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.