Transcript Film/Screen Imaging

Screen / Film Imaging
Roland Wong, Sc.M., D.A.B.M.P,
D.A.B.R.
Outline
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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
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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
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The radiation intensity from a
point source decreases with the
square of the distance
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E2 = E1 ∙ (D1/D2)2
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This relationship is only valid for point sources
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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
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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
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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
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Cassette
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Light-tight and ensures screen
contact with film
ID flash card area on back
Front surface of carbon fiber
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1 or 2 Intensifying Screens
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Convert x-rays to visible light
Mounted on layers of
compressed foam
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Sheet of film
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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
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Intensifying Screens
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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
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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
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Since early 70’s: rare earth
phosphor
Lanthanide series: Z = 57 – 71
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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
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Top coat
Phosphor and binder
Adhesive
Support
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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.
Screen Composition
PROTECTIVE LAYER < 1 mil
PHOSPHOR LAYER ~ 4 to 6 mil
TiO2 REFLECTIVE LAYER ~ 1 mil
PLASTIC BASE ~ 10 mil
Screen Function & Geometry
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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
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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 xrays 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
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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 Thickness Effect
Screen Function & Geometry
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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.
Spatial Resolution Depends
Upon Screen Thickness
Reflective Layer of
Screens
Crossover of Light Through One Emulsion
To The Emulsion on the Other Side
SCREEN
FILM
EMULSION
BASE
FILM
EMULSION
SCREEN
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)
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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)
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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.
Conversion Efficiency
CaWO4 Low Conversion
Efficiency Screens
15 INCIDENT
X-RAYS &
ONLY 3
INTERACT IN
SCREEN
PRODUCE
3000 LIGHT
PHOTONS
WHICH
DARKEN
FILM
Rare Earth High Conversion
Efficiency Screens
5 INCIDENT
X-RAYS &
ONLY 1
INTERACTS IN
SCREEN
PRODUCE
3000 LIGHT
PHOTONS
WHICH
DARKEN
FILM
Absorption Efficiency (AE)
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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)
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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 xray photons
S-F systems are considered
energy detectors
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., pp. 154-155.
Absorption Efficiency
Thicker Screens Have Higher
Absorption Efficiency
Overall Efficiency
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Spatial resolution of
film is high
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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
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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
4000
5000
Wavelength (Angstroms)
6000
Intensifying Screen Materials
F
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L
M
S
E
N
S
I
T
I
V
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T
Y
SILVER
HALIDE FILM
PANCHROMATIC
FILM
ORTHOCHROMATIC
FILM
3000
UV
4000
BLUE
5000
GREEN
6000
YELLOW
WAVELENGTH (ANGSTROMS)
7000
RED
MASS ATTENUATION COEFFCIENT (CM2/GM)
K-Edge of Phosphor Material Makes
Screens are kVp Dependent
1000
K-EDGE OF SCREENS
100
10
1
0
20
40
60
80
100
X-RAY ENERGY (keV)
La2O2S
Gd2O2S
CaWO4
120
SPEED vs. X-RAY kVp
R
E
L
A
T
I
V
E
S
P
E
E
D
1.0
Gd2O2S FILMSCREEN SYSTEM
0.5
0
40
60
80
100
TUBE POTENTIAL (kVp)
120
Noise Effects of CE vs. AE
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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
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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
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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
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Quantum Mottle
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Screen Mottle
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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.
Quantum Mottle
Nx
N = # PHOTONS / mm2
100%
N
Different Screen-Film Combinations have
Mottle that have Different Appearances
MAGNIFIED VIEW OF UNIFORMLY EXPOSED FILMS
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.
R
E
L
A
T
I
V
E
N
O
I
S
E
WHITE NOISE
FILM-SCREEN NOISE
...MOSTLY Q.M.
GRAIN NOISE
SPATIAL FREQUENCY (LP/mm)
RADIOGRAPHIC MOTTLE
(IMAGE NOICE)
THREE MAIN COMPONENTS OF MOTTLE
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QUANTUM MOTTLE
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SCREEN MOTTLE
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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
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1 or 2 layers of film emulsion
coated onto a flexible Mylar
plastic sheet
Emulsion: silver halide (AgBr
and AgI) bound in a gelatin
base
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Emulsion of an exposed sheet
of film contains the latent
image
Latent image rendered visible
through film processing by
chemical reduction of silver
halide into metallic silver
grains
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 157.
Optical Density
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Increased x-ray exposure → developed film becomes
darker
Degree of darkness of the film is quantified by the optical
density (OD) which is measured with a densitometer
Transmittance (T) is the fraction of incident light passing
through the film
= I/I0 where I – intensity measured at a particular location
on film and I0 – intensity of light measured with no film in
densitometer
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 158.
Optical Density
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As OD increases, transmittance decreases
OD = -log10(T) = log10(1/T) = log10(I0/I), inverse relationship is T = 10-OD
The OD of superimposed films is additive
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 158.
The Hurter and Driffield (H&D) Curve
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H&D (characteristic) curve
describes how film responds to
x-ray exposure
Non-linear, sigmoidal shape
log10-log10 plot (OD vs. log
exposure)
Film base → OD = 0.11 – 0.15
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 159.
The Hurter and Driffield (H&D) Curve
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Fogging due to long storage,
heat and low background
exposure
Base + Fog ≤ 0.20 OD
Toe
Linear region
Shoulder
Fast films requires less
exposure to achieve a given
OD; slow films require more
exposure
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 159.
Contrast of Film (Average Gradient)
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Contrast of film is related to the
slope of the H&D curve:
 Higher slope have higher
contrast
 Reduced slope have lower
contrast
Average gradient =
[OD2-OD1]/[log10(E2)-log10(E1)]
 OD2 = 2.0 + B + F
 OD1 = 0.25 + B + F
Range from 2.5 – 3.5
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., pp. 160.
Contrast of Film (Average Gradient)
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Describes the contrast properties of the film-screen system
Important to obtain well controlled exposure levels to ensure good
contrast
Film manufacturer physically controls contrast of film by varying
silver halide grain size distribution
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., pp. 161.
Sensitivity or Speed
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From H&D curve, as the speed
of SF system increases, the
amount of x-ray exposure
required to achieve same OD
decreases
Faster (higher-speed) SF
systems result in lower patient
doses but in general exhibit
more quantum mottle (noise)
than slower systems
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 162.
Sensitivity or Speed
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Absolute speed = 1 / Exposure
(R) required to achieve OD =
1.0 + B + F
1,667 R-1 (1/0.0006R) and 500
R-1 (1/0.002R)
Relative speed of a SF
combination– relative to a
common standard (100 speed),
commercially used
Most US institutions that use
screen-film use 400 speed for
general radiography
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 162.
Sensitivity or Speed
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100-speed – detail work
(thinner screens, slower, better
spatial resolution)
600-speed – angiography
(thicker screens, decreased
spatial resolution)
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 162.
Latitude
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Horizontal shift between 2
H&D curves – systems differ in
speed
Systems with different contrast
have H&D curves with different
slopes
Latitude is the range of x-ray
exposures that deliver ODs in
the usable range
Latitude is also called dynamic
range
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 162.
Latitude
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System A has higher contrast
but reduced latitude
It is more difficult to consistently
achieve proper exposures with
low-latitude SF systems.
Chest radiography needs a
high-latitude system to achieve
adequate contrast in both the
mediastinum and lung fields
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 162.
The Screen-Film System
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Film emulsion should be sensitive to light emitted by
screen
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CaWO4 emits blue light to which film is sensitive
Gd2O2S:Tb emits green light
Wavelength sensitizers added to film
 green: orthochromatic
 red: panchromatic
Screens and films usually purchased in combination
The Screen-Film System

Reciprocity law of film states that the
relationship between exposure and
OD should remain constant
regardless of the exposure rate

Reciprocity law failure: at long and
short exposure times, the OD at a
given kVp and mAs is not constant
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2st ed., p. 163.
Contrast and Dose

Through adjusting the kVp
(quality) and mAs (quantity),
the technologist is adjusting
subject contrast with respect to
the S-F H&D
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Technique still an art, but:
 Technique chart
 Phototimer
 Different body habitus
Keep exposure time short
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., pp. 165.
Contrast and Dose


kVp ↑ → dose and contrast ↓
Classic compromise between
image contrast and patient
dose
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., pp. 166.
Scattered Radiation

m(CS) ≈ m(PE)

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Tissue @ 26 keV
Bone @ 35 keV
Most radiographic interactions
produce scattered photons
Scattered photons → violation
of the basic principle of
projection imaging: misinformation reducing contrast
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 167.
Scattered Radiation

Scatter-to-Primary ratio (S/P)
 Area of collimated x-ray
field
 Object thickness
 kVp of x-ray beam
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 167.
Scattered Radiation
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Loss of contrast
In the absence of scatter:
 C0 = [A-B]/A
In the presence of scatter:
 C = [(A+S)-(B+S)]/(A+S)
 C = [A-B]/(A+S)
 C < C0 → contrast
decreases
 C = [A-B]/[A(1+{S/A})]
 C = C0/(1+{S/P})
S/P ↑ → contrast ↓
1/(1+{S/P}): contrast reduction
factor
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 168.
The Antiscatter Grid
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Between object and detector
Uses geometry to ↓ scatter
Thin lead septa separated by
aluminum or carbon fiber
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Grid ratio (GR) = H/W = septa
height/interspace width
8:1, 10:1 and 12:1 common
5:1 for mammography
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↑ GR → ↓ S/P
↑ GR → ↑ dose
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., pp. 168-169.
Grid Construction
The Antiscatter Grid
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
↑ GR → ↑ clean-up of scatter
striking the grid at large
angles, less effective for
smaller angles
Grid frequency: lines/cm
 grid freq. doesn’t alter S/P
 60 lines/cm
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., pp. 170.
The Antiscatter Grid
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

Stationary grids: lines appear
on image
Bucky: reciprocating grid
Bucky factor =
 dosew grid/dosew/o grid
Bucky factors:
 5:1
3
 8:1
4
 12:1
5
 16:1
6
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., pp. 171.
Parallel Grid
Grid Ratio
Parallel Grid
Parallel Grid
Focused Grid
Focused Grids
Focused 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 xrays 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 mispositioning


Upside down: severe loss of
OD at margins


Crooked & off-center: general
decrease of OD across entire
image


Off-focus: loss at lateral edges
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., pp. 172.
Grid Artifacts and Air Gap


Air gap: ↓ S/P, but ↑ M, ↓ FOV
and ↓ MTF (unless very small
focal spot used)
Not used all that often in
radiography, used in
mammography (magnification)
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd 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