Transmission, Projection Imaging Image based on: •Transmission imaging (c.f. emission, reflection) •X-ray attenuated as it passes Film-Screen Radiography through patient ∝ e-μx •Negative image recorded on film •Optical density (darkness) ∝ David Dubowitz MD PhD 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 size “r-squared-rule”
Large focal spot: •Edges of projection indistinct •Exaggerated by magnification •Results in geometric blurring X-ray Exposure ∝ 1/r2
•Reduce geometric blurring with: •Reduced focal spot size •AND •Reduced magnification
Mrs. Röntgen’s Hand
X-ray + photographic film
1 Amplification with film+screen
X-ray + photographic film + intensifying screen
Film-screen geometry Screen thickness
~15% conversion efficiency i.e. % x-ray photons ~5% conversion absorbed by efficiency screen i.e. % x-ray photons becoming light photons
Thicker screens are more efficient….but….more blurred
Film/Screen Efficiency
• Absorption Efficiency of the Screen screen characteristics – 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 As detection increases...image definition decreases & v.v.
2 Film-screen combinations Hurter Driffield (HD) Plot (or “characteristic curve”)
Note: OD is a log measurement Note: log [exposure]
~ 1000x more photons Log-Log plot of optical transmission vs. x-ray exposure
“Optical Density” Measuring Optical Density
Increase in sensitivity, Hurter Driffield (HD) Plot No change in contrast
Contrast ∝ slope A more sensitive than B Exposure > or < linear (“higher speed”)…. Lt shift region reduces contrast Less x-rays for same OD For radiographic film: Gradient ~2.5 – 3.5 Slope (contrast) unchanged
3 Increase in sensitivity, Photosensitivity of silver halide 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)
film processing & characteristic plot Film Sensitometer for Quality Control
Affected by:
•Concentration- •Temperature- •Time-
of developer
higher concentration / higher temp / longer developer time Æ more metallic Ag deposited Æ darker film (increased density + more base fog)
Quality Control Automated Film Processor
•Process film strips •Check Temperature •Check Development time •Check for artifacts •Monitor results daily to detect trends
~ 90 sec total time Vital for Consistent Results ~ 22 sec developer time Need consistent results Needs QC! Needs regular cleaning
4 Common 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
70kVp 70kVp 5.5mAs 11mAs Effect of mAs on Optical Density
70kVp 70kVp 22mAs 40mAs
Effect of kVp on Optical Density
Double mAs, Doubles Optical No change in contrast
5 Photon Energy vs. kVp 55kVp 65kVp 25mAs 25mAs
75kVp 85kVp 25mAs 25mAs
Contrast & Scatter
Scattered photons: •Different angle •Lower energy
15% increase kVp ≈ 2x mAs 15% decrease kVp ≈ ½x mAs
Pelvis 10” x 12” Grid No Grid 8:1 Grid Introduced 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
6 Grid Errors Air Gap
Fewer scattered photons reach film Increases contrast
But…… Reduced source – object distance increases dose Magnifies image
kVp vs. skin dose Tissue Contrast
•Improved by: Reducing kVp increases Lower kVp (less scatter) contrast Smaller x-ray field (less scatter) Contrast media (high atomic number of iodine -> PE effect) But…… Noise reduction (“contast” is actually contrast-to noise) increases skin dose Grid or air gap (reduce scattered photons reaching film ) Proper positioning (reduce superposition, grid artifacts)
Film-screen combinations Choosing a grid
Choose a film-screen combination to match X-ray machine near patient (divergent beam) the clinical need: • Use focused grid X-ray machine far from patient (~Parallel beam) E.g. • Use parallel grid • Abdo / pelvis: Sensitive film-screen, High kvP (= more scatter) needs higher ratio grid reduced dose, some loss of detail. (<90kVp – use 8:1, >90kVp use 12:1) • Chest: Single phosphor / UV screens • Remember: 80kvP 8:1 grid, 120kVp 12:1 grid inproves detail Grid increases radiation dose (“Bucky Factor”) • Mammo: Single phosphor / Single emulsion • Avoid grid if not really needed (e.g. pediatrics) high resolution film Static grid lines can reduce detail • Dental: No screen at all • Blur grid lines with moving grid (Hollis Potter, 1920)
7 Sample Q’s Sample Q’s
2002 G39: 2002 G52: In an x-ray machine with a tungsten target, The output of a fluoroscopic unit is 10 mR/min at increasing the kVp from 100 to 150 will increase 50 cm. The output at 75 cm : all of the following except: A. 15.0 A. The total number of x-rays emitted. B. 7.5 B. The maximum energy of the x-rays. C. 6.6 C. The average energy of the spectrum. D. 5.0 D. The energy of the characteristic x-rays. E. 4.4 E. The heat units generated (for the same mAs).
Sample Q’s Sample Q’s
2002 G57: 2002 G73: Which graph represents the shape of an H&D curve? At x-ray energies between 40 and 100 keV,_ absorbs . more energy than _ per gram.
A. fat, muscle B. muscle, bone C. iodine, bone D. fat, air E. air, muscle
Sample Q’s Sample Q’s
2002 G75: 2002 G76: A grid improves the quality of a diagnostic x-ray Regarding geometric unsharpness, which of the primarily by attenuating _ . following is false?
A. Primary photons It is: B. Compton scattered photons A. Inversely proportional to focal spot size. C. Compton electrons B. Directly proportional to object-film distance. D. Photoelectrons C. Inversely proportional to focal spot-object distance. E. Coherent scattered photons D. Characterized by penumbra width.
8 Sample Q’s Sample Q’s 2002 G77: The purpose of a screen is to: 2002 G78: A film of optical density (OD) 0.75 is placed over 1. Convert x-rays to light photons. another identicalfilm.The OD of the pair is_. 2. Reduce scatter reaching the film. 3. Reduce patient's exposure. A. 0.75 4. Increase radiographic resolution. B. 1.0 A. 1,2,3,4 C. 1.5 B. 2 only D. 1.75 C. 2,4 E. 2.25 D. 1,3 E. 4 only
Sample Q’s Sample Q’s 2002 D12: 2002 D1: A newly installed bucky radiographic system produces For a 70 kVp x-ray beam, the mass abdominal images that are of acceptable density over attenuationcoefficientwhich varies the most from the spine and progressively lighter toward both muscle tissue is: lateral edges of the film. The most likely reason for this finding is improper: A. Air. B. Aluminum. A. Collimator tracking. C. Bone. B. Focal distance for grid. D. Fat. C. Grid ratio. E. Iodine. D. kVp calibration of the system. E. Programming of the AEC system.
Sample Q’s Sample Q’s
2002 D13: 2002 D15: The impression of noise in an x-ray image is: The penumbra associated with the image of the edge of an object placed 50 cm above the film plane, for an A. Increased by increasing the film speed in a screen-film SID of 100 cm, and a focal spot size of 1.0 mm is mm. cassette. B. Decreased by increasing the film speed in a screen-film A. 0.01 cassette. C. Increased by decreasing the focal-spot size. B. 0.1 D. Decreased by decreasing the focal-spot size. C. 1.0 E. Mainly determined by imperfections in the image D. 10 receptor.
9 Sample Q’s Sample Q’s
2002 D17: 2002 D18: A measurement of the cardiac dimensions is obtained The main reason a 12:1 grid is never used with from a chest film. The SID is 72" and the heart is portable radiography is: midway in a 14" chest. The distance between the chest changer surface and the film is 3". The A. There will be too much grid cut-off if the grid is not dimensions on the film are than the actual anatomy. positioned properly. B. The output of portable x-ray units is too low. A. 32% larger C. It is necessary to keep exposure times under lOInS. B. 16% larger D. The increased scatter makes for wider latitude C. 8% smaller radiographs which are undesirable for portable chest D. 16% smaller x-rays. E. 32% smaller E. High ratio grids may only be used at high kVp. Portable x-ray units only go up to 90 kVp.
Sample Q’s Sample Q’s 2002 D23: 2002 D19: Consider the three characteristic curves in the diagram. Which statement is false? 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. 0.2 B. 0.5 A. System B has the highest contrast. C. 1.0 B. System C has the widest latitude. D. 4.0 C. System A has the highest maximum density. E. 20.0 D. System B has the highest base-fog density. E. System C is the fastest.
Sample Q’s 2002 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 gradient B. High gamma C. Slow speed D. Long latitude E. Low fog
10 Answers Sample Q’s 2002 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’s Sample Q’s 2002 G52:E 2002 G57:C 2 The absorbed dose per unit exposure is called the f- By the inverse square law: I75=I50 x (50 / 75) =4.4 mR/min. 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’s Sample Q’s 2002 G73:D 2002 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.
11 Sample Q’s Sample Q’s 2002 G76:A 2002 G77:D Geometric sharpness or edge gradient is reduced by Convert x-rays to light photons. minimizing magnification. Reduce patient's exposure
Sample Q’s Sample Q’s 2002 G78:C 2002 D1:E Optical density is additive. It is the log of The high atomic number of iodine (53) yields an appreciable photoelectric attenuation at 70 kVp. The (incident/transmitted) light intensity. 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’s Sample Q’s 2002 D12:B 2002 D13:A Cut-off is caused because the grid strips no longer "aim" Increasing the film speed in a screen-film cassette reduces the number of x-ray photons absorbed by the at the focal-spot. 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.
12 Sample Q’s Sample Q’s 2002 D15:C 2002 D17:B By similar triangle geometry, the penumbra is equal to The heart-is situated 10" from the film (14"/2 + 3"). The the focal spot size, since source-object and object-film magnification factor is 72/62 =1.16. distances are equal.
Sample Q’s Sample Q’s 2002 D18:A 2002 D19:D The higher the grid ratio, the more sensitive it is to grid Bucky factors of 3 to 6 are expected for grids. cut-off. Slight angulation and/or decentering will cause major density changes on the radiograph.
Sample Q’s Sample Q’s 2002 D23:E 2002 D24:D System A is the fastest, i.e., requires the lowest Long-latitude film has a low gradient and a low gamma, so exposure to give a net density (density minus base + fog) large changes in exposure make small changes in film of 1.0. density. Note that this is also low in contrast.
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