Download Screen Film Radiology

Screen Film Radiology
Mohammad Reza AY, PhD
Department of Medical Physics, Tehran University of Medical Sciences, Tehran, Iran
Division of Nuclear Medicine, Geneva University Hospital, Geneva, Switzerland
Objectives of Lecture
v Understand
the principles of basic geometric principles
and apply to projection imaging
v How screen-film detector systems work
v Define the characteristics of screens and films
v Understand the relation between contrast and dose in
radiography
v Significance of scattered radiation in projection
radiography
2
1
1. Projection Radiography
v Projection
imaging is the
acquisition of a 2D image of a
patient’s 3D anatomy
v Projection radiography is a
transmission imaging
procedure
v The optical density at any
location on the film
corresponds to the
attenuation characteristics (eµx) of the patient at that
location
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 146.
3
2. Basic Geometric Principles
v Similar
v
v
triangles (geometry)
a/A = b/B = c/C = h/H
d/D = e/E = f/F = g/G
v Similar
triangles are
encountered when determining
the image magnification and
when evaluating image
unsharpness caused by focal
spot size and patient motion
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 147.
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2
2. Basic Geometric Principles
v
v
a/A = b/B = c/C = h/H
d/D = e/E = f/F = g/G
v Magnification
v
v
v
(M)
Occurs because x-ray beam
diverges from focal spot to
image plane
For a point source,
v M = I/O = SID/SOD
largest when object closest
to focal spot and
approaches value of 1 when
object at image plane
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 147.
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2. Basic Geometric Principles
v For
a extended source (focal spot),
v Penumbra or blur (f)
v
edge gradient blurring due to
finite size of focal spot (F)
v
f/F = OID/SOD
v
f/F = (SID-SOD)/SOD
v
f/F = (SID/SOD)-1
v
f = F(M-1)
v
v
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.
6
3
3. The Screen-Film Cassette
Cassette
v
v
v
Light-tight and ensures screen
contact with film
Front surface - carbon fiber
ID flash card area on back
1 or 2 Intensifying Screens
v
v
Convert x-rays to visible light
Mounted on layers of
compressed foam (produces
force)
Sheet of film
v
v
v
Register the x-ray distribution
Chemically processed
Storage and display
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p 148.
7
4. Characteristics of Intensifying Screens
v Film
relatively insensitive to x-rays, requires a lot of x-ray energy to
produced a properly exposed x-ray film
v Patient receives a large dose
v To
reduce dose and exposure times, screens are used
made of scintillating material: phosphor
v Screens
v When
x-rays interact with phosphor, visible or UV light is emitted
Light emitted darkens the film
v → Screen-film detectors are considered an Indirect detector
Using filmfilm-screen versus film only reduces
radiation dose to patient by a factor of 50!
8
4
4. Screen Composition and Construction
20th century: calcium
tungstate, CaWO4
v Early
v Since early 70’s: rare earth
phosphor
v Lanthanide series: Z = 57 – 71
v Gd2O2S:Tb
(gadolinium
oxysulfide: terbium) - common
v LaOBr:Tm (lanthanum
oxybromide: thulium)
v YTaO4:Nb (yttrium tantalate:
niobium)
c.f. http://www.ktf- split.hr/periodni/en/
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4. Screen Composition and Construction
v Top
coat
v Phosphor
and binder
v Adhesive
v Support
thickness (g/cm2)
expressed as mass thickness =
thickness (cm) · density (g/cm3)
v Phosphor
v
v General
radiography: each of
two screens around 60 mg/cm2
v Mammography: single screen of
35 mg/cm2 used
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 150.
Cross-sectional image of
an intensifying screen
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5
4. Intensifying Screen Function and Geometry
v Function: absorb x-rays, convert to visible or UV light which exposes the
film emulsion
v Conversion efficiency of a phosphor = fraction of absorbed energy
emitted as UV or visible light
v CaWO4 ≈ 5% intrinsic conversion efficiency
v Gd2O2S:Tb ≈ 15% intrinsic conversion efficiency
50,000 eV x-ray x 0.15 = 7500 eV
Green light, 2.7 eV
7500 eV / 2.7 eV/photon
= 2,800 photons
200-1000 photons reach film after diffusing through phosphor layer
and being reflected at the interface layers
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4. Intensifying Screen Function and Geometry
v Quantum
Detective Efficiency
(QDE) of a screen = fraction of
incident x-rays photons that
interact with it
v QDE
increases with screen
thickness
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p 151.
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4. Intensifying Screen Function and Geometry
v Thicker
screens absorb greater
amount of x-rays, but a greater
lateral spread of the visible light
occurs (isotropic diffusion) causing
blurring and reducing spatial
resolution
Cross-section of a screen-film cassette
vA
thin screen results in less x-ray
absorption but less lateral spread of
light and better spatial resolution
v For
maximum resolution, a singlescreen cassette is used
v
v
X-rays first traverse the film and
then strike screen
Less light spread and maximum
spatial resolution
↑
Radiography
↑
Mammography
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p 152.
13
4. Intensifying Screen Function and Geometry
v As
screen thickness ↑ QDE ↑ and screen sensitivity ↑, but
light-diffusion increases
v Compromise between sensitivity and resolution
c.f. http://www.sprawls.org/resources/RADDETAIL/classroom.htm
c.f. http://www.rad-icon.com/pdf/Radicon_AN07.pdf
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4. Intensifying Screen Function and Geometry
v Modulation
Transfer Function
(MTF) describes the resolution
properties of an imaging
system
v The
MTF illustrates the
fraction (or %) of an object’s
contrast that is recorded by the
imaging system as a function of
object size (spatial frequency)
vF
(linepairs or cycles/mm)
F=1/2∆, ∆ = object size
v As screen thickness ↑ MTF ↓
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p 152.
15
4. Intensifying Screen Function and Geometry
v Crossover
or print-through:
light from top screen
penetrates the film base and
exposes the bottom emulsion
or vice versa
v Due to type of film grain, not a
problem now
c.f. http://www.sprawls.org/resources/RADDETAIL/classroom.htm
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4. Conversion Efficiency (CE)
Total conversion efficiency (CE) of a screen-film combination
refers to the ability of the screen or screens to convert the energy
deposited by the absorbed x-rays into film darkening or optical
density
CE depends on:
v Intrinsic conversion efficiency of phosphor
v Efficiency of light propagation through the screen to film
emulsion layer
v 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.
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4. Conversion Efficiency (CE)
Light propagation in screen (diffuses in all directions)
Distance from absorption to film
Light-absorbing dye reduces lateral distance: CE ↓ (slow), Spatial resolution or MTF ↑
Reflective layer redirect light photons: CE ↑ (fast), Spatial resolution or MTF ↓
Screen is a linear device at a given x-ray energy
v
Number of x-ray photons doubles, light intensity produced by screen also doubles
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 153.
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4. Absorption Efficiency (AE)
v The
absorption efficiency or
QDE describes how efficiently
the screen detects x-ray photons
that are incident upon it
v X-ray
photon absorbed by the
screen deposits its energy and
some fraction of energy is
converted to light photons
v Screen-film
systems are energy
detectors
The number of light photons
produced in the screen is
determined by the total
amount of x-ray energy
absorbed by the screen, not
by the number of x-ray
photons
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 154-155.
19
4. Overall Efficiency of a Screen-Film System
v Total
efficiency = AE · CE
vA
SF system increases x-ray
detection efficiency compared to
film only (29.5% vs. 0.65% at 80
kVp)
v Using
film-screen versus film only
reduces radiation dose to patient by
a factor of 50!
v Intensification
factor (IF) = ratio of
energy absorption of 120 mg/cm2
phosphor vs. 0.80 mg/cm2 AgBr
v An IF of 50 is achieved for Gd2O2S over the x-ray
energy spectra commonly used for diagnostic
examination
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 156.
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10
4. Noise Effects of Changing CE vs. AE
Noise: local variations in film OD, not representing variations of
attenuation in patient
Includes random noise caused by factors such as
v Statistical fluctuation in x-ray quantity interacting with
screens
v
v
Statistical fluctuation in fraction of light emitted by the
screen that is absorbed by the film emulsion
Statistical fluctuation in the distribution of silver halide
grains in film emulsion
The visual perception of noise is reduced (better image quality)
when the number of detected x-ray photons increases (more in
Chapter 10)
21
Noise Effects on Image Quality
22
c.f. http://www.sprawls.org/resources/IMGCHAR/module/#20
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4. Noise Effects of Changing CE vs. AE
What happens to noise in image when the CE is increased
(or fast film screen system) by adding reflective layer?
If “speed” of the SF system is increased by increasing the CE
(so that each detected x-ray photon becomes more efficient
at darkening the film):
v
few x-ray photons are required to achieve same film
darkening (as before increasing CE), so noise increases
v
Therefore, increasing the CE to increase the speed of a SF
system will increase the noise in the images
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4. Noise Effects of Changing CE vs. AE
What happens to noise in image when the AE is
increased (thicker screen)?
If AE is increased, 10% more x-ray photons detected, then
reduction of 10% in incident x-ray beam is required to
deliver same amount of film darkening (as before
increasing AE)
v
v
Since the fraction of increase in x-ray photon detection and
reduction in incident x-ray intensity is same, the total
number of detected x-ray photons is the same. No change
in noise
However, spatial resolution will get worse with thicker
screens
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12
5. Characteristics of Film
v1
or 2 layers of film emulsion
coated onto a flexible Mylar plastic
sheet
v Emulsion: silver halide (AgBr and
AgI) bound in a gelatin base
Tubular
grains
Cubic
grains
v Emulsion
of an exposed sheet of
film contains the latent image
v 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.
25
Optical Density
v Increased
x-ray exposure → developed film
becomes darker
v Degree of darkness of the film is quantified by the
optical density (OD) which is measured with a
densitometer
v Transmittance (T) is the fraction of incident light
passing through the film
v T = 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.
26
13
Optical Density
v OD = -log10(T)
-OD
= log10(1/T) = log10(I0/I), inverse relationship is T
= 10
v As OD increases, transmittance decreases
v The OD of superimposed films is additive
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 158.
27
The Hurter and Driffield (H&D) Curve
v H&D
(characteristic) curve
describes how film responds to
x-ray exposure
v Non-linear,
sigmoidal shape
v log10-log10 plot (OD vs. log
relative exposure)
v Film
base → OD = 0.11 – 0.15
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 159.
28
14
The Hurter and Driffield (H&D) Curve
v Fogging
due to long storage,
heat and low background
exposure
v Base
+ Fog ≤ 0.20 OD
v Toe
v Linear
region
v Shoulder
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 159.
29
Contrast of Film (Average Gradient)
v Contrast
of film is related to
the slope of the H&D curve:
v
Higher slope have higher
contrast
v
Reduced slope have
lower contrast
v Overall
contrast given by
Average gradient =
v [OD2-OD1]/[log10(E2)log10(E1)]
v
OD2 = 2.0 + B + F
v
OD1 = 0.25 + B + F
v Range from 2.5 – 3.5
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 160.
30
15
Contrast of Film (Average Gradient)
v Describes
the contrast properties of the film-screen system
v Important to obtain well controlled exposure levels to ensure
good contrast
v Film manufacturer physically controls contrast on film by
varying the size distribution of the sliver grains
31
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 161.
Sensitivity or Speed
v As
the speed of SF system
increases, the amount of x-ray
exposure required to achieve
same OD decreases
v Fast
films requires less
exposure to achieve a given OD;
slow films require more
exposure
v 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.
32
16
Sensitivity or Speed
v Absolute
speed = 1 / Exposure
(R) required to achieve OD = 1.0 +
B+F
v Relative
speed of a SF
combination– relative to a common
standard (100 speed),
commercially used
v 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.
33
Sensitivity or Speed
v 100-speed
– detail work
bony radiographs of
extremities, (thinner screens,
slower, better spatial
resolution)
v 600-speed
– angiography
(thicker screens, decreased
spatial resolution)
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 162.
34
17
Latitude (Dynamic Range)
v Horizontal
shift between 2
H&D curves – systems differ
in speed
v Systems
with different
contrast have H&D curves
with different slopes
v Latitude
is the range of xray exposures that deliver
ODs in the usable range
v Latitude
is also called
dynamic range
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 162.
35
Latitude (Dynamic Range)
v System
A has higher
contrast but reduced latitude
v It
is more difficult to
consistently achieve proper
exposures with low-latitude
SF systems.
v 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.
36
18
6. The Screen-Film System
v Film
emulsion should be sensitive to light emitted
by screen
v CaWO4
emits blue light to which film is sensitive
v Gd2O2S:Tb
emits green light
v Wavelength
sensitizers added to film
v Screens
and films usually purchased in
combination since matching of spectral sensitivity
very important
37
Reciprocity Law of Film
v Reciprocity
law of film states that
the relationship between exposure
and OD should remain constant
regardless of the exposure rate
v Reciprocity
law failure: at long
and short exposure times, the film
becomes less efficient at using the
light incident on it and lower ODs
result
v This
is a factor in mammography
when long exposure times are
needed for large and dense
breasts
All points exposed exactly the same with different
exposure rate
38
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2st ed., p. 163.
19
7. Contrast and Dose in Radiography
v The
SF system governs the overall detector contrast
v The contrast of a specific radiographic study depends on the
requirements of the study, total exposure time, radiation dose, size of
patient and so on…
v The
kVp (quality) and mAs (quantity) are adjusted by the
technologist to adjust the subject contrast
v
v
Quality – energy or penetrating power of x-ray beam (As kVp
increases, HVL also increases)
Quantity – number of x-ray photons
Technique still an art, but:
v
v
Technique chart
Phototimer (automatic technique)
39
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 165.
Contrast and Dose in Radiography (2)
v kVp
↑ → dose and contrast ↓
v Classic compromise between image contrast and patient
dose
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 165-166.
40
20
8. Scattered Radiation in Projection Radiography
v Most
radiographic interactions
produce scattered photons
v Scattered photons → violation of
the basic principle of projection
imaging: mis-information reducing
contrast
v The
scattered photon if detected
by film causes film darkening but
provides no useful information to
the image
v Scatter-to-primary
(S/P) ratio
refers to how many scattered xray photons there are for every
primary photon
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 167.
41
Scattered Radiation in Projection Radiography
Scatter-to-Primary ratio (S/P)
v
Area of collimated x-ray
field
v
Object thickness
v
kVp of x-ray beam
v
As FOV is reduced, scatter
is reduced
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 167.
42
21
Scattered Radiation in Projection Radiography
v Scatter
radiation causes loss
of contrast
v In
the absence of scatter, for
two adjacent areas
transmitting photon fluences
of A and B, the contrast is:
v
C0 = [A-B]/A
v In
the presence of scatter:
v
C = C0 x [1 / (1 + S/P)]
v S/P ↑ → contrast ↓
v 1/(1+S/P): contrast reduction
factor
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 168.
43
The Antiscatter Grid
v The
antiscatter grid is used to combat
the effects of scatter
v Between object and detector
v Uses geometry to ↓ scatter
v Thin lead septa separated by aluminum
or carbon fiber, aligned with focal spot
v Grid
ratio (GR) = H/W = septa
height/interspace width
v 8:1, 10:1 and 12:1 common, 5:1 for
mammography
v↑
v↑
GR → ↓ S/P
GR → ↑ dose
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 168-169.
44
22
The Antiscatter Grid
GR → ↑ clean-up of
scatter striking the grid at
large angles, less effective
for smaller angles
v↑
v
v 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.
45
The Antiscatter Grid
v Stationary
grids: lines
appear on image
v Bucky: device that moves
grid
v Moving grid bars on visible
on image
v Bucky
v
factor =
dosew grid/dosew/o grid
v Bucky
v
factors:
Range from 3 to 5
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 171.
46
23
Grid Artifacts
v Most
grid artifacts due to
mispositioning
v Upside
down: severe loss of
OD at margins
v Crooked
& off-center:
general decrease of OD
across entire image
v Off-focus:
edges
loss at lateral
c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 172.
47
Air Gaps
v Air
gap: ↓ S/P, but ↑ M, ↓
FOV and ↓ MTF (unless very
small focal spot used)
v Not
used all that often in
radiography except chest
radiography, used in
mammography
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 173.
48
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IMAGE QUALITY
INFLUENCE OF ENERGY (kV)
Bone
Soft tissue
THE CORRECT ENERGY
Good difference between dense and soft tissues
Best contrast
TOO HIGH ENERGY
Very few photons are absorbed
Small difference between dense and soft tissues
Low contrast
TOO LOW ENERGY
Most of the photons are absorbed
Low signal
Dose to the patient with no usable output
• High energy may be necessary in dense or thick areas (abdomen, pelvis)
• Very low energies are rarely of use and are harmful to the patient
25
INFLUENCE OF ENERGY (kV)
kV
Loss of contrast
Pale image
Loss of contrast
Dark image
High
High contrast
Dark image
Lack of photons
going through
Pale and loss of
information
Low
Low
High
mAs
INFLUENCE OF ENERGY (kV)
Low kV
High kV
Patient dose
Penetration
Contrast
The choice of kV depends on the patient anatomy and the object to be imaged
• Thick or dense part of the body: High kV,
ex. Abdomen, pelvis - 80 to 100 kV
• Small or flat parts of the body: Low kV,
ex. Breast, hand - 30 to 50 kV
Then the necessary mAs is elected to produce the clinically useful image
26
INFLUENCE OF FOCAL SPOT SIZE: PENUMBRA
Fine (small) focal spot
Sharp projection from a fine focal
spot
Dark
Dark
Clear
Large focal spot
Large focal spot is
responsible for blurr,
leading to inability to
distinguish between
small, close objects
Dark
Dark
Clear
Gray
Gray
LOSS OF SPATIAL RESOLUTION
Penumbra
INFLUENCE OF FOCAL SPOT SIZE: PENUMBRA
Increased
SOD
Decreased
OID
SOD
SID
OID
The blurr effect, or Penumbra, can be decreased by:
• Increasing the Source to Object Distance
• Placing the image receptor as close as possible to the patient
27
MAGNIFICATION EFFECT
long OID
short OID
There is always a magnification, but the same object may appear with different size
depending on its location in the body
The differential magnification is also reduced by using long SOD
and image receptor close to the patient
SIGNAL TO NOISE RATIO (SNR)
IMPROVING SNR
Less
Noise
The better the SNR
The better the visibility
LOSS OF CONTRAST
RESOLUTION
Signal
Inherent object contrast
Amplitude of the signal (difference in contrast versus the local background)
SNR =
Amplitude of the noise (random fluctuation of the signal)
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INCREASING THE SIGNAL - CONTRAST AGENTS
A way to improve SNR
Early phase:
opacification of the
general arterial
system
Injection of an Iodine
solution in the antecubital vein (arm)
Latter phase:
opacification of the
urinary system
(IVU)
Local injection
of Iodine
through a
catheter
Local vascular
opacification:
here the
coronary arteries
Catheter
Thanks For Your Attention
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