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Physics Board Review Film and Screen Film Systems CR, and DR

Physics Board Review Film and Screen Film Systems CR, and DR. Norbert Pelc. Department of Radiology Stanford University School of Medicine. Department of Radiology. Film. Radiographic film consists of • 150  m base (mylar) • 10  m thick emulsion(s)

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Physics Board Review Film and Screen Film Systems CR, and DR

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  1. Physics Board ReviewFilm and Screen Film SystemsCR, and DR Norbert Pelc Department of Radiology Stanford University School of Medicine

  2. Department of Radiology Film Radiographic film consists of • 150 m base (mylar) • 10m thick emulsion(s) silver halide (mostly iodide, some bromide) grains 1-1.5 m is size, 107 Ag atoms per grain deposited on one or both sides of the base • protective coating Grain sensitization • ionization with light or x-ray generates a free electron • e- + Ag+ -> Ag • grains with a few reduced Ag atoms are "developable” • takes multiple light photons or a single absorbed x-ray

  3. Department of Radiology Film Development • preferential reduction of Ag in exposed grains • steps: develop, fix, wash • developed grain is a speck of silver that absorbs light (black) • increased development temperature or time -> blacker • processors have to be well maintained and QC'd chemicals, temperature, clean parts

  4. Io It Department of Radiology Optical Density A measure of the transparency of an object • Transmittance = fraction of light transmitted (It/Io) • Optical density OD = log10 (Io/It) increasing OD -> decreasing transmittance -> darker OD=1 -> 10% transmittance, OD=2 -> 1% transmittance If you stack objects OD's add • Useful OD range is 0.3 to ~3 (50% to 0.1% transmittance) OD>2.2 or so requires a 'hot' light

  5. Department of Radiology Characteristic (H and D) Curves Plot of OD vs. log (exposure) Base plus fog - OD without any exposure Toe - region at low OD before the linear portion Shoulder - region at high OD after the linear portion Speed: 1/(exposure in Roentgens that produces OD=1)

  6. Contrast vs Latitude Department of Radiology Image contrast = difference in OD (subject contrast)x(slope of H&D curve) Gamma: highest slope of H & D curve (3 is typical) Gradient or mean slope (from OD=0.25 to 2.0 above base + fog) Latitude: range of exposure levels that can be imaged dynamic range inversely related to contrast High gamma or gradient implies high contrast, low latitude

  7. Film is a poor x-ray detector • • poor quantum detection efficiency (QDE) • • one interacting x-ray -> one hot electron • many ionizations but all in the same grain • one developable grain Intensifying screen • QDE better than film (40% vs 5%) • absorb x-rays in a phosphor layer • convert energy to visible light photons (conversion efficiency= 10-20%) • one interacting x-ray -> one hot electron many many light photons (~ 3 eV each) many developable grains Department of Radiology Intensifying Screens

  8. Department of Radiology Intensification factor (IF) IF = (exposure with film alone)/(exposure with screen-film) = (speed of film-screen)/(speed of film alone) • typically 30-50 ~ 8 from QDE (does not increase quantum noise) rest from more grains per detected x-ray (increases noise) • reduced patient dose • shorter exposure time

  9. Department of Radiology Intensifying Screen phosphors CaWO4 emits blue light (high energy per light photon) W K-edge (~70 keV) is too high modest conversion efficiency gadolinium oxysulfide (Gd2O2S:Tb) (rare earth screen) emits green light - must be matched to film type higher conversion efficiency than CaWO4 Gd K-edge at 50 keV same QDE with thinner screen - higher resolution other phosphors LaOBr (blue) Y2O2S:Tb, La2O2S:Tb (green)

  10. Department of Radiology Intensifying Screens Speed can be increased by • increased QDE (no noise increase) increased  - same spatial resolution double screen - some loss of spatial resolution increased thickness - even more loss of spatial resolution • increased conversion efficiency (noise increases) • increased film speed (noise increases)

  11. Intensifying Screens Department of Radiology Advantages • reduced dose • shorter exposure time • less motion blurring Disadvantage • lower spatial resolution due to light spread

  12. Computed radiography (CR) - Photostimulable phosphors Department of Radiology exposure: some deposited energy is stored in “traps” readout: electrons excited out of traps (with scanning laser) emit light which is digitized erase: using bright light to empty all traps advantages: very wide dynamic range no retakes image processing PACS compatibility disadvantage: modest QDE

  13. amorphous silicon a-Si layer contact leads contact fingers pixel circuit charge storage TFT switch x-ray absorber (converter) glass substrate Flat panel detectors

  14. Department of Radiology Digital radiography (DR) • digitization is earlier in the acquisition process (vs. CR) • “instant” readout (some are fluoro capable) Indirect conversion scintillation phosphor (CsI, Gd2O2S) on top of flat panel flat panel pixel array, each pixel has a photodiode x-rays -> light -> electrons -> electrical signal Direct conversion semiconductor on top of panel x-rays -> electron-hole pairs -> electrical signal selenium (mammo, extremities), HgI2?

  15. Digital radiography Department of Radiology advantages: very wide dynamic range no retakes image processing PACS compatibility better DQE than film/screen or CR (dose reduction?) advanced apps (e.g., tomosynthesis, dual energy) disadvantages: lower limiting resolution than film/screen cost

  16. Other DR systems Department of Radiology Semiconductors with charge detectors Phosphor screen with silicon diode array Scanning silicon arrays (mammo)

  17. Q1. Two screens have the same absorption efficiency. Screen A has better Conversion Efficiency than B. If used with the same film and reach the same OD: a. A requires shorter exposure time than B b. the images have equal quantum mottle c. A has higher contrast than B d. A has better resolution than B e. a and b Q2. Two screens have the same conversion efficiency. Screen A has better absorption efficiency than B. If used with the same film and reach the same OD: a. A requires shorter exposure time than B b. the images have equal quantum mottle c. A has higher contrast than B d. A has better resolution than B e. a and b adapted from recalls

  18. Q3. The advantage of screen-film radiography compared to direct film exposure is? a. better spatial resolution b. better contrast resolution (lower noise) c. lower radiation dose d. lower cost Q4. Computed radiography (CR) systems use: a. lasers to print out images b. semiconductors to convert x-rays to electric charge c. photostimulable phosphors d. scintillators with photodiodes to convert x-rays to electric charge Q5. The advantage of Computed Radiography (CR) over film-screen radiography is: a. wider dynamic range b. better contrast c. better spatial resolution d. lower dose for the same image quality adapted from recalls

  19. Q6. If a photostimulable phosphor cassette is opened in the exam room and then closed before a radiograph is obtained, the image could deteriorate because a. the plate would be exposed and the image would be black b. dust will get onto the plate causing artifacts c. photostimulable phosphors are hygroscopic (damaged by humidity) Q7. What type of screen-film system is used for chest radiography? a. double screen - double emulsion film b. single screen – double emulsion film c. double screen – single emulsion film d. single screen – single emulsion film Q8. If a screen-film cassette has poor film-screen contact, the impact is: a. loss of optical density from light loss b. streak artifacts from light leaks c. poor spatial resolution d. retained signal from prior exposures adapted from recalls

  20. Q9. Compared to screen-film chest radiography, mammography uses: a. lower kVp, b. more sensitive (faster) film c. thicker, higher conversion efficiency phosphors d. wider latitude film Q10. Quantum mottle in images made with an indirect flat panel detector is primarily determined by: a. number of x-rays transmitted through the patient b. number of xrays absorbed by the phosphor c. efficiency of the conversion of energy to light by the CsI phosphor d. noise figure of the flat panel readout circuit Q11. The instrument used to measure the density of an already-processed film? a. densitometer b. dosimeter c. sensitometer d. spectrometer adapted from recalls

  21. Q12. Which is true regarding light boxes and OD? a. OD’s greater that 1 should be viewed with a hot light b. mammography light boxes should be dimmer than conventional ones c. fluorescent lights in light boxes lose luminance with time d. masking unused regions is of no benefit Q13. Two films with optical density of 1 are stacked on top of each other, what is the transmittance? a. 10%, b. 2%, c. 1%, d. 0.1% Q14. Which of these increases geometric unsharpness? a. increased thickness of intensifying screen b. increased SOD c. increased focal spot size d. increased film speed Q15. What is the effect of increasing the film speed in mammography? a. decreased quantum mottle b. increased spatial resolution c. decreased dose d. increased contrast adapted from recalls

  22. Q16. A radiograph transmits 10% of the light from a viewbox with an illumination level of 400 lux. What is the optical density of the radiograph? a. 2.0 b. 1.0 c. 0.3 d. 2.0/400 e. 1.0/400 Q17. An image acquired using CR has too little contrast. What should you do? a. retake the image with a lower kVp b. retake the image with a higher mAs c. adjust the display Q18. What material could be used in a direct conversion digital radiography system a. selenium photoconductor b. photodiodes c. photostimulable phosphor

  23. Answers Q1: a Q2: e Q3: c Q4: c Q5: a Q6: b Q7: a Q8: c Q9: d Q10: b Q11: a Q12: c Q13: c Q14: c Q15: c Q16: b Q17: c Q18: a

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