Screen / Film Imaging

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.


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


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


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)


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


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


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


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



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


Screen Thickness Effect


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 ↓


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


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


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


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


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


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


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


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


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


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


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)


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


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


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


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


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


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


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


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


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


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


Parallel Grid

Grid Ratio

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


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)


Alternative to Grids

Air Gaps of 20-30 cm can be effective in reducing scattered radiation to the image receptor.

IMAGE RECEPTOR

Screen / Film Imaging

Documents

Transcript of Screen / Film Imaging