Upload
espen
View
48
Download
2
Embed Size (px)
DESCRIPTION
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 - PowerPoint PPT Presentation
Citation preview
Screen / Film Imaging
Roland Wong, Sc.M., D.A.B.M.P, D.A.B.R.
Outline
Projection Radiography Projection Radiography Basic Geometric Principles Basic Geometric Principles Inverse Square Law Inverse Square Law The Film-Screen Cassette The Film-Screen Cassette Characteristics of Screens Characteristics of Screens Characteristics of Film Characteristics of Film The Screen-Film System The Screen-Film System Contrast and Dose in Radiography Contrast and Dose in Radiography Scattered Radiation in Projection RadiographyScattered Radiation in Projection Radiography
Projection Radiography Acquisition of 2D transmission Acquisition of 2D transmission
image through a 3D object image through a 3D object → → information compression information compression
The measured x-ray intensity The measured x-ray intensity (signal) determined by the (signal) determined by the attenuation (attenuation (I = II = I00 e e
--(E)(E)··x x ) )
characteristics along a straight line characteristics along a straight line through the patient from x-ray tube through the patient from x-ray tube focal spot to the corresponding focal spot to the corresponding location on the detector location on the detector
Image detector records the Image detector records the attenuation modulated x-ray attenuation modulated x-ray distribution as film emulsion distribution as film emulsion exposure → optical density (OD)exposure → optical density (OD)
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 146. 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 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 146. ed., p 146.
Inverse Square Law The radiation intensity from a The radiation intensity from a
point source decreases with the point source decreases with the square of the distance square of the distance
EE22 = E = E11 ∙ (D ∙ (D11/D/D22))
22
This relationship is only valid for point sources This relationship is only valid for point sources
Thus this relationship would not be valid near a patient injected with Thus this relationship would not be valid near a patient injected with radioactive materialradioactive material
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 757. ed., p 757.
Geometric Principles Similar triangles (geometry) Similar triangles (geometry)
3 angles of one = 3 angles of 3 angles of one = 3 angles of the other the other
a/A = b/B = c/C = h/H a/A = b/B = c/C = h/H d/D = e/E = f/F = g/G d/D = e/E = f/F = g/G
Magnification Magnification beam diverges from focal spot beam diverges from focal spot M = I/O = SID/SOD M = I/O = SID/SOD largest when object closest to largest when object closest to
focal spot and → 1 at image focal spot and → 1 at image planeplane
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 147. ed., p 147.
Geometric Principles
Penumbra Penumbra edge gradient blurring due to edge gradient blurring due to
finite size of focal spot (F) finite size of focal spot (F) f/F = OID/SOD f/F = OID/SOD f/F = (SID-SOD)/SOD f/F = (SID-SOD)/SOD f/F = (SID/SOD)-1 f/F = (SID/SOD)-1 f = F(M-1) f = F(M-1) f or blur increases with F and M f or blur increases with F and M f can be decreased by keeping f can be decreased by keeping
object close to image plane object close to image plane (OID)(OID)
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 147. ed., p 147.
The Film/Screen Cassette Cassette Cassette
Light-tight and ensures screen Light-tight and ensures screen contact with film contact with film
ID flash card area on back ID flash card area on back Front surface of carbon fiber Front surface of carbon fiber
1 or 2 Intensifying Screens 1 or 2 Intensifying Screens Convert x-rays to visible light Convert x-rays to visible light Mounted on layers of Mounted on layers of
compressed foam compressed foam
Sheet of film Sheet of film Indirectly records the x-ray Indirectly records the x-ray
distribution distribution Chemically processed Chemically processed Storage and displayStorage and display
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 148. ed., p 148.
Intensifying Screens
Film relatively insensitive to x-rays Film relatively insensitive to x-rays Patient receives a large dose Patient receives a large dose Screens made of scintillating material: Screens made of scintillating material:
phosphorphosphor
Intensifying Screens X-rays absorbed by phosphor create visible X-rays absorbed by phosphor create visible
light through photoelectrons, Compton light through photoelectrons, Compton electrons and delta-rays which excite rare electrons and delta-rays which excite rare earth atoms that emit EM radiation in the UV earth atoms that emit EM radiation in the UV and visible regions and visible regions ≈ ≈ 5% of film darkening due to direct x-ray 5% of film darkening due to direct x-ray
interaction with film interaction with film → → IndirectIndirect detector detector Reduce radiation burden to patient up to 50XReduce radiation burden to patient up to 50X
Screen Composition Early 20Early 20thth century: calcium century: calcium
tungstate, CaWOtungstate, CaWO4 4
Light emissions in the blues Light emissions in the blues and UV.and UV.
Film had to be sensitive to blue Film had to be sensitive to blue light and UV.light and UV.
Permitted “safelight” in the red. Permitted “safelight” in the red. Complete darkness not Complete darkness not required. required.
c.f. c.f. http://www.ktf-split.hr/periodni/en/http://www.ktf-split.hr/periodni/en/
Screen Composition Since early 70’s: rare earth Since early 70’s: rare earth
phosphor phosphor Lanthanide series: Z = 57 – 71 Lanthanide series: Z = 57 – 71 GdGd22OO22S:Tb (gadolinium S:Tb (gadolinium
oxysulfide: terbium) oxysulfide: terbium) LaOBr:Tm (lanthanum LaOBr:Tm (lanthanum
oxybromide: thulium) oxybromide: thulium) YTaOYTaO44:Nb (yttrium tantalate: :Nb (yttrium tantalate:
niobium)niobium) Emissions are in the green part Emissions are in the green part
of spectrum.of spectrum. Film had to be green sensitive.Film had to be green sensitive. Screens emitted many more Screens emitted many more
photons. photons. c.f. c.f. http://www.ktf-split.hr/periodni/en/http://www.ktf-split.hr/periodni/en/
Screen Composition Top coat Top coat Phosphor and binder Phosphor and binder Adhesive Adhesive Support Support Phosphor thickness expressed Phosphor thickness expressed
as mass thickness = thickness as mass thickness = thickness (cm) ∙ density (g/cm(cm) ∙ density (g/cm33) )
General radiography: each of General radiography: each of two screens around 60 mg/cmtwo screens around 60 mg/cm22
Mammography: single screen Mammography: single screen of 35 mg/cmof 35 mg/cm22 used used
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 150. ed., p 150.
Screen Composition
PLASTIC BASE ~ 10 mil
TiO2 REFLECTIVE LAYER ~ 1 mil
PHOSPHOR LAYER ~ 4 to 6 mil
PROTECTIVE LAYER < 1 mil
Screen Function & Geometry
Function: absorb x-rays and convert to light Function: absorb x-rays and convert to light Conversion efficiency = fraction of absorbed energy Conversion efficiency = fraction of absorbed energy
emitted as UV or visible light emitted as UV or visible light CaWOCaWO44 ≈ 5% intrinsic efficiency ≈ 5% intrinsic efficiency
GdGd22OO22S:Tb ≈ 15% intrinsic efficiency S:Tb ≈ 15% intrinsic efficiency
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 151. ed., p 151.
Screen Function & Geometry GdGd22OO22S:Tb – S:Tb –
545 nm (green), 2.7 eV 545 nm (green), 2.7 eV 50,000 eV x 0.15 = 7500 eV 50,000 eV x 0.15 = 7500 eV 7500 eV / 2.7 eV/photon 7500 eV / 2.7 eV/photon = 2,800 photons = 2,800 photons 200-1000 photons reach film 200-1000 photons reach film
Quantum Detective Efficiency Quantum Detective Efficiency (QDE) of a screen = fraction of x-(QDE) of a screen = fraction of x-rays photons attenuated by the rays photons attenuated by the screens screens
QDE increases with screen QDE increases with screen thickness thickness
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 151. ed., p 151.
Screen Function & Geometry
Light-spreading within Light-spreading within phosphor (isotropic diffusion) phosphor (isotropic diffusion) causing blurring of imaged causing blurring of imaged object at detector object at detector
As screen thickness ↑ QDE ↑ As screen thickness ↑ QDE ↑ and screen sensitivity ↑, but and screen sensitivity ↑, but light-diffusion increaseslight-diffusion increases
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 152. ed., p 152.
Screen Thickness Effect
Screen Function & Geometry
Crossover or print-throughCrossover or print-through: light : light from top screen penetrates the from top screen penetrates the film base and exposes the film base and exposes the bottom emulsion bottom emulsion
Modulation Transfer Function Modulation Transfer Function (MTF) describes the degree of (MTF) describes the degree of image sharpness or spatial image sharpness or spatial resolution resolution
As screen thickness ↑ MTF ↓As screen thickness ↑ MTF ↓
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 152. ed., p 152.
Spatial Resolution Depends Upon Screen Thickness
Reflective Layer of Screens
SCREEN
SCREEN
FILM EMULSION
FILM EMULSION
BASE
Crossover of Light Through One Emulsion To The Emulsion on the Other Side
Film Cassette
Parallax
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.
Effect of Dyes in the Screen
Conversion Efficiency (CE) Total conversion efficiency (CE) is the ability of Total conversion efficiency (CE) is the ability of
screen-film combination to convert the energy screen-film combination to convert the energy deposited by the absorbed x-rays into film darkening deposited by the absorbed x-rays into film darkening or OD or OD Intrinsic conversion efficiency of phosphor (DQE & Intrinsic conversion efficiency of phosphor (DQE &
light emission efficiency)light emission efficiency) Efficiency of light propagation through the screen Efficiency of light propagation through the screen
to film emulsion layer to film emulsion layer Efficiency of the film emulsion in absorbing the Efficiency of the film emulsion in absorbing the
emitted lightemitted light
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 153. ed., p 153.
Conversion Efficiency (CE)
Light propagation in screen Light propagation in screen Distance from absorption to film Distance from absorption to film Light-absorbing dye: CE ↓, MTF ↑Light-absorbing dye: CE ↓, MTF ↑ Reflective layer: CE ↑, MTF ↓Reflective layer: CE ↑, MTF ↓
Screen a linear device at a given x-ray energyScreen a linear device at a given x-ray energy
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p 153. ed., p 153.
Conversion Efficiency
CaWO4 Low Conversion Efficiency Screens
PRODUCE3000 LIGHTPHOTONS
WHICHDARKEN
FILM
15 INCIDENTX-RAYS &ONLY 3
INTERACT INSCREEN
Rare Earth High Conversion Efficiency Screens
PRODUCE3000 LIGHTPHOTONS
WHICHDARKEN
FILM
5 INCIDENTX-RAYS &ONLY 1
INTERACTS INSCREEN
Absorption Efficiency (AE)
The absorption efficiency or The absorption efficiency or QDE describes how efficiently QDE describes how efficiently the screen detects x-ray the screen detects x-ray photons that are incident upon photons that are incident upon it it
X-ray beam is polychromatic X-ray beam is polychromatic and has a broad spectrum of and has a broad spectrum of energies energies
X-ray photon absorbed by the X-ray photon absorbed by the screen deposits its energy and screen deposits its energy and some fraction of energy is some fraction of energy is converted to light photonsconverted to light photons
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 154-155. ed., pp. 154-155.
Absorption Efficiency (AE)
The number of light photons The number of light photons produced in the screen is produced in the screen is determined by the total amount determined by the total amount of x-ray energy absorbed by of x-ray energy absorbed by the screen, not by the number the screen, not by the number of x-ray photons of x-ray photons
S-F systems are considered S-F systems are considered energy detectorsenergy detectors
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 154-155. ed., pp. 154-155.
Absorption Efficiency
Thicker Screens Have Higher Absorption Efficiency
Overall EfficiencySpatial resolution of Spatial resolution of
film is high film is high Screens used to Screens used to
reduce dose reduce dose Exposure times Exposure times
shorter shorter Reduced costs for Reduced costs for
equipment and equipment and shieldingshielding
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 156. ed., p. 156.
Overall Efficiency Total efficiency = AE ∙ CE Total efficiency = AE ∙ CE Intensification factor (IF) = Intensification factor (IF) =
ratio of energy absorption ratio of energy absorption of 120 mg/cmof 120 mg/cm22 phosphor phosphor vs. 0.80 mg/cmvs. 0.80 mg/cm22 AgBr AgBr (film emulsion) (film emulsion)
Example: 80 kVp Example: 80 kVp GdGd22OO22S:Tb detects S:Tb detects
29.5% 29.5% AgBr detects 0.65% AgBr detects 0.65% IF = 29.5%/0.65% = IF = 29.5%/0.65% =
45.445.4
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 156. 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.
Wavelength (Angstroms)
4000 5000 6000
Y2O2S
La2O2S
Gd2O2S
CaWO4
Intensifying Screen Materials
3000 4000 5000 6000 7000
WAVELENGTH (ANGSTROMS)
UV GREENBLUE YELLOW RED
FILM
SENSITIVITY
SILVER HALIDE FILM
PANCHROMATIC FILM
ORTHOCHROMATIC FILM
K-Edge of Phosphor Material Makes Screens are kVp Dependent
La2O2S Gd2O2S CaWO4
40 60 80 100 120
1.0
0.5
0
RELAT I VE
SPEED
TUBE POTENTIAL (kVp)
Gd2O2S FILM-SCREEN SYSTEM
SPEED vs. X-RAY kVp
Noise Effects of CE vs. AE Noise: local variations in film OD not representing variations of Noise: local variations in film OD not representing variations of
attenuation occurring in the object, includes random noise caused attenuation occurring in the object, includes random noise caused by factors such as by factors such as Statistical fluctuation in x-ray quantity interacting with screens Statistical fluctuation in x-ray quantity interacting with screens Statistical fluctuation in fraction of light emitted by the screen Statistical fluctuation in fraction of light emitted by the screen
that is absorbed by the film emulsion that is absorbed by the film emulsion Statistical fluctuation in the distribution of silver halide grains in Statistical fluctuation in the distribution of silver halide grains in
film emulsion film emulsion
The visual perception of noise is reduced (better image quality) The visual perception of noise is reduced (better image quality) when the number of detected x-ray photons increaseswhen the number of detected x-ray photons increases
Noise Effects of Changing CE vs. AENoise Effects of Changing CE vs. AE
What happens to noise in image when the CE is What happens to noise in image when the CE is increased? increased? ↑ ↑ CE => fewer x-ray photons are required to achieve CE => fewer x-ray photons are required to achieve
same OD on film so noise increases same OD on film so noise increases What happens to noise in image when the AE is What happens to noise in image when the AE is
increased? increased? ΔΔ AE => noise unchanged (↑ AE => ↓ mAs, so same AE => noise unchanged (↑ AE => ↓ mAs, so same
number of x-ray) photons absorbed) number of x-ray) photons absorbed) If ↑ AE through ↑ screen thickness => ↓ spatial If ↑ AE through ↑ screen thickness => ↓ spatial
resolutionresolution
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
MAGNIFIED VIEW OF UNIFORMLY EXPOSED FILMS
Different Screen-Film Combinations have Mottle that have Different Appearances
Mottle in a Clinical Image
Advantages of Rare Earth Screens
For the same thickness as CaWO4, rare earth decreases the patient dose, with the same resolution and more quantum mottle.
For the same patient dose, Rare earth screens are thinner, thus have better resolution and more quantum mottle.
SPATIAL FREQUENCY (LP/mm)
RELATIVE
NOISE
WHITE NOISE
GRAIN NOISE
FILM-SCREEN NOISE ...MOSTLY Q.M.
RADIOGRAPHIC MOTTLE (IMAGE NOICE)
THREE MAIN COMPONENTS OF MOTTLE QUANTUM MOTTLE
SCREEN MOTTLE
GRAIN MOTTLE
QUANTUM MOTTLE IS VARIATION IN # PHOTONS / mm2 USED TO FORM IMAGE
SCREEN MOTTLE IS VARIATION IN PHOSPHOR THICKNESS & DENSITY
GRAIN MOTTLE IS VARIATION IN # SILVER GRAINS IN FILM / mm2
Basic Principles
Film determines the image contrast. Only the screen determines spatial resolution. The combination of film and Screen determines the speed (dose) & quantum mottle. Film processing affects everything except resolution.
Conclusions about Quantum Mottle (Q.M.)
For a give type of phosphor, the thickness of the intensifying screen does not increase QM – Only speed (dose)
Screens with low spatial resolution decrase QM – noise is blurred out.
Changing to a faster film does increase QM.
Conclusions about Q.M.
High contrast film increases visibility of QM
Changing film processing can affect QM.
Changing from CaWO4 to various rare earth screens increases Q.M.
Q.M. is only important when trying to visualize low contrast objects.
Film Composition & Function 1 or 2 layers of film emulsion 1 or 2 layers of film emulsion
coated onto a flexible Mylar coated onto a flexible Mylar plastic sheet plastic sheet
Emulsion: silver halide (AgBr Emulsion: silver halide (AgBr and AgI) bound in a gelatin base and AgI) bound in a gelatin base
Emulsion of an exposed sheet of Emulsion of an exposed sheet of
film contains the latent image film contains the latent image Latent image rendered visible Latent image rendered visible
through film processing by through film processing by chemical reduction of silver chemical reduction of silver halide into metallic silver grainshalide into metallic silver grains
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 157. ed., p. 157.
Optical Density
Increased x-ray exposure → developed film becomes Increased x-ray exposure → developed film becomes darker darker
Degree of darkness of the film is quantified by the optical Degree of darkness of the film is quantified by the optical density (OD) which is measured with a density (OD) which is measured with a densitometer densitometer
Transmittance (T) is the fraction of incident light passing Transmittance (T) is the fraction of incident light passing through the film through the film
= I/I= I/I00 where I – intensity measured at a particular location where I – intensity measured at a particular location
on film and Ion film and I00 – intensity of light measured with no film in – intensity of light measured with no film in
densitometer densitometer
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 158.Imaging, 2nd ed., p. 158.
Optical Density As OD increases, transmittance decreases As OD increases, transmittance decreases OD = -logOD = -log1010(T) = log(T) = log1010(1/T) = log(1/T) = log1010(I(I00/I), inverse relationship is T = 10/I), inverse relationship is T = 10-OD-OD
The OD of superimposed films is additive The OD of superimposed films is additive
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 158.Imaging, 2nd ed., p. 158.
The Hurter and Driffield (H&D) CurveThe Hurter and Driffield (H&D) Curve
H&D (characteristic) curve H&D (characteristic) curve describes how film responds to describes how film responds to x-ray exposure x-ray exposure
Non-linear, sigmoidal shape Non-linear, sigmoidal shape loglog1010-log-log1010 plot (OD vs. log plot (OD vs. log
exposure) exposure) Film base → OD = 0.11 – 0.15Film base → OD = 0.11 – 0.15
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 159. ed., p. 159.
The Hurter and Driffield (H&D) CurveThe Hurter and Driffield (H&D) Curve
Fogging due to long storage, Fogging due to long storage, heat and low background heat and low background exposure exposure
Base + Fog ≤ 0.20 OD Base + Fog ≤ 0.20 OD Toe Toe Linear region Linear region Shoulder Shoulder Fast films requires less Fast films requires less
exposure to achieve a given exposure to achieve a given OD; slow films require more OD; slow films require more exposureexposure
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 159. ed., p. 159.
Contrast of Film (Average Gradient)
Contrast of film is related to Contrast of film is related to the slope of the H&D curve: the slope of the H&D curve: Higher slope have higher Higher slope have higher
contrast contrast Reduced slope have lower Reduced slope have lower
contrast contrast Average gradient = Average gradient = [OD[OD22-OD-OD11]/[log]/[log1010(E(E22)-log)-log1010(E(E11)] )]
ODOD22 = 2.0 + B + F = 2.0 + B + F
ODOD11 = 0.25 + B + F = 0.25 + B + F
Range from 2.5 – 3.5Range from 2.5 – 3.5
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 160. ed., pp. 160.
Contrast of Film (Average Gradient)
Describes the contrast properties of the film-screen system Describes the contrast properties of the film-screen system Important to obtain well controlled exposure levels to ensure good Important to obtain well controlled exposure levels to ensure good
contrast contrast Film manufacturer physically controls contrast of film by varying Film manufacturer physically controls contrast of film by varying
silver halide grain size distributionsilver halide grain size distribution
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 161. ed., pp. 161.
Sensitivity or Speed From H&D curve, as the speed From H&D curve, as the speed
of SF system increases, the of SF system increases, the amount of x-ray exposure amount of x-ray exposure required to achieve same OD required to achieve same OD decreases decreases
Faster (higher-speed) SF Faster (higher-speed) SF systems result in lower patient systems result in lower patient doses but in general exhibit doses but in general exhibit more quantum mottle (noise) more quantum mottle (noise) than slower systemsthan slower systems
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 162. ed., p. 162.
Sensitivity or Speed Absolute speed = 1 / Exposure Absolute speed = 1 / Exposure
(R) required to achieve OD = (R) required to achieve OD = 1.0 + B + F 1.0 + B + F
1,667 R1,667 R-1-1 (1/0.0006R) and 500 (1/0.0006R) and 500 RR-1-1 (1/0.002R) (1/0.002R)
Relative speed of a SF Relative speed of a SF combination– relative to a combination– relative to a common standard (100 speed), common standard (100 speed), commercially used commercially used
Most US institutions that use Most US institutions that use screen-film use 400 speed for screen-film use 400 speed for general radiography general radiography
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 162. ed., p. 162.
Sensitivity or Speed
100-speed – detail work 100-speed – detail work (thinner screens, slower, better (thinner screens, slower, better spatial resolution) spatial resolution)
600-speed – angiography 600-speed – angiography (thicker screens, decreased (thicker screens, decreased spatial resolution)spatial resolution)
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 162. ed., p. 162.
Latitude Horizontal shift between 2 Horizontal shift between 2
H&D curves – systems differ in H&D curves – systems differ in speed speed
Systems with different contrast Systems with different contrast have H&D curves with different have H&D curves with different slopes slopes
Latitude is the range of x-ray Latitude is the range of x-ray exposures that deliver ODs in exposures that deliver ODs in the usable range the usable range
Latitude is also called dynamic Latitude is also called dynamic rangerange
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 162. ed., p. 162.
Latitude System A has higher contrast System A has higher contrast
but reduced but reduced latitude latitude It is more difficult to consistently It is more difficult to consistently
achieve proper exposures with achieve proper exposures with low-latitude SF systems. low-latitude SF systems.
Chest radiography needs a Chest radiography needs a high-latitude system to achieve high-latitude system to achieve adequate contrast in both the adequate contrast in both the mediastinum and lung fieldsmediastinum and lung fields
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 162. ed., p. 162.
The Screen-Film System Film emulsion should be sensitive to light emitted by Film emulsion should be sensitive to light emitted by
screen screen CaWO4 emits blue light to which film is sensitive CaWO4 emits blue light to which film is sensitive GdGd22OO22S:Tb emits green lightS:Tb emits green light
Wavelength sensitizers added to film Wavelength sensitizers added to film green: orthochromatic green: orthochromatic red: panchromatic red: panchromatic
Screens and films usually purchased in combinationScreens and films usually purchased in combination
The Screen-Film System
Reciprocity law of film states that the Reciprocity law of film states that the relationship between exposure and relationship between exposure and OD should remain constant OD should remain constant regardless of the exposure rate regardless of the exposure rate
Reciprocity law failure: at long and Reciprocity law failure: at long and
short exposure times, the OD at a short exposure times, the OD at a given kVp and mAs is not constantgiven kVp and mAs is not constant
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2stst ed., p. 163. ed., p. 163.
Contrast and Dose Through adjusting the kVp Through adjusting the kVp
(quality) and mAs (quantity), (quality) and mAs (quantity), the technologist is adjusting the technologist is adjusting subject contrast with respect to subject contrast with respect to the S-F H&D the S-F H&D
Technique still an art, but: Technique still an art, but:
Technique chart Technique chart Phototimer Phototimer Different body habitus Different body habitus
Keep exposure time shortKeep exposure time short
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 165. ed., pp. 165.
Contrast and Dose
kVp ↑ → dose and contrast ↓ kVp ↑ → dose and contrast ↓ Classic compromise between Classic compromise between
image contrast and patient image contrast and patient dosedose
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 166. ed., pp. 166.
Scattered Radiation mm(CS) ≈ (CS) ≈ mm(PE) (PE)
Tissue @ 26 keV Tissue @ 26 keV Bone @ 35 keV Bone @ 35 keV
Most radiographic interactions Most radiographic interactions produce scattered photons produce scattered photons
Scattered photons → violation Scattered photons → violation of the basic principle of of the basic principle of projection imaging: mis-projection imaging: mis-information reducing contrastinformation reducing contrast
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 167. ed., p. 167.
Scattered Radiation
Scatter-to-Primary ratio (S/P) Scatter-to-Primary ratio (S/P) Area of collimated x-ray Area of collimated x-ray
field field Object thickness Object thickness kVp of x-ray beamkVp of x-ray beam
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 167. ed., p. 167.
Scattered Radiation Loss of contrast Loss of contrast In the absence of scatter: In the absence of scatter:
CC00 = [A-B]/A = [A-B]/A In the presence of scatter: In the presence of scatter:
C = [(A+S)-(B+S)]/(A+S) C = [(A+S)-(B+S)]/(A+S) C = [A-B]/(A+S) C = [A-B]/(A+S) C < CC < C0 0 → contrast → contrast
decreases decreases C = [A-B]/[A(1+{S/A})] C = [A-B]/[A(1+{S/A})] C = CC = C00/(1+{S/P}) /(1+{S/P})
S/P ↑ → contrast ↓ S/P ↑ → contrast ↓ 1/(1+{S/P}): contrast reduction 1/(1+{S/P}): contrast reduction
factor factor
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 168. ed., p. 168.
The Antiscatter Grid Between object and detector Between object and detector Uses geometry to ↓ scatter Uses geometry to ↓ scatter Thin lead septa separated by Thin lead septa separated by
aluminum or carbon fiber aluminum or carbon fiber Grid ratio (GR) = H/W = septa Grid ratio (GR) = H/W = septa
height/interspace width height/interspace width 8:1, 10:1 and 12:1 common 8:1, 10:1 and 12:1 common 5:1 for mammography 5:1 for mammography ↑ ↑ GR → ↓ S/P GR → ↓ S/P ↑ ↑ GR → ↑ dose GR → ↑ dose
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 168-169. ed., pp. 168-169.
Grid Construction
The Antiscatter Grid ↑ ↑ GR → ↑ clean-up of scatter GR → ↑ clean-up of scatter
striking the grid at large striking the grid at large angles, less effective for angles, less effective for smaller angles smaller angles
Grid frequency: lines/cm Grid frequency: lines/cm grid freq. doesn’t alter S/P grid freq. doesn’t alter S/P 60 lines/cm60 lines/cm
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 170. ed., pp. 170.
The Antiscatter Grid Stationary grids: lines appear Stationary grids: lines appear
on image on image Bucky: reciprocating grid Bucky: reciprocating grid Bucky factor = Bucky factor =
dosedosew gridw grid/dose/dosew/o grid w/o grid
Bucky factors: Bucky factors: 5:1 5:1 3 3 8:18:1 4 4 12:112:1 5 5 16:116:1 6 6
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 171. ed., pp. 171.
Parallel Grid
Grid Ratio
Parallel Grid
Parallel Grid
Focused Grid
Focused GridsFocused grids have a range of focal distances.
Distance de-centering, Lateral de-centering & a combination of both cause cut-off of primary radiation.
Upside down focused grids show only a narrow area in the center of the image receptor.
Crossed Grids
Crossed Grids
The effective grid ratio of two crossed grids is the sum of the individual ratios.
Crossed grids clean up (remove) the scattered radiation in two orthogonal directions.
Crossed grids are more sensitive to improper alignment.
Focused Grid at the Focal Point
Focused Grid – Upside Down
Focused Grid – Distance De-centered
Focused Grid – High and to Left
Focused Grid – Low and to the Right
Contrast Improvement Factor (K)
The removal of scattered x-rays by the grid improves the contrast.
Ratio of contrast with scatter plus grid devided by contrast with scatter without grid is “K”
K = CSG
CSNG
Contrast Improvement Factor of Grids
Bucky Factor (B)
Because the grid removes scattered x-rays that would have exposed the film,Fewer x-rays reach the image receptor. Radiation dose must be increased to
maintain the film’s OD.
B = (DOSEWITH GRID) / (DOSENO GRID)
Extra Radiation Needed With Use of a
Grid
Extra Radiation Needed With Use of a Grid
SKIN ENTRANCE EXPOSURE FOR 3M ABDOMEN PHANTOM VS. kVp
X-RAY TUBE POTENTIAL (kVp)
Dependence of “B” & “K”
“B” and “K” Depend on grid ratio Both increase with higher g
“B” and “K” Depend upon ( S / P )Both increase with more scatter.
“B” & “K” are nearly the same numerically. “B” > “K”
A typical value of “B” IS ~ 3 – 5
Grid Artifacts and Air Gap
Most grid artifacts due to mis-Most grid artifacts due to mis-positioning positioning
Upside down: severe loss of Upside down: severe loss of
OD at margins OD at margins Crooked & off-center: general Crooked & off-center: general
decrease of OD across entire decrease of OD across entire image image
Off-focus: loss at lateral edgesOff-focus: loss at lateral edges
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 172. ed., pp. 172.
Grid Artifacts and Air Gap
Air gap: ↓ S/P, but ↑ M, ↓ FOV Air gap: ↓ S/P, but ↑ M, ↓ FOV and ↓ MTF (unless very small and ↓ MTF (unless very small focal spot used) focal spot used)
Not used all that often in Not used all that often in radiography, used in radiography, used in mammography (magnification)mammography (magnification)
c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., pp. 173. ed., pp. 173.
Alternative to Grids
Air Gaps of 20-30 cm can be effective in reducing scattered radiation to the image receptor.
IMAGE RECEPTOR