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The
Tutorial
X-ray
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questionnaire
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on
45 1-456.
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The
of the
beam.
The
information
quantity
After reading
tbis article
and taking The test, t/,c
isms
of x-ray
.
Be
scattering,
leigh
be
in
imaging
and the relative
probability
of photoelectric
interac-
various
procedures
LS a function
of the
. Be able
types,
of x-ray
the
and
number
absnber.
to predict
and
x-ray
.
following
interaction
Auger
image
dose
as use
range,
photoof atimage
appropriate
and
of contrast
the
modes
is to optimize
the
the
by
ac-
contrast
scattered
radiation
material
and
in
dedicated
and
degree
quality
pair
the
and
production
in diagnostic
differences
in these
diagnostic
are
utility
types
of the
presented
but
of x-ray
inter-
examination.
do
not
Ray-
occur
to any
sig-
radiography.
U INTRODUCTION
Understanding
of x-ray
specific
exposure
together
with
ray
the
interaction
kilovolt
will
of the
predominate.
to obtain
tient
Finally,
the
tissues
The
imaging
and
are
examination.
The objective
of this
interactions
the
and
highest
these
for
peak),
quality
a variety
x-ray
being
of reasons.
target
and
imaged,
predominant
impact
on patient
dose,
utility.
An understanding
radiologist
medical
is important
(eg,
characteristics
that
a substantial
diagnostic
dose.
interactions
parameters
Selection
filter
all affect
x-ray
the
interaction
the quality
of the radiograph,
of these
issues
enhances
the
diagnostic
types
of interactions
often
the
topic
for
article
is to provide
mechanisms
by which
images
are
some
at the
fundamental
of the
physics
an overview
these
lowest
to the
of x-
in turn
and
ability
its ultiof the
possible
of the
various
occur.
type
will
pa-
understanding
portion
of the
interactions
of
combinations,
types
of
board
of x-ray
In addition,
x
the
type
exploit
the
electrons.
Understand
of each
nostic
in-
characteristic
and
on
the
of second-
the
rays
ener-
understand
radiations
eluding
the
the
characteristics
ary
patient
such
equipment
scattering
have
mate
atomic
calculate
gies,
minimizing
photo-
which
would
to predominate
tiOflS
while
techniques
to improve
nificant
flOde
energs
tissues
major
and
predominant
obtain
of
four
influenced
energy
the
imaging
to
percentage
are
scattering,
are
diagnostic
are
attenuation
the
the
x-ray
of attenuation
interactions
in diagnostic
x-ray
Imaging
the
scattering
challenges
controlling
image.
absorption,
to predict
interaction
likely
the
of the
by
There
degree
is largely
incident
Compton
The
In the
Compton
the
is delivered
scattering,
in
image
from
recorded.
production.
composition.
and
mammography
scatter-
production.
able
between
actions
Compton
pair
by
image
successfully
involved
tissue
One
quisition
the
pair
removed
interactions,
photoelectric
and
mechan-
Rayleigh
including
ing,
tenuation.
will.
the
and
absorption
not
(coherent)
and
mechanisms
energy
are
Rayleigh
or fluoroscopic
are
of the
that
absorption,
electric
that
content
photons
interactions:
x-ray
in a radiograph
of x rays
information
noninteracting
predominant
Understand
PhD
diagnostic
result
electric
I
Residents
Interactions1
Jerrold
x-ray
reader
for
credit
hour
in category
tI.)e AAIIA Physician
Recognition
Physics
AAPM/RSNA
impact
of interaction
quality
in diag-
Index
terms:
Radiography
RadloGraphics
1998;
Physics
18:457-468
radiology.
‘From
CA 9581.
revision
the
1)cpartment
From
requested
the
ol Radiology.
AAPM/RSNA
October
22 and
School
Phvsics
received
of Medicine.
Tutorial
at the
November
University
1996
RSNA
19; accepted
of California
scientific
November
l)avis.
231 5 Stockton
assembly.
28. Address
Received
reprint
Blvd.
September
requests
Sacramento,
19.
1997:
to the
author.
. RSNA.
1998
457
Scattered
photon
1.
Figure
ing.
Rayleigh
Diagram
dent
photon
with
an
tered
scatter-
shows
the
atom
and
photon
the
being
2
with approximately
wavelength.
Rayleigh
tered
photons
emitted
tion,
seat-
Incident
photon
emitted
the same
seat-
are typically
in the
fairly
mci-
X1 interacting
forward
diree-
to
trajee-
close
the
tory of the incident
photon.
K, L, and M are electron
shells.
(Redrawn,
with permission,
from
reference
discussions
of x-ray
of the
factors
interaction
influence
the
nally,
examples
tions
that
can
1
8.)
that
and
x-ray
how
images
are
of differences
be
affect
the
these
presented.
in x-ray
exploited
mode
interactions
Fiinterac-
to enhance
utility
of a diagnostic
examination
presented.
The specific
modes
of x-ray
clinical
tions
include
Rayleigh
scattering,
the
are
interac-
Compton
the incident
photon.
This type of interaction
has a low probability
of occurrence
in the diagnostic
energy
range
and typically
accounts
for
less than 5% of x-ray interactions.
Rayleigh
interactions
are also referred
to as coherent
scattering
or classical
scattering.
During
a Rayleigh
interaction,
the electric
field of the electromagnetic
wave of the mci-
scattering,
photoelectric
absorption,
and pair
production.
This article
discusses
the importance
of these
interactions
in medical
imaging,
some of their characteristics,
and their relative
probability
of occurrence
in the energy
range
of x rays used for diagnostic
purposes
(ie, the
diagnostic
energy
range).
(Further
reading
on
these
topics
can be found
in references
1-8.)
dent
U RAYLEIGH
SCATTERING
In Rayleigh
scattering,
the incident
photon
interacts
with,
and “excites,
the total atom,
as
opposed
to individual
electrons
as is common
with the other
types of x-ray interactions.
Because
this interaction
occurs
mainly
with very
low energy
diagnostic
x rays, such as those
used in mammography
(15-30
key),
there
is
no ionization
and the scattered
photon
is emitted with essentially
no loss in energy
relative
to
details
“
2
the
and
atom
ting
in a
photon
expends
energy,
which
causes
all
electrons
in the scattering
atom to oscillate
radiate
in phase.
The electron
cloud
of the
immediately
reradiates
this energy,
emita photon
of the same energy
but typically
slightly
different
direction
(Fig i).
U COMPTON
SCAfl’ERING
Described
by Arthur
Compton
of Compton
by
interacting
with
Imaging
& Therapeutic
Technology
the
(3),
the
factors
of occurrence
imaging.
X-ray
scattering
do so
free
or valence
shell
electrons,
in which
the incident
photon
energy
greatly
exceeds
the binding
energy
of
the valence
shell electron
that is ejected.
For
example,
a iOO-keV
photon
interacting
with a
water
molecule
in soft tissue
primarily
does so
Compton
which
the binding
the water
molecules
U
so-called
in i923
and
that contribute
to its probability
are very important
to medical
photons
undergoing
Compton
through
458
scattering
scattering
energy
(ie,
interactions,
of the
hydrogen
in
electrons
in
and oxygen
Volume
18
Number
2
Valence electrons
Figure
2.
ing. Diagram
Compton
electron
dent
(Ee.)
Compton
photon
with
Angle
of deflection
are electron
..- .
is insignificant
compared
with
the
mci-
-
photon
(E1)
tive
scattermci-
energy
E()
emission
of
a Compton
scattered
photon
E5 emerging
at an angle 8 relative to the trajectory
of the
incident
photon.
K, L, and M
Scattered
-%
the
interacting
with the valence
shell electron,
which
results
in the ejection
of the Compton ejected
electron
E and
the simultaneous
Incident
photon
(E0)
atoms)
shows
to the incident
the incident
shells.
with
permission,
enee
8.)
photon
photon
(Redrawn,
from
is denoted
undergoes
refer-
as 0.
Compton
dent photon
energy.
In fact, Compton
scattering predominates
not only in the diagnostic
energy
range
of x rays in tissue
(ie, above
30 key)
but continues
to predominate
well beyond
diagnostic
energies
of x rays (to approximately
While
30
The probability
of Compton
scattering
is
proportional
to the number
of electrons
per
gram.
The number
of electrons
per gram
is
fairly constant
in most materials
with the exception
of hydrogen,
which,
because
of its lack
of neutrons,
results
in an approximate
doubling
of electron
density.
Thus,
hydrogenous
materi-
MeV).
When
Compton
scattering
does
oc-
cur at the lower
x-ray energies
associated
with
diagnostic
imaging
(25- 1 50 kVp),
the majority
of the incident
photon
energy
interacting
with
the loosely
bound
electron
is transferred
to the
scattered
photon,
which,
when
detected
by
the image
receptor,
contributes
to image
degradation
by reducing
the primary
photon
attenuation differences
of the tissues.
The components
of Compton
scattering
to
keep
track of during
the interaction
are as follows: The incident
photon
wavelength
is denoted
as X1 and its energy,
as E0. The energy
of the Compton
electron
is denoted
as E,
whereas
the scattered
photon
wavelength
is
denoted
as X2 with energy
equal
to
The
angle of deflection
of the scattered
photon
rela-
March-April
1998
scattering,
the
photon
and
simultaneously.
tening
als
have
resultant
Compton
scattered
ejected
(or recoil)
electron
The process
of Compton
is illustrated
a higher
in Figure
probability
appear
scat-
2.
of a Compton
scat-
ten interaction
than nonhydnogenous
materials
of equal
mass.
However,
in radiology,
we do
not usually
compare
equal
masses.
We usually
compare
regions
of an image
that correspond
to irradiation
of adjacent
volumes
of tissue.
Therefore,
density-that
is, the mass contained
within
a given
volume-plays
an important
Bushberg
U
RadioGraphics
U
459
role.
One can
of water
glass
density
radiographically
because
of the
of the
ice
compared
surrounding
water
Once
a Compton
the
with
(Fig
ice in a
in
that
of the
is ejected
from
3).
electron
kinetic
energy
through
and ionization
of atoms
in the sunmaterial.
The Compton
scattered
on the other
hand,
can traverse
the medium
without
interaction
or
atom,
it loses
excitation
rounding
photon,
through
may
visualize
differences
undergo
photon
its
any
of a number
of additional
including
a subsequent
interactions,
Compton
or, if the
scattering,
photoelectric
photon
energy
is quite
:1
absorption,
low, Rayleigh
scattering.
As with
and
all types
momentum
energy
must
of the
sum
of the
and
the
of interactions,
be
incident
energy
conserved.
photon
of the
kinetic
both
energy
Thus,
is equal
E)
scattered
of the
energy
the
to the
photon
ejected
Figure
3. Radiograph
(acquired
at 125 kVp with
an antiscatter
grid) of two ice cubes
in a plastic
eontamer
of water.
The ice cubes
can be visualized
because
of their lower
electron
density
relative
to that
of liquid
water.
The small radiolucent
objects
seen
at
electron
several
the
locations
are
the
result
of
air
bubbles
in
water.
E:
The
binding
ejected
energies
We
of the
can
the
that
convert
and
electron
compared
involved,
a photon
using
energy
is so small,
(i)
E5. + Ee_
=
U
with
it can
be
between
its energy
that
was
the
other
AX(flfl,)
the
wavelength
of
volts
by
equation:
As the
(2)
1.24i’X.
scattered
tered
more
‘2
AX:
These
and
=
X1 + AX.
trajectory
of the scattered
angle
0 of scatter
of the
an analysis
in wavelength
incident
of energy
between
(3)
photon
relative
photon.
conservation,
the
ton
for
incident
Derived
the
and
deto the
from
change
Comp-
Imaging
& Therapeutic
Technology
are
likely
thus
of the
image
observability
to be
increas-
receptor
of contrast.
scattering
transferred
angle,
to the
In
the
energy
frac-
scattered
pho-
with increasing
incident
for higher
energy
photons,
of the
scat(Fig
is transferred
photon
the
to
the
Compton
scattered
electron.
For example,
at a
60#{176}
scattering
angle,
the ratio of the scattered
tron
energy
E
E.
is 0.9
or
0. 1 or
to that
90%
10%
at
of the
100
Compton
keV
but
elec-
approxi-
at 5 MeV.
Conservation
of energy
and momentum
tate certain
limits on both scattering
angle
energy
transfer.
For example,
the maximum
ergy
transfer
thus
the
ton
energy)
scattered
U
more
receptor,
a given
Thus,
majority
increases,
direction
much
exposure
of energy
energy.
are
the
decreases
scatter.
460
(4)
electrons
forward
image
overall
addition,
mately
The wavelength
pends
on the
by the
cosO).
-
energy
and
the
the
reducing
tion
photon
photons
by
photon
X.
(1
photons
toward
detected
ing the
The lower
energy
of the scattered
photon
relative to the incident
photon
can alternatively
be
expressed
as the increase
in wavelength
of the
scattered
photon
relative
to the wavelength
of
the incident
photon.
Stated
another
way, the
scattered
photon
wavelength
S
equal to the
incident
photon
wavelength
A plus the change
in wavelength
be expressed
0.00243
=
incident
both
4).
E(ke)
can
ignored.
in kiloelectron
conversion
ton scattered
photons
following
equation:
to the
maximum
occurs
In fact,
photon
the
Compton
electron
reduction
in incident
with
(and
pho-
a 180#{176}photon
maximum
is limited
energy
to
backof the
5 1 1 keV,
Volume
dieand
en-
which
18
Number
2
By using
I 00
keV
90
Equation
photon
to the
change
80
is 0.02031
70
thus
(2).
60
c
a.
20
calculated
0
30
60
90
Scatter
Figure
Graph
ifiustrates
as a function
20,
140-keV
and
normalized
to 100%.
University
in tissue.
(Courtesy
180
seatfor
curve
is
M. Boone,
School
tons.
Even
of Medicine,
Davis.)
at a 90#{176}
scattering
angle
and
a maxi-
mum
of 255 keV during
a backscatter
event.
These
maximum
limits on photon
energy
are
maintained
even when
extremely
high energy
photons
(eg, therapeutic
energy
range)
interact
through
a Compton
scattering
event.
The scatten angle for the ejected
electron
may occur
at
angle
up
scattered
to 90#{176}
and
photon
backscatter.
at any
In contrast
practically
electron
will
discussed
concepts.
undergoing
outer
shell
binding
energy
scattered
of the
phoejected
an
of less
80-keV
scattering
of an oxygen
than
is
previously
the
is the
Compton
By substituting
of the
change
March-April
maximum
ejected
an
atom
with
The
first
a
transferred
0 (minimum
cited
maximal
the
energy
to
L shell).
incident
the
state
The
photon
atom
with
energy
of the
loss,
x-ray
the
scat-
energy
photon
shell
a lower
fills
is
ates
vacancy,
from
cascade
levels
occurs.
leased
as either
vacancy,
in-
ex-
inner
shell.
An
from
a
transition
is filled
energy
higher
this
and
energy
creating
This
which
higher
from
The
in an
atom.
After
in an ionized
binding
the
stable
another
energy.
is left
a vacancy
with
energetically
electrons
electron
electron.”
i80#{176}for
K or
the
nearby
pho-
with
10 eV.
energy
the
electron
question
is “What
is the minimum
energy
of
the scattered
photon,”
and the second
question
is “What
that
is
Equation
completely
absorbed
and the electron
is
ejected
(now
referred
to as an ejected
photoelectron)
with a kinetic
energy
equal
to the incident
photon
energy
minus
the binding
energy of the ejected
electron.
For photoelectric
absorption
to occur,
the photon
energy
must
be at least equal
to or greater
than the binding
energy
of the electron
that is ejected.
The ejected
electron
is most likely one
whose
binding
energy
is closest
to, but less
than,
scattering
of the
Consider
with
teraction,
Compton
some
Compton
electron
Compton
up to a 180#{176}
absorbed.
involving
to reinforce
of the
energy
be locally
A problem
that
angle
to the
all the
presented
ton
using
U PHOTOELECTRIC
ABSORPTION
Photoelectric
absorption
is another
mechanism
of x-ray attenuation
important
to diagnostic
mmaging in which
the incident
photon
interacts
with a tightly
bound
electron
(typically
one
from
occurs
ton,
by
nm,
energy
tered
photons
still have a relatively
high
and thus a good probability
of detection.
Each
ofJohn
of Radiology,
of California,
150
relative
Compton
of scattering
angle
photons
Department
120
(degrees)
Angle
4.
ter probability
any
photon
keV
remembering
added
AX of 0.00486
scattered
as 61
when
-
0
PhD,
The
80-
of an
which,
in wavelength
nm.
Now,
wavelength
ejected
electron
is equal
to 19 keY (80 keV
61 key).
This exercise
illustrates
an important
concept
about
the energy
of the scattered
pho-
10
80,
the
nm,
Compton
ejected
electron
is equal
to the difference between
the incident
and scattered
photon energies,
one can readily
see that the kinetic energy
associated
with the Compton
50
2
(2),
is 0.0155
a more
crein turn
by
levels.
Thus,
an
to lower
energy
difference
in energy
characteristic
x rays
is re-
or Auger
electrons.
energy
scattered
photon)
into Equation
(4), the
in wavelength
is equal
to 0.00486
nm.
1998
Bushberg
U
RadioGraphics
U
461
1-
Binding
Energy
(keV)
100 keV
incident
photon
Characteristic
A:O.6keV(N-’-M)
B:4.4keV(M--L)
<
X3<
22<
(L-.-K)
C:29keV
Figure
5. Photoelectric
absorption.
Diagram
shows a 100-keV photon
undergoing
photoelectric
absorption
with an iodine
atom.
In this case, the K-shell
electron
is ejected
with a kinetic
energy
equal
to the difference
between
the incident
photon
energy
and the K-shell
binding
energy
of 34 keV or 66 keV. The vacancy
created
in the K shell results
in the transition
of an electron
from the L shell to the K shell. The difference
in their binding energies
(ie, 34 and
5 key)
results
in a 29-keV
Ka characteristic
x ray. This electron
cascade
will continue,
resulting
in the production
of other
characteristic
x rays of lower
energies.
Note that the sum of the characteristie x-ray energies
equals
the binding
energy
of the ejected
photoelectrons.
Although
not shown
in this diagram,
Auger
electrons
of various
energies
could
be emitted
in lieu of the characteristic
x-ray emissions.
(Redrawn
and modified,
with permission,
from reference
8.)
To conserve
transition,
energy
during
characteristic
each
x rays
are
electron
emitted
with an energy
that equals
the difference
between
the binding
energies
of the electrons
from the initial and fmal shells.
Consider
the
transition
of an electron
from the L shell to the
K shell in iodine,
in which
the K-shell
binding
energy
is 34 keV and the L-shell
binding
energy
is 5 keV. A Ka characteristic
x ray of 34
5 =
29 keY will be released
(Fig 5). The nomenclatune used to identify
the characteristic
x rays
subsequently
Auger
electron
is established
istic
-
U
such
that
the
capital
letter
mdi-
Another
form
of energy
dissipation
is Auger
In this process,
the energy
that otherwise
would
appear
as a characteristic
x ray after an electron
transition
is transferred
to a orbital
electron
whose
binding
energy
is
electron
less
than
that
x ray
minus
characteristic
x ray
and
the
binding
energy
of the
from
would
have a kinetic
energy
of 29
0.6 = 28.4
keY. Insofar
as the electron
binding
energies
of
hydrogen
and oxygen
in tissue
are very low
and the kinetic
energy
associated
with Auger
electrons
would
be lower
than the alternative
characteristic
x-ray energy,
both the character-
Imaging
M shell
to the
a indicates
electron
was
& Therapeutic
L shell,
in which
the
that the origin
of the casthe adjacent
M shell. A K,
Technology
electron.
of the
ejected.
The kinetic
energy
of the
is equal
to that of the character-
example,
as an alternative
to a Ka characteristic
x-ray emission
of 29 keY,
that energy
may be used to eject an M-shell
electron
with a binding
energy
of 0.6 keY
within
the same
atom.
The Auger
electron
the
ejected
emission.
cates the final destination
of the cascading
electron
and the subscript
Greek
letter
indicates whether
the transition
occurred
from an
adjacent
or nonadjacent
shell. For example,
an
L0 characteristic
x ray indicates
a transition
subscript
cading
462
characteristic
x ray indicates
an electron
transition to the K shell from a nonadjacent
shell
(eg, the M shell).
For
-
Volume
18
Number
2
istic
x-ray
will
be locally
photon
The benefit
that there
are
tons
to
and
Auger
electron
an atomic
emissions
absorbed.
of photoelectric
no additional
degrade
the
absorption
nonprimary
image;
however,
is
pho-
the
local
deposition
of energy
increases
the radiation
dose in a relatively
small area, and this effect
must be considered
with respect
to its impact
on dosimetry.
The laws of conservation
of energy dictate
that the sum of the characteristic
x-ray
and
binding
Auger
electron
energy
of the
energies
ejected
equals
the
photoelectron.
The probability
of Auger
electron
emission
increases
as the atomic
number
(2) of the absorber
increases,
and thus this process
does
not occur
frequently
for x-ray interactions
in
soft
tissue.
The probability
of photoelectric
increases
dramatically
with the
of the absorber
(ie, proportional
versely,
the
probability
absorption
atomic
number
to Z3). Con-
of photoelectric
absorp-
tion decreases
dramatically
with increasing
incident
photon
energy
(ie, proportional
to
lIE03). Thus,
for a given
absorbing
material,
there
is generally
a rapid decrease
in attenuation as photon
energy
is increased.
However,
at photon
energies
equal
to the binding
energy
of inner
shell electrons,
there
is a rapid and
dramatic
increase
in attenuation.
This rapid
increase
is referred
to as an absorption
edge,
at
which
point
the number
of electrons
available
for interaction
dramatically
increases,
resulting
in a rapid rise in the attenuation
cross section.
The phenomenon
of the absorption
edge is
used in radiographic
contrast
agents
such as iodine and barium.
For these
materials,
the absorption
edges
of 33 and 37 keY, respectively,
create
substantially
increased
values
of x-ray attenuation
relative
to that of surrounding
tissues. The high atomic
number
of these
contrast agents
also dramatically
increases
the
probability
creases
wise
of photoelectric
scattered
degrade
radiation,
the
radiograph.
absorption
which
and
would
In fact,
de-
other-
photo-
electric
absorption
is the primary
mode
of interaction
of diagnostic
x rays with screen
phosphors,
contrast
materials,
and bone.
A problem
involving
photoelectric
absorption is presented
to reinforce
some of these
concepts.
Consider
a contrast
material
A with
March-April
1998
number
of 25, which
with
50-keV
photons,
terial
B with
an atomic
is irradiated
and another
contrast
ma-
of 50, which
is
irradiated
with 100-keY photons.
What is the
probability
of photoelectric
absorption
in material A relative
to material
B for the same thickness of material?
Because
the probability
of
photoelectric
absorption
is proportional
to Z3/
E03, the probability
of photoelectric
absorption
with respect
to atomic
number
changes
as the
ratio
of (25/50)
= ‘/8 or 0. 125.
The probability
of photoelectric
absorption
with respect
to
photon
energy
changes
as i/(50/iOO)3,
which
is equal
to 8. Thus,
the overall
effect
of a decrease
to one-eighth
number
in
photoelectric
absorp-
tion probability
for a material
of lower
atomic
number,
combined
with an eightfold
increase
in photoelectric
absorption
probability
associated with the lower-energy
photons,
results
in
no net difference
in the probability
of photoelectric
absorption.
At photon
energies
below
50 keV, the photoelectric
process
plays an important
role in
imaging
soft tissue.
The photoelectric
absorption process
can be used to amplify
differences
in attenuation
between
tissues
with slightly
dif-
ferent
atomic
contrast.
knowledge
ploited
numbers,
An excellent
thus
of differential
to improve
improving
example
absorption
subject
contrast
mammography.
The development
of the
tube
targets
and molybdenum
cated
mammography
subject
of how
this
can
be
is seen
ex-
in
molybdenum
x-ray
filters
for dedi-
systems
is a case
in point.
Characteristic
x rays of i7 and 19.5 keV are
produced
in the output
energy
spectrum
of the
x-ray
tube.
The tube port,
which
is made
of beryllium,
has low atomic
number
(Z = 4) to al-
low
essentially
all the photons
of importance
to be transmitted.
Unfortunately,
a preponderance
of low-energy
photons
(<1 5 key) are simply absorbed
in the breast
and a preponderance of high-energy
photons
(>20 key) reduce
the subject
contrast.
Use of a molybdenum
filten (typically
25-30
im thick)
allows
the preferential
transmission
of the desired
characteristic x rays because
their energies
are just below
Bushberg
U
RadioGraphics
U
463
Relative Photon
Intensity
1E
6
I
1.1
2
0iL
Photon
Mass
attenuation
coeffIcient
Energy
spectrum
Mo target
I
(keV)
Mass attenuation
Molybdenum
-
30 kVp unfiltered
Bremsstrahlung
- Aluminum
coefficient
srna
80
(cm/gm)
60
(cm2!
\j<
40
2:
gm)
0
Photon
Energy
(key)
Photon
Energy
3
3
with 0.03 mm Mo filter
Mo target
(keV)
Mo target
with 0.5 mm Al filter
4
Relative
Intensity
.
-
10
15
Photon
absorption
ray
photons
edge.
are
gies just beyond
creased
attenuation
sorption
reduces
--_-__.
25
(key)
30
35
energies,
absorbed,
and,
the
x-
at ener-
K-absorption
edge,
indue to photoelectric
abthe transmission
of higherenergy
photons
through
the filter.
A pseudomonoenergetic
spectrum
is thus achieved;
this
spectrum
maximizes
the subject
contrast
of
the soft tissues
of the breast
with a minimal
radiation
dose.
Aluminum
is the material
most commonly
used
for
x-ray
applications,
of the
transmission
quently
shows
results
in suboptimal
allowing
of high-energy
photons
is illustrated
in subject
with
at-
increased
and
contrast
a mammography
conse-
(Fig
6).
20
25
30
Energy (key)
at 30 kVp, with
filter directly
that
was
35
attenuation
curves
and
below
on the left and
imaged
with
both
molybde-
mammograms
of the
same
spec-
breast
oh-
tamed
approximately
10 years apart.
The
change
in imaging
techniques
over the period
results
in substantial
improvement
in subject
contrast.
The mammogram
obtained
in the
early
1980s
used 30 kYp. The higher
effective
energy,
pression
spectrum,
15
Photon
and aluminum
filters (Fig 7).
The clinical
impact
of the x-ray energy
trum
can also be seen in Figure
8, which
is used for mammography.
edge of aluminum
is not at
of interest
for mammographic
in radiography;
10
num
of the
a reduction
effect
phantom
how-
filters
which
tenuation
This
the
tube
ever, molybdenum
The K-absorption
the energy
range
5
of an unfiltered
molybdenum
(Mo) target
molybdenum
filter and a 0.5-mm
aluminum
with permission,
from reference
8.)
At lower
readily
Intensity
20
Energy
Figure
6.
Output
spectrum
filtered
spectra
for a 0.03-mm
right,
respectively.
(Redrawn,
the
Photon
I
-
.
5
Relative
Ii
2
Photon
together
era
and
resulted
with
absence
screen-film
technology
of “substantial”
in reduced
subject
comcontrast
8a). The contemporary
mammogram
was
obtained
with 26 kYp. A lower
tube voltage
and more
accurate
automatic
exposure
control
results
in a energy
spectrum
“tuned”
to provide
a beam
of lower
effective
energy
(Fig 8b). A
(Fig
substantial
improvement
in subject
and
radio-
by enhancements
in screen-film
technology,
such as increasing
the gradient
of the characteristic
curve
response
of the film and greater
breast
compres-
graphic
contrast
is achieved
sion.
464
U
Imaging
& Therapeutic
Technology
Volume
18
Number
2
a.
Figure
merized
b.
7.
Images
methyl
of an American
methacrylate
(Lucite)
College
were
of Radiology
obtained
mammography
with
target
and molybdenum
filter (a) and with a molybdenum
provement
in subject
contrast
for the calcification
specks
the test phantom
is readily
evident
in a.
test
phantom
a mammography
unit with
target
and aluminum
and other
test objects
made
of poly-
a molybdenum
filter (b).
embedded
The
imwithin
Figure
8.
Mammograms
of the same breast imaged
10 years apart. (a) Mammognam
acquired
with a mammography
unit with a three-phase
six-pulse
generator
(CGR Sentograph
500T; GE Medical
Systems,
Milwaukee, Wis) at 30 kVp and a Min-R screen
with Ortho
M
film (Eastman
Kodak,
Rochester,
NY) demonstrates
the
image
quality
resulting
from older
imaging
techniques,
including
higher
effective
energies,
film with lower
contrast
and lower
speed,
and minimal
breast
cornpression.
(b) Mammogram
acquired
10 years
later with
a mammography
unit with a high-frequency
generator
(CGR Sentograph
600HT; GE Medical Systems)
at 26
kVp and a Kodak Min-R screen with Microvision
film
(Du Pont, Wilmington,
Del) demonstrates
greater
subject
contrast
and
was
obtained
at lower
dose.
The
much improved
image was produced
with a lower effeetive energy x-ray beam, higher speed screen-film
technology,
and better breast compression.
(Modified,
with
a.
March-April
permission,
from
reference
8.)
b.
1998
Bushberg
U
RadioGrapbics
U
465
The photoelectric
when lower energy
terials
of high
diagnostic
process
photons
atomic
energy
predominates
interact
with
number
range,
(Fig
the
ma-
photoelectric
pro-
PRODUCTION
converted
namely
a high-energy
into
the
-U
U
.b
0
P.
20
C.)
0
B
50i
.
Cl)
0
2
a.
Inpalrproductlon,
den the influence
Nuclear
Medicine
9). In the
cess predominates
in materials
such as lead
used in protective
aprons,
contrast
agents,
and
the sodium
iodide
in the crystals
used in gamma scintigraphic
cameras.
Conversely,
Compton scattering
wifi predominate
at higher
photon energies
with materials
of lower
atomic
numbers
such as tissue
and air.
U PAIR
Diagnostic
Radiology
photon,
of the atomic
a matter
electron
and
un-
nucleus,
antimatter
pair,
The
andpositron.
is
keV
thresh-
old photon
energy
required
for this interaction
is 1 .02 MeV, which
is equal
to the rest mass energy equivalent
of the positron-electron
pair.
Figure
9.
Graph
plots
tion of the photoelectric
the
(right
processes
The
sues
electron
loses
its
energy
kinetic
through
and ionization
and becomes
ated with another
atom or is eventually
sorbed
into the free electron
pool.
The
tron (a form of antimatter)
also loses its
excitation
energy
by
is much
excitation
and
different
than
positron
will eventually
tron in an annihilation
of the
combine
reaction,
ab-
posiits fate
electron.
The
with an elecin which
the
rest mass energy
of the positron-electron
pair is
completely
converted
into electromagnetic
radiation
in the form of two 51 1-keV photons.
These photons,
to one another,
attenuation
emitted
at approximately
180#{176}
are referred
to as annihilation
and
MeY. After
cay.
a pair production
The
resultant
required
for pair
pair production
U CONCLUSIONS
Four types
of x-ray
unless
viewed:
high
does
not
energies
of 1 .02
scattering
energies
energies
become
greatly
MeY.
In fact,
significant
exceed
the minimum
As previously
stated,
predominates
and
the
beyond
in tissue
up
photon
energy
Compton
at diagnostic
to approximately
tis-
in excess
the extremely
to occur.
for various
reaction,
any enof 1 .02 MeY is distributed
as Idnetic energy
to the positron-electron
pair.
Positrons,
however,
are important
in nuclear
imaging,
as certain
types
of radioactive
materials emit positrons
as a form of radioactive
deergy
ted from the
tron emission
production
and Compton
as a function
of energy.
When diagnostic
energy photons
interact
with materials
of low atomic
number
(eg, soft tissues),
the Compton
process
dominates.
(Redrawn
and modified,
with permission, from reference
8.)
are illustrated
in Figure
10.
The process
of pair production
is of no consequence
in diagnostic
radiography
because
of
radiation
of contnibu-
scale)
kinetic
but
ionization,
that
associ-
scale)
percentage
(left
30
annihilation
patient
can
tomographic
Rayleigh
radiation
emit-
be detected
by posi(PET) scanners
(4).
interaction
scattering,
have
been
Compton
photoelectric
absorption,
and pair
tion. As one can see from the summary
attenuation
coefficients
in soft tissue
as
tion of energy
(Fig 1 1), pair production
not contribute
substantially
to the types
ing,
teractions
important
in the
energy
re-
scatterproducof mass
a funcdoes
of in-
range
used
in diagnostic
radiology.
Photoelectric
absorption is an important
process
in diagnostic
studies in which
contrast
agents
are employed
because
466
U
Imaging
& Therapeutic
Technology
of the high
atomic
number
of the
Volume
18
Number
2
n
and
incident
photon
(Negation)
A
(Positron)
Annihilation
4rMe”H800
Radiation
B
10.
Pair production.
Diagram illustrates
the pair production
process
in which a high-energy
incident
photon,
under the influence
of the atomic nucleus,
is converted
to a matter and antimatter
pair. The electron
expends
its kinetic
energy
by excitation
and ionization
as does the positron.
However,
when the positron
comes
to rest, it combines
with an electron,
producing
the two 51 1-keV annihilation
radiation
photons.
K, L,
and M are electron
shells. (Redrawn,
with permission,
from reference
8.)
Figure
10
E
U
3
C
0
U
1
/TOt1
0
0
U
\
C
0
0
,.-.-
Photoelectric
C
1m...I..uI.__
Compton
Rayleigh<1
<
U)
0.01
Pair
..
0.003
0.001
10
100
N
Energy
March-April
1998
Figure
Rayleigh,
1,000
(keV)
10,000
11.
Graph
the
Compton,
and total mass at-
pair production,
tenuation
coefficients
(Z
7) as a function
Bushberg
plots
photoelectric,
U
for
soft
tissue
of energy.
RadioGraphics
U
467
absorber.
It is also important
in special
applications such as mammography,
in which
the
photoelectric
absorption
process
aids in the
ability
to amplify
subtle
differences
in tissue
attenuation.
Compton
scattering
interactions
predominate
energy
contrast
tissue
over
the
majority
of the
range
in soft tissue
is chiefly
derived
density.
Detection
Acknowledginent
ofJ. Anthony
Seibert,
the 1996 AAPM/RSNA
are greatly
in which
subject
from differences
1 . Anderson
DA. Absorption
of ionizing
radiation.
Baltimore,
Md: University
Park Press,
1984.
2. Evans RE. The atomic
nucleus.
Malabar,
Fl:
Knieger,
1982.
3. Compton
MI. A quantum
theory
of the scattering of x-rays
by light elements.
Phys Rev 1923;
in
scattered
photons
by the image
receptor
results
in a loss
of radiographic
contrast.
An understanding
of how these
interactions
occur
and their effects
on subject
contrast
and
21:483.
4. Votaw
dose allow one to control
the imaging
acquisition variables
and x-ray production
equipment
to yield the best possible
diagnostic
mnformation at the lowest
possible
patient
dose.
Furthen reading
on this and related
topics
can be
found
in several
excellent
texts
in medical
physics
5.
JR. Physics
of PET. RadioGraphics
15:1179-1190.
Hendee
WR, Ritenour
ER. Medical
imaging
This
(5-8).
468
U
Imaging
article
meets
To obtain
& Therapeutic
the
credit,
criteria
see
for
the
1.0
credit
questionnaire
Technology
hour
1995;
physics.
3rd ed. St Louis,
Mo: Mosby-Year
Book,
1992.
6. Sprawls
P Jr. Physical
principals
of medical
imaging.
2nd ed. Gaithersburg,
Mo: Aspen,
1993.
7. Bushong
SC. Radiologic
science
for technologists: physics,
biology,
and protection.
6th ed.
St Louis,
Mo: Mosby,
1997.
8. Bushberg
JT, Seibert
JA, Leidholdt
EM, Boone
JM. The essential
physics
of medical
imaging.
Baltimore,
Awara
appreciated.
U REFERENCES
diagnostic
of Compton
The comments
and suggestions
PhD, who served
as chair
for
Physics
Tutorial
for Residents
in category
on pp
Md:
I of the AMA
Williams
Physician
& Wilkins,
199-I.
‘s Recognition
45 1-456
Volume
18
Number
2