SID.
If there was a centering or tube angle problem
there would more likely be a noticeable density loss on one side or the other.
focused grid had been placed upside down
only the central portion of the image (along the long axis of the image receptor) would have been imaged
crescent-shaped kink, or crinkle, marks
caused by acutely bending the x-ray film.
an artifact will usually appear as a plus-density (dark) artifact if it is produced:
before exposure
an artifact will usually appear as a minus-density (light) artifact if it is produced:
after exposure.
X-ray film emulsion is very sensitive to mishandling, particularly after
Which of the following terms is used to describe unsharp edges of tiny radiographic details?
geometric blur
the area of blurriness that may be associated with small image details
blurriness can be produced by
using a large focal spot, or by diffused fluorescent light from intensifying screens
The image proper (ie, without blur)
a grainy appearance caused by fast imaging systems
If the grid failed to move during the exposure
there would be grid lines throughout
If the central ray was off-center
there would be uniform loss of density.
When a slow screen-film system is used with a fast screen-film automatic exposure control system, the resulting images
are too light
When an AEC (phototimer or ionization chamber) is used
the system is programmed for the use of a particular screen-film speed (eg, 400 speed).
If a slower-speed screen 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
if the AEC is programmed for a 400 screen-film combination, and if a 200-speed screen image receptor is placed in the bucky tray, the resulting radiograph:
will have half the required radiographic density
Foreshortening can be caused by
the radiographic object being placed at an angle to the IR
to reduce shape distortion
Aligning the x-ray tube, anatomic part, and image recorder so that they are parallel
Tube angulation causes elongation of the part
Size distortion (magnification)
is inversely proportional to SID and directly proportional to OID
Decreasing the SID and increasing the OID serve to
increase size distortion
What are the effects of scattered radiation on the radiographic image?
It produces fog
Scattered radiation
produced as x-ray photons travel through matter, interact with atoms, and are scattered (change direction).
If these scattered rays are energetic enough to exit the body:
they will strike the IR from all different angles
scattered rays
do not carry useful information and merely produce a flat, gray (low-contrast) fog over the image.
Grid cutoff
increases contrast and is caused by an improper relationship between the x-ray tube and the grid, resulting in absorption of some of the useful/primary beam
Treelike, branching black marks on a radiograph are usually the result of
static electrical discharge.
Problems with static electricity are especially prevalent during
cold, dry weather and can be produced by simply removing a sweater in the darkroom.
Grid cutoff due to off-centering would result in
overall loss of density
Over- or underexposure under the anode is usually the result of
exceeding the focusing distance limits in addition to being off-center
Foreshortening of an anatomic structure means that
it is projected on the IR smaller than its actual size
What effect will a stained intensifying screen have on the finished radiograph?
Decreased density
If an intensifying screen becomes stained, either partly or wholly, the stained area:
will not react to x-ray photons as completely and will emit less light.
the film emulsion adjacent to the stained area(s) will exhibit:
decreased density on the finished radiograph.
how would changing exposure factors affect film
The original mAs (regulating radiographic density) was 200. The original kVp (regulating radiographic contrast) was 90. The mAs was cut in half, to 100, causing a decrease in density. The kVp was increased (by 15%) to compensate for the density loss and thereby increase the scale of contrast.
developer temperature
1. The development process slows at the lower temperature, and a much lighter radiograph results.
2. at too high temperature, the development process is accelerated and the radiograph is too dark
A black crescent usually results from bending
after exposure
A characteristic curve compares
the exposure, given the film with the resultant density
A characteristic curve has three portions:
the toe, the straight-line portion (region of correct exposure), and the shoulder
A characteristic curve is used to
predict the speed, contrast, and exposure latitude of a particular film emulsion.
A decrease in source-image distance causes
magnification and blurriness of recorded detail.
a destructive disease such as osteoporosis will decrease
A grid is usually employed
1. when radiographing a large or dense body part.
2. when using high kilovoltage.
A grid is
a thin wafer placed between the patient and the IR to collect scattered radiation. It is made of alternating strips of lead and a radiolucent material such as plastic or aluminum.
A higher milliamperage with a short exposure time is generally
A histogram is a
graphic representation showing the distribution of pixel values
A microswitch initiates and terminates replenishment as
it senses the beginning and end of each film.
A radiograph exposed using a 12:1 ratio grid may exhibit a loss of density at its lateral edges because the
SID was too great
A technique chart should include which of the following information?
1. Recommended SID
2. Grid ratio
3. Screen-film combination
According to the line focus principle, an anode with a small angle provides
1. improved recorded detail.
2. improved heat capacity.
The line focus principle illustrates that as the target angle decreases, the effective focal spot decreases (providing improved recorded detail), but the actual area of electron interaction remains much larger (allowing for greater heat capacity). It must be remembered, however, that a steep (small) target angle increases the heel effect, and part coverage may be compromised.
an increase in bone size and density
All of the following are related to recorded detail
A. motion.
B. film-screen contact.
Although SID affects exposure rate (according to the inverse square law of radiation), and therefore affects density (according to the density maintenance formula), it is not used to regulate
radiographic density
An additive pathology such as Paget's disease will increase
An increase in exposure factors is usually required in which of the following circumstances?
1. Edema
2. Ascites
3. Acromegaly
An increase in kVp with appropriate compensation of mAs will result in
increased exposure latitude
An overall image density arising from factors other than the light or radiation used to expose the image is called
anatomic parts measuring in excess of 10 cm require
a grid.
Anything other than intensifying screen light or primary x-radiation is
undesirable in terms of image exposure
anything foreign to the image; the term could include fog, but it also covers many physical interferences.
As focal spot size increases, recorded detail
decreases because more penumbra is produced.
As focal-object distance and SID decrease, so does
As kVp is increased,
x-ray photons begin to interact with atoms of tissue via the Compton scattered interaction
As SID increases, so does
recorded detail, because magnification is decreased
As temperature increases,
contrast decreases.
As the kilovoltage is increased
more penetration will occur and a greater range of densities (grays) will be apparent in the image
a collapsed or airless lung; it requires an increase in exposure factors.
Beam restrictors function to
limit the x-ray field size, thereby reducing the production of scattered radiation and shortening the scale of contrast.
Because mammographic techniques operate at very low kVp levels, the cassette front material
becomes especially important.
because of the target angle,
penumbral blur varies along the longitudinal tube axis, being greater at the cathode end of the image and less at the anode end of the image.
Because shorter SIDs and larger IR sizes require greater divergence of the x-ray beam to provide coverage,
the anode heel effect will be accentuated.
is a chronic dilatation of the bronchi with accumulation of fluid
Change is affected in
average exposure level and exposure's latitude; these changes will be reflected in the images informational numbers ("S number," "Exposure Index," etc)
changing from single-phase to three-phase, 12-pulse equipment; therefore the new mAs should be
half the original
Comparison of Technical Factors Required
Single-Phase (x mAs)
Three-Phase, 6-Pulse (2/3x mAs)
Three-Phase, 12-Pulse (1/2x mAs)
Compensating filters
can be accommodated by tracks in the tube head.
Compensating filters
can be wedge-shaped (with the thicker part of the wedge paralleling the thinner body part), thus compensating for greater or lesser tissue densities (as in a large decubitus abdomen)
Crescent-shaped black marks on the finished radiograph are usually due to
bending the film acutely
Degree of accuracy in positioning and centering
can have a significant effect on histogram appearance (as well as patient dose)
Differential absorption refers to
the different attenuation, or absorption, properties of adjacent body tissues
Each film passing through the processor solutions
takes with it a certain amount of solution
a chronic pulmonary disease characterized by an increase in the size of the air-containing terminal bronchioles.
exposure factors that would properly expose one part will
severely overexpose or underexpose the neighboring part (as with lungs vs thoracic spine)
Factors that affect screen speed will also affect
radiographic density.
Focal spot size is thus inversely related to
radiographic sharpness or recorded detail.
Focal spot size
one of the geometric factors affecting recorded detail; it has no effect on the scale of contrast.
Focal-object distance and SID are inversely proportional to
Geometric unsharpness is influenced by which of the following?
1. Distance from object to image
2. Distance from focus to object
3. Distance from focus to image
Geometric unsharpness is most likely to be greater
at the cathode end of the image
Going from three-phase, 12-pulse to single-phase requires
twice the mAs.
Greater latitude is available to the radiographer when using
1. high kVp factors.
2. a slow film-screen combination.
Guide shoes
also called deflector plates, serve to keep the film on its proper course by directing or guiding it around corners.
he type of chemistry used in the automatic processor and especially the temperature of the solution can have a big impact on
the resulting image contrast.
High radiographic contrast also results when
radiographing anatomic parts that have high subject contrast, such as the chest.
High radiographic contrast is, in part, a result of
lower energy photons (lower kVp).
Higher contrast is
shorter scale contrast; it is present in an image that has few shades of gray between white and black.
Histogram analysis and use of the appropriate LUT together function to
produce predictable image quality in CR.
How can the radiographer reduce the amount of scattered radiation generated during a radiographic examination?
1. Use optimum kVp.
2. Collimate closely.
How would the introduction of a 6-inch OID affect image contrast?
Contrast would be increased
ID can affect contrast when it is used as
an air gap.
If 10% of the illuminator's light passes through the image, that image has a density of
If a 6-inch air gap (OID) is introduced between the part and IR
much of the scattered radiation emitted from the body will not reach the IR
If intensifying-screen phosphors are covered with dust
either they will not fluoresce or their fluorescence will not reach the IR emulsion
If single-emulsion film was loaded into its cassette with the emulsion facing away from the intensifying screen, the resulting image would demonstrate
decreased density
If the central ray was off-center, there would be
uniform loss of density.
If the exposure is then made, scattered radiation from the tabletop (where there is no absorber) will undercut the part, causing
excessive image density.
if the film were loaded with the antihalation side against the light-emitting screen, the film emulsion would
receive very little exposure
If the grid failed to move during the exposure, there would be
grid lines throughout.
if the grid is not used within its recommended SID (focus) range (ie, if the SID is too great or too little), there will be
a decrease in density at the periphery of the image.
If the quantity of black metallic silver on a particular radiograph is such that it allows 1% of the illuminator light to pass through the image, that image has a density of
if the screen is scratched and phosphors are removed,
there will be no fluorescence to expose the IR.
If the x-ray beam is not centered to the grid, or if the x-ray tube and grid surface are not parallel (level), there will be a fairly uniform decrease in
radiographic density across the entire image.
If the x-ray tube is angled significantly across the lead strips of a focused grid, there is
uniform loss of density (grid cutoff)
If, for example, a blue-sensitive emulsion were matched with green-emitting screens, the resulting radiograph would be
underexposed because the blue-sensitive film emulsion was not responsive to the green-emitting phosphors.
If, however, a lead rubber mat is placed on the overhanging illuminated area, most of this scatter will be
Improper spectral matching between rare earth intensifying screens and film emulsion results in
insufficient density
In addition, as the kVp and scale of grays increase,
the exposure latitude increases; the "margin for error" in technical factors becomes greater.
In which of the following examinations would a cassette front with very low absorption properties be especially desirable?
In which of the following ways can higher radiographic contrast be obtained in abdominal radiography?
1. By using lower kilovoltage
2. By using a contrast medium
3. By limiting the field size
Inadequate penetration and high contrast are a result of insufficient
Increased collimation is important in
the control of patient dose and scattered radiation, not differential absorption.
Insufficient or excessive distance with focused grids causes
loss of density (grid cutoff) along the periphery of the image.
intensifying screens
amplify the action of x-rays
It is essential that the light-sensitive emulsion be placed against the
light-emitting screen
It is the function of radiographic contrast to
make details visible
It must be remembered, however, that a steep (small) target angle increases
the heel effect, and part coverage may be compromised
Log relative exposure
the amount of exposure required to produce a given density as measured on the sensitometric graph
is said to be the greatest enemy of recorded detail because it completely obliterates image sharpness
Multiple myeloma
a condition characterized by infiltration and destruction of bone and marrow.
OID acts as a
low-ratio grid and increases image contrast.
OID is used to effect
an increase in contrast in the absence of a grid, usually in chest radiography
OID may be said to be directly proportional to
can affect contrast when it is used as an air gap.
One way to determine the quantity of replenisher solution to be added is by
the length of the film entering the processor.
Optical density
normal radiographic density.
optimal kVp is recommended to reduce
the production of scattered radiation
or exostosis, is a (usually benign) bony tumor that can develop on bone
osteomalacia, fibrosarcoma, and paralytic ileus (obstruction), result in a
decrease in body tissue density.
a softening of bone so that it becomes flexible, brittle, and deformed
Other factors affecting histogram appearance and, therefore, these informational numbers, include
selection of the correct processing algorithm (e.g., chest vs femur vs cervical spine), changes in scatter, SID, OID, and collimation.
Parts generally requiring the use of a grid include
the skull, spine, ribs, pelvis, shoulder, and femur.
Penetration and contrast are a function of
penetrometer (aluminum step wedge)
used to illustrate the effect of kVp on contrast
is inflammation of the lung(s) with accumulation of fluid.
a collection of air or gas in the pleural cavity.
Poor screen-film contact
reduces recorded detail because of the degree of light diffusion in the areas of poor contact. Areas of poor contact appear blurry.
pulmonary emphysema
a chronic disease characterized by overdistention of the alveoli with air.
Radiographic contrast is described as
the difference between densities, or scale of grays, in the radiographic image.
Rare earth-type phosphors
absorb x-rays more efficiently and convert their energy into fluorescent light; they therefore affect both screen speed and radiographic density
Recorded detail is directly related to
source-image distance (SID)
Replenishment is also determined by
the number of films (volume) processed.
Replenishment is also essential for
maintaining each solution's level of concentration, to maintain solution activity and avoid chemical fog from exhausted solutions.
Restricting the size of the field will also function to
increase contrast because less scattered radiation will be generated.
SID is directly related to
recorded detail.
Since pathology can alter the degree of attenuation of the x-ray beam, it can affect
Single-emulsion film has an antihalation backing that
efficiently absorbs reflected (crossover) light.
Single-emulsion film is used for particular examinations, such as
Single-emulsion film is used
with a cassette that has a single intensifying screen and provides better detail than typical double-emulsion film and two-screen cassettes
Special plastics that resist impact and heat softening, such as polystyrene and polycarbonate, are frequently used as
cassette front material
straight-line portion
follows the toe; portion of correct exposure and extends from about 0.25 to 2.5
Subcutaneous emphysema
is a pathologic distention of tissues with air
Technique charts are
exposure factor guides that help technologists produce radiographs with consistent density and contrast
Technique charts do not take into account
the nature of the part (disease, atrophy, etc)
The abdomen has low
subject contrast, and therefore abdominal radiographs will tend to have very low contrast unless technical factors are selected to increase contrast.
The amount of replenisher solution added as a film enters the automatic processor is related to the
1. size of the film.
2. number of films processed.
The amount of scattered radiation generated in a given exposure increases as three factors increase:
the kVp, the field size, and the thickness and density of the tissues.
The anode heel effect becomes more pronounced as
the SID decreases, as IR size increases, and as target angle decreases.
The CR laser scanner recognizes the various tissue density values and constructs a
representative gray-scale histogram
The crossover rollers
are located at the top of the processor, out of the solution, and direct the film from one solution tank to the next. These are the racks that need daily cleaning to avoid chemical or emulsion buildup on their surface. Chemical or emulsion buildup on roller surfaces can cause film artifacts.
The curve then bends and levels off at the
shoulder (Dmax) portion of the curve.
The effect that differential absorption has on radiographic contrast of a high subject contrast part can be minimized by
1. using a compensating filter.
2. using high-kVp exposure factors.
The fact that x-ray intensity across the primary beam can vary as much as 45% describes the
anode heel effect
the function of grids is to
collect scattered radiation, they serve to shorten the scale of contrast.
The function of the developer solution chemicals is to
reduce the latent image to a manifest image
The greater the number of films processed,
the lower the required replenishment rate
The larger the part,
the more scattered radiation is generated.
The latent image is
the invisible image produced within the film emulsion as a result of exposure to radiation
The line focus principle illustrates that as the target angle decreases,
the effective focal spot decreases (providing improved recorded detail), but the actual area of electron interaction remains much larger (allowing for greater heat capacity).
The major exception to this rule is
the chest
The mAs selected is directly proportional to
radiographic density (ie, if the mAs is cut in half, radiographic density will be halved)
The OID is thus acting as a
low-ratio grid and increasing image contrast.
The photoelectric effect
the interaction between x-ray photons and matter that occurs at low kVp levels—levels that tend to produce short-scale contrast.
The presence of dust or scratches on intensifying screens will cause
decreased density in those areas of the image
The principal quantitative factor regulating radiographic (or optical) density is
The processor rollers that are out of solution and function to transfer the film from one solution to another are the
crossover rollers
the SID needs to be increased 7 inches for every
1 inch of OID.
The spongy layer behind each intensifying screen helps ensure
good screen-film contact and therefore good recorded detail. The spongy layer is unrelated to radiographic density.
The sum of subject contrast and film contrast equals
radiographic contrast.
The technical factors mAs, kVp, and grid ratio have no effect on
recorded detail.
The thickness of the phosphor layer affects
speed and density similarly: As the thickness of the phosphor layer increases, speed and density increase.
The toe
occurs immediately after base-plus fog, whose density must not exceed 0.2.
The use of soft, low-energy x-ray photons is the underlying principle of
mammography; any attenuation of the beam would be most undesirable
The x-ray tube anode is designed according to the
line focus principle, that is, with the focal track beveled
They suggest a group of exposure factors to be used at
a particular SID with a particular grid ratio, screen-film combination, focal spot size, and central ray angulation.
This allows a larger actual focal spot to project a
smaller effective focal spot, resulting in improved recorded detail with less blur
This effect can be minimized by the use of
a compensating filter or by the use of high kilovoltage (for more uniform penetration).
This is frequently helpful in
lateral lumbar spines and AP shoulders
This is termed
long scale or low contrast
To better demonstrate high contrast within a viscus
a contrast medium such as barium, iodine, or air can be used.
To produce high radiographic contrast in abdominal radiography,
lower kVp should be used.
Tube current affects
radiographic density and is unrelated to recorded detail.
Turnaround assemblies
are located at the bottom of each solution tank and function to direct the film from a downward to an upward motion
Two parts with widely differing absorption characteristics will produce
a high radiographic contrast.
unexposed crystals are removed from the film during
the fixing process.
What should be done to correct for magnification when using air-gap technique?
Increase SID
When changing from single-phase to 3-phase, 12-pulse equipment,
only one-half of the original mAs is required.
When changing from single-phase to three-phase, six-pulse equipment,
two thirds of the original mAs are required to produce a radiograph with similar density
When going from three-phase, six-pulse to single-phase, add
one-third more mAs.
When the collimated field must extend past the edge of the body, allowing primary radiation to strike the tabletop, as in a lateral lumbar spine, what may be done to prevent excessive radiographic density due to undercutting?
Use lead rubber to absorb tabletop primary radiation
When the primary beam is restricted to an area near the periphery of the body, sometimes
part of the illuminated area overhangs the edge of the body
Which of the following adult radiographic examinations usually require(s) use of a grid?
1. Ribs
2. Vertebrae
3. Shoulder
Which of the following can affect histogram appearance?
1. Centering accuracy
2. Positioning accuracy
3. Processing algorithm accuracy
Which of the following can affect radiographic contrast?
1. Processing
2. Pathology
3. OID
Which of the following conditions require(s) a decrease in technical factors?
1. Emphysema
2. Osteomalacia
Which of the following factors affect(s) both radiographic density and intensifying screen speed?
1. Thickness of phosphor layer
2. Type of phosphors used
Which of the following function(s) to reduce the amount of scattered radiation reaching the IR?
1. Grid devices
2. Beam restrictors
Which of the following is most likely to occur as a result of using a 30-inch SID with a 14 x 17-inch IR to radiograph a fairly homogeneous structure?
Density variation between opposite ends of the IR
Which of the following methods can be effectively used to decrease differential absorption, providing a longer scale of contrast in the diagnostic range?
1. Using high kVp and low mAs factors
2. Using compensating filtration
Which of the following pathologic conditions are considered additive conditions with respect to selection of exposure factors?
1. Osteoma
2. Bronchiectasis
3. Pneumonia
Which of the following pathologic conditions require(s) a decrease in exposure factors?
1. Pneumothorax
2. Emphysema
3. Multiple myeloma
Which of the following radiographic accessories functions to produce uniform density on a radiograph?
Compensating filter
Which of the following will have an effect on radiographic contrast?
1. Beam restriction
2. Grids
Which portion of the characteristic curve would most likely represent a density of 1.0?
Straight-line portion
Which type of error results in grid cutoff at the periphery of the radiographic image?
While Compton scatter reduces patient dose, compared to photoelectric interactions
it can pose a significant radiation hazard to personnel during fluoroscopic procedures.
white crescent occurs if the film is bent
before exposure
With a 6-inch OID, the SID is usually increased from 6 feet to
10 feet (120 inches)
With three-phase equipment x-ray intensity is
significantly greater.
With three-phase equipment, the voltage never drops to
Which of the following pathologic conditions require(s) a decrease in exposure factors?
1. Pneumothorax
2. Emphysema
3. Multiple myeloma
All 3 conditions --
All three pathologic conditions involve processes that render tissues more easily penetrated by the x-ray beam. Pneumothorax is a collection of air or gas in the pleural cavity. Emphysema is a chronic pulmonary disease characterized by an increase in the size of the air-containing terminal bronchioles. These two conditions add air to the tissues, making them more easily penetrated. Multiple myeloma is a condition characterized by infiltration and destruction of bone and marrow. Each of these conditions requires that factors be decreased from the normal to avoid overexposure.
What will result from using single-emulsion film in an image receptor having two intensifying screens?
A. Double exposure
B. Decreased density
C. Increased recorded detail
D. Greater latitude
B --
Dual-screen cassettes are made to be used with dual-emulsion film. The fluorescing screens are adjacent to the emulsions. If single-emulsion film is placed in a dual-screen cassette, the emulsion will receive only one-half of the intended exposure, and the resulting image will exhibit decreased density.
If a 6-inch OID is introduced during a particular radiographic examination, what change in SID will be necessary to overcome objectionable magnification?
SID must be increased by 42 inches , or 3.5 feet --

As OID is increased, recorded detail is diminished as a result of magnification distortion. If the OID cannot be minimized, an increase in SID is required to reduce the effect of magnification distortion. However, the relationship between OID and SID is not an equal relationship.
In fact, to compensate for every 1 inch of OID, an increase of 7 inches of SID is required. Therefore, an OID of 6 inches requires an SID increase of 42 inches. That is why, a chest radiograph with a 6-inches air gap is usually performed at a 10-foot SID.
Compared to a low ratio grid, a high ratio grid will
1. absorb more primary radiation.
2. absorb more scattered radiation.
3. allow more centering latitude
1 & 2 only --
Grid ratio is defined as the height of the lead strips to the width of the interspace material (see the figure below). The higher the lead strips (or the smaller the distance between the strips), the greater the grid ratio and the greater the percentage of scattered radiation absorbed. However, a grid does absorb some primary radiation as well. The higher the lead strips, the more critical the need for accurate centering, as the lead strips will more readily trap photons whose direction do not parallel them.
The effect that differential absorption has on radiographic contrast of a high subject contrast part can be minimized by
1. using a compensating filter.
2. using high-kVp exposure factors.
3. increased collimation.
1 & 2 only --
Differential absorption refers to the different attenuation, or absorption, properties of adjacent body tissues. Two parts with widely differing absorption characteristics will produce a high radiographic contrast. Frequently, exposure factors that would properly expose one part will severely overexpose or underexpose the neighboring part (as with lungs vs thoracic spine). This effect can be minimized by the use of a compensating filter or by the use of high kilovoltage (for more uniform penetration).
Increased collimation is important in the control of patient dose and scattered radiation, not differential absorption.