Screen / Film Imaging Roland Wong, Sc.M., D.A.B.M.P, D.A.B.R.
Outline • Projection Radiography • Basic Geometric Principles • Inverse Square Law • The Film-Screen Cassette • Characteristics of Screens • Characteristics of Film • The Screen-Film System • Contrast and Dose in Radiography • Scattered Radiation in Projection Radiography
Projection Radiography • Acquisition of 2D transmission image through a 3D object → information compression • The measured x-ray intensity (signal) determined by the attenuation (I = I0 e-(E)·x ) characteristics along a straight line through the patient from x-ray tube focal spot to the corresponding location on the detector • Image detector records the attenuation modulated x-ray distribution as film emulsion exposure → optical density (OD) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 146.
Projection Radiography • In the ideal world, the focal spot is a geometric point. • Initial imaging was direct film exposure. • Very high spatial resolution. • Very high dose. • Dental radiography is still direct film exposure. c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 146.
Inverse Square Law • The radiation intensity from a point source decreases with the square of the distance • E2 = E1 ∙ (D1/D2)2 • This relationship is only valid for point sources • Thus this relationship would not be valid near a patient injected with radioactive material c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 757.
Geometric Principles • Similar triangles (geometry) • 3 angles of one = 3 angles of the other • a/A = b/B = c/C = h/H • d/D = e/E = f/F = g/G • Magnification • beam diverges from focal spot • M = I/O = SID/SOD • largest when object closest to focal spot and → 1 at image plane c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 147.
Geometric Principles • Penumbra • edge gradient blurring due to finite size of focal spot (F) • f/F = OID/SOD • f/F = (SID-SOD)/SOD • f/F = (SID/SOD)-1 • f = F(M-1) • f or blur increases with F and M • f can be decreased by keeping object close to image plane (OID) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 147.
The Film/Screen Cassette • Cassette • Light-tight and ensures screen contact with film • ID flash card area on back • Front surface of carbon fiber • 1 or 2 Intensifying Screens • Convert x-rays to visible light • Mounted on layers of compressed foam • Sheet of film • Indirectly records the x-ray distribution • Chemically processed • Storage and display c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 148.
Intensifying Screens • Film relatively insensitive to x-rays • Patient receives a large dose • Screens made of scintillating material: phosphor
Intensifying Screens • X-rays absorbed by phosphor create visible light through photoelectrons, Compton electrons and delta-rays which excite rare earth atoms that emit EM radiation in the UV and visible regions • ≈ 5% of film darkening due to direct x-ray interaction with film • → Indirect detector • Reduce radiation burden to patient up to 50X
Screen Composition • Early 20th century: calcium tungstate, CaWO4 • Light emissions in the blues and UV. • Film had to be sensitive to blue light and UV. • Permitted “safelight” in the red. Complete darkness not required. c.f. http://www.ktf-split.hr/periodni/en/
Screen Composition • Since early 70’s: rare earth phosphor • Lanthanide series: Z = 57 – 71 • Gd2O2S:Tb (gadolinium oxysulfide: terbium) • LaOBr:Tm (lanthanum oxybromide: thulium) • YTaO4:Nb (yttrium tantalate: niobium) • Emissions are in the green part of spectrum. • Film had to be green sensitive. • Screens emitted many more photons. c.f. http://www.ktf-split.hr/periodni/en/
Screen Composition • Top coat • Phosphor and binder • Adhesive • Support • Phosphor thickness expressed as mass thickness = thickness (cm) ∙ density (g/cm3) • General radiography: each of two screens around 60 mg/cm2 • Mammography: single screen of 35 mg/cm2 used c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 150.
PROTECTIVE LAYER < 1 mil PHOSPHOR LAYER ~ 4 to 6 mil TiO2 REFLECTIVE LAYER ~ 1 mil PLASTIC BASE ~ 10 mil Screen Composition
Screen Function & Geometry • Function: absorb x-rays and convert to light • Conversion efficiency = fraction of absorbed energy emitted as UV or visible light • CaWO4 ≈ 5% intrinsic efficiency • Gd2O2S:Tb ≈ 15% intrinsic efficiency c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 151.
Screen Function & Geometry • Gd2O2S:Tb – • 545 nm (green), 2.7 eV • 50,000 eV x 0.15 = 7500 eV • 7500 eV / 2.7 eV/photon • = 2,800 photons • 200-1000 photons reach film • Quantum Detective Efficiency (QDE) of a screen = fraction of x-rays photons attenuated by the screens • QDE increases with screen thickness c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 151.
Screen Function & Geometry • Light-spreading within phosphor (isotropic diffusion) causing blurring of imaged object at detector • As screen thickness ↑ QDE ↑ and screen sensitivity ↑, but light-diffusion increases c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 152.
Screen Function & Geometry • Crossover or print-through: light from top screen penetrates the film base and exposes the bottom emulsion • Modulation Transfer Function (MTF) describes the degree of image sharpness or spatial resolution • As screen thickness ↑ MTF ↓ c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 152.
SCREEN FILM EMULSION BASE FILM EMULSION SCREEN Crossover of Light Through One Emulsion To The Emulsion on the Other Side
Summary of Screen Effects on Spatial Resolution • Thin screens have better spatial resolution. • Thin screens have less absorption efficiency – More patient radiation dose. • Reflective layer reduces patient radiation dose – But worsens the spatial resolution. • Dyes can be added to screens to decrease light spread & improve spatial resolution – more patient dose.
Conversion Efficiency (CE) • Total conversion efficiency (CE) is the ability of screen-film combination to convert the energy deposited by the absorbed x-rays into film darkening or OD • Intrinsic conversion efficiency of phosphor (DQE & light emission efficiency) • Efficiency of light propagation through the screen to film emulsion layer • Efficiency of the film emulsion in absorbing the emitted light c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 153.
Conversion Efficiency (CE) • Light propagation in screen • Distance from absorption to film • Light-absorbing dye: CE ↓, MTF ↑ • Reflective layer: CE ↑, MTF ↓ • Screen a linear device at a given x-ray energy c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 153.
CaWO4 Low Conversion Efficiency Screens PRODUCE3000 LIGHTPHOTONSWHICHDARKENFILM 15 INCIDENTX-RAYS &ONLY 3INTERACT INSCREEN
Rare Earth High Conversion Efficiency Screens PRODUCE3000 LIGHTPHOTONSWHICHDARKENFILM 5 INCIDENTX-RAYS &ONLY 1INTERACTS INSCREEN
Absorption Efficiency (AE) • The absorption efficiency or QDE describes how efficiently the screen detects x-ray photons that are incident upon it • X-ray beam is polychromatic and has a broad spectrum of energies • X-ray photon absorbed by the screen deposits its energy and some fraction of energy is converted to light photons c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 154-155.
Absorption Efficiency (AE) • The number of light photons produced in the screen is determined by the total amount of x-ray energy absorbed by the screen, not by the number of x-ray photons • S-F systems are considered energy detectors c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 154-155.
Overall Efficiency • Spatial resolution of film is high • Screens used to reduce dose • Exposure times shorter • Reduced costs for equipment and shielding c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 156.
Overall Efficiency • Total efficiency = AE ∙ CE • Intensification factor (IF) = ratio of energy absorption of 120 mg/cm2 phosphor vs. 0.80 mg/cm2 AgBr (film emulsion) • Example: 80 kVp • Gd2O2S:Tb detects 29.5% • AgBr detects 0.65% • IF = 29.5%/0.65% = 45.4 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 156.
Matching Screen Light to Film Response • If film sensitivity is not matched to screen light => Patient radiation dose increases. • Some of the light from the screen can be lost if the film is not sensitive to the total spectrum of emission. • CaWO4 screens can emit continuous blue light. • Rare earth phosphors emit discrete hues in the green, yellow or UV.
CaWO4 Gd2O2S La2O2S Y2O2S 5000 6000 4000 Wavelength (Angstroms)
FILM SENSITIVITY SILVER HALIDE FILM PANCHROMATIC FILM ORTHOCHROMATIC FILM 3000 4000 5000 6000 7000 YELLOW UV BLUE GREEN RED WAVELENGTH (ANGSTROMS)
K-Edge of Phosphor Material Makes Screens are kVp Dependent La2O2S Gd2O2S CaWO4
SPEED vs. X-RAY kVp RELAT I VE SPEED 1.0 Gd2O2S FILM-SCREEN SYSTEM 0.5 0 40 60 80 100 120 TUBE POTENTIAL (kVp)
Noise Effects of CE vs. AE • Noise: local variations in film OD not representing variations of attenuation occurring in the object, includes random noise caused by factors such as • Statistical fluctuation in x-ray quantity interacting with screens • Statistical fluctuation in fraction of light emitted by the screen that is absorbed by the film emulsion • Statistical fluctuation in the distribution of silver halide grains in film emulsion • The visual perception of noise is reduced (better image quality) when the number of detected x-ray photons increases
Noise Effects of Changing CE vs. AE • What happens to noise in image when the CE is increased? • ↑ CE => fewer x-ray photons are required to achieve same OD on film so noise increases • What happens to noise in image when the AE is increased? • Δ AE => noise unchanged (↑ AE => ↓ mAs, so same number of x-ray) photons absorbed) • If ↑ AE through ↑ screen thickness => ↓ spatial resolution
Radiographic Mottle (Image Noise) • Three Main Components of Mottle • Quantum Mottle • Screen Mottle • Grain Mottle
Radiographic Mottle (Image Noise) • Quantum mottle is the variation in the # photons/ mm2 used to form the image. • Screen Mottle is the variation in phosphor thickness and density. • Grain mottle is the variation in # silver grains in film / mm2.
N x 100% N Quantum Mottle N = # PHOTONS / mm2