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Physics
for Residents
: : Tutorial
This
artide
meets
for
criteria
hour
Clinical
Principles1
the
Applications
of Basic
X-ray
Physics
1.0 credIt
in category
1 of
Beth
A. Scbueler,
PhD
The
application
of basic
the AMA
Physician’s
Recognition
Award.
To obtain
credit,
see
the questionnaire
pp
quires
on
consideration
any
725-730.
given
each
radiographic
of these
current,
LEARNING
After
reading
(focal
and
taking
reader
.
selection
how
of x-ray
factors
density
and
Be able
the
I
to identify
by
technique
equipment
An
is
principles
and
factors.
Be able
basic
application.
.
Be able
to compare
scatter reduction
mechanisms
with respect
various
to image
patient
technique
including
measures
often
of image
factors
(source-object
patient
dose
of exposure
while
and
parameter
contrast,
density,
mo-
minimizing
patient
expo-
involves
quality
radiation
consideration
of
and exposure.
quality
dose.
and
#{149}
INTRODUCTION
in a
how
to identify the
types of x-ray
generators
and to select
the appropriate
type for a
particular
radiographic
.
quality,
of
geometry
of
tube
equipment
influence
for evaluation
unsharpness,
of radiographic
and
For
application
characteristics
also
re-
voltage,
In addition,
design)
of image
and geometric
various
basic
distance)
The basis
and
factors-tube
receptor.
generator
optimization
between
exposure
receptor
of the radiograph.
Selection
trade-offs
x-ray
radiography
interrelationships.
understanding
the
and image
use,
complex
contrast.
of unsharpness
radiograph
and learn
the visibility of detail
four
grid
is the
The
to clinical
have
time-determine
source-image
unsharpness,
sure.
image
causes
affected
tion
exposure
influences
size,
and
the quality
will:
Understand
exposure
spot
principles
that
proper
is essential.
to the patient
distance
article
the test, the
selection
.
tbis
physics
factors
examination,
factors
and
exposure
OBJECTIVES
x-ray
of many
understanding
production
of basic
and
x-ray
interaction.
to produce
physics
includes
Radiographic
an
image
that
knowledge
imaging
allows
the
of the
involves
principles
application
radiologist
of x-ray
of these
to visualize
the
basic
internal
anatomy
of the patient
so that a diagnosis
can be made.
For each radiographic
examination,
the operator
has control
over several
parameters
that affect
the appearance
of
the image,
including
the x-ray tube voltage,
tube current,
exposure
time, and distance.
In addition,
the design
of the radiographic
equipment
(x-ray tube and generator)
and
properties
of the particular
patient
and examination
(tissue,
contrast
media
used,
and
motion)
affect
radiographic
image
quality.
An understanding
of how x rays are produced
and interact
in tissue
is essential
to determine
the appropriate
selection
of technique
factors
and equipment
design
for a particular
clinical
examination.
The appearance
of a radiograph
is described
by various
image
quality
elements,
which
include
image
density,
contrast,
blur, and noise.
These
factors
describe
various
characteristics
themselves
ally
Index
has
RadloGraphics
the
RSNA
Physics
ceived
©RSNA,
that provide
interrelated
a detrimental
terms:
‘From
are
Physics
1998;
Department
November
effect
on
the
other
and comparing
in one image
factors.
Image
images.
The
quality
factor
evaluation
must
also
55905.
From
factors
gener-
include
‘ Radiography
18:731-744
of Diagnostic
Tutorial
a means
for evaluating
so that an improvement
at the
24; accepted
1996
Radiology,
RSNA
November
scientific
28.
Mayo
Clinic,
assembly.
Address
reprint
200
First
Received
requests
St, SW,
March
Rochester,
MN
1 3, 1998;
revision
to the
requested
the
AAPM/
May
27 and
re-
author.
1998
731
Figure
1. Radiographic
strate
how radiographic
was acquired
at 70 kVp
density
and the 10-kVp
rule. (a-c)
Lateral
density
varies
with different
milliampere-second
and 16 mAs. With the kilovoltage
unchanged,
with half the milliampere-seconds
the milliampere-seconds
or 32 mAs.
obtained
twice
diographic
density
and 16 mAs. With
10-kVp
reduction
varies
with different
the miffiampere-seconds
to 60 kVp,
or 8 mAs,
radiograph
exposure
as tow as possible.
article
examines
compromises
associated with the choice
of various
technique
and
equipment
design
factors
to provide
a guide
for
proper
selection
of x-ray technique,
focal spot
size, x-ray generator
type, and scatter
rejection
method.
Several
standard
texts provide
addiThis
information
and
on
image
principles
quality
of basic
x-ray
(1-7).
a SELECTION
OF X-RAY EXPOSURE
FACTORS
X-ray exposure
factors
include
the peak tube
voltage,
tube current,
and time that are selected
on the control
panel
of the x-ray machine
to produce
the desired
radiograph.
The
selection
of these
factors
affects
the image
density
and
tient
contrast
of the
radiograph
and
the
pa-
exposure.
. Control
of Image
Density
The primary
control
of image
density
ing of the radiograph)
is the product
current
and
exposure
time,
expressed
(blackenof tube
as milli-
ampere-seconds
(mAs).
Increasing
the mifiiampere-seconds
wifi proportionally
increase
the
number
of x rays that reach
the patient
and the
image
receptor.
Changes
in the tube voltage
also
affect
density,
but
in this
the tube voltage
will greatly
sure to the patient
and the
rays
through
changes
image
the
patient.
case
increase
increasing
the
transmission
As a result,
in tube voltage
cause
density.
The relationship
and
by observing
that the
overexposed
radiographs
patient
physics
phantom
demonradiograph
(b)
radiograph
(a) was
normal
Lateral
consideration
of the radiation
exposure
to the
patient.
Unfortunately,
image
quality
improvements
frequently
result
in greater
patient
exposure. Therefore,
it is important
to consider
ways to optimize
image
quality
while
keeping
tional
of a skull
values.
The
the underexposed
the overexposed
radiograph
(c) was obtained
at
radiographs
of a skull phantom
demonstrate
how rakilovoltage
values.
The normal
radiograph
(e) was acquired
at 70 kVp
unchanged,
the underexposed
radiograph
(d) was acquired
with a
(d-f)
and the overexposed
kVp. The 10-kVp
rule is demonstrated
are similar and the densities
of the
radiographs
expo-
of x
small
large changes
in
between
mdli-
(0
was acquired
with
a 10-kVp
densities
of the underexposed
(c, f) are similar.
ampere-seconds
and
tube
increase
voltage
that
#{149}Imaging
& Therapeutic
Technology
(a,
d)
results
in production
of equivalent
image
density
is
known
as the 10-kVp
rule: An increase
of 10
kVp is equivalent
to doubling
the milliampereseconds.
Figure
1 shows
how kilovoltage
and milliampere-seconds
can be manipulated
to change
image density.
As predicted
by the 10-kVp
rule, a
decrease
of 10 kVp produces
an image
with
density
similar
to that achieved
by reducing
the
milliampere-seconds
from
16 mAs
to
8 mAs,
and an increase
of 10 kVp produces
with density
similar
to that achieved
bling the miuiampere-seconds
from
an image
by dou16 mAs to
32 mAs.
It should
be noted
that the 10-kVp
rule
does not apply
for radiographs
acquired
at <60
kVp or >100 kVp or of small body parts such as
the extremities.
. Tube
Voltage
Selection
Selection
of tube voltage
is the primary
of controlling
contrast
in a radiograph.
contrast
is defmed
as the
difference
method
Image
in radio-
graphic
density
of adjacent
anatomic
structures.
The formation
of image
contrast
depends
on two independent
factors:
film contrast
and
subject
contrast.
Film contrast
depends
on the
characteristics
of the film used and how it is
processed,
which
is described
by the characteristic
curve.
Subject
contrast
is defmed
as the
relative
radiation
intensities
of the x-ray beam
exiting
the
patient.
The
subject
contrast
is
larger
if x-ray penetration
through
an object
is
much
different
from the penetration
through
adjacent
background
tissue.
The penetrability,
or penetrating
power,
is determined
by the effective
energy
of the x-ray beam:
Higher-energy
x-ray beams
penetrate
matter
farther
than towenergy
beams
do.
Because
x-ray
beam energy
directly
affected
by changing
the tube
the latter
is a major
factor
in determining
graphic
contrast.
732
to 80
radiographs
Volume
is
voltage,
radio-
18
Number
3
.
a.
d.
b.
-
c.
May-June
f.
1998
Schueler
U
RadioGrapbics
#{149}733
-
.5.
...
4
Figure
2. Effect
torn was acquired
of tube
voltage
quired
at 100 kVp
in a large reduction
and 9 mAs.
in patient
at 70 kVp
is 1 1 5 mR
370 mR
Figure
tem
3.
shows
anatomic
(0.297
x 10’ C/kg),
(0.955 x io- C/kg).
Characteristic
curve
change
in film contrast
areas
with
different
on contrast
and
60 mAs.
In addition
exposure.
whereas
with
relative
represented
by the solid and dashed
ference
in optical
densities
between
Low-contrast
to a reduction
The entrance
the
for a screen-film
and dose.
(b)
skin
exposure
(a)
High-contrast
radiograph
of a skull phanradiograph
of a skull phantom
was ac-
in contrast,
skin exposure
produced
the
increase
in kilovoltage
for the low-contrast
in the
high-contrast
radiograph
is
Shoulder
sys-
exposure.
attenuation
results
radiograph
Two
are
lines. A larger
the two areas
dii-
indicates higher
contrast
is present
in the image.
When
the
anatomic
areas are properly
exposed,
the optical
densities fall within
the linear
portion
of the characteristic
curve
and the contrast
is greatest.
If the anatomic
areas
are overexposed,
the optical
densities
fall within
the
shoulder
portion
of the curve
and contrast
is reduced.
C,)
0
C)
C.
0
Use
of high
tube
voltage
results
in a reduc-
tion in contrast,
compared
with that achieved
with low kilovoltage
techniques.
This effect
is
demonstrated
by the two radiographs
in Figure
2. The image
obtained
at 100 kVp has substantially reduced
contrast,
compared
with that
seen in the 70-kVp
image.
The loss of contrast
results
.
in the
visibility
of detail
Exposure
in
areas such as the frontal
sinus.
However,
when
the milliampere-seconds
is adjusted
so that the
amount
of radiation
reaching
the image
receptor is the same,
the 100-kVp
technique
requires
nique.
more
beam
a much
. Milliampere-Seconds
Selection
Selection
of milhiampere-seconds
affects
image
density,
as demonstrated
in Figure
la-ic.
In addition,
milliampere-seconds
selection
influences
contrast
in a secondary
way. For screen-
compared
734
in a decrease
log Relative
Imaging
lower
with
radiation
that
& Therapeutic
exposure
needed
in the
Technology
to the
70-kVp
patient
tech-
The higher
penetrating,
is absorbed
kilovoltage
x-ray beam
is
so a smaller
fraction
of the
by the patient.
Volume
18
Number
3
.
4’..J
.
1)‘
a.
b.
Figure
4.
Loss of contrast
due to improper
exposure.
Underexposed
radiograph
of a skull
phantom
acquired
at 70 kVp and 30 mAs (a)
and the overexposed
radiograph
acquired
at 70
kVp and 1 20 mAs (c) are lower in contrast
cornpared
with the normal
exposure
acquired
at 70
kVp and 60 mAs (t).
the
difference
anatomic
in optical
areas
wilt
be
densities
the
between
largest
est contrast.
Film contrast
posure
results
in densities
for
the
two
high-
is reduced
when
exthat lie in the toe or
shoulder
regions
of the curve.
The effect
of Under- and overexposure
on contrast
in a clinical
image
is demonstrated
in Figure
4.
. FOCAL
SPOT SELECTION
The choice
of focal spot size primarily
influences
the amount
of geometric
unsharpness
a radiograph.
However,
focal spot selection
C.
film radiography,
both
underexposure
(mifiiampere-seconds
too low) or overexposure
(millampere-seconds
too high)
result
in a reduction
in film contrast.
The relationship
between
film
contrast
and density
can be understood
in
terms
of the characteristic
curve
(Fig 3). The
characteristic
curve
(or Hurter
and Driffield
[H&D]
curve)
describes
the relationship
between
optical
density
and exposure.
The curve
has
three
regions
that
correspond
1998
influences
the
amount
of motion
blur
in an
image,
since
tube current
the selection
limits the maximum
and tube voltage
settings,
thereby
affecting
exposure
the
time.
In addition,
design
of the x-ray
tant consideration,
tube anode
is also
because
the anode
may
focal
influence
the
field coverage,
vided
by the
spot
and heat
x-ray tube.
size,
capacity
the
an imporangle
radiation
that
are
pro-
to different
exposure
levels.
For low- and high-exposure
levels,
the slope
of the curve
is relatively
small.
These
portions
of the curve
are the toe and
shoulder
regions.
In between
the toe and
shoulder,
the curve
is a straight
line with
a
steep
slope.
Within
the straight-line
portion,
May-June
also
in
.
Focal
Spot
The x-ray focal
rays.
Instead,
size.
This
Blur
spot is not
a point
it is a rectangular
causes
a point
Schueler
source
region
in an
object
of x
of finite
to appear
#{149}RadioGrapbics
U
735
F
Focal
Spot
I
SOD
SID
6a.
Object
Plane
OlD
1
Image Plane
Bf
5.
Figures
5, 6.
how
(5) Focal spot blur. Diagram
the focal spot blur in the image
_____________
plane (Bj7 increases
as the object is moved
6b
closer to the focal spot. OlD = object-image
distance,
SID
= source-image
distance,
SOD = source-object
distance.
illustrates
(6) Effect of focal spot size and magnification
on blur. (a, b) Radiograph
of the sella turcica,
obtained
with a small focal spot of nominal
size 0.3 mm (measured
size, 0.5 mm) (a), exhibits
greater
detail than
the radiograph
obtained
with a large focal spot of nominal
size 1.0
mm (measured
size, 1 .8 mm) (b). Both a and b have the same magnification
(M = 2). (c) Radiograph
obtained
with a large focal spot of
nominal
size 1 .0 mm (measured
size, 1 .8 mm) but with the object
in
contact
with the image receptor
(M = 1 . 1) is relatively
sharp cornpared
with b, even though
a large focal spot was used.
blurred
the
the
on
the
image.
The
amount
of blur
ing Bf
by
where
SID
the
magnification
M (M
=
source-image
distance):
Bf0
=
=
Bf/M
=Fx(1
R
Imaging
& Therapeutic
SID/SOD,
F x (OlD/SOD),
where
F = focal spot size, OlD = object-image
distance,
and SOD = source-object
distance.
To
compare
the focal
spot
blur to the size of the
object
itself, we calculate
the blur in the plane
of the object
(Bf).
Bf0 is determined
by divid-
736
=
in
image plane (BJ7 can be calculated
from
two similar
triangles
shown
in Figure
5:
Bf
_____
Technology
- 1/il).
The effect
of focal spot size and magnification on blur in a clinical
image
is demonstrated
in Figure
6. For the same magnification,
the focal spot
blur wifi increase
as the focal spot size
increases
(Fig 6a, 6b). In Figure
6b, bone
margins are indistinct
and some fme structures
blend into the background.
In addition
to the
Volume
18
Number
3
0.7
0.6
Composite
0.6
-
______
0.5
--..---.-
1.0 mm Focal
0.5
Spot
0.4
E
E
0.4
0.3
I-
0.3
0.2
0.2
Detail
Screen
0.1
0
1
1.2
1.4
1.6
1.8
2
2.2
2.4
1
1.2
1.4
Magnification
2
2.2
2.4
Magnification
Figure
7. Blur in the object
plane
as a function
of magnification.
cal spot and a high speed
screen
has a minimum
composite
blur
1 .0-mm
focal spot and a detail screen
has a minimum
composite
spot
blur also depends
on the
6c). When
there
is no magnification
(M = 1), the focal
spot blur is zero. if
magnification
is increased
by either
moving
the
object
away from the receptor
or moving
the
focal spot closer
to the object,
the focal spot
blur will increase.
Blur due to the image
receptor
will also
contribute
to the total image
blur in a radiograph.
Receptor
blur is primarily
caused
by the
spreading
of light photons
formed
by x rays interacting
with the intensifying
screen.
Because
the spreading
of emitted
light increases
as the
distance
between
the x-ray interaction
and film
increases,
the amount
of blur depends
on the
magnification
thickness
size,
the
(Fig
of the
screen
phosphor
layer.
A
thick,
high-speed
screen
has an inherent
blur
(Br) of approximately
0.7 mm,
whereas
the
blur from a thin,
detail
screen
is 0.2-0.3
mm.
As with focal spot blur, it is more
clinically
retevant
to calculate
the amount
of blur in the ohject plane
because
it can be compared
with the
size of the object
itself. The receptor
blur in
the object
plane
(Br0)
is determined
by dividing the inherent
blur in the image
plane
by the
magnification:
Br
=
Br/M.
1998
(a) A radiographic
system with a 1 .0-mm
at a magnification
of 1.5. (b) A system
with
blur when
no magnification
is used.
foa
the sum of the two components
squared.
The
contribution
to B from the two sources
depends
on the magnification
of the object.
As
magnification
increases,
focal spot blur increases
while
receptor
blur decreases.
The relationship
between
total
image
fication
can be demonstrated
cal spot-receptor
combination
graph
(8). Figure
7a shows
blur
and
magni-
for a particular
foin the form of a
the composite
im-
age blur for a radiographic
system
when
a 1.0mm focal spot and a high-speed
screen
are
used.
The curve
representing
focal spot blur
shows
how geometric
unsharpness
increases
with magnification.
The curve
representing
receptor blur shows improvement
in detail
with
magnification.
The composite
of the two mdicates
that the total image
blur decreases
then
increases
with
nification
is used,
magnification.
receptor
When
the magnification
size becomes
the major
When
blur
little
mag-
dominates.
is large,
the
determining
focal
factor
spot
in
the total image
blur. For this system,
a magnification
of 1 .5 will produce
the sharpest
radiograph.
Figure
7b demonstrates
the blur-magnification
relationship
when
a detail
screen
and
1 .0-mm focal spot are used. The detail screen
results
in tess receptor
blur compared
with
that produced
by the high-speed
screen.
For
this
equation
shows
that receptor
blur in the
object
plane
wifi decrease
as an object
is magnified.
The total image
blur in a radiograph
(H) is a
composite
of the focal spot blur and the receptor blur. It is calculated
as the square
root of
This
May-June
1.8
b.
a.
focal
1.6
system,
produced
ing the
receptor
tance.
the
sharpest
if magnification
object
as close
and increasing
Schueler
radiograph
wifi
be
is minimized
by placas possible
to the image
the source-image
ills-
#{149}RadioGrapbks
#{149}737
. Motion
Blur
Another
component
tal image
blur
that
minimized
by using
possible.
However,
factors
to produce
may
result
contributes
is patient
in an
the
to the
motion.
Motion
shortest
exposure
the selection
the shortest
increase
to-
blur
is
Focal
of technical
exposure
time
in focal
spot
Spot
blur.
Rotating
Anode
We
have seen that geometric
unsharpness
is decreased
by using a small focal spot. A small focal spot concentrates
heat on a smaller
area of
the anode
and results
in a tower
heat capacity.
A tow heat capacity
limits technique
settings
to
low power
or tow tube voltage
and tube current. With technique
selection
limited
to low
values,
exposure
time must to increased
to produce
adequate
image
density.
A large focal spot
can be used with higher
tube voltage
and tube
current
settings
for a shorter
exposure
and minimized
motion
blur.
In clinical
practice,
the compromise
tween
geometric
unsharpness
and
spot
blur.
When
large
focal
spot
tube voltage
the exposure
motion
with
values
time.
blur
higher
should
used
to
reduce
is a problem,
tube
be used
I
blur
focal
be
I
I
bemotion
small
should
I
Cathode
se-
spot
Anode
Angle
time
can be handled
by tailoring
the focal spot
lection
to the requirements
of the particular
amination.
When
image
detail
is important,
focal
Track
time
current
exthe
I
I
I’.
the
Focal
Size
I
and
Figure
to minimize
ing
8.
. Anode
Angle
The surface
of an x-ray tube anode
is angled
with respect
to the central
axis of the x-ray
beam
(Fig 8). Tubes
are produced
with anode
angles
that range
from 7#{176}
to 20#{176}.
This
angulation permits
larger
heat loading
while
minimizing the effective
focal spot size, which
is the
size of the x-ray source
as viewed
from the image. The angled
surface
increases
the width
of
the anode
focal spot track,
which
is defined
by
the surface
area impacted
by electrons
from
the filament
as the anode
rotates.
As a result,
the amount
of anode
angulation
influences
the
heat capacity
of the x-ray tube.
In addition,
the
anode
angle determines
the size of the area
of
Diagram
assembly
anode
tween
covered
by the x-ray beam,
since
the edge
the anode
wilt limit the angle of the emitted
Effective
Spot
the anode
depicts
the side view of a rotatof an x-ray tube. The angle be-
surface
and the central
axis is de-
fined as the anode
angle.
The effective
focal spot
size is the length
and width
of the x-ray beam
projected
down
the central
axis.
field coverage
is limited.
An x-ray
duced
with an anode
with a large
a larger
area,
low
because
spot
track.
anode
the
rate
on
For
heat capacity
and
as cine angiography,
small
anode
angles
are
phy
requires
large
field
ode
angles
are
of the
the
the
procovers
dissipation
width
practice,
depends
examination.
high
such
of heat
small
In clinical
angle
graphic
but
of the
beam
angle
is
focal
choice
particular
of
radio-
applications
requiring
small field coverage,
x-ray tubes
with
used.
General
coverage,
radiograso large
x
rays.
For a given
effective
focal spot size, the
choice
of the anode
angulation
is a compromise between
heat capacity
and field coverage.
As shown
in Figure
9, anodes
with small angles
provide
the highest
heat capacity,
but radiation
U GENERATOR
The x-ray generator
ity
and
technique
generator
promise
patient
SELECTION
design
exposure.
and focal
selection
of various
affects
Just
image
as with
#{149}Imaging
& Therapeutic
Technology
qual-
x-ray
spot selection,
the goat of
is to choose
the best comfactors
depending
on
particular
radiographic
examination.
The
basic types
of generators
are single
phase,
three
phase,
high frequency,
and constant
738
an-
needed.
Volume
18
the
four
po-
Number
3
Large
Anode
Angle
Small
Anode
Angle
I
Focal
Track
Spot
Width
Focal Spot
Track Width
/
/
/
/
I
/
I
I
I
/
I
I
Field
7
Coverage
Field
a.
Coverage
b.
Figure
9.
Comparison
size, the choice
of the
age. (a) Diagram
vides
a large
of small and large anode
angles.
For
anode
angulation
is a trade-off
between
depicts
focal
spot
the side view
track
width
of an anode
for a high
heat
with
a given
effective
heat capacity
a small
capacity,
angle.
but
the
focal spot
and field cover-
The small
resultant
angle
radiation
profield
coverage
is limited. (b) Diagram
depicts
the side view of an anode with a large angle. The
large angle provides
larger radiation
field coverage,
but the rate of heat dissipation
is low
because
of the small width of the focal spot track.
I
fsJ#{149}SstJ#{149}SsJ#{149}CsJ\
I
.
- -
-
.
Single
full wave
phase
rectified
(two pulse)
Three phase
Three
phase
(six pulse)
I
. Patient
Exposure
and
Exposure
Time
Considerations
of patient
dose and exposure
time can be evaluated
by examining
the generator
voltage
waveform.
Figure
10 shows
representative
voltage
waveforms
for each generator type. The voltage
ripple
is defmed
as the
percentage
(twelve
pulse)
and
difference
minimum
single-phase
between
voltages
the
in the
generator
maximum
waveform.
exhibits
The
a 100%
voltage
ripple,
since the voltage
varies
from zero to the
peak value.
The three-phase,
six-pulse
generaHigh frequency
tor
has
a lower
Three-phase,
voltage
ripple
12-pulse
of
generators
1 3%-25%.
have
a ripple
Figure
10.
Diagram illustrates
representative
voltage waveforms
for single-phase,
three-phase
(sixpulse
and 12-pulse),
high-frequency,
and constant
of 3%- 10%, which
is similar
to the voltage
ripple
in high-frequency
generators
(4%-15%).
Constant
potential
generators
have no ripple.
A generator
with a large voltage
ripple
requires
a higher
patient
exposure
to produce
a
radiograph
at a certain
kilovoltage
selection.
These
types
of generators
produce
many low-
potential
energy
0,
-
Constant potential
0
>
Time
x-ray
generators.
tential.
Selection
criteria
of patient
exposure
and
exposure
reproducibility,
low unit cost.
May-June
1998
include
exposure
compact
minimization
time, good
size, and
x rays
that
do
not
contribute
age because
they are absorbed
Therefore,
the highest
patient
needed
when
a single-phase
Schueler
to the
by the
exposure
generator,
U
im-
patient.
is
which
RadioGraphics
U
739
has
100%
stant
voltage
potential
substantial
reduction
Generator
also
is used.
longer
in greater
Use
provides
in patient
with
large
types
require
sults
ripple,
generator
Primary
ripple
times,
blur.
most
dose.
voltage
exposure
motion
of a con-
the
This
X-rays
which
re-
is because
the
low-voltage
portion
of the exposure
pulses
does not deliver
a significant
exposure
to the
image
receptor,
thus the exposure
must be
lengthened
to produce
proper
image
density.
The total exposure
time required
when
a
single-phase
generator
used is longest
of the
four generator
types.
Applications
in which
rapidly
moving
structures
are imaged,
such as
cine angiography,
need
a generator
capable
of
producing
very short
exposure
times.
A constant potential
generator
is capable
of the
shortest
exposure
pulses
of approximately
J
111111111
Image
Receptor
msec.
An additional
generator
property
affecting
exposure
time is the generator
power
rating.
Use of a generator
with a higher
power
rating
allows
for selection
of higher
tube voltage
and
tube current
exposure
factors
so that exposure
times
can be shorter
for a desired
milliampereseconds.
. Exposure
Reproducibility
Good
exposure
reproducibility
produce
images
with uniform
reduce
the number
of retakes.
is also
phy
critical
because
for
digital
High-frequency
and
erators
provide
superior
to that
phase
generators.
three-phase
can
potential
reproducibility
with singleis because
output
the
ability
to compensate
for
variations
from
the
desired
that
discussion,
provide
the
conlowest
high
cost
of the
are
less
system.
High-fre-
much
more
compact
expensive.
In addition,
generators
can
be
in
designed
from either
singleor three-phase
line
age or from a battery
or charged
capacitor
for mobile
radiographic
units.
to
voltbank
and
directly
for
radiation
run
is
on
are
in-
time
sudden
line
voltage
changes.
High-frequency
generators
use closed-loop
regulation
to sense
the tube
voltage
and tube current
continuously
and to
correct
and
quency
generators
size and relatively
in-
that
or three-
the input
line voltage.
Voltage
regulators
cluded
in these
circuits,
but the response
limits
size
gen-
single-
depends
preceding
generators
high-frequency
constant
This
potential
of scattered
patient
exposure,
shortest
exposure
time, and
good
reproducibility.
However,
disadvantages
of constant
potential
generators
are the large
be-
cause
the amount
receptor.
. Size
and
Cost
As evident
from the
angiogravoltage
images
exposure
available
power
helps
reduce
reaches
the
is required
to
contrast
and to
Reproducibility
in tube
tween
mask and contrast
complete
subtraction.
11.
Cross-sectioned
diagram
shows
how a
grid placed between
the patient
and image receptor
Figure
stant
subtraction
differences
11111
0.5
settings.
U SCATI’ER
A large
fraction
undergo
REJECTION
of the x rays entering
Compton
interactions,
which
a patient
produce
scattered
x rays. The scattered
photons
are emitted in all directions,
but they tend to be scattered
in a more forward
direction,
as the energy
of the primary
beam is increased.
When
the primary x-ray beam
enters
hone surrounded
by soft
tissue,
the
radiographic
density
change
between
the soft tissue and bone should
be very large.
However,
the high contrast
is reduced
by scattered x rays, which
strike the image
receptor
740
U
Imaging
& Therapeutic
Technology
Volume
18
Number
3
a.
b.
Comparison
of grid and “nongrid”
techniques.
Both radiographs
of a skull phantom
were
with 90 kVp, 105-cm source-image
distance,
and 80-cm source-object
distance.
Radiograph
obtained
with a grid (grid ratio of 12:1 [grid thickness:interspace
width])
(a) demonstrates
a
noticeable
improvement
in contrast
compared
with the nongrid
radiograph
(b). In addition,
a substantial
increase
in patient
dose was required
for the grid radiograph
(150 mR [0.387
x 10
C/kg])
compared
with the nongrid
radiograph
(33 mR [0.085 x 10’ C/kg]).
Figure
12.
obtained
can
the shadow
of the bone.
Several
methods
be used to reduce
the amount
of scattered
x
The increase
in contrast
is achieved,
ever,
at the expense
of increased
patient
howdose.
rays
that
The lead strips
of the grid absorb
some
radiation
that would
have reached
the
receptor;
thus, an increase
in exposure
quired
to achieve
the same film density.
entrance
skin exposure
for the radiograph
taken
with the grid is 150 mR (0.387
x
kg), whereas
the skin exposure
produced
obtaining
the radiograph
without
a grid
of the
image
is reThe
stantially
C/kg).
within
reach
the
receptor.
oils are the use of grids
tion to limit the volume
reduces
the production
Two
of these
meth-
or an air gap. Collimaof irradiated
tissue also
of scattered
x rays.
. Grids
The most common
method
of reducing
the
level of scattered
radiation
reaching
the image
receptor
is use of grids (9). A grid is constructed
of alternating
strips oftead
and nonabsorbing
interspace
material
and is placed
between
the
patient
and image receptor.
The strips are arranged
a line
rected
to transmit
from
at an
the
only
x-ray
angle
are
those
source
x rays
(Fig
preferentially
directed
1 1). X rays
absorbed
in
1998
at 33 mR
(0.085
x 10
Cl
in
is sub-
The ratio of the exposure
required
with grid
use and without
grid use is called
the Bucky
factor.
The Bucky
factor
is higher
for higher
tio grids and higher
energy
exposures.
ra-
diby
the grid. Because
most scattered
x rays are emitted at an angle to the primary
beam direction,
a
large fraction
of the scattered
radiation
is absorbed.
Figure
12 demonstrates
the contrast
improvement
that can be obtained
by using a grid.
May-June
tower
10
. Air Gap Technique
Another
method
of reducing
tered
radiation
that reaches
is to place
a gap
between
Schueler
the level
the image
the
patient
U
of scatreceptor
and
RadioGraphics
the
#{149}
741
receptor
(10). Because
are emitted
at an angle
most scattered
x rays
to the direction
of the
primary
beam,
a large
fraction
will not strike
the receptor
if it is separated
from the patient
by
a sufficient
distance
(Fig
13).
However,
Primary
X-rays
pri-
mary x rays
source
wifi
directed
in a line from
the x-ray
not be affected.
The typical
air gap
distance
is 1 5-45
cm, which
wifi also introduce
some
magnification
and limit the field of
view of the subject.
The change
in contrast
that results
from use of an air gap is shown
in
Figure
14.
Patient
Clinical
Applications
Both grid and air gap techniques
are effective
means
of controlling
scattered
radiation
and
improving
contrast
in a radiograph.
To select
S
which
method
is best for a specific
application, we must examine
the trade-offs
involved.
Grids do not require
use of magnification,
so
focal spot blur is reduced.
When
an air gap is
used,
a small focal spot is generally
needed
to
minimize the geometric
unsharpness.
The advantages
of the air gap technique
include
use
of lower
milliampere-second
values,
compared
with that needed
for the grid technique,
which
results
in less
tube
tient exposure
may
gap, but the amount
the source-to-patient
loading.
In addition,
the
be reduced
by using
of reduction
depends
distance
used.
Air
Gap
$
Figure
13.
Cross-sectioned
an air gap placed
between
diagram
shows
how
the patient
and image
receptor
helps reduce
the amount
of scattered
radiation that reaches
the receptor.
pa-
an air
on
For most radiographic
examinations,
grid
use is common.
However,
there
are several
applications
in which
the air gap technique
offers
some
advantages
over grid use. One such application
is cerebral
angiography
(1 1). When
the
air gap technique
is used in cerebral
angiography, the geometric
magnification
is generally
adjusted
to 1 .5- 1 .8. Some
radiologists
prefer
the magnified
images,
which
are free of grid
lines. Compared
with a grid technique,
the increase
in patient
exposure
resulting
from positioning
the
patient
closer
to the
x-ray
source
approximately
the same as the exposure
increase
required
when
a grid is used. Another
air gap application
is chest
radiography
(12).
this
case,
because
a large
source-image
tance
(6 or 10 feet) is typically
used, the
an air gap can substantially
reduce
patient
and the resulting
magnification
is slight.
742
#{149}
Imaging
& Therapeutic
Technology
is
. SUBJECT
CONTRAST
The amount
of subject
contrast
produced
is affected by both physical characteristics
of the
object
ray
and
penetrating
beam.
ness,
Object
physical
number
(Z).
different
sorb
In
use of
dose
density,
Two
voltage
erator,
effective
areas
include
x-
thick-
atomic
that
have either
or Z will ab-
densities,
amounts
characteristics
of the
include
and
tissue
thicknesses,
different
of radiation.
X-ray
tube
and
voltage
beam
the
waveform
produced
by the x-ray genplus filtration
(ie, added
and inherent
attenuating
dis-
characteristics
characteristics
material
in the
path
of the
x-ray
beam).
As demonstrated
in Figure
2, kilovoltage has a substantial
effect
on image
contrast.
In addition,
factors
such as filtration
and voltage waveform
shape
also alter the distribution
of x-ray energies
in the beam.
The
way
in which
gether
to produce
mined
by Compton
these
subject
and
factors
contrast
photoelectric
Volume
come
to-
is deter-
interac-
18
Number
3
a.
b.
Figure
14.
Comparison
of air gap and “non-air
gap”
tom were
acquired
at 90 kVp without
a grid. Radiograph
strates
a noticeable
improvement
in contrast
compared
ample,
the source-image
distance
was adjusted
so that
proximately
the
Effective
Densities
Materials
same
to
better
compare
techniques.
Both
numbers,
and
ties
are
very
ton
interactions
a patient
Effective
Atomic
Number
Physical
Density
(g/cm3)
beam
1.00
1.00
0.91
3.50
4.93
0.0013
7.5
5.9
56.0
53.0
7.6
ray
not
for
the
Compton
interactions
manly
on
dence
on atomic
tissue
occurrence
strong
ray
in tissue.
probability
occurring
density,
number
with
energy.
The
teractions
increases
as energy
on
of
body
little
depen-
being
energy.
interactions
atomic
has
number
probability
and
a
of photoelectric
as Z increases
increases.
and
de-
sity.
Radiography
tissue
requires
to radiographic
technique.
ids,
May-June
and
fat
1998
have
relatively
special
Muscle,
low
attention
tissue
effective
can
be
the
attenuation
imaged
so that
The
has
Because
example
x-
of softin which
with
the
the
a
and
we
need
barium.
dennormally
material
maximize
Use
of 60-75
it produces
many
iodine
and
structures
contrast
to
is in-
contrast
number
of the
iodinated
the
object
contrast
used
atomic
size
into
of the
subject
iodine
absorption.
because
introduced
commonly
a high
the
small,
above
most
contain
usually
able
anode
to change
imaged
ential
. Soft-Tissue
Imaging
of soft
maxi-
a tow-energy
Media
Iodine
in-
To
kVp are used with
and filter to produce
agents
materials
x-
An
inter-
types.
x rays.
creased.
The
interac-
is mammography
Contrast
pri-
tissue
of 25-30
tube
when
x-ray
Compton
used.
factors
x-ray
to occur
as photoelectric
effect,
radiography
Contrast
depends
very
or x-ray
of photoelectric
dependence
creases
The
likely
However,
be
densiComp-
a high-energy
as effective
must
and
As a result,
distinguishing
technique
special
numbers
more
photoelectric
beam
tissue
.
of x rays
this exis ap-
(Table).
with
keY).
are
mize
atomic
are
(>40
tions
their
similar
low-energy
tions
(b). For
radiographs
is imaged
actions
7.4
Water
Muscle
Fat
Barium
Iodine
Air
of a skull phanair gap (a) demon-
contrast.
Atomic
Numbers
and Physical
of Human
Tissues
and Contrast
Substance
radiographs
obtained
with a 25-cm
with a contact
radiograph
magnification
in the two
kVp
K-absorption
are
the
x rays
edge
differ-
is preferjust
of 33 keV.
fluatomic
Schueler
U
RadioGraphics
I
743
x-ray
K-absorption
absorption
ergy
is equal
(The
edge is an abrupt
increase
that occurs
when
the x-ray
to or
energy
binding
The
atomic
slightly
greater
of the K-shell
number
than
in
en-
electrons.)
density
of barium
and
case,
normally
fluid-filled
cavities
are
absorption
difference
in density.
.
results
from
the
3.
Baltimore,
Md: Williams
Bushong
SC. Radiologic
gists: physics,
biology,
a
4.
5.
application
clinical
of x-ray
many
interrelated
technique,
scatter
factors
focal
rejection
generally
for
of image
posure.
size,
method.
involves
selecting
these
quality
without
principles
to
7.
of
including
exposure
generator
type,
8.
and
Selection
of these
compromises
that
patient
9.
cx-
10.
P. Physical
principles
of medical
imag-
Va: American
College
of Radiology,
Bucky
1 1.
1 . Arnmann
E. X-ray generator
and AEC design.
In: Seibert
JA, Barnes
GT, Gould
RG, eds.
Specification,
acceptance
testing,
and quality
control
of diagnostic
x-ray imaging
equipment.
12.
G.
A grating
diaphragm
to cut
off sec-
ondary
rays from the object.
Arch Roentgen
Ray 1913; 18:6-9.
Gould RG, Hale J. Control
of scattered
radiation
REFERENCES
U
Sprawls
x-ray imPhysics,
1984.
is optimization
excessive
considerations
ing. 2nd ed. Gaithersburg,
Md: Aspen,
1993.
Sprawls
P, Lamel DA. Principles
of imaging,
Sequence
142 (case no. 8042). Diagnostic
Radiological Health Sciences
Learning
Laboratory.
Reston,
particular
characterisexamination.
The
factors
imaging
designs.
In:
film mamand mcdiMadison,
Wis:
physics
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Medical
Physics,
1991; 47-66.
Hasegawa
BH. The physics
of medical
aging.
2nd
ed. Madison,
Wis: Medical
1991.
cat
large
consideration
factors,
spot
should
be based
on the
tics of a given
radiographic
basis
physics
requires
St Louis, Mo: Mosby,
1988.
Curry
TS, Dowdey
JE, Murry RC. Christensen’s
physics
of diagnostic
radiology.
4th ed. Philadetphia,
Pa: Lea & Febiger,
1990.
Gauntt
DM. Mammography
x-ray generators:
mography:
6.
radiography
& Wilkins,
1994.
science
for technoloand protection.
4th ed.
conventional
and high frequency
Barnes GT, Frey GD, eds. Screen
filled
CONCLUSIONS
The
20. WoodPhysics,
1994;
of
233-266.
Bushberg
JT, Seibert JA, Leidholdt
EM, Boone
JM. The essential
physics
of medical
imaging.
with air. Even though
the effective
atomic
number
of air is similar
to that of soft tissue,
differential
No.
2.
the
are similar
to those
of iodine,
but the size of
the structures
normally
imaged
with barium
contrast
agents
are generally
large. Therefore,
high kilovoltage
technique
is used
to penetrate
the contrast
agent
and better
visualize
the lumen.
Air can also be used as a contrast
agent.
In
this
Medical
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hnaging
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Technology
hour
in category
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1 oftbe
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Volume
18
Number
3