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FilmFilm--Screen RadiographyScreen Radiography

David Dubowitz MD PhDDavid Dubowitz MD PhD

Transmission, Projection ImagingImage based on:

•Transmission imaging (c.f. emission, reflection)

•X-ray attenuated as it passes through patient ∝ e-μx

•Negative image recorded on film•Optical density (darkness) ∝log [transmission of radiation]

•Projection of 3D anatomy onto 2D image

•Divergent beam, so image is magnified ∝ SID/SOD

•Irradiation ∝ 1/(distance)2

Focal spot sizeLarge focal spot:

•Edges of projection indistinct

•Exaggerated by magnification

•Results in geometric blurring

•Reduce geometric blurring with:

•Reduced focal spot size

•AND

•Reduced magnification

“r-squared-rule”

X-ray Exposure ∝ 1/r2

Mrs. Röntgen’s Hand

X-ray + photographic film

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Amplification with film+screen

X-ray + photographic film + intensifying screen

Film-screen geometry Screen thickness

Thicker screens are more efficient….but….more blurred

i.e. % x-ray photons absorbed by screen

~15% conversion efficiency

~5% conversion efficiency

i.e. % x-ray photons becoming light photons

Film/Screen Efficiency• Absorption Efficiency of the Screen

– Quantum Detection Efficiency (QDE):Fraction of incident x-rays that interact with it

• Intrinsic Conversion Efficiency of the Screen– Fraction of absorbed x-ray energy converted to light photons– CaWO4 = 5%, Gd2O2S:Tb = 15%

• Total Efficiency of Film/Screen Combination– Equals Absorption Efficiency x Conversion Efficiency– Ability to convert absorbed x-ray energy into film darkening– Also called Intensification Factor– 50 times greater than film alone

We use screens because they reduce dose even though they degrade image quality

screen characteristics

As detection increases...image definition decreases & v.v.

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Film-screen combinations

~ 1000x more photons

Hurter Driffield (HD) Plot(or “characteristic curve”)

Note: log [exposure] Note: OD is a log measurement

Log-Log plot of optical transmission vs. x-ray exposure

“Optical Density” Measuring Optical Density

Hurter Driffield (HD) Plot

Contrast ∝ slope

Exposure > or < linear region reduces contrast

For radiographic film: Gradient ~2.5 – 3.5

Increase in sensitivity, No change in contrast

A more sensitive than B (“higher speed”)…. Lt shift

Less x-rays for same OD

Slope (contrast) unchanged

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Increase in sensitivity, Increase in contrast

A more sensitive than B (“higher speed”)…. Lt shift

Slope (contrast) increased

Film latitude reduced (i.e. reduced dynamic range of exposures)

Photosensitivity of silver halidePhotosensitivity of silver halide

film processing & characteristic plot

higher concentration / higher temp / longer developer timemore metallic Ag depositeddarker film (increased density + more base fog)

Affected by:

•Concentration-•Temperature-•Time-

of developer

Film Sensitometer for Quality Film Sensitometer for Quality ControlControl

•Process film strips•Check Temperature•Check Development time•Check for artifacts•Monitor results daily to detect trends

Vital for Consistent Results

Quality ControlQuality Control

~ 90 sec total time~ 90 sec total time~ 22 sec developer time~ 22 sec developer timeNeed Need consistent resultsconsistent resultsNeeds QC!Needs QC!Needs regular cleaningNeeds regular cleaning

Automated Film ProcessorAutomated Film Processor

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Common Processing ArtifactsCommon Processing Artifacts Noise• “Noise”: local variation in OD not due to attenuation in patient.• Random variation in # xray photons interacting with screen.• Random variation in fraction of light emitted by xray photons

interacting with screen.

• “faster” (more xray sensitive) screen – Each xray photon more efficient at blackening film. Fluctuations in xray photons more readily seen on film…… INCREASE IN NOISE

• Thicker (more xray absorbent) screen – Total number of x-ray photons detected to produce a given OD is unchanged. Random fluctuations in xray photons have same effect on film…… NO CHANGE IN NOISE

Effect of mAs on Effect of mAs on Optical DensityOptical Density

70kVp

5.5mAs

70kVp

11mAs

70kVp

22mAs

70kVp

40mAs

Double mAs, Doubles Optical No change in contrast

Effect of kVp on Effect of kVp on Optical DensityOptical Density

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55kVp

25mAs

65kVp

25mAs

75kVp

25mAs

85kVp

25mAs

Photon Energy vs. kVp

15% increase kVp ≈ 2x mAs15% decrease kVp ≈ ½x mAs

Contrast & Scatter

Scattered photons:•Different angle•Lower energy

GridIntroduced by Gustav Bucky 1913

•Alternating Pb/Al layers (or carbon)

•Fewer scattered photons reach film

•Increases contrast

•Increases dose (Bucky factor 3-6)

•Height:interspace = grid ratio

•Typically 6:1 – 12:1 (lower ratio is more forgiving!)

•Parallel, focused, crossed

Pelvis 10” x 12”No Grid 8:1 Grid

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Grid Errors Air GapFewer scattered photons reach film

Increases contrast

But……

Reduced source – object distance increases dose

Magnifies image

kVp vs. skin dose

Reducing kVp increases contrast

But……

increases skin dose

Tissue Contrast

• Improved by:Lower kVp (less scatter)

Smaller x-ray field (less scatter)

Contrast media (high atomic number of iodine -> PE effect)

Noise reduction (“contast” is actually contrast-to noise)

Grid or air gap (reduce scattered photons reaching film )

Proper positioning (reduce superposition, grid artifacts)

Film-screen combinationsChoose a film-screen combination to match

the clinical need:

E.g.• Abdo / pelvis: Sensitive film-screen,

reduced dose, some loss of detail.• Chest: Single phosphor / UV screens

inproves detail• Mammo: Single phosphor / Single emulsion

high resolution film• Dental: No screen at all

Choosing a gridX-ray machine near patient (divergent beam)• Use focused gridX-ray machine far from patient (~Parallel beam)• Use parallel gridHigh kvP (= more scatter) needs higher ratio grid

(<90kVp – use 8:1, >90kVp use 12:1)• Remember: 80kvP 8:1 grid, 120kVp 12:1 gridGrid increases radiation dose (“Bucky Factor”)• Avoid grid if not really needed (e.g. pediatrics)Static grid lines can reduce detail• Blur grid lines with moving grid (Hollis Potter, 1920)

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Sample Q’s2002 G39:

In an x-ray machine with a tungsten target, increasing the kVp from 100 to 150 will increase all of the following except:

A. The total number of x-rays emitted.B. The maximum energy of the x-rays.C. The average energy of the spectrum.D. The energy of the characteristic x-rays.E. The heat units generated (for the same mAs).

Sample Q’s2002 G52:

The output of a fluoroscopic unit is 10 mR/min at 50 cm. The output at 75 cm :

A. 15.0B. 7.5C. 6.6D. 5.0E. 4.4

Sample Q’s2002 G57:

At x-ray energies between 40 and 100 keV,_ absorbs more energy than _ per gram.

A. fat, muscleB. muscle, boneC. iodine, boneD. fat, airE. air, muscle

Sample Q’s2002 G73:

Which graph represents the shape of an H&D curve?.

Sample Q’s2002 G75:

A grid improves the quality of a diagnostic x-ray primarily by attenuating _ .

A. Primary photonsB. Compton scattered photonsC. Compton electronsD. PhotoelectronsE. Coherent scattered photons

Sample Q’s2002 G76:

Regarding geometric unsharpness, which of the following is false?

It is:A. Inversely proportional to focal spot size.B. Directly proportional to object-film distance.C. Inversely proportional to focal spot-object distance.D. Characterized by penumbra width.

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Sample Q’s2002 G77:

The purpose of a screen is to:

1. Convert x-rays to light photons.2. Reduce scatter reaching the film.3. Reduce patient's exposure.4. Increase radiographic resolution.

A. 1,2,3,4B. 2 onlyC. 2,4D. 1,3E. 4 only

Sample Q’s2002 G78:

A film of optical density (OD) 0.75 is placed over another identicalfilm.The OD of the pair is_.

A. 0.75B. 1.0C. 1.5D. 1.75E. 2.25

Sample Q’s2002 D1:

For a 70 kVp x-ray beam, the mass attenuationcoefficientwhich varies the most from muscle tissue is:

A. Air.B. Aluminum.C. Bone.D. Fat.E. Iodine.

Sample Q’s2002 D12:

A newly installed bucky radiographic system produces abdominal images that are of acceptable density over the spine and progressively lighter toward both lateral edges of the film. The most likely reason for this finding is improper:

A. Collimator tracking.B. Focal distance for grid.C. Grid ratio.D. kVp calibration of the system.E. Programming of the AEC system.

Sample Q’s2002 D13:

The impression of noise in an x-ray image is:

A. Increased by increasing the film speed in a screen-film cassette.

B. Decreased by increasing the film speed in a screen-film cassette.

C. Increased by decreasing the focal-spot size.D. Decreased by decreasing the focal-spot size.E. Mainly determined by imperfections in the image

receptor.

Sample Q’s2002 D15:

The penumbra associated with the image of the edge of an object placed 50 cm above the film plane, for an SID of 100 cm, and a focal spot size of 1.0 mm is mm.

A. 0.01B. 0.1C. 1.0D. 10

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Sample Q’s2002 D17:

A measurement of the cardiac dimensions is obtained from a chest film. The SID is 72" and the heart is midway in a 14" chest. The distance between the chest changer surface and the film is 3". The dimensions on the film are than the actual anatomy.

A. 32% largerB. 16% largerC. 8% smallerD. 16% smallerE. 32% smaller

Sample Q’s2002 D18:

The main reason a 12:1 grid is never used with portable radiography is:

A. There will be too much grid cut-off if the grid is not positioned properly.

B. The output of portable x-ray units is too low.C. It is necessary to keep exposure times under lOInS.D. The increased scatter makes for wider latitude

radiographs which are undesirable for portable chest x-rays.

E. High ratio grids may only be used at high kVp. Portable x-ray units only go up to 90 kVp.

Sample Q’s2002 D19:

Abdominal radiographs taken at high kVp without a grid can be expected to have considerable scatter.The ratio (SIP) of scattered photons (S) to unscatteredprimary photons (P) reaching a screen-film combination without a grid is about

A. 0.2B. 0.5C. 1.0D. 4.0E. 20.0

Sample Q’s2002 D23:

Consider the three characteristic curves in the diagram. Which statement is false?

A. System B has the highest contrast.B. System C has the widest latitude.C. System A has the highest maximum density.D. System B has the highest base-fog density.E. System C is the fastest.

Sample Q’s2002 D24:

In some situations, e.g., a chest exam, it is important to see radiographic anatomy in both high- and low-density regions. To aid in this, one could choose a film with a

A. High gradientB. High gammaC. Slow speedD. Long latitudeE. Low fog

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Answers Sample Q’s2002 G39:D

At 100 kVp, the highest characteristic x-rays (69 kV) are already present, so the characteristic peaks will remain the same, although their intensity will increase.

Sample Q’s2002 G52:EBy the inverse square law: I75=I50 x (50 / 75)2 =4.4 mR/min.

Sample Q’s2002 G57:C

The absorbed dose per unit exposure is called the f-factor, and depends on both Z and the photon energy. f factors listed from lowest to highest are:

fat, air, muscle, bone, and iodine.

Sample Q’s2002 G73:D

Sample Q’s2002 G75:B

Compton-scattered photons travel in random directions, and contain no useful diagnostic information. The grid absorbs most of these photons, and thus improves image contrast.

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Sample Q’s2002 G76:A

Geometric sharpness or edge gradient is reduced by minimizing magnification.

Sample Q’s2002 G77:D

Convert x-rays to light photons.Reduce patient's exposure

Sample Q’s2002 G78:C

Optical density is additive. It is the log of (incident/transmitted) light intensity.

Sample Q’s2002 D1:E

The high atomic number of iodine (53) yields an appreciable photoelectric attenuation at 70 kVp. The other materials have much lower atomic numbers and less photoelectric attenuation. The Compton components of mass attenuation coefficients for all the materials are similar.

Sample Q’s2002 D12:B

Cut-off is caused because the grid strips no longer "aim" at the focal-spot.

Sample Q’s2002 D13:A

Increasing the film speed in a screen-film cassette reduces the number of x-ray photons absorbed by the screens to produce the same density on the film. Thus, quantum noise increases. Noise is due to the statistics of x-ray detection; it is therefore independent of focal-spot size.

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Sample Q’s2002 D15:C

By similar triangle geometry, the penumbra is equal to the focal spot size, since source-object and object-film distances are equal.

Sample Q’s2002 D17:B

The heart-is situated 10" from the film (14"/2 + 3"). The magnification factor is 72/62 =1.16.

Sample Q’s2002 D18:A

The higher the grid ratio, the more sensitive it is to grid cut-off. Slight angulation and/or decentering will cause major density changes on the radiograph.

Sample Q’s2002 D19:D

Bucky factors of 3 to 6 are expected for grids.

Sample Q’s2002 D23:E

System A is the fastest, i.e., requires the lowest exposure to give a net density (density minus base + fog) of 1.0.

Sample Q’s2002 D24:D

Long-latitude film has a low gradient and a low gamma, so large changes in exposure make small changes in film density. Note that this is also low in contrast.